DEPARTMENT OF THE INTERIOR

Roy 0. West, Secretary

U. S. GEOLOGICAL SURVEY 

George Otis Smith, Director

Bulletin 803

GEOGRAPHY, GEOLOGY, AND MINERAL RESOURCES 

OF THE PORTNEUF QUADRANGLE, IDAHO

GBOKGE ROGERS MANSFIELD

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON

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CONTENTS

Abstract. ___-_------.- ________________--_----_--_----,_________-_ 1

Introduction __ _____________________---_---_-__..__--_____________ I

Scope of the work._____________. ______________________________ 1

Location and area of the quadrangle_____-_--_______-__ _________ _ 2

Field work. __----_- ___ ________--__-_-___--__-__ _ ___ _ __. 3

Historical notes. ______________________________________________ 3

The Hayden surveys___________________-______________________ 5

Later geologic work. _____________-_-___-___________________. 6

Geography- __------_ __.__-_____. __ _._ _______ 0

Summary of regional physiographic development. ______________ _ 6

Principal topographic units-_---__-_-_-___-_-_-_-_-___________-_ 7

Blackfoot Mountains. _____________________________________ 8

Blackfoot Valley__ __ _ __ _.-______ _ _ ___ ___ ____ _ 8

Chesterfield Range... __ _ ____.__-__ __ _. __ ____ __ _ 8

Soda Springs Hills __ _____________________________________ 10

Portneuf Valley.. ______ __ __ ___ _.._ ___ __ ___ __ _ 10

Portneuf Mountains_-_--__-_---_-__--_-_--------_--____--- 10

Drainage- _.-.-_-___--__-.-.-_---_-_-_-_--------__---__-___--- 11

Rivers _____-_-__-__--______-_-----------------_--_____--- 11

Springs ____-_-_-___--__--__---_-----__---____-________-_- 12

Climate. _______ ... ____ _. _ _____ 12

Vegetation _______-_-_---_-_-___---_----_--__----____-_____--,. 13

Settlements and industries_______-__-___-__________ .______.___.__ 13

Transportation facilities __ ____-_----_---_---_-____.. _ ____ _, 13

Geology.-..-.- ---------------------------------------..------_---- 14

Stratigraphy . __ ... .. 14

The stratigraphic column. _-___-___._____________.___._..., 14

Cambrian system____. ____________________________________ 16

General features ___._._____._-_____.________._______.-__ 15

Brigham quartzite. _ _-_--___-_-.-_-____.______.____ 16

Langston limestone. _.__--____-_-_-___--___________,_ 16

Ute limestone __ ___-___-_.-__.__-_____-_-__________ 16

Blacksmith limestone... _______________________________ 17

Bloomington formation __ _____.__-.-___-___________._- 17

Nounan limestone.. _ __________________________________ 17

St. Charles limestone..-.-..-.... _-_-_-___-.__________. 18

UndifTerentiated Cambrian.. _.__-_____-_-_.___.. ______ 18

Ordovician system. _____________________________ _ _ ______ 19

Garden City limestone-.-.-----.---.---.---..^..!.-,.-- 19

Swan Peak quartzite----_---_-------_-_-_-______I__-_- 20

Fish Haven dolomite_---_--------------__--__________, 20

Silurian system. _________.___--____-_-__________________ 20

Laketown dolomite _ _.___--_-_-_-_-_______________ _ 21

Devonian system... _______________________________________ '21

Jefferson limestone___-_-__-_-__-_-_-_-_-_____________, 21

Threeforks limestone.. _-__----_-_--__________ _ __.__.. 21
IV . CONTENTS

Geology Continued.

Stratigraphy Continued. Page

Carboniferous system.-____________________________________ 22

Mississippian series_______-_________________-__-___-___ 22

Madison limestone-__--____________-___-__-____-_- 23

Brazer limestone._________________________________ 25

Pennsylvanian series.-__________________.______________ 26

Wells formation______________.____________________ 26

Permian series._________-_-______--__---___-___-_-__-_ 27

Phosphoria formation,._____________________________ 27

 Triassic system______________-__-____-..-_-__._____________ 29

Lower Triassic series__-_---________________---________ 30

Woodside shale.___--___-______-_-___--------__-__ 30

Thaynes group_______.___________ _______________ 31

Timothy sandstone..._____.-_____-_____.-__-__---- 33

Triassic (?) formations..____-___--__-__---___--_--_____---_ 34

Higham grit._________________________________________ 34

Jurassic system.___________________________________________ 35 f

Cretaceous system._______________________________________ 35 c

Cretaceous or Tertiary rocks___ __--_____________-_-_--_-___ 35

Tertiary system..___-___________---______________---____-_ 36

Pliocene (?) series._____________________________________ 36

Salt Lake formation.______________________________ 36

Quaternary system._______________________________________ 37

Pleistocene series.._-___-___-_____-_________-_-_-_-_-__ 37

Travertine-_----_--_-__-___---__-_-___--_-____--- 37

Hill wash and older alluvium________.______________ 38

Recent series_____-_---_--___-____-_-__-___-_----_-__- 39

 Alluvium_ _____________________________________ 39

Igneous rocks.._______________________________________________ 40

General occurrence._______________________________________ 40

Hornblende andesite porphyry.--__-_-_-_________--__-______ 40

Rhyolite... ... ___--.._________________----_._--_ 41

Olivine basalt.....__._____-.__..______..____....-__---.- 42

Relations of igneous rocks to sediments______________________ 44

Relations of igneous rocks to each other..__.____.__-____-_--_ 44

General igneous activity of the region______________________ 44

Age of the igneous rocks_________________._________________ 45

Structure_ -____________ ____________________________________ 45

Earlier geologic work--_-____________._.._.___._-_-__-____. 45

General features._-_--_._-__-___-__-._.-__-______.___--_-_ 46

Structure contours__-------_-_-__-_-_________-._-----_-_-_ 47

Detailed descriptions-___--__---_-__-___________-_-______-- 47

Corral Creek anticline._________________________________ 48

Nigger Creek syncline...___________..__ _______.________ 48

Grizzly Creek anticline._______________________________ 49

Graves Creek syncline.________________________________ 51

Twentyfourmile anticline_____________________________ 52

Rock Creek syncline..--__-_____-______________--_--_-_ 53

Otherfolds---_---------__-____________.__,----__..-_.- 55

Bannock overthrust.__________________________________ 55

Other faults.______.*__________________.--_____-..- 59

Age of the folding._____ ____________________________' 61

, ; Age of the faulting.____________________________._ 62

Relation of structure to phosphate beds__________,--__-__.___ 63
CONTENTS

Geology Continued.

Geologic history__________-____________.___-._----_----_-_-_-_- 63

Summary of record..___-_____________-_-___--__-_-_-__-__- 63

Proterozoic era_______-____________-__-_-_---_-_-_-__-_-- 63

Paleozoic era_-____---_--________---------_---.----_--_---- 63

\ Mesozoic era_ ..............................j....^........ 65

Cenozoic era...____________________________________________ 66

Mineral resources ____________-_-_--__-____-____-.-..--__-_-_-__-_-_ 67

Phosphate rock__________-____-_________-_--__.._-_-__-_-__--_- 67

General character._._-________________-__-_---__---__.-___ 67

Iron and alumina content,_________________..----_-_____-___ 68

Vanadium content. ....................................... 68

Distribution of phosphate in Portneuf quadrangle..._._..__... 68

Description of townships that contain phosphate___.____-_._ 69

T. 5 S., R. 38 E.._......_......_..._..__.--.......-... 69

T. 6 S., R. 38 E._.___ ................................. 72

T. 5 S., R. 39 E_ -.-_-.__.--- --_ -._ -. 73

T. 6S., R. 39E. - - ,- .. - -_ - 77

T. 5 S., R. 40 E. _ ..............._........_ 81

T. 6 S., R. 40 E_._ " - - - -_ - 86

T. 7 S., R. 40 E....................................... 88

Government investigations relating to phosphate._____________ 89

Geological Survey_..-______-_--_---___---------..____ 89

Bureau of Soils..--------_--_------_-----------------__ 90

Bureau of Standards__-_-__-_----_---__----___.___-__ 90

Bureau of Mines----._-_____-_--_________---_____,____ 91

Comparison of phosphate rock from different fields.___________ 91

Occurrence and physical characteristics._________________ 91

Chemical and mineral composition___,___----__________ 93

Phosphate reserves of the United States______-_--_-_.______ 94

Florida..........-_--______._______.._.-___-...___..__ 94

Western States.______-._________-_____--_-__-_.__-__. 94

Revised estimates of tonnage..___.._________________________ 95

Further notes on phosphate__._________-__,__---___-_____- 95

Limestone___. .-.........-.........-^.......-.-............ 96

"Earlier Paleozoic limestones.._._-__________,-_-_._-.-__.-__ 96

Carboniferous limestones.._______._.____-..,__.._.__.___.__ 96

Triassic limestones.____-__-__.___-_-_._______--______-_.__ 97

\ Tertiary limestones___.____________-__.__----_-__-__-._. 97

\ Quaternary limestones. .................................... 97

\ Road metal____._.-____.------_____-__-__--_-_-_--_-__-___--_ 97

V Building stone....-___.__-..______.-__-..____-_--____-___--. 98

/ Quartzite ......_____......__._._..__.......___._..... 98

Water resources____-._,--___._____-________________-____-_. 99

Surface water..........--_--____,_._-_-____^4-V-:.-----r --- 99

Blackfoot River...__ ...........................L:l;.'.. 99

Portneuf River-_------___-_----__--_._-- __-_..--..I 100

' Ground water.._____--_--__.-__----____-__-_--_-_-__-___- 102

General distribution and availability......._____-___--.__ 102

Springs.____--_---___-__-__-__.____---_-__.___-_- 103

General character and distribution______-_____-___. 103

Mineral springs._-___.___-___-______--___-____--_. 103

f Thermal springs._-_____-_--_-__-___---__---__--__ 104
VI CONTENTS

Mineral resources Continued.

Water resources Continued. Page

Utilization of water.__________________.-__--___-____---___ 104

Domestic use._--____-___-____-___-_----__--_-----__-_ 104

Irrigation.___________________________________________ 104

Portneuf-Marsh Valley works_________---____-____ 105

Empire Irrigation District___________J__--_-______ 105

Water power________________________________________ 106

Blackfoot River....._________________--._ 106

Portneuf River________-___-_-__-_______________ 107

Index. __---_--._________________________________________ 109

ILLUSTRATIONS

Pag* 

PLATE 1. Geologic map of the Portneuf quadrangle.____-_-_-_-___ In pocket.

2. Geologic structure sections in the Portneuf quadrangle,___ In pocket.

3. A, Ridge of Brazer limestone northwest of Monroe Canyon, 

T. 7 S., R. 40 E.; JB, Travertine cliff with cave at mouth of 

Eighteenmile Canyon, sec. 12, T. 7 S., R. 39 E.; C, View 

near forks of Eighteenmile Creek, T. 7 S., R. 40 E_ ._____._ 26

4. A, Terraces at mouth of Twentyfourmile Canyon, Chesterfield; 

B, Hill of Swan Peak quartzite in sec. 35, T. 7 S., R. 38 E.; 

C, Fold in Madison limestone uncovered by erosion of travertine, sec. 19, T. 7 S., R. 40 E____._____________,. 26

5. A, Summit of. ridge in SE. Y± sec. 20, T. 7 S., R. 40 E.; B,

Ledges of Rex chert in sec. 10, T. 6 S., R. 39 E__.___--____ 26

6. A, Ledges of Salt Lake formation, sec. 2, T. 5 S., R. 38 E,, 

Paradise Valley quadrangle; B, Massive ledges of Salt Lake 

formation by road in NW. % sec. 6, T. 5 S., R. 38 £._.____ 26

7. A, Spring cone of travertine in sec. 15, T. 5 S., R. 40 E.; B, 

Prospect made by Geological Survey party in phosphatic 

shales in SE. % sec. 34, T. 4 S., R. 40 E.; (7, Canyon of 

Blackfoot River below dam in sees. 11 and 12, T. 5 S., R. 

40 E., Cranes Flat quadrangle_-------__--_-_--_--------_ 26

8. Geologic map and structure section of T. 5 S., R. 40 E____ In pocket. 

FIGURE 1. Index map showing the position of the Portneuf quadrangle

with reference to previously surveyed areas ______________ 4

2. Sketch map of part of the Bannock overthrust, showing its relations to the Putnam overthrust and to later-mapped portions in the Portneuf, Paradise Valley, and Ammon quad3. Map showing areas of classified phosphate land in townships

included wholly or in part in the Portneuf quadrangle.___. 73
GEOGRAPHY, GEOLOGY, AND MINERAL RESOURCES 

OF THE PORTNEUF QUADRANGLE, IDAHO

By GEORGE ROGERS MANSFIELD

ABSTRACT

In this report the interesting history of the Portneuf quadrangle Is briefly 

sketched and its geographic features are systematically described. About 

half of the area is a much eroded mountainous upland whose ridges trend 

northwest and have a maximum altitude of about 7,300 feet, the remainder 

forming broad lowlands 1,000 to 2,000 feet lower. The climate is semiarid, with 

&n average rainfall ranging from 13.88 inches at Pocatello to 17.26 inches at 

Blackfoot Dam.

The geologic formations include representatives of all the Paleozoic and later 

systems except the Jurassic, Cretaceous, and early and middle parts of the 

Tertiary. These are systematically described. The rocks are folded and broken 

and have a noticeable tendency toward eastward inclination or overturning, a 

common structural habit of the general region. Many of the structural features 

are concealed or obscured by a widespread blanket of Tertiary rocks, but they 

are described in such detail as is practicable. A noteworthy structural feature 

is the occurrence of a window in the Bannock overthrust where the uparched 

thrust plane has been cut through by the erosion of the upper fault block. The 

geologic history is briefly outlined.

The mineral resources of the quadrangle are phosphate rock, which is the 

most valuable, limestone, road metal, building stone, and quartzite. Phosphate is discussed both in connection with the description of seven individual 

townships and in some of its broader relations. All these resources, which are 

treated in the order named, are present in sufficient quantity to supply more 

than any probable local need over a long period. Water resources are described 

in the final chapter. Surface waters are insufficient for irrigation within the 

quadrangle, because under the present plan of utilization much of the available 

supply is distributed outside. Ground water Is apparently abundant, but as yet 

is little utilized. The opportunities for power development within the quadrangle are not promising.

INTRODUCTION ^ y. ., 

SCOPE OF THE WORK

The study of the Portneuf quadrangle, Idaho, is one of a series 

which the United States Geological Survey has planned for the 

Rocky Mountain region in the general vicinity of the Idaho-Wyo1
2 POBTNEUF QUADRANGLE, IDAHO

ming border. Detailed examination has revealed the fact that this 

region contains tremendously thick accumulations of sedimentary 

rocks, geologic structure of great complexity, igneous rocks in considerable volume and some variety, and mineral resources of great 

value.

The work previously accomplished in this region, chiefly under 

the direction of the writer, comprises the mapping of the Fort Hall 

Indian Reservation, with an area of about 800 square miles, and of 

the Cranes Flat, Henry, Lanes Creek, Freedom, Slug Creek, Crow 

Creek, and Montpelier quadrangles, which together have an area of 

about 2,200 square miles, a total of 3,000 square miles. In addition 

the Randolph quadrangle, Utah, which adjoins the Montpelier quadrangle on the south and has an area of 892 square miles, has been 

mapped by G. B. Richardson. The writer has also made reconnaissance studies of areas in the Willow Creek, Pine Creek, and 

Teton Basin districts and on the Continental Divide in Idaho and 

the Salt River Range in Wyoming. Official reports covering various 

phases of the work and a number of scientific papers relating to 

the region have already been published. The Fort Hall Indian 

Reservation is the subject of a separate bulletin,1 and the area of the 

seven designated quadrangles in Idaho is described in detail in a 

professional paper of the Geological Survey.2

As much of the country supposed to contain valuable phosphates 

has not been mapped, it has been thought worth while to continue 

the detailed mapping of areal units, not only for the sake of outlining and determining the character of the phosphate reserves but also 

to supply a basis for broader regional studies which it is hoped may 

shed additional light upon the history and development of the 

Rocky Mountains.

A fairly complete bibliography of the subjects covered by this 

report is given in the professional paper above referred to (see also 

Professional Paper 56), so that a bibliographic list in this bulletin 

is unnecessary.

LOCATION AND ABEA OF THE QUADRANGLE

The Portneuf quadrangle is in southeastern Idaho between meridians 111° 45' and 112° west longitude and parallels 42° 45' and 

43° north latitude and comprises about 219 square miles. Its west 

edge is about 23 miles due east of Pocatello, with which it has con1 Mansfield, G. R., Geography, geology, and mineral resources of the Fort Hall Indian 

Reservation, Idaho, with a chapter on water resources by W. B. Heroy: TJ. S. Geol. Survey 

Bull. 713, 1920.

3 Mansfield, G. R., Geography, geology, and mineral resources of part of southeastern 

Idaho: U. S, Geol. Survey Prof. Paper 152, 1927.
INTRODUCTION <5

\

nections by road and by rail, but as these routes have to follow the 

windings of Portneuf Canyon distances along them are nearly three 

times as great as air lines.

The quadrangle includes in the northwestern part a strip about 6 

miles long and 3 miles wide belonging to the Fort Hall Indian 

Reservation, which was mapped by a Geological Survey party in 

charge of the writer in 1913. On the east it is adjoined by the Henry 

quadrangle, which, with six other quadrangles on the north, east, 

and southeast, was mapped in the same way in 1916 and earlier 

years. The Portneuf quadrangle thus serves as a connecting link 

between two previously surveyed areas and helps to provide a continuous geologic section from the Snake River Plains of southern 

Idaho eastward to the ranges of western Wyoming. (See fig. 1 

and pi. 1.)

FIELD WORK

The topographic control work was done by L. F. Briggs and F. L. 

Whaley and the topographic mapping by C. G. Anderson and J. L. 

Lewis, all of the Geological Survey, in 1914. In 1913 T. 5 S. and 

part of T. 6 S., R. 38 E., were mapped geologically as part of the 

Fort Hall Indian Reservation, and in 1916 T. 5 S., R. 40 E., with some 

adjacent territory, was mapped by the writer and P. V. Roundy in 

connection with the survey of the Henry and Cranes Flat quadrangles. In 1923 the writer, assisted by H. C. Mansfield and later 

by W. B. Lang, spent two months reviewing portions of the previously mapped areas and completing the geologic mapping of the 

Portneuf quadrangle. Later in the season they carried the work 

forward into the Paradise Valley quadrangle, on the north. In 

1925 parts of two days were spent measuring the Paleozoic section 

north of Monroe Canyon and examining the beds north and south of 

Tenmile Pass.

HISTORICAL NOTES

The Portneuf quadrangle lies in an area important as hunting and 

trapping grounds to the Indians, trappers, and earlier explorers. As 

general accounts of the early history of the region have been given 

elsewhere,3 only a few supplemental notes need be added here. The 

name Portneuf, which has come down from early days, is that of 

one of the trappers.

In the discussion of the Fort Hall Indian Reservation the writer 

noted the activities of Captain Bonneville in the region of Snake 

and Portneuf Rivers and expressed the opinion that Bonneville

»Mansfield, G. R., Geography of southeastern Idaho: Assoc. Am. Geographers Annals, 

vol. 15, No. 2, pp. 51-64, 1925. See also U. S. Geol. Survey Bull. 713, pp. 12-14, 1920; 

Prof. Paper 152, pp. 2-5. 1927.
PORTNEUF QUADRANGLE, IDAHO

25 MILES

FIGUBB 1. Index map showing the position of the Portneuf quadrangle with reference 

to previously surveyed areas
INTBODUCTIOET 5

wintered near the large springs on the Snake Biver flat within the,, 

reservation at the heads of Clear Creek and Spring Creek. Irving * 

quotes the captain as saying that the springs where he wintered 

" gush out of the earth in sufficient quantity to .turn a mill and furnish beautiful streams, clear as crystal and full of trout of large 

size." So far as size and character are concerned Bonneville's description would certainly apply to the springs mentioned. In talking 

about the matter with H. Van Slooten, of Bancroft, however, the 

writer learned that local opinion favored the springs in sees. 29 

and 30, T. 7 S., K. 39 E., as the probable site of Bonneville's winter 

camp. Upon a review of Irving's account the geographic setting 

there given was seen to fit the upper Portneuf region somewhat 

better than the lower Portneuf. It is quite probable that the lastnamed springs were the actual site of Bonneville's camp and that 

Portneuf Valley was the scene of some of his hunting exploits.

Portneuf Valley and Portneuf Canyon were traversed by the Astorians on their way to the mouth of Columbia Kiver. Irving s gives 

a vivid description of their journey with its hardships and of the 

life and habits of the Indians and trappers whom they met. Nathaniel J. Wyeth, a trader from Boston and the founder of Fort Hall, 

came here in 1834, joining Bonneville in buffalo hunting on the upper 

Blackfoot, and J. C. Fremont in 1843 opassed through this region in 

connection with his explorations farther west.

The old emigrant trail to Snake and Columbia Rivers crossed 

Portneuf Valley and wound along Portneuf Canyon. This route was 

later selected by a transcontinental railroad and now constitutes part 

of a great artery of interstate commerce.

THE HAYDEN SURVEYS

The earliest geologic descriptions of the region that include the 

Portneuf quadrangle were given by members of the Hayden survey. 

In 1871 a party under Hayden traveled northward through Cache 

Valley and along Marsh Creek and the lower Portneuf on the way to 

the Yellowstone Park area. The next autumn the same party, returning from the field, came through the upper Portneuf Valley to the 

bend of Bear River and proceeded along Bear River and Bear Lake 

to Evanston, where the party disbanded. Geologic observations along 

the route are recorded in the Fifth Annual Report of the Hayden 

Survey, published in 1872, but few of them apply directly to the 

Portneuf quadrangle.

* Irving, Washington, The adventures of Captain Bonneville, U. S. A., In the Rocky 

Mountains and the Far West, Pawnee ed., vol. 1, pp. 324-325, 1898.

 Irving, Washington, Astoria, 1836.
6 PORTNEUF QUADRANGLE, IDAHO

 In 1872 the Snake Eiver division of the Hay den survey, with 

Frank H. Bradley as geologist, passed northward from Ogden 

through Malade Valley and down the Portneuf to Snake River. The 

party went up Ross Fork to Fort Hall, which was then situated in the 

valley of Lincoln Creek, and thence northward and northeastward 

into the Teton Range, the Yellowstone country, and the headwaters 

of Snake River. On the return the party again visited Fort Hall and 

spent a number of days exploring the region of the divides between 

Ross Fork, the Portneuf, and Lincoln Valley. (See the Sixth Annual 

Report, published in 18T3.)

In 1877 the Green River division of the Hayden survey, A. C. 

Peale, geologist, completed a geologic map that includes the area of 

the Portneuf quadrangle. Peale's report, in the Eleventh Annual 

Report, published in 1879, contains, brief descriptions of parts of the 

area, but his party covered so large a territory that he could devote 

little time or space to this particular area. His map, which shows 

valuable generalizations, nevertheless requires considerable correction, as will be noted in later pages.

LATER GEOLOGIC WORK

The east side of the Portneuf quadrangle was included in a 

reconnaissance by Schultz and Richards 6 in 1911. Their work covered principally Tps. 5, 6, and 7 S., R. 40 E., with a little adjacent 

territory on the west and south. They noted the general relations of 

the Carboniferous and Triassic rocks in those townships and obtained 

some structural data, to which further reference will be made. The 

subsequent activities of the Geological Survey in the quadrangle have 

been mentioned under the heading " Field work."

GEOGRAPHY

SUMMARY OF REGIONAL PHYSIOGRAPHIC DEVELOPMENT

In a previous report 7 the writer has given an extended geographic 

account of the general region that includes the Portneuf quadrangle. 

It is shown there that the country has a complex erosional history, 

including many stages, the more recent of which are given for reference in the following table, though not all of them are represented 

in the Portneuf quadrangle. The stages recognized in an earlier and 

less comprehensive study of the Fort Hall Indian Reservation are 

also cited for comparison.

8 Schultz, A. R., and Richards, R. W., A geologic reconnaissance in southeastern Idaho: 

TJ. S..Geol. Survey Bull. 530, pp. 267-284, 1913.

7 Mansfield, G. R., Geography, geology, and mineral resources of part of southeastern 

Idaho : U. S. Geol. Survey Prof. Paper 152, 1927.
GEOGRAPHY

Erosional stages in southeastern Idaho

Epoch

Recent.

Pleistocene.

Miocene.

Eocene.

Professional Paper 152, 1927

Postglacial.

Blackfoot cycle (St. Charles glacial episode).

Dry Fork.

Elk Valley. '

Gannett.

Deposition of Salt Lake formation.

Tygee cycle?

Middle Miocene deformation.

Snowdrift peneplain.

Bulletin 713, 1920

Spring Creek.

Oibson.

The Snowdrift peneplain is not represented within the quadrangle, 

but some remnants may exist in the higher parts of the Portneuf 

Mountains in the west and southwest, not yet studied except in 

earlier reconnaissance surveys. Middle Miocene deformation is supposed to have permitted the erosion (Tygee cycle?) that carved the 

valleys and low hills that were subsequently blanketed by the Salt 

Lake formation. The Gannett erosion surface is possibly represented 

by the even-topped limestone ridge southwest of Sawmill and 

Graves Creeks in Tps. 5 and 6 S., K. 39 E., and southeastward. Elsewhere the country has been too much worn down to preserve it. The 

remaining cycles are well represented and correspond as indicated 

to those previously described in the Fort Hall Indian Keservation. 

Within the limits of the quadrangle, however, there is no direct evidence of glaciation.

PRINCIPAL TOPOGBAPHIC UNITS

The Portneuf quadrangle includes parts of five mountain ranges 

and of two intermontane valleys so arranged that the northeast half 

is chiefly mountains and the southwest half chiefly lowlands. The 

mountains are principally strike ridges with northwesterly trend 

and with maximum altitudes of about 7,300 feet above sea level. 

The older rock structure, however, is largely concealed by later beds, 

which upon erosion have produced smoothly rounded surfaces that 

are noAv cut by early mature canyons with a maximum depth exceeding 1,200 feet. The lowlands are young plains in part basaltcovered and 5,300 feet or more in altitude. The principal topo-
)

8 PORTNEUF QUADRANGLE, IDAHO \

I

graphic units of the quadrangle from northeast to southwest are the 

Blackfoot Mountains, Blackfoot Valley, Chesterfield Range and 

Soda Springs Hills, Portneuf Valley, and Portneuf Mountains. j

Blackfoot Moimtains. The Blackfoot Mountains as defined by the 

writer 8 extend northwestward nearly 30 miles from Wilson Creek and 

Blackfoot River, in the southwestern part of the Cranes Flat quad- trangle, and they are about 15 miles in greatest breadth just north of 

the Paradise Valley quadrangle. The group as a whole is relatively 

low, rising 600 feet above Willow Creek on the northeast and 1,400 ( 

feet above Blackfoot Valley on the southwest. Recent geologic and 

physiographic work in the Paradise Valley and Ammon quadrangles 

shows that they agree in topographic development with other mountains of similar altitude in the general region, such as the Chesterfield { 

Range. The drainage goes chiefly to Blackfoot River, but some goes 

into Willow Creek or other tributaries of Snake River.

Only the extreme southwestern part of the Blackfoot Mountains, 

comprising perhaps 3 square miles, enters the Portneuf quadrangle, 

but the group occupies much of the Paradise Valley quadrangle, on 

the north. Rich phosphate deposits occur in these mountains, but 

they are not exposed within the Portneuf quadrangle, although they 

are present within minable depth.

Blackfoot Valley. Blackfoot Valley, which in the adjacent Henry 

and Cranes Flat quadrangles maintains a width of 6 miles or more, 

becomes constricted to less than a mile in width in the northeastern 

part of the Portneuf quadrangle. It is undoubtedly underlain by 

basalt, which is more or less overspread by wind-blown or alluvial 

deposits. The basalt is an extension of the Blackfoot lava field, 

which forms a prominent topographic feature of the Henry and 

Cranes Flat quadrangles. Blackfoot Valley is a young basaltic 

plain in which ledges appear here and there at the surface and as 

practically continuous cliffs along the stream courses. Its altitude 

is about 6,100 feet. Its tributary, Corral Valley, has the same character.

Grizzly and Thompson Creeks, which are tributary to the Black- ; 

foot system through Corral Creek, have alluviated valleys that are j 

now submaturely dissected. i

Chesterfield Range. The principal mountain mass in the Portneuf quadrangle has been named by the writer 9 the Chesterfield 

Range, from the town of Chesterfield, at its southwestern base. The 

range as a whole is nearly 25 miles long and 10 miles wide. It extends northwestward from Tenmile Pass, in the southeast corner of 

the quadrangle, to the broad sag west of Morgan, in the southwest8 Op. cit. (Prof. Paper 152), p. 34. 

» Idem, p, 35.
GEOGRAPHY 9

ern part of the Paradise Valley quadrangle. It is the northern part 

of the mountain group called by Peale 10 the Soda Springs Hills, but 

its topographic and geologic isolation make a separate designation 

more appropriate.

The range is described by Peale as "somewhat plateaullke," but 

it is composed of several ridges of sedimentary rocks that reach 

maximum altitudes of about 7,300 feet and rise about 1,000 to 1,200 

feet above the valleys on the east but nearly 2,000 feet above the floor 

of Portneuf Valley on the west. A prominent sedimentary ridge in 

the southwestern part of T. 7 S., K. 40 E., is shown in Plate 3, A. 

The plateaulike character noted by Peale is with little doubt due to 

the preservation of the Elk Valley and Dry Fork erosion surfaces. 

The relationship of these erosion surfaces is well shown near the 

forks of Eighteenmile Creek, in sees. 3, 4, 9, 10, and 15, T. 7 S., E. 

40.E. The ridge top in sees. 3,10, and 15, corresponding with neighboring ridge tops though not so high as some of them, is assigned to 

the Elk Valley erosion surface. The row of projecting spurs at the 

west and southwest comprises the remnants of an old valley floor of 

Dry Fork age now cut by youthful and early mature valleys of the 

Blackfoot cycle. (See pi. 3, C.) Near the end of the Blackfoot 

cycle or after it the lower course of Eighteenmile Creek became obstructed by the development of a large travertine terrace across the 

mouth of its canyon. This terrace stands about 100 ^eet above the 

adjacent slopes with its upper 50 feet an almost sheer cliff. A similar 

obstruction has formed at the mouth of Little Flat Canyon, about a 

mile and a half to the southeast. (See pi. 3, B.) Above these terraces young alluvial plains have been formed.

The alluviation stage, which was a late feature of the Blackfoot 

cycle and is represented throughout the region by valley fillings and 

alluvial fans, was succeeded by an epoch of moister climate probably 

associated with the last glacial stage. During this moister epoch 

the alluviated areas were in places submaturely dissected, the remnants of the former surfaces being preserved as flats or gentle slopes 

at corresponding heights above the later valley floors. The town of 

Chesterfield stands upon such a remnant or terrace 50 feet or more 

above the valley of Twentyfourmile Creek, which lies on the east 

side of the town. The terrace here rises 126 feet in 0.9 mile, as 

indicated by two bench marks. About 100 feet higher than this 

terrace is another similar terrace developed locally near the mouth of 

Twentyfourmile Canyon, probably recording a substage in the 

Blackfoot cycle. (See pi. 4, A.) Similar terraces or substages have 

been noted elsewhere in the general region.

10 Peale, A. C., Report of the Green River Division: U.' S. Geol. and Geog. Survey Terr. 

Eleventh Ann. Rept., p. 596, 1879.
10 PORTNEUF QUADRANGLE, IDAHO

The Chesterfield Range supports some timber, and parts of it are 

utilized for grazing. Ranches are scattered here and there at favorable places. Phosphate beds occur in several areas more fully described on pages 69 to 89.

Soda Springs Hills. The group of rocky hills here called the Soda 

Springs Hills constitutes the southern member of the group to which 

Peale gave this name. It lies southeast of the Chesterfield Range 

and extends from Tenmile Pass to the bend of Bear River west of 

Soda Springs, a distance of about 10 miles. These hills have not 

been mapped or studied in detail. Only the northern tip of the 

group, composed of older limestones, enters the quadrangle.

Portneuf Valley. The lowland called Portneuf Valley occupies 

perhaps a third of the quadrangle. It is very narrow at the northwest corner, but it broadens at first slowly and then rapidly southeastward and attains a width of nearly 10 miles where it crosses the 

south boundary. The valley is bordered here and there, as at the 

mouths of Twentyfourmile and Eighteenmile Creeks, with submaturely dissected alluvial fans. The southern part of the valley is 

a young basalt plain with somewhat uneven surface and with a 

general altitude between 5,400 and 5,500 feet. The basalt appears 

at the surface at many places, but elsewhere it is covered with a 

mantle of wind-blown soil or alluvium thick enough to be generally 

cultivated. The northern part of the valley is a young alluvial plain, 

from a few feet to 100 feet lower than the southern part, and is 

somewhat marshy in the vicinity of Portneuf River. A marshy area 

in T. 6 S., R. 38 E., has been converted into a reservoir by damming 

the river. In sees. 23, 25, and 26, T. 7 S., R. 38 E., the flood plain of 

the Portneuf is bordered by bluffs 15 to 20 feet high. Elsewhere its

borders are less clearly defined.

Portneuf Valley is continuous southward with the portion of Bear 

River Valley that lies west of the Bear River Range, but the drainage 

of the two valleys is separated by a low divide.

Portnewf Moimtcdns. As defined by the writer 11 the Portneuf 

Mountains extend from the north end of Cache Valley to the valley 

of Blackfoot River in the Fort Hall Indian Reservation. They lie 

between a bend of Portneuf Canyon, extended southward by Marsh 

Valley, on the west, and Portneuf and Bear River Valleys, on the east. 

Portneuf River cuts a gorge across them midway of their length.

The Portneuf Mountains have not been studied in detail, though 

parts of them have been described in connection with the Fort Hall 

Indian Reservation.12 They are high enough in places to preserve 

parts of the Gannett erosion surface and possibly even remnants of

"Op. clt. (Prof. Paper 132), p. 37.

12 U. S. Geol. Survey Bull. 713, pp. 71-104,. 1920.
GEOGRAPHY 11

the Snowdrift peneplain. The later erosion surfaces are undoubtedly 

represented. Their higher parts support some good timber, but. 

they are principally utilized for grazing. Mineral prospects have 

been opened in them here and there, and some ore has been shipped,13 

but the outlook for any substantial mineral development, aside from 

that of the phosphate deposits in their northern part, is poor.

The western part of the Portneuf quadrangle contains some of the 

eastern foothills of the northern part of the Portneuf Mountains,, 

comprising interstream ridges and spurs and some isolated knobs.

DRAINAGE

Rivers. Most of the drainage of the quadrangle is tributary to> 

Blackfoot and Portneuf Kivers, though Tenmile Creek, which 

crosses the southeast corner, belongs to the Bear Eiver system. This; 

creek is intermittent within the Portneuf quadrangle. The Chesterfield Kange is the principal divide and is so situated that probably two-thirds of all the drainage goes to the Portneuf. Thestreams are mostly in a youthful or submature stage, except that in; 

the larger alluviated valleys there are graded reaches in which thestreams have the sluggish and winding courses that are characteristic 

of topographic old age.

Blackfoot Kiver, which makes only a short loop into the northeast: 

corner of the quadrangle, is one of the principal tributaries of SnakeEiver in this region. Its flow is described on page 99. Its principal 

tributaries, parts of whose courses are within the quadrangle, are 

Corral and Gtraves Creeks. Grizzly Creek, which is joined byThompson Creek, is tributary to Corral Creek, and Sunday and Warbonnet Creeks are tributary to Graves Creek. Except the lowercourse of Sunday Creek these are all permanent streams. Warbonnet Creek heads against Twentyfourmile Creek and is followed 

part way by the road from Chesterfield to the Austin ranch and. 

beyond.

Portneuf Kiver (see p. 100) is also directly tributary to the Snake 

and in its middle and lower courses is comparable to the Blackfooir 

in size, but in this quadrangle, above the springs in sees. 19 and 30,. 

T. 7 S., R. 39 E., it is small, sluggish, and winding and is bordered! 

here and there by marshy areas.

Although several permanent streams rise in the Chesterfield Range 

and are ultimately tributary to the Portneuf drainage system, few 

of them join the Portneuf as surface streams. Most of their water 

is diverted for irrigation, and the rest sinks into the alluvium or 

beneath the basalt. The principal tributaries on the east side of the18 U. S. Geol. Survey Bull. 713, p. 116, 1920. 

6148 29 2
12 PORTNEUF QUADRANGLE, IDAHO

valley are Monroe and Little Flat Canyons and Eighteenmile, Moses, 

and Twentyfourmile Creeks. The reservoirs on Twentyfourmile 

Creek shown on the map have been practically abandoned, though the 

larger one still had some water in 1923.

On the west sicle of the valley Topons Creek is the largest tributary. Smith Creek and King Creek are permanent streams, though 

their waters are diverted and do not ordinarily reach the river 

directly.

Springs. Many springs are distributed over the quadrangle, some 

of which formerly deposited calcium carbonate in large amounts (see 

p. 103) and still deposit some. Mention may be made of those on 

Grizzly and Twentyfourmile Creeks and those along Portneuf Biver 

from sec. 19, T. 7 S., R. 39 E., southwestward, all of which give rise 

to considerable streams. These last-mentioned springs probably have 

historic interest in connection with the activities of Captain Bonneville in the early days of western exploration.

CLIMATE

A somewhat extended discussion of the climate of southeastern 

Idaho has already been published,14 from which the following data 

pertinent to the quadrangle are abstracted.

Climatic data bearing on Portneuf quadrangle, Idaho

Station

Blackfoot. Dam ______ _____

Chesterfield _ . _ . ___ .........

Fort Hall. .......................... .

South eastern Idaho.. __ . ..........

Precipitation, 

mean 

annual 

(inches)

17.26 

13.74 

9.81 

13.88 

13.96

Average 

number 

of days 

having precipitation 

Of 0.01 

inch or 

more

66 

68 

76 

93

77

Annual 

snowfall unmelted 

(inches)

81.1 

52.8 

28.1 

45.0 

62.4

Temperature (°F.)

Highest 

average annual

5)6 

99 

100 

102 

99.2

Lowest 

average annual

-42 

-48 

-28 

-20

-38.0

Mean 

annual

37.8 

40.0 

45.4 

47.5 

41.1

Direction of 

prevailing 

wind

SW. 

W. 

SW. 

SE. 

SW.

Length 

of 

growing 

vseason 

(days)

48 

65 

121 

158 

87

The data for Pocatello and for southeastern Idaho as a whole are 

more reliable than those for the other three stations named, one of 

which covers a period of only 4 to 6 years, whereas the Pocatello 

record covers about 21 years and the figures for southeastern Idaho 

comprise observations at 16 stat.ions from Border (Bear Kiver 

Valley) and Bedford (Star Valley) on the east to Idaho Falls on 

the west, some of which at the time of compilation (1926) had 

records covering periods of 25 years.

"Mansfield, G. R., Geography, geology, and mineral resources of part of southeastern 

Idaho: U. S. Geol. Survey Prof. Paper 152, pp. 37-44, 1927; Climate of southeastern 

Idaho: Assoc. Am. Geographers Annals, vol. 11, pp. 7&-92,, 1921.
GEOGBAPHY 13 

VEGETATION

The vegetation adapts itsejf closely to the climatic conditions, varying in character and quantity with exposure to sun and w,ind and 

with the distribution of moisture and temperature. The moister 

bottoms are covered with reeds, rushes, and wild grasses, which make 

an inferior hay. The lower mountain slopes and foothills together 

with the dr.ier bottoms are largely covered with sagebrush, interspersed with grasses and weeds that are utilized for grazing.

The eastern foothills of the Portneuf Mountains and the western 

slopes and rounded summits of much of the Chesterfield Range are 

practically devoid of trees, but willows are to be found along the 

streams. The eastern slopes of the Chesterfield Range support many 

groves of aspen (Popuhis tretrwloides) and somewhat tangled 

growths of brush. The higher slopes at the headwaters of Graves 

and Sawmill Creeks, particularly the east slope of the rocky ridge 

that forms the crest of the range in Tps. 5 and 6 S., R. 39 E., support 

some t,imber, chiefly lodgepole pine (Pinus contorta), Douglas fir 

(Pseudotsuga, taxifolia) , Engelmann spruce (Picea engebnanni) , and 

possibly other conifers. Some logging was in progress in this area 

in 1923; the logs were being hauled out northward and sawed for 

Jocal use.

SETTLEMENTS AND INDUSTRIES

The town of Chesterfield (population 690 in 1920) is practically 

the only settlement in the quadrangle. Mail and stage connections 

are maintained between this town and Bancroft (population 374), 

on the Oregon Short Line Railroad, about 10 miles to the south. 

Ranches, many of them deserted in 1923, are scattered over Portneuf 

Valley and along the foothills. The mountain region is practically 

uninhabited, although there are some fenced areas in the hills. 

Farming and stock-raising are the principal industries. The phosphate beds, described in detail on pages 69 to 89, may form the basis 

of phosphate mining at a future time.

TRANSPORTATION FACILITIES

The Oregon Short Line Railroad, which enters the southwestern 

part of the quadrangle, supplies the principal outlet for the country. 

A State highway follows the line of the railroad. Many roads, most 

of them passable by automobiles, traverse Portneuf Valley.

The Chesterfield Range forms a barrier not crossed by any good 

road between Tenmile Pass, at the southeast, and the pass near the 

line between Bingham and Caribou Counties, iy2 miles north of 

the quadrangle. Similarly, the Portneuf Mountains on the west are 

crossed by no road between the canyon of Portneuf River, at the
14 PORTNEUF QUADRANGLE, IDAHO

south, and the road between Ross Fork and the Blackfoot Dam, near 

the Bannock-Bingham County line, at the north, which is part of 

the old Lander Trail from Wyoming to Snake River and the Pacific. 

There are roads that lead into or across the Chesterfield Range, but 

the through roads may not safely be traveled by automobiles and 

are hardly passable for wagons.

GEOLOGY

STRATIGRAPHY 

THE STRATIGRAPHIC COLUMN

A detailed account of the stratigraphy of the general region of 

which the Portneuf quadrangle forms a part has been given in the 

larger report already cited, so that only brief descriptions of the 

formations represented are needed here. The stratigraphic column 

of the Portneuf quadrangle is fairly complete as high as the Jurassic. No Cretaceous sedimentary rocks are known, but a fault 

breccia recognized at several places is assigned to the mountainbuilding epoch that closed the Cretaceous period. No early Tertiary beds have been found, but the Salt Lake formation (Pliocene?) covers large areas. Quaternary beds of three sorts have 

been differentiated.

Because of the detailed studies made in neighboring areas and 

already published, few sections were measured in this quadrangle, 

but a detailed section was measured across one of the ridges of the 

Chesterfield Range on the north side of Monroe Canyon, in T. 7 

S., R. 40 E., where excellent exposures of beds ranging in age from 

Silurian to Mississippian are to be seen. The following table summarizes the stratigraphy that is represented in the quadrangle. The 

distribution of the several formations is shown on Plate 1.

Stratigraphy of the Portneuf quadrangle

Quaternary:

Recent: Alluvium. 

Pleistocene:

Hill wash and older alluvium. 

Travertine. 

Unconformity.

TfcrHnrv Thickness ieruaiy. (feet)

Pliocene (?) : Salt Lake formation (chiefly

conglomerate) __________________ 0-7504- 

Unconformity.

Tertiary or Cretaceous: Fault breccia (chert fragments). 

Unconformity. 

Triassic (?): Higham grit________________ 200±r.
STRATIGRAPHY 15

Triassic: ,.. , Thickness Lower Triassic: (feet)

Timothy sandstone______________ 800+

Unconformity.

Thaynes group:

Portneuf limestone_____ 2, 600± 

Fort Hall formation (sandstone, shale,

and limestone)_____________ 800-1,000 

Boss Fork limestone_______ _ 1, 800-2, 600 

Woodside shale_____________ 2,000±

Unconformity. 

Carboniferous:

Permian: Phosphoria formation (chert and

phosphatic shale and sandstone) _1_ 680± 

Unconformity.

Pennsylvanian: Wells formation (sandstone 

and limestone)___________ __ - 2,100-3,000

Unconformity. 

Mississippian:

Brazer limestone______ __ 2, 088+ 

Unconformity.

Madison limestone_ _ 612 

Unconformity. 

Devonian:

Upper Devonian : Threeforks limestone 180 

Middle Devonian : Jefferson limestone.- 935 

Unconformity.

Silurian : Laketown dolomite- _____ _ 485+ 

Ordovician:

Upper Ordovician: Fish Haven dolomite____ 700 

Unconformity. 

' Lower Ordovician:

Swan Peak quartzite ____________ 700 

Garden City limestone____________ 1,130 

Unconformity. 

Cambrian:

Upper Cambrian : St. Charles limestone____ 1, 300 

Middle Cambrian:

Nounan limestone_______________ 300± 

Bloomington formation (limestone and

shale) __ __________ 400± 

Blacksmith limestone_____________ 350± 

Ute limestone__________________ 400± 

Langstori limestone______________ 450± 

Middle and Lower Cambrian: Brigham 

quartzite- _______________ 1, 000-1, 600

CAMBRIAN SYSTEM

General feahcres. The oldest rocks recognized in the quadrangle 

are of Cambrian age and consist .of quartzite, limestone, shale, and
16 PORTNETJF QUADRANGLE, IDAHO

sandstone. In general, they represent Walcott's 15 sections in Blacksmith Fork, Utah, and 5 miles west of Liberty, Idaho, as modified by 

Richardson.16 They occur at intervals for 8 miles along the southwest border of the quadrangle in the foothills of the Portneuf 

Range and are best exposed on the north side of King Creek in sec. 

15, T. 7 S., R. 38 E., where practically the entire sequence is represented. The beds on the north side of the creek are here offset nearly 

half a mile eastward with respect to those on the south side by a 

fault that is followed by the course of the stream.

The boundaries indicated for the respective formations were drawn 

largely on lithologic data, which seemed fairly satisfactory, but 

fossils were collected at several localities that confirmed in a general 

way the conclusions shown in the mapping. Edwin Kirk, who kindly 

examined the collections, makes the following comment regarding 

them:

It is a question whether the Cambrian formations of Walcott in Blacksmith 

Fork can be carried far laterally. The Cambrian varies greatly in lithology 

and the faunas are not sharply delimited. It is possible to make close correlations only when considerable collections of well-preserved fossils are available.

BrigTiam quartzite. ^s, the name implies, the Brigham quartzite 

is chiefly a massive quartzite, generally vitreous, and reddish, pinkish, purplish, or yellowish. Locally it contains pebbly bands, and 

the upper part consists largely of hard sandy micaceous shale. Its 

thickness in the general region ranges from 1,000 to 1,600 feet. In 

the Portneuf quadrangle the Brigham quartzite is exposed only in 

small areas north and south of Topons Creek, though it lies close 

to the western boundary much of the way southward from these 

areas. The stratigraphic section rises eastward here, so that the 

uppermost beds of the formation lie nearest to the quadrangle.

Lamgston limestone. Beds assigned to the Langston limestone occur on the south side of Topons Canyon, where with beds of the Ute 

limestone they are included in a partly exposed syncline. They also 

occur in the King Creek section just west of the quadrangle boundary. 

The rock is a somewhat magnesian limestone with fine to medium 

granular crystalline texture and with glittering steel-gray color on 

fresh fracture. It weathers brown to gray. No fossils were observed 

in it in the Portneuf quadrangle, but it is moderately fossiliferous 

at its type locality in Blacksmith Fork, Utah. Its thickness in the 

Bang Creek section is about 450 feet.

Ute limestone. In the Liberty section of the Ute limestone the 

lowermost beds, which are highly fossiliferous paper shales, are des15 Walcott, C. D., Cambrian geology and paleontology: Smithsonlan Misc. Coll., vol. 53, 

pp. 5-9, 190-200, 1908; Cambrian Bracbiopoda: U. S. Geol. Survey Mon. 51, pp. 148-153, 

1912.

16 Richardson, G. B., The Paleozoic section in northern Utah : Am. Jour. Sci., 4th ser., 

vol. 36, pp. 406-416, 1913.
STBATIGRAPHY 17

ignated the Spence shale member. In the general region of southeastern Idaho it has not been practicable to differentiate this member 

as a cartographic unit. In the King Creek section of the Portneuf 

quadrangle there is a topographic depression where the shale might 

be, but the shale itself was not recognized. The formation, however, 

here consists of bluish gray limestone with interbedded greenishgray papery shales that are more noticeable in the talus slopes than 

in the ledges. A few fossils were collected from the limestone, including Zacanthoides idahoensis Walcott and Batliyuriscus sp., said 

by Edwin Kirk to be characteristic of the Ute. The thickness of 

the formation at King Creek is about 400 feet.

Blacksmith limestone. The beds that constitute the Blacksmith 

limestone are similar to the upper beds of the Ute, but the bedding is 

more massive and the boundary between the two formations is usually 

marked by low cliffs. In the King Creek section no definite boundary was obtained and no satisfactory fossils were collected. The 

lower boundary was drawn at the point where the interbedding of 

shales with limestones ceases and the limestone beds become more 

massive. There are no noteworthy cliffs, but well-defined ledges 

occur. Near the top the beds are coarsely oolitic. The upper boundary was drawn at a topographic depression which it was thought 

might mark the position of the Hodges shale member of the Bloomington formation. The thickness of the Blacksmith limestone as 

thus mapped is about 350 feet.

Bloomington formation. The presence of the Hodges shale member at the base of the Bloomington formation was suggested by shaly 

float on the east side of the topographic depression used as the upper 

boundary of the Blacksmith limestone. The beds on the east consist 

in part of coarsely oolitic gray limestone and in part of rock that is 

not oolitic. The top of the formation was mapped at a second topographic depression that was thought to correspond with the shaly 

rocks that elsewhere constitute the upper beds of the formation. 

Although the Bloomington formation at its type locality is abundantly fossiliferous, no satisfactory collection was obtained here. 

The thickness of the formation as mapped is about 400 feet.

Noiman limestone. The Nounan limestone ordinarily consists of 

massively bedded, whitish or light gray, somewhat coarsely crystalline dolomitic limestone that sparkles on freshly fractured surfaces. 

It occurs in beds about 18 inches thick and alternating with these 

are dark gray beds of similar character and thickness with branching calcitic forms that are probably organic replacements. In the 

King Creek section beds of these types occur together with some 

oolitic beds. The thickness of the formation as mapped is about 

300 feet.
18 PORTNEUF QUADRANGLE, IDAHO

St. Charles limestone. In the Cambrian section north of King 

Creek only the Worm Creek quartzite member of the St. Charles 

formation is exposed. The quartzite forms the eastern tip of the 

hill, which ends abruptly. From conditions south of the canyon it is 

inferred that the limestone beds that normally make up the greater 

part of the formation have been faulted' out or down so that they are 

now concealed by hill wash. Practically the full thickness of the 

quartzite member is preserved, however.

South of the King Creek Canyon, in the SE. 14 SW. % sec. 22 and 

the NE. 1,4 NW. *4 sec. 27, T. 7 S., R. 38 E., what is probably a very 

nearly complete section of the formation is exposed. Here the 

quartzite member is about 300 feet thick and the limestone member 

1,000 feet thick. Northward from this locality a fault cuts obliquely 

.across the formation, apparently shutting it out entirely at the south 

wall of the King Creek Canyon and introducing on the east the 

Garden City limestone (Ordovician).

The quartzite that forms a prominent knoll in the SE. ^ SW. ^4 

sec. 22 is pinkish or whitish and less vitreous than either the Brigham quartzite or the Swan Peak quartzite. The quartzite member is 

composed of successive beds a few inches thick and weathers into 

rough angular blocks. The limestone, which is in beds half an inch 

to 3 inches thick, is gray with yellowish spots and cherty, weathering 

with a rough cherty surface, and is decorated at the locality named 

with orange-colored lichens.

Recent work by Ulrich, Walcott,17 and Resser, mostly unpublished, 

shows that the St. Charles limestone includes beds now believed to be 

of " Ozarkian " age.

Undifferentiated Cambrian. In sees. 3,10,11,14, and 15, T. 7 S., 

R. 38 E., ledges of limestone with Cambrian lithology are exposed. Determinable fossils are apparently few; the fragmentary 

remains that were collected proved to be of no value. Float pieces 

of quartzite, probably representing the Brigham quartzite, were 

found in the NW. ^4 SE. 14 sec. 10 with limestone fragments both 

to the "north west and the southeast. If this material represents an 

actual occurrence of the Brigham, the pieces of quartzite would mark 

the location of a minor anticlinal fold in the Cambrian succession or, 

perhaps, a fault. More probably these pieces as well as the blocks 

of Swan Peak quartzite in the NE. y± NE. 14 sec. 15 are to be considered as debris weathered out of the Salt Lake formation. They 

have been so mapped.

The undifferentiated Cambrian is distinguished on the map by a 

different pattern from those of the designated formations.

17 Walcott, C. D., Cambrian geology and paleontology, Cambnan and Ozarkian trilobites: 

Smithsonlan Misc. Coll., vol. 75, p. 127, 1925.
STRATIGRAPHY 19

In the SW. % SW. 14 and the SE. % sec. 18, T. 5 S., R. 39 E., 

masses of Cambrian limestone large enough to be considered ledges 

appear in the midst of the Salt Lake formation. If they are in fact 

ledges, their position athwart the strike of Carboniferous rocks 

indicates the presence of a concealed fault. The Salt Lake formation, however, contains at some localities boulders of Ordovician 

quartzite and other rocks 4 or 5 feet in diameter. There is therefore the possibility that these supposed ledges may be large detached 

masses included in the Salt Lake formation.

ORDOVICIAN SYSTEM

Ordovician rocks are exposed only in the southwestern part of the 

quadrangle, where all three of the subdivisions distinguished for the 

general region by Richardson 18 have been identified. They form 

part of the King Creek section on the south side of the canyon and 

comprise a number of rocky promontories and isolated knobs. Distinctions here too have been made largely on lithologic grounds, for 

there has been little difficulty in recognizing the formations. Fossils 

are poorly preserved and the collections were highly unsatisfactory.

Garden City limestone. A characteristic feature of the Garden 

City limestone is the presence, at intervals throughout the formation, 

of beds of conglomerate or breccia, consisting of elongated fragments 

of gray or yellowish-gray limestone of various sizes, the largest 2 or 

3 inches long, embedded in a matrix of similar composition. There 

are also streaks of yellowish-brown chert approximately parallel to 

the bedding and beds of purer limestone, some of which are rather 

massive, a foot or more thick. Annelid borings were noted at one 

locality. The Garden City limestone carries a Beekmantown fauna, 

but no satisfactory collections were obtained in the Portneuf 

quadrangle.

In the King Creek section limestone beds in the NE. % sec. 22, 

T. 7 S., R. 38 E., are assigned to the Garden City. They are not 

well exposed, but from their relations to well-recognized Swan Peak 

beds on the east and from their lithology near the fault that cuts 

out the Worm Creek quartzite member of the St. Charles there seems 

little doubt of the correctness of the assignment. The Garden City 

limestone appears in the SW. i/4 sec. 23, NW. i/4 sec. 26, SE. 1/4 sec. 

27, and NE. 14 sec. 34. The largest exposure is in the E. 1^ sec. 3, 

T. 8 S., R. 38 E., and adjoining territory on the north, where the 

exposed thickness is about 1,130 feet.

18 RlchardsoD, G. B.> The Paleozoic section irt northern Utah: Am. Jour. Sci., 4thi ser., 

vol. 38, pp. 406-416, 1913.
-20 PORTNEUF QUADRANGLE, IDAHO

Swan Peak quartzite. The whitish, vitreous, blocky appearance 

of the Swan Peak quartzite serves in.most places to distinguish it 

from the Brigham and "Worm Creek quartzites below and from

 quartzitic sandstones of Pennsylvanian age above. Locally the Swan 

Peak beds are pinkish or reddish and softer, and there the distinction 

is more difficult. The formation makes rough, blocky, and ledgy hills 

that are conspicuous topographic features. (See pi. 4, B.) It underlies the rocky knoll in sees. 13 and 14, T. 7 S., R. 38 E., and forms 

"the east end of the ridge south of King Creek. It constitutes the 

eastern part of the knoll in the SW. % sec. 23 and the southeast tip 

of the hill in sec. 26. The rocky hills and ridges in sees. 35 and 36 

and farther south are composed mainly of Swan Peak quartzite, and 

so are some small rocky knobs in sees. 5 and 8, T. 8 S., R. 39 E., near 

the south boundary of the quadrangle. The total thickness is not 

exposed, but the formation is probably not less than 700 feet thick. 

The fauna of the Swan Peak quartzite has been tentatively referred 

to the Chazy by Ulrich and Kirk,

Fish Haven dolomite. Beds assigned to the Fish Haven dolomite 

; are exposed at only a few scattered localities in the southwestern part 

of the quadrangle. The largest area is the southeastward-projecting 

knoll in the NW. % sec. 14, T. 7 S., R. 38 E. Here the beds are 

inassive bluish-gray limestone striking N. 42° W. and dipping 45° 

SW. The lithology suggests the Fish Haven, and the strike is at 

sharp variance with that of beds believed to be of Middle Cambrian 

age that make up the main hill from which the knoll projects. It is 

therefore believed that the supposed Fish Haven beds are faulted 

against the Cambrian beds. The thickness of the (Fish Haven at 

this locality is at least 700 feet. At other localities, which are in 

sees. 5 and 8, T. 8 S., R. 39 E., the relations of the rock to the Swan 

Peak quartzite are such as to leave little doubt of the identity of the 

formation. The exposures at these localities are so small that no idea 

of the thickness of the formation here can be obtained.

The Fish Haven dolomite is normally a dark-gray to blue-black, 

locally cherty dolomite about 500 feet thick and carries a fauna of 

Richmond age.

SILURIAN SYSTEM

The Silurian system in southeastern Idaho is represented by a 

single formation that in the regions thus far studied is confined to 

small, disconnected areas. It is better developed in the Randolph 

quadrangle, Utah, where it was originally studied by Richardson.19

19 Richardson, G. B., op. cit.
STKATIGRAPHY 21

In the Portneuf quadrangle the only exposures are in a small area 

on the southwest flank of the Chesterfield Eange.

Laketown dolomite. In sees. 29 and 32, T. 7 S., R. 40 E., the beds

 exposed near the base of the slope and just above the hill wash consist

 of light-gray to white porous crystalline rock that weathers with a 

rough and cavernous surface and closely resembles the Laketown

 dolomite in general lithology. They occupy a position appropriate 

for this formation with reference to overlying formations and are 

therefore confidently assigned to the Laketown. The beds comprise 

INos. 1 to 3 of the section measured north of Monroe Canyon (p. 23), 

with a thickness of 485 feet, but the base is not exposed. A single 

lot of poorly preserved fossils contained Favosites fwvosus .(Goldfuss) and Pentamerus cf. P. oblongus Sowerby, as identified by 

Edwin Kirk, who assigns the formation to the Niagaran epoch.

In this general region the Devonian system is represented by two 

formations, both of which are exposed in the Portneuf quadrangle 

along the southwest flank of the Chesterfield Range north and south 

of Monroe Canyon, where they occupy parts of the lower slopes and 

foothills. These are the Jefferson limestone and the Threeforks 

limestone. Devonian rocks also continue southeastward into the 

Soda Springs Hills beyond the limits of the quadrangle.

Jefferson limestone. The contact zone between the Laketown and 

the Jefferson is readily distinguished by the marked change ,in lithology, the Jefferson weathering to a chocolate-brown and having a 

dark-gray color on fresh fracture. The rock is dolomitic and much 

broken and is seamed here and there with calcite. There are, however, some light-colored beds that in general appearance are not 

unlike the Laketown or some of the beds of the Madison. Poorly 

preserved fossils were found, among which Edw.in Kirk identified 

Pachyphyllwm woodmani White, Atrypa reticularis (Linnaeus), 

Stromatopora sp., Diphyllum sp., and Cladopora? sp.

Beds here assigned to the Jefferson include Nos. 4 to 13 of the 

section measured north of Monroe Canyon (p. 23) and have a 

thickness of 935 feet.

Threeforks limestone. The contact zone between the Jefferson and 

the Threeforks is also fairly well distinguished, the Jefferson making good ledges and furnishing large blocky float pieces, whereas 

the Threeforks is thinner bedded, makes no ledges, and supplies 

a light, somewhat varicolored float of sandy limestone or limy 

sandstone in flat pieces generally not more than 3 or 4 inches across.
22

In the Monroe Canyon section (p. 23) the beds here assigned to 

the Threeforks include Nos. 14 to 16 and have a thickness of 180 

feet. The contact zone of the Threeforks with the overlying Madison and probably some of the Madison beds as well are covered 

with debris of the Salt Lake iformation, which must once have 

covered most if not all of the rocks now exposed in the section.

Southeast of Monroe Canyon the Threeforks limestone diminishes 

rapidly in thickness, and in the NE. 14 sec. 4, T. 8 S., R. 40 E., 

it is represented by only a few feet of beds. Southwest of Tenmile 

Pass it seems to be entirely absent, and the relations of the .Madison 

to the Jefferson indicate unconformity rather than faulting. It 

is therefore believed that the Threeforks limestone may once have 

been much thicker but that it had been largely eroded away at the 

time of deposition of the Madison.

CARBONIFEROUS SYSTEM

A full sequence of Carboniferous formations is present in the 

Portneuf quadrangle, but they are not all exposed at any one locality. 

The Chesterfield Eange is apparently composed largely of Carboniferous rocks, though these are much concealed by Tertiary beds. 

Carboniferous beds also underlie considerable tracts in the Blackf oot 

Mountains, especially in the Paradise Valley quadrangle, on the 

north. They consist chiefly of limestone with some sandstones and 

with subordinate cherty and shaly members, including economically 

important beds of phosphate rock. The aggregate exposed thickness 

of the system is nearly 6,200 feet.

MISSISSIPPIAN SERIES

The Mississippian series includes the Madison limestone below and 

the Brazer limestone above. Both formations are characterized by 

rather pure limestone, but both include rock of other types, and there 

are also faunal differences. The Mississippian series is well represented in the section measured north of Monroe Canyon in T. 1 S., 

R. 40 E. This section also contains Devonian and Silurian beds, but 

it is inserted here because of the great preponderance in it of Mississippian beds. The section was measured in a direction at right 

angles to the strike with a 5-foot rule to which was attached a clinometer. Offsets were made here and there along the strike in order to 

utilize more favorable ground for travel or to visit more favorable 

exposures.
STRATIGKAPHY 23

Section of Paleozoic beds north of Monroe Canyon in sees. 82 and 28, T. 7 S.,

R. 40 E.,

Field 

No. Thickness

Concealed; top of section. Feet 

36 Limestone float, bluish gray, with low-lying ledges.................................... 5+

35 Limestone, massive, dense, medium gray, fairly free from chert. Corals and brachiopods of Brazer age__----__________.____..___________... 425 

34 Limestone, dark bluish gray, with cherty nodules; weathers sandy; strike N. 35° W.,

dip 45° E.. ...... ................. ...................................... 80

33 Limestone, dark bluish gray, cherty, streaked with calcite, coarser textured and

lighter gray in the upper part; fossiliferous; strike N. 35° W., dip 68° E............. 100

32 Limestone, massive, gray, forming crest of ridge, cherty nodules, fossiliferous____ . 230 

31 Limestone float, gray, similar to beds below___._:_._.______.___ 160 

30 Limestone, massive, bluish gray, beds as much as 4 feet thick, chert nodules and bands

about 20 feet above base_...._..__._____.__..__________.__ 60 

29 Limestone, massive, light gray, relatively pure, fossiliferous-_...__...._..._ 100 

28 Limestone, very massive, sandy streaks, cross-bedding . _ ____. ___ 76 

.27 Limestone, light gray, massive, but with some thin sandy beds, fossiliferous; 15 feet

above base many shell fragments..___ ..... _.......__....__ 120

26 Sandy and limy beds, yellowish, thin bedded....._...______________ 58

25 Limestone, light gray, massive, with chert streaks and nodules in bands at 4 to 6 inch

intervals, fossiliferous...__._.____-._-_._-__ . . ___ . 40 

24 Limestone, dove-colored to drab, massive, beds 1 to 10 feet thick; strike N. 35° W.,

dip 40° E--.--.........._.........._ .....--.__._ - ._.--__.... -.____.. 42

23 Limestone, light gray, sandy; weathers into angular blocks; strike N. 35° W., dip

25° E........................................................................I..... 108

22 Sandstone, weathering yellowish and reddish, thin bedded; no ledges exposed; float

intermingled with chert fragments; more calcareous toward top....________ 405 

21 Limestone, platy, sandy and cherty, weathering pink to purplish, indistinct fossils;

freshly broken rock has ortor of petroleum; no ledges.._____.._......_ 80

20 Limestone, light gray, thin bedded; noledges; weathersdrabtopinkorreddish...._.__ 195

19 Limestone, gray, massive, finer and more sandy toward the top; float becoming thinbedded and weathering drab to pink; Madison fossils.----.--.-.--.-_.... .... 390

]S Limestone, gray, more or less concealed by Salt Lake formation; strike N. 35°-40° W.,

dip30°E.--'-.-..... .-.. .-.-...;.-. -....-.........-. .............. 25

17 Limestone, massive bed, projecting through cover of Salt Lake formation; base of

Madison (?)-.....................-..-.-........-.....-..-...... .............-.. 2

16 Dfibris of Salt Lake formation covering older beds....._____......_.._. ... 10

.15 Calcareous beds, reddish and yellowish, rather fine float, pieces not usually larger than

3 or 4 inches; no ledges, much more reddish yellow near top..... .._.. 140 

14 Limestone, gray, weathering pinkish or purplish in pieces 1 foot or more in diameter;

no ledges------..-..---.........---.-.-.-.--.-.-.----------------.-----_---------- 30

13 Limestone, gray ledges, individual beds as much as 1 foot thick; lower beds are lime- , stone breccia; fine banding on weathered surface.......____ _.__ 70

12 Limestone, light brownish gray, weathering to thin slabs; no ledges; finer float red,

pink, drab, oryellow...________...... ...... . . . 295

11 Limestone, grayish brown, beds 1 foot or more thick, fine banded; strike N. 30° W.,

dip 30° E........................................... ........ ...........'...... 110

10 Limestone, brownish, highly fractured, a few chert nodules as much as 6 inches in

diameter, beds as much as VA feet thick, finely banded on weathered surface... -.. 30 

9 Limestone, massive, brownish gray, much fractured; contains small linear and branching organisms and a few corals of Jefferson age....__.. ... ..... . . 80 

8 Limestone, light grayish brown, massively but indistinctly bedded and highly fractured, black cherty patches__.....___.. ... . 55 

7 Limestone, like beds below but weathering smooth and fractured; fewer ledges.... 20 

6 Limestone, massive; some beds as much as3H feet thick... . 77 

5 Limestone, grayish brown, massive; well-marked ledges. . .. 48 

4 Limestone, dark brownish gray, much fractured and seamed with calcite, beds 1 foot

or nwre thick, Jefferson lithology; strike N. 35° W., dip 20° E....................... 150

3 Dolomite, gray, crystalline; no good ledges; Laketown lithology. ... 120 

2 Dolomite, massive, like No. 1; contains many poorly preserved fossils, some silicifled,

of Laketown age__...____ 250 

1 Dolomite, gray, finely crystalline, massive, beds as much as 2}^ feet thick; strike N.

40° W., dip 21° E..A.. ...... ... --. .........:. - .-. 115

Covered area, base of slope and bottom of measured section.

4,300

Madison limestone. In the Portneuf quadrangle the Madison 

limestone is exposed only on the southwest flank of the Chesterfield 

Kange and Soda Springs Hills, southeastward from Little Flat 

Canyon, in sec. 19, T. 7< S., E. 40 E. The northernmost exposure 

is in a gully carved in the face of the travertine cliff at the mouth 

of the canyon, where a sharp and broken fold of the Madison beds,
24 PORTNEUF QUADRANGLE, IDAHO

formerly concealed by travertine, has been uncovered. (See pi. 4, C.} 

The Madison limestone as here exposed consists mainly of thinbedded dark bluish-gray limestone that weathers brown or gray, but 

at the base there are beds of more massive lighter-colored limestone, 

some of which is crinoidal and coarsely crystalline. Below lie the 

varicolored beds of the Threeforks, in which the dip and strike arenot well shown.

The strike of the Madison is about N. 45° W. The dip in thesouthern part of the exposure is 30°-35° NE., locally as low as 12°; 

in the northern part it is considerably steeper, about 59° on the point 

south of Little Flat Canyon. This difference in dip is in accord 

with the facts stated in the discussion of the Threeforks limestone,, 

where the unconformable relation of the two formations was pointed 

out. Southeast of Tenmile Pass the basal beds of the Madison consist of a breccia, apparently of sedimentary origin, composed of a 

light-gray matrix of Madison lithology with fragments of chocolate-colored Jefferson or of varicolored Threeforks. The Madison, 

here rests directly upon the Jefferson.

The top of the Madison as here mapped is composed of thin-bedded gray limestone that lies just beneath a dark cherty and somewhat phosphatic shale, thought to represent the phosphatic member 

of the Brazer limestone, which is recognized at several places farther 

south. This shale occurs pretty uniformly along the hillside in T. T 

S., R. 40 E., and in general the two formations seem fairly conformable. On the spur south of Little Flat Canyon, however, the Madison dips much more steeply than the Brazer, so that it is probable 

that here as elsewhere farther south the two formations are really 

unconformable.

In the Monroe Canyon section the beds here assigned to the Madison include Nos. IT to 20 and have a thickness of 612 feet. The 

Madison limestone is not so well exposed on the slope northeast of 

Monroe Canyon as it is on the slopes to the southeast, where the 

relation of the upper beds to the overlying cherty beds of the Brazer 

is better shown.

Fossils were collected at several horizons in, the Madison from 

which G. H. Girty has identified the following forms:

Syringopora surcularia. 

Triplophyllum excavatum. 

Lithostrotionella n. sp. 

Crania sp.

Leptaena analoga. 

Schuchertella chemungensis. 

Ohonetes loganensis. 

Productus sp. 

Avonia sp.

Pustula aff. P. concentrica. 

Camarotoechia metallica. 

Dielasma sp. 

Spirifer centronatus. 

Reticularia cooperensis. 

Nucula sp. 

Parallelodon sp. 

Schizodus sp. 

Cypricardinia aff. C. scitula.
STEATIGEAPHY 25

Straparollus ophirensis. 

Naticopsis sp. 

Loxonema sp. 

Orthoceras sp. 

Grifflthides sp. 

Ostracoda undet.

Bellerophon mansfieldianus. 

Bucanopsis sp. 

Pleurotomaria, several sp. 

Platyceras sp. 

Euomphalus utahensis. 

Euomphalus luxus?

Brazer limestone. Considerable areas in the Chesterfield Kangeare underlain by the Brazer limestone. It is exposed on both sides, 

of Tenmile Pass at the southeast and in company with older formations it extends thence northwestward, forming a high, rocky ridge - 

between that pass and Little Flat Canyon. It is also exposed in 

small areas at the heads of Tenmile Creek (sec. 11, T. 7 S., K. 40 E.) 

and Eighteenmile Creek (sec. 33, T. 6 S., R. 40 E.). Farther northwest a larger area extends as far as the main canyon of Thompson 

Creek, in sec. 20, T. 6 S., E. 40 E. The Brazer emerges from a Tertiary cover in the northwest corner of sec. 18, T. 6 S., R. 40 E., and 

extends as far northwestward as the southeast corner of sec. 19, T. 

5 S., K. 39 E., where it again passes under cover. Beds assigned to* 

the Brazer also occupy part of the slope in the W. % sec. 18, T. 6, 

S., K. 39 E.

The Brazer limestone is characterized chiefly by massive, generally 

light-gray limestone that is prominent as a ridge and cliff maker 

(see pis. 3, J., and 5, A) and that in certain beds is highly fossiliferous.

According to G. H. Girty corals play a much more prominent part 

in the Brazer fauna than in most faunas of Carboniferous age in 

North America. Especially conspicuous and characteristic are cup, 

or horn corals distinguished by their large size and numerous slendersepta. Some of these have been identified as Cya>thophyllum\? mviltfylamellat, and large specimens reach a length of 8 inches and a diameter - 

of 2y2 inches. In some beds these are very plentiful associated with 

slender tubular colonies (Syringopora] and other colonies of the 

Litkostrotion type, some forming hemispherical masses and others 

having a bushy habit. The massive forms all appear to belong to the - 

genus Lithostrotion&lla, the others to Lithostrotion. Collection from 

a number of localities yielded the following forms :

Syringopora aff. S. surcularia. 

Lithostrotion sp. 

Lithostrotionella n. sp. 

Cyathophyllum? multilamella. 

Anisotrypa sp. 

Batostomella sp. 

Fenestella, several sp. 

Streblotrypa sp.

Productus brazerianus. 

Productus aff. P. inflatus. 

Avonia? n. sp. 

Pustula n. sp. 

Dielasma n. sp. 

Spirifer brazerianus. 

Spirifer aff. S. bifurcatus. 

Cliothyridina sublamellosa.
26 POETNEUF QTJADKANGLE, IDAHO

In the Monroe Canyon section beds here assigned to the Brazer 

include Nos. 21 to 36 and have a total thickness of 2,088 feet without 

including the top of the formation. Exposures in Little Flat Canyon, however, indicate that the contact between the Wells and Brazer 

is not far back of the southwest edge of the Tertiary cover.

The cherty and phosphatic beds at the base of the Brazer are 

represented by the 80 feet of beds in No. 21 of the measured section. 

Above these comes 405 feet of thin-bedded sandstone, which is better exposed on the slopes southeast of the canyon, where there are 

well-marked ledges and where the change to the overlying massive 

limestone is rather abrupt. Although sandy beds have been noted 

elsewhere in the Brazer, sandstone of this type is unusual in this formation. Possibly its absence in other localities may be due to the 

same unconformable relations that render the occurrence of the underlying cherty and phosphatic beds somewhat sporadic.

PENNSYLVANIAN SEKIKS

Wells formation. The rocks of the Pennsylvanian series all belong to the Wells formation, which, like the Brazer limestone, is a 

prominent ridge-maker and is well exposed in several parts of the 

Chesterfield Range. The actual distribution of the Wells formation in the Portneuf quadrangle is undoubtedly much wider than is 

shown by the exposed areas. A blanket of Tertiary sediments now 

conceals extensive structural features in which the Wells must be 

abundantly represented. Small areas assigned to the Wells occur 

in sec. 26, T. 7 S., R. 40 E., ancl in sees. 16 and 17 of the same township, where the Tertiary cover is cut through by Little Flat Canyon, 

and a single ledge is exposed in the SE. ^ NE. % sec. 7, on the 

north side of Eighteenmile Canyon. A larger area occurs in sees. 

25 and 26 and adjacent territory, T. 6 S., R. 39 E. A small ledge in 

the NW. ^4 SW. % sec. 24 is probably to be assigned to the Wells, 

but if so its presence there indicates a fold or fault intervening between it and the larger area just mentioned. The favored interpretation, shown in structure section F-G, Plate 2, is consistent with the 

known structural features of the quadrangle. The Wells is exposed continuously in a relatively narrow belt that extends with 

curving trend first northward and then northwestward from the 

SE. % SW. % sec. 28, T. 6 S., R. 40 E., as far as the SE. % sec. 20, 

T. 5 S., R. 39 E., where it passes again beneath Tertiary beds. This 

belt forms much of the crest of the main ridge and divide of the 

Chesterfield Range. The Wells is also exposed in small areas in 

sees. 5, 6, 15, and 17, T. 6 S., R. 39 E.; and a larger area extends 

northwestward from sec. 18 of the same township to sees. 35 and 36,
XT. S. GEOLOGICAL SURVEY BULLETIN 803 PLATE 3

A. RIDGE OF BRAZER LIMESTONE NORTHWEST OF MONROE CANYON, T. 7 S.,

R. 40 E.

View southeastward from vicinity of Little Flat Canyon

K. TRAVERTINE CLIFF WITH CAVE AT MOUTH OF EIGHTEEN MILE CANYON, 

SEC. 12, T. 7 S., R. 39 E.

C. VIEW NEAR FORKS OF EIGHTEENMILE CREEK, T. 7 S., R. 40 E. 

Shows former valley of Dry Fork are fa) now cut by valleys of the Blackfoot cycle (b)
TJ. S. GEOLOGICAL SURVEY BULLETIN 803 PLATE 4

A. TERRACES AT MOUTH OF TWKNTYFOUHMILE CANYON, CHESTERFIELD 

The hills at the right consist mostly of Ihe Salt Lake formation. Note the smooth slopes

li. HILL ()!< SWAN PEAK QIJAHT/ITK liN SKC. :(5, T. 7 S., \\. :i(l E. C. FOLD IN MADISON LIMESTONE UNCOVERED BY EROSION OF TRAVERTINE, SEC. 19, T. 7 S., R. 40 E.
U. S. GEOLOGICAL SURVEY BULLETIN 803 PLATE 5

A. SUMMIT OF RIDGE IN SE. % SEC. 20, T. 7 S , JR. 40 E.

B. LEDGES OF REX CHERT IN SEC. 10, T. 6 S., H. 39 E.
U. S. GEOLOGICAL SURVEY BULLETIN 803 PLATE 0

 " *..

A. LEDGES OF SALT LAKE FORMATION, SEC. 2, T. 5 S., R. 38 E., PARADISE 

...... VALLEY QUADRANGLE ,JT

. MASSIVE LEDGES OF SALT LAKE FORMATION BY ROAD IN NW. M SEC. 6, 

. T. 5 S.. R. 39 E.
U. S. GEOLOGICAL SURVEY BULLETIN 803 PLATE 7

A. SPRING CONE OF TRAVERTINE IN SEC. 15, T. 5 S., R. 40 E. B. PROSPECT MADE BY GEOLOGICAL SURVEY PARTY IN PHOSPHATIC SHALES 

IN SE. \i SEC. 34, T. 4 S., H. 40 E.

C. CANYON OF RLACKFOOT RIVER BELOW DAM IN SECS. 11 AND 12, T. 5 S., R. 40 E., CRANES FLAT QUADRANGLE
STRATIGRAPHY 27

T. 5 S., R. 38 E. There is also an area in sees. 30 and 31, T. 5 S., R. 

39 E, and a small interrupted strip in sees. 29, 30, and 19, T. 5 S., 

R. 40 E. . . The threefold subdivision of the Wells noted in other parts of the 

region is observed in the Portneuf quadrangle. The lower beds of the 

Wells are massive limestones, generally more sandy and cherty than 

the massive beds in the upper part of the Brazer. The cherts are. 

either characteristically ovoid, 4 inches or more long, and concentrically banded, or discrete irregular masses rather than bands -or 

streaks, though some of these are present. The occurrence of branch-, 

ing Bryozoa and of the big brachiopod /Spirifer rockymontcwm (S. 

opimus Hall var. occidentals Girty) 20 are also distinctive features of 

the lower Wells.

The middle sandstone member forms steep slopes strewn with 

debris but makes few or inconspicuous ledges except in the steepersided gullies. On the east slope of the main divide in the Chesterfield 

Range the vegetation is so dense that there are few exposures, and 

even the debris is largely concealed.

The upper siliceous limestone member forms prominent ledges or 

knobs along the hillsides, which are emphasized by the relative weakness of the overlying phosphatic shale beds.

A number of fossil collections were made from the Wells formation, which have been identified by G. H. Girty as follows:

Stenopora aff. S. carbonaria. 

Rhombopora lepidodendroides. 

Cystodictya sp. 

Productus cora. 

Producing coloradoensis. 

Pustula nebraskensis. 

Marginifera splendens?

Spirifer opimus var. occidentalis. 

Spirifer carneratus. 

Spiriferina spinosa? 

Composita subtilita. 

Cleiothyridina orbicularis. 

Aviculipecten sp. 

Phillipsia sp.

The thickness of the Wells as exposed in Tps. 5 and 6 S., R. 39 E., 

ranges from about 2,100 to 3,000 feet, as shown on Plate 1 and in 

structure sections A-B and C-D, Plate 2. There is little discordance 

in strike and dip between adjacent beds of the Wells and the underlying Brazer, but the boundary of the Wells transgresses certain beds 

of the Brazer, and parts of the Brazer represented in the Preuss 

and Webster Ranges, farther southeast, are absent here. It is therefore concluded that the Wells and Brazer are unconformable.

PERMIAN SERIES

PhospJioria formation. The Permian strata constitute a single 

formation, the Phosphoria, which, though not of great thickness, 

maintains a nearly uniform character over wide areas. The rocks

> U. S. Geol. Survey Prof. Paper 152, pp. 433, 434, 1927. 

6148 29 3
28 PORTNEUF QUADRANGLE, IDAHO

are exposed in narrow bands along the flanks of the larger, more 

simple folds and in more complex crumplings in the smaller folds 

or along the borders of faulted areas. The Phosphoria formation is 

of great economic and scientific interest because it contains the extensive deposits of high-grade phosphate rock that constitute the chief 

mineral resource of the region. There are four principal bands of 

the Phosphoria formation within the Portneuf quadrangle, besides 

several minor occurrences. In the northeasternmost band, which includes parts of sees. 9, 15, and 16, T. 5 S., R. 40 E., only the upper 

cherty member of the formation is exposed. A second band, in which 

the phosphatic shales are intermittently exposed, extends northwestward from the NW. ^ sec. 33 in the same township. A third 

band, the longest within the quadrangle, extends 5 miles northwestward from the SW. % SE. 14 sec. 1, T. 6 S., R. 39 E. A number 

of exposures related to broken folds occupy an area that includes 

the junction of Tps. 5 and 6 S., Rs. 38 and 39 E., and extends southeastward. Isolated areas of chert in sec. 8, T. 6 S., R. 39 E., indicate 

that considerable areas now occupied by Tertiary beds are probably 

underlain by the Phosphoria. In sees. 24 and 25 of the same township the formation is exposed in a strip more than a mile long. In 

sec. 10 of the same township, where the East Fork of Twentyfourmile 

Creek cuts through the ridge, ledges of chert assigned to the Phosphoria are conspicuously exposed on the north side of the canyon. 

(See pi. 5, B.) Further reference to this exposure is made on page 53.

The lower portion or phosphatic shale member of the Phosphoria 

formation ordinarily consists of 75 to 180 feet of yellowish to brown 

phosphatic sandstone, dark-brown to black phosphatic shale, beds 

of brown or black fetid limestone, and one to three economically 

important beds of phosphate rock. The phosphate rock is characterized by gray, brown, or black color, fine to coarse oolitic texture, 

and a strong fetid odor when freshly broken. Weathered fragments or float pieces have a characteristic bluish-white bloom, many 

with white reticulate markings. The thickest and richest bed of 

phosphate rock is usually at or near the base and ranges in thickness 

from 4 to 7 feet or even more over large areas. The other beds of 

possible economic importance are thinner and occur near the top 

and middle of the shale interval. Details of phosphate occurrences 

are given in the discussion of individual townships.

The phosphate rock itself is practically nonfossiliferous, so far 

as observed, although a few discinoids and bone fragments have 

been found, and locally in the region there is a bed that contains 

many shell fragments close to the base. The beds that accompany 

the phosphate rock are, however, to a certain extent fossiliferous. 

The following fossils have been identified by G. H. Girty from localities within or near the Portneuf quadrangle:
Lingula carbonaria? 

Lingulidiscina missouriensis. 

Chonetes ostiolatus. 

Productus geniculatus. 

Productus eucharis. 

Productus phosphaticus.

STRATIGRAPHY 29

Pustnla montpelierensis.

Pugnax weeksi.

Leda obesa.

Aviculipecten montpelierensis.

Omphalotrochus conoideus.

Omphalotrochus ferried.

The Kex chert member, which constitutes the upper division of 

the Phosporia formation, is generally conspicuous because its 

superior hardness enables it to stand out in strong cliffs, ledges, or 

prominent knolls. The phosphatic shale, on the other hand, is 

topographically weak and in many places eroded into gullies and 

depressions. In the Portneuf quadrangle, the limestone f acies, which 

appears in the E-ex chert member in other areas, was not found. The 

principal exposures show the flinty shale that is characteristic of 

the member in much of the general region, but locally, as in sec. 10, 

T. 6 S., R. 39 E., the chert forms massive ledges.

In the limestone facies of the Rex in other parts of the general 

region fossils are abundant. Characteristic fossils of this horizon, 

according to G. H. Girty, include the following:

Productus multistriatus. 

Pustula subhorridus.

Spiriferina pulchra. 

Composita subtilita var.

Spirifer pseudocameratus.

At some places where massive chert beds are exposed sponge spicules and. casts of crinoid stems have been found, but in the Portneuf 

quadrangle no fossils were seen in the Rex.

The thickness of the Rex as exposed in sec. 29, T. 5 S., R. 40 E., 

is about 500 feet.

TRIASSIC SYSTEM

Beds assigned to the Triassic system are exposed at many places 

in the northern third of the quadrangle, especially in the northeastern part. In the general region of southeastern Idaho,21 not including the Portneuf quadrangle, three divisions with an aggregate 

maximum thickness of 5,350 feet, have been distinguished and 

assigned to the Lower Triassic series namely, the Woodside shale, 

Thaynes group, and Timothy sandstone. In addition three other 

formations of uncertain age with a combined thickness of 550 feet 

are provisionally assigned to the Triassic. These are the Higham 

grit, Deadman limestone, and Wood shale. All but the last two of 

these subdivisions are represented in the Portneuf quadrangle.

The relations of the Triassic to the Permian are not fully understood. .The field relations suggest conformity between the two, for 

the boundary between the Woodside and Phosphoria is generally

81 Mansfield, G. R., Geography, geology, and mineral resources of part of southeastern 

Idaho: U. S. Geol. Survey Prof. Paper 152, pp. 85-95, 1927.
30 POETNEUP QTTADKANGLE, IDAHO

very regular and the attitudes of the two formations correspond 

closely. There is no conglomerate at the base of the Woodside and 

no evidence of erosion in the Phosphoria preceding the deposition 

of the Woodside, unless the somewhat variable thickness of the 

Phosphoria is so considered. On the other hand, there are striking 

fauna! and lithologic differences between the two formations, and it 

is believed that a stratigraphic break (unconformity) of some 

magnitude intervenes. The nomenclature of the Triassic in southeastern Idaho and the relations of the Triassic to the Permian are 

more fully discussed in the larger work to which reference has 

already been made.22

LOWER TRIASSIC SERIES

The formations of the general region assigned to the Lower Triassic outside the Portneuf quadrangle include 3,850 to 5,350 feet of 

shale, calcareous beds, and sandstone grouped in three units, of which 

the Woodside shale and Thaynes group are fossiliferous at certain 

horizons. The age determination is based upon ammonite zones, 

which occur 1,000 to 2,250 feet above the top of the Paleozoic formations. The fossils of the overlying 3,100 feet of sediments are less 

distinctive, and the faunal relations of some of them have not been 

fully studied. It is therefore possible, though perhaps not probable, 

that some of these strata may be of later age than Lower Triassic. 

The Timothy sandstone, the third member of the series, has thus 

far yielded no determinable fossils. The thicknesses here given for 

the Portneuf quadrangle are somewhat greater than those for the 

general region.

WOODSIDE SHALE

The principal exposures of Woodside shale in the Portneuf quadrangle are in sees. 25 .and 36, T. 5 S., R. 38 E.; in a belt extending 

southeastward from sec. 21, T. 5 S., R. 39 E., into sec. T, T. 6 S., R. 40 

E.; and in several large areas in T. 5 S., R. 40 E. The rocks are 

characteristically platy calcareous shale with some limestone. They 

are usually distinct lithologically from the underlying Phosphoria 

formation but pass upward conformably into an overlying group 

of generally similar beds from which they are distinguished chiefly 

on the basis of the contained fossils, although there are some lithologic differences.

The Woodside shale as a whole is characterized by olive-drab 

platy shale with alternating thin beds of brownish-gray limestone 

that have locally a faint purplish tint. The shale is silicequs and 

calcareous, so that it is hard and rings under the hammer. It 

weathers into rusty-brown, yellowish, or even black fragments that

M Mansfield, G. R.,. op. cit, pp. 372-373.
STRATIGRAPHY 31

on breaking show the characteristic olive-drab color, often with 

dendrites. The limestone on weathering preserves the purplish-gray 

or brownish-gray color and assumes a sort of velvety surface. Topographically the Woodside is represented by rounded but steep slopes 

strewn with .platy debris but with relatively few ledges. Near the 

top of the formation, however, there are massive limestone beds that 

contain' abundant pelecypods and that locally form conspicuous 

ledges.

The thickness of the Woodside shale in the Portneuf quadrangle 

is about 2,000 feet.

The shale is sparingly fossiliferous, and the forms contained are 

poorly preserved. Linguloid brachiopods and pelecypods, chiefly of 

the genus Myalina, are occasionally found. The limestone at some 

horizons, especially in the upper part of the formation, is highly 

fossiliferous. A Myalina zone and .a zone of brachiopods, chiefly 

species of T&rebratula, occur just below the top of the formation and 

serve as convenient guides.

The age of the Woodside shale is someAvhat in doubt. Its lithologic 

and faunal similarity to the deposits of the Thaynes group have 

caused it to be grouped with those deposits as Lower Triassic rather 

than with the Paleozoic rocks. Hence its base is provisionally 

regarded as the base of the Triassic in this region.

THAYNES GROUP

The °Thaynes, which had formerly been considered a single formation, was designated a group by the writer 23 in connection with his 

studies in the Fort Hall Indian Reservation. Three formations 

the Ross Fork limestone, the Fort Hall formation, and the Portneuf 

limestone were differentiated on both lithologic and faunal grounds, 

and descriptions of them were published in the paper cited and in 

the writer's account of the Fort Hall Indian Reservation.24 As an 

attempt has been made to differentiate the formations on the map 

(pi. 1), brief descriptions are given below. Collections from the 

Woodside shale and Thaynes group in the Portneuf quadrangle are 

very meager. For details regarding the faunas of these formations 

the reader is referred to the report just cited or to the writer's report 

on the general region.25

The Ross Fork limestone, which takes its name from Ross Fork 

Creek, in the Fort Hall Indian Reservation, lies conformably upon

23 Mansfield, G. R., Subdivisions of the Thaynes limestone and Nugget sandstone, Mesozolc, in the Fort Hall Indian Reservation, Idaho: Washington Acad. Sci. Jour., vol. 6, 

pp. 31-42, 1916.

34 Mansfield, G. R., Geography, geology, and mineral resources of the Fort Hall Indian 

Reservation: IL. S. Geol. Survey Bull. 713, pp. 46-50, 1920.

25 Mansfield, G. R., Geography, geology, and mineral resources of part of southeastern 

Idaho: U. S. Geol. Survey Prof. Paper 152, pp. 93, 94, 434-436, 1927.
32 PORTNEUF QUADRANGLE, IDAHO

the Woodside shale and includes at its base the " Meek&ceras beds " of 

the Hayden Survey. These are gray to reddish-brown limestones 

about 50 feet thick that contain numerous ammonites, the chambered 

shells of which appear on the weathered surface of the rock. In 

this region the fossils do not weather out so readily and the beds 

are not so conspicuous as in the Georgetown district, farther southeast. Above the Meekocerits zone for about 800 feet are massive 

and thin-bedded gray to brown limestones which contain many 

brachiopods, chiefly Pugnax and terebratuloids, and pelecypods, 

Myalina and others, with intervening calcareous shales. The lithology of this part of the formation is much like that of the Woodside. 

The presence of a small brachiopod in the massive limestone near the 

base is a convenient guide where the Meekoceras zone is not available. 

The upper 500 feet of the Ross Fork limestone consists of a dense 

calcareous gray to olive-greenish shale that weathers brown to yellow. 

This shale forms conspicuous cliffs and is mainly nonfossiliferous. 

The Fort Hall formation, named from the site of old Fort Hall 

on Lincoln Creek, in the Fort Hall Indian Reservation, is conformable with the Ross Fork limestone and at the type locality consists 

of four parts. At the base is a soft, sugary yellow calcareous sandstone, about 50 feet thick, which at one locality includes a bed of 

sandy limestone 15 feet thick containing a plicated oyster-like shell. 

Next comes about 100 feet of gray or yellowish siliceous dense limestone, which contains large pectinoids and irregular cherty nodules 

and streaks that weather with a rough surface and project along 

bedding planes. This limestone forms rough ledges and high points. 

The third part, observed only locally, is composed.of about 50 feet 

of sandy and shaly gray limestone and includes an oolitic bed 6 to

10 feet thick. The remainder of the section, estimated to be about 

600 feet thick, consists of yellow to grayish cherty and sandy limestones in thin beds, which are represented chiefly by fairly smooth 

slopes strewn with yellow and reddish sandy and cherty float fragments.

The Portneuf limestone, named from Portneuf River, near the 

head of which the formation is well exposed, is a massive siliceous 

and cherty gray to yellowish limestone. The chert occurs in rounded 

and elongated nodules and in streaks. Silicified fossils, including 

Spiriferina n. sp. ?, Terebratula semisimplex and other terebratuloids, and Myophwia lineataf, project from the weathered surfaces. 

The formation is fairly resistant to erosion and is estimated to be 

about 1,500 feet thick. In the Lanes Creek, Freedom, and Montpelier quadrangles, farther east, a well developed red-bed member, 

which consists of interbedded red sandstones and shales and ranges 

in thickness from 200 to 1,000 feet, occurs in the midst of the Port-
STRATIGRAPHY 33

neuf limestone. This was not recognized in the Fort Hall Indian 

Reservation.

In the Portneuf quadrangle the differentiation of the three formations above described is not so readily made as in the Fort 

Hall Indian Reservation. The boundary between the Ross Fork 

and the Fort Hall was drawn in the field where the thin, platy, and 

calcareous beds of the Rosa Fork, which locally form strong ledges, 

,ire succeeded by thicker-bedded and softer yellowish sandstones 

that weather into pinkish or reddish rounded blocks. The thinnerbedded material at the top of the Ross Fork as here mapped contains 

many pectinoids, large and small. The large pectinoids are with 

little doubt Avicutipecten idalioensis,. so that the particular beds 

mentioned should be assigned to the Fort Hall rather than to the 

Ross Fork. The lithologic boundary selected, however, is a much

more readily recognized cartographic boundary, and its use does 

not materially misrepresent the actual distribution of the formations. The upper beds of the Fort Hall as here mapped are massive 

and cherty and in general similar in lithology to beds of the lower 

part of the Portneuf. The cherts of the upper part of the Fort 

Hall, however, are irregular, streaky, and sandy and are developed 

in greater abundance than in the Portneuf beds, in which they are 

more nodular. The Portneuf beds also are more massive and contain 

silicified fossils here and there which project from weathered surfaces. The red-bed member of the Portneuf, recognized in the 

Lanes Creek, Freedom, and Montpelier quadrangles, was not identified in the Portneuf quadrangle. The principal occurrence of the 

formation is in the slope southwest of Grizzly Creek, in T. 5 S., Rs. 

39 and 40 E., where brush and aspen groves are very thick and exposures are poor.

The thicknesses of the respective formations of the Thaynes 

group as exposed in T. 6 S., R. 39 E., are approximately as follows: 

Ross Fork limestone, 1,800 to 2,600 feet; Fort Hall formation, 800 

to 1,000 feet; and Portneuf limestone, 2,600 feet. The variation 

in thickness of the Ross Fork and Fort Hall is probably due in 

part at least to the intense folding which they have experienced. 

The thickness given for the Portneuf may be excessive because the 

structural features that include the Portneuf in the northwest part 

of T. 5 S., R. 40 E., are faulted and more or less concealeld by later 

igneous and sedimentary rock.

TIMOTHY SANDSTONE

The Timothy sandstone is typically a somewhat sugary yellowish 

to grayish rock of generally uniform character in beds 1 to 3 inches 

thick or locally more massive. In some places it weathers with
34 PORTNEUF QUADRANGLE, IDAHO

a pinkish tinge; in others beds assigned to this formation are reddish 

or purplish. It is usually less resistant to weathering than the limestone below or the grit above and hence occupies depressions or 

relatively smooth slopes, on which exposures are few and poor. The 

formation as exposed elsewhere in the region is, at least locally, unconformable upon the Thaynes group. The only occurrences of the 

 Timothy in the Portneuf quadrangle are in the NW. % NW. 14 

sec. 18, T. 5 S., R. 40 E., overlapping a little into adjacent sections. 

The breadth of the principal exposure and the dip shown at this 

locality suggest that the thickness of the Timothy may be as much 

as 2,000 feet. This estimate is probably excessive. The thickness in 

the Fort Hall Indian Reservation is only 800 feet, and the greater 

apparent thickness here may be due to the position of the beds in a 

deeply folded and somewhat faulted syncline.

TRIASSIC ( ?) FORMATIONS

The great thickness of the formations assigned to the Lower Triassip in the general region, the unconformity at the base of the Timothy 

sandstone, and the more marked unconformity below the Higham 

grit suggest that the Higham and the overlying Deadman limestone 

and Wood shale may be of later age than Triassic. Nevertheless, 

the full representation of Jurassic beds in the general region, together with certain similarities between the stratigraphic sequence 

of the Idaho section and that of sections in Wyoming and Utah have 

led to the view that these three formations represent Middle and 

Upper Triassic sedimentation, although no fossils have been found 

in them. In the larger work already mentioned 26 the writer has 

discussed these relations and has presented a table that shows tentative correlations of the Idaho formations with those at specified 

localities in Wyoming and Utah. In the Portneuf quadrangle only 

the Higham grit is exposed.

Higham grit. The Higham grit is normally a coarse white gritty 

or conglomeratic sandstone, locally quartzitic, with subangular fragments or pebbles of quartzite. No other rock type has been observed 

as a constituent. The rocks weather pinkish and locally reddish or 

purplish. The material appears to have been derived from early 

Paleozoic or older quartzite. The grit is distinct lithologically from 

the other rocks of the region and is prominent topographically. It 

forms strike ridges that are marked by rough, craggy ledges in many 

places. The lower 10 to 20 feet at some localities, as on the North Fork 

of Boulder Creek, in the Freedom quadrangle, is fine grained, purplish 

or yellowish, and more or less ferruginous. The formation appears 

to lie conformably on the Timothy sandstone, but several features

^Mansfleld, G. R., op. cit. (Prof. Paper J52), pp. 190-192, 373, 374.
STRATIGBAPHY 35

suggest that the base of the grit probably marks a pronounced 

stratigraphic break. These are the abrupt and striking change in 

lithology from preceding formations; the strongly ferruginous character of the basal beds; and the southward thinning of the underlying Timothy sandstone.

The only exposures of the Higham grit in the Portneuf quadrangle 

are in a single area occupying parts of the SW. 1/4 sec. 7 and the 

NW. 14 sec. 18, T. 5 S., R. 40 E., where the formation is represented 

by perhaps 200 feet of beds. The overlying Deadman limestone and 

Wood shale are absent in the Portneuf quadrangle but are exposed in 

the Cranes Flat quadrangle.

JURASSIC SYSTEM

The Jurassic system in this general region is represented by' the 

Nugget sandstone, the Twin Creek limestone, the Preuss' sandstone, 

and the Stump sandstone, with an aggregate thickness of about 

6,500 feet. None of these formations are exposed in the Portneuf 

quadrangle, and probably all are absent though they appear in the 

Cranes Flat quadrangle on the northeast.

ORETAOEOTJS SYSTEM

In the general region beds assigned tentatively to the Cretaceous 

system have been included in the Gannett group, which has been 

subdivided into five formations having an aggregate thickness of 

3,300 feet, and younger rocks assigned unqualifiedly to the Cretaceous 

have been included in the Wayan formation, comprising 11,800 feet 

of beds. The rocks are nonmarine sediments composed of conglomerate, limestone, sandstone, and more or less carbonaceous shale. 

None of these, however, are believed to be present in the Portneuf 

quadrangle, but they underlie extensive areas in the Cranes Flat 

quadrangle.

CRETACEOUS OR TERTIARY ROCKS

In sec. 13, T. 6 S., R. 39 E., a conspicuous ledge of siliceous and 

cherty breccia is cut into a gateway by a small canyon, utilized by 

the road that runs northeastward from Chesterfield to Grizzly Creek 

and Blackfoot River. The breccia is well exposed on both sides of 

the gateAvay and consists of cherty material similar to that of the 

more massive facies of the Rex chert, solidly cemented by silica. The 

breccia is massive rather than bedded and weathers with a somewhat 

waxy-looking surface. A similar breccia occurs in sec. 20, T. 6 S., 

R. 40 E.; sees. 10, 3, and 4, T. 6 S., R. 39 E.; and sees. 19, 32, and 33, 

T. 5 S., R. 39 E. Breccia of this sort was first observed by the writer 

in 1913 in the SW. Vi SW. ^ sec. 17, T. 5 S., R. 38 E., in the Fort
36 PORTNEUF QUADRANGLE, IDAHO

Hall Indian Reservation. Later similar breccia was found at several 

places along the trace of the Bannock overthrust in the Montpelier 

quadrangle, and it is believed that the breccia in the last-named area 

at least is related either to the Bannock fault or to some associated 

fault. The date of the Bannock overthrust is not known closely, but 

it occurred during the mountain-building epoch that followed the 

deposition of the Wayan formation and preceded the deposition of 

the Wasatch that is, during the Laramide revolution. The age of 

the breccia would therefore be late Cretaceous or early Tertiary.

TERTIARY SYSTEM.

In the general region the Tertiary system is represented only by 

the Wasatch formation (Eocene) and the Salt Lake formation 

(Pliocene?). Of these only the Salt Lake formation is exposed in 

the Portneuf quadrangle.

PLIOCENE (?) SERIES

--& 

The Pliocene (?) rocks are in general massively bedded and relatively soft. They overlie unconformably all older formations with 

which they are associated.

Salt La/ce formation. As most commonly encountered the Salt 

Lake formation consists of light-gray, buff, cor locally pinkish conglomerate, in which the matrix is white, relatively soft, loose textured, and calcareous. Most of the pebbles are of local materials and 

rather angular, though many are subangular or even rounded. There 

is a wide range in size, some of the boulders being 4 or 5 feet in 

diameter and many of the fragments less than an inch. (See pi. 6, A.) 

In the Portneuf quadrangle the materials consist chiefly of early 

Paleozoic rocks, the most conspicuous and most widely distributed 

being pebbles and boulders of Swan Peak quartzite. Hills covered 

with conglomerate of the Salt Lake formation are strewn with countless pebbles and boulders of sandstone, quartzite, limestone, and 

chert. Ledges are few and in general poorly exposed. The massive 

character of the formation is, however, well shown by an exceptional 

exposure in the NW. 14 sec. 6, T. 5 S., E. 39 E., V/2 miles north of 

the Portneuf quadrangle, along the road from Ross Fork to the 

Blackfoot dam. (See pi. 6, B.) Ordinarily the hills underlain by 

the Salt Lake formation are smoothly rounded (pi. 3, <?), though 

valley sides cut in it are steep in many places.

in addition to the conglomerate there are beds of white marl, calcareous clay, sandstone, grit, and volcanic ash. In the Portneuf 

quadrangle a few areas of limestone and sandstone have been distinguished in the mapping. The conglomerate, however, is here the
STRATIGRAPHY 37

predominating representative of the formation. Soils weathered 

from the formation are commonly white or light colored.

A few fresh-water gastropods were collected from the Salt Lake 

formation in the Portneuf quadrangle. These proved to be undeterminable. In previous publications relating to the general region the Salt Lake formation has been tentatively referred to the 

Pliocene. No evidence has yet been found that necessitates a change 

in this reference.

The Chesterfield Range is largely underlain by the Salt Lake formation, though ledges and larger masses of older rocks emerge from 

cover here and there. Considerable areas in the foothills of the 

Portneuf Kange are also underlain by these beds.

In the Fort Hall Indian Reservation large areas were mapped 2T 

as Tertiary or Quaternary upon the assumption that on many slopes 

where pebbles and boulders of the Salt Lake formation are strewn 

material of'Tertiary, age had been reworked and redistributed in 

Quaternary time. From wider experience in the general region the 

writer has concluded that most of these slopes are in reality developed upon Tertiary material and that they should be mapped accordingly. The mapping of the part of the reservation that lies 

within the Portneuf quadrangle has been changed to conform with 

this view.

The beds of the Salt Lake formation are as a rule nearly horizontal, though dips as high as 47° have been found locally. Some 

canyons cut in them are 750 feet or more deep. As the formation 

has experienced vigorous and protracted erosion the original thickness must have greatly exceeded this figure.

QUATERNARY SYSTEM

The rocks of the Quaternary system consist for the most part of 

unconsolidated gravel, sand, and mud that occupy the larger valley 

bottoms, and. of coarser or finer debris, less well stratified, that has 

accumulated here and there along the lower slopes above the valley 

bottoms. Glacial deposits are absent, but there are a number o»f 

accumulations of travertine. The Quaternary deposits are all unconformable upon older rocks. For mapping they are divided into three 

groups, as described below.

PLEISTOCENE SERIES

Travertine. White spongy or porous deposits of calcium carbonate have been formed at several places in the quadrangle by mineralized springs, some of which are still depositing, though on a 

reduced scale. The rock thus formed, which is called travertine, is,

27 U. S. Geol. Survey Bull. 713, pi. 3, 1920.
38 POBTNEUF QUADRANGLE, IDAHO

in some places coarse textured, Open, or even cavernous (see pi. 3, B) ; 

elsewhere it is finer textured and harder. Locally it is stained yellowish or reddish by iron oxide.

Terraces of travertine at the mouths of Little Flat and Eighteenmile Canyons form rather striking topographic features. Another 

conspicuous area is in sec. 15, T. 5 S., R. 40 E., where a welldeveloped spring cone has been formed. (See pi. 7, A.} Other 

areas of some extent occur in sees. 29, 30, and 32 of the same township, in sees. 10 and 16, T. 6 S., E. 39 E., and along the east shore 

of Portneuf Eeservoir from the SW. % sec. 19, T. 6 S., R. 39 E., 

northwestward as far as the SE. 1/4 sec. 11, T. 5- S., R. 38 E. There 

are a number of minor occurrences, some of which have not been 

mapped.

Hill wash and older alluvium. The lower slopes of many of the 

hills are covered with rock fragments and soil that conceal the underlying formations. This material is in many places poorly assorted or 

without definite arrangement. Elsewhere it merges with fluviatile 

deposits and forms alluvial fans that have considerable areal extent. 

The upper limit of the deposits as mapped is the general line that 

marks the outcrop of the older, underlying formations, or the line 

where the debris of these rocks is sufficiently characteristic and abundant to indicate the strong probability of their occurrence beneath. 

The line is somewhat generalized, especially where it lies between the 

Salt Lake formation and the hill wash. The bouldery nature of 

the slopes underlain by the Salt Lake renders difficult or impossible 

any sharp distinction between the two formations. The lower limit, 

also somewhat generalized, is the line of demarkation of these deposits 

from those of the valley bottom, which are generally finer and better 

sorted.

The materials of the hill wash and older alluvium are mostly local 

or derived from near-by sources. They range in coarseness from 

boulders to fine soil and are usually not well rounded. The deposit 

has been cut away by stream action to a greater or less extent, so that 

the present surface is uneven, with low-lying spurs and interfingering 

depressions. The low-lying spurs and higher slopes of the deposit 

are generally covered with sagebrush where they have not been 

cleared for agriculture. Many of the depressions are grass covered, 

and those large enough to be separately indicated are mapped with 

the alluvium.

Some of the finer material mapped as hill wash has an uneven 

surface and probably represents accumulations of wind-blown dust 

such as those that have been recognized in the Henry and Cranes Flat 

quadrangles. The general characteristics of the hill wash indicate
 STRATIGRAPHY 39

that it is a product of more arid climatic conditions than those of 

to-day.

The principal accumulations of hill wash in the Portneuf quadrangle are in the valleys of Corral and Grizzly Creeks and in 

Portneuf Valley. The alluvial fan on which Chesterfield is situated 

and that in the vicinity of Eighteenmile Canyon, both of which have 

been partly cut away by later erosion, are noteworthy. Fans also

 occur along the west side of Portneuf Valley. The thickness of the 

deposit differs from place to place. West of the mouth of Eighteen-

 mile Canyon it probably exceeds 100 feet.

The climatic oscillations of the Quaternary period in southeastern 

Idaho have probably differed in degree rather than in character from 

those of the Bonneville Basin near Great Salt Lake as described by 

Gilbert. The epoch of deposition of the hill wash here is believed to 

correspond with Gilbert's pre-Bonneville epoch.28

RECENT SERIES

Alluvium. In the broader valley bottoms and along the courses 

of many of the larger streams are narrow bands or broader areas of 

grassy meadow land with finger-like extensions up the tributary 

streams. The materials that underlie these areas are commonly well 

sorted and fine or locally gravelly. Along Blackfoot River there are 

stony flood-plain deposits. Elsewhere,. as along Portneuf River, 

there are marshy areas of greater or less extent.

The principal area of alluvium is in Portneuf Valley northwest of 

a line that extends northeastward from the NE. % sec. 26, T. 7 S., 

R. 38 E. The alluvial part of the valley is here about 6 miles wide. 

The northwest extension of this area, which is relatively narrow, is 

partly flooded by the Portneuf Reservoir. Areas of alluvium, which 

because of their situation above the general level of the valley may 

be called'"perched," lie above the travertine deposits in Eighteenmile and Little Flat Canyons. In Grizzly Creek and its tributary 

Thompson Creek there is another fairly large, area of alluvium.

Between the deposition of the hill wash and older alluvium and 

ihat of the alluvium here described there interposed an epoch of 

moister climate. No lacustrine stages such as those in Bear Lake 

Valley described by the writer 29 have been recognized in the Portneuf 

quadrangle, but it is believed that aside from this feature the physiographic development of this area is generally parallel to that of 

the broader area previously described and that the age of the 

alluvium may be considered Recent.

28 Gilbert, G. K.( Lake Bonneville: U. S. Geol. Survey Mon. 1, p. 221, 1890. 

20 Op. cit. (Prof. Paper 152), p. 30.
40 PORTNEUF QUADRANGLE, IDAHO

IGNEOUS BOCKS 

GENERAL OCCURRENCE

Igneous rocks occupy considerable areas in two widely separated 

parts of the quadrangle. The southern part of Portneuf Valley, embracing more than 30 square miles within the area mapped, is occupied 

by a great field of basalt. The northern and northeastern parts of 

the quadrangle contain many exposures of both rhyolite and basalt. 

These exposures are scattered rather than continuous, and some are 

only a few feet across, but others occupy as much as 2 square miles. 

Some of the basalt is contemporaneous or nearly so with the rhyolite, 

but much of it is younger. A single occurrence of andesitic rock was 

noted. This is probably the oldest of the igneous rocks. Although 

beds of volcanic ash are well exposed in the Paradise Valley quadrangle, less than a mile north of the area here considered, such beds 

have not been recognized in the Portneuf quadrangle, and if present 

they are to be found in areas mapped as the Salt Lake formation.

HORNBLENDE ANDESITE PORPHYRY

Character. In digging a prospect in the phosphatic shale in the 

SE. 14 NE. 1/4 sec. 30, T. 5 S., R. 40 E. (see p. 82), very much wealthered andesitic material was encountered at depths of 2 to 5 feet. The 

original andesite had decomposed into yellowish gravel, which now 

underlies and truncates the deformed phosphatic shale. The andesite had doubtless been intruded into the shale as a dike. Its extent 

and thickness were not determined. The weathered products resemble closely the similar weathered material of the hornblende andesite 

porphyry that is exposed at Sugar Loaf Mountain, in sec. 25, T.

2 S., R. 41 E., in the Cranes Flat quadrangle. The andesite is there 

seen in all stages from fresh rock to gravelly waste. Several dikes 

of generally similar rock are exposed in neighboring sections of the 

same township and in T. 2 S., R. 42 E.

The hornblende andesite porphyry in these localities is a gray 

crystalline rock that weathers pinkish, yellowish, or greenish. It is 

mottled with light or dark spots, which represent crystals of feldspar, hornblende, or biotite. The following petrographic description 

of a specimen from sec. 22, T. 2 S., R. 42 E., as kindly given by E. S. 

Larsen, shows the general character of a rock similar to that from 

\vhich the weathered material in the Portneuf quadrangle is supposedly derived.

Macroscopically the rock is light gray and shows plagioclase phenocrysts 

as much as 1 centimeter across, prominent but smaller hornblende prisms, and 

a little biotite in a groundmass that has the appearance of a microgranular 

rock. The study of the thin section shows that the rock contains large pheno-
IGNEOUS BOOKS 41

crysts of andesine and pale-green zoned hornblende. There are a few crystals 

of apatite with smaller crystals of plagioclase, biotite, and iron ore in a fine 

groundmass that is chiefly plagioclase but that probably includes some orthoclase and quartz.

Relative age. In the general region the hornblende andesite 

porphyry has been found in contact with only two formations. In 

the Cranes Flat quadrangle it is associated with the Homer limestone member of the Wayan formation (Lower ? Cretaceous), and in 

the Portneuf quadrangle it is intruded into the phosphatic shale of 

the Phosphoria formation. Its deeply weathered condition suggests 

that it is probably older than the other igneous rocks, none of which 

show so much alteration. Difference in weathering, however, depends 

upon so many variable factors that it may prove deceptive as a guide 

to the relative age of igneous rocks,

EHTOLITE

Chwactei\ The rhyolite of the Portneuf quadrangle has not been 

studied separately from that of the general region. Many specimens 

from different localities in southeastern Idaho show little variation. 

According to Mr. Larsen their chief differences are textural. They 

probably represent closely related flows. They are nearly white to 

pale Quaker drab, pink, gray, or darker colored, rather porous 

fluidal rhyolites, which include a few crystals of quartz and orthoclase with a little plagioclase. They carry also a Very little biotite, 

which is partly altered, besides zircon, apatite, and iron ore. In some 

parts of the region the rock is glassy, spherulitic, or pumiceous, but 

in the Portneuf quadrangle such rock is practically absent, though 

the glassy type is represented in a small exposure in the SW. y± sec. 

11, T. 5 S., R. 39 E.

Distribution. The rhyolite exposures are all in T. 5 S., Rs. 39 and 

40 E. In some places, as in sec. 8, T. 5 S., R. 39 E., they consist of 

ledges barely protruding from the Salt Lake formation or forming 

low-lying points like rocky islands in that formation. In other 

places, as in sees. 18, 19, 20, and 21, T. 5 S., R. 40 E., the rhyolite 

forms masses of greater or less extent that overlie Triassic sediments. 

Elsewhere, as in sees. 11, 13, and 24, T. 5 S., R. 39 E., the rhyolite 

occurs in intimate association with basalt, with which it appears to 

be interbedded.

Mode of occurrence. In all these occurrences the rhyolite is in the 

form of flows that lie nearly horizontal or dip as much as 25°. Although rhyolitic cones are prominent features of the landscape in the 

Henry quadrangle, on the east, none occur in the Portneuf quadrangle.

Thickness. The thickness of the rhyolite differs locally, and its 

maximum has not been determined. In some places the flows are
42 PORTISTEtrF QUADRANGLE, IDAHO

less than 50 feet thick; elsewhere valleys more than 200 feet deep 

have been excavated in them.

Relative age. The rhyolite and hornblende andesite porphyry do 

not occur in proximity, and their relations to each other are unknown. 

From the much more weathered condition of the hornblende andesite 

porphyry it is believed to be the older. The relations between the 

"rhyolite and basalt are relatively clear and indicate that the two rocks 

were, in part at least, practically contemporaneous, though some 

rhyolite is older than some basalt. The big rhyolitic cones south of 

the Blackfoot Kiver Reservoir, in the Henry quadrangle, are practically surrounded by basalt. On the north slope of Middle Cone, 

in the same locality, ledges of basalt with inclusions of rhyolite and 

of rhyolite with inclusions of basalt have been found. In sec. 11, 

T. 5 S., E. 39 E., just north of the Portneuf quadrangle, the rhyolite 

has been intruded by basalt, as indicated by a small dike of basalt 

with chilled edges and vesicular center between plates of rhyolite.

OLIVTNE BASALT

Character. All the basalts of the region apparently contain olivine 

and may be classed as olivine basalts. There are many differences 

in the color and texture of the rock in the general region, but in the 

Portneuf quadrangle there is little variety in either texture or appearance. The rock may be described as aphanitic vesicular basalt. 

In thin section under the microscope, as described by J. F, Hunter, 

it is hypocrystalline, subophitic, and somewhat porphyritic, showing 

a few scattered phenocrysts of plagioclase, olivine, and augite. These 

are but little larger than the individuals of the groundmass. The 

groundmass is made up chiefly of laths of plagioclase, irregular grains 

of augite, olivine, magnetite, and a small amount of glass. The 

plagioclase has approximately the composition of labradorite.

Mode of occurrence. Practically all the basalt exposed in the 

.Portneuf quadrangle is in the form of flows. These occur either on 

top or on the sides of hills, where they are in many places slightly 

warped or tilted, or else in the valleys, where they are generally 

horizontal, though even here some exposures show pronounced dips. 

.The. disturbance of the hill basalts, however, has been due to earth 

movements, whereas that of the valley basalts is the result of flowage 

or of conditions accompanying the escape from the lava of imprisoned 

gases or vapors.

ThicTcn&ss. The thickness of the basalt doubtless varies considerably from place to place, especially where it was poured out over 

high ground, from which it would tend to run off. In the broad 

valleys, however, greater uniformity in thickness and greater thickness may be expected. In a well in the Henry quadrangle (sec. 30,
IGNEOUS BOOKS 43

T. 7 S., R. 42 E.) basalt lying beneath 12 feet of soil and including 

two layers of red ash was at least 279 feet thick. In .the Portneuf 

quadrangle well data are not available, but more than 100 feet of 

basalt is exposed in the canyon of Blackfoot River about. a mile 

northeast of the quadrangle, where several flows may be recognized. 

(See pi. 7, C.)

Relative age. The warped or tilted basalts that now lie on some 

of the hills are obviously older than the valley basalts, which have 

not been deformed. Thus there is evidence in the Portneuf quadrangle of at least two epochs of basaltic extrusion. Some basaltic 

eruptions on hilltops or hillsides in the general region 'probably belong with the later series. The earlier epoch was closely related 

to rhyolitic extrusion, as pointed out in the description of the rhyolite, The later epoch occurred after rhyolitic activity had ceased in

the Portneuf quadrangle.

Source. The older, hill basalts have come in part from local fissures, as indicated by the small basaltic dike mentioned in connection with the rhyolite, although no local fissures aside from the one 

occupied by that dike have been recognized. They may also have 

come in part from vents now more or less concealed by later flows 

or sediments. Some evidence of such vents occurs in the Paradise 

Valley quadrangle, on the north, where the basalt and rhyolite flows 

are associated with much reddish scoriaceous material similar to 

that which now forms basaltic cones in the adjoining Henry 

quadrangle.

The valley basalts in the Portneuf quadrangle are mostly extensions of the Blackfoot lava field, which occupies much of the Henry 

quadrangle and part of the Cranes Flat quadrangle. The basalt of 

Blackfoot and Corral Valleys, in T. 5 S., R. 40 E., came from the 

northern part of that field and.at the time of its extrusion was fluid 

enough to cover wide areas with a practically horizontal surface. 

The basalt in Portneuf Valley may have come in part from craters 

situated a few miles south of the quadrangle, but if so flows from 

these sources were covered by the flood of lava that poured in from 

easterly sources through Tenmile Pass. The basalt, which, in a 

succession of flows, was dammed back along the east flanks of the 

Chesterfield Range and the Soda Springs Hills, found outlet through 

this pass and then deployed in a broad. fan which heads at the pass 

and radiates forward into Portneuf Valley. A similar outpouring 

westward occurred at the gap occupied by Bear River at the south 

end of the Soda Springs Hills, but there the fanlike deployment of 

the basalt is not so evident.

/Surface features. The surface of the basalt in Portneuf Valley, 

though generally even, is marked here and there by broken mounds

6148 29 4
44 PORTNEUF QUADRANGLE, IDAHO

of basalt with steep outward radial dips and rude radial cracks, the 

whole probably representing former blisters 25 feet or more in diameter and 6 or 8 feet high on the surface of the formerly molten or 

viscous lava. Several of these occur in the vicinity of the southeast 

corner of sec. 31, T. Y S., R. 40 E. The lines of sinks and other evidences of subterranean channels, in the basalt that are common in the 

Henry quadrangle were not observed in the Portneuf quadrangle, 

and if present are covered with soil. Ledges and boulders of basalt 

are sufficiently abundant, especially in the southeastern part of the 

quadrangle, to make the roads rough in places.

Distribution. The general distribution of the basalt has been outlined in the preceding discussion. The hill basalts in the Portneuf 

quadrangle are confined almost entirely to T..5 S., R. 39 E. The 

valley basalts are exposed in T. 5 S., R. 40 E., and T. 7 S., Rs. 38 

to 40 E.

RELATIONS OF IGNEOUS ROCKS TO SEDIMENTS

The hornblende andesite porphyry is intrusive into the Phosphoria 

formation. The rhyolite has been extruded upon Triassic formations and is interbedded with the Salt Lake formation. It is overlain here and there by hill wash. The older basalt, like the rhyolite, 

is interbedded with parts of the Salt Lake formation. The la.ter 

basalts overlie the Salt Lake and earlier formations here and there 

and are themselves overlain in places by hill wash or wind-blown 

soil.

RELATIONS OF IGNEOUS ROCKS TO EACH OTHER

Details of the relations of the igneous rocks to each other have

already been given. From the evidence presented in ,the Portneuf 

and Cranes Flat quadrangles it appears that the intrusion of hornblende andesite porphyry was not closely related to the other igneous 

activity of the region and may have been considerably earlier than 

the rhyolitic or the basaltic extrusion. Evidence from the Henry and 

Cranes Flat quadrangles taken in conjunction with .that in the Portneuf quadrangle shows that an epoch of dominantly rhyolitic eruption preceded an epoch in which rhyolite and basalt were extruded 

with some alternation, which was followed by a .time in which basalt 

was the dominant extrusive.

GENERAL IGNEOUS ACTIVITY OF THE REGION

The igneous activity of the Portneuf quadrangle has been summarized in the preceding paragraph. When the broader region of 

which this area forms a part is considered, the story is longer and 

more complex. The evidence seems clear for at least five epochs of
STRUCTURE 45

igneous activity.30 To .this may be added the somewhat uncertain 

evidence of three other epochs. The latest phases of igneous activity thus far recognized seem to have been rhyolitic or latitic rather 

than basaltic. Elsewhere in southeastern Idaho, as in the Craters of 

,the Moon National Monument, there are very late basaltic extrusions 

which may prove to be the youngest volcanic features of the region.

AGE OF THE IGNEOUS ROCKS

From evidence available in the region outside the Portneuf quadrangle it has been concluded that the hornblende andesite porphyry 

is probably not younger than early Pliocene and may be as old as 

early Eocene. The rhyolites and earlier basalts are closely associated with ,the Salt Lake formation and appear to represent products 

of igneous activity occurring in the later part of the epoch of deposition of that formation. As the Salt Lake formation is tentatively 

considered of Pliocene age the rhyolite and earlier basalt are also 

Pliocene (?). The later basalts, which occupy valleys developed in 

the Blackfoot cycle (see p. 7), are overlapped here and there by 

early Quaternary sediments and hence may be considered Pleistocene.

STBTTCTUBE 

EARLIER GEOLOGIC WORK

The earliest geologic map that includes the Portneuf quadrangle is 

that by Peale, published in 18T9, to which reference has already been 

made (p. 6). The field work on which this map is based was done 

in 1877.31 Considering the fact that Peale made few direct geologic 

observations within the quadrangle his generalizations are surprisingly accurate. He noted, for example, that the structure of the 

upper Portneuf Valley is in part anticlinal 32 and that the hills in 

the northeastern part of the quadrangle are Jurassic.33 These rocks 

are now considered Lower Triassic, but they were for a time grouped 

with the so-called " Jura-Trias." The main mass of the Chesterfield 

Kange he correctly ascribed to the Carboniferous, and he noted the 

probability of synclinal structure between these Carboniferous rocks 

and the ridge now called Keservoir Mountain, in the adjoining 

Henry quadrangle on the east. He thought there were no faults 

because there was apparently room enough for the folds then 

recognized.

The principal errors of Peale's mapping in the light of more 

recent work are the designation of the rocks west of Portneuf Valley

80 See U. S. Geol. Survey Prof. Paper 152, pp. 123-125, 1927.

ffl U. S. Geol. and Geog. Survey Terr. Eleventh Ann. Rept., pp. 511-646, 1879.

33 Idem, p. 563.

88 Idem, p. 596.
46 PORTNEUF QUADBAlsrGLE, IDAHO

as Carboniferous, the omission of the Tertiary rocks that are abundant in the southern and central parts of the Chesterfield Range, and 

the failure to recognize the general complexity of the structure of 

that range.

Schultz and Richards 3 * made little advance over Peale's. mapping 

except the marking of faults along the northeast and southwest .sides 

of the main ridge of the Chesterfield Range, which they considered a 

monocline with northwest trend and southwest dip. They correlated 

these faults with the Bannock overthrust of Richards and Mansfield.3 *

The writer's work in the Fort Hall Indian Reservation in 1913 3<J 

included parts of Tps. 5 and 6 S., R. 38 E., in the Portneuf quadrangle. The adjustment of this work to the later and larger-scale 

topographic map now available, together with some revision of field 

mapping and of interpretation, has been accompanied by appropriate 

changes in the geologic map here presented as Plate 1.

GENERAL FEATURES

The detailed work of 1913, 1916, and 1923 has shown that the 

geologic structure of the Portneuf quadrangle is far more complex 

than Peale supposed. The mountainous mass occupying more than 

the northeast half of the quadrangle and including parts of the 

Blackfoot Mountains, the Chesterfield Range, and the Soda Spring 

Hills contains no less than six major folds, all of which are cut to a 

greater or less extent by faults. The structure of Portneuf Valley 

is largely concealed by Tertiary and later sediments and by basaltr 

but from exposures along the northeast side there seems to be justification for Peale's view that the structure, in that part of the valley 

at least, may be anticlinal. The breadth of the valley farther south 

and the general complexity of the structure on the northeast suggest that the valley area may likewise be underlain by a number of 

large folds broken by faults. This suggestion is further strengthened by the evidences of folding and faulting in the foothills of 

the Portneuf Mountains along the west border of the quadrangle. 

The folds east of Portneuf Valley trend generally northwest. 

Although they are apparently open and upright in some places, as 

seen in the geologic structure sections (pi. 2), they are elsewhere 

markedly inclined or even overturned northeastward. The trend 

of the folds west of Portneuf Valley is more directly north or even 

a little northeast, but the same tendency for eastward inclination is 

apparently present.

34 Schultz, A. R., and Richards, R. W., A geologic reconnaissance in. southeastern Idaho: 

U. S. Geol. Survey Bull. 530, pp. 267-284, 1913.

85 Richards, R. W., and Mansfield, G. R., The Bannock overthrust, a major fault In southeastern Idaho and northeastern Utah: Jour. Geology, vol. 20, pp. 681-709, 1912.

» U. S. Geol. Survey Bull. 713, 1920.
STEUCTUEE . 47

One of the most remarkable structural features of the quadrangle 

is an overthrust fault that is believed to pass beneath the Chesterfield Range and Blackfoot Mountains and is locally eroded through, 

exposing rocks of the underlying block through a " window " in the 

upper block. Such features have been observed in connection with 

the Bannock overthrust. It is therefore concluded that this fault 

is either a part of the Bannock overthrust or of a closely related 

thrust fault. Other faults of presumably more recent date have dislocated parts of the folds contained in the upper thrust block and 

have doubtless affected the underlying block as well. All these faults 

have an important bearing on the commercial availability of the 

phosphate rock associated with the folds.

Another noteworthy feature of the quadrangle is the remarkable

 extension of Tertiary and later cover that conceals the structure 

over large areas. Fortunately, sufficient data are available, even in 

;areas: rather completely covered, to afford a basis for structural 

inferences that are probably true in a broad way.

STRUCTURE CONTOURS

As classification of phosphate lands is dependent in part upon the

 depth at which the phosphate occurs, it has been deemed advisable 

to show as far as practicable by structure contours on the map the 

approximate position of the. phosphate beds beneath the surface. 

In this rugged region the altitude of the surface changes so greatly 

from place to place that it was thought best to use sea level as a

 datum plane. The structure contours were therefore drawn by 

projecting to the surface at the lines* of the structure sections the 

rsuccessive positions of the top of the phosphatic shale member of the 

Phosphoria formation at altitudes ranging from 3,000 to 6,000 feet 

^ibove sea level and connecting the points at each altitude. The lines 

thus drawn are necessarily very much generalized, but they serve to 

show approximately the areas within which phosphate may presumably be found at accessible depth. The approximate depth of the 

phosphate at any place along a structure-contour line may be found 

!>y. subtracting the figure which designates that structure contour 

from the figure which represents the surface contour at the same 

place.

DETAILED DESCRIPTIONS

The detailed descriptions that follow are based on the geologic 

structure sections given in Plate 2. The folds are treated in order 

from northeast to southwest. Much of the faulting is discussed in

 connection with the folds, but independent consideration is given 

4o the Bannock overthrust and to certain other faults.
48 PORTNEUF QUADRANGLE, IDAHO

Corral Creek afiiticline. The principal fold in the northeast corner 

of the Portneuf quadrangle is the Corral Creek anticline, named 

from Corral Creek, which joins Blackfoot River at the west base of 

the ridge that forms the west limb of the fold. This fold, which 

has a width of about 2 miles, emerges from cover near the center of 

sec. 15, T. 5 S., R. 40 E., continues northwestward about 4^2 miles,: 

and passes again beneath cover in the Paradise Valley quadrangle. 

It is upright and symmetrical but is broken along its west flank in 

Blackfoot Valley by a fault, believed to be normal, that brings the 

Portneuf limestone on the west into contact with the lower part of 

the Ross Fork limestone on the east. The downthrow to the west 

at the line of structure section A-B is estimated at about 4,000 feet,. 

whereas at the line D-E it is about 2,000 feet. Thus the throw of 

the fault appears to diminish southeastward. On the east side the 

Corral Creek anticline adjoins a syncline that extends from the 

Paradise Valley quadrangle into the Cranes Flat..and Henry,quadrangles and is thought to be the continuation of the Reservoir syncline 

of the Henry quadrangle.

The Corral Creek anticline pitches toward the northwest at an 

average rate of about 725 feet to the mile. It has commercial importance ,in that it makes valuable phosphate deposits relatively 

accessible. It is supposedly cut at unknown depth by the Bannock 

overthrust.

Nigger Creek syncline. Immediately southwest of the Corral 

Creek anticline there is a syncline whose surface is drained in part 

by Nigger Creek. This fold may therefore appropriately be called 

the Nigger Creek syncline. Its enters the area of the Portneuf quadrangle beneath cover from the southeast and its extension in that

direction is unknown. Its presence at the line of structure section 

F-G is inferred from the attitudes of the Ross Fork limestone and 

Fort Hall formation in areas half a mile or more north of this lineWithin the Portneuf quadrangle the fold has a curving trend toward 

the northwest for a distance of about 7 miles. In sees. 17 and 18r 

T. 5 S., R. 40 E., it is offset by a transverse fault with downthrow to 

the north. This fault, which is entirely concealed, must exist, for 

the Portneuf limestone occurs on the north side of Nigger Creek in 

the strike of the Ross Fork limestone on the south side. The fault 

is believed to be normal and to be related to the strike fault that 

cuts off the west flank of the Corral Creek ant.icline. The Nigger 

Creek syncline is offset by a second cross fault in sec. 7, T. 5 S., R40 E., by which Portneuf limestone on the north is brought upward 

with respect to Timothy sandstone and Higham grit on the south. A 

short distance north of this fault the fold passes beneath cover which 

continues far into the Paradise Valley quadrangle. The east side
STRUCTURE 49

of the fold for at least part of its course ,is marked by the strike 

fault above mentioned.

At the line of structure section F-G the Nigger Creek syncline is 

little more than hajlf a mile wide, and the uppermost of the preTertiary beds in it presumably belong to the Fort Hall formation. 

It is believed to be unsymmetrical, with eastward .inclination of its 

axial plane. At the line of structure section D-E the fold is somewhat wider, and its unsymmetrical character is clearly; shown, the 

west limb being strongly overturned toward the northeast. At least 

one minor anticlinal fold is there developed ,in its troughlike bottom. 

The Fort Hall formation still comprises its uppermost pre-Tertiary 

beds. At the line of structure section A-B the fold is as much as 

2 miles wide and its uppermost beds, though concealed, are believed 

to be the Timothy sandstone, for the Higham gr.it, next above the

Timothy, is exposed within about 100 yards north of the line of 

the section. The greater width and higher stratigraphic content of 

the'fold-at-this line are .believed -to ibe due in part to the combined 

influence of the fault in Nigger Creek and of the strike fault, which 

it joins. Both of these faults produce a greater downthrow toward 

the northwest. This effect may also be due in part to a gentle northwesterly pitch.

The Nigger Creek syncjine, like the Corral Creek anticline, is believed to be cut off at unknown depth by the Bannock overthrust. 

Even without this disadvantage the Nigger Creek syncline ,is so 

deep that the phosphate beds that would normally underlie it would 

be too deep to be considered commercially available.

Grizzly Creek anticline. The Grizzly Creek anticline enters the 

Portneuf quadrangle beneath cover in about sees. 22 and 15, T. 6 S., 

K. 40 E., and extends southeastward from this area under cover an 

unknown distance. Within the Portneuf quadrangle it has a generally northwest trend for 9 miles or more and probably extends a 

considerable distance into the Paradise Valley quadrangle, but for 

much of its course it is concealed by Tertiary or later beds and by 

igneous rocks. It is named from Grizzly Creek, which follows the 

general strike of the fold.

The position and character of this fold at the line of structure 

section F-G are inferred from exposures of Triassic formations a 

quarter of a mile and more north of the line. It is about a mile 

wide and is presumably inclined eastward here, as it is farther north.

At the line of structure section D-E the fold is about 2 miles wide 

and its crest is broken by a fault. This fault, which is believed to 

be normal, has a downthrow to the southwest estimated at nearly 

2,500 feet. It is nowhere exposed but is covered by Quaternary 

sediments, by weathered products of the formations affected, or by
50 POETNEUF QUADRANGLE, IDAHO

igneous rocks. Several areas of travertine along the northeast side 

of the valley of Grizzly Creek are thought to indicate the approximate position of its trace. There is no evidence of its presence 

near the line F-G, so it doubtless originates somewhere between F-G 

and D-E. Likewise it dies out northwestward. Its maximum throw 

is reached at about the line D-E, where beds of the upper part of 

the Woodside are brought into contact with beds of the middle to 

the upper part of the Wells.

The undue width of the strips of territory occupied by Ross Fork 

limestone southwest of the fault and by Woodside shale northeast 

of it is explained by drag folds near the crest of the fold, which are 

necessitated by the sharp changes in dip near the contact of the 

Woodside and Koss Fork at the southwest and within the Woodside 

at the northeast. Such drag folds within larger folds have been 

recognized and were photographed at a number of places in the 

quadrangles on the east and southeast previously mapped by the 

writer, and his associates. 37 Their presence in an area so strongly 

folded as the Portneuf quadrangle is therefore to be expected.

The Grizzly Creek anticline brings to the surface the phosphatic 

shale member of the Phosphoria formation for a distance- of about 

2 miles along the northeast side of the.valley. The phosphatic shale 

occupies depressions just back of a series of rocky knolls, and in 1916 

was prospected by the Geological Survey party. (See p. 82.) The 

adjacent Nigger»Creek syncline is so deep that the phosphate beds 

descend steeply into it. Hence the area in which they may be 

regarded as accessible by mining is relatively narrow.

At the line of structure section A-B the Grizzly Creek anticline 

is little, more than a mile wide. The area occupied by the Ross Fork

limestone as projected beneath cover to the line of the section is 

apparently too narrow to allow for the double thickness of this 

formation necessitated by the fold. It is believed, therefore, that 

the fault already discussed in connection with structure section D-E 

is also present here but with much diminished throw.

This fold appears to have reached its maximum development at 

about the line of o structure section D-E, i where the crest was arched

higher than elsewhere and where correspondingly lower beds were 

exposed. The extra width of the fold near this line is due not only 

to the arching mentioned but also to the faulting above described.

The Grizzly Creek anticline is economically important because of 

the phosphate beds that it contains. These are accessible in the 

higher parts of the arch, as already explained, but toward the north 

and the south they would normally lie too deep to be commercially 

available. There is the further probability that the phosphate in

37 See U. S. Geol. Survey Prof. Paper 152, pis. 48, B, and 50, B, 1927-
STRUCTIJEE 51

the deeper portions of the fold may be cut out by the Bannock 

overthrust.

Graves Greek syncline. The next fold in.order southwest of the 

Grizzly Creek anticline is the Graves Creek syncline, named from 

Graves Creek, which has excavated a valley in its southwest limb. 

This fold enters the Portneuf quadrangle beneath cover in about 

the NE. % sec. 27, T. 6 S., R. 40 E., southeastward from which it 

extends beneath cover an unknown distance. Within the quadrangle 

it extends northwestward about 4 miles before emerging, and at a 

distance of about 914 miles it again passes beneath cover. There is 

every reason for believing that it continues well into the Paradise 

Valley quadrangle, but its northwestward extension has not been 

identified.

At the line of structure section F-G the position and character 

of the Graves Creek syncline can be inferred only from the position 

and attitude of the Wells formation, which there constitutes part 

of its southwest limb and dips southwestward, and from conditions 

indicated by structure section C-D-E on the northwest. At line 

F-G the fold is only about a mile wide, whereas farther northwest 

it is considerably wider. It is therefore likely that the syncline dies 

out toward the southeast, or perhaps it is affected by cross buckling 

or warping in that direction.

At the line of structure section C-D-E the Graves -Creek syncline 

is about 2 miles wide, and its axial plane is inclined strongly northeastward. The highest beds contained in it belong to the Portneuf 

limestone, of which only the lower member appears to be represented. 

About a mile southeast of the point marked D in the section the 

Fort Hall formation seems to transgress the Eoss Fork limestone, 

suggesting the presence of a minor thrust fault in that part of the 

fold. The Phosphoria formation with its phosphate beds is exposed 

along the southwest flank of the syncline. This formation is here 

overturned and dips southwestward at an angle of about 50°. In 

the trough of this fold the phosphate beds would normally lie too 

deep to be considered available for mining.

At the line of structure section A-B the Graves Creek syncline is 

2 miles or more wide and, though inclined eastward, as at the line 

C-D-E, seems to be less closely compressed, perhaps because the 

fault dies out in the adjacent Grizzly Creek anticline. The Phosphoria continues to be exposed along the southwest limb, and the 

highest beds are Portneuf. Although none of the red beds of the 

Portneuf have been recognized here, the lower member is thought 

to be somewhat more fully preserved than at the line C-D-E. The 

greater width of the syncline at C-D-E and A-B than at F-G, together with the presence of successively higher beds toward the
52 POETNEUF QUADRANGLE, IDAHO

northwest, indicates that the syncline pitches gently in that direction, though it has not been practicable to measure the rate.

The lower part of the Graves Creek syncline is with little doubt 

cut off by the Bannock overthrust, which locally rises to the surface 

in the next fold to the southwest. One effect of this fault, so far as 

this syncline is concerned, is to make very doubtful the existence of 

phosphate in normal position in its deeper parts.

Twentyfourmile anticline. The name of the Twentyfourmile 

anticline is taken from Twentyfourmile Creek, which, with its East 

Fork, has excavated valleys along both limbs of the fold for a 

number of miles. This is the largest fold in the Portneuf quadrangle, and because of its intimate relations with the Bannock overthrust it is in some respects also the most interesting structural 

feature in the quadrangle.

The line of structure section H-I has been extended to include 

exposures of the Brazer limestone in the adjoining Henry quadrangle. 

These exposures clearly indicate that the general structure observed 

farther northwest continues southeastward into the Henry quadrangle. Unfortunately the fold passes beneath cover in that direction beyond the first mile or two. In the Portneuf quadrangle, too, 

it is largely concealed by Tertiary beds, which lie unconformably 

upon the eroded older strata. After continuing northwestward 

about 14 miles from the line H-I the anticline passes beneath cover, 

perhaps to reappear in one of the broken folds of the Fort Hall Indian 

Reservation beyond, but its identity in any extended portions has 

not been established. Upon the assumption that at the line H-I 

it is a closely folded anticline overturned toward the northeast, an 

assumption which seems justified from conditions farther northwest, 

the Twentyfourmile anticline would be nearly 2% miles wide where 

it enters the quadrangle, and the lowest beds exposed in it would be 

beds of the Brazer limestone.

At the line of structure section F-G the conditions are somewhat 

similar. Only the Wells formation, which makes up part of the 

northeast limb, is exposed, but the southwesterly dip of these beds, 

about 35°, indicates clearly that the fold is strongly overturned toward the northeast. The arched plane of the Bannock overthrust 

is supposed to rise near the surface at this line and perhaps even to 

have been exposed by the erosion of the fold, but, if this is true, the 

exposure has been concealed by the deposition of the Salt Lake formation. From the occurrence of the siliceous fault breccia indicated on 

the map by the symbol KTb at places within 1 mile southeast of the 

line F-G and iy2 miles northwest of this line it is thought that the 

upper block of the Bannock overthrust may be broken through or 

faulted along the crest of the arched thrust plane. This interpretation is indicated in the structure section.
STRUCTURE 53

At the line of structure section C-D the Twentyfourmile anticline 

is about 3y2 miles wide. The northeast limb is fully exposed and also 

a small part of the southwest limb. The dips of the beds in the two 

limbs are roughly parallel, about 40° SW.<, so that the fold as a 

whole is strongly overturned toward the northeast. The most interesting feature of the fold at this line is the occurrence in the midst 

of it of massive ledges undoubtedly belonging to the Hex chert member of the Phosphoria formation and surrounded on all sides by 

Brazer limestone. The chert dips southwestAvard somewhat more 

strongly than the inclosing limestone. It is this occurrence which, as 

.above stated, has led to the view that the Bannock overthrust or 

some related fault with folded thrust plane underlies the Chesterfield 

Range. The Bannock overthrust is more fully described on page 55.

At the line of structure section A-B the anticline is a little narrower 

than at C-D and has about the same degree of northeastward overturning. The lower block of the Bannock overthrust is not exposed, 

but its presence near the surface is suggested by the ledges of cherty 

fault breccia along the crest of the arched Brazer limestone at places 

intermediate between the exposure of Rex chert noted above and the 

line A-B and by the similar ledge in the SE. % sec. 19, T. 5 S., R. 39 

E., iy2 miles northwest of that line. These exposures seem to indicate 

;a more or less continuous break in the limestone along the line supposedly marking the trend of the arch in the folded thrust plane. 

This interpretation is indicated on the map and in structure section 

A-B.

The group of structure sections above reviewed seems to indicate 

that the Twentyfourmile anticline originates at a point some distance 

southeast of the place where the fold enters the Portneuf quadrangle. 

It pitches on the whole rather gently northwestward but is wider and 

more deeply eroded at the line C-D than elsewhere. This effect may 

"be due to a slight upwarping of the axis of the fold at that place, 

but the upwarping is not sufficient to disturb the strike of the beds 

.along the exposed northeast limb of the fold except to a very minor

 degree.

Phosphate beds are exposed for about 5 miles along the northeast 

Hank of the anticline and locally along the southwest flank. It is 

believed that they follow both sides of the fold beneath cover. Their 

actual position in these places would have to be determined by

 drilling,.but their approximate positions may be inferred from the 

measured dips and the known thicknesses of the rock formations. 

Because of the presence of the Bannock overthrust beneath at least 

certain parts of the fold the beds, which descend steeply, are in all 

probability cut off at moderate depths in such places.

Rock Creek syncline. Southwest of the Twentyfourmile anticline the structure of the underlying rock is largely concealed by
54 POBTXEUF QUADKANGLE, IDAHO

Tertiary beds, but exposures here and there indicate the presence 

of at least one more large fold. The largest exposed area is in the 

vicinity of Rock Creek in the southeast corner of T. 5 S., R. 38 E, 

The fold with which these exposures are chiefly connected may therefore appropriately be called the Rock Creek syncline.

The presence of this syncline at the line of structure section H-I 

is inferred from the exposures of older Paleozoic rocks in eastward 

ascending order along the west base of the Chesterfield Range north 

of Tenmile Pass and of the Brazer limestone along the line H-I 

(extended) in the adjoining Henry quadrangle. The indicated 

structure is postulated from the measured dips and from the knosvii 

thicknesses of the formations involved. It is believed that this fold 

passes southeastward an unknown distance under cover. At the line 

H-I it is about 2 miles wide. The depth of the syncline is unknown, 

but a reasonable interpretation of the available data shows that the 

Phosphoria formation and some of the lower beds of the Woodside 

are probably present. The actual position of the phosphate beds 'in 

this part of the fold can be determined only by drilling, but their 

approximate position is shown in the structure section.

At the line of structure section F-G the Phosphoria and Wells 

formations are exposed with gentle northeasterly dips near the west 

end of the line, and the Wells formation with steeper westerly dips 

appears near the middle. On the basis of the known folds farther 

northeast, of which the last-named Wells exposures are presumably 

a part, there is little, if any, room for beds higher than the Phosphoria in the Rock Creek syncline at the west. Thus the syncline is 

supposed to be relatively shallow and broadly opened.

At the line of structure section C-D the Rock Creek syncline is 

largely concealed by Tertiary beds, and its shape and character are 

inferred on the basis of available measurements of dip and thickness 

and the structure of neighboring beds on the east. The presence of 

a subsidiary anticline within the syncline is suggested by the folded 

and faulted condition of the syncline farther northwest and by the 

fact that the scattered exposures of Rex chert and Woodside shale 

within a mile to the northwest of the line C-D appear to indicate a 

shallow rather than a deep fold. The west limb of the syncline is 

slightly faulted and overturned toward the northeast.

At the line of structure section A-B the Rock Creek syncline is 

affected by shallow minor folds, some of which may be of the drag 

fold type, and by faulting. There are two synclinal axes, which, 

presumably join farther northwest beneath cover, with an intervening anticlinal axis. The faults, which appear to include both 

normal and reverse types, do not create great displacements, though 

they introduce considerable complexity into the mapping, especially-
STRUCTURE 55

in sec. 6, T. 6 S., R. 39 E. The character and direction of the minor 

folds are indicated by axes drawn on the geologic map. The narrowing of the Rock Creek syncline, as outlined by exposures of 

phosphatic shale at Rock Creek in sec. 31, T. 5 S., R. 39 E., suggests 

a local transverse warping of the fold in the manner tentatively 

indicated on the map.

The Rock Creek syncline apparently deepens and widens toward 

the northwest and is doubtless represented by one of the folds in 

the Fort Hall Indian Reservation, but the structure in the intervening areas is concealed. This fold has some importance as a 

possible commercial source of phosphate, of which the most readily 

accessible beds are those in the areas immediately adjoining Rock 

Creek. It is thought that phosphate would probably be found beneath 

cover along the trend of the axis as indicated on the map, 'but its 

depth, extent, and character could be determined only by boring.

Other folds. The southwest ends of the structure-section lines all 

show strata dipping in such a manner as to constitute the northeast 

limb of an anticline, the crest of which would lie beneath cover near 

the eastern border of Portneuf Valley. This mapping supports 

Peale's view of anticlinal structure at the north end of the valley. If 

no significant faults exist other folds similar to those above described 

doubtless underlie Portneuf Valley.

The foothills of the Portneuf Mountains west of Portneuf Valley, 

so far as they extend into the Portneuf quadrangle, include only 

parts or fragments of folds. The beds exposed are all of Cambrian 

and Ordovician formations with general easterly dips, and the 

stratigraphic succession rises in general in that direction.

The Swan Peak quartzite in sec. 35, T. 7 S., R. 38 E., and sec. 1, 

T. 8 S., R. 38 E., lies in a syncline in which the axis strikes northwest 

and the west limb dips more steeply than the east limb. The exposures of Swan Peak quartzite in sees. 23,13, and 14, T. 7 S., R. 38 E., 

suggest similar structure, but here the east limb as represented by 

the knoll in sees. 13 and 14 dips more steeply.

The Cambrian beds on the south side of Topons Creek have 

synclinal structure.

Bannock overthrust. The name Bannock overthrust was originally given by Richards and Mansfield 3S to a group of thrust faults, 

which, upon detailed study, seemed to mark the edge of a great block 

or slice of the earth's crust that had moved east-northeastward with 

respect to underlying rocks for possibly 35 miles or more. Starting 

in northeastern Utah the sinuous course of this overthrust was traced 

northward and northwestward intermittently for about 270 miles. 

Much of the faulted area lies in the group of seven quadrangles east

38 Richards, R. W., and Mansfield, G. R., Jour. Geology, vol. 20, pp. 681-707, 1912.
56 POBTNEUF QUADRANGLE, IDAHO

and southeast of the Portneuf quadrangle described in the author's 

larger work,39 and the reader is therefore referred to that publication 

for a detailed description of this great structural feature. In the 

paper cited suggestion was made of a possible connection between 

the Bannock overthrust and the Putnam overthrust of the Fort. Hall 

Indian Reservation.40

One of the remarkable characteristics of the Bannock overthrust isthe fact that its thrust plane is systematically warped or gently 

folded so that the overlying block is cut through here and there b> 

erosion, thus exposing the rocks of the underlying block. The exposure of the Rex chert in the midst of Brazer limestone in the canyon 

of the East Fork of Twentyfourmile Creek in sec. 10, T. 6 S., R. 39> 

E., in the Portneuf quadrangle, is thought to be similar evidence of 

the presence of the Bannock overthrust plane beneath the Chesterfield Range. The alternative supposition, that the chert ledges are* 

an eroded remnant of a fault block let down into its present position,, 

has little to support it. The occurrence of a great thrust fault in. 

the western parts of Tps. 3 and 4 S., R. 40 E., in the Paradise Valleyquadrangle, on the north, is believed to strengthen the view first 

stated. The ledges or masses of Cambrian rocks in sec. 18, T. 5 S., 

R. 39 E., to which reference was made on page 19, lie on both sides of 

the line (extended) which has already been mentioned as marking 

the position of ledges of siliceous fault breccia presumably associated with the Bannock overthrust. It therefore seems highly probable that these occurrences are ledges of Cambrian rocks and that 

they are definitely related in some way to the Bannock overthrust,. 

either as a part of the upper block or as a slice of older rock 

heaped upon the upper block by a subsidiary thrust fault. The thick

cover of Tertiary sediments, however^ effectually conceals the structure in the vicinity of these ledges.

In sec. 20, T. 5 S., R. 38 E., in the Fort Hall Indian Reservation,, 

there is a thrust fault, part of the Putnam overthrust, that brings 

Cambrian and Ordovician rocks into contact with Wells and Phosphoria beds. This fault, which passes under cover both eastward 

and westward with only scant exposure, is now believed to represent 

a westward extension of the fault zone of the Bannock overthrust.- 

The relations of the Putnam overthrust and of the window in the 

Portneuf quadrangle to other parts of the Bannock overthrust are 

shown in Figure 2. The window and the axis of the Twentyfourmileanticline lie in the northwest extension of the axis of an anticline 

in the fault plane previously mapped on the basis of evidence in

88 Mansfield, G. R., Geography, geology, and mineral resources of part of southeastern 

Idaho: U. S. Geol. Survey Prof. Paper 152, 1927.

40 See also Mansfield, G. E., Geography, geology, and mineral resources of the Fort Hall/ 

Indian Reservation, Idaho : U. S. Geol. Survey Bull. 713, pp. 62-65, 101-102, 1920.
STBUCTUEE 57

>> v» i MADISON VTETQN*!

V. *\ » TYDTrtnaO '

FIODBB 2. Sketch map of part of the Bannock overthrust, showing its relations to 

the Putnam overthrust and to later-mapped portions In the Portneuf, Paradise 

Valley, and Ammon quadrangles. Short arrows show dip of fault planes
58 PORTNEUF QUADRANGLE, IDAHO

Bear River Valley near Nounan and southeast of Soda Springs. 

The branches shown in the Paradise Valley and Ammon quadrangles 

were mapped in 1923 and 1925.

The eastern edge of the upper block of the Bannock overthrust 

as thus far mapped passes northwestward through the center of the 

Cranes Flat quadrangle and continues thence into Taylor Mountain 

(Blackfoot Peak), about 25 miles north of the Portneuf quadrangle, 

where St. John 41 many years ago recognized evidences of faulting. 

The western edge of the thrust plane or the line of origin of the fault 

has not been found. Indeed, it is doubtful if even detailed work 

will bring it to light unless through some fortunate combination of 

circumstances. Doubtless it lies some distance west of the Portneuf 

quadrangle.' Possibly the hot springs (Lava Hot Springs) and 

travertine terraces in Portneuf Canj^on, 8 miles or more south of the 

southwest corner of the Portneuf quadrangle, may have some bearing 

on the question.

The Bannock overthrust passes beneath cover of Tertiary sediments 

and lavas in T. 1 S., R. 38 E., in the Ammon quadrangle. A fault 

of similar habit is reported by Kirkham 42 on the Continental Divide 

near Medicine Lodge Creek. This lies in the strike of the Bannock 

overthrust and is intermediate in position between that fault and the 

Philipsburg faults previously described by Calkins,43 with which 

it is also in line. Possible connections of these faults with. one 

another and with other great overthrusts farther north are thus 

suggested.

The Bannock overthrust is introduced into the structure sections 

A-B to H-I, Plate 2, without attempt to indicate its position otherwise than to mark its rise to the surface in the Twentyfourmile

anticline. Some undulation in the fault plane is to be expected in 

view of its known flexures in the region farther east. Its relation 

to the folds of the upper block is purely conjectural. No information regarding the nature of the underlying block is available except 

the occurrence of the chert ledges above mentioned and of the 

Triassic and Jurassic beds that emerge from beneath the upper block 

in Tps. 3 and 4 S., R. 40 E., in the Paradise Valley quadrangle. The 

occurrence of these rocks suggests that the underlying block is composed of Paleozoic and Mesozoic rocks much like those of the upper 

block. Their constitution and structure at any given place, however, 

are indeterminable except by drilling.

The effect of the Bannock overthrust upon the occurrence and 

accessibility of phosphate beds is with little doubt to cut off the

41 St. John, Orestes, U. S. Geol. and Geog. Survey Terr. Eleventh Ann. Rept., pp. 340, 

341, 356, 1879.

42 Kirkham, V. R. D., personal communication.

48 Bmmons, W. H., and Calkins, P. C., Geology and ore deposits of the Philipsburg quadrangle, Mont.: U. S. Geol. Survey Prof. Paper 78, p. 146, 1913.
STRUCTURE 59

bottoms of the deeper phosphate-bearing synclines and to render 

generally indeterminate the position of possible phosphate beds in 

.the lower block.

Other faults. Most of the faults mapped in the Chesterfield Range 

have already been described in connection with the description of

 the folds with which they are associated. A rather persistent fault

 of small throw, presumably a minor thrust, extends southeastward 

from sec. 1, T. 6 S., R. 38 E., and disappears beneath cover in sec. 

17, T. 6 S., R. 39 E. It follows the phosphatic shale of the Phosphoria formation, which it cuts out completely at some places. It 

as offset by a normal fault in the SW. ^ sec. 6, T. 6 S., R. 39 E., and 

by two small transverse faults in sec. 7. A similar and possibly 

.related fault cuts out the phosphate beds in the SE. % sec. 25 of the 

isame township.

The complex of faults in sec. 6, T. 6 S., R. 39 E., and sec. 31, T.

 5 S., R. 39 E., has been mentioned briefly in the discussion of the 

Hock Creek syncline. They probably include both normal and reverse types. Their relations are not fully understood but are thought 

to be correctly represented within the limitations imposed by time 

.and scale.

The Little Gray fault of the Henry quadrangle, one of the transverse normal faults that there produces marked geologic and topographic effects, may enter the Portneuf quadrangle. If so, it either

 dies out or is cut off by the fault that separates the Nigger Creek 

.syncline from the Corral Creek anticline. It is represented on the 

map in sec. 15, T. 5 S., R. 40 E., merely as a suggested extension. 

'The Pelican fault of the Henry quadrangle has not been identified 

in the Portneuf quadrangle, but there are some suggestions that it 

may be present. (See p. 87.)

Reference has been made (p. 46) to the mapping of a fault by 

;Schultz and Richards along the west base of the Chesterfield Range,

 which they thought might be related to the Bannock overthrust. The 

possible extension o»f the Bannock overthrust in this region has 

.already been discussed. There remains the question of possible 

normal faulting along the west base of this range.

At two places the western slope of the range is marked by a steep 

.rocky ridge composed of Paleozoic rocks, which represent the east 

.limb of an anticline whose west limb, if present, must lie beneath 

.the later cover of the floor of Portneuf Valley. The general abruptness of the west slope in each of these ridges is noteworthy and in 

itself suggestive of faulting, but no fault breccia and no slickensides 

.have been observed in confirmation of the suggestion. On the other 

"hand, there are two very striking features in the southern ridge that 

give support to the idea of faulting. These are the travertine ter6148 29 5
60 POBTNEUF QUADRANGLE, IDAHO

races that form such conspicuous ledges at the mouths of Little Flat 

and Eighteenmile Canyons, in T. 7 S., Rs. 39 and 40 E. (See pi. 

3, B.) Although travertine terraces are not a feature of the landscape in connection with the northern ridge, there is a considerable 

mass of this material more or less concealed by hill wash near the 

base of this ridge and exposed for nearly 3 miles northwestward from 

the dam on the northeast shore of Portneuf Reservoir.

Another line of evidence is suggestive in this connection. The 

lowest beds exposed just north of Monroe Canyon, in T. 7 S., R. 

40 E., belong to the Laketown dolomite, and the series of formations 

descends stratigraphically westward. Structurally the beds are rising westward toward an anticlinal axis, and the next formations in 

order should be of Ordovician and Cambrian age. These beds, 

together with the overlying Devonian and Carboniferous formations, 

are normally resistant to erosion and form generally rugged topography. It is therefore strange that the west flank of the ridge should 

cease so abruptly and that these resistant formations should be low 

enough to be concealed by Quaternary sediments and basalt unless 

they had been dropped down by faulting. On the whole the idea 

of a normal fault with downthrow to the west along the west base of 

the Chesterfield Range is believed to be justified, although direct 

evidence of its presence is lacking. It is therefore tentatively represented on the map.

With southwestward-dipping normal faults both to the northeast 

and to the southwest the Chesterfield Range would seem to be a 

horst that is, an eroded fault block composed of folded and faulted 

older strata, standing relatively higher than neighboring blocks on

both sides.

Reference to faults in the foothills of the Portneuf Mountains west 

of Portneuf Valley has already been made in the discussion of the 

Cambrian and Ordovician formations. The most noteworthy of 

these faults is the one followed by King Creek, which causes the 

Cambrian beds on the north side of the creek to be offset about half 

a mile eastward with respect to those on the south side. The fault 

is presumed to be normal with the downthrow on the south side. 

The dips of the beds on opposite sides of the fault differ, those on 

the south side being somewhat steeper, though all are easterly. 

Upon the assumption that the fault plane is vertical and that the 

dip of the beds averages 30°, the vertical displacement would approximate 1,500 feet.

Another fault of earlier date affected by the above-mentioned displacement is the one described on page 18 as obliquely cutting out the 

limestone of the St. Charles formation south of King Creek. This 

fault is believed to be the same as the one that brings Fish Haven
STRUCTURE 61

dolomite into contact with undifferentiated Cambrian rocks in the 

NW. !/! sec. 14, T. 7 S., R. 38 E. The fault is regarded as normal, with 

downthrow to the east. The vertical displacement is not determinable from present data but is probably less than 1,500 feet.

In sec. 35, T. 7 S., R. 38 E., and sec. 2^ T. 8 S., R. 38 E., there is 

a broad depression underlain by Swan Peak quartzite and bounded 

on the east side by an abrupt ridge composed of eastward-dipping 

Swan Peak quartzite. The physiographic relations suggest faulting. It is thought that the fault mentioned in the preceding paragraph may extend southward to this locality. This interpretation 

is therefore tentatively represented on the map.

In the NE. % sec. 8, T. 8 S., R. 39 E., there is an Isolated knoll 

of eastward-dipping Swan Peak quartzite. Half a mile or more

farther west is a group of ledges and knolls composed of eastwarddipping Fish Haven dolomite with Swan Peak quartzite at the 

west. The strike of the first-mentioned knoll of quartzite is at 

variance with the strike of the rocks in the knolls fartffer west. The 

knolls and ledges are isolated from one another by basalt, so that 

the relation of the easternmost knoll to the others is concealed. 

Folding, either open or close, may account for the arrangement 

seen. If the folding were close the dips would indicate westward 

inclination of the folds involved, which is unusual in the region. If 

the folding were open one would expect to find some Fish Haven 

dolomite accompanying the easternmost quartzite knoll. The dolomite may indeed be present beneath the basalt. On the other hand, 

the eastward repetition of the Swan Peak quartzite may be explained 

by normal faulting with downthrow to the west. This is thought 

to be the more probable explanation, and it is therefore the one 

selected for representation on the map.

AGE OF THE FOLDING

The folds of the Portneuf quadrangle involve rocks ranging in 

age from Middle Cambrian to probably Middle or Upper Triassic. 

Elsewhere in the general region the highest rocks folded to the degree 

indicated by the geologic structure sections belong to the Wayan 

formation, of Lower (?) Cretaceous age. The mountain building that 

produced these folds was therefore later than the deposition of these 

rocks. It is believed to have been a part of the great mountainbuilding disturbance which came in the interval between the end of 

the Cretaceous period and the early part of the Tertiary, generally 

known as the Laramide revolution.

A subsequent epoch of less intensive disturbance is suggested by 

the arching of the plane of the Bannock overthrust and by the dips 

now observed in the Salt Lake formation and some of the associated
62 PORTNEUF QUADRANGLE, IDAHO

lavas. These dips are commonly as much as 10° and locally as 

great as 29°. On the present supposition that the Salt Lake formation is of Pliocene (?) age this second disturbance is presumably to 

be assigned to the end of the Pliocene.

Some evidence of additional earlier epochs of mountain building ( 

is furnished by rocks exposed in quadrangles farther east, but the 

effects of these earlier disturbances were largely obliterated by the 

more intensive activity of the Laramide revolution.

AGE OF THE FAULTING

The Bannock overthrust is also ascribed to the Laramide epoch 

and probably belongs to the later part of that epoch, because it did 

not occur until the rocks in what is now the upper block had been 

intensely folded indeed, folded practically as much as those of the 

lower block. This is shown in the Montpelier quadrangle, where 

the folds of both blocks may be observed. The folding of the fault 

plane, however, is much more open, more in accord with that of the 

late Pliocene disturbance. It is therefore believed that the folding 

of the fault plane was much later than the production of the thrust 

fault itself.

The other thrust faults associated with the folding are believed to 

have been caused by the Laramide disturbance.

The normal faults differ among themselves in age, some being cut 

by others. In general they are younger than the Laramide disturbance and hence fall chiefly within the Tertiary period. They ; 

were probably produced during the epoch of block faulting that 

gave rise to much of the so-called "basin and range" structure, of 

which the Chesterfield Range is here considered an outlying representative. This structure is believed to have been developed in 

about middle Miocene time.

In addition to these faults there are some in the general region 

associated with igneous rocks and Tertiary strata that occurred 

subsequent to the deposition of the Salt Lake formation and hence 

presumably in early Pleistocene time. To this group probably belongs the fault in the NW. % sec. 6, T. 6 S., R. 39 E., by which 

the Rex chert on the north is separated from the Salt Lake formation 

on the south. The Salt Lake formation from exposures in the 

SW. Vi sec. 6 is known to overlie the Rex, and its preservation 

south of the fault line mentioned is believed to be due to downfaulting. As this fault is itself offset by the north-south fault that 

breaks across the structure in the west half of the section it also may 

be assigned to the post-Tertiary group. Faults of probably Recent [ 

age have been identified in the Montpelier quadrangle. The movements that produced the Pleistocene and Recent faults may have
GEOLOGIC HISTOEY 63

refreshed earlier faults and thus may have given rise to some of 

the travertine deposits which are conspicuous here and there in the 

Portneuf quadrangle.

RELATION OF STRUCTURE TO PHOSPHATE IJEDS

The folds and faults of the Portneuf quadrangle may be regarded 

as having been an influential factor in the preservation of the phosphate beds. These beds, which were originally nearly horizontal, 

might with little doubt have been eroded and to a greater or less 

extent destroyed in the normal course of weathering and removal of 

rock debris by the attack of streams. By means of these structural 

features, however, considerable bodies of phosphate rock have been 

depressed below the level of erosion and thus preserved for an indefinite period. The presence of the Bannock overthrust, on the

other hand, has diminished the amount of this phosphate that might 

otherwise have been ultimately available.

GEOLOGIC HISTORY

Swnmary of record. The latest page of the geologic history of 

the Portneuf quadrangle, in so far as the record may be deciphered 

from the evidence at hand, is the geography of the region as it 

appears to-day. This has been discussed in a preceding section of 

this report. The business of historical geology is to reconstruct and 

interpret in orderly sequence the geography of past epochs. With 

even so great a body of sedimentary rocks as is found in southeastern 

Idaho the record is very imperfect, for gaps represent the loss of 

many pages and even of whole chapters and volumes of the story. 

The total thickness of the strata of southeastern Idaho is about 46,000 

feet, or more than 8.7 miles. For the Portneuf quadrangle the 

record is less complete, all sediments of the Cretaceous and Jurassic 

periods and some of the later Triassic and earlier Tertiary beds 

being absent.

Proterozoic era. No rocks of pre-Cambrian age are exposed in 

southeastern Idaho, so that inferences regarding the Proterozoic 

era must be drawn from the evidence furnished by neighboring 

regions. This evidence suggests that southeastern Idaho, including 

the Portneuf quadrangle, was land throughout much of that time, 

but that there was continental deposition, deformation, erosion, and 

even perhaps glaciation, as in later periods, and that the great 

continental elements of higher and lower land were already blocked 

out before the Paleozoic era began.

Paileozoic era. The Paleozoic record is dominantly marine and 

thus indicates progressive subsidence of the region. The variations 

in character of the sediments, however, and the numerous strati-
64 PORTNEUF QUADRANGLE, IDAHO

graphic breaks show that there were at times interruptions in subsidence and even reversals of movement, with erosion. These disturbances were gentle, but were fairly well distributed throughout 

the era. A noteworthy feature is the general dominance of limestone-making processes.

Early in the Cambrian the relatively low and long-eroded surface 

of southeastern Idaho began to subside and receive sandy sediments, 

at first probably nonmarine but later marine. The invasion of 

the sea may not have begun until the later part of Lower Cambrian 

time or the early part of Middle Cambrian time. The sediments 

of the Middle Cambrian were chiefly limy, but at the beginning 

of the Upper Cambrian the sea became more restricted and sands 

were for a time again the predominant sediments. This condition 

was succeeded by the renewal of limestone deposition and even 

more extended invasion of the sea the first great transcontinental 

marine inundation of Paleozoic time.

A brief erosion interval with little change in the attitude of the 

land separates the Upper Cambrian from the Ordovician, when 

limestone making again took place. A reversal of subsidence in the 

later part of Lower Ordovician time brought first a change in 

sedimentation, the deposition of the Swan Peak quartzite, and then 

an erosion interval that lasted throughout much of Ordovician 

time. Toward the end of the Ordovician period subsidence with 

deposition of limestone was resumed.

The Silurian period in southeastern Idaho is represented by a 

single formation, the Laketown dolomite, reported by Kirk to carry 

Niagaran fossils. In spite of its apparent conformity with the underlying Ordovician Fish Haven dolomite a stratigraphic break of 

some magnitude intervenes. The Silurian sea was widespread in the 

west, but toward the end of the period it withdrew from the region 

now occupied by the Rocky Mountains, and that entire region became 

land.

These conditions continued until Middle Devonian time, when 

the region was again occupied by the sea, and limestone making was 

resumed without any marked discordance in attitude of the later 

with the earlier beds. From Middle Devonian through Upper Devonian time the region of deposition was extensively flooded. Gentle 

earth movements during the period are indicated by changes in the 

composition and appearance of the beds.

Mild disturbances of the land surface with ensuing erosion are 

believed to have intervened between the deposition of the Devonian 

and that of the Carboniferous formations of southeastern Idaho, 

though the rocks of the two systems lie with little discordance of 

attitude. There is evidence also of interruption in deposition between 

the two formations of the Mississippian series, though here again there
GEOLOGIC HISTORY 65

is no conspicuous discordance of the strata. Limestone forms the 

main body of the rocks of this series, but shale and sandstone are also 

present, indicating variations in conditions of deposition. Another 

crustal disturbance marked the interval between upper Mississippian 

and Pennsylvanian deposition. The beds of the Pennsylvanian epoch 

were more sandy than those of the Mississippian epoch, and in all 

three epochs of the Carboniferous period subsidence of the sea bottom 

was intermittent or there were minor oscillatory movements. An unconformity separates the Permian from the Pennsylvanian beds. 

The region receiving deposits was cut off from the sea on the east, 

south, and west (in part) but had northward and westward connections with the Pacific. Under special conditions of deposition not 

yet well understood beds of phosphate rock that promise to be of great 

economic importance were formed, and the overlying strata of the 

same period include beds of massive chert and flinty shale. The 

history of the Permian in this region thus constitutes a series of interesting stratigraphic problems.

Mesozoic era. The change from the Paleozoic to the Mesozoic in 

southeastern Idaho is marked by strong faunal contrasts rather than 

by any noteworthy discordance of strata. The Mesozoic is distinguished bj alternations on a great scale between marine and nonmarine conditions of deposition. The later part of the era witnessed 

the accumulation of enormous thicknesses of continental sediments.

Though the North American continent as a whole was a relatively 

broad land area in the Triassic period, southeastern Idaho was 

for a time occupied by an arm of the sea connecting with the Pacific 

at the southwest. In Lower Triassic time open-sea conditions prevailed in southeastern Idaho, but farther east there were shallow 

marginal seas that dried up at certain seasons and lagoons or 

estuaries that gave rise to red beds. The shore line of the sea 

advanced or retreated from time to time, and these movements 

caused some interfingering of marine and red-bed sediments. At 

least once during the epoch the sea withdrew sufficiently to permit 

erosion. After the deposition of the Lower Triassic formations 

there was widespread erosion, followed by continental deposition, 

chiefly of the desert type. Of the material then laid down only a 

small area of Higham grit, in sees. 7 and 18, T. 5 S., R. 40 E., now 

remains in the Portneuf quadrangle, and no further deposition is 

recorded there in the exposed formations until the later part of the 

Tertiary period.

The Jurassic and Cretaceous periods were represented in neighboring areas by thick and extensive formations of both marine and 

aonmarine types, and there seems every probability that these formations were once represented in the Portneuf quadrangle but that 

they have been removed by erosion. It is possible that such beds may
66 POKTNEUF QUADRANGLE, IDAHO

be concealed in the lower block overridden by the Bannock overthrust, but no direct evidence of them is available. In the Paradise 

Valley quadrangle they are found emerging from beneath the upperblock.

In Jurassic time marine invasions twice alternated with desert 

continental conditions. The end of the Jurassic was marked by soabrupt a change in the character of the sediments as to suggest 

mountain building in neighboring areas. The Cretaceous left no 

record in the immediate region. At its end came the great Laramide 

revolution, during which the complicated structural features of the 

region were produced. The great depth of the accumulated sediments and their position are thought to have had a directive influence 

upon the mountain-building forces.

The interval between the Cretaceous and Tertiary deposition in 

southeastern Idaho was occupied by erosion, but in neighboring 

parts of southwestern Wyoming the coal-bearing Evanston formation, 

was laid down in early Tertiary or late Cretaceous time. The nature 

of this formation suggests that the reduction of the adjacent 

mountains of Idaho was well advanced by the beginning of the 

Wasatch Eocene.

Cenozoic era. The Cenozoic in southeastern Idaho was largely an 

era of erosion interrupted from time to time by crustal. disturbances 

and by the deposition of continental sediments. No beds of" 

Fort Union age are present and no other Eocene beds are exposed in the Portneuf quadrangle, though beds assigned to theWasatch formation occur in some of the neighboring quadrangles.

The Miocene was an epoch of erosion. Although it was a time of 

great volcanic activity in many parts of the West, no such activity 

is recorded in southeastern Idaho. Crustal disturbances in the middle part of the Miocene are thought to have uplifted the old, worndown surface, remnants of which in neighboring quadrangles have1 

been called the Snowdrift peneplain, and to have permitted in the 

later part of the epoch the development of broad and deep vallejrs. 

To these disturbances also are attributed the development of the 

"basin and range" structure of which the Chesterfield Kange is an 

outlying representative.

The erosion of the Miocene may have continued into Pliocene time,, 

but it eventually gave way to the conditions that permitted the Salt 

Lake formation to accumulate until it blanketed many valleys and 

some of the lower hills. Some climatic fluctuations during the 

period are suggested by the nature of the Salt Lake formation. There 

was also volcanic activity on a considerable scale. Further uplift 

came toward the end of the Pliocene. According to recent unpublished work by Alden the final activities of the Pliocene were probably
MINERAL RESOURCES 67

 erosionaH and included the development of the Gannett erosion 

surface.

The Quaternary history of the region is largely one of erosion interrupted by uplift and modified by climatic changes and volcanic outbursts, which have caused the silting up of valleys. The Elk Valley, 

Dry Fork, and Blackfoot erosion cycles have produced the present 

topographic features. Late faulting has affected the basalt and even 

.the slopes of hill wash in some parts of the general region and has 

revived some of the earlier faults. Recent erosion has accomplished 

little change since the Blackfoot cycle.

MINERAL RESOURCES

No metalliferous deposits are known in the Portneuf quadrangle.

and none of commercial value are to be expected, for the rocks exposed 

are not of the type that is generally favorable for the development

 of such deposits. It is true that in the foothills of the Portneuf 

Mountains, especially in the southwestern part of the quadrangle, the 

.sedimentary rocks are of types similar to those in the Bear River 

Range, west, of Bear Lake, in which small pockets of lead and copper

 ore have been found. These deposits, however, though extensively 

prospected, have not proved to be commercially valuable.

There are a number of nonmetallic resources that are potentially 

valuable though not of immediate commercial importance. These 

.are phosphate rock, limestone, basalt, silica (quartzite), and water.

PHOSPHATE BOCK 

GENERAL CHARACTER

An extended description of the western phosphate rock, dealing 

with its texture, appearance, chemical composition, mode of occurrence, origin, and exploitation, has been given in Professional Paper 

152, and in the present paper only a brief summary is necessary. 

Phosphate rock in southeastern Idaho occurs at two horizons, upper 

Mississippian and Permian, but the Permian beds are the only ones 

of much commercial importance and the only ones that have been 

recognized in the Portneuf quadrangle. This rock is characterized

 chiefly by its oolitic texture (composed of small rounded grains like 

fish roe) and usual dark color and by its odor (resembling that of 

'Crude petroleum) when freshly broken. This odor is not distinctive

 of phosphate, for many limestones when freshly broken have a 

similar odor. Weathered material is usually distinguished by a 

bluish-white coating or bloom that in some specimens has a netlike 

pattern. The phosphate rock is a bedded deposit of marine origin 

;and must be mined in the same manner as coal.
68 PORTNEUF QUADRANGLE, IDAHO

Phosphate rock is really a mixture or " solid solution " of several 

phosphatic minerals, but its chemical composition is approximately 

that of tricalcium phosphate, Ca 3 (P04 ) 2 - Material of this general 

composition is known under the trade name of " bone phosphate of 

lime " (abbreviated B. P. L.). There are many accessory constituents, 

among which vanadium is noteworthy. Some tendency toward 

enrichment by weathering has been observed. From tests made some 

years ago in the Geological Survey laboratory the average density of 

the I'ock is approximately 2.90, and the weight of a cubic foot of the 

more massive rock, such as is now being shipped from the mines in 

this region, is about 180 pounds.

IRON AND ALUMINA CONTENT

Iron and alumina in phosphate rock in excess of 3 to 4.5 per cent 

in the eastern, fields are considered as placing the ore below foreign 

contract standards. Besides being impurities that increase the quantity of acid necessary for treatment, these substances are supposed 

to produce, in the present process of superphosphate manufacture, 

deliquescent salts, which render the drying and shipment of the 

product difficult. Samples from the western fields that have been 

tested in the laboratory of the Geological Survey have shown generally less than 1 per cent of either radicle computed in the oxide 

form. Of the samples thus far analyzed from the Portneuf quadrangle .one composite has yielded 4.08 per cent for both taken 

together. (See p. 84.)

VANADIUM CONTENT

No systematic search for vanadium in western phosphate rock 

has been made, but Geological Survey chemists have obtained this 

metal in some analyses, the discovery dating from 1911, when it was 

found in material from the vicinity of Driggs, Idaho. Samples 

of phosphate from Montpelier Canyon contained as much as 0.52 

per cent of V2 05 . Three samples from sec. 25, T. 6 S., E. 39 E., 

in the Portneuf quadrangle, contained 0.17, 0.4, and 0.5 per cent of 

V2 05 . Although these quantities seem small in comparison with 

the content of phosphoric acid usually present in the rock, the 

vanadium can be precipitated from solutions derived from phosphate rock in making superphosphates, so that the western phosphate 

rock may be classed as a possible commercial source of vanadium.

DISTRIBUTION OF PHOSPHATE IN PORTNEUF QUADRANGLE

The rocks of the Portneuf Mountains west of Portneuf Valley, 

within the Portneuf quadrangle, are all older than the Permian. 

The rocks beneath the basalt and alluvium of Portneuf Valley are
MINERAL RESOURCES 69

also probably pre-Permian, to judge by the exposures along the 

sides of the valley, but on the supposition of faulting along the east 

side of the valley, discussed on page 59, it is possible that rocks of 

Permian age or even younger may be present in the valley. The 

expense of exploration beneath existing cover practically precludes 

any search for them there, and their discovery must await the drilling for some other purpose of a well deep enough to penetrate the 

underlying strata at ,a favorable locality. The Chesterfield Range, 

though extensively covered by Tertiary beds, is known to contain 

folds favorable for the preservation of phosphate beds, and phosphate rock is actually exposed in four bands of differing extent 

in connection with these folds. Phosphate rock is also present at 

accessible depths in the Blackfoot Mountains, in the northeastern 

part of the quadrangle. The details of distribution, together with 

data on the character and quantity of phosphate present, are given 

in the descriptions of the several townships that follow.

DESCRIPTION OF TOWNSHIPS THAT CONTAIN PHOSPHATE 

T. 5 S., E. 38 E.

A detailed description of T. 5 S., R. 38 E., is given in the writer's 

account of the Fort Hall Indian Reservation 44 and need not be 

repeated here. The southeastern part of the township is included 

in the Portneuf quadrangle. Much of the area so included is covered 

with Tertiary and Quaternary sediments, but phosphate beds are 

exposed in sec. 36 and vicinity.

Field work in 1923. The field work of 1923 necessitated a few 

changes in the earlier mapping, but these do not seriously affect the 

published interpretations. Much of the area here mapped as Salt 

Lake formation was mapped in the earlier report as Tertiary or 

Quaternary. (See p. 51.) A slight change in the mapping of the 

phosphate beds in the SE. % sec. 36 has also been made necessary 

by work in the adjacent townships. The phosphate-bearing syncline is not closed off at the south, as tentatively mapped in the 

previous report, but continues southeastward into T. 6 S., R. 39 E. 

There is also some hitherto unsuspected minor folding, involving 

phosphate beds, on the south side of Rock Creek in the southeast 

corner of the township. The principal phosphate-bearing fold in 

the township is the Rock Creek syncline, described on page 53. It 

is probably to be regarded as a synclinorium, rather than a simple 

syncline, so that its structure is somewhat more complex than was 

formerly supposed.

44 U. S. Geol. Survey Bull. 7.13.. pp. 39, 40, 99-103, 1920.
70 PORTNEUF QUADRANGLE, IDAHO

Results of previous prospecting. In 1913 the Geological Survey 

party prospected the phosphate in sec. 36 of this township by means 

of a trench and six test pits. The trench, which began with the lower 

limestone bed of the Phosphoria formation, was 68 feet long, from 

2^ to 4 feet deep, 2% feet wide, and at right angles to the strike 

of the beds. The strike of the lower limestone bed was N. 89° W., 

and the dip 20° N. The pits, which were dug in the line of continuation of the strike of the trench across the phosphatic shale and ranged 

in depth from 2^2 to 4i£ feet, were spaced at the following intervals: 

No. 1, 40 feet north of the end of the trench; No. 2, 15 feet north of 

No. 1; No. 3, 25 feet north of No. 2; No. 4, 45 feet north of No. 3; 

No. 5, 20 feet north of No. 4; No. 6, 135 feet north of No. 5. The 

hill slope above was all strewn with float of Rex chert. The results 

follow:

Partial section of phosphate shales in NE. % SE. ty sec. 36, T. 5 S., R. 

compiled from trench and six test pits

E.,

Character of beds

Soil and other material. 

Shale, broken pieces, black to brown, somewhat phosphatic; contains

Shale, black. ____ . _ . ______________ . __________ .

Limestone, black, fetid, fossiliferous; "cap lime". _ ..--... _ ........ ...

Pa05

Per cent

31.40

( 33. 02 

\ 35. 46

33. 14

Equivalent to 

Ca3(P04)2

Per cent

68.7

72.1 

77.4

74.1

Thickness

Ft. in. 

2±

48

1

1

7

18±

1

7

16

3

3

2

1

10

1 9

6

10

1

15

2 11

2 1

2 1 

2

2 

6

9

3

146 5

The phosphate beds near the northwest corner of sec. 18, in sec. 

13 of the adjoining township, were also sampled by the Geological 

Survey party in 1913. The prospecting trench was about 30 feet 

long and from 3^ to 5^ feet deep, and its strike was N. 19° E. The 

strike of the phosphate beds was N. 46° W., and the dip 82° SW. 

The results of this work are also given for comparison.
MINERAL KESOUECES 71

Partial section of phosphate shale about 20 feet west of section corner of NE. 

NE. % sec. 13, T. 5 8., R. 37 E.

Character of beds P2 05

Equivalent to 

Ca8 (P04)j

Thickness

Shale, brown to light, sandy, broken, top not exposed.

Phosphate rock, black, fine grained, hard, cherty (?).....................

Broken zone, shale and phosphate rock, brown to black.................

Phosphate rock, dark brown, finely oolitic, somewhat shaly, much broken,

with interbedded shale.-...___._________________... 

Shale, phosphatic, black to brown, with a few narrow bands of phosphate,

much broken_....._...__..__........_______....

Broken zone, mingled phosphate rock and shale, brown to yellowish._. 

Phosphate rock, fine to coarse oolitic texture, beds up to 3 inches thick,

dark gray to black, weathering yellowish, and in broken blocks coated

white, including 

Inches 

Samples......................................................... 22

Sample4......................................................... 22

Samples.......................................................... 22

Phosphate rock, medium oolitic, brownish gray, with a few yellow sandy

streaks; beds have a maximum thickness of 1M inches, grading into

thin shaly phosphate (sample 2).___.i___........__... ....

Shale, yellowish brown, finely banded, iron stained, much broken and

containing scattered phosphate nodules and streaks__________. 

Chert, with phosphatic nodules and fragments of discinoids, much broken. 

Shale, yellowish brown, sandy, with irregular phosphate streaks . 

Phosphate rock, medium to fine, oolitic; contains yellow sandy streaks

(sample 1)...._.____._.____________________. 

Sandy beds, yellowish brown, scattered phosphate nodules, banded chert,

black, broken; base not seen....__....._____________......

Per cent Per cent

34.65

34.59

32.64

75.71

75.57

71.31

52.31

22.97 50.18

Ft. in. 

1 3

1 8

2 8

4 9

1 3

5 6

2 6

10

3

1 5

26 9

Phosphate deposits. The summary for this township given in the 

earlier report cited is applicable here and with minor corrections is 

repeated for reference.

A thick bed of high-grade phosphate rock has been found and sampled in 

sees. 18 and 36. The outcrop of the phosphate shales in sees. 17 and 18 is 

alined well with that in sec. 36. [Later work suggests that both are parts of 

the Rock Creek syncline.] Postphosphate rocks lie on the northeast side 

of this line. These facts suggest that much of the northeastern part of the 

township may be underlain by valuable phosphate rock within workable limits. 

However, the extensive cover and the complex structure of the exposed parts 

of the older sedimentary rocks make it unwise to estimate the phosphate 

resources of the covered area except of those parts relatively near the exposed 

areas. It is estimated that about 8,120 acres! are underlain by available phosphate rock. The complexly faulted small areas of Rex chert in sees. 28, 29, 32, 

and 33 are not included in this estimate.

About 250 acres included in the above figures lies east of the 

reservation boundary in sees. 25 and 36.

In the prospect made by the survey party just oiver the township line from 

the northwest corner of sec. 18 the phosphate bed was found to have a thickness of 66 inches and an average content of 33.96 per cent phosphorus pentoxide 

(PzOs) equivalent to 74.39 per cent tricalcium phosphate (Ca3 (P04)2). In the 

NE. 3/4 SE. i/4 sec. 36 in this township the prospect made by the survey party 

showed the phosphate bed ta.be 73 inches thick with an average content of 

34.37 per cent phosphorus pentoxide, equivalent to 74.5 per cent tricalciumj 

phosphate. The weight of such rock from previous determination is about 180 

pounds per cubic foot. On the assumption that the lower thickness is the 

average for the area, it is estimated that this township contains as a minimam
72 PORTNEUF QUADRANGLE, IDAHO

148,008,900 long tons of high-grade phosphate rock, at a depth of 5,000 feet or 

less. The estimate assumes that the rocks are horizontal. The rocks are, 

however, inclined at different angles, so that the estimate should fall somewhat 

below the actual content of the phosphate beds. In view of the complex structure of the township it seems wise to leave this difference as an additional 

margin of safety for the estimate given.

The small areas of phosphate in the SE. % SE. 14 sec. 36 were not 

included in the above estimate and would tend slightly to increase 

it. On the other hand, it now seems reasonably certain that the 

Bannock overthrust or a closely related thrust fault underlies this 

township. Its presence may adversely affect the phosphate content 

of the Eock Creek syncline, thus tending to reduce the estimate. As 

from existing data there is no other than an arbitrary basis for revising the estimate, it is believed better to let the figures stand as 

given. The areas in this township officially classified as phosphate 

land are shown in Figure 3.

T. 6 S., R. 38 E.

The two northern tiers of sections in T. 6 S., Bf 38 E., were described in the Fort Hall report, previously cited. The Portneuf 

quadrangle includes most of the eastern half of the township. The 

only exposures of older rock within the area here mapped are the 

Cambrian beds north and south of the mouth of Topons Canyon and 

the Carboniferous beds in the northeastern part. The rest of the area 

is occupied by Tertiary and Quaternary sediments and by Portneuf 

Reservoir. West of Portneuf Kiver and Portneuf Reservoir the preTertiary rocks are of Cambrian and Ordovician age. Between these 

and the Carboniferous rocks on the east there is believed to be a 

normal fault. (See pp. 59 and 60.)

Phosphate deposits. The phosphate beds in sec. 1 were not recognized in the more cursory mapping of 1913 but were followed out 

in some detail in 1923. The area there underlain by phosphate is 

estimated at approximately 80 acres. No prospecting of the phosphate beds in this township was attempted, but assuming the thickness of the main phosphate bed to be 6 feet, as indicated in the 

prospect in sec. 36, T. 5 S., R. 38 E. (the adjoining township), and 

estimating in the same way as before, we find that the 80 acres 

would contain about 1,845,000 long tons of high-grade phosphate 

rock. The computation is made on the assumption that the beds 

are horizontal, whereas they are inclined, so that the actual quantity 

present might be greater than the figure given. On the other hand, 

the phosphate-bearing area (see fig. 3) is in part concealed by Tertiary beds and is traversed by at least two faults, which may cut out
MINERAL RESOURCES 73

part of the phosphate. The figure given is, therefore, conservative 

and reasonable. None of this phosphate is deeper than a few hundred 

feet.

T. 5 8., R. 39 E.

More than half of T. 5 S., R. 39 E., including a strip iy2 miles 

wide in the Paradise Valley quadrangle, is occupied by Tertiary 

and Quaternary sediments with associated igneous rocks, which com-
74 ' POETNEUF QUADRANGLE, IDAHO

pletely conceal the pre-Tertiary formations. These earlier rocks 

crop out southeast of an irregular line that extends northeastward! 

from the NW. 1/4 sec. 30 to the NE. Vi sec. 24.

The township lies mostly in the interior of the Chesterfield Range,, 

though it extends down to Blackf oot Valley at the north and northeast. It is entered in sees. 1, 5, and 6 by the road from Blackfoot 

Dam to Ross Fork, one of the few thoroughfares of the region and 

locally a good automobile road. There are two other roads, both 

of which are very poor and in places practically impassable for automobiles. One of these connects the town of Chesterfield with theAustin ranch and Ross Fork road by way of Twentyfourmile and 

Warbonnet Creeks, and the other connects both Chesterfield and 

Corral Creek with Blackfoot Valley by way of Grizzly, and lower 

Graves Creeks.

/Structural details. The township is crossed by or contains parts 

of all the major folds of the Chesterfield Range, though such parts 

of the Nigger Creek syncline as may be present are concealed by 

Tertiary and Quaternary rocks. The general structural relations 

are shown by the geologic structure section A-B, Plate 2.

On the basis of the available exposures the lowest beds of theGrizzly Creek anticline brought to the erosion surface, upon which 

the Tertiary and Quaternary beds lie, are identified as the Ross 

Fork limestone. The space available for the folded Ross Fork beds 

is apparently too narrow to accommodate them without some break. 

It is therefore believed that the fault recognized on Grizzly Creek 

in T. 5 S., R. 40 E., continues into T. 5 S., R. 39 E., but with diminished throw.

The highest beds in the Graves Creek syncline are Portneuf limestone. The phosphate beds, if present in normal sequence, would 

therefore lie very deep in the trough portions of this fold. The lower 

part of this syncline' has doubtless been cut off by the Bannock overthrust and phosphate is probably absent from the area occupied by it, 

except along its west flank, where it merges with the adjacent 

anticline.

Beds of Brazer limestone occupy the core or central area of theTwentyfourmile anticline. The Bannock overthrust probably cuts 

out much of this fold, and to judge from exposures farther south its 

fault plane is thought to approach relatively near the surface alongthe crest of the fold. Exposures of cherty breccia along the samegeneral line suggest the cracking through or faulting of the upper 

block, as indicated in the map and section. Phosphate beds areexposed along both limbs of the anticline.

The Rock Creek syncline is shallow and affects only the southwest 

corner of the township, sees. 30 and 31. The syncline is actually a.
MINERAL RESOURCES 75

synclinorium consisting of two roughly parallel shallow. synclines 

separated by a low anticline, broken and modified by minor folds. 

The transverse anticline shown on the map is apparently a local upwarping of the synclinorium, which makes it narrower by bringing 

the phosphate beds on opposite sides closer together. The cross fold 

dies out rapidly eastward, for the boundary between the Wells and 

Brazer formations is only slightly if at all affected by it. The east 

side of the synclinorium is broken by two faults, which cause the 

beds on the west to be lowered with respect to those on the east and 

are therefore presumed to be normal. The older of the two where 

i,t crosses the canyon has a westerly dip of about 30° and is more 

gently inclined than the younger fault, against which it terminates 

at the north.

Phosphate deposits. The Phosphoria formation, dipping about 

50° W., enters the SW. % sec. 35 and extends northwestward .to the 

SW. y± sec. 21, where it passes beneath Tertiary cover, thus forming 

a belt roughly 3% miles long. On the basis of the postulated structure, as shown in section A-B (pi. 2), the belt underlain by phosphate 

would be about 800 feet wide, but it broadens southward so that at 

the line C-D it may be more than half a mile wide.

A second area underlain by the Phosphoria formation occupies 

parts of sees. 30 and 31. In sec. 30 the Rex chert is downfaulted 

against the Wells formation on the east, and the phosphate beds are 

not exposed, but lower beds of the Woodside just overlap the township line. All three formations pass beneath cover in .the northern 

half of the section. In sec. 31 the phosphatic shale member is 

exposed in contact with the Wells on both sides of the syncline, but 

the beds along the east side are much broken and sheared and vary in 

thickness in such manner as to indicate slight faulting. On the west 

side they are likewise interrupted by a fault that breaks across twominor folds and slightly offsets the phosphatic shale. A subordinate 

syncline on the township line near the southwest corner contains some 

of the phosphatic shales for a distance of about a quarter of a miler 

but the size of this area as mapped may be somewhat exaggerated..

No prospecting in this township was done by the Geological Surveyparty, but two prospects in sec. 31 were visited. One in the NE. 14 

SW. y^ by the roadside in the canyon bottom on the ranch of F. L. 

Corneilison, is a hole about 25 feet deep in a nearly vertical- zone of 

crushed phosphatic shale, from which some very large nodules of 

impure limestone have been taken. One of these nodules, an eggshaped mass 3^ feet long, 18 inches wide, and 18 inches high, lay on 

the dump. At the mouth of the pit about 5 feet of finely oolitic 

phosphate in shattered beds was exposed. A rough sample comprising the upper 18 inches was taken and later analyzed at the labora6148 29 6
76 POETNEUF QUADRANGLE, IDAHO

tory of the Geological Survey. It proved to be very poor, containing 

only 13.27 per cent P2 O5 , equivalent to 29.83 per cent Ca3 (P04 ) 2 . 

The material was too dirty to provide a satisfactory sample. The 

other prospect was on a low point above Corneilison's ranch, in the 

NW. 14 SE. 14 of the section. It was a caved pit near the contact of 

the Kex chert with the phosphatic shale. Some hard lumps of phosphate rock were lying on the dump, but no samples were taken.

In 1913 the Geological Survey party prospected the phosphatic 

shale in the NE. % SE. 14 sec, 36 of the adjacent township, T. 5 S., 

R. 38 E., by means of a trench and six pits. The results of this 

prospecting, tabulated on page TO, may be regarded as showing in 

detail the general character of the phosphatic shales in the southwestern part of T. 5 S., R. 39 E.

The phosphate band extending from sec. 35 to sec. 21 was not 

prospected, but float fragments collected from two localities in the 

SE. % SW. 14 sec. 35 and the SE. 14 NE. % sec. 34 yielded 37.10 and 

36.87 per cent P2O5 , equivalent to 81 and 79.10 per cent Ca3 (PO4 ) 2 , 

respectively. These samples, which may have been somewhat enriched by weathering, show nevertheless that high-grade phosphate is 

present.

The Geological Survey prospect in sec. 36, T. 5 S., R. 38 E., 

showed the presence of a phosphate bed 73 inches thick with an 

average content of 34.37 per cent of phosphorus pentoxide, equivalent 

to 74.5 per cent tricalcium phosphate. A similar group of prospects 

made by the Geological Survey in 1916 in the SE. 14 NE. 1/4 sec. 30, 

T. 5 S., R. 40 E., indicated the presence of a phosphate bed that for 

the purposes of computation could be assumed to have a thickness of 

at least 5 feet and a phosphate content greater than 70 per cent of

tricalcium phosphate. (See p. 84.) Previous work in the Idaho 

phosphate field has shown that the phosphate rock has a tendency 

to preserve its thickness and quality over wide areas. It seems 

reasonable to believe, therefore, that T. 5 S., R. 39 - E., which lies 

between the two prospects mentioned, should contain rock of thickness and quality corresponding with that at the localities cited. For 

purposes of computation it is therefore assumed that the principal 

phosphate bed in this township is at least 5% feet thick and that it 

contains at least 70 per cent of tricalcium phosphate. The depth 

limit for such a bed under current regulations of the Geological 

Survey regarding land classification 45 is 4,500 feet. From structure 

sections A-B and C-D (pi. 2) it appears that all the phosphate 

present in the area not concealed by Tertiary and later rocks lies 

within 3,000 feet of the surface and therefore well within the depth 

limit assigned.

46 Smith, G. 0., and others, Classification of the public lands: U. S. Geol. Survey Bull. 

537, pp. 129-132, 1913.
MINERAL RESOURCES 77

It would be possible with a fair degree of assurance to project 

the phosphate bed exposed in the SW. y± sec,. 21 northwestward 

along its strike beneath cover through the NW. ^ sec. 7. There are, 

however, uncertainties of structure inherent in the general structural 

complexity of the region, especially in relation to the possible existence of the overthrust fault block to which the Cambrian beds in 

sec. 18 may belong (see p. 56). that render this action inadvisable. 

Because of these uncertainties and of the relative abundance of 

phosphate rock in this field the areas here regarded for purposes 

of classification as phosphate land are restricted rather closely to 

the sections in which the phosphate-bearing folds are actually exposed. On this basis it is estimated that T. 5 S., R. 39 E., contains 

about 1,400 acres underlain by accessible phosphate. The area withdrawn, as shown in Figure 3, is somewhat larger than this because 

the borders of the withdrawn lands do not follow the outcrop or 

structure lines of the phosphate bed but necessarily conform with 

the legal subdivisions, the smallest of which is 40 acres. On the 

assumption, as in previous computations, that a cubic foot of phosphate weighs 180 pounds, the 1,400 acres would contain about 

26,953,000 long tons of phosphate rock if the bed were horizontal. 

The fact that the beds dip at various angles would tend to increase 

this estimate, but the presence of minor faults, folds, and crushed 

zones,-such as are known to occur in at least part of the area, would 

tend to offset this increase. It is therefore thought that the estimate given is reasonable.

T. 6 S., H. 39 E.

T. 6 S., R. 39 E., the central township of the quadrangle, extends 

from Portneuf Valley at the southwest through the foothills to the 

highest ridge of the Chesterfield Range at the northeast. It contains the town of Chesterfield, the largest settlement in the Portneuf 

quadrangle, which is connected by good roads w.ith other parts of 

Portneuf Valley and with Bancroft and other shipping points on 

the Oregon Short Line Railroad, 8 to 10 miles to the south. Poor 

roads across the range pass northward and northeastward from 

Chesterfield.

Pre-Tertiary rocks are exposed only in the northeastern, northwestern, and southeastern parts of the township. Nearly threequarters of the township is occupied by Tertiary and Quaternary 

rocks.

Stt^ucturol details. The Graves Creek syncline ,is represented by 

part of its western limb. The Twentyf ourmile anticline and the Rock 

Creek syncline also cross the township. The general character of 

these folds is displayed in sections C-D and A-B, Plate 2. The plane
78 PORTNEUF QUADRANGLE, IDAHO

i

of the Bannock overthrust is brought to the surface in sec. 10 by the 

arching and erosion of the thrust plane. (See p. 53.)

The highest beds of the Graves Creek syncljne, which crop out a 

feAv hundred feet northeast of sec. 1, are Portneuf limestone. The 

phosphate beds, even if present in normal position, would therefore 

lie too deep in the central portions of this fold to be considered accessible. There is great probability that they are cut out by the 

Bannock overthrust. They are present, however, along the southwest 

limb of the syncline where it merges w.ith the Twentyfourmile 

anticline.

The core of the Twentyfourmile anticline consists of Brazer limestone with a subordinate mass of Hex chert (presumably belonging 

to the lower fault block) exposed in the midst of it. The southwest 

limb of the fold is largely concealed, but it is believed to contain the 

Phosphoria formation in normal order.

For reasons stated on page 54 the Rock Creek syncline is thought 

to be relatively shallow and to contain two subordinate synclines. 

The west limb of the fold, which is partly exposed northwestward 

from sec. 18, is broken by a fault, supposedly a thrust, that locally 

cuts out the phosphatic shale. In sec. 6 this fault participates with 

other faults and minor folds in producing a highly complex structure. 

The phosphatic shale and upper beds of the Wells are there arched 

in a small anticline extending northwestward from the SW. %= 

SE. 14. This anticline is broken down on the southwest limb by a 

supposedly normal fault that introduces at the west a fragmentary, 

transverse syncline containing some beds as high as the lower part 

of the Woodside. Farther northwest this fault brings beds of the 

Salt Lake at the south into contact with Rex chert at the north.

Both the folds and the fault just described are again cut by a normal 

fault with downthrow to the west. This is apparently the youngest 

fault in the group. Probably none of the faults have a throw greater 

than a few hundred feet.

In the SE. ^ sec. 25 the phosphate beds are partly cut out by a 

minor fault believed to be a thrust.

Phosphate deposits. A belt of phosphate rock extends northwestward through parts of sees. 1, 2, and 12. The phosphate rock is here 

overturned and dips southwestward beneath older rocks. It doubtless underlies the Rex area in sec. 10, if the interpretation above suggested for this area is correct. It also enters sec. 6 from the northwest and extends southeastward beneath cover an unknown distance 

but with little doubt crossing the entire township and including the 

exposures in sees. 24 and 25. According to present data the Rock 

Creek syncline narrows greatly southeastward, so that the phosphatic 

shale may be absent in some parts of the fold.
MINERAL RESOURCES 79

No prospecting was done by the Geological Survey party in sec. 6, 

but the character of the phosphate in that area, where not too much 

broken and infiltered with dirt, is doubtless represented by the results 

obtained by prospecting in sec. 36, T. 5 S., K. 38 E., to which reference has already been made.

In 1923 the Geological Survey party made two prospects in the 

NW. 14 sec. 17 and the NE. % sec. 18, near the section .line. By 

means of horses, plow, and scraper a trench across the general strike

 of the beds, about 100 feet long, 4% feet wide, and 5 feet deep, was 

made at the first-named locality. From float at the surface this 

seemed a favorable place. The overburden proved very thick. 

Some reddish-weathering brown shale or impure limestone and some 

whitish and yellowish Aveathering rock appeared in the bottom of the 

cut, but none of the dark and black phosphate usually seen. At 

the southwest end of the trench some pieces 'of gray-weathering 

hard phosphate, apparently representing a thin bed near the base

-of the section, were encountered early in the plowing, but the bed 

itself was not exposed. The results here were so unpromising that 

further work was abandoned, and a new trench, the second above

 mentioned, was started at a neighboring locality. This trench was 

30 feet long, 4 feet deep, and 5 feet wide. The overburden was 2 

feet or more thick. No bedrock was exposed except some decomposed, yellowish-weathering, impure limestone. At the. place where 

phosphate should normally occur, near the top of the underlying 

Wells formation, there was an earthy jumble of phosphatic sand

-and fragments of phosphate rock with red clay. This was deepened 

"2 or 3 feet Avith pick and shovel, but no solid rock or change of 

"material was noted. Float of hard rock lay on the surface next 

to the exposed upper beds of the Wells at this locality, as in the

-other trench, and some material of this character appeared in the 

cut, but no bed of it Avas exposed. A random sample of float fragments Avas taken for analysis. This proved to be low-grade material containing only 25.63 per cent of phosphorus pentoxide,

 equivalent to 55.95 per cent of tricalcium phosphate.

The phosphate bed in sees. 1 and 2 Avas not prospected, but in 1923 

the Geological Survey party dug a trench in the phosphatic shale in 

the SE. 14 NW. ^4 sec. 25. The upper beds of the Wells formation 

here strike about N. 42° W. and dip about 18° NE. The trench, 

which was nearly at right angles to the strike of the rocks, Avas 30 

feet long, 2 feet wide, and 4 feet 8 inches deep at the deepest place. 

'The results of the prospecting are tabulated below. All the phosphate rock exposed Avas very dirty.
80 PORTNEUF QUADRANGLE, IDAHO

Section of phosphate beds in the 8E. % NW. % sec. 25, T. 6 8., R. 39 E., Portneuf

quadrangle,

Locality 

No.

M-92-23

Character of material

Not exposed. 

Phosphate rock, grayish brown, medium oolitic, beds -rV to }^ 

inch thick; includes limestone nodule 6 inches thick: 

Sample 3, hand-picked material across the part of the section

Shale, brown...... . _ . __ . . ..... . __ ..

P2 05

Per cent 

31. 98

30.09

31.07

Ca3 (PO<)2

Per cent 

09. 80

65. 68

07. 80

Thickness

Ft. in..

2 4

2

4

4 8:

The pieces taken for sample 3 Avere selected so as to be as free as 

possible from dirt. The part of the section represented by sample 

2 was too dirty to make hand sorting worth while. As it proved, 

the hand-sorted material was only slightly richer than the unsorted 

sample. Alumina and iron are low in the three samples, rangingfrom 0.68 per cent in sample 3 (selected material) to 0.92 per cent 

in sample 1. Chromic oxide, Cr2 03 , is present in all three samples, 

but in little more than traces. Vanadium calculated as pentoxide 

(V2Og ) is present in all samples. (See p. 68.) Fluorine is present 

in amounts less than 0.04 per cent.

In view of the results of prospecting in sec. 36, T. 5 S., K. 38 E.,. 

and sec. 30, T. 5 S., R. 40 E., above cited, together with those justgiven, it seems fair to assume for this township the presence of a. 

5-foot bed of phosphate of at least 70 per cent grade. The depth, 

limit for such a bed under current regulations is 4,000 feet.

Although the phosphate is believed to be continuous throughout 

the Rock Creek syncline, the abundant cover and the uncertainties 

regarding concealed structure render it inadvisable to apply thetonnage computation to areas much to the southeast of the line C-D. 

(See pi. 1.) The phosphate-bearing area, which is bounded approximately by the Wells formation on the southwest and by the6,000-foot structure contour on the northeast, is therefore arbitrarily cut off about half a mile southeast of the Rex exposure in the 

SE. 14 sec. 18. Similarly the phosphate-bearing area in sees. 24 and 

25 is cut off at the section and township lines respectively. The Rex. 

area in sec. 10 is thought to indicate the presence of accessible phosphate rock in the lower thrust block of the Bannock overthrust. In 

sees. 1-3 and 12 the southwest limit of the phosphate-bearing area, 

assumed for purposes of computation, is the projected line along 

which the phosphate as shown in structure section C-D is cut off at 

depth. The northeastern limit would normally be the contact of the 

phosphatic shale with the Rex, but as an additional margin of safety 

the Rex and Woodside contact is taken instead. The areas thus cle-
MINERAL RESOURCES 81

limited taken together contain about 3,300 acres. As the withdrawn 

lands must follow the boundaries of legal subdivisions their acreage 

as shown in Figure 3 is somewhat larger.

If the weight of a cubic foot of phosphate rock is assumed as before to be 180 pounds. 3,300 acres underlain by a 5-foot bed would 

contain approximately 57,756,000 long tons of phosphate rock if 

the strata were horizontal. Allowance for the dip would increase 

this figure, but complexities of structure, with consequent shattering or injury to the phosphate rock, would tend to offset this increase. The estimate as given is therefore considered reasonable.

T. 5 S.. R. 40 E.

T. 5 S., R. 40 E., lies mostly in the Portneuf quadrangle but includes 

strips a mile or more wide in the Paradise Valley, Cranes Flat, and 

Henry quadrangles. It contains parts of the Blackfoot Mountains, 

Reservoir Mountain, the Blackfoot lava field, and the Chesterfield 

Range. Blackfoot River, which leaves the reservoir of the same 

name in sec. 12, makes a broad loop westward through the northern 

part of the township. Railroad facilities are 25 miles or more away, 

but fair roads connect the township with Soda Springs, Blackfoot, 

and Fort Hall, which are the chief available shipping points.

/Sfo'luctural details. The principal structural features of the township comprise parts of six large folds. One of these, entirely outside 

the Portneuf quadrangle, is an anticline mostly exposed in the Cranes 

Flat and Paradise Valley quadrangles but occupying parts of sees. 1 

and 2 of this township. The west limb is faulted, but the lower 

member of the Phosphoria is preserved along the fault, making available the fine section described below from sec. 34 of the adjoining 

township. A second fold, which also lies completely outside the Portneuf quadrangle, occupies sees. 1, 2, 11, and 12 and is thought to be 

the continuation of the Reservoir syncline of the Henry quadrangle. 

The Thaynes and Woodside beds in sees. 24, 25, and 36 are also probably parts of the same fold. It consists mainly of beds of the Thaynes 

group, including some beds of the Portneuf limestone. Much of 

this fold is concealed by basalt, and it is thought to be offset beneath 

cover by the Little Gray fault. (See p. 59.) The loAver part of it is 

probably cut off by the Bannock overthrust. The other folds, named 

in order southwestward, are the Corral Creek anticline, the Nigger 

Creek syncline, the Grizzly Creek anticline, and the Graves Creek 

syncline. These folds are described under their respective names 

on pages 48 to 53. (See also pi. 8.)

Phospliate deposits. A small area of phosphate-bearing shale occurs in the NW. % NE. % and the NE. 1/4 NW. ^ sec. 2, just inside
82 POETNEUF QUADRANGLE, IDAHO

the township line, where the first anticline mentioned in the preceding paragraph is faulted against the adjacent syncline. The 

phosphate here terminates against the fault and is probably confined 

to the area designated. The syncline just mentioned is so deep that 

any phosphate it might otherwise contain is probably cut out by the 

Bannock overthrust.

The Corral Creek anticline contains a considerable body of phosphate most readily accessible along its crest. The area containing 

this phosphate is about 1*4 miles wide in sees. 14, 15, and 16 and 

extends northwestward 4 miles to the northwest corner of the township, where it is about a mile wide. This phosphate-bearing area 

may extend southeastward from sees. 14r-16 with the same general 

trend, but a thick cover of basalt overlies that portion of the 

area and renders any phosphate that it may contain practically 

inaccessible.

The Grizzly Creek anticline contains phosphate in at least the 

upper portions of the fold. The area affected is approximately 

three-quarters of a mile wide and extends northwestward about 3^ 

miles from the township line in sees. 32 and 33.

Before 1916 no prospecting for phosphate had been done in this 

township. In that year the Geological Survey party prospected 

and sampled the phosphatic shale on the crest of the knoll in the 

SE. 14 NE. % sec. 30. The Wells in prominent ledges forms the 

west edge of the crest and the west slope of the hill. The phosphatic 

shale occupies the sag and furnishes an abundance of excellent float. 

The Eex forms the eastern part of the hill.

The overburden proved to be 2 feet or more thick, and below it 

a much weathered biotite-hornblende andesite was encountered in a

gently inclined dike, which cut out much of the phosphatic shale, 

leaving an eastward-pointing wedge of this rock between the westward-dipping andesite and the nearly vertical beds of the upper 

part of the Wells. The phosphatic shale in this wedge had the 

structure of a syncline overturned eastward. This structure, however, is local, for the distribution of the shale as mapped shows that 

it forms a steeply inclined stratum with dips ranging on both sides 

of the vertical, depending upon the relative position of the stratum 

measured with regard to the fold in which it appears to be involved. 

Four trenches were dug. No. 1, at the base of the section, was 

26^/2 feet long, 2y2 feet wide, and 5y2 feet in maximum depth, with a 

strike N. 53° E. No. 2, which was in line with No. 1 but 6 feet from 

it, was 8 feet long, 2% feet wide, and 3*4 feet deep. No. 3 was offset 

31 feet north of No. 2 but was so placed as to continue the phosphate 

section upward. It was 40 feet long, 2*4 feet wide, and 4% feet
MINERAL RESOURCES 83

in maximum depth, with a strike N. 46° E. No. 4 was offset about 

100 feet N. 83° E. from No. 3. It was about 6 feet long, 2 feet wide, 

and 2 feet deep, and 165 feet N. 56° E. from the top of the Wells as 

exposed in No. 1. The lithologic details observed in the four trenches 

and the chemical determinations made from the samples collected 

in them are given below.

Partial section of phosphatic shales exposed in four trenches vn SE. % NE. % 

sec. 80, T. 5 8., R. 40 E., Idaho

Trench No. 4

Field

No. of

specimen

PsOj

Equivalent to Thickness

Per cent Per cent Ft. in.

M-136-16 Limestone, brown, with phosphatic streaks, top not exposed. ... . .. 10 

Phosphate rock, thin bedded, coarsely oolitic, with shaly streaks,

brownish gray.... .._....__.... .. . 6

Shale, phosphatic, brown, somewhat oolitic..........- .....

Sample? Phosphate rock, dense, siliceous, black; one bed____.____ 24.95 54.5 2 

Shale, phosphatic, brown..._____.....___._......__..

Phosphate rock, coarsely oolitic, gray to black, layers A 1 to 1J^

inches thick___________________________ 

Limestone, brownish gray, with oolitic grains; one bed_____ ___............ 3

Shale, phosphatic, calcareous.._________________ 

Limestone, brown, somewhat shaly; base not exposed...._.

Trench No. 3

Soil and overburden, not measured.

Limestone, brown, not well exposed, about........ . . . 2 6

Phosphate rock, much broken, mingled with soil, some pieces 

coarsely oolitic, black, about.._______....___................. . . 3

Samples 6 Phosphate rock, brownish gray, thin bedded, finely oolitic, part 1 22 07 4s fl 1 

and 5. shaly, some coarsely oolitic layers, occasional calcareous nodules; > fo'oi on'o H 

2 samples, 2 feet each.......................................... I 18"" M- 6 }

Limestone, brown, weathering black, massively bedded...... --..-..... 1 9 

Sample 4 Phosphate rock, gray to black, thin-bedded, finely oolitic, with

some shaly material weathering reddish blacker gray...... . 31.40 . 68.6 2 1

Weathered zone with brownish soil. Fragments of phosphate 

rock, shale, and brown limestone._____................ .............. . 1 9

Phosphate rock, thin bedded, dark gray, with shaly streaks, 

finely oolitic____-----___________________... ___-.. . 5

Phosphate rock, thin bedded, finely oolitic, with shaly streaks 

and calcareous layers; much weathered and broken........ _.. _. . ... 1 10

Soil zone, brown, powdery, with pieces of phosphate rock and 

limestone.... ........_______________... ... .... 10

Phosphate rock, grayish brown, thin bedded, finely oolitic, with 

calcareous beds____.._____________________ ._.... . 9

Limestone, brown, with phosphatic streaks, much broken.. .... .. ..... 1. 3

Soil zone, brownish gray, powdery, with fragments of brown limestone and phosphatic shale________..____..__.. --------. --- 2

Phosphate rock, brownish gray, shaly. with nodules of limestone, finely oolitic, mingled with earth, about.. .._._........ . 1 9

Limestone, brown, shaly, with streaks of phosphate rock, much 

broken.._____....._____..__._.___._....................... 1 4

Phosphate rock, brownish gray, shaly, finely oolitic, much broken.. __. _ ... 1 6

Phosphate rock, very shaly, much broken, mingled with soil, 

apparently in syncline__.._...._--------__..__ ..._-._. ---------- 1 3

Soil zone, brown......_....__________....._...... _-. .. -... ... 6

Limestone, brown, impure_______....._........._......- .. .... 3

Phosphate rock, brownish gray, shaly, finely oolitic, much broken.... ..-,-- 7

Limestone, brown, shaly, thin calcite veins.---------_ ....-. . .- 6

Trench No. 2

[Oniy overburden and decomposed andesite exposed; no measurements.]
PORTNETJF QUADRANGLE, IDAHO

Partial section of phosphatio shales exposed in four trenches in 8E. 

sec. 80, T. 5 8., R. 40 E., Idaho Continued

NE. V*

Trench No. 1

Field 

No. of 

specimen

: Sample 3 

-Sample 2 

 Sample 1

Andesite, weathered into yellowish gravel, to depth of at least 2 

feet below overburden. 

[Phosphate rock, brownish gray, thin bedded, medium oolitic, 

1 much brecciated- ________________________

1 Phosphate rock, dark gray, hard, dense, finely oolitic, single bed.. 

(Phosphate rock, brownish gray, thin bedded, medium oolitic .... 

Phosphate rock, brownish gray, coarse to medium oolitic, beds t"s 

to \Vi inches thick, possibly in part equivalent to those below ... 

Phosphate rock, brownish gray, coarse to medium oolitic, beds y% 

to \Vi inches thick, somewhat folded.. _____________

Shatter zone, soil with fragments of limestone and phosphatic 

material __ . ___________ . ______________

Limestone, top of Wells formation.

PsOs

Per cent 

i 32. 87

32.53 

31.33

Equivalent to 

CaaCPOOs

Per cent

71.7

71.0 

70.5

Thickness

Ft. in. 

f t

< 4^ 

1 7M

2 

1 4

1 3

The distance across the outcrop of the phosphatic shale is about 

170 feet. In spite of the generally steep dip this is probably somewhat greater than the actual thickness of the shale member, because 

of the disturbances above noted. In the trenches described a thickness of about 41 feet of strata was measured. Of this thickness 24 

feet 7 inches was phosphate rock, some of it possibly duplicated and 

nearly half of it too broken or dirty for sampling. The IS1/^ feet 

sampled averaged about 60 per cent tricalcium phosphate. Near 

the base a bed 5 feet 4 inches thick with possibly some duplication 

averaged 71.1 per cent tricalcium phosphate.

The iron and alumina content of the phosphate sampled appears 

to be within the prescribed limit (p. 68). A composite of samples 

2 and 3 yielded 1.48 per cent of iron and alumina together. A composite of the somewhat poorer samples 5 and 6 yielded 1.04 per cent 

Pe2O3 and 3.04 per cent A12O3 .

In 1916 a prospect was made in the phosphatic shale in sec.-34, 

T. 4 S., R. 40 E. (See pi. 7, B.) The results of this work are given 

here for comparison.

Partial section of phosphatic shale in the SE. "-

R. 40 E., Idaho

[Analyst, R. M. Kamrn]

8E. sec. 34, T. 4

Field

No. of

specimen

M-129-16

Sample 1

Samples 3, 

4, and 5.

Clay, sandy, yellow, top not exposed.

Phosphate rock, brownish gray, somewhat shaly, finely oolitic...

Limestone, brown.. ________________________

Phosphate rock, brownish gray, thin bedded, partly shaly, medium to finely oolitic; 3 samples, about 20 inches each.

PjOs

Per cent

28.66

25.02

( 29. 82 

\ 24. 45 

I 21. 73

Equivalent to

CasCPOOa

Per cent

62.7

54.6

65.1 

57.9 

47.4

ThickFt. in.

1 3

2

2

1 4

1 10

3M

\5 2
MINERAL RESOURCES 85

.Partial section of phospJwtic shale in the SE. y± SE. 

R. 40 E., Idaho Continued

sec. 34, T. 4 S..

Field 

No. of 

specimen

Sample 6

: Samples 7 

and 8-

 Sample 11

.-Samples 

12, 13, 

and 14.

[Phosphate rock, beds 1A 1 inches thick, brownish gray, medium to 

\ finely oolitic.

Phosphate rock, brownish gray, in beds J£ to A 1 inch thick, finely 

oolitic; 2 samples; 22.inches.each.

[Phosphate rock, in beds % to 1 inch thick, dark gray, finel y oolitic. 

|Phosphate rock, brecciated and shattered, weathered almost

[Phosphate rock, black with yellow specks, coarsely oolitic, in beds 

1 Ji to 1 inch thick; 3 samples, about 21 inches each.

Phosphate rock, shalv, finely oolitic, and brown shale. ...........

Limestone, gray, top of Wells formation, not measured; strike NT. 

41° W., dip 62° S.

PaOs

Per cent 

\ 33. 22

/ 31.33 

I 30. 20

28.28

28.33

I 30. 03

f 32. 92 

\ 33.34

130.21

Equivalent to 

Ca3 (P04)j

Per cent 

72.5

68.3 

65.9

61.8

61.8

65.5

71.8

72.7

65.9

Thickness

Ft. in,

. !

8

1 10 

1 10

1 4

2

2

h

33 V/i

Stratigraphically above the section just given but 138 feet farther

 down the slope, corresponding to about 112 feet above the highest 

bed included in the section, is a bed of hard phosphate 10 inches 

thick, \vhich forms a ledge. Sample 15, taken from this ledge, 

yielded 20.70 per cent phosphorus pentoxide, equivalent to 45.2 per

 cent tricalcium phosphate. The phosphatic shale at this locality 

.appears from the above data to be at least 145 feet thick.

The section above described is unusually rich in phosphate, containing at least 31 feet 6' inches of phosphate rock, of which 24 feet 

'6 inches was sampled and found to average about 63 per cent trical- 

<cium phosphate. One of the richest beds is a few feet above the 

middle of the trenched portion of the section. It is 1 foot 8 inches 

thick and averages 72.5 per cent tricalcium phosphate. Another'bed 

just above is thicker but of poorer quality, being 5 feet 2 inches

 thick and averaging 56.8 per cent tricalcium phosphate. The most 

important bed commercially is near the base; this bed is 5 feet 5 

inches thick and averages 70.1 per cent tricalcium phosphate. Besides these beds there are five others, ranging from 13 inches to 3 feet 

8 inches in thickness and from 61.8 to 67.1 per cent (average of samples 7 and 8) tricalcium phosphate.

The shatter zones and the proximity of the fault already mentioned 

; suggest that some of the phosphate beds may be repeated, but the 

Kthologic details above given differ sufficiently to show that there is 

probably no duplication.

Ten of the samples taken from the trench above described were

 divided into four sets of composites and tested for iron and alumina. 

'The composite including samples 3, 4, and 5 yielded 0.51 per cent
86 POETFEUF QUADRANGLE, IDAHO

Fe2 03 and 2.56 per cent Al^Og. As the combined amount of these 

two substances present was less than the limit usually allowed 

(p. 68), the iron and alumina were not separately determined for 

the remaining composites but were grouped in a single determination for each. The composite including samples 7 and 8 yielded 

only 1.96 per cent of iron and alumina; the composite including 

samples 1.0 and 11, 1.92 per cent; and the composite including samples 12 to 14, 1.19 per cent.

From the data given above it seems safe to assume for purposes 

of computation that T. 5 S., R. 40 E., contains a bed of workable 

phosphate at least 5 feet thick and averaging TO or more per cent 

tricalcium phosphate. The depth limit of classification for such a 

bed under current regulations is 4,000 feet.

On the basis of the existing classifications 6,760 acres in this 

township is reserved as phosphate land. (See fig. 3.) The boundaries of this area, which have to follow public-land subdivisions, include more territory than is needed for the computation of an 

estimate of tonnage. The actual area so employed is 4,570 acres.

If the weight of a cubic foot of phosphate rock in the ground, 

is assumed to be about 180 pounds, a horizontal bed 5 feet thick 

underlying 4,570 acres would contain about 79,983,000 long tons. 

Allowance for the dip of the beds would increase this estimate, 

but uncertainties of structure, especially the attitude of the Bannock, 

overthrust, may affect it adversely. The estimate as given is believed to be moderate and is therefore retained.

T. e s., R. 40 E.

T. 6 S.. R, 40 E., overlaps the Henry quadrangle on the east about: 

2 miles. It includes some of the western foothills of Reservoir 

Mountain, much of Corral Creek valley, and part of the Chesterfield 

Range. A road through Corral Creek valley connects the townshipwith Soda Springs between 12 and 15 miles to the south, and with 

Fort Hall and Blackfoot, 25 miles or more to the northwest. Poor 

roads, hardly more than trails, cross the Chesterfield Range to Portneuf Valley by way of the divide southeast of Sawmill Creek and upThompson Creek.

Structural details. The Woodside shale crops out in the E. y2 . 

sees. 1 and 12, and beds of Wells and Phosphoria in sees. 12 and 13. 

The western part of the Wells ledge, which constitutes the westernmost exposure of the older rocks in Reservoir Mountain, is brecciated 

and contains much chert and secondary quartz. There is travertine 

both north and south of this ledge that suggests faulting. The Rex. 

chert, however, occupies the next knoll south of the Wells along its 

strike and indicates the presence of an anticline that pitches gently
MINERAL RESOURCES 87

southward. It is inferred that the anticline is broken by a fault that 

brings in higher beds beneath cover on the west.

A fault that offsets eastward the structure of Reservoir Mountain 

is believed ta enter the township in the northeast corner of sec. 24, 

but it passes beneath cover and has not been identified farther west. 

It is known in the Henry quadrangle as the Pelican fault. The eastward displacement of the southern parts of the Rock Creek syncline 

and the Twentyfourmile anticline may be due to this fault rather 

than to bending as suggested by the mapping of their axes. The fault 

in King Creek on the west side of the quadrangle is suggestive in 

this connection, but the ridge of Carboniferous rocks that crosses 

the township from sec. 7 to sec. 33 is apparently not offset, and if a 

transverse fault does cut it the dislocation has not been recognized. 

However, this relationship was not suspected in the field, and no 

search for pertinent evidence was made. Reservoir Mountain and 

the structural features associated with it have been described elsewhere.46

The other folds represented in the township, named in order southwestward, are the Nigger Creek syncline, the Grizzly Creek anticline, 

the Graves Creek syncline, the Twentyfourmile anticline, and the 

Rock Creek syncline. These folds are so concealed by Tertiary and 

Quaternary rocks that exposures only here and there are available 

to show the general position and character of the folds. In drawing 

the geologic structure section F-G these exposures have been employed in conjunction with the knowledge of the folds as more fully 

exposed in Tps. 5 and 6 S., R. 39 E., and T. 5 S., R, 40 E.

Phosphate deposits. The phosphatic shale is exposed in this 

township only in the SE. 14 sec. 12 and the NE. 14 sec. 13. Phosphate is doubtless associated with the Twentyfourmile anticline in 

sees. 7, 8, and 17 and with the Rock Creek syncline in sees. 30 and 31, 

as indicated in the structure section F-G. It is estimated that at 

the line F-G the phosphate beds of the anticline would lie between 

500 and 1,000 feet east of the 6,000-foot structure contour in sec. 17. 

Similarly it is believed that in the Rock Creek syncline the phosphate 

would lie approximately between the 6,000-foot structure contour 

lines drawn on the two sides of the axis. In each of these places the 

position of the phosphate can be determined only by boring or mining operations.

The phosphate of the Rock Creek syncline was prospected by the 

Geological Survey party in sec. 25, T. 6 S., R. 39 E., where a partial 

section exposed at least 4 feet of phosphate averaging 66 per cent 

or more of tricalcium phosphate. (See p. 80.) In 1916 the Geological Survey party prospected the phosphatic shale in the NE. 14

« U. S. Geol. Survey Prof. Paper 152, pp. 34, 164, 1927.
88 POKTNEUF QTJADKAFGLE, IDAHO

NE. % sec. 30, T. 6 S., R. 41 E., where a bed of phosphate at least: 

7 feet thick was found, 5 feet of which contained 70 per cent or 

more of tricalcium phosphate. It is reasonable to suppose that thephosphate present in T. 6 S., R. 40 E., is not less than 4 feet thick 

and will average more than 66 per cent of tricalcium phosphate.

The phosphate rock in the Rock Creek syncline and the Twenty - 

fourmile anticline is not considered sufficiently available to classify, 

because it is so thoroughly concealed and because any development 

of it must undoubtedly be delayed until other regions where it may 

be located without boring are exploited. The existing classified phosphate land in this township includes 2,160 acres in sees. 1, 12, 13, 24, 

and 25. (See fig. 3.) For computing tonnage, however, it has been 

thought best to limit the area considered to a strip about 0.7 mile 

Avide lying along the township line in sees. 1, 12, 13, and 24, and 

comprising 1,160 acres. From the above-mentioned data obtained 

in sec. 30, T. 6 S., R. 41 E., it is assumed that this area is underlain 

by a 5-foot bed of phosphate, which, on the same basis as previous 

computations, should yield about 20,300,000 long tons of phosphate 

rock if horizontal. Allowing an increase of 20 per cent on account 

of the dip, which ranges from 19° to 40° and is in some places overturned, the partial estimate for the tonnage of this township would 

amount to 24,360,000 long tons.

T. 7 S., R. 40 E.

T. 7 S., R. 40 E., overlaps the Henry quadrangle about iy2 miles 

and includes most of the south end of the Chesterfield Range with 

adjacent portions of Portneuf Valley on the west and the Blackfoot 

lava field on the east. The range is not crossed by any road here, 

but the higher parts of it are accessible by trails up most of the 

canyons. A road through Tenmile Pass gives access from one side 

of the township to the other at the south. Bancroft, on the Oregon 

Short Line Railroad about five miles southwest of the township, is 

the nearest shipping point.

Structural details. The older rocks of the Chesterfield Range are 

largely concealed by Tertiary beds. Nevertheless, exposures near the 

margins afford a fairly satisfactory basis for drawing the geologic 

structure section, H-I (pi. 2), in which it appears that folds recognized farther northwest continue into this township. Thus with 

little doubt the Rock Creek syncline and Twentyfourmile anticline 

are present, and in the northeastern part of the township probably 

some of the Graves Creek syncline is represented.

Phosphate deposits. Phosphate is not exposed in the township, 

but, as suggested by the structure section, it probably underlies an 

area roughly bounded by the two 6,000-foot structure contours that
MINERAL RESOURCES

outline the position of the Rock Creek syncline. It also probably 

follows the east limb of the Twentyfourmile anticline northwestward 

from about the NE. % sec. 13. Here, as in T. 6 S., R. 40 E., any 

actual location of phosphate beds must be determined by boring or 

by mining operations.

No definite information is available about the character of the 

phosphate in this township. On the basis of work done in neighboring townships, however, it is believed that the phosphate rock herewill prove to be 4 feet or more thick and of 70 per cent quality. The^ 

general continuity and uniformity of character of the phosphatethroughout the region of which this township forms a part supports, 

this assumption.

There is with little doubt a considerable tonnage of phosphate in» 

the township that may ultimately prove minable. Nevertheless, no? 

land here is classified as phosphate land, and no estimate of tonnageis attempted, because of the extent and thickness of the Tertiary 

cover and because the probability of exploiting the phosphate is; 

very remote.

GOVERNMENT INVESTIGATIONS RELATING TO PHOSPHATE

The United States Geological Survey and the Bureau of Soils haveengaged in phosphate investigations for many years. More recently 

the Bureau of Standards and Bureau of Mines have also taken up thiswork.

GEOLOGICAL SUKVEY

The Geological Survey has studied the phosphate fields of thecountry to determine the distribution, character, relative abundance,, 

and workability of the phosphate rock and particularly to obtain data 

for the classification of public lands. The present report is an example of such a study expanded to include a description and discussion of the geologic features of an entire quadrangle. Other brieferstudies involve merely the examination and prospecting of specified! 

tracts of land preliminary to the determination of their legal 

status or the feasibility of their development under Federal lease. 

Work of this type has recently been done both in Florida and in the 

Western States. 47 In addition the Geological Survey has for manyyears collected statistics and trade notes relating to the phosphateindustry, which it has published in the chapters on phosphate rock in. 

the annual volumes of Mineral Resources. Upon the transfer of thedivision of mineral resources of the Geological Survey to the Department of Commerce, July 1,1925, this work was assigned to the Bureau, 

of Mines.

47 See chapters on phosphate rock in Mineral Resources for 1923, issued by the United 

States Geological Survey, and for 1924, issued by the Bureau of Mines.
90 PORTNEITF QUADRANGLE, IDAHO

BUREAU OF SOILS

For a number of years scientists of the Bureau of Soils, Department 

of Agriculture, have been experimenting on the volatilization of phosphoric acid from phosphate rock with a view to reducing the cost of 

extraction. From a brief account of some of these experiments 48 the 

following excerpt is taken.

The raw materials used in the volatilization process of preparing phosphoric 

acid are phosphate rock, sand, and coke. When these are ignited at the proper 

temperature under reducing conditions, phosphorus is evolved in the elemental 

state, and it is this process that is now in use for the commercial preparation 

of this material. If the ignition is made in a type of furnace from which the 

air is not excluded, the evolved phosphorus is at once oxidized to phosphorus 

pentoxide and escapes from the furnace as dense white fumes. Owing to the 

hygroscopic nature of this fume it readily reacts with the moisture of the air 

or with moisture driven off from the charge and may be readily recovered 

as a solution of phosphoric acid by passing through a Cottrell precipitator.

The paper discusses the reactions that may occur when calcium 

phosphate is ignited with silica and carbon either singly or in combination under different temperatures or other conditions.

BUREAU OF STANDARDS

With the cooperation of a number of fertilizer companies the 

Bureau of Standards, Department of Commerce, in 1924 analyzed 

its standard phosphate-rock sample No. 56.49 The work brought 

out a number of interesting facts concerning the procedure for the 

determination of moisture, phosphoric acid, and " soluble .iron and 

alumina " by the old methods and resulted in the discovery of desirable changes in these methods as well as the development of certain new methods. Although special attention was paid to the 

usual determinations just mentioned, methods for the less commonly 

determined ferric oxide, alumina, and lime were also included.

The recommendations of the authors are summarized as follows:

The use of boric acid is desirable in preparing solutions for the analysis of 

phosphate rock, for this lessens the hydrofluoric acid attack on glassware and 

prevents the interference of this acid in determinations of phosphorus.

Results obtained by solution of the ammonium-phosphomolybdate precipitate 

and a single precipitation with magnesia mixture can not be correct except 

through compensating errors, for the precipitate always contains molybdenum 

and is rarely of ideal composition. If magnesia mixture is added too slowly 

to neutral or ammoniacal solutions of phosphate the results are usually low. 

For accurate analyses solution of the magnesium ammonium phosphate precipitate is recommended, followed by the addition of 2 to 3 cubic centimeters 

of magnesia mixture and reprecipitation by ammonia.

48 Ross, W. H., Mehring, A. L., and Jones, R. M., Preparation of phosphoric acid: Jour. 

Ind. and Eng. Chemistry, vol. 16, pp. 563-566, 1924. See also U. S. Dept. Agr. Bull. 

1179, 53 pp., 11 pis., 12 figs., December, 1923.

«Lundell, G. E. F., and Hoffman, .T. I., The analysis of phosphate rock: Jour. Assoc. 

Off. Agr. Chemists, vol. 3, pp. 184-206, 1924.
MINERAL KESOURCES 91

In the alkalimetric method for phosphorus the solution should not be heated 

after the addition of the molybdate reagent. The 23:1 ratio can not be used 

in calculating the phosphorus titre of the sodium hydroxide solution unless the 

method is carefully worked out and followed to the letter.

It is not difficult to get concordant results for " soluble iron." The determination of "soluble alumina," on the other hand, requires rigid attention to a 

definitely defined method of solution and careful analysis if comparable results 

are to be obtained.

BUHEAU OF MINES

As stated above the Bureau of Mines, Department of Commerce, 

has taken over the statistical.work relating to phosphates and has assumed responsibility for preparing and publishing the annual statements. Requests for these publications should hereafter be addressed 

to the Director, U. S. Bureau of Mines, Washington, D. C.

COMPARISON OF PHOSPHATE ROCK FROM DIFFERENT FIELDS

Occurrence and physical characteristics. The Florida phosphate 

is distinguished from that of the other fields in the United States 

by its generally lighter color, by the finer texture and more nearly 

structureless character of individual masses, and by the more common 

association of bones, teeth, and fragments of replaced shell with 

the rock. The hard rock is largely a residual accumulation upon 

an irregular limestone floor of Eocene age, of phosphatic debris 

gathered by the a.gencies of solution, secondary deposition, and 

slumping from phosphatic strata of Miocene age. The direction of 

concentration has been mainly downward, with little lateral transportation of the phosphate-bearing materials or solutions. The same 

causes have accentuated the irregularities of the floor on which the 

phosphate has accumulated. The result is extreme irregularity in 

thickness and quality of the phosphate-bearing mass or matrix. 

Only intensive prospecting by drilling can determine the extent, 

thickness, and quality of the phosphate in any tract of supposedly 

phosphate-bearing land.

The land pebble is a concentration of phosphate rock by the same 

processes and from the same sources as the hard rock, with the added 

features of erosion and redeposition. That is to say, portions of 

the Miocene phosphate-bearing strata and accumulations of hard 

rock have both been eroded and redeposited as part of a neAv group 

of marine beds of Pliocene age. The floor on which this deposit is 

laid down is more even and smooth than the floor of the hard-rock 

deposits. The formation itself is also more evenly distributed 

through the region in which it occurs. Thus prospecting to determine the character of any particular tract need not be so intensive 

or expensive as in the hard-rock field. Nevertheless, the distribution 

of the phosphatic matrix within the formation is decidedly uneven? 

6148 29 7
92 POETNBUF QUADRANGLE, IDAHO

as regards both thickness and quality, so that careful prospecting 

must precede commercial development.

In both the hard rock and the pebble fields considerable overburden must be removed, and the matrix includes much clay and 

sand, together with boulders of limestone and chert, so that thorough 

washing is necessary to separate the phosphate from its unprofitable 

matrix. Some soft phosphate is lost by this process, but the washings 

from the rock as mined are accumulated in worked-out pits, so that 

some of this material may be recovered later should the market 

justify the undertaking.

i The South Carolina rock is similar to some phases of the Florida 

hard rock, but it does not have such an irregular base.

The Tennessee brown phosphate is also a residual deposit but generally is of more granular appearance than the Florida rock, and 

its color is darker, being dark brown or black to yellow. It also 

has accumulated upon a limestone floor whose inequalities have been 

accentuated by solution. The several phosphatic formations that 

have contributed to it are much older than the Florida rock, being 

of Ordovician age.

The Tennessee blue phosphate js an original marine deposit, 

corresponding in a way with the land-pebble deposit of Florida. It 

consists of earlier residual brown phosphate, rearranged and redeposited by the sea, together with some additional phosphatic material 

that was present in the Devonian or Mississippian waters in which 

it was laid down. Its texture ,is finely granular or even oolitic in 

appearance, and its color is black to dark blue, weathering brown.

The Tennessee brown phosphate must be prospected, mined, and 

washed in much the same manner as the Florida rock. The blue 

rock, on the other hand, is mined by underground methods like coal. 

Unfortunately the bed is rarely more than 3 feet thick and usually 

less, and much barren material must be moved in order to get the 

desired phosphate.

The western phosphate rock is much like the Tennessee blue rock 

in appearance, having a characteristic oolitic texture and a black to 

brown or grayish color. It is more or less friable, though some beds 

are hard and dense. It is an original marine deposit, there having 

been, so far as known, no adequate previous accumulation of residual 

phosphate, such as was available for rearrangement and redeposition 

when the Tennessee blue phosphate and the Florida land pebble were 

laid down. There was, to be sure, a comparatively small and .irregularly distributed quantity of phosphate laid down in the beginning of 

the upper Mississippian, but this is of the same general nature as the 

Permian deposits .and could have contributed little to the widespread 

and relatively abundant phosphatic accumulations of the later epoch. 

The western phosphate must be mined like coal, by underground
MINERAL RESOURCES 93

methods, but the productive beds are so thick, usually 5 feet or more, 

that much of the development work in a given bed can be carried on 

in the phosphate rock itself. Thus little waste material need be 

handled. Moreover, the western rock, like the Tennessee blue rock, 

needs no washing. The rock thus far mined is relatively dry, but 

some of the producers run it through a crusher and a dryer before 

shipping in order to command a higher price.

Chemical and mineral composition. Although the types of phosphate rock from the several fields differ in physical appearance, they 

are much alike in chemical and mineral composition. So far as optical properties are concerned more attention has been paid to the 

Florida and western phosphate rock than to material from other 

fields. Both isotropic and doubly refracting minerals or mixtures of 

minerals occur in these phosphates. It is thought that the isotropic 

mineral may be collophanite and that the doubly refracting substances may correspond with one or more of the minerals from the 

French phosphorites called dahllite, francolite, or quercyite, described 

by La Croix.50 .The great difficulty in studying the mineralogy of 

the phosphates is to obtain pure material for each mineral to serve 

as a standard of comparison.

The following table includes typical high-grade material from 

each of the more important fields; complete analyses of Tennessee 

rock are not available.

Analyses of typical phosphate rock from various fields

SiOi.. .................

Ala03 -.

Fe^Ot. .................

MgO. . . - 

CaO.-. .

NaaO.-. ... ... ... ... Kto. ......... ..:... ...

HaO.. .....:.. ......

TiOi........... ... .... .

COa.... ............. ...

PtOi. ............ ......

803 . ...................

¥............ ....

C}.. ...................

South 

Carolina 

land 

rock °

13.03

.40 

1.38 

Trace. 

39.10

2.43

3.05 

27.23 

1.45

5. 68 

5.25

99.00

Florida

Hard 

rock 6

3.44 

1.49 

1.43 

.23 

48.81 

Trace. 

Trace. 

.90 

1.98 

.13 

2.71 

35.95 

.10 

2.55

99.70

Land 

pebble c

4.34

2.53 

1.35 

.21 

47.95

.63 

3.15

2.19 

34.72

2.72

99.79

Tennessee

Brown rock <*

32.39

Blue 

rock 

30.30

Western field /

2.62 

.46 

.97 

.40 

.35 

48.91 

.97 

.34 

1.02 

1.34 

None. 

2.42 

33.61 

2.16 

.40 

Trace. 

Not determined.

98.97

« U. S. Oeol. Survey Bull. 580, p. 196, analysis 4. 

» U. S. Geol. Survey Bull. 604, p. 72, analysis 6.

 U. S. Oeol. Survey Bull. 604, p. 80, analysis 38.

d U. S. Bur. Soils Bull. 81, p. 9, average of 6 analyses.

 U. S. Bur. Soils Bull. 81, average of 8 analyses. 

/ U. S. Qeol. Survey Bull. 430, p. 465, analysis 4.

50 La Croix, A., Sur la constitution mine'ralogique des phosphorites franchises: Compt. 

Rend., vol. 150, p. 1213, 1910; Mingralogie de la France, vol. 4, pt. 2, p. 555, 1910. See 

also Rogers,, A. F., Nature and origin of the " brown rock " phosphorites of Tennessee 

[abstract] : Geol. Soc. America Bull., vol. 39, pp. 279-280, 1928.
94

Among the interesting comparisons that may be drawn from the 

above table are the relatively low amounts of silica and insoluble 

material and of iron and alumina in the western rock as compared 

with Florida and South Carolina rock. Lime and phosphoric acid 

are comparable in all except the South Carolina rock. Fluorine is 

present in both Florida and western rock, but it is less conspicuous in 

the latter.

PHOSPHATE RESERVES OF THE UNITED STATES

s.

The Government now has two extensive phosphate reserves, one in 

Florida and the other in the Western States.

FLORIDA

The first phosphate Avithdrawal in Florida, which included 2,400 

acres, was made on July 2, 1910. Further withdrawals, Avith some 

subsequent restorations, have since been made, and the total lands 

withdrawn on November 1, 1928, comprised 67,076 acres, in parcels 

ranging in size from a fractional subdivision of less than 40 acres to 

as much as 400 acres and distributed in approximately 295 townships 

in about 31 counties, extending from Apalachicola River at the northwest as far south as Fort Myers. The belt of country thus included 

ranges from 10 to nearly 100 miles in width. Only 120 acres in 

Florida has thus far been classified as phosphate land. -

WESTERN STATES

Since December 9, 1908, the Government has maintained a phosphate reserve in a group of Western States. The great bulk of the 

phosphate-bearing area in these States is public land, though some 

has passed into private ownership. The acreage of the reserve has 

changed from time to time in accordance with the results of geologic 

examinations of the withdrawn lands, but the total has remained 

large.

Oiitstandmff ivithdrawn pliospjiate lands in Western States, Nov. 1, 1.92S

Acres

Idaho_______________________________ 391,532 

Montana_______________________________ 279,944 

Utah _______________________________ 301,945

Wyoming______________________________ 989,289

1, 962, 710

In addition to this land, areas amounting to 160 acres in Utah, 

268,299 acres in Idaho, 25,293 acres in Wyoming, and 3,833 acres in 

Montana, or 297,585 acres in all, have been examined in detail and 

formally classified as phosphate land. The total classified and 

withdrawn lands thus amount to 2.260,295 acres. Not all this terri-
MINERAL RESOURCES 95

tory contains high-grade rock, but the estimates given below, which 

are conservative and incomplete, show that a vast tonnage of highgrade rock may be expected.

Under the act of July IT, 1914, agricultural entries may be made 

upon withdrawn phosphate lands, but the mineral rights are reserved to the United States.

REVISED ESTIMATES OF TONNAGE

Tonnage estimates of phosphate in the United States have been 

given in previous reports. 51 There has been some change in the status 

of the acreage reserved since the publication of the report cited, 

and the tonnage figures need to be increased by the additional tonnage in western lands classified early in 1925 and decreased by the 

quantity of rock mined in the several fields in 1925 and 1926 (figures 

for phosphate rock mined in 1927 not available). The corrected 

figures are given in the table below.

Estimate of phosphate rock in the United States available December 31, 1926,

in long tons

State Quantity

20, 000, 000

288, 000, 000

878, 000

8, 786, 000

83, 018, 000

400, 682, 000

State

Utah...............................

Quantity

4, 997, 809, 000

391, 323, 000

326, 745, 000

115, 754, 000

6, 232, 313, 000

The Government-owned deposits are available for lease under the 

phosphate provisions of the- general leasing law (act of February 

25, 1920.; Public. No. 146). (See General Land Office Circular 696.)

Idaho's position among the phosphate-bearing States is preeminent, 

but her phosphate resources have as yet hardly been scratched

FURTHER NOTES ON PHOSPHATE

The annual chapters on phosphate in Mineral Resources of the 

United States contain many items of interest to phosphate producers. 

For example, those for 1922-1924 contain, in addition to statistical 

matter, bibliographies, lists and brief descriptions of patents, accounts 

of Government investigations, and of commercial processes for 

treating phosphate rock, together with current notes on foreign 

phosphate deposits.

51 See especially U. S. Bur. Mines Mineral Resources for 1924, pt. 2, pp. 87-88, 1925.
96 POET1STEUF QUADRANGLE, IDAHO

LIMESTONE

Limestone is one of the most abundant and widely distributed 

mineral resources of the Portneuf quadrangle. Its formation was 

recurrent at intervals from the Cambrian to the Quaternary. There 

are many textural varieties, and they range in composition from 

rocks that might be classed as calcareous shale and sandstone to nearly 

pure limestone. Some beds are well crystallized, but there are no 

rocks that could be classed as marble.

Earlier Paleozoic limestones. The exposed earlier Paleozoic limestones are confined to the southwestern part of the quadrangle south 

of Topons Creek, where they occupy foothills of the Portneuf 

Mountains, and the southwest flank of the Chesterfield Range between Little Flat Canyon and Tenmile Pass. They occur also in 

the Soda Springs Hills southeast of Tenm,ile Pass. These earlier 

limestones are more or less magnesian, and some of them have been 

called dolomites. Such are the Fish Haven and Laketown dolomites 

(pp. 20 and 21), each of which, at localities in the Randolph quadrangle, Utah, contains, according to analyses by W. C. Wheeler, 

formerly of the Geological Survey, 21.3 per cent magnesia (MgO). 

The Jefferson limestone, with 19.2 per cent of MgO, is hardly less 

magnesian.52

Limestone probably to be assigned to the early Paleozo.ic but rated 

as " high-calcium " was burned prior to 1920 at Pebble, Bannock 

County, just west of the quadrangle, by the White Elk Lime Co., of 

Pocatello. Since that date small quantities of limestone from the 

same property have been sold to sugar factories.

Carboniferous limestones. The Carboniferous limestones are better adapted for general purposes than most of the others. The 

Madison and Brazer limestones and the lower part of the Wells format,ion contain many beds that appear to be relatively pure limestone. The distribution of these formations in the Portneuf quadrangle may be seen by reference to Plate 1. On the whole the Brazer 

beds appear to be more nearly pure than the others, though this formation includes beds of sandstone and possibly shale in addition to 

limestone. The Brazer limestone is the source of the lime formerly 

burned at the limekilns at Montpel,ier and is locally reported to yield 

a fine product. An analysis made in the Geological Survey laboratory of an average sampjle from the Montpelier quarry is given in 

the following table. The sample represents the 20 feet of thickbedded, broken, and calcite-seamed rock exposed along the quarry 

wall.

62 Analyses cited by Richardson, G. B., Am. Jour. Sci., 4th ser., vol. 36, pp. 406-415, 

1913.
MINERAL RESOURCES 97

Analysis of Wmesione from quairy at Hontpelier, T. 13 S., R. 44 E., Idalw

[W. C. Wheeler, analyst]

Silica (SiO.)____________________________ 2.55 

Alumina (Al2 0s)_____________________-___ .43 

Iron oxide (Fe2 03, total)________ __ .44 

Magnesia (MgO)________________ _ 1.35 

Lime (CaO)____________________________ 51.96 

Carbon dioxide (COs)_______________________ 41.08 

Loss on ignition less C02 ________________ 1. 80

99. 61

As ,this limestone contains 93 per cent of calcium carbonate the 

product obtained by proper burning would be classed as high-calcium 

lime. The ledges of Brazer limestone northeast of Monroe Canyon 

in T. 7 S., R. 40 E., shown in Plates 3, A, and 5, JL, are very striking 

topographic features and are readily accessible from that canyon.

Triassio limestones. The upper beds of the Woodside shale and 

the lower beds of the Thaynes group locally consist of relatively pure 

limestone in quantity apparently sufficient for commercial purposes. 

The localities where these rocks occur, however, are relatively remote 

from railway transportation and from settlements, though they are 

easily accessible by roads. No tests or analyses of these limestones 

have yet been made.

The Por.tneuf limestone of the Thaynes group has massive beds 

that are exposed in T. 5 S., Rs. 39 and 40 E., and T. 6 S., R. 40 E., but 

these are probably too siliceous to have commercial value.

Tertiatfy limestones. The Salt Lake formation contains marls and 

limestones whose commercial value has not been investigated, but 

some of them might possibly be utilized. Beds of .these rocks occur 

at the narrows of Portneuf River in sees. 15 and 22, T. 5 S., R. 38 E., 

also in sees. 19 and 30, T. 6 S., R. 40 E., and sec. 10, T. 7 S., R. 40 E. 

(See also sec. 13, T. 6 S., R. 39 E.)

Quaternary limestones. The deposits of travertine described on 

page 37 are also possible sources of commercial limestone. An 

analysis of such material from Steamboat Spring, on Bear River near 

Soda Springs, made years ago and cited in Fremont's report,50 shows 

92.55 per cent of carbonate of calcium. This figure suggests that the 

travertine could supply large quantities of high-grade lime.

ROAD METAL

Supplies of rock suitable for road metal are very abundant in the 

Portneuf quadrangle, but comparatively little use of them has yet 

been made. Doubtless as funds become available the standards of

63 Fremont, 3. C., Narrative of an exploring expedition to the Rocky Mountains in the 

year 1842 and to Oregon and northern California in the years 1843-44, p. 131. London, 

1846.
98 PORTNEUF QUADRANGLE, IDAHO

road construction there will be raised. The improvement of the State 

highway eastward from Pocatello was in progress in 1925 but had 

not then reached the portion that crosses the southwest corner of the 

quadrangle. The more accessible materials suitable for road construction are Paleozoic limestone, quartzite, and basalt, all of which 

are desirable but require crushing. The Rex chert member of the 

Phosphoria formation, though less accessible, would also serve the 

purpose well. The weak material from the Thaynes and Woodside 

formations, which is locally used in many parts of the general 

region, is available in the northeastern part of the quadrangle. It is, 

however, better adapted for patching and temporary repairs than 

for permanent road construction.

BUILDING STONE

None of the rocks exposed in the Portneuf quadrangle have been 

used to any extent for building, nevertheless some of them may in 

time prove useful for that purpose. For example, the Brazer limestone contains thick beds of uniform character, relatively free from 

chert and other impurities, that might be worked into blocks suitable 

for building. (See pi. 5, A.) The rhyolite of the northeastern 

part of the quadrangle is also suggested as a possibility, though it 

has not been examined with this use in mind. This rock has been 

used for building in Pocatello, Blackfoot, Idaho Falls, and Rexburg. 

Basalt has been used in constructing a few buildings. Thus Loughlin 54 notes that

The Zion Cooperative Mercantile Institution building at Idaho Falls, built in 

188-1, has a front entirely of basalt in dressed blocks, with both rock face and 

finely tooled surface; some blocks are 6 feet long. The stone is of nearly black

color and contains several small to large irregularly spaced vesicles. It gives 

a somber appearance but shows no weathering effects after 30 years of exposure, and door sills where loaded trucks have been passing back and forth 

'have not suffered any noticeable abrasion. Basalt has also been used, and 

with good effect, in the base of the Methodist Episcopal church at Levviston, 

erected in 1907. Here the blocks are mostly small, with rock face exposed.

QUARTZITE

Iii the southwest corner of the quadrangle there are hills of Swan 

Peak quartzite favorably situated with respect to transportation 

both by highway and by railroad. In Bear Lake County this rock 

has been quarried in Worm Canyon, west of Bear Lake, for road 

metal. The Swan Peak quartzite is ordinarily rather pure and vitreous and should prove a valuable resource. Pure quartzite may be 

crushed or ground and used in a variety of ways,55 such as the manu54 LoughJin, G. F., U. S. Geol. Survey Mineral Resources for 1913, pt. 2, p. 1370, 1914.

55 See chapters on silica in Mineral Resources.
MINERAL RESOURCES 99

facture of pottery, paints, and scouring soaps, as a road filler, as a 

polisher, as a scouring agent, for " frosting " glass, for chemical and 

metallurgical use, and for other purposes.

WATER RESOURCES 

SURFACE WATER

The surface water of the Portneuf quadrangle is practically all 

gathered in two streams, Blackfoot and Portneuf Rivers, botli of 

which are tributary to Snake -River. A little water is contributed 

to the Bear Kiver drainage system by Tenmile Creek, but most of this 

sinks beneath the basalt at or near Tenmile Pass and needs no further 

consideration here. The Blackfoot and the Portneuf have for a 

number of years been systematically measured by the Geological

Survey in cooperation with the State of Idaho, and four gaging stations were formerly maintained on Portneuf River and Topons Creek 

within the quadrangle. These have been discontinued. The nearest 

active station on the Blackfoot is at Rocky Ford crossing, in sec. 11, 

T. 5 S., R. 40 K, about 200 feet below the wagon bridge below Blackfoot Dam and about 1% miles east of the northeast corner of the 

quadrangle. The nearest active station on the Portneuf is at the 

Oregon Short Line Railroad bridge in sec. 23, T. 9 S., R. 37 E., a 

quarter of a mile west of Topaz station and 6 miles southwest of 

McCammon.

Blackfoot River. Records at the Rocky Ford station have been 

obtained continuously since July 15, 1908. Those for the 10-year 

period 1914-1924 are summarized in the following table. 50 For details the reader is referred to the reports cited.

Maximum, minimum, and mean annual discharge and animal run-off of Blackfoot Rivei', Rocky Fora station, years ending September SO, 191^ to 1924

Year

1914.............

1915.............

1916.............

19)7.............

19)8.............

1919.............

Discharge (second-feet)

Maximum

1,060 

928 

799 

750

Minimum

29 

44 

35 

3 

23 

17

Mean

299 

408 

290 

247 

259 

251

Total 

run-off 

(acrefeet)

216, 000 

295, 000 

210, 000 

179, 000 

188,000 

182, 000

Year

1920... ..........

1921.............

1922.............

1923.............

1924.............

Discharge (second-feet)

Maximum

1,120 

1,190 

1,010 

1,010 

936

Minimum

8.2 

9 

17 

22 

6

Mean

201 

220 

360 

308 

193

Total 

run-off 

(acre 

feet)

146, 000 

160, 000 

261,000 

223, 000 

140, 000

The maximum recorded daily discharge during this period Avas 

1,190 second-feet, in May, 1921, and the minimum was 3 second-feet, 

in May, 1917. The flow of the river is regulated by the Blackloot 

River Reservoir, in the adjacent Henry and Cranes Flat quadrangles.

511 Compiled from U. S. Geol. Survey Water-Supply Papers 39?., 4.13, 443, 463, 483, 513, 

and 533, and £rom Idaho Dept. Reclamation First, Second, and Third Bienu. Repts.
100 PORTNEUF QUADRANGLE, IDAHO

The Blackfoot receives only one unnamed tributary of small volume 

between the reservoir and the place where it enters the Portneuf 

quadrangle. Within the quadrangle it is joined by Corral and Nigger Creeks. These streams have not been measured, but their combined flow would total perhaps 25 to 50 second-feet, making the 

mean annual discharge of the river as it leaves this quadrangle 

probably about 300 second-feet.

Portneuf River. The principal stream of the quadrangle is Portneuf River, which enters at the northwest corner, traverses the 

western part, and leaves at the southwest corner. Its waters are impounded in Portneuf Reservoir. (See p. 105.) Three gaging stations 

were maintained for a time on Portneuf River within the quadrangle 

and one on Topons Creek, a tributary, a short distance west of the 

quadrangle. The records of these stations are summarized below, and 

in addition the record of the station at Topaz previously mentioned 

is given.57

The first station was on Portneuf River at Faulkner's ranch, just 

above the ford near the ranch house, about 7 miles northwest of 

Chesterfield. Incomplete measurements are available from April 28, 

1912, to September 30,1913.

Annual discharge of Portneuf River above reservoir northwest of Chesterfield,

Idaho

Period

1912 (Apr. 28 to Sept. 30).. ..........................

Oct. 1, 1912 to Sept. 30, 1913.... _. ._.... ......

Discharge (second-feet)

Maximum

66 

131

Minimum

10

Mean

24.7 

17.7

Total 

run-off 

(acre-feet)

7,510 

12,800

The second station was in sec. 30, T. 6 S.. R. 39 E., about a quarter 

of a mile below the dam, where a small flume crosses the river, and 

about 2y2 miles from Chesterfield post office. The flow as measured 

is somewhat greater than that of the river itself because some of 

the water from Topons Creek is diverted into the reservoir. The 

results of measurements for the years 1912 to 1915 are summarized 

in the following table:

Annual discharge of Portneuf River below the reservoir near Chesterfield, 

Idaho, for the years ending September 30, IV12 to 1915

Period

1912 (May 23 to Sept. 30)...............................

1913............. ....................................

1914..-....... .............................. .......

1915 (station discontinued Apr. 9).... _ _ _ ....... .

Discharge (second-feet)

Maximum

44 

46 

81 

7.1

Minimum

2.6 

3.3

1.5

Mean

12.0 

8.99 

13.35 

4.32

Total 

run- off 

(acre-feet)

2,390 

6,500 

9,311 

1,650

See papers cited in footnote 56; also Water-Supply Papers 292, 312, 332, and 362-B.
MINERAL RESOURCES 101

The station on Topons Creek was situated in sec. 34, T. 6 S., R. 38 

E., at the Butterfield ranch, about half a mile below the head of the 

diversion canal into Portneuf Reservoir and about 7 miles west of 

Chesterfield. The record is summarized in the next table.

Discharge of Topons Creek west of Chesterfield^ Idalho, for the yews ending 

September 30, '1912 to 1914

Period

1912 (May to Sept. 30)... ............................ ..

1913................. .. ....-.. .................

Discharge (second-feet)

Maximum

355 

162 

220

Minimum

12

Mean

96.3 

30.5 

41.7

Total 

run-ofl 

(acre-feet)

29,200 

22, 000 

30, 100

The third station on the Portneuf was situated at W. A. Gamble's 

ranch, in sec. 26, T. 7 S., E. 38 E., about 3 miles north of Pebble post 

office and was established September 8, 1910. Its record is summarized as follows:

Annual discharge of Portneuf River near Pebble, Idaho, for the years ending 

September 80, 1910 to 1913

Period

1910 (Sept. 8-30)....... ............. .. .. . 

1911...................... _ . _ .......................

1912................................................ .

Discharge (second-feet)

Maximum

66 

624 

365 

537

Minimum

39 

32 

39 

34

Mean

47.6 

98.6 

93.6 

89.6

Total 

run-off 

(acre-feet)

10, 970 

71, 300 

68, 000 

57,800

The location of the station at Topaz is given on page 99. Its record 

is summarized in the following table:

Annual discharge of Portneuf River at Topaz, Idaho, for the years ending 

September 30, 1913 and 1914, and 1920 to 1924

Period

1913 (Jan. 12 to Sept. 30).... ...........................

1914....................................................

1919-20 (July 20, 1919, to Sept. 30, 1920).. .... .. 

1921....................................................

1922....................................................

1923 (Apr. 1 to Sept. 30)................................

1924....................................................

Discharge (second-feet)

Maximum

902 

770 

853 

771 

822 

562 

605

Minimum

128 

163 

152 

160 

168 

190 

121

Mean

233 

292 

227 

279 

260 

310 

205

Total 

run-off 

(acre-feet)

123, 000 

211,000 

165, 000 

202, 000 

188, 000 

112,000 

149, 000

The records of flow of Portneuf River differ in completeness and 

in accuracy. From the records given in the reports cited it appears 

that the Portneuf enters the quadrangle as a small stream with a 

mean annual discharge of about 20 second-feet. At the reservoir
102 PORTNEUF QUADRANGLE, IDAHO

there is some gain from waters diverted from Topons Creek, but 

there are Josses into irrigation ditches just above and below the dam, 

so that the resulting flow is somewhat less than the volume of the 

river above the reservoir. Additional water enters below the dam, 

however, from Topons Creek, which is a tributary twice or three 

times the volume of the upper Portneuf, and possibly some water 

enters from Smith and King Creeks, but these streams together with 

Topons Creek are largely diverted for irrigation. Farther south 

the flow of the river is considerably augmented by springs. (See pp. 

5,12, and 103.) Thus at the station near Pebble its volume is four or 

five times as large as where it enters the quadrangle. Beyond the 

quadrangle the river receives further contributions from tributaries 

and springs, and there are few significant diversions until it reaches 

the vicinity of Topaz, where several large canals receive water from 

it. At this station the river has been increased in volume ten to 

fifteen times and is nearly as large as the Blackfoot at Rocky Ford 

station.

GROUND WATER 

GENERAL DISTRIBUTION AND AVAILABILITY

No special study of the ground-water conditions in the quadrangle 

was made, but the supply is believed to be Avell distributed and probably sufficient to meet the domestic needs of a considerably greater 

number of residents than are now living there.

In the mountainous parts of the area the rocks are folded and 

faulted, and the ground-water conditions are correspondingly complex. The water-bearing beds are fractured or dislocated so that the 

water is returned to the surface at many places in the form of

springs. The location of springs with regard to lines of faulting 

is in some places very striking, as, for example, the spring in 

the East Fork of Twentyfourmile Creek or possibly the springs 

that built the travertine deposits at the mouths of Little Flat and 

Eighteemnile Canyons. Elsewhere the connection of springs with 

structure lines is not so obvious and their occurrence may be due to 

other causes. The relatively large number of permanent streams 

within the mountains bears witness to the presence and wide distribution of ground water there.

The valleys of Blackfoot River and Corral Creek at the northeast are underlain by basalt and have as yet no wells within the limits 

of the quadrangle. The position of the water table is not known, 

but the course of Corral Creek suggests that it may be relatively 

shallow near the bases of the hills and may slope gently northeastward to a maximum depth indicated by the surface of Blackfoot 

River. Thus the water table might range in depth from 25 to 150
MINERAL RESOURCES 103

feet. These figures are comparable to those of many wells in Tps. 

7 and 8 S., Rs. 41 and 42 E., records of which are available at the 

Geological Survey. 38

The basalt-covered part of Portneuf Valley is in large measure 

dependent upon ground water for its supplies both for domestic use 

and on the farm. So far as known the supplies available for these 

purposes are adequate. A considerable number of wells are in use, 

but their logs are not available for this report. In the area mapped 

as alluvium the water table is relatively near the surface. This area 

consists almost entirely of meadow land, considerable parts of which

are marshy.

SPRINGS

General character and distribution, The springs of the Portneuf

quadrangle are widely distributed and as a rule are not characterized 

by unusual thermal conditions or mineral content. A few are of sufficient size to give rise at once to considerable streams. Among these 

are the Dairy Springs, in the SW. ty SE. % sec. 31, T. 5 S., R. 40 E., 

tributary to Grizzly Creek, the spring in the center of the SW. % sec. 

10, T. 6 S., R. 39 E., which supplies much of the flow of TAventyfourmile Creek, and the springs along Portneuf River southAvestward from 

sees. 19 and 20, T. 7 S., R. 39 E., Avhich greatly increase the flow 

of that stream. These springs have not been measured to determine 

their discharge, but probably none of them, except possibly some of 

those along the Portneuf, floAv more than 20 second-feet.

Mineral springs. A feAv of the springs of the quadrangle should 

be classified as mineral springs because of the deposits of calcium 

carbonate that they have made in the past and are to a certain extent 

still making. The most conspicuous accumulations of traA^ertine, 

composed chiefly of calcium carbonate, are those at the mouths of 

Eighteenmile and Little Flat Canyons, described on page 9. These 

are not noAv identified Avith any active spring. On the other hand, the 

travertine area near the forks of Twentyfourmile Creek is probably 

still in the process of deposition. The main source of the water 

is the spring in sec. 10, T. 6 S., R. 39 E. The travertine area begins 

a short distance beloAV this spring, and the Avaters are divided by the 

travertine into interlacing streams. The proximity of this spring 

to the place Avhere the arched plane of the Bannock overthrust crosses 

the canyon points to a probable causal relation betAveen the fault 

and the spring. In the NE. 1/4 sec. 16 another group of springs issues 

from the hillside and by interlacing streams contributes both to the 

accumulation of the travertine and to the floAv of the creek. No fault 

has been recognized here, but the spring Avater issues from beds of 

the Salt Lake formation, which conceal the underlying structure.

58 Unpublished report by I-I. T. Stearns.
104 POBTNEUF QUADRANGLE, IDAHO

An interesting and somewhat conspicuous spring is situated in the 

SE. 14 NW. % sec. 15, T. 5 S., E. 40 E., on the north side of Blackfoot 

River. (See pi. 7, A.) This spring has built a cone about 12 feet 

high and 75 feet in diameter. It has a crater about 13 feet in diameter 

and 33 feet deep as determined by sounding. The water, which fills ' 

the crater, is carbonated and foul from decaying algae. Its temperature is cool but was not measured.

Thermal springs. No sharp line can be drawn between mineral 

and thermal springs, for those described as mineral may once have 

been thermal and those described as thermal have usually some 

readily recognizable mineral constituents. It is convenient, however, 

to distinguish as thermal those springs which still have water with ; 

temperature noticeably warm or higher than that of most of the .: 

springs of the surrounding country. Possibly the spring in sec. 15, 

T. 5 S-, E. 40 E., just described, might better have been included 

here. There is, however, one distinctly thermal spring in the quad- ; 

rangle that deserves mention. This is commonly known as Eichey's ; 

Hot Spring and is situated on the north side of Portneuf Eiver near 

the center of sec. 34, T. 7 S., E. 38 E., at the contact of the lime- ,\ 

stone and the basalt. The water feels comfortably warm and is ;\ 

carbonated but does not contain noticeable amounts of sulphur or 4 

iron. No samples were collected for analysis. The spring has been ^ 

walled in to form a pool a few feet across. Diagonally across the 

river to the southeast is a spring of clear cold water issuing from the *" 

basalt. The two springs, though relatively near each other, are * 

obviously supplied by different sources, the thermal spring doubtless 

representing some deep fracture or possible fault not otherwise 

recognized.

UTILIZATION OF WATER 

DOMESTIC USE

Settlers along the foothills or in the alluvial parts of Portneuf 

Valley obtain their, water for domestic use from surface bodies or 

shallow wells. In the basalt-covered area dependence must be placed 

on wells presumably somewhat deeper. So far as known the supply 

of ground water for this purpose is adequate.

IRRIGATION

Practically all the streams that enter Portneuf Valley are utilized 

extensively for irrigation. Their diversion provides sufficient water 

for irrigating most of the lands lying immediately along the foot 

slopes of the hills, but the supply is inadequate for large areas 

throughout the valley. Under these conditions much of the land in 

Portneuf Valley and southward is at present suitable only for dry
MINERAL RESOURCES 105

farms. Two irrigation projects have been initiated, one of which has 

been operative for many years but affords little relief for lands 

within the Portneuf quadrangle; the other is as yet undeveloped. 

These are respectively the works of the Portneuf-Marsh Valley Irrigation Co. and the project of the Empire Irrigation District.

Portneuf-Marsh Valley works. According to unpublished information available in the files of the Geological Survey, a mutual 

association known as the Portneuf-Marsh Valley irrigation Co. was 

organized June 22, 1908, to build a reservoir and canal system to 

irrigate land in the vicinity of Downey, Idaho. Except minor lateral 

canals the projected works were practically completed March 1, 

1912, and on March 20, 1915, the system was turned over to the 

Portneuf-Marsh Canal Co. for operation. The area irrigable under 

the project includes a total of 15,600 acres, of which more than 11,000 

acres was patented to the State under the Carey Act on March 29, 

1918.

The reservoir included in this project, part of an intake canal 

from Topons Creek, and part of the outlet canal are shown on 

Plate 1. The reservoir is created by a dam 50 feet high across Portneuf Eiver in sec. 24, T. 6 S., R. 38 E., and sec. 19, T. 6 S., R. 39 E. 

It has a storage capacity of 28,000 acre-feet and a surface area at 

high-water line of 1,500 acres. It is 35 miles above the point 

of diversion of the company^ main canal measured along the general 

course of the valley. Many small ditches divert water from the 

river below the reservoir, but the larger canals are located between 

McCammon and Dempsey. The discharge at Topaz, 1*4 miles above 

the diversion canal of the company, represents the water available 

for this company and for the other companies below.

Empire Irrigation District. The Empire Irrigation District was 

organized about 1923 with headquarters at Bancroft, Idaho, for the 

purpose of irrigating between 35,000 and 55,000 acres of land in 

Tps. 6, 7, and 8 S., Rs. 38, 39, and 40 E. It is proposed to divert 

water from Biackfoot River by means of a tunnel 3 ;820 feet long 

near Morgan Bridge, in sec. 19, T. 4 S., R. 39 E., in the Paradise 

Valley quadrangle, and to lead it by canal southward along the 

east side of Portneuf Valley and adjoining territory to a point in 

sec. 30, T. 9 S., R. 40 E., a distance of about 32 miles in a direct 

line south-southeast from the diversion point but several times 

greater if measured along the right of way of the proposed canal. 

The project also includes the diversion of water from Biackfoot 

River in sec. 14, T. 7 S., R. 42 E., and the use of reservoir sites in 

the Fivemile Meadow district in T. 8 S., Rs. 41 and 42 E., for the 

irrigation of additional lands in Tps. 8 and 9 S., Rs. 41 and 42 E. 

The surplus waters of the eastern areas are to be available for the 

western areas by a connecting system of ditches.
106 PORTNEUF QUADRANGLE, IDAHO

As the water of the Blackfoot is practically all appropriated, 

its use by the Empire Irrigation District is contingent upon the 

purchase by that district of 110.000 acre-feet of storage in the reservoir at American Falls constructed by the United States Bureau 

of Reclamation. This storage supply will then be exchanged for 

an equal quantity of storage in Jackson Lake Reservoir, which 

will be utilized to obtain the release of Blackfoot River water for 

the Empire district. Plans and survej^s had been completed, rights 

of way obtained, and an allotment of the required storage capacity 

in the American Falls Reservoir had been obtained, but at the time 

of writing (April, 1926) the financing necessary for the purchase 

of this desired storage had not been arranged. The location of the 

right of way for the canal in Portneuf Valley within the limits 

of the quadrangle is shown on Plate 1.

The soil is good, consisting in part of volcanic ash and in part of 

alluvium. The growing season, however, is relatively short, the average being only about 65 days, as indicated by records of the United 

States Weather Bureau at Chesterfield. The mean annual rainfall 

there is 13.74 inches (p. 12). These meteorologic conditions indicate 

that dry-farm crops can be raised without irrigation during years 

when the growing season is unusually long. The section is fairly 

well settled, and the chief crops produced are hay and grain. With 

an adequate water supply for irrigation, sugar beets probably could, 

be grown successfully in most years.

WATER POWER

Blackfoot River. Power-site reserves have been located alongBlackfoot River in T. 5 S., K. 40 K, partly in the Portneuf quadrangle. This section of the river lies in a canyon which has a lowgradient 59 and other physical features unfavorable to the development of water power. Furthermore, the operation of the Blackfoot 

Reservoir immediately above causes abrupt changes in the stream 

flow. For about 10 per cent of the time the flow is 15 second-feet 

or less, and the 10-year average for the months November to March, 

inclusive, a period during which under present market conditions 

the .demand for power is high, was only 85 second-feet. These 

conditions make the potential power of the Blackfoot in this quadrangle of little commercial importance. The same is true of the 

section farther downstream, where other physical conditions of the 

river are materially more favorable to power development.

The lands suitable for power sites have been reserved for the purpose of maintaining the interest of the public in the development 

of water power, and the Federal Power Commission is authorized

60 Unpublished reports of Ralf R. Woolley and W. G. I-Ioyt.
WATER RESOURCES 107

to issue licenses for such development on terms designed to protect 

the public interests.

Portneuf River. As stated by Heroy,00 Portneuf River in the 

upper part of its course flows through a broad marshy valley with 

slight fall. The discharge is small, and no feasible power sites are 

recognized within the limits of the Portneuf quadrangle. After 

leaving the Portneuf quadrangle th© river has, in many places, a 

steady gradient and affords opportunity for the development of a 

small amount of power in the section between Pebble and Pocatello. 

West of Pocatello the river has only a slight fall. The normal flow 

during the irrigation season is largely diverted, and as the Portneuf 

Reservoir of the Portneuf-Marsh Valley Irrigation Co. stores much 

of the winter flow, the possibility of developing power on the lower 

portion of Portneuf River is unattractive.

00 Heroy, W. B., Water resources of the Fort Hall Indian Reservation: U. S. Geol. Survey Bull. 713, p. 148, 1920.

6148 29 8

INDEX

Page. 

Alluvium, occurrence and features of........ 39

Arkansas, tonnage of phosphate rock in..... 95

Astorians, Portneuf River Valley traversed

by.............................. 5

Bannock overthrust, description of.......... 55-59

Blackfoot Mountains, features of...._..... 8

Blackfoot River, canyon of, plate showing... 26 

power sites on........_.__....._ 106-107

tributaries and flow of__..._....._ 99-100

Blackfoot Valley, features of................. 8

Blacksmith limestone, occurrence and, features of................_....... 17

Bloomington formation, occurrence and features of..............._....... 17

Bonneville, Capt. B. L. E., winter camp of.. 3,5

Brazer limestone, occurrence and features of. 25-26 

plate showing......_...__.__..... 26

Breccia northeast of Chesterfield, nature of. 35-36

Brigham quartzite, occurrence and features

of............................... 16

Building stone, occurrence and use of........ 98

Bureau of Mines, information on production

of phosphates issued by......... 91,95

Bureau of Soils, experiments by, on the extraction of phosphoric acid...... 90

Bureau of Standards, methods for analysis of

phosphate rock improved by.... 90-91

Cambrian system, formations of............. 15-19

Cenozoic era, events of.---........-.-......- 66-67

Chesterfield Range, features of...__...... 8-10

Climate of the region......._.............. 12

Corral Creek anticline, description of__.... 48

Cretaceous rocks, absence of................. 35

Drainage of the quadrangle.................. 11-12

Eighteenmile Creek, view near forks of...... 26

Empire Irrigation District project, description of......................... 105-106

Faulting, age of............................. 62-63

Faults, minor, description of................ 59-61

Field work, record of...__.............._ 3

Fish Haven dolomite, occurrence and features of....__.................. 20

Florida, phosphate lands withdrawn in...... 94

phosphate rock in, composition of....... 93-94

physical features of.................. 91-92

tonnage of............................ 95

Fold in Madison limestone, plate showing.. _ 26 

Folding, age of...............__.__.._ 61-62

Fort Hall formation, occurrence and features

of-.-.-..-...-.........-.-......- 32,33

Garden City limestone, occurrence and features of.......................... 19

Geography of the quadrangle................ 6-14

Page.

Geologic map and structure section of T. 5

S., R. 40 E................. In pocket.

Geologic map of the quadrangle........ In pocket.

Geologic structure sections in the quadrangle.........._........ In pocket.

Geological Survey, study of phosphate fields

by.............................. 89

Girty, G. H., fossils determined by....'24-25,27,29

Government withdrawals of phosphate lands,

location and extent of........... 94-95

Graves Creek syncline, description of........ 51-52

Grizzly Creek anticline, description of....... 49-51

Ground water, distribution and availability

of............................. 102-103

Hayden surveys, reconnaissance by......... 5-6

Higham grit, occurrence and features of...... 34-35

Hill wash, occurrence and features of ------ 38-39

Historical notes on the area.................. 3,5

History, geologic, outline of... ___. _..... 63-67

Hornblende andesite porphyry, nature and

age of..........._._-.-....... 40-41

Idaho, phosphate lands withdrawn in....... 94

tonnage of phosphate rock in............ 95

Igneous rocks, age of........._............. 45

nature and occurrence of_.. __........ 40-45

relations to each other and to sediments.. 44

Industries of the quadrangle....... _....... 13

Irrigation, utilization of water for.......... 104-105

Jefferson limestone, occurrence and features

of............................... 21

Jurassic rocks, absence of.___...._...... 35

Kentucky, tonnage of phosphate rock in_.. 95

Laketown dolomite", occurrence and features

of............................... 21

Land pebble, nature of...................... 91-92

Langston limestone, occurrence and features

of.............................. 16

Limestones, Carboniferous, occurrence and

composition of_................ 96-97

earlier Paleozoic, occurrence and use of.. 96 

Quaternary, occurrence and quality of... 97 

Tertiary, occurrence of.................. 97

Triassic, occurrence and quality of....... 97

Location of the quadrangle.................. 2-3

Loughlin, G. F., quoted..................... 98

Lundell, G. E. F., and Hoffman, J. I.,

quoted.......................... 90-9i

Madison limestone, fold in, plate showing... 26 

occurrence and features of............... 23-25

Mesozoic era, events of.............-.--.-..- C5-66

Mississippian rocks, occurrence and features

of..................-.-.-----..- 22-26

Monroe Canyon, section of Paleozoic bods

north of......................... 23

109
no INDEX

Page

Montana, phosphate lands withdrawn in.... 94 

tonnage of phosphate rock in...._.... 95

Nigger Creek syncline, description of..___ 48-49 

Nounan limestone, occurrence and features

of............................... 17

Olivine basalt, age and source of._........ 43

 . distribution and surface features of....... 43-44

nature and occurrence of_____... 42

Ordovician system, formations of_____. 19-20

Paleozoic era, events of.._...._______ 63-65

Peale, A. C., geologic mapping by....._.. 45-46

Pennsylvanian rocks, occurrence and features

of............................... 26-27

Permian rocks, occurrence and features of.... 27-29

Phosphate rock, depth to_..__........ 47

distribution of...____...__......... 68-69

exposure of..______________. 50 

iron and alumina in..._________. 68 

.nature of.._________________ 67-68 

relation of structure to beds of..... __... 63

vanadium in.._.____________. 68 

western, composition of____...._.. 93-94

physical features of....._____. 92-93

Phosphatic shales, prospect in, made by 

Geological Survey party, plate 

showing..___._......__.. . 26

Phosphoria formation, occurrence and features of.......................... 27-29

Physiographic development of the region.... 6-7

Pliocene (?) series, occurrence of______- 36-37 

Pbrtneuf limestone, occurrence and features

of............................... 32-33

Portneuf Mountains, features of..___.... 10-11

Portneuf River, flow of._................ 100-102

possibility of power development on..... 107

Portneuf Valley, features of....._.___... 10

Portneuf-Marsh Valley irrigation works, description of.....-..-....--..----- 105

Proterozoic era, events of.__....__.... 63

Quartzite, occurrence and use of.._____. 98-99 

Quaternary deposits, nature of..._.__.... 37-39

Rex chert, plate showing___._....- 26 

Rhyolite, nature and age of..____.._.. 41-42 

Rivers of the quadrangle_. _________ 11-12 

Road metal, occurrence of..______._ 97-98 

Roads and railroads in the quadrangle___ 13-14 

Rock Creek syncline, description of____ 53-55 

Ross, W. H., Mehring, A. L., and Jones, R.

M., quoted-....:..............., 90

Ross Fork limestone, occurrence and features

of.............'................ 31-32,33

St. Charles limestone, occurrence and features of........................... 18

Salt Lake formation, nature and occurrence

of................................ 36-37

plates showing..._____.___.__. 26 

Schultz, A. R., and Richards, R. W., reconnaissance by......_...--......- 6

work of._____.__....__.......... 46

Scope of the survey.....___..__........ 1-2

Settlements in the quadrangle...__.__... 13

Page 

Soda Springs Hills, features of.........___ 10

South Carolina, phosphate rock in, composition of.......................... 93-94

phosphate rock in, physical features of. - 92 

tonnage of.....___.......__.__ 95

Springs of the quadrangle, nature and distribution of................... 12,103-104

Stratigraphy of the quadrangle.........__ 14-45

Structure contours, method of drawing... _ 47 

Structure of the rocks....__...........__ 45-63

Swan Peak quartzite, occurrence and fea-'

tures of___..............___ 20

plate showing__..................._. 28

T. 5 S., R. 38 E., field work in............... 69-71

phosphate deposits in..............._.. 71-72

T. 5 S., R. 39 E., features of................. 73-74

phosphate deposits in................._ 75-77

structural details in................___ 74-75

T. 5 S.,R. 40 E., features of................. ' 81

phosphate deposits in_ _.........._. 81-86

structural details in_....--....-.....-.- 81

T. 6 S., R. 38 E., phosphate deposits in...... 72-73

T. 6S., R. 39 E., features of................. 77

 phosphate deposits in_...........___ 78-81

structural details in__......_____ 77-78

T. 6S., R. 40 E., features of................. 86

phosphate deposits in._...._______ 87-88

structural details in......_._.___ 86-87

T. 7 S., R. 40 E., features of................. 88

phosphate deposits in.............__ 88-89

structural features in. __._._____ 88 

Tennessee, phosphate rocks in, composition

of....... ..................... 93

phosphate rocks in, physical features of. 92 

tonnage of...______.____ 95 

Terraces at mouth of Twentyfourmile

Canyon, plate showing....___ 26

Thaynes group, formations of.......... __ 31-34

Threeforks limestone, occurrence and features

Of..... 21-22

Timothy sandstone, occurrence and features

of........ ... I......... 33-34

Topography of the quadrangle...._____ 7-11

Topons Creek, discharge of __.____ 101

Travertine, occurrence of. _____. _____ 37-38

plates showing. ______ _______ 2S

Triassic system, formations of.....___?v. 29-34

Triassic (?) formations, occurrence of. ___ 34-35 

Twentyfourmile anticline, description of.... 52-53

Utah, phosphate lands withdrawn in___ 94

tonnage of phosphate rock in... ____ 95

Ute limestone, occurrence and features of.... 16-17

Vanadium, occurrence of....__..____ 80 

Vegetation of the quadrangle..__..._ 13

Water, domestic supplies of...._.__ 104

Water power, possible sites for........ _. 106-107

Water resources of the quadrangle.___... 99-107 

Wells formation, occurrence and features of.. 26-27 

Western States, phosphate lands withdrawn

in............................... 94-95

Woodside shale, occurrence and features of.. 30-31

Wyoming, phosphate lands withdrawn in... '94

tonnage of phosphate rock in....._ . 95

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