If you have no further use for this report after it has served your purpose, please return it to 

the Geological Survey, using the official mailing label at the end.

UNITED STATES DEPARTMENT OF

IGNEOUS

MOUNT

GEOLOGICAL

GEOLOGY AND

OF THE

THE INTERIOR

STRUCTURE

TAYLOR VOLCANIC FIELD

NEW MEXICO

SURVEY PROFESSIONAL PAPER 189— B

IGNEOUS GEOLOGY AND STRUCTURE OF THE MOUNT TAYLOR VOLCANIC FIELD,

NEW MEXICO

By CHARLES B. HUNT

ABSTKACT

The Mount Tftylor volcanic field is situated in northwestern 

New Mexico and embraces a part of the south flank of the San 

Juan Basin and the adjoining part of the block-faulted Rio 

Grande Valley. Most of the volcanic field is in the Colorado 

Plateau province, but the eastern part is in the Basin and Range 

province. The boundary between the two provinces is clearly 

marked topographically and geologically. A variety of land 

forms and several different types of drainage patterns represented 

in the area are due to differences in resistance and structural 

attitude of the rocks.

The sedimentary rocks in the field consist chiefly of Upper 

Cretaceous sandstone and shale, the Santa Fe formation of late 

Miocene and Pliocene age, and some younger deposits of probable 

Pleistocene age; also small areas of the Morrison formation, of 

Upper Jurassic age. Volcanic rocks were erupted at different 

times in this area. Probably the first eruptions were those of 

the Mount Taylor volcano, in Miocene time, which began with 

rhyolitic tuffs and trachyte, continued with a series of latites, 

and concluded with porphyritic andesite. The tuffs, trachyte, 

and latites apparently came from a central pipe and crater in 

Mount Taylor, but the porphyritic andesite was erupted mostly 

from fissures radiating from the pipe. The tuff and lava erupted 

by Mount Taylor accumulated in a cone whose top has been 

considerably lowered by erosion. The cone is estimated to have 

included originally at least 12^ cubic miles of the eruptives.

The Mount Taylor eruptions were followed, probably in 

Pliocene time, by a large number of basalt sheets erupted by small 

volcanoes distributed around Mount Taylor. These sheets of 

basalt were erupted on erosion surfaces cut around the Mount 

Taylor cone. A few of the sheets preceded the latest flows of 

porphyritic andesite from Mount Taylor, and one preceded 

the rhyoliUijj- but most of them were erupted after Mount Taylor 

had become quiescent. The sheets were supplied by many small 

volcanoes, mostly of the central type, although a few fissure 

eruptions are knojyn.

Since the eruptions streams have cut downward nearly 2,000 

feet, and the lava sheets now cap high mesas. On these lavacapped mesas the volcanoes are marked by cones of lava and 

scoriae. Where the lava has been completely removed the old 

volcanoes are marked by isolated necks. The most conspicuous 

necks are composed almost entirely of basalt; others include 

basaltic breccia with the basalt, and still others are mostly 

basaltic breccia. There is practically no deformation of the 

country rock adjacent to these intrusions. The breccia is 

believed to be a nonextruded byproduct of a quiet intrusive 

process.

Folding has been the predominant type of structural deformation of the rocks in the Colorado Plateau portion of the Mount 

Taylor volcanic field, but faulting has been the predominant 

type in the Basin and Range portion. The Mount Taylor 

eruptions jferb preceded by regional northward tilting, by 

folding of 'the McCarty syncline, and by folding and associated 

faulting west of the syncline and gentle warping east of the 

syncline. Probably moist of the displacement along the faults 

150009—38

in the Basin and Range part of the field preceded the Mount 

Taylor eruptions also, but after those eruptions there was renewed 

minor folding and faulting in the McCarty syncline, minor 

faulting in the gently warped area east of the syneline, and 

continued faulting in the eastern part of the volcanic field- The 

faulting in this part of the area continued during the deposition 

of the Santa Fe formation and some deposits that are probably 

younger than the Santa Fe.

INTRODUCTION

LOCATION OF THE AREA

The Mount Taylor volcanic field, named from Mount 

Taylor, its largest volcano and most conspicuous topographic feature, includes an area of about 2,000 scuare 

miles in northwestern New Mexico midway between 

Albuquerque and Gallup. The area lies north of th« 

valley of the Rio San Jose. This valley is used by the 

Atchison, Topeka & Santa Fe Railway and United 

States Highway 66 for their approach to the Continental Divide, which crosses the northwest corner cf the 

volcanic field. (See fig. 3.)

From the village of Grant, 15 miles southwest of 

Mount Taylor, the western and southwestern parts of 

the volcanic field are easily accessible. A Forest Service road from Grant extends within 4 miles of Mount 

Taylor Peak. A graded road, not fully shown on 

plate 7, leads from Grant to San Mateo, whence other 

roads lead northward along the west side of Mesa 

Chivato and eastward onto the mesa. Laguna and the 

Laguna Indian pueblo, south of the field, are the best 

starting points for visiting the east side of Mount Taylor 

and Mesa Chivato. Stage roads lead northward as far 

as Marquez. New Mexico State Highway 6, the most 

direct road west from Albuquerque, crosses the southeast corner of the area shown on plate 7. The northern 

parts of the area can best be reached from San Yfidro, 

on State Highway 44, or from Cabezon, which is connected by a fair road with that highway.

PRESENT INVESTIGATION

The field investigation on which this report is l^sed 

was begun in 1930, with the assistance of D. A, Andrews, R. W. Brown, and W. S. Pike, Jr. In that year 

the portion of the volcanic field north and northwest of 

Mesa Chivato and also a strip about 10 miles wide along 

the east side of the mesa, including the many volcanic 

necks, were mapped, In 1931 the field investigation

51
52 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

was resumed with the assistance of H. R. Joesting, 

R. C. Becker, J. W. Wyckoff, and G. F. Taylor; the 

country along the south side of Mount Taylor and east 

of the mountain as far as the eastern border of the Upper 

Cretaceous formations was mapped.

The field investigations of 1930 and 1931 formed a 

part of ths Geological Survey's regular program of surdeterminations. During these two seasons several trips 

were made into the flanks of the Mount Taylor volcano, 

but only very general relations were noted. The numerous volcanic necks northeast of Mount Taylor were 

examined more carefully in connection with the detailed 

mapping.

Returning in 1932, I was able to spend 3 weeks

JD

= ^Sandia 

I fV!t/s / 

AMiguerque/

FIGUBE 3.—Index map of New Mexico showing the location of the Mount Taylor volcanic field (stippled) and the approximate boundaries

of the physiographic provinces.

veys for the claszification of public lands with reference 

to fuel and other mineral resources. The energies of the 

field parties were therefore confined mostly to the mapping of the sedimentary rocks. The mapping was done 

by plane-table methods on a scale of 1:62,500 and consisted of careful lateral tracing of formation boundaries 

and carrying altitudes on these horizons for structural

in studying the Mount Taylor volcano and the later 

eruptives immediately surrounding it. I iring this 

study sketch maps were made of the field relations, and 

another week was spent hi obtaining additional information on the volcanic necks east of Mesa Chivato.

The present report therefore includes the results of 

both detailed and reconnaissance studies, and in the
CONTENTS

Page

Abstract.... __________ ______-_______________________________________________-__--___----____-.-.---_.--_-------- 51

Introducticjtr^v- ___--____________________________________________________________-_-__----___------_-----__----- 51

Location of the area__-____________________________________________________________---___-----_---_---_----- 51

Present investigation---__________-_________-_________________-________________-__----__-_---------_-__---.-- 51

Previous investigations._____________________________________________________-_--_-----_-------------__----- 53

Geography— ———— ____________________________________________________-___-_--__-- — ———————— __--—— 54

Geomorphic location-____________-________________________________________.._______-----__-------------_----- 54

Land forms._______________________________________________________________________________________________ 54

Drainage _________________..___________________________________________________--_---__-------_-----_-.----- 55

Climate and vegetation______________________!_____________________________________-_-___---_-_--_----_----- 56

Summary o,f the late Tertiary and Quaternary history_____________________________________---___---------___--.----- 56

Chronology of events.--______-_________________________________________--__------_--------------~---------- 56

Age alignments.__________________________________________________________________________________________ 56

Volcanic activity_______________________________________________________________________________________________ 57

General chronology._______________________________________________________-______---____-----_----_-_----- 57

The Mount Taylor volcano______________________________________.._________________-_-_-_-----------__-----_ 58

Rhyolitic tuff.___________________________________„______________-_---_-__---_--------------- 58

Trachyte--__________.______________._______________________________________ 60

Latite series. _________________________________________________.._-_____________--_--__---__-_--_-__----- 61

Porphyritic andesite series._____________________________________--_______----------_-_----__------------ 62

Summary of Mount Taylor eruptions______________________________-_______-_____------_------------------ 63

Sheet basalt.____________.__________________________._____________________________ 64

Basalt in the amphitheater_________________________________________.-___________-__-____------_--._------ 65

Vents on the north flank of Mount Taylor______-____________________-________-___-___-_------------_----- 65

Flows on Mesa Chivato_______________________________________________________---____-_--_---_--------- 65

La Jara Mesa._____________________________________________________________---_--___---_---_--_-_----- 66

Grant Ridges._________________________________________________________________ — __ 66

Horace Mesa_________________________________________________________________---_-__--__-------------- 67

The volcanic necks____________________________________________________________---_-__--_---_-__-_----_- 67

Cabezon Peak__________________________________________________________-_--__-__----_------------- 68

Cerro de los Cuates________________________________________________________________________________ 68

Neck in Gonzales Canyon___________________________________________________________________________ 68

La Sefiora Peak-_________________________________________________________-_--_-------------__----- 69

Seboyeta Peak_.______________________________________________________________ 69

Basaltic rocks by New Mexico State Highway 6___--__-__-____-_--__-_----------_---------.------------ 70

Sierra Alesna._________________________________________________________-__-----_-----_-_--_--_----- 70

Other volcanic necks_________________________________________________-__-_--_-____-_---_----------- 70

Basaljp;i_kes-__-__ _________________________________________________________________________________________ 71

Origi^i of the breccia.______________________________________________________________________________________ 72

Summary of basalt eruptions_i_________________________________________________ __________________________ 73

Recent flows.________-_______ r ___________________________________________________-_____-_-_---_-_--------- 73

Structural geology.____________________________________________________________________________________________ 73

General structural relations of the volcanic field-_________________-______-________--_------__------------------ 73

Colorado Plateau portion of the field___-________________________________________-_------__---_-_------------- 74

McCarty syncline--_______________________________________._______________-____---_-_----___---_-_----- 74

Minor folds within McCarty syncline_-_____________________-_____________----------_--------------_----- 74

Fo^ch northwest of McCarty syncline_____________________________________--_-_----__--------_------_---_ 75

(i«ii%le warping east of McCarty syncline-___________________________.--___---_-----------------_--------- 75

Basins and Range portion of the field_______________-__________________________-_-_-_-----_-_--------_-------- 76

General structural relations_____._________________________________-____________-_--____-_---_---_--_-_-- 76

Faults_________________________________________________________________________._-______---- 76

Folds.-.._____________________________________________________---________-_ 76

Age of the deformation_-_--___________________________________-___________________-______----_-_-_---_----- 77

Possible significance of the structural features-___________________________-__-__---_-_------_------_---------— 78

Index--.._._________________________________________________________.____________ 79

in
ILLUSTRATIONS

PLATE 7. Map showing structure and reconnaissance areal geology of the volcanic rocks of the Mount

N. Mex__. __ __________________________________________________________________________________ In pocket

8. A, View of Mount Taylor; B, View looking northeast across erosion surface in Lobo Canyon________________'-_ 54

9. Map showing drainage in Mount Taylor volcanic field_-________--___-_____-_-_-__--^-_-_--------_-__----- 54

10. A, Porphyritic andesite flows near base of north flank of Mount Taylor; B, V-shaped valley cut in porphyritic

andesite. _________________________________________________________________________________________ 54

11. A, Cross section of basaltic neck exposed by erosion in Grant Ridges; B, Cliff face exposing basaltic breccia on

rhyolitic tuff near the volcanic neck________________________-_____--___________-_-_-----__---_-_--__--. 54

12. A, Basaltic lava and cinder cone on the sheet basalt capping Horace Mesa; B, View looking northeast f roar,, top

of volcanic neck east of La Abra de los Cerros; C, View looking south from Cerro de los Cuates. ___________ i 1.:-- 62

13. A, Cabezon Peak viewed from the bluffs above the village of Cabezon; B, Closer view of south side of Cafcezon

Peak showing fairly uniform vertical columns capped with a layer of scoriaceous basalt_ _________ ______^^-_'_. 62

14. A, Volcanic neck east of La Abra de los Cerros; B, Xenolith of fossiliferous Cretaceous sandstone in basaltic breccia

in neck in Gonzales Canyon_______________________________________________-____-____--____-__------- 70

15. A, B, Views of Evans Peak____________._ __ ______ _ _____________ ____ _______ __ __ _ __ ___ ___ 70

16. A, View looking southwest toward Seboyeta Peak; B, Xenoliths of sedimentary rock in breccia along east side of

Seboyeta Peak______ ____ ___ __ _____________ ___ _ __ ____ _ _ ___ _ ____________________ 70

17. A, Dike of basalt and basaltic breccia intruding Cretaceous shale; B, Banded basaltic breccia lying at outer contact 

of large breccia mass; C, D, Angular unconformity between Santa Fe formation and underlying Cretaceous 

rocks _ ____________________ ______ _______________________________________________________________ 70

18. A, Block diagram showing structural surface of part of Antonio Sedillo Grant at southwest corner of the a^e^; B, 

Block diagram showing structural surface of most of north hah* of Basin and Range part of Mount Taylor volcanic field_______ __ _____________ _ ____ _ ________________________ _ ____ _ __ _ __ -_ ___ __- 78

19. A, Flat-lying unbroken Santa Fe formation crossing a normal fault in Cretaceous shale; B, Basaltic lava in Fanta

Fe formation tilted and broken by small fault_________-._____-_________---_--_-------_-__--_------_---- 78

FIGURE 3. Index map of New Mexico showing location of Mount Taylor volcanic field- __________-______-_-___--___---_ 52

4. Diagrammatic view of part of Mount Taylor volcanic field_____________ ___________________________________ 54

5. Sketch showing topographic relations in Lobo Canyon-________-___--_--___-_----_--_---_-_--_-------.-_--- 55

6. Pedestal of alluvium about 50 feet high left standing in Chico Arroyo, 3 miles west of Cerro de los Cuates_ _ _ _ — _ _ _ 56

7. Arroyo of Rio Puerco 2 miles north of Ojito ranch______________________-_--_-__-_______---___----------- 56

8. Sequence of late Tertiary and Quaternary events in Mount Taylor volcanic field. _ ____________________________ 57

9. Hypothetical profile of Mount Taylor___-_____________-______-______--__--_---__-_---_--___------------ 58

10. Geologic sketch map of amphitheater and surrounding rim of Mount Taylor volcano_______ ____________ _______ 59

11. Sketch map showing intrusive basaltic breccia in Gonzales Canyon_________________________-______________-_ 69

12. Diagrammatic section through north side of Juan Tafoya Peak__________________-_-___-__-_-__---_---_-_-_ 71

13. Diagram of columnar jointing in intrusions 4 miles east-northeast of La Sefiora Peak. ___________________ ___ r __ 71

14. Sketch from photograph showing fault relations half a mile west of San Ysidro and 1,000 feet north of Statef ^4ghway 44 ____________________________________________________________________________________ _^._-_.,_ 77

15. Diagrammatic section illustrating faulting east of Mesa Lucero_-___________-__-___--_--_--_---_____---__->- 77

IV
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 53

descriptions an effort is made to indicate the degree of 

completeness of the observations.

PREVIOUS INVESTIGATIONS

Numerous reports contain references and descriptions 

of the Mount Taylor volcanic field. The earliest 

describe the general appearance of the area as viewed 

from the nearby routes of travel. Subsequent reports 

give fulled accounts of the geology, but most of them 

were written to emphasize some specific problem of the 

area. The reports are summarized as follows:

Simpson, J. H., Journal of a military reconnaissance from Santa 

Fe, N. Mex., to the Navajo country: 31st Gong., 1st sess., S. Ex. 

Doc. 64, pp. 56-138, 146-148, 1850. Mount Taylor is named in 

honor of th^ President, and there is brief mention of the kinds of 

rock enoouritered along the line of march.

Marcou, Jules, Re'sume of a geological reconnaissance extending * * * t0 the pueblo de los Angeles in California: U. S. 

Pacific R. R. Expl., 33d Gong., 2d sess., S. Ex. Doc. 78, vol. 3, 

pt. 4, pp. 165-171, 1856. Mount Taylor is identified as a volcanic 

cone. On the basis of fossils Jurassic and Cretaceous ages are 

assigned to the older rocks exposed in the valley south of Mount 

Taylor.

Marcou, Jules, Geology of North America, Zurich, 1858. This 

report contains the first geologic map and cross section of this 

part of the country. In the cross section Mount Taylor is shown 

as a volcano.

Newberry, J. S., Geological report, in Ives, J. C., Report upon 

the Colorado River of the West, pt. 3, pp. 65-66, 1861. Contains 

a description of Mount Taylor as viewed from the San Jose 

Valley.

Newberry, J. S., Geological report, in Macomb, J. N., Report 

of the exploring expedition from Santa Fe, N. Mex., to junction 

of the Grand and Green Rivers of the great Colorado of the West 

in 1859, pp. 9-118, 1876. Describes the appearance of Mount 

Taylor as viewed from the northeast corner of the volcanic field. 

Howell, E. E., U. S. Geog. and Geol. Surveys W. 100th Mer. 

Rept., vol. 3, pp. 227-301, 1875. Recognizes the fact that the 

eruptions of the Mount Taylor volcano were later than the folding 

of the Cretaceous strata and that the sheet basalts were later than 

Mount Taylor. Also calls attention to faulting in the eastern 

part of the area.

Dutton, C. E., Mount Taylor and the Zufii Plateau: U. S. 

Geol. Surveiy 6th Ann. Rept., pp. 105-198, 1885. The first complete desertion of the geology of the volcanic field is in this 

report. Discussion is centered around the volcanic rocks, however, with only passing mention of the sedimentary rocks. Includes a very satisfactory geologic map with topographic contours 

covering Mount Taylor and the Zufii Mountains.

Emmons, S. F., and others, Geological guidebook of the Rocky 

Mountain excursion (5th Internat. Geol. Cong., Washington, 

1891), pp. 468-471, John Wiley & Sons, 1893. Includes a description, by G. K. Gilbert, of the appearance of the Mount 

Taylor area as viewed from the Rio San Jose.

Herrick, C. L., The geology of the San Pedro and Albuquerque 

districts [N. Mex.]: Denison Univ., Sci. Lab., Bull., vol. 11, pp. 

93-116, 1898. Discusses stratigraphy of the Cretaceous strata. 

Herrick, C. L., and Johnson, D. W., The geology of the Albuquerque sheet [N. Mex.]: Denison Univ., Sci. Lab., Bull., 

vol. 11, art. 9, pp. 175-239, 1900; New Mexico Univ., Hadley 

Lab., Bull., vol. 2, pt. 1, pp. 1-67, 32 pis., with colored geologic 

map, 1900. Discusses stratigraphy and paleontology of the 

Cretaceous &Sta.

Herrick, C. L.; Reports of a geologic reconnaissance in western

Socorro and Valencia Counties, N. Mex.: Am. Geologist, vol. 

25, pp. 331-346, pis. 8-60, 1900. Discusses stratigraphy of the 

Cretaceous strata.

Johnson, D. W., Block mountains in New Mexico: Am. Geologist, vol. 31, p. 137, 1903, Briefly mentions block faulting along 

the Rio Puerco in relation to the regional structure bf the Rio 

Grande Valley at Albuquerque.

Reagan, A. B., Geology of the Jemez-Albuquerque region, N. 

Mex.: Am. Geologist, vol. 31, pp. 67-111, 7 pis., 1903; Indiana 

Acad. Sci. Proc., 1902, pp. 187-197, 1903. Briefly mentions 

Cretaceous stratigraphy of the eastern part of the area in connection with regional correlation.

Schrader, F. C., The Durango-Gallup coal field of Colorado 

and New Mexico: U. S. Geol. Survey Bull. 285, pp. 2^1-25,8, 

1906. Demonstrates the continuity of the coal field between 

Durango, Colo., and Gallup, N. Mex.

Johnson, D. W., Volcanic necks of the Mount Taylor region, 

N. Mex.: Geol. Soc. America Bull., vol. 18, pp. 303-324, 1907. 

Demonstrates that the volcanic necks east of Mesa Chivto are 

correctly interpreted as such and are not remnants of fiSws ®r 

laccoliths.

Shimer, H. W., and Blodgett, M. E., The stratigraphy of the 

Mount Taylor region, N. Mex.: Am. Jour. Sci., 4th se^., vol. 

25, pp. 53-67, 1908. Describes and discusses significance of 

fossils collected during the measurement of three seetfins of 

Cretaceous rocks in the Rio Puerco Valley.

Gardner, J. H., The coal field between Gallup and San Mateo, 

N. Mex.: U. S. Geol. Survey Bull. 341, pp. 364-378; 190P; The 

coal field between San Mateo and Cuba, N. Mex.: U. S Geol. 

Survey Bull. 381, pp. 461-473, 1910. The coal-bearing Mesaverde formation is traced from Gallup to Cuba acro^* the 

northern part of the volcanic field.

Darton, N. H., A reconnaissance of northwestern New 1-fexico 

and northern Arizona: U. S. Geol. Survey Bull. 435, p. 69. 1910. 

Gives general statements concerning the stratigraphy and structure of the eastern part of the Mount Taylor area.

Lee, W. T., Stratigraphy of the coal fields of northern central 

New Mexico: Geol. Soc. America Bull., vol. 23, pp. 62^-629, 

1912. Discusses the regional correlation of the Upper Cretaceous 

formations present in the area.

Stanton, T. W., Some variations in Upper Cretaceous stratigraphy: Washington Acad. Sci. Jour., vol. 3, pp. 60-61, 1913. 

Discusses the regional correlation of the Upper Cretaceous formations present in the area.

Lee, W. T., Relations of the Cretaceous formations to the 

Rocky Mountains in Colorado and New Mexico: U. S. Geol. 

Survey Prof. Paper 95, pp. 27-58,1916. Discusses the regional 

correlation and sedimentation of the Upper Cretaceous formations in this area.

Lee, W. T., and Knowlton, F. H., Geology and paleontology 

of the Raton Mesa and other regions in Colorado and New 

Mexico: U. S. Geol. Survey Prof. Paper 101, pp. 195-198, 1917. 

Discusses the regional correlation of the Upper Cretaceous f®rmations present in this area.

Darton, N. H., Geologic structure of parts of New Mexico: 

U. S. Geol. Survey Bull. 726, pp. 257-265, 1922; "Red Beds" 

and associated formations in New Mexico: U. S. Geol. S irvey 

Bull. 794, pp. 109-137, 1928. Describes the regional structure 

of the southern part of the area.

Bryan, Kirk, Historic evidence on changes in the channel of 

the Rio Puerco, a tributary of the Rio Grande in New Mexico: 

Jour. Geology, vol. 36, pp. 265-282, 1928. Evidence shows 

that before 1885 the valley floor of the Rio Puerco was e true 

flood plain. Between 1885 and 1890 the channel was deepened 

and widened, a process still continuing. Six small towns have 

been abandoned, and the others impoverished by the erosion,
54 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

F*K

GEOGEAPHY 

GEOMORFHIG LOCATION

The Mount Taylor field overlaps the southeast 

border of the San Juan Basra and the northwest border

of the block-faulted Rio

' '•":, 1|§ Grande Valley. (See fig. 

| s J 3.) It therefore includes 

3 g | portions of both the Colo11 "i rado Plateaus and the 

03 °. .u Basin and Range prov- 

^ ~. § inces. The boundary beJ-1 § tween these two provinces 

•§ ^ 1 is clearly marked geologio » «- cally in the area, a feature +Z rrt *" *^ '

> 8 s of some interest in view of

e] t-4 ^}

° ° g the uncertain boundary

| "11 that must generally be

P | c? drawn elsewhere in the

g 2 2 State. The northeastern 03 O IO

§ 11 part of the area adjoins

1,3 g the Nacimiento uplift of

ww| the Southern Rocky

D. I g Mountain province.

LAND FORMS

The most prominent 

topographic feature in the 

area is the mountain for 

which the field was named. 

Viewed from a distance 

^ g ^ 1 M°unt Taylor has a dis11 3 S tinctly conical form, with 

S ^_ "^ 1 extensive flat-topped foot5 i I ® hills spreading away from 

S ^ .2 1 its base. (See pi. 8, A, and 

I 111 fig. 4.) The top of the

•*i M « I cone consists of a series of 

12 '§• 11 peaks distributed along an

* »6 a arc concave to the east. 

| w 11 Mount Taylor Peak, the 

> I °. .a highest, stands on the west

-I g « ^ side of the arc and rises 

^ | s £ about 11,350 feet above sea 

o | o level, a mile higher than the 

•£•§•§ ro ro Rio San Jose 12 miles to

^ I ^ the south. Within the arc 

1 -g b- is the crater of the old vol- 

| §•! cano. The crater has been 

enlarged by erosion to form 

an amphitheater covering 

nearly 5 square miles and 

about 2,500 feet deep, 

drained eastward by Water 

Canyon Creek, which has 

cut its way through the east 

flank of the cone. In this 

report the term "Mount

H>> r . H

§rf :

P .. 1C fl

f £3

1 S 

5 O 'S 

> 03

Taylor" refers to the whole conical mountain mass and 

thus includes all the peaks in the arc, the amphitheater, 

and the rugged slopes down to the level of the foothills. 

The highest peak is called Mount Taylor Peak.

Mount Taylor is surrounded by broad, flat-topped 

foothills capped with sheet basalt. (See fig. 4.) Mesa 

Chivato, the largest, extends northeastwari for about 

25 miles from the base of Mount Taylor and at most 

places is more than 10 miles wide. From a distance 

Mesa Chivato appears as a nearly flat plateau, but closer 

views reveal scores of lava and cinder cones, some a few 

hundred feet high, marking the vents that supplied the 

basaltic lava cap. The top of the. mesa is about 2,000 

feet above the surrounding country and 3,000 feet 

below the peaks on Mount Taylor. Th<? northern, 

eastern, and western limits of Mesa Chivato are distinct geologically and topographically, but the southern 

limit will be drawn arbitrarily at Water Canyon and 

the canyon that heads southeast of San Mateo.

South of Mount Taylor flat-topped foothills capped 

with lava, mostly basalt, extend 10 miles from the base 

of the cone to the valley of the Rio San Jose, the principal stream of this part of the area. The^e foothills, 

deeply dissected by canyons, are exceedingly rough. 

San Fidel Mesa, between Water and Rinconada Canyons, 

is somewhat lower than Mesa Chivato, but Horace and 

La Jara Mesas, southwest of Mount Taylor, have about 

the same altitude as Mesa Chivato and slope gently away 

from Mount Taylor. The Grant Ridges are smaller 

isolated mesas along the northwest side of the valley 

heading northeast of Grant. Mesa Prieta, 15 miles east 

of Mesa Chivato, is another high mesa capped with 

basalt, but its top is considerably below Mesa Chivato.

The boundaries of the foothills around Mount Taylor 

are located with fair accuracy on plate 7, but no instrumental control was extended to the tops of the broad 

foothills. The topography of Mount Taylor as shown 

on plate 7 was sketched without instrumental control 

on the United States Geological Survey reconnaissance 

topographic map. The map, as thus mod;fied, represents some improvement over the earlier reconnaissance 

map but still leaves much to be desired in the way of 

precise locations and delineation of land forms.

The lavas capping the mesas are the most resistant 

rocks in the area. Erosion of the mesas har progressed 

almost entirely by removal of the relatively soft sedimentary rocks, thus undercutting the resistant lava caps.

Erosion by the Rio Puerco has produced a broad valley 

nearly 2,000 feet below the lava-capped Me^as Chivato 

and Prieta. The sedimentary rocks in the valley have 

been eroded to low relief, which emphasizes th e height of a 

dozen peaks, volcanic necks of resistant lava, some of 

which rise nearly 2,000 feet above the valley bottom. 

(See pi. 12, 0.) ^

Elsewhere in the Mount Taylor volcanic field the 

surface rocks are entirely sedimentary and consist of 

moderately resistant sandstones interbedded with 

thicker bodies of easily eroded shale. The character of
GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 8

A. VIEW OF MOUNT TAYLOR FROM A POSITION 25 MILES TO THE SOUTH. 

Photograph by W. S. Pike, Jr.

B. VIEW LOOKING NORTHEAST ACROSS EROSION SURFACE IN LORO CANYON. 

This surface, capped with a very thin mantle of gravel, truncates northeastward-dipping beds of sandstone and shale. The high peak in the distance is Mount Taylor.

GEOLOGICAL SUBVEY PEOFESSIONAL PAPEE 189 PLATE 10

A. PORPHYRITIC ANDESITE FLOWS NEAR BASE OF NORTH FLANK OF MOUNT TAYLOR. 

View looking southwest near center of south line of sec. 9, T. 12 N., R. 7 W.

B. V-SHAPED VALLEY CUT IN PORPHYRITIC ANDESITE, 

View near center of south line of sec. 31, T. 12 N., R. 7 W. The sky line at the head of the valley marks the rim of the amphitheater of the Mount Taylor volcano.
GEOLOGICAL StTRVE? PROFESSIONAL PAPER 189 PLATE 11

A. CROSS SECTION OF BASALTIC NECK EXPOSED BY EROSION IN GRANT RIDGES, 6 MILES NORTHEAST OF GRANT.

View looking southwest in sec. 3, T. 11 N., R. 9 W. Basalt in core is jointed in vertical columns and surrounded by a thin zone of platy jointing developed along the 

contact with white tuff. Bedded breccia overlies the tuff, and basaltic lava overlies the breccia on the far side of the cone. The tuff, chiefly rhyolitic, was erupted 

from Mount Taylor.

•If

B. CLIFF FACE EXPOSING BASALTIC BRECCIA ON RHYOLITIC TUFF NEAR THE VOLCANIC NECK.

In sec. 3, T. 11 N., R. 9 W.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX.

the topography has been controlled by the number, 

thickness, and structural attitude of the sandstones. 

Northwest and southeast of Mount Taylor, where there 

are many thick beds of sandstone with gentle dips, 

there are prominent mesas with steplike benches around 

them. In the eastern part of the volcanic field a miniature Basin and Range topography has been formed 

where thick sandstone beds, steeply tilted in successive 

fault blocks, produce sharp parallel ridges between 

valleys eroded in the shale.

Lobo Canyon contains a notable exception to the general rule of structural control of the topography; there 

an erosion surface that slopes southwest smoothly truncates folded beds of sandstone and shale. (See pi. 8, B, 

and fig. 5.) A small, inconspicuous remnant of this

age has been diverted to join a tributary nearer the 

head of the master stream, the Rio San Jose.

DRAINAGE

The variety of land forms determines an equal variety 

of drainage patterns. Plate 9, showing the drrinage 

system in the volcanic field, reveals four conspicuous 

types of pattern, and there are numerous minor variations in each type.

The Rio San Jose and Rio Puerco are the master 

streams draining the volcanic field. The Rio San Jose 

flows eastward with an average gradient slightly less 

than 25 feet to the mile and joins the Rio Puerco at 

the southeast corner of the field. The Rio Puerco flows 

southward across the eastern part of the field with an 

average gradient of about 20 feet to the mile.

FIGURE 5.—Sketch showing topographic relations in Lobo Canyon. The erosion surface (A) was developed by drainage that formerly went southwest between tie Grant 

Ridges and Horace Mesa. Lobo Canyon Creek, which now flows aronnd the northwest side of the Grant Ridges, trenches an alluvial plain (B). An alluvial far (C) has 

been built out on the plain. Hogbacks of eastward-dipping Upper Cretaceous formations (E) protrude above the alluvium. Sheet basalt or andesite caps the high 

mesas, above which rise basaltic lava cones (D) that mark the vents from which the lava forming the sheets was erupted.

surface (A, fig. 5) lies south of Lobo Canyon Creek 

between the Grant Ridges and Horace Mesa. This 

outlier is on the divide between Lobo Canyon and 

streams draining to the southwest. Earlier drainage on 

the surface probably went southwestward from Lobo 

Canyon, between the Grant Ridges and Horace Mesa. 

Landslides in the narrow canyon that separates the 

Grant Ridges from Horace Mesa may have diverted 

the drainage to form Lobo Canyon Creek, which now 

flows around the northwest side of the Grant Ridges. 

This hypothesis is suggested by the accumulation of 

lava boulders, some 20 feet in diameter, along the south 

edge of the outlier and also by the presence of numerous 

landslides along the southwest side of Horace Mesa. 

The conditions are unusual, however, in that the drainIn the southwest quarter of the field streams vdth. a 

radial pattern drain Mount Taylor. The amphitheater, at the hub of the radial pattern, is drained by 

Water Canyon Creek. This creek receives numerous 

headward tributaries in the amphitheater, and the arcuate divide separating the heads of these tributaries 

from the heads of surrounding streams include^1 the 

series of peaks that form the summit of Mount Taylor. 

The valley bottoms of most of these radial streams are 

relatively wide and flat in the lower courses (pi. 10, A) 

but are commonly narrow and V-shaped toward their 

heads (pi. 10, B}.

A group of nearly parallel streams flow northeastward 

on a dip slope of resistant sandstone in T. 15 N., Rs. 8 

and 9 W. Where the sandstone passes beneath the
56 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

overlying and more easily eroded shale the streams 

unite and meander widely in broad flood plains prior to 

joining the Chico Arroyo.

The drainage has a crudely dendritic pattern at the 

northwest corner of the volcanic field, on the Pacific 

side of the Continental Divide. A similar pattern is 

better developed at the northeast corner of the field, 

where tributaries of the Rio Puerco and Chico Arroyo 

flow in broad shale valleys between discontinuous sandstone ridges.

A rectangular drainage pattern has developed in the 

tributaries of the Rio Puerco in the block-faulted southeastern part of the volcanic field. The river and its 

principal tributaries have, in general, cut across the 

structural trends, but the second-order tributaries distinctly follow the fault-line valleys. Where the streams 

flow across the Tertiary deposits, however, there is no 

structural control, because most of these beds are later

FIGURE 6.—Pedestal of alluvium about 50 feet high left standing in Chico Arroyo, 3 

miles west of Cerro de los Cuates. Probably the result of the cutting off of a 

meander while tbe arroyo was narrower. Sketch from a photograph.

than the faulting and consist of relatively homogeneous 

material in which the stream courses are not controlled. 

Nearly all the streams in the Mount Taylor volcanic 

field once flowed on broad alluvial flood plains, but 

they are now entrenched in arroyos that are cut in the 

alluvium. Some of the arroyos are 50 feet deep (fig. 6). 

This erosion has lowered the water table and impoverished the country by impairing the growth of plants 

dependent on ground water. Bryan * has effectively 

described the result of arroyo cutting by the Puerco 

and pointed out, among other disastrous effects, the 

abandonment of six small towns in the eastern part of 

this volcanic field. The ruins of the settlements are 

still to be seen along the flood plain, and some of the 

houses are being undercut and removed by widening of 

the arroyo (fig. 7).

CLIMATE AND VEGETATION

The climate of the Mount Taylor volcanic field is 

arid. No measurements of rainfall have been recorded 

very near the field, but the average for western New 

Mexico is generally given as ranging from 10 to 15

1 Bryan, Kirk, Historic evidence on changes in the channel of the Rio Puereo, a 

tributary of the Bio Orrande in New Mexico: Jour. Geology, vol. 36, p. 281, 192S.

inches.2 There is some variation of rainfall within the 

field, for Mount Taylor rises several thousand feet 

above the surrounding area and receives numerous local 

showers that do not extend to the lower country. 

The daily range of temperature is considerable. In 

the summer the nights are cool but the days r,re warmed 

by a persistent sunshine. Local rains are frequent in 

the summer, and the temperature may change quickly 

at any locality clouded and showered by a storm. In 

winter freezing temperatures are reported as common 

at night but the abundant sunny days are commonly 

balmy.

Most of the volcanic field is less than 7,500 feet above 

sea level and is within the Upper Sonorar life zone. 

Over most of this part of the field sagebrush, the picturesque cane cactus, and several other cacti and yuccas 

are associated with the junipers, nut pines, and grama 

grass that characterize the life zone. The broad flattopped foothills around Mount Taylor and most of 

Mount Taylor itself to an altitude of about 9,500 feet 

support an open, grassy forest, chiefly of yellow pine,

FIGUKE 7.—Arroyo of Rio Puerco 2 miles north of Ojito ranch. T'le arroyo here is 

about 50 feet deep and is gradually being cut laterally under the house built on the 

alluvial flood plain. Sketch from a photograph.

which is characteristic of the Transition zone. Above 

this is the Canadian zone, containing mainly spruces, 

firs, and aspens.

SUMMARY OF THE LATE TERTIARY AND 

QUATERNARY HISTORY

CHRONOLOGY OF EVENTS

The sequence of late Tertiary and Quaternary events 

in the Mount Taylor volcanic field is summarized diagrammatically in figure 8.

AGE ASSIGNMENTS

The Santa Fe formation is of considerable assistance 

in studying the chronology of events in the Mount 

Taylor volcanic field and provides the only evidence 

for determining their approximate age. The formation is generally regarded as of upper Miocene and 

Pliocene age, on the evidence of vertebrate fossils 

collected by earlier investigators from the formation 

near Santa Fe. The Santa Fe formation and overlying younger deposits of probable Pleistocene age 

occupy a belt 20 miles wide adjoining the east border 

of this field and are present in outliers within the field

2 Bailey, Vernon, Life zones and crop zones of New Mexico: TJ. S. Pept. Agr, 

North American Fauna, no. 35, p. 9,1913.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 57

near the east border. In this field the sediments of 

the formation are mostly clastic and probably were 

deposited by streams flowing from mountains to the 

north. The sediments apparently were deposited in 

and gradually overlapped the sides of the topographic

too little for all of Quaternary time and in addition 

an indefinite fraction of late Tertiary time. Accordingly, some uncertainty is introduced regarding the 

age of the beds overlying the Santa Fe in this field, 

and these beds may be as young as Pleistocene.

Time of 

events

§1

0 ®

si

Ol

a

£ 

a

a

i

<3

Elevation of drainage around base of Mount Taylor 

cone with respect to present stream levels

Drainage at present level

Drainage about 100 feet above present level

Drainage about 750 feet above present level

Erosion surface about 2,000 feet above present 

drainage developed around Mount Taylor

Irregular topor'-iphy with drainage abort 2.000 

feet ab;ve present level

Probablo irregular topography witli drainage 

more than 2,000 feet above present level

Structural movements

*° a-&

• g,|| 

•2 a 2

III

m

p:

S

-^ .-'o M 2 s" 0> !jo ® 9 !_ 3 

|g|||

&l?i-di

w .S £ 5X1 § S ^•Cfl _a>S

a £ "3 2 5 .a 6c a 6c 3

11

Deposition San ofof andmation y

dedifferentiated 

:

Volcanic activity

Sheet basalt

*

''

Mount Taylor

•'S >>> 2

H "c3 !i &

I,

FIGUKE 8.—Sequence of late Tertiary and Quaternary events in the Mount Taylor volcanic field. The length of line represents approximately the duration of each activity; 

solid lines indicate fairly definite information; dotted lines indicate considerable uncertainty. Age assignments are based upon the Santa Fe formation and the younger 

undiflerentiated deposits of Pleistocene (?) age.

depression that resulted from the downfaulting of the 

Rio Grande Valley.

Probably only the upper part of the Santa Fe formation overlaps westward into the Mount Taylor field. 

Locally, thin deposits lie on pediment surfaces sloping 

south from the Nacimiento Mountains, and these 

surfaces are in places only 100 feet above the present 

drainage level. The possibility must be considered 

that unless the original thickness was far greater than 

that of any of the preserved remnants, only slightly 

more than 100 feet of downcutting has taken place in 

the region since Santa Fe time. But this is probably

VOLCANIC ACTIVITY

GENEEAL CHEONOLOGY

The Mount Taylor volcanic field was the scene of 

frequent volcanic activity during late Tertiary time, 

probably beginning in the Miocene epoch, and, with 

some interruptions, this activity has continued nearly 

to the present time. The eruptions were preceded 

by the deformation that uplifted the Zufii Mountains 

and Mesa Lucero (fig. 3). In the Mount Taylor field 

this deformation produced the regional northward 

dip, the folding of the McCarty syncHne, and prol <\bly 

most of the other folds and faults associated with that
58 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

syncline. Except for some local basalt flows, the earliest eruptions came from the Mount Taylor volcano, 

the only large volcano in the field. The volcano 

erupted first rhyolitic tuff and trachyte, next a series of 

latites and trachytes, and finally porphyritic aiidesite. 

These eruptives accumulated to form the cone of 

Mount Taylor. The erosion surfaces that were subsequently developed around the cone were flooded with 

sheets of nonporphyritic basalt and andesite erupted 

by a large number of vents distributed throughout the 

volcanic field. A few of these sheets were erupted 

prior to the latest Mount Taylor eruptions, but most 

of them were erupted after Mount Taylor had become 

quiescent. The sheets commonly overlap the edges 

of the Mount Taylor cone and spread out on the erosion 

surfaces sloping from it. The maximum eruptions of 

the sheets occurred prior to the final structural movements in the field, but there were minor basaltic 

eruptions as late as recent time.

THE MOUNT TAYLOR VOLCANO

With the possible exception of some very local eruptions of basalt, the Mount Taylor volcano was the 

earliest and certainly the most active center of volcanism in this area. The volcano still retains its conical 

form, but its top has been lowered by erosion. (See 

fig. 9.) The crater probably covered originally an area 

of less than 1 square mile, but erosion has enlarged it

FIGURE 9.—Hypothetical profile of Mount Taylor showing the probable mass above 

the present summit that has been removed by erosion.

to an amphitheater covering nearly 5 square miles. 

(See fig. 10.) Lavas and tuffs accumulated around the 

old crater until a cone at least 3,000 feet high had been 

built. There is no way of determining the original extent of these lavas and tuffs, but today they are confined to the cone, whose base has a radius of about 8 

miles. (See also p. 65.)

The amphitheater is drained to the east by Water 

Canyon Creek, which is incised into the east flank of the 

cone. The rim of the amphitheater is about 2,500 feet 

above the creek, and the lavas and tuffs erupted from 

the crater are fairly well exposed along the sides and in 

the bottom of the amphitheater.

In the eastern part of the amphitheater there is a 

considerable area of folded sedimentary rocks belonging 

to the Mesaverde formation. The original crater undoubtedly lay west of this outcrop. The lavas in the 

cone slope in all directions from a point half a mile west 

of the junction of the North and South Forks of Water 

Canyon Creek. A system of dikes radiates from the 

same point, and immediately north of it is the only 

breccia found in or on the Mount Taylor volcano.

Eruptions from this crater began with vitric tuffs, 

mostly rhyolitic, and at least one lava flow of porphyritic

trachyte, and these were followed by a series1 of porphyritic latites and trachytes. These early eruptions apparently came from a central crater. The final eruptions of the volcano were porphyritic anc^sites that 

were probably supplied mostly by the radiating dikes,

RHYOIJETIC TUFF

The earliest definite eruptions (p. 59) from the 

Mount Taylor volcano were pyroclastic materials that, 

so far as known, are now entirely vitric rhyolitic tuffs. 

The tuffs are well exposed in the eastern part of the 

amphitheater and around the outer fringe of the Mount 

Taylor cone.

In the amphitheater (fig. 10) east of the folded sedimentary rocks and north of the creek about 400 feet 

of tuff is exposed. The base is concealed, but it is 

probable that the underlying rock is Mesaverde sandstone or shale. Most of the tuff is volcanic dust, but 

there is considerable ash and some lapilli. Both the 

dust and the coarser material are vitric. No undoubted 

rock fragments and only a very few crystal?-, probably 

feldspar, were observed. The tuff is distinctly bedded, 

with the coarser material segregated in layer1 . No evidence indicating flowage was found; the exposure is 

clearly pyroclastic. The beds in the lowe1" part dip 

about 10° E. Upward the dip increases, and near the 

top the beds dip 25°. The obvious suggestion of these 

dips is that a cone was being built, but two alternatives 

seem equally possible—(1) an early tuffaceous cone 

was built around the crater, and later pyroclastic rocks 

accumulated on the flanks and beyond the limits of the 

early cone; or (2) as the top of the exposure is about 

a quarter of a mile north of the base the difference in 

dip may be due to location and the result of structural 

movements. The dips are in the same direction as 

those of the sedimentary rocks immediately to the west 

but are less in amount.

Tuff that is probably rhyolitic and composed mostly 

of dust is locally exposed along each side of Water Canyon where the creek is incised into the flanl* of Mount 

Taylor. About midway between the amphitheater 

and the mouth of the canyon 150 feet of tuff is well 

exposed beneath the lava that caps the no^th canyon 

wall. A short distance upstream from this locality tuff 

is exposed at a lower altitude in the creek bottom. 

The total thickness in this part of the canyon is probably at least 200 feet. About half a mile above the 

canyon mouth, however, the tuff at the base of the 

lavas is only 30 feet thick, and near the mouth practically no tuff is present at this horizon. However, there 

is another 100 feet of tuff above the lowest lava. 

Nearly all the tuff at these outcrops is of dust size. 

Locally thin beds of ash consisting of various types of 

glass, much of it black, make dark streaks across the 

lighter outcrops.

Outcrops similar to those in Water Canyon are found 

in Lumber Canyon, the next canyon to the southwest.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. HEX. 59

Here, as in Water Canyon, rhyolitic tuff lies at the base 

of the eruptive series and thins from about 100 feet at 

the head of the canyon to 20 feet at the mouth. Most 

of the rhyolitic tuff is of dust size and light-colored. 

The deposit is massive, with lapilli of obsidian and 

pumice and bombs of pumice scattered irregularly 

through it. Nonporphyritic andesite, belonging to the 

post-Mount Taylor sheet eruptives, overlies the tuff 

and is overlain by another 100 feet of tuff.

A very few feet of rhyolitic tuff, with the base concealed, is exposed beneath the oldest lava along Raton 

Springs Canyon half a mile north of the south line of

tuffs southward from the experiment station is younger 

than the porphyritic lavas.

About 2,000 feet south of the experiment station the 

rhyolitic tuff overlies a small basalt flow. Although at 

several localities basalt preceded the porphyritic 

andesite lavas from Mount Taylor, this is the only 

locality found where basalt preceded the rhyolitic tuff. 

This tuff along the west side of Rinconada Canyon can 

be traced southward nearly to the south line of T. 11 N., 

R. 8W.

Tuff is excellently exposed at several localities in the 

vicinity of Lobo Canyon, southwest of Mount Taylor.

EXPLANATION

VOLCANIC ROCKS

Rhyolitic tuff, porphyritic trachyte and fatite 

(So/id lines ind/cate rocks that 

have been identified Ay thin 

sect/on; c/as/iect'//nes, rocks 

megrascapica/fy identified}

Mesaverde formation 

(So/id lines shew areas of/mown 

outcrops; cfasfied fines ind-'caie 

probaA/e ac/c/tt/ona/extent)

White area in amphitheater contains f&iv outcrops, butprofosxb/y 

/s composed mostly of porfihyrit/c /atite.

Boundaries between rock t)f>GO 

are approximate on/y.

Spring

z Miles

FIGURE 10.—Geologic sketch map of the amphitheater and surrounding rim of the Mount Taylor volcano.

T. 11 N., R. 7 W. It thins out southward, and in the 

mesa to the south younger eruptives rest directly 

upon sedimentary rocks.

Along the west side of Rinconada Canyon are several 

fine exposures of thick bodies of rhyolitic tuff. Near 

the head of the canyon about 500 feet of tuff is overlain 

by two or more flows of porphyritic andesite, erupted 

from Mount Taylor. Although exposures are discontinuous, these flows seem not to extend as far south as 

the experiment station of the New Mexico Agricultural 

College. The southward slope of these lavas would 

carry them below the top of the tuffs just south of the 

experiment station, so that possibly the upper part of the

Along the south side of La Jara Mesa, west of Mount 

Taylor, near the west line of T. 12 N., R. 8 W., 200 feet 

of tuff, probably rhyolitic, is exposed beneath the capping sheet of andesite as shown in the following section:

Section of eruptive rocks on south side of La Jara Mesa

Feet

Andesitic lava, apparently a single flow, nonporphyritic; 

central part massive; top irregular, scoriaceous, and 

ropy; bottom 1 foot scoriaceous, with vesicles as much as 

a quarter of an inch in diameter_____________________ 70

Agglomerate, red, loosely cemented, angular fragments of 

scoriacous andesite; fragments small, few of them exceeding a quarter of an inch in diameter___ ________________ 2J.

Volcanic dust with a little ash; vitric, white, massive_____ 12
60 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

Section of eruptive rocks on south side of La Jara 

Mesa—Continued

Feet

Volcanic ash, white, massive, bombs and lapilli as much as 

6 inches in diameter irregularly distributed; bombs and 

lapilli of pumice, gray glass, and black glass; vitric 

throughout _______________________________________ 48

Tuff, bedded, white; predominantly dust, but ash, lapilli, 

and bombs sorted in beds.______________________.___ 55

Dust, ash, lapilli, and bombs, white, in part at least 

rhyolitic; generally massive but locally in irregular beds. 92

Base concealed by talus.

There is probably at least 75 feet of tuff concealed by 

the talus below the outcrop. The tuff probably rests 

upon sedimentary rocks, for there is little likelihood that 

a rock as resistant as lava would be concealed in the 

steep slope of material so easily eroded. A bomb of 

pumice from the lowest unit, analyzed by J. G. Fairchild 

in the chemical laboratory of the Geological Survey, 

contains 72 ± 2 percent of SiO2 . The index of refraction 

of the pumice is 1.495 ± 0.005, and the index of the black 

obsidian is practically the same. The refractive indices 

of other tuffs examined differ little from this figure, and 

it is inferred that they are generally rhyolitic.

This deposit thins westward, and probably most of 

the thinning takes place where the lava truncates the 

steepest dips in the sedimentary rocks. The exposed 

tuff is not tilted, like the sedimentary rocks, but lies 

nearly flat. Just how the thinning takes place was not 

determined. Only the youngest tuffs are exposed, and 

the oldest deposits conceivably may be involved in the 

folding. But the suggestion is clear that these tuffs are 

later than the deformation of the underlying sedimentary rocks.

About 1% miles east of this locality, just east of the 

cinder cone, tuff is exposed about 300 feet below the 

base of the lava. It seems likely that the tuff thickens 

toward the head of the canyon.

Abundant rhyolitic tuff is exposed in the sides of the 

highest mesa in the Grant Ridges, at the south side of 

Lobo Canyon. Plate 11, A, shows rhyolite tuff overlain by sheets of basalt and intruded by a plug that 

contributed to the sheets. The rhyolite tuff is about 

300 feet thick and rests in nearly horizontal position 

upon tilted sedimentary rocks, although the contact 

was not found. Plate 11,5, shows a closer view of the 

tuff. The material is white, mostly dust, but ash, 

lapilli, and bombs are sorted in beds. No undoubted 

fragments of sedimentary rock were found. Most of 

the bombs consist of frothy pumice drawn out in intimately interwoven thin fibers, but a few are perlitic 

black obsidian. The tuff was not found beneath the 

sheet basalt that caps the other mesas of the Grant 

Ridges, and, though it may be present and concealed 

by talus, the thickness certainly decreases southwestward. The striking conical form assumed by the 

sheet eruptives and the complete absence of such structure in the tuff, the thinning of the tuff to the southwest, 

and the persistence of similar thick tuffs to the northNa2O-___.

K2O_---_.

H

/~»

TiO2 _— ..

P,OS_____.

.__ 2.71

.__ 4.36

1

QC

.__ 2.67

.__ .73

MnO___.

C02 -- —

.___ 0.30

.__. 2.56

101. 08

east toward Mount Taylor indicate with some certainty 

that their source was the Mount Taylor volcr.no and not 

the local vents that supplied the sheet eruptJves.

Irregular masses of igneous rock, in part at least of 

latitic composition, are associated with the tuff at the 

northeast end of the Grant Ridges. The material is 

poorly exposed, and no means was found to determine 

whether it is a lava flow or an intrusior. At one 

locality horizontal columnar joints truncated by crude 

plates are exposed for a width of 6 feet, suggesting 

intrusion of a dike, but similar relations were observed 

in definite flows on the north flank of Mount Taylor. 

It was probably some of this material that WP-S collected 

by J. S. Diller and analyzed by T. M. Chatard in the 

chemical laboratory of the Geological Surrey.3 The 

analysis showed the following composition:

Analysis of dacite 6 miles northeast of Grart

SiO2 ________ 49.80

A12O3 _------ 15.33

FeO---_-_._ 7.44

MgO----_-_ 6.61

CaO________ 7. 19

Thin beds of light-colored volcanic dust and ash are 

locally interbedded with the sheet eruptives on Mesa 

Chivato. In general these tuffs are less than 10 feet 

thick. They closely resemble the rhyolitic tuff associated with the Mount Taylor volcano and were 

probably derived from it.

TRACHYTE

At the junction of the North and South Forks of 

Water Canyon Creek (fig. 10) a mass of porphyritic 

trachyte lies immediately east of the supposed site of 

the old crater of the volcano. The mass may be an 

intrusion that solidified in the eastern part c* the pipe 

and crater, but it is improbable that such a p] ug should 

be formed of trachyte and not of latite or andesite, 

which make up most of the cone. If the trachyte is 

a lava flow, as seems probable, it is older than the flows 

and some of the rhyolitic tuff in the northeast rim of 

the amphitheater but is probably younger than the 

earliest rhyolite tuffs. A comparatively smrll volume 

of this early trachyte must have been erupted, for it 

was found at only the one locality mentioned. It was 

among the earliest if not the earliest of the lavas, and 

the fact that it is intermediate in composition between 

the rhyolite and latite adds interest to its position in 

the eruptive sequence.

In the hand specimen the trachyte is li^ht gray, 

speckled with feldspar crystals that are clear except 

where stained with yellow or red iron oxide. The 

phenocrysts attain a maximum size of about 3 millimeters. Minute vesicles are uniformly distributed. 

Crude flow lines, produced by parallelism of the laths

3 Clarke, F. W., Analyses of rocks and minerals from the laboratory of the United 

States Geological Survey, 1880-1908: U. S. Geol. Survey Bull. 419, p. 123, analysis D. 

1910.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 61

of oligoclase, can be seen in thin sections. The trachyte 

is porphyritic, containing phenocrysts of sanidine and 

oligoclase. The proportion of sanidine to oligoclase 

is estimated at about 3 to 1. A few scattered grains 

of quartz may be secondary. Magnetite is finely disseminated through the groundmass and hi a few phenocrysts. There is only a trace of ferromagnesian 

minerals.

LATTTE SERIES

The latite series attains a maximum thickness of 

about 1,000 feet and makes up a considerable part of the 

Mount Taylor cone. The series includes chiefly latites, 

but near the top there are at least one local trachyte 

flow and some andesite. All these lavas are poorly exposed, but a general composite section can be reconstructed from the several outcrops in the amphitheater.

The oldest exposed lava in the latite series crops out 

near the base of the north rim of the amphitheater 

about 2 miles east-northeast of Mount Taylor Peak 

(fig. 10). The exposure is about 300 feet above the 

trachyte at the junction of the forks of Water Canyon 

Creek and about half a mile farther from the crater. 

The rock is porphyritic. Phenocrysts form about 25 

percent of it and consist chiefly of sodic oligoclase with 

minor quantities of augite and magnetite. The groundmass is microfelsitic and has an index of refraction 

lower than that of balsam.

Another flow of porphyritic latite, probably slightly 

higher in the series, is exoosed near the base of the rim 

north of the crater. This rock also has about 25 percent of phenocrysts, mostly corroded crystals of andesine. Orthoclase, augite, magnetite, and highly altered 

biotite are present in minor amounts as phenocrysts, 

and there is a little olivine. The groundmass is a feltlike mass of orthoclase and oligoclase in nearly equal 

amounts and a little magnetite.

The upper part though not the top of the latite series 

is exposed in the western part of the amphitheater about 

half a mile east-northeast of Mount Taylor Peak. This 

outcrop is nearly 1,000 feet higher than the early 

trachyte and is very near the crater. It is porphyritic 

latite consisting of about 15 percent of phenocrysts set 

in a glassy groundmass. Subhedral sodic oligoclase is 

the most abundant phenocryst, although euhedral 

biotite is common, and there are a few crystals of 

anhedral quartz and magnetite. The glassy groundmass has an index of refraction lower than that of 

balsam. It commonly has a spherulitic structure.

Irregular masses of latite high in the series are exposed on the ridges in the southwestern part of the 

amphitheater. The percentage of potash feldspar is 

uncertain, because it rarely occurs as phenocrysts and 

is nearly indistinguishable in the finely felted groundmass.

The top of the latite series is exposed along the southeastern rim of the amphitheater about 2% miles eastsoutheast of Mount Taylor Peak, where it is overlain

by porphyritic andesite. The top is about 500 feet 

above the trachyte, and projection of the dips indicates 

that near the crater the top of the latite series was about 

1,000 feet above the trachyte.

These upper lavas of the latite series are porpl yritic 

and holocrystalline. They contain about 15 percent of 

phenocrysts, mostly corroded oligoclase. Magnetite 

and altered biotite are less common. The groundmass 

consists of euhedral sanidine and oligoclase, the former 

predominating, and traces of anhedral magnetite. A 

partial analysis of a lava about 400 feet below the top 

of the latite series showed K2O, 4.92 percent; Na2O, 

5.78 percent; and CaO, 1.68 percent.

The top of the latite series is also locally exposed 

around the flanks of the cone where valleys have been 

eroded through the later flows of porphyritic andesite. 

Exposures were found at the localities indicated on 

plate 7, but probably there are many others.

Trachyte is exposed along the Forest Service, trail 

half a mile south of Mount Taylor Peak, where it is 

probably overlain by porphyritic andesite, although 

the contact was not found. The rock contains 

phenocrysts of hypersthene, augite, magnetite, potash 

feldspar, and andesine, with the potash feldspar predominating. The phenocrysts are 3 millimeters in maximum size and make up about 35 percent of the rock. 

The groundmass is nearly all sanidine, with very minor 

quantities of oligoclase, augite, and magnetite. The 

rock is similar hi composition to the trachyte at the 

forks of Water Canyon Creek. The trachyte in the 

amphitheater, however, lies below the latite series, 

whereas the trachyte just described is at the top. .This 

is the only locality found where the latite series is known 

to include trachyte. It is probably a local eruption, 

because at the other localities examined the top of the 

series is latite.

In the southeastern part of T. 12 N., R. 8 W., in the 

canyons 2 miles west of Mount Taylor Peak, a lightcolored lava of vitric latite lies at the top of the series 

and is overlain by porphyritic andesite. The latite is 

porphyritic, with phenocrysts of orthoclase, plagioclase 

(probably oligoclase), and biotite in nearly equal 

amounts and minor quantities of augite and magnetite. 

The phenocrysts attain a maximum size of about 3 

millimeters and form about 20 percent of the rock. 

The groundmass is mostly glass but includes some 

crystalline oligoclase.

The top of the latite series is also exposed in the 

valleys on the north flank of the cone about IK miles 

north-northeast of Lamasca Peak. The exposures are 

deeply weathered and discolored by hydrous iron oxide. 

The rock contains about 25 percent of phenocrysts of 

sodic andesine and biotite in nearly equal amounts and 

traces of augite and magnetite. The groundmass 

includes oligoclase and orthoclase in nearly equal 

amounts. The overlying flows are porphyritic andesite 

belonging to the succeeding series.
CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

Irregular masses of latite that are poorly exposed with 

the tuffs at the northeast end of the Grant Ridges may 

be flows of this series. They are described on page 60.

The dike a third of a mile northeast of Mount Taylor 

Peak trending nearly northwest for half a mile is the 

only dike in this area known to be latitic. It intrudes 

the latite series within the amphitheater but was not 

found to cut the rim or outer flank of the cone. The 

dike may have supplied some of the later lavas in the 

latite series. It is porphyritic, with phenocrysts of 

sanidine, oligoclase, and biotite in nearly equal amounts 

and a little magnetite. The phenocrysts attain a maximum size of about 3 millimeters and constitute about 

20 percent of the rock. The groundmass is a felt of 

oligoclase, sanidine, and glass in nearly equal amounts.

Another dike, probably latitic, less than a mile south 

of east of Mount Taylor Peak, trends slightly east of 

north. It intrudes the latite series and may have supplied some of the later flows in that series. The rock 

includes about 40 percent of phenocrysts, about half of 

which are oligoclase, potash feldspar, and perthite in 

nearly equal amounts. Biotite constitutes about a 

quarter of the phenocrysts, and the remainder are 

quartz, hornblende, and magnetite in nearly equal 

amounts. The groundmass consists of glass and a felt 

of oligoclase and potash feldspar.

Several rock samples were collected by J. S. Diller 

from the east side of Mount Taylor, presumably from 

the latite series in Water Canyon or the amphitheater. 

Analyses of the samples were made T. M. Chatard in 

the chemical laboratory of the Geological Survey and 

showed the following composition: 4

Analyses of rocks from east side of Mount Taylor

Si02 _ __________________________

Al»0,_ ____-_---__-__-_____-____

Fe_O8 _ — — — _ _____________

FeO___ — — _ — — — ____ — __

MgO— — - __________ ______

CaO___________.___________ _ _

Na_O__-_ — -------- _ --______

K2 0___— ______________________

H2 0_ __________________________

TiOg.— _ — _ — —— ____________

P_0____________________________

MnO_ _________________________

A

68.40

17.99

2.66

1.63

.49

.67

4.54

3. 54

.52

.21

100. 65

B

65. 51

16.89

1.41

2. 52

.39

1. 19

6.42

5.02

. 16

.92

.07

.31

100. 81

C

65.78

17.32

3.68

.46

.47

1. 66

5.23

4. 64

. 14

.27

. 13

.32

100. 10

A. Lava, canyon on east side of San Mateo Mountain.

B. Andesite, canyon on east side of San Mateo Mountain.

C. Quartz latite, canyon east side of San Mateo Mountain.

POKPHYRITIC ANDESITE SERIES

Probably all the eruptives described above, except 

possibly the latest flows, were poured out from the 

central vent. The final series of eruptions of the 

Mount Taylor volcano consisted chiefly of porphyritic

• Clarke, F. W., Analyses of rocks and minerals from the laboratory of the United 

States Geological Survey; 1880-1908: U. S. Qeol. Survey Bull. 419, p. 123, 1910.

andesite and were probably supplied mostly by the 

dikes that radiate from the crater and pipe below. 

Four lines of evidence indicate this derivation. (1) 

With only a few exceptions the dikes are jorphyritic 

andesite identical in composition with the bulk of the 

final series of eruptives; (2) no dikes were found to 

intrude the latest flows, although a more complete 

survey may reveal some; (3) what is believed to be the 

crater consists only of breccia, and no central core of 

porphyritic andesite was found; (4) it is very improbable that so many dikes would be intruded so near the 

surface without reaching the surface and erupting.

The dikes are distributed radially within the amphitheater of the volcano (fig. 10). They are more resistant to erosion than the other rocks in the amphitheater and stand up as high walls, the highest estimated at 100 feet.

Most of the dikes are 20 to 30 feet in width, but a 

maximum of 60 feet was observed. Within the amphitheater the contact between the dikes and the early 

series of eruptives is generally sharp and marked by a 

thin zone of platy jointing and more or less black 

perlitic glass. Around the rim, however, particularly 

in the southwestern part, where the dikes are in contact 

with the latest series of eruptives, the contacts are 

commonly very indistinct, and locally th<^ jointed 

structure of the flow and that of the dike grade into 

each other. Horizontal columnar joints are well developed in all the dikes observed. So far as could be 

determined the dikes stand vertical or nearly so.

Fifty dikes were found and mapped, and thin sections 

from 14 were examined. The easternmost dike in the 

amphitheater, just north of Water Canyon Creek east 

of the Mesaverde outcrop, is probably related to the 

later sheet eruptives and is not described h^re. Two 

latitic dikes have already been described. Eleven of 

the dikes are very similar in composition, containing 

phenocrysts of andesine (rarely oligoclase or labradorite), biotite, augite, magnetite, and rarely olivine. 

The phenocrysts of augite and magnetite are commonly 

in nearly equal amounts, but biotite is the predominant 

mafic mineral. There is generally two or three times 

as much feldspar as mafic minerals among the phenocrysts. The groundmass commonly consists of more 

or less glass with tiny laths of oligoclase. Where the 

oligoclase is anhedral and the groundmass is feltlike 

there may be some potash feldspar.

Except in the bottoms of some of the largest valleys, 

flows of the porphyritic andesite series form the surface 

of most of the flanks of the Mount Taylor cone. This 

series apparently constitutes a smaller volume of eruptives than the latite series.

One of the thickest accumulations of thes^ andesitic 

lavas is along the south rim of the amphitheater, 2 

miles southeast of Mount Taylor Peak. The thickness here is estimated at 500 feet and represents several 

flows, certainly not less than three. Exposures are not
GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 12

A. BASALTIC LAVA AND CINDER CONE ON THE SHEET BASALT CAPPING HORACE MESA. 

View in center of sec. 3, T. 11 N., R. 8 W.

B. VIEW LOOKING NORTHEAST FROM TOP OF 'J7 ' ANIC NECK EAST OF LA ABRA DE LOS CERROS.

Cabezon Peak (right) is composed almost wholly of basalt. Cerro Chato (left) is mostly basaltic breccia. Road in foreground serves as scale for nearby buttes of

Cretaceous shale.

C. VIEW LOOKING SOUTH FROM CERRO DE LOS CUATES.

The mesa across the Rio Puerco is capped with'Cretaceous sandstone.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 13

A. CABEZON PEAK VIEWED FROM THE BLUFFS ABOVE THE VILLAGE OF CABEZON. 

The peak is a volcanic neck of basalt jointed in nearly vertical columns but capped with highly scoriaceous basalt. Photograph by W. T. Lee.

B. CLOSER VIEW OF SOUTH SIDE OF CABEZON PEAK. 

Showing fairly uniform vertical columns capped with a layer of scoriaceous basalt at the top of the neck.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELp, N. MEX. 63

sufficiently good to determine with certainty the limits 

of individual flows. They rest upon porphyritic latite 

of the preceding series. Thin sections of three flows 

show very similar composition. Each includes about 

35 percent of phenocrysts, mostly of andesine but with 

minor amounts of augite and magnetite. One rock 

has a few olivine pheiiocrysts. The groundmass is 

practically all tiny laths of oligoclase with a little glass. 

A sample of the highest flow was analyzed for alkali 

content by E. T. Erickson in the chemical laboratory of 

the Geological Survey, with the following result: K2O, 

3.42 percent; Na2O, 5.62 percent; CaO, 4.00 percent.

Half a mile southeast of Mount Taylor Peak andesite 

is exposed above the latite series. The contact with 

the trachyte, here the top of the latite series (p. 61), was 

not found. At least two flows are present. The lower 

one is nonporphyritic and estimated to include about 

75 percent of oligoclase, 20 percent of glass, and 5 percent of magnetite. There are few crystals of ferrornagnesian minerals. Some crystals of oligoclase are 

fairly large, but there is complete gradation from these 

to microcrystalline size. The younger andesite is distinctly porphyritic, with about 15 percent of phenocrysts, mostly of sodic andesine and minor quantities 

of augite and only traces of magnetite. Most of the 

rock consists of tiny laths of oligoclase in dark glass.

Mount Taylor Peak, at the west rim of the amphitheater, is capped with similar porphyritic andesite. 

Probably the flows were once connected but have been 

separated by erosion.

A valley north of Mount Taylor Peak exposes the 

top of the latite series. Lamasca Peak, next to the 

northeast, is capped by flows which megascopically 

resemble the porphyritic andesite series.

Another valley at the rim east of Lamasca Peak 

exposes lavas that are probably latitic and may be connected with the exposed top of the latite series \% 

miles north-northeast of Lamasca Peak (p. 61). Porphyritic andesite caps the ridge east of the valley and 

covers most of the north and east sides of the cone, but 

the latite may be exposed in the deeper canyons in the 

part of the cone mapped as porphyritic andesite. 

Three thin sections from these flows indicate little variation in the lithology. The rocks are porphyritic, and 

most of the phenocrysts are andesine, but there are 

minor quantities of augite and magnetite and traces of 

hypersthene and biotite. The groundmass makes up 

about 65 percent of the rock and consists of felted 

feldspar and glass. Oligoclase can ordinarily be recognized in the groundmass, but considerable potash feldspar is probably occult in the glass.

Much later sheets of basalt and andesite from extraneous sources overlap the edges of the cone on the 

north, east, and west sides. There are similar overlaps 

along the south side, but the numerous canyons there 

provide opportunity for examining the outer fringe of 

flows from Mount Taylor.

150009—38———2

Along the west side of Kinconada Canyon the porphyritic andesites are overlapped by the sheet eruptives. 

(See pi. 7.) At the head of the canyon the porphyritic 

andesite series consists of two flows resting upon rhyolitic tuffs. The lower flow, about 100 feet thick, has 

very irregular columnar jointing and is not markedly 

scoriaceous. It contains only a few large corroded 

phenocrysts of calcic andesine set in a felted grouper 

mass of feldspar and glass. There is little magi;ej$te 

or ferromagnesian mineral. The material has a distinct 

flow structure. The upper flow is highly scoriaceous 

and about 40 feet thick. It is abundantly porphyritic, 

with many euhedral phenocrysts of calcic andesine $nd 

minor quantities of olivine, augite, and magnetite, ja a 

groundmass of dark glass. There is only a very sliglit 

suggestion of flow structure.

Porphyritic andesite, interpreted as derived from 

Mount Taylor, is exposed beneath later sheet erupfives 

in Raton Springs Canyon. The rock approaches a 

basalt, having phenocrysts of labradorite, but the 

groundmass is a less calcic plagioclase. Olivine and 

augite are subordinate as phenocrysts; augite and magnetite are common in the groundmass.

Where Lumber Canyon and Water Canyon cut the 

southeast fringe of the Mount Taylor cone the apparent order of eruptives is reversed. PorphjTitic 

andesite, believed to have been erupted from Mount 

Taylor, overlies nonporphyritic andesite that may have 

come from the plug of similar andesite in the eastern part 

of the amphitheater. The nonporphyritic andesite is 

probably related to the generally later sheet eruptives. 

The porphyritic andesite contains phenocrysts, chiefly 

of calcic andesine, with traces of olivine, augite, and 

magnetite. The groundmass is mostly calcic oligoclase 

with minor and nearly equal quantities of augite, olivine, 

and magnetite.

SUMMARY OP MOUNT TAYLOR ERUPTIONS

The eruptions of the Mount Taylor volcano began 

probably in late Miocene time, subsequent to the major 

movements of folding and faulting in the central andu 

western part of the volcanic field. (See fig. 8.?) Tlie 

volcano broke out in the deepest part of the McCarty 

syncline, which was the center of maximum intensity 

of the early folding. Inasmuch as both the volcano 

and the fold are unusual features of the southera San 

Juan Basin, their association seems to indicate more 

than fortuitous coincidence. The volcanic activity 

followed the major structural movements, but the time 

interval is not known. It is interesting, though purely 

speculative, to consider the possibility that the structural movements and fusion of the deep-seated magma 

were contemporaneous and related, and that the time 

interval between the structural movements and surface 

eruptions was that required for the lava to find its way 

to the surface.
64 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

The eruptions occurred in a fairly well defined sequence of composition. The earliest eruptions were 

more or less violent ejections of rhyolitic tuffs. Next 

followed relatively quiet eruptions of porphyritic lavas 

that are separable into two and possibly three series 

on the basis of their content of potash feldspar.. The 

porphyritic trachyte, interpreted as a flow (p. 60), 

probably followed much of the rhyolitic tuff but has 

only a small volume. This was followed by a large 

volume of porphyritic latite—interrupted, however, by 

at least one more flow of porphyritic trachyte. The 

latite hi turn was followed by a slightly smaller volume 

of porphyritic andesite. The following table gives 

estimates of the relative volumes of these materials:

Estimated volume of Mount Taylor eruptives

Rock series

Rhvolitic tuff _ ___ ___ ___

Trachyte. ___ __ ___

Porphyritic andesite series _ _ -

Maximum 

observed 

thickness 

(feet)

500

(?)

1,000

500

Minimum 

area covered (square 

miles)

155

. 1

60

11Q

Estimated 

volume 

(cubic 

miles)

5. 1

.05?

4.0

3.6

The figures indicate that a minimum total volume 

of about 12}o cubic miles of lava and tuff was erupted 

from the Mount Taylor volcano, a volume sufficient 

to provide a 6-inch blanket over the entire State of 

New Mexico.

Probably the rhyolite and trachyte and most of the 

latite were erupted from the central crater, whereas 

most of the porphyritic andesite and possibly the 

youngest latite were erupted by dikes intruded radially 

around the main pipe. The eruptives now make a 

cone 3,000 feet high with a base covering about 150 

square miles. The cone is what is left of the volcano 

after prolonged erosion has accomplished about 2,000 

feet of valley cutting in the surrounding country. 

Certainly the original volcano was higher and probably 

covered a larger area than the preserved remnant. 

Figure 9 shows the probable minimum amount of 

erosion that has altered the cone. The figure indicates 

,i lowering of the top by nearly 1,000 feet but does not 

consider the possibility of lowering of the flanks. During this period of erosion successive pediments, or 

erosion surfaces, were developed around the cone. It 

was mostly on these surfaces that the sheet basalts were 

erupted, and these later lavas have proved so resistant 

to erosion that several of the old surfaces are still preserved under the lava capping.

SHEET BASALT

Some of the sheet eruptives preceded the latest flows 

from Mount Taylor, but most of them were poured out 

after Mount Taylor had become quiescent. The sheets 

spread out on erosion surfaces developed around the 

base of the Mount Taylor cone, and most of them were

erupted from central vents, though a few came from 

fissures. Where the sheets are still preserved as the 

caps of mesas the vents are marked by conical piles of 

lava and cinders as much as 500 feet high. (See pi. 12, 

A.) Where erosion has removed the lava cap and 

lowered the country rock the vents are marked by 

picturesque necks, columns of lava and breccia that 

solidified in the pipes without reaching the surface. 

(See pis. 12, B, C; 13, A; 16, A.) Locally a relief as 

great as 1,800 feet has been produced around lava 

necks. The breccia necks (pi. 12, B} are less resistant 

and topographically are not so prominent. Several of 

the necks are elongate and dikelike; others are apparently cylindrical.

Johnson 5 showed conclusively that the irterpretation 

of the necks as such is inescapable. Vert; cal contacts 

with platy jointing can commonly be found. In addition, erosion has produced all gradations from the lava 

cones, with the lava sheets completely around the vents 

(pi. 12, A), through cones with the lava removed from 

only one side, exposing the pipe (pi. 11, A), to necks 

from which the surrounding sheets of lava have been 

completely removed (pis. 12, B, C; 13, A', 15, A; 16, A).

Most of the vents are northeast of Mount Taylor, 

but a few are scattered around the south and west sides. 

The old cone is nearly surrounded by the sheets, which, 

on all sides except the southeast, overlap the earlier 

porphyritic lavas from Mount Taylor. Some of the 

lava sheets broke out from vents on the flanks of the 

Mount Taylor volcano, adding their lavas to the cone.

Numerous dikes were intruded at the time of the 

sheet eruptions. Most of them trend es.st of north, 

nearly parallel to the strike of the faults in the eastern 

part of the volcanic field and to the strike of one set of 

faults in the central and western parts. However, only 

one dike was definitely intruded along a fault surface. 

The necks of dikelike form generally trend east of north, 

but no systematic distribution of these necks was 

determined. The alinement of Sleeping Found Butte, 

Juan Tafoya Peak, and Santa Rosa Peal' is obvious, 

but the trend of this alinement is nearly north-south, 

inclined 15°-20° to other regional trends. Detailed 

mapping of the cones on Mesa Chivato might provide 

the clue for determining a system to the location of the 

volcanoes, for the vents there are more closely spaced 

than elsewhere.

The basaltic lavas erupted by these volcanoes flowed 

as sheets. They generally contain a very few phenocrysts, mostly olivine. The feldspar in the lavas is 

generally either calcic andesine or sodic labradorite, 

and, depending upon which predominates, the individual flows are designated respectively ande^te or basalt. 

There is commonly a glassy groundmass, however, and 

the feldspar of this groundmass is presumably less 

calcic than the larger, determinable crystals. More

5 Johnson, D. W., Volcanic necks of the Mount Taylor region, N. Mex.: Geol. 

Soc. America Bull., vol. 18, pp. 303-321,1907.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 65

comprehensive studies, therefore, may prove that the 

basaltic type is less abundant than is indicated in this 

report. This lava series is loosely designated basaltic 

to avoid confusing it with the porphyritic andesite 

series erupted by Mount Taylor. Tremendous quantities of basaltic breccia in the pipes and in some of the 

dikes consist of fragments of basalt and sedimentary 

rocks in a friable glassy matrix. Apparently very little 

of the breccia was extruded. A few thin beds of tuff, 

probably rhyolitic, are interbedded with some of the 

lava sheets on Mesa Chivato. This tuff may have 

been a result of minor explosions of the Mount Taylor 

volcano near the end of its activity.

BASALT IN THE AMPHITHEATER

In the eastern part of the amphitheater (fig. 10) a 

mass of basalt intrudes folded sedimentary rocks. The 

intrusion is bordered on three sides by platy joints, but 

near the top the central core consists of highly scoriaceous basalt. The dip of the platy jointing converges 

upward, suggesting that the intrusion narrowed upward, but study of jointing in the necks northeast of 

Mount Taylor discounts this evidence. For example, 

the zone of platy jointing on the north side of the intrusive mass dips 55° N., but it may have developed 

by consolidation of the basalt against earlier intrusives, 

as basaltic breccia. Such relations are common in 

many of the necks, and so the jointing cannot be regarded as necessarily indicative of the limits and shape 

of the whole intrusion.

Whether or not the intrusion domed the sediments 

cannot be determined. Elsewhere in this field the intrusion of these basaltic rocks did not deform the country rock.

The intrusive mass is olivine basalt in which small 

phenocrysts of olivine and augite form about 10 percent 

of the rock. Augite but not olivine is present in the 

groundmass, which contains also about three times as 

much labradorite as augite and minor quantities of 

magnetite.

This intrusion may mark the source of some of the 

olivine andesite sheets that underlie the latest porphyritic andesite on the southeast flank of Mount Taylor 

(p. 63). But the careful tracing of individual flows 

necessary to decide this point was not attempted during 

this study.

VENTS ON THE NORTH FLANK OF MOUNT TAYLOR

Eruptions broke out from several vents on the northeast flank of the Mount Taylor cone. Basaltic lava 

and cinder cones were built upon the earlier porphyritic 

andesite flows of the volcano, and tongues of basalt 

were poured northward down the slope. These eruptives are younger than the porphyritic andesites, but 

their age relative to the sheets on Mesa Chivato is not 

known. At least 300 feet of lava from these vents 

accumulated locally on the lower flanks of Mount Taylor. Just southwest of the Fernandez camp there are 

probably six or seven flows. These lavas thin yestward (and probably eastward), and in two canyons the 

underlying porphyritic andesite is exposed.

These basaltic lavas are all dark gray and dense. 

The more scoriaceous portions of the flows weather red, 

apparently because of oxidation of the abundant magaetite. There are a few large phenocrysts of olivine, the 

only megascopic mineral in the flows. The rocks .consist of sodic labradorite or calcic andesme, olirifle, 

augite, and magnetite, with more or less glass. The 

plagioclase generally forms more than 50 percent of the 

rock, and olivine is generally the most common mafic 

mineral. Magnetite ranges from a trace to about 10 

percent.

FLOWS ON MESA CHIVATO

Mesa Chivato is a part of the highest and no doubt 

oldest pediment developed around the Mount Taylor 

cone, and many small volcanoes flooded it with street 

basalt. Probably these eruptions occurred soon riter 

the pediment was formed, because the smooth surface 

beveling resistant sandstone and easily eroded shale 

would not otherwise be preserved.

No detailed work was done over most of the interior 

of Mesa Chivato, and neither the number nor the exact 

location of the volcanoes is known. About 50 cones 

were counted in a view from the north flank of Mount 

Taylor. They seem more or less clustered, with 

broad lava flats separating them. Each of the few 

cones examined is composed of basaltic lava and cinders 

of scoria. No craters were found.

Some vitric and some lithic tuffs commonly underlie 

the basaltic sheets, and locally pumiceous tuff is irterbedded with the flows. The source of the tuffs is not 

known. The lithic tuffs were probably erupted by the 

vsnts that supplied the basaltic sheets, for lapilli 

of scoriaceous basalt are common, and such material 

was not erupted by the Mount Taylor volcano. However, tlie pumice is apparently rhyolitic and wpuld 

represent a separate, extreme differentiate if erupted 

by these vents. It may have been thrown out du*ing 

the final stages of the Mount Taylor volcano or may 

have an extraneous source.

These sheet eruptives include both andesite and 

basalt. At Garter Mesa, 2K miles southeast of Marquez, the lava is andesitic, being composed rougljly of 

40 percent of calcic andesme, 10 percent of olivine, 

10 percent of augite, 5 percent of magnetite, and 

30 percent of glass. There are a few large phenocrysts of olivine. At the southeast corner of Mesa 

Chivato, 4% miles east of Picacho Peak, are two basaltic 

flows. Both have distinct amygdules coated ydth 

zeolites. The lower flow is composed roughly of 65 

percent of labradorite, 15 percent of olivine, traces 

of magnetite, and glass. The upper flow is competed 

roughly of 50 percent of sodic labradorite, 5 percent of
66 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

magnetite, and a feltlike mass of glass and tiny anhedral crystals of mafic minerals.

Andesite occurs in the lower part of Canyon Colorado, near the north base of the Mount Taylor cone. 

The flow apparently came from a cone just west of the 

creek. The flows slope 5° S. on the south side of the 

cone and extend a few hundred feet up the creek, overlapping the eroded flows of porphyritic andesite. To 

the north, the flowTs slope nearly 12° near the cone but 

flatten downstream and extend onto Mesa Chivato. 

The flows are andesitic, dark gray, and nonporphyritic. 

The composition is roughly 75 percent of calcic andesine, 10 percent of olivine, 10 percent of augite, and 5 

percent of magnetite.

Erosion around the edges of Mesa Chivato has locally 

exposed the roots of cinder cones and has therefore produced natural cross sections of these volcanoes. At one 

of these, near the head of Marquez Canyon, the exposure 

shows an inner core of dense basalt locally chilled against 

an outer zone of basaltic breccia. The plug is 800 to 

1,000 feet in diameter, and the exposed section is about 

600 feet high. It intruded thick sandstone and shale 

of the Mesaverde formation, built a cone on the high 

surface of the mesa, and contributed to the lavas on 

that surface. Undisturbed and unaltered sandstone of 

the Mesaverde is exposed 30 feet from the breccia, but 

the contact is concealed. The dense central core grades 

upward to scoriaceous and even ropy basaltic lavas that 

cap the mesa. The scoriaceous parts of the outcrop are 

commonly weathered to brick red, but the dense portion is deep gray. The core is jointed in columns that, 

near the base of the exposure, dip 30° towrard the outer 

contact. Upward the columns are masked, but to judge 

from other localities they probably assume a nearly vertical position. The columnar jointing gives way to 

platy jointing at 3 to 5 inches in from, the contact with 

basaltic breccia. The contact between the platy-jointed 

basalt and the breccia is sharp but is crenulated for a 

width of 2% inches, whereas the boundary between the 

platy jointing and the columnar jointing is regular and 

does not follow these crenulations. The breccia consists of an irregular assemblage of ropy, scoriaceous 

basalt, dejise basalt, and boulders of sedimentary rock 

in an extremely friable glassy matrix. Boulders of 

sandstone are also present in the core cf the plug, but 

they are less abundant toward the center and top.

LA JARA MESA

The pediment north of Mount Taylor was probably 

continuous around the west side of the mountain and 

across La Jara Mesa. Near Mount Taylor the pediment is cut across rhyolitic tuffs and porphyritic andesite erupted by Mount Taylor, but to the west the surface truncates folded sedimentary rocks. Four vents 

in the southern part of the mesa supplied most of the 

basaltic lava cap. These lavas spread out in sheets, 

flowed against the erosion scarp developed at the west

edge of the Mount Taylor cone, and entered some of 

the valleys eroded in its flanks. In the eastern part of 

the mesa the sheets probably overlap porphyritic andesite erupted by Mount Taylor. Elsewhere they rest 

upon the rhyolitic tuff, except in the western part of 

the mesa, where they cover the truncated edges of the 

folded sedimentary rocks.

In the southern part of the mesa, near the west line 

of T. 12 N., R. 8 W., the lava is dark-gray hypersthene 

andesite. The main part of the flows is de-nse, but the 

upper part and bottom are highly scoriaceous. The 

dense portions of the flows are jointed in vertical columns. The thickness of individual flows ranges from 

about 50 to about 80 feet. The rock is composed of 

about 40 percent calcic of andesine, 10 percent of hypersthene, 5 percent of magnetite, and 45 percent of 

cryptocrystalline groundmass.

GRANT RIDGES

The Grant Ridges are three lava-capped mesas north 

of Grant and southwest of Lobo Canyor. The lava 

sheets on these mesas are basaltic, were undoubtedly 

connected, and may have been continuous with the 

basaltic lavas that cap Horace Mesa and possibly La 

Jara Mesa. The lavas on the ridges were supplied by 

at least four vents on the most northeasterly mesa. 

One of the vents is located along the southeast side of 

the mesa and provides one of the most instructive 

exposures in the volcanic field.

At this exposure erosion has developed a natural 

cross section of the volcano (pi. 11, A). Here a cylindrical pipe of basalt about 500 feet in diameter intruded 

the exposed tuffs and erupted onto the erosion surface 

cut on the tuff. Within the pipe basalt in contact with 

tuff is jointed in plates to a depth of a few inches. The 

remainder of the basalt is jointed in vertical columns. 

These relations are common in the Mount Taylor 

volcanic field and suggest that cooling in the pipes 

progressed downward from the craters rather than 

horizontally from, the contacts. The flat-lying beds of 

tuff at the locality shown in plate 11, A, illustrate the 

general absence of structural deformation accompanying the basalt intrusions in this volcanic field. Some 

important exceptions, however, are noted in descriptions of other localities. Above the tuff can be seen a 

cross section of the cone, which is composed of scoria 

and short flows of basaltic lava. The scoria consists of 

basaltic boulders, mostly less than a foot in diameter, 

in a friable glassy matrix. A very few small fragments 

of sedimentary rocks were found with the scoria. Exceedingly fluid basalt, erupted after the scoria, flowed 

southwest in sheets less than 100 feet thick for at least 

7 miles down a \% percent grade.

At the other vents kt the Grant Ridges thin bands of 

perlitic glass, locally squeezed into mhiute isoclinal 

folds, form the contact between the basrlt intrusions 

and the earlier rhyolitic tuff.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 67

The lava flows are mcstly basalt consisting of about 

30 percent of sodic labradorite, 5 to 10 percent of 

olivine, 10 percent of augite, and 5 percent of magnetite 

in a cryptocrystalline and glassy groundmass. Some 

andesite was found which consisted of about 60 percent 

of calcic andesine, 5 percent of olivine, 5 percent of 

augite, and 5 percent of magnetite in a cryptocrystalline 

groundmass.

Several basalt samples were collected by J. S. Diller 

"6 miles northeast of Grant",6 presumably from the 

northeasterly mesa of the Grant Kidges. They were 

analyzed by T. M. Chatard in the chemical laboratory 

of the Geological Survey with the following results:

Analyses of basalt from Grant Ridges

SiOs— _ _-------.___.__

Al2 03 _________--_-_ ___._.

Fe2O3 _ __ ___ __.___.__

FeO__- — __ — — .___-__

MgO_— __ -.---_..______

CaO--_--_._. _____.._.____

Na20. __.______-__.__._._

K2 0___ -_-__---__._-____

H2 0-_-------_--_. ______

Ti02 _____ _____________

P206 - — ------------

Cr2 03 - — - — ---. .--___

MnO___ — ----.__-__

(CoNi)O. ________________

BaO~___ — _-__—-. ------

C02—- — -_ —— -.-___-

A

47.54

16.73

6. 69

6. 67

6.38

8.74

2.81

1. 10

.36

2. 76

.51

10

.03

(?)

100. 51

B

47.06

7.77

1.30

8. 15

13.52

19.33

.33

. 11

.20

1. 82

.06

Trace

.20

Trace

199. 85

C

52.54

01 OK

oo

12 34.

3.55

.42

. 26

Undet.

Trace?

100. 65

A. Basalt 6 miles! northeast of Grant. Contains chiefly 

plagioclase, augite, and olivine, with much magnetite. 

B. Augite from A. 

C. Feldspar from A. Analysis on 0.75 gram of material.

HORACE MESA

The erosion surface beneath the sheet eruptives on 

Horace Mesa lies at about the same altitude as the 

surface in Mesa Chivato and La Jara Mesa and was 

probably originally a connected part of that surface. 

Where remote from the Mount Taylor cone the surface 

slopes about 50 feet to the mile away from the cone, 

but the gradient increases toward the cone. The surface is cut across the outer fringe of tuffs and porphyritic lavas erupted by Mount Taylor, but to the south 

it truncates tilted sedimentary rocks.

The lavas capping Horace Mesa were erupted from 

four vents of the central type and from one fissure. 

Plate 12, A, is a view of one of the cones. The descriptions of basaltic lava and cones on La Jara Mesa and 

Mesa Chivato apply to Horace Mesa, and only unusual 

features will be mentioned here.

Along the west side of Rinconada Canyon, about 

2,000 feet south of the experiment station of the New 

Mexico Agricultural College, is one of the few exposures 

of basalt erupted from fissures in this field. Here a

6 Clarke, F. W.. Analyses of rocks and minerals from the laboratory or the United 

States Geological Survey, 1880-1908: U. S. Geol. Survey Bull. 419, p. 123, 1910,

dike is exposed reaching the erosion surface on which it 

was erupted. An elongate cone has been built, marked 

chiefly by highly scoriaceous basalt and loose cinders. 

Apparently only a single flow was supplied by the 

fissure. Three-quarters of a mile south of this locality 

there is another exposure of basalt, probably eruf 4/ed 

along the same fissure. At this exposure there are two 

cones, each about 250 feet high.

Just east of the northernmost exposure of the eruptive is an outcrop of basalt interpreted as part of an 

early flow. The outcrop is1 about 200 feet wide on an 

eastward-trending promontory. The basalt lies upon 

6 feet of light-colored tuff, which in turn overlies 

Cretaceous sandstone. The tuff has been baked brick 

red by the basalt. The flow is about 30 feet thick, is 

jointed in vertical columns, and locally has a highly 

scoriaceous upper surface. Above the flow is rhycTite 

tuff erupted by Mount Taylor. This overlying tuff is 

not baked. At other localities basalt is known to have 

preceded the porphyritic andesite from Mount Taylor, 

but this is the only known locality where basalt preceded 

the rhyolite.

The basalt sheets on Horace Mesa were erupted after 

the folding of the McCarty syncline and also after the 

eruptions of Mount Taylor. But along the west side 

of the mesa a later fault, trending nearly due north, 

displaces the lava about 150 feet down to the west.

THE VOLCANIC NECKS

The sheet eruptives originally covered a much 

greater area than at present. At several localities 

cited above erosion has exposed natural cross sections 

of the volcanoes, but locally the lava has been completely removed from around the volcanic centers leaving only the resistant core. These cores, or necks, are 

striking topographic features in the area, particularly 

in the Rio Puerco Valley northeast of Mount Taylor. 

The general aspect of the necks has been discussed by 

Button,7 and subsequently Johnson 8 described many 

of the individual necks in more detail. The prerent 

study confirms the essential conclusions of the earlier 

investigators.

Presumably the basalt sheets that cap Mesa Chivato 

and Mesa Prieta were once more or less connected on 

surfaces that were continuous across what is now the 

intervening Rio Puerco Valley, and the necks are the 

roots of volcanoes that supplied the sheets. There are 

about 50 necks of various sizes and dimensions distributed through the volcanic field. Most of them are 

east of Mesa Chivato, but two are west of the mesa and 

two others south of Mount Taylor.

Some of the necks are composed mostly of baealt, 

others are mostly basaltic breccia, but most of them are 

a mixture of basalt and basaltic breccia. The breccia

' Button, C. E., Mount Taylor and the Zuni Plateau: U. S. Geol. Survey 6th Ann. 

Kept., pp. 166-179, 1885.

8 Johnson, D. W., Volcanic necks of the Mount Taylor region, N, Mex.: GeoJ, 

§90. America Bull., vol. 18, pp. 303-324, 1907,
68 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

is very friable and so easily eroded that necks of breccia 

are less conspicuous topographically than the others. 

The breccia consists of inclusions of sedimentary rock 

and basalt in a friable glassy matrix that commonly has 

a distinct flow structure. The basalt inclusions are in 

general highly scoriaceous and commonly have flow 

structure parallel to the flow of the matrix. This 

parallelism of flow structure in separated basalt inclusions indicates that they were plastic or liquid masses 

during the formation of the breccia.

With few exceptions the country rock adjoining the 

intrusions was not deformed. At two localities minor 

tilting of adjacent beds occurred, but there was no 

contact metamorphism except a very slight induration.

CABEZOHT PEAK

Cabezon Peak (pi. 13, A) is the highest and most 

impressive of the necks, rising nearly 2,000 feet above 

the Rio Puerco, and the protruding basaltic core is 

about 1,500 feet in diameter. The diameter of the pipe 

is probably at least 2,000 feet, as indicated by poor 

exposures of basaltic breccia around the base of the 

basalt core. The country rocks intruded by the neck 

are Cretaceous sandstone and sjiale that dip gently 

northward, and although the contact is not exposed 

there is no indication of deformation by the intrusion. 

The exposed neck is nearly cylindrical and is about 800 

feet high. It consists of dense columnar-jointed basalt 

below and highly scoriaceous basalt on top (pi. 13, 5). 

The joints in the dense basalt immediately below the 

scoriaceous top are nearly vertical, but downward they 

flare slightly toward the peripheral contact. It is not 

certain whether this flaring is due to greater depth, as 

Johnson 9 has suggested, or whether the base of the 

neck is closer than the top to the side contacts. The 

jointed columns range from a few feet to 8 feet in 

diameter. Throughout the neck the jointing is much 

more regular than is common in the other necks, 

probably because of the absence of basaltic breccia 

except along the contacts.

Cabezon Peak is slightly higher than the basalt 

sheets on Mesa Prieta, but these sheets rise gently 

northward and if projected would approximately coincide with the top of the peak. The thick cap of scoria 

on top of the peak is probably a remnant of a cone 

built on the surface of eruption.

The basalt at Cabezon Peak consists of about 60 

percent of calcic plagioclase (labradorite?), 15 percent 

of olivine, 20 percent of augite, and 5 percent of magnetite. The feldspar is anhedral, with indistinct 

borders. The olivine is commonly in considerably 

resorbed or altered phenocrysts. The augite is commonly in small euhedral cyrstals, but the magnetite is 

disseminated in slightly larger crystals.

• Johnson, D. W., Volcanic necks of the Mount Taylor region, N. Mes.: Geol. Soc. 

America Bull., vol. 18, p. 322, 1907.

CERRO DE IOG CJATES

The Cerro de los Cuates, 4 miles west of Cabezon 

Peak, is an elongate mass of intrusive basalt now rising 

about 500 feet above the nearby country. The exposed 

main intrusive trends east of north and is estimated to 

be about 1,200 feet long and 500 feet wide. The adjoining rocks are Cretaceous sandstone and eandy shale. 

A narrow dike, about 28 inches wide, can be traced 

northward for about 2 miles from a point near the 

north base of the neck. Probably the neck is an 

expanded part of the dike and thus appears to have 

been erupted along a fissure. This association of dike 

and neck is closer than elsewhere in this volcanic field, 

for most of the necks are of the central type.

There is little basaltic breccia at the Cerro de los 

Cuates except along the side contacts, which are best exposed on the east side. Jointing at high angles to the 

side contacts is well developed in the basalt. Toward 

the top of the north peak, however, the joints dip into 

the neck, but the significance of this attitude is uncertain, because in other necks the intrusion of basalt was 

preceded by basaltic breccia, and so this upward flaring 

of the joints may reflect contact with an overlying 

breccia mass.

Petrographically the basalt at the Cerro de los 

Cuates is like that at Cabezon Peak. In the one thin 

section examined the following percentages of essential 

minerals were estimated: Labradorite, f5 percent; 

olivine, 15 percent; augite, 15 percent; magnetite, 

nearly 5 percent. The feldspar is in small euhedral 

laths. The magnetite is finely disseminated, though 

locally it is concentrated as aggregates of minute 

crystals.

KECK IN GONZA1ES CAWYON

An inconspicuous intrusive mass of basaltic breccia in 

Gonzales Canyon, about 2 miles west of C^sa Salazar, 

provides some of the most interesting exposures in the 

volcanic field. (Se: e fig. 11.) The intrusion is nearly 

circular and about 500 feet in diameter, and no basalt 

core is exposed. The exposures are about 1,500 feet 

below the erosion surface on which tjie sheet basalts 

were erupted. The country rock is sandstone, separated from lower sandstones by 800 feet of shale and 

from higher sandstones by 400 feet of sandy shale.

The basaltic breccia consists of a friable glassy matrix 

containing fragments of scoriaceous basalt, and abundant xenoliths of sedimentary, rock as much as 3K feet 

in diameter. Most of the xenoliths are sardstone, and 

several facts indicate that most of them v^ere derived 

from the sandstone adjoining the intrusion: (1) Some 

of the sandstone xenoliths are abundantly fossiliferous 

(pi. 14, B}, and the fossils indicate a stratigraphic position no lower than the exposed sandstone; (2) sandstone 

xenoliths near the contacts of the intrusion are commonly larger, more angular, and more abundant than 

at the center of the intrusion, suggesting a gradual
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. HEX.

incorporation of the sandstone by the intrusive; (3) the 

sandstone surrounding the intrusion is highly fractured 

for a width of 250 feet, and breccia has been squeezed 

into some of the fractures.

The disturbed zone around the intrusion consists of a 

series of very small ringlike (concentrically arranged) 

faults whose traces are roughly parallel to the sides of 

the intrusion. The sandstone is downfaulted, and the 

faulted blocks are tilted toward the intrusion, producing 

a total subsidence of nearly 100 feet. Two narrow 

ring dikes of basaltic breccia were intruded along minor 

fractures.

The basaltic breccia at this intrusion is entirely 

similar to breccias at the large necks definitely related

FIGUEE 11.—Sketch map showing intrusive basaltic breccia in Gonzales Canyon. 

Tbr, Basaltic breccia; Kg, Mesaverde sandstone with strikes and dips indicated; 

Qal, alluvium.

to the sheet basalt series and is assumed to be a part of 

that series. The intrusion, however, may not have 

penetrated much higher than its present position. 

The breccia is interpreted as formed by the alteration 

of a roof rock foundering above a rising column of basalt 

(p. 72).

LA SEWORA PEAK

The neck referred to by Button and Johnson as Great 

Neck is locally called La Sefiora Peak. It is an intrusive 

mass of basalt and basaltic breccia. It is about threequarters of a mile in diameter and may extend northward for another mile and connect with the basaltic

breccia exposed there. Most of the intrusive is basaltic 

breccia, but toward the south there is increasingly more 

basalt, culminating in the peak. The basalt in the p^ak 

is jointed in columns that are vertical except near the 

base, where they flare toward the contacts. The top 

of the neck, as at Cabezon Peak, consists of scoria, which 

is probably a remnant of the cone or crater above the 

neck.

Sedimentary rocks, are exposed near the south side of 

the intrusive and are neither baked nor tilted. However, near the north base of the peak there is in the 

basalt a shale xenolith thoroughly indurated by baking. 

Although the metamorphism consisted only of the induration and baldng, this xenolith is in striking contrast 

to the unaltered country rock along the contact? of 

these intrusives. The metamorphism of the xenolith is 

attributed to its position above the basalt.

A puzzling exposure in the valley along the west side 

of La Sefiora Peak shows banded basaltic breccia oT^erlain by scoriaceous basalt and dipping 25° away from the 

peak. The contact between this breccia and the 

country rock is concealed except on the hillside above 

the valley, where the contact dips 20° into the breccia. 

Apparently this mass is now completely isolated from 

the basalt peak. The suggestion is strong that this is 

a surface flow, but better exposures of rocks in similar 

relations elsewhere indicate that the mass is intrusive.

SEBOYETA PEAK

Seboyeta Peak (pi. 16, .4), 2 miles east of the village 

of Seboyeta, is an elongate intrusive of andeintic 

breccia and andesite penetrating shale and tl inbedded limestone. It trends slightly east of ncHh 

and is about 1,500 feet long and about 800 feet wide. 

A nearly vertical contact with the shale is locally 

exposed along the east side of the neck. Probably 

the intrusive continues to the north and south as 

narrow dikes.

The neck is composed mostly of breccia intruded by 

irregular narrow dikes of andesite (pi. 16, B). The 

breccia consists of a chaotic assemblage of boulders of 

sedimentary rock and scoriaceous andesite in varying 

proportions in a glassy matrix. Banding is crude and 

nearly vertical.

A series of andesite dikes ranging in width from 1 to 

10 feet intrudes the breccia in the southern half of the 

neck. These dikes trend at an angle of 45° to the 

long axis of the neck and are spaced fairly regularly at 

intervals of 50 to 100 feet. In the north half of the 

neck the dikes are more irregular in trend and vary 

considerably in thickness. There are in addition many 

irregular stringers of andesite in the breccia, probably 

for the most part offshoots from the dikes. An 

interesting relation was found where one of these dikes 

intruding breccia is deflected around a boulder of 

sandstone,
70 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

The andesite at Seboyeta Peak consists roughly of 

45 percent of calcic andesine, 10 percent of augite, 15 

percent of olivine, and 5 percent of magnetite in an 

indeterminate groundmass that probably is mostly 

feldspar.

BASALTIC ROCKS BY NEW MEXICO STATE HIGHWAY 6

Where New Mexico State Highway 6 crosses the 

Rio Puerco there is exposed a conical mass of basaltic 

bfeccia and basalt south of the highway and a flow 

of basalt just north of the highway. The contacts of 

the conical mass are concealed by alluvium. At the 

edge of the alluvium interbedded basalt and banded 

basaltic breccia dip 45°-70° into the cone. The top 

of the cone is red, highly scoriaceous basalt. Whether 

this cone is truly a lava and cinder cone partly concealed by alluvium or merely a neck left protruding 

by erosion of the surrounding country cannot be fully 

determined. The basaltic flow with no other known 

nearby source suggests that the conical mass is a lava 

and cinder cone. However, masses similar to this 

cone and known to be necks are exposed in other 

parts of the Mount Taylor field.

The basaltic flow north of the highway overlies 

part of the Santa Fe formation and is overlain by higher 

beds of that formation (pi. 19, J5). At the largest 

exposure adjacent to the highway the lava is tilted 

above 8° S., and a small fault displaces the north edge 

of the exposure. Other exposures to the northeast 

show that the dip of the lava changes to east in conformity with the change in dip of the Santa Fe beds. 

The flow is much lower than the sheet basalts nearer 

Mount Taylor, but this may be due partly to downfaulting (p. 77). The flow is probably either contemporaneous with or younger than the sheet basalts around 

Mount Taylor.

SIERRA ALESNA

The Sierra Alesna lies northwest of Mount Taylor, 

about 10 miles north of San Mateo. The neck intrudes 

sandstone and shale and rises 1,200 feet above the surrounding area. The country rock is well exposed on 

the north side, about 600 feet below the top of the neck, 

and though within 175 feet of the neck is neither tilted 

nor baked. The intrusive mass is nearly circular and 

therefore of the central type, but several outliers of 

andesite south of the neck are undoubtedly connected 

with it by a dike trending slightly east of north.

The neck is composed entirely of dense andesite 

jointed in nearly vertical columns. Near the base the 

joints dip about 70° toward the contact, but they curve 

upward and at the top dip about 80°. These joints 

produce the structural control that has given the neck 

its awl shape, from which the Spanish name for the 

peak was taken. Around the base of the neck the 

joints are a little over 1 foot in diameter. They are 

four- to six-sided, though most have only four sides.

The andesite consists of about 40 percent of oligoclase, 10 percent of hypersthene, 35 percent of augite, 

5 percent of olivine, and 10 percent of magnetite.

OTHER VOLANIC NECKS

The other necks are also composed of es.rlj basaltic 

breccia and later basalt intruded into Upper Cretaceous 

sandstone and shale. A few of the necks are elongate 

and may have been erupted from fissures, but most of 

them are cylindrical (pis. 12, B; 15, A). There is a 

very slight upward drag of strata in contact with the 

neck east of La Abra de los Cerros. At the small 

andesite intrusions 4 miles east-northeast of La Senora 

shale in contact with the andesite is indurated for about 

1 inch from the contact, but at 8 inches from the contact no alteration is discernible. Elsewhere the intrusions neither tilted nor altered the adjoining country 

rock. A contributing reason for the general absence of 

metamorphism, however, is probably the common occurrence of breccia between the basalt and the country 

rock.

The basaltic breccia consists of xenoliths, some of 

them many feet in diameter, of sedimentary rocks and 

basalt. Most of the basalt inclusions are highly scoriaceous, but some are dense. The fragments of sedimentary rock are generally well rounded and enclosed 

by a zone of alteration. The matrix of tH breccia is 

friable glass, commonly with distinct flovT structure. 

Much of the breccia shows banding, which is at least 

partly due to varying proportions of sedimentary and 

basalt inclusions. The bands are commonly tilted but. 

not folded. Isolated inclusions of basalt as a rule show 

a flow structure mutually parallel and parallel to the 

banding. At Santa Rosa Peak, as at La P^ilora Peak 

(p. 69), the banded breccia dips at fairly steep angles 

away from the neck. More commonly, however, the 

breccia along a contact dips toward the neck, generally 

between 10° and 40° (pis. 14, A; 17, J5). Breccia is 

generally present along the contacts, but irregular 

masses of breccia also occur throughout some of the 

necks.

Many of the necks have cores of basalt, and this 

basalt was intruded after the breccia rras formed. 

Some of these cores contain little or no breccia, as at 

Evans Peak (pi. 15, A, I?); others contain abundant 

breccia—for example, the neck west of La Abra de los 

Cerros and Cerro Chato. Still other necks contain 

intermediate proportions of breccia and basalt. Wherever breccia is scarce the basalt is jointed in fairly 

regular columns, and near the contacts the columns are 

parallel to platy joints that are developed along the 

contact. At these localities, therefore, cooling of the 

basalt progressed downward from the crater rather 

than inward from the contacts. Where breccia is associated with the basalt there is generall^ a zone of 

platy-jointed basalt next to .the breccia, and the columns 

show a strong tendency to curve toward the plates.
GEOLOGICAL SURVEY

A. VOLCANIC NECK EAST OF LA ABRA DE LOS CERROS.View looking east. The country rock is Cretaceous"shale (Ku). There is a vertical contact between the shale and the banded basaltic breccia 

(Tbr) that dips into the neck. Irregular masses of andesite or basalt (Tan) with columnar jointing intrude the breccia.

PROFESSIONAL PAPER 189 PLATE 14

B. XENOLITH OF FOSSILIKEROUS CRETACEOUS SANDSTONE 

IN BASALTIC BRECCIA IN NECK IN GONZALES CANYON.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 15

*

A. VIEW FROM THE EAST. 

The neck is composed of andesite (possibly basalt) and protrudes about 1,100 feet above the level of the foreground. Photograph by D. A. Andrews.

B DETAIL OF COLUMNAR JOINTING AND OUTER SHELL OF IRREGULAR PLATY JOINTING.

Photograph by R. C. Hecker.

VIEWS OF EVANS PEAK.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 16

v4. VIEW LOOKING SOUTHWEST TOWARD SEBOYETA PEAK. 

The peak is composed mostly of andesitic breccia intruded by andesite dikes, some of which can be seen at the left protruding above the lower part of the neck.

B. XENOLITHS OF SEDIMENTARY ROCK IN BRECCIA ALONG EAST SIDE OF SEBOYETA PEAK. 

Andesite dikes, protruding as dark masses, can be seen at the upper left.
GEOLOGICAL SURVEY ' PROFESSIONAL PAPER 189 PLATE 17

A. DIKE OF BASALT AND BASALTIC BRECCIA INTRUDING 

CRETACEOUS SHALE.

In sec. 3, T. 14 N., R. 3 W.

C. ANGULAR UNCONFORMITY BETWEEN SANTA FE FORMATION AND UNDERLYING CRETACEOUS

ROCKS.

The Santa Fe (Tsf) dips 10° S. 20° W.; the Cretaceous rocks (Ku) dip 18° S. 50° E. View looking south in center of sec. 3,

T. 14 N., R. 1 E.

B BANDED BASALTIC BRECCIA LYING AT OUTER CONTACT 

OF A LARGE BRECCIA MASS.

The banded breccia dips into the mass. In sec. 19, T. 14 N., R. 4 W.

D. ANGULAR UNCONFORMITY BETWEEN SANTA FE FORMATION AND UNDERLYING CRETACEOUS

ROCKS.

The Santa Fe (Tsf) dips 7° S.; the Cretaceous rocks (Ku) dip 30° S. View looking east in center of sec. 4, T. 14 N., R. 1 E.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 71

These relations occur around xenolithic masses of 

breccia as well as against the breccia along the contacts 

(fig. 12).

Even in the very small intrusive 4 miles east-northeast of La Sefiora Peak there is evidence of considerable loss of heat upward. The rock is andesite jointed

N. S.

25feet

Ku

FIGUBE 12.—Diagrammatic section through north side of Juan Tafoya Peak. Ku, 

Cretaceous shale and sandstone; Tt>r, basaltic breccia; Te, basalt. The trend of 

columnar joints in the basalt is indicated by the broken lines.

in columns that in plan radiate from the center of 

the mass (fig. 13). In cross section, however, the 

columns are horizontal against the outer contact with 

Cretaceous shale, but inward the columns curve upward 

and at the center are nearly vertical. At Picacho Peak, 

southeast of Mount Taylor, the joint relations provide 

a notable exception to these general descriptions. The 

intrusive mass is elongate and consists wholly of dense 

andesite jointed in columns that are horizontal and 

normal to the side contacts.

The basalt in most of the necks contains 60 to 70 

percent of sodic labradorite or calcic andesine, 15 to 

25 percent of augite, 5 to 15 percent of olivine, and 

magnetite. The olivine occurs commonly in large 

phenocrysts: one that was collected is nearly 1 inch 

long. Where basalt solidified against breccia considerable glass developed, but where basalt solidified 

against basalt there is little glass.

BASALT DIKES

Numerous basalt dikes, probably related to the basaltic sheet eruptions, are exposed in the Mount Taylor 

volcanic field. All trend north, conforming to the 

regional strike of the faults, but only one (p. 72) is 

known to have been intruded along a fault surface. 

The dikes are most abundant east of Mount Taylor 

and Mesa Chivato, but a few are locally exposed south 

of the mountain. The dikes are thin, most of them 

about 2 feet thick, but some attain a maximum of 

about 30 feet.

On the north line of sec. 2, T. 14 N., R. 4 W., about 

3 miles west of Casa Salazar, a nearly vertical dike 

trends slightly east of north. The dike intrudes shale

and can be traced almost continuously for several hundred feet. It is nowhere more than 2 feet in tl ickness 

and generally about 18 inches. Locally the dike is 

composed wholly of basalt horizontally jointed in small 

columns normal to the side contacts. Elsewhere basaltic breccia is associated with the basalt, occurring 

along one side of the dike or more commonly in the 

middle. Plate 17, A, shows an outcrop where basaltic 

breccia separates two basalt portions of the dike, a 

relation that persists for a considerable distance along 

the outcrop. This outcrop is particularly interesting 

in connection with the necks, because the breccia, like 

that in the necks, contains small xenoliths of sedimentary rocks and highly scoriaceous basalt in «, friable 

glass matrix. On each side of the breccia there is 

basalt 6 to 8 inches wide jointed in horizontal columns. 

The basalt in contact with the shale is dense and nonscoriaceous, but there are vesicles along its contact 

with breccia, strongly suggesting that the breccia allowed release of pressure existing in the fluid basalt. 

This exposure is about 1,500 feet below the erosion 

surface on which the sheet basalts were eruptec1 . The 

lithology of this dike suggests that it is related to the 

sheet basalts and to the necks, but it may have been 

intruded during the later period of minor faulting that 

occurred in this part of the volcanic field wl en the 

drainage system was between 100 

and 900 feet above its present 

position (p. 75). The basalt in 

this dike consists chiefly of calcic 

andesine with some augite and 

traces of magnetite in a glassy 

groundmass.

A basalt dike trending east of 

north crops out at intervals near or 

along a fault about 3 miles westnorthwest of Casa Salazar. If the 

dike is continuous, as seems likely, ^ 

its thickness ranges from about 18 u"

, i n f i TI • i FIGURE 13.—Diagram of co- inches to 6 teet. It is composed ]umnar joilltjng in intrumostly of basalt crudely jointed

in horizontal columns. At one

locality a dikelike offshoot extends

a few hundred feet to the west.

Where it joins the main dike there

is considerable breccia, and the associated basalt is very

irregularly jointed.

At Paguate Indian pueblo a dike crops out ttat can 

be traced southward for about 2 miles and northward 

for about 1 mile. The dike ranges from 2 to 12 feet in 

width and intrudes thick sandstones and inter! ^dded 

shale. It is mostly breccia, consisting almost wholly 

of friable glass containing small fragments of sedimentary and igneous rock, with locally a ver^ thin 

border zone of basalt. The dike is more easily eroded 

than the enclosing sandstones and forms a depression 

where it cuts them. It is about as susceptible to erosions 4 miles east-northeast 

of La Sefiora Peak. A, Plan 

of radial jointing*; B, Eastwest cross sectioi showing 

joints curving upward near 

center of intrusion.
72 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

sion as the soft shale, and where shale is the country 

rock exposures of the dike are few except in arroyos. 

No evidence of alteration of the country rock was 

observed anywhere along the dike.

Only one dike is known in the severely faulted portion 

of the area. The outcrop, a mile southeast of the Ojo 

Escondido fault, 4% miles east of Suwanee, was found 

by G. F. Taylor. The dike trends about N. 25°-30° E. 

and dips 65° NW. The trend closely parallels that of 

the many nearby faults, but no displacement was 

found along the dike. The rock is finely crystalline 

andesite composed mostly of calcic andesine, considerable augite, a little magnetite, and traces of olivine.

Another dike crops out about 1 mile northeast of 

McCarty, south of Mount Taylor, and there are 

several others near Chico Arroyo, north of Mesa 

Chivato. Most of these dikes parallel the trend of 

nearby faults, but, so far as known, none of them follow 

fault surfaces.

At the mouth of Einconada Canyon, south of Mount 

Taylor, H. R. Joes ting and R. C. Becker found Cretaceous sandstone and shale displaced 150 feet by a 

fault along which a basaltic dike was intruded. The 

dike is 30 feet wide and closely follows changes in 

trend of the fault. Toward the south end of the exposure the dike widens considerably, and higher volcanic rocks funnel out over tuffs, about 70 feet thick, 

that probably belong to the rhyolite tuff series erupted 

by Mount Taylor. The higher volcanic rocks may be 

a remnant of an elongate cone built by a fissure eruption. Both the fault and the dike die out beneath the 

cone and do not extend farther south. This is the only 

known locality in the Mount Taylor volcanic field 

where a dike follows a fault fissure. The occurrence 

is also unusual for this field in that the basalt is relatively very coarsely crystalline and could be described 

as dolerite. It is composed of sodic labradorite, augite, 

and minor quantities of olivine and magnetite.

ORIGIN OF THE BRECCIA

It seems probable that the breccia is a nonextruded 

byproduct of the intrusive process. My reasoning is 

not that certain facts indicate an intrusive process but 

that those facts are difficult to explain by a theory of 

pyroclastic origin. The evidence may be summarized 

as follows:

1. Under a theory of pyroclastic origin each breccia 

mass must represent several falls of fragmental material. 

But the material now preserved could not have fallen 

back into the vents after each explosion, for the bands 

are a conformable series. Each breccia mass must 

have fallen back as a large unit block (pi. 17, B), a landslip from the cone. But none of the preserved cones 

are composed of such material.

2. If the tremendous quantity of breccia now preserved in the necks represents pyroclastic material 

that fell back into the vents some pryoclastic material

should be preserved with the lavas. There is practically none. Thin beds of light-colored tuff locally 

interbedded with the basalt sheets bear no resemblance 

to the basaltic breccia. Locally where there are natural 

cross sections of the volcanoes, as in Marquez Canyon, 

large masses of breccia are preserved along tl Q, walls of 

the pipe, but little or no breccia occurs with the overlying flows. Some basaltic flow breccia exposed in the 

Grant Eidges (pi. 11, A) differs from the breccia in the 

necks by the comparative absence of a cementing glass 

matrix and very low proportion of sedimertary rock 

fragments. It consists almost wholly of boulders of 

frothy basalt.

3. As the breccia is lighter than the basalt the breccia 

masses, if pyroclastic, must have fallen into open conduits. But some of the conduits are ver^ narrow 

fissures many hundreds of feet deep in poorly consolidated shale (p. 71; pi. 17, A). It is unlikely that they 

could remain open to collect debris from tie surface.

4. At the breccia neck in Gonzales Canyon most of 

the fragments are sandstone, in part fossiliferous and 

surely derived from the adjacent sandstone formation 

(p. 78). These inclusions are practically at their original 

location. Furthermore, they grade from large angular 

blocks near the side contacts to fine particles in the 

interior of the intrusion.

These relations pre more consistent with an assumption that the breccia is of nonextrusive origin. But the 

rising columns of basalt did not fold or fault the adjacent 

country rock and produced practically no drag: along the 

side contacts, so the breccia is attributed to progressive 

alteration and incorporation of roof rock by the rising 

intrusions. Each of the strata is visualized as being 

progressively more altered and deformed as the intrusions rose, and at any given time there was complete 

gradation from undisturbed roof rock through breccia 

to relatively unadulterated basalt.

Whatever hypothesis is proposed must confine the 

disturbing effects of the intrusions to a pipelike zone 

through. the roof rock, and the subsequent intrusion 

must occupy all of that pipe, because the sedimentary 

rocks beside the intrusions are not disturbed. Probably 

the pipe of the deformed strata was a long slender cone, 

and in each stratum the firpt eflect of thy intrusion was 

at a point—the apex of the cone. As intrusion continued the cone of disturbance was extended upward, 

and in each of the strata the disturbed area was 

extended laterally.

Probably the first effect was a baking and induration. 

This would culminate in abundant jointing and, in 

water-bearing sandstone, further fracturing by steam 

explosions. A push upward or a downward settling at 

this stage would develop minor faults across and ring 

faults around the fractured zone, contributing to further 

break-up of the roof. The upper part of the laiTa column, 

probably more frothy and richer in volatile constituents 

than the main column, could be intruded alon^ the frac-
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 73

ture planes, and the addition of this material would convert the breccia to a plastic mass. Considerable churning of the plastic breccia would comminute the fragments of sedimentary rock and intimately mix the fine 

particles with the enclosing medium. Irregular stringers 

of basalt intruded at this stage would be churned in 

with the breccia and form isolated and relatively pure 

masses of basalt in the breccia. Rise of the basalt 

column through this mass would squeeze the breccia 

against the side contacts and there develop the banding 

and flow lines.

This hypothesis does not satisfactorily explain why 

the banding and flow lines of the breccia masses so 

commonly dip into the intrusions. But to assume that 

the masses were rotated to this attitude by rise of the 

adjacent basalt column is as reasonable as to assume 

that they descended from the surface into open conduits, 

as required by the theory of pyroclastic origin.

^Basalt in the breccia is commonly highly scoriaceous, 

and this is the chief basis for the pyroclastic hypothesis. 

However, the superior jointing of basalt in contact with 

breccia indicates that the breccia was more potent in 

promoting consolidation than the sandstone or shale of 

the country rock. Furthermore, basalt in contact with 

breccia commonly developed vesicles, but basalt of the 

same intrusion in contact with sedimentary rocks is 

dense. Apparently, therefore, the breccia allowed relief of pressures and release of any gases that may have 

been contained in adjacent basalt, and this facilitated 

consolidation with development of vesicles.

SUMMARY OF BASALT ERUPTIONS

Sheets of basalt were erupted from a large number of 

vents distributed around Mount Taylor. One local 

basalt eruption preceded the rhyolitic tuff, and a few 

basalt flows preceded the later flows of the porphyritic 

andesite series, but the greatest volume of basalt was 

erupted shortly after the Mount Taylor volcano had 

become quiescent. Most of the flows spread out in 

sheets on erosion surfaces developed around the Mount 

Taylor cone and locally overlapped the sides of the 

cone. The sheets are preserved today capping high 

mesas that extend as foothills away from the Mount 

Taylor cone. Most of the sheets were supplied by 

eruptions of the central type, though a few fissure 

eruptions are known. Apparently the basalt made its 

way through the upper part of the crust by some process 

other than forcible injection, because even where the 

country rock is soft shale the eruptions caused no 

deformation.

RECENT FLOWS

Very recent flows of basalt are found in the valley 

bottom of the Rio San Jose along the south border of 

the volcanic field. These flows were given only casual 

attention, because none were supplied by vents within 

the field. Some of the vents are along the east side of

the Zuni Mountains, others are north of the villlage qf 

Bluewater, and others are near Laguna Pueblo. The 

flows have exceedingly fresh surfaces, which, combined 

with their position in the valley bottom, make them 

appear as if they had been flowing only yesterday. 

These flows are more ferromagnesian than most of the 

sheet eruptives of the Mount Taylor volcanic field. 

They consist of labradorite with a high percentage of 

olivine, slightly less augite, and some magnetite.

STRUCTURAL GEOLOGY

GENERAL STRUCTURAL RELATIONS OF THE VOLCANIC FIELD

Most of the Mount Taylor volcanic field is in the 

Colorado Plateaus province, but the field extends eastward into the Basin and Range province. (See fig. 3.)

The Colorado Plateaus are represented in northwestern New Mexico by the San Juan Basin, a broad structural depression. In the interior of the basin the beds 

are nearly flat, but on all sides they rise gently to the 

bordering mountain uplifts. The north flank of the 

basin, in southwestern Colorado, is a steep monoclinal 

flexure produced by the uplift of the San Juan Mountains. A monocline forming the west flank of the basin 

is produced by the Defiance uplift and by a laccolithic 

injection at Carrizo Mountain. The east flank of the 

basin is in part a steep monocline along the Nacimiento 

Mountains, a range of the southern Rocky Mountains, 

at the northeast corner of the Mount Taylor volcanic 

field. From the base of the monocline westward the 

beds dip gently toward the interior of the basin. The 

Nacimiento Mountains consist of a central core of 

basement rocks uplifted in an asymmetric anticline, 

with local reverse faulting along the steep west flank. 

On the south side alone the basin is not bordered by a 

monoclinal fold. A gentle northward dip prevail? there, 

produced by the upli e t of the Zuni Mountains and Mesa 

Lucero. The Zuni, Lucero, and Nacimiento uplifts 

may have been more or less contemporary in the interval between early Eocene and late Miocene time. 10

The south flank of the San Juan Basin, on which 

Mount Taylor is located, departs considerably from a 

simple basinward dip. West of the Zuni Mountains is 

the Gallup-Zuni Basin, a northward-plunging asymmetric syncline. Immediately east of the Zuili Mountains is the northward-plunging McCarty syncline, 

a broad, shallow, symmetrical fold. The McCarty 

syncline extends northward into the south flank of the 

San Juan Basin and plunges steeply under the Mount 

Taylor volcano. The area shown on plate 7 therefore 

includes the northern extension of the McCarty S7ncliiie 

and parts of the north flanks of the uplifts on each side 

of the syncline.

The central and western parts of the Mount Taylor 

volcanic field, limited eastward by the Alamosa fault, 

San Ignacio monocline, and Reservoir fault, are in the

10 Hunt, C. B., Tertiary structural history of part of northwestern New Mexico 

[abstract]: Washington Acad. Sci. Jour., vol. 24, pp. 188-189,1934.
74 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

San Juan Basin and constitute the portion of the 

volcanic field in the Colorado Plateaus.

The southwest quarter of New Mexico is in the Basin 

and Range province. It is an area of abundant faulting 

and more or less folding. In the southern part of the 

State, west of the Rio Grande Valley, there are overthrusts combined with folds and intersecting sets of 

normal faults. Along the valley, however, there are 

only broad northward-trending folds broken by normal 

strike faults that produce horsts and grabens. The 

folded and faulted belt of the Rio Grande Valley extends 

northward across the eastern part of the Mount Taylor 

volcanic field to the south tip of the Nacimiento 

Mountains.

In the Mount Taylor volcanic field, east of the 

Alamosa fault, San Ignacio monocline, and Reservoir 

fault, the block-faulted Cretaceous strata are exposed, 

but farther east they are concealed by the fluviatile 

Santa Fe formation. This part of the volcanic field is 

the east edge of a graben that extends 25 miles eastward 

past Albuquerque to Sandia Mountain. The Santa Fe 

formation, deposited in the topographic depression that 

resulted from the graben faulting, thickens rapidly 

eastward. The part of the Mount Taylor volcanic 

field lying in the graben constitutes the Basin and 

Range portion of the field.

The structural deformation of the Mount Taylor 

volcanic field began probably in late Miocene time and 

continued through the Pliocene (p. 56, fig. 8). In the 

Colorado Plateau portion of the field the movements 

began with the uplift of the Zuni Mountains and Mesa 

Lucero, which produced the regional northward dip 

and the folding of the McCarty syncline. The date of 

the folding and faulting northwest of the syncline and 

the gentle warping just east of it is uncertain, but they 

were probably contemporaneous with these early 

movements. This deformation preceded the eruptions 

of Mount Taylor. Minor faulting east of the McCarty 

syncline and minor folding and faulting within the 

syncline occurred much later, after the eruption of the 

post-Mount Taylor sheets of basalt. In the Basin 

and Range portion of the field deformation was nearly 

completed before eruption of the sheet basalts and 

deposition of the Santa Fe formation, but some movements continued until after the eruptions, while the 

Santa Fe was being deposited.

COLORADO PLATEAU PORTION OF THE FIELD

McCARTY SYNCLINE

The largest fold in the Mount Taylor volcanic field 

is the McCarty syncline, the axis of which trends about 

N. 20° E. across the central part of the field. (See 

pi. 7 and fig. 3.) South of this field the syncline is 

shallow and symmetrical, with flanks rising gently to 

the bordering uplifts, the structural relief for 10 miles 

on each side of the axis being only about 600 feet. At 

the south border of the Mount Taylor field there is an

abrupt northward plunge. Under Mount Taylor the 

structural relief for 10 miles on each si'de of the axis is 

at least 1,500 feet. North of Mount Taylor the northward plunge of the syncline is considerably less than 

the regional dip, and near the north border of the 

volcanic field the syncline dies out into the regional 

dip. Near Mount Taylor the syncline is strongly 

asymmetric, the west flank being a steeply dipping 

monocline. This monocline extends from the south 

side of T. 12 N., R. 9 W., to the north side of T. 14 N., 

R. 8 W. Many dip faults, mostly small, fracture the 

northern part of the monocline. Two miles west of 

San Mateo there is a strike fault with a downthrow of 

about 700 feet to the east, which apparently takes up 

most of the monoclinal displacement and dies out under 

La Jara Mesa. South of this mesa a second monocline 

produces about 1,600 feet of structural relief in about 

1 mile. This monocline (fig. 5) is probably faulted, 

but the outcrops are so few that detailed recorstruction 

of it was not attempted.

Apparently the east flank of the McCarty syncline 

in the Mount Taylor field is fairly regular, and it dips 

westward at about 200 feet to the mile.

The sheets of basalt that followed the eruptions of 

Mount Taylor lie on erosion surfaces cut across the 

edges of strata dipping into the deepest part of the 

McCarty syncline. On La Jara Mesa the erosion 

surface and overlying eruptives truncate the nonocline 

at the south side of the mesa (fig. 5) and the strike 

fault at the north side. Erosion surfaces and overlying 

sheet basalts truncate the northward-plunging syncline 

along the south side of San Fidel Mesa. Similar 

truncation of the east flank of the syncline is seen in 

the canyons that have been eroded into the east side 

of Mesa Chivato.

MINOR FOLBS WITHIN McCARTY SYNCLINE

There are several minor folds south of Mount Taylor 

in the McCarty syncline. At the head of Einconada 

Canyon, in the eastern part of T. 11 N., R. P W., is a 

dome covering about \% square miles and having a 

structural relief of about 1,000 feet (pi. 7).

The attitude of the Mesaverde beds at the head of the 

north fork of Lobo Canyon, in the southern part of 

T. 12 N., R. 8 W., indicates another closed fold. 

Because of the uncertain position of the exposed rocks 

in the Mesaverde formation, this fold was not contoured. 

Probably, however, the structural relief of this fold is 

not as great as that of the fold at the head of E,inconada 

Canyon.

The location of these two folds at the heads of the 

two largest canyons draining the southwest, side of 

Mount Tayior is puzzling. No direct evidence was 

found indicating that the two folds are later than the 

eruptives capping the canyon rims. Although the 

outcrops of the flows nearly parallel the strike of flanks
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 75

of the folds, these outcrops are so far from the area of 

steepest dips that no disturbance of the eruptives was 

noted. It would seem reasonable to interpret these 

domes as later than the lavas, including the sheet 

basalts, and also to regard the canyons as having been 

eroded because of the disruption of the resistant lava 

cap. But the basalts overly ing the dome in Canyon 

Seco, near the south line of T. 11 N., R. 7 W., are not 

folded and thereby discount that interpretation.

A remarkable feature of the dome in the southeastern 

part of T. 11 N., R. 8 W., is the abrupt flexure of the 

west flank. There is no intermediate dip between the 

flat beds to the west and the nearly vertical beds of the 

west flank. This abrupt change in dip and the fact 

that these domes produce considerable structural relief 

within very small areas suggest forces of igneous 

injection, but no evidence was found of alteration or 

baking of beds on the crests of the domes.

Folded sedimentary rocks belonging to the Mesaverde formation are exposed in the bottom of the 

Mount Taylor amphitheater near the east side. The 

easternmost beds of this exposure dip 20° E. They 

are overlain by not less than 400 feet of rhyolitic tuff. 

The tuff also dips to the east, but the dip ranges from 

10° to 20° and may be depositional rather than structural. The source of the tuff was the crater of the 

Mount Taylor volcano, not more than 2 miles to the 

west, and the tuff may have accumulated on dip slopes. 

No exposures of tuff were found associated with the 

steeper northward-dipping beds of the northernmost 

part of this Mesaverde exposure (p. 58).

That some of the faulting within the McCarty 

syncline was later than the formation of the high erosion 

surfaces and of the sheet basalt is indicated by the 

relations along the southwest side of Horace Mesa, 

where a fault trending slightly west of north drops the 

erosion surface and overlying basalt 150 feet on the west.

FOLDS NORTHWEST OF McCARTY SYNCLINE

Northwest of the McCarty syncline the prevailing 

northward dip is interrupted by several folds. In the 

Mount Taylor volcanic field the folds are large and 

prominent structural features, as shown by the structure contours on plate 7. Westward, however, folding 

progressively diminishes, and near the west border of 

the adjoining area, mapped by Sears,11 only broad and 

very gentle undulations interrupt the regional dip. 

There is a similar diminution of intensity of folding to 

the northwest. 12 The folds in the western part of the 

Mount Taylor volcanic field therefore represent a maximum of the deformation and are not representative of 

the south flank of the San Juan Basin.

The two sets of faults that are present in the Mount 

Taylor volcanic field northwest of the McCarty syncline

« Sears, 3. D., The coal field from Gallup eastward toward Mount Taylor, N. Mex.: 

U. S. Geol. Survey Bull. 860-A, pi. 1, 1934.

is Hunt, C. B., and Dane, C. H., Preliminary map showing geologic structure of the 

southern part of the San Juan Basin, N. Mex., U. S. Geol. Survey, 1933.

are found over much of the south flank of the basin. 

The faults are crowded in the areas of steepest folding. 

To the west and northwest, concomitant with the diminishing intensity of folding, the faults not only become progressively fewer but produce less displacement. 

The close relationship between degree of folding and 

degree of faulting strongly suggests an intimate genetic 

relationship between the two types of deformation. It 

seems likely that they were contemporaneous, but it is 

impossible to date the movement closely. The youngest beds involved are of lower Pierre age. Probably, 

however, the movements were much later than early 

Pierre time and were related to the folding of the 

McCarty syncline, the most southeasterly and largest 

of the folds, regarded as later than early Eocene and 

earlier than late Miocene (p. 57, fig. 8).

GENTLE WARPING EAST OF McCARTY SYNCLINE

East of the McCarty syncline the Cretaceou^1 rocks 

are very gently folded. These rocks rise toward the 

east and southeast, where the undulations are terminated abruptly by faults and a steep eastward-dipping 

monocline. This border of gentle folding lies along the 

Alamosa fault, San Ignacio monocline, and reservoir 

fault, at the southeast rim of the San Juan Basin.

Sheet basalts lie on erosion surfaces that truncate 

the northward-dipping and gently undulating strata 

east of the McCarty syncline. The warping is accordingly interpreted as probably contemporaneous with 

the folding of the syncline.

The faults in the north half of this portion of the field 

are paralleled by several basaltic dikes. There is, however, no alinement of the necks, and apparently none 

of these intrusions followed the faults as conduits to 

the surface. The faults seeni related to the blocV faults 

in the Basin and Range part of the field, for ttey certainly show a marked parallelism of trends.

Little evidence is available to limit the date of the 

faulting in this gently warped part of the area. Along 

the north side of Chico Arroyo, 5% miles west of the 

Cerro de los Cuates, Cretaceous rocks are displaced 50 

feet by a fault. However, gravel about 70 feet above 

the flood plain and 100 feet above the arroyo lies undisturbed across the fault. The final movements on 

this fault had therefore taken place prior to tl n, time 

when the stream .flowed 100 feet above its present position. If this faulting is contemporaneous with the 

fault along the southwest side of Horace Mesa the 

faulting occurred while the drainage system vas between 100 and 900 feet above its present position.

The part of the area between Mesa Chivato and Mesa 

Prieta was the scene of extensive volcanism when the 

sheets of basalt were erupted. These volcanic flows 

thus rose through the gently warped series of rooks and 

not through the folded and faulted area northwest of 

the McCarty syncline or the severely faultec1 Basin 

and Range part of the volcanic field.
76 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

BASIN AND RANGE PORTION OF THE FIELD

GENERAL STRUCTURAL RELATIONS

The most striking structural feature of the Mount 

Taylor volcanic field is the sharp boundary between the 

Basin and Range and Colorado Plateau portions of the 

field. The boundary is marked by the Alamosa fault, 

San Ignacio monocline, and Reservoir fault (pi. 7), 

which have dropped the much-faulted and steeply tilted 

rocks to the east with respect to the gently warped 

rocks of the plateau. There is practically no transition 

zone between the areas showing the two types of 

deformation.

The trace of the Alamosa fault was not found, but its 

position locally can be determined within a few hundred 

feet. The displacement is between 2,000 and 2,500 

feet (pi. 18, A). The fault dies out toward the north, 

but the displacement is maintained by the faulted San 

Ignacio monocline, which has an average dip of about 

40° E. (pi. 18, B). The monocline in turn dies out 

northward in a series of small faulted folds (pi. 7). 

Where the San Ignacio monocline dies out an arbitrary 

boundary line between the two provinces must be drawn 

eastward to the Reservoir fault, which, like the other 

boundary faults, drops the rocks in the Basin and 

Range province with respect to the Colorado Plateau 

part of the volcanic field. The Reservoir fault dies out 

northward in the southward-plunging folds produced 

by the Nacimiento uplift.

Several hundred faults are exposed in the Basin and 

Range portion of the Mount Taylor volcanic field. 

Most of these are small, but some produce displacements of a few thousand feet. So far as known all the 

faults are normal, and the displacement is generally 

down to the west. The faults commonly dip west at 

only moderate angles. The Ojo Escondido fault, for 

example, at one locality produces a displacement of 

3,000 feet, and the fault dips only 45° W. Several of 

the smaller faults, such as the Ojito fault, have equally 

moderate west dips.

Evidently the fault surfaces are very irregular. The 

dips of some faults change as much as 35° in short 

distances along the strike. Other faults have sinuous 

traces across areas of low relief. Striae on the fault 

surfaces can rarely be found, because the faulted rocks 

are very friable. The few striae observed are at high 

angles. These relations suggest that'movement on the 

faults was chiefly vertical.

There is a striking parallelism of the fault traces, the 

vast majority of which trend N. 10°-15° E. The 

apparent fault dips show less parallelism.

Very few of the faulted blocks have been folded. In 

general there is a fairly uniform east clip within any 

single block, but the amount of dip in different blocks 

is highly variable.

The following descriptions are confined to only a few 

of the most striking structural features.

FAULTS

The Ojo Escondido fault, in the Antonio Sedillo 

grant, pursues a sinuous course across an are??, of little 

topographic relief. It is a normal fault, with the west 

block dropped. The displacement increases northward 

to about 3,500 feet where the fault passes under cover 

of the Santa Fe formation. Locally along the fault 

there is a shattered zone, as much as 300 feet wide. 

The material in this zone ranges in size from finely 

crushed rock to boulders measurable in scores of feet. 

Where the zone thins out and the fracture is a single 

clean break the fracture surface dips 70°-45° W.

The Suwanee fault, about 2% miles west of the Ojo 

Escondido fault, also has considerable stratigraphic displacement down to the west. Where it emerges from 

alluvial cover at the south, the trend of the fault is 

N. 10° E. and the displacement is 500 feet. A mile to 

the north the fault trends N. 20° E., and where the 

fault passes under cover at the north the displacement 

is increased to nearly 1,200 feet.

East of San Ignacio there are nine nearly parallel 

faults, in general producing steps down to the west. 

The most westerly of this group, the Ojito fault, has a 

stratigraphic displacement of 500 feet and dip^1 45°-50° 

W. Most of the other faults dip between 70° and 85°.

The San Fernando fault is traceable for onH a short 

distance in T. 12 N., R. 1 W. It is worthy of special 

mention because with the exception of the boundary 

faults it is the one fault known to have corsiderable 

stratigraphic displacement down on the east side. The 

amount of the displacement is uncertain, although it is 

probably near 1,500 feet. Locally along the fault there 

are zones of shatter 200 feet wide. Southward the 

fault branches into several minor faults. To the north 

it passes under cover.

The Garcia fault, concealed except in sees. 30 and 31, 

T. 14 N., R. 1 E., has a minimum stratigrs.phic displacement of 2,200 feet down to the west. The true 

figure is probably nearer 3,000 feet. The fault is poorly 

exposed and passes under the cover of the Santa Fe 

formation both to the north and to the soutl.

FOUDS

The southern part of the block that is faulted down by 

the Ojo Escondido fault has been folded into r, syncline 

whose axis trends about N. 45° E. and plunges northeastward into the fault.

In the southeastern part of T. 12 N., R. 3 W., there 

is a northward-trending syncline a short distance east 

of the San Ignacio monocline.

At the north edge of the Basin and Range p^,rt of the 

volcanic field two synclines and an anticline plunge 

southward. These are exceedingly sharp folds, but 

they are within the area deformed by the Nacimiento 

uplift and can be regarded as features related to that 

uplift, so that similar folds are not to be expocted farther south in the area of block faulting.
IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 77

Lucero Mesa

AGE OF THE DEFORMATION

The structural movements in the Basin and Range 

part of the Mount Taylor volcanic field can be closely 

dated with respect to the Santa Fe formation, generally 

regarded as of late Miocene and Pliocene age, which in 

this field is overlain by deposits of probable Pleistocene 

age (p. 56). The major movements of faulting had 

clearly preceded Santa Fe deposition in the eastern part 

of the volcanic field, but the latest movements involved 

the lower part of the formation. There is invariably a 

distinct angular unconformity between the Santa Fe 

and the Cretaceous beds, and the latter dip 

much more steeply. Rarely is there a pronounced difference in strike. Faults displacing the Santa Fe beds almost invariably 

displace the Cretaceous by considerably 

greater amounts.

These relations are well exposed throughout the northeastern part of the field. For 

example, in sec. 4, T. 14 N., R. 1 E., the 

Cretaceous has been tilted 30° S., but the overlying base of the Santa Fe has been tilted 

only 7° in the same direction (pi. 17, Z>). A mile east 

of this locality the Cretaceous beds strike N. 40° E. 

and dip 18° SE., whereas the overlying Santa Fe strikes 

N. 70° W. and dips 10° SW. (pi. 17, C}.

The best example of the relation of the Santa Fe to 

the faulting is found just outside the mapped area, a 

few hundred feet north of State Highway 44, about half 

a mile west of San Ysidro. Here Upper Cretaceous 

beds have been faulted down about 2,500 feet on the 

east against Triassic or Permian red shale, whereas the 

Santa Fe formation and the erosion surface beneath it 

have been displaced only 7 feet by the fault (fig. 14).

basalts were roughly contemporaneous with the overlap 

of the Santa Fe into the Basin and Range part of the 

Mount Taylor volcanic field, and that the basalts were 

erupted after the major fault movements but were 

involved in the latest deformation. The basalts and 

underlying erosion surfaces extend eastward nearly to 

the edge of the plateau, as at Mesa Lucero, south of the 

field, and at Mesa Prieta. Figure 15 illustrate? these 

relations. Evidently most of the movement along the 

fault shown in the figure preceded the cutting of the 

erosion surface beneath the sheet basalt. If it were

Sheet basaltCPIiocene?)

FIGUBE 14.—Sketch from photograph showing fault relations half a mile west of San 

Ysidro and 1,000 feet north of State Highway 44. Cretaceous rocks (Ku) faulted 

down about 2,500 feet against Permian or Triassic (P"E). The base of the Santa 

Fe formation (Tsf) has been displaced 7 feet.

Locally the Santa Fe formation lies nearly horizontal 

on faulted and tilted Cretaceous formations, as at the 

locality in the Antonio Sedillo grant shown in plate 

19, A

The Santa Fe does not extend westward into the 

plateau part of the volcanic field, nor are the sheet 

basalts known to extend eastward out of the plateau. 

Indirect evidence, however, indicates that the sheet

Pl'ocene?',

FIGCJKE 15.—Diagrammatic section illustrating faulting east of Mesa Lueero. After Darfton. The 

basalt and the erosion surface on which it lies are probably younger than the fault at the right. 

(See also fig. 8, p. 57.)

assumed that the faulting followed the cutting of the 

erosion surface, then a reconstruction of the pr3-fault 

relations would show Cretaceous rocks capping a westward-facing scarp 2,000 feet higher than the erosion 

surface. This scarp would have been about 50 miles 

long, and the assumed later faulting would have coincided with it. This assumption of the fault coinciding 

with an extensive 2,000-foot scarp is improbable.

Other evidence, however, cited below, indicates that 

the basalt did not entirely escape the fault movements.

1. One tilted and faulted basalt flow, interbedded with 

the lower part of the Santa Fe, is known in the Basin 

and Range part of the volcanic field (p. 70).

2. The erosion surface beneath the Santa Fe at the 

fault locality near Highway 44, half a mile west of 

San Ysidro slopes away from Sierrita Mesa, in the 

Nacimiento Mountains. The surface is about f0 feet 

above the flood plain of Arroyo Salado and about 100 

feet above the stream bed and is only 2 to 6 mile^1 from 

the nuclear area. Certainly this surface is younger than 

the basalt-capped surface on Mesa Prieta, wlich is 

about 900 feet above the present drainage level and 20 

to 30 miles from any known nuclear area. The lower 

erosion surface is displaced 7 feet by a large fault. 

The formation of the higher surface therefore must 

have preceded the latest structural movements in the 

Basin and Range part of the field.

3. Minor faults in the eastern part of the CoT orado 

Plateau portion of the volcanic field are parallel to the 

set of faults in the Basin and Range part of the field, 

and these minor faults locally displace the old sheet 

basalts (p. 75).

The structural history of the Mount Taylor volcanic 

field may, then, be summarized as follows: The northward tilting of the southern San Juan Basin, the devel-
78 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

opment of the McCarty syncline, and probably also the 

folding and associated faulting northwest of the McCarty syncline and gentle warping in the eastern part 

of the Colorado Plateau portion of the volcanic field had 

all been concluded before Mount Taylor erupted. This 

deformation ended probably before late Miocene time. 

However, it began later than early Eocene time, because 

far to the north, in the interior of the San Juan Basin, 

the Wasatch formation was tilted northward. Probably 

the faulting in the Basin and Range part of the volcanic field began at about the same time, but this 

faulting continued through later Tertiary time. The 

eruptions of Mount Taylor and the later sheet basalts 

were followed by minor folding and faulting within the 

McCarty syncline, south of Mount Taylor, minor 

faulting in the gently warped area east of the syncline, 

and continued faulting and tilting in the Basin and 

Range part of the volcanic field. These final movements 

began in Pliocene time and probably continued into 

Pleistocene time.

POSSIBLE SIGNIFICANCE OF THE STRUCTURAL FEATURES.

The presence in the southern San Juan Basin of 

broad areas with gentle homoclinal dips implies the 

absence of active compressive stresses in the stratified 

rocks. It is hardly conceivable that the weak stratified upper crust of the southern San Juan Basin could 

transmit stresses without considerable deformation. 

Probably there was compression deep in the crystalline 

basement, but deformation of the weak stratified 

formations was apparently in the main an adjustment 

to irregularities developed in the upper part of the 

basement rocks. Renick 13 suggests that the Nacimiento 

uplift was thrust west because the basement rocks were 

uplifted sufficiently to allow their being pushed westward against the weak stratified formations. The 

same interpretation may be applied to the linear 

folding in the Gallup-Zuni Basin, west of the Zufii 

uplift. 14

The stratified formations overlying an arch of basement rocks would be stretched and fractured, probably 

by normal faulting. The close relationship of faulting 

to folding in the northwestern part of the Mount Taylor 

volcanic field has already been cited (p. 75) and perhaps resulted from such a process. The deformation 

of the southern San Juan Basin was at a maximum 

beneath the Mount Taylor volcano and diminished 

progressively westward, suggesting that the assumed 

deep stresses were active from the east. The asymmetry of the Zufii and Nacimiento uplifts and the

i' Renick, B. C., Geology and ground-water resources of western Sandoval County, 

N. Mex.: U. S. Geol. Survey Water-Supply Paper 620, p. 71, 1931.

14 Sears, J. D., Geology and coal resources ol the Gallup-Zuni Basin, N. Mex.: 

U. S. Geol. Survey Bull. 797, pi. 1, 1925.

westward thrusting of the Nacimiento further suggest 

active forces directed westward.

The following facts collectively suggest that at least 

the stratified rocks in the Basin and Range portion of 

the Mount Taylor volcanic field were deformed by 

horizontal tensional stresses or by stresses directed 

vertically:

1. So far as known the faults are all normal faults 

on which there was very little horizontal movement 

(p. 76).

2. The faults trend nearly parallel to the western 

limit of the faulting. Experiments indicate that horizontal shearing would have developed en echelor faulting.

3. The width of the faulted area apparently has been 

expanded by faulting and diminished by folding and 

tilting. To determine roughly the relative amounts 

of the expanding and shortening a computation was 

made along a 6-mile line approximately coinciding with 

the south face of the block shown on plate 18, A. It 

was assumed that the dip of the fault surfaces continued 

at depth as observed at the surface. The computation 

accordingly indicates an east-west shortening of 1,600 

feet by folding. The widening by faulting amounts to 

2,800 feet. The net widening therefore would be 1,200 

feet in 6 miles, or nearly 4 percent. A second computation was made along a line 5 miles in length about the 

position of the south face of the block shown in plate 

18, B. This computation indicates a widening by 

faulting of 1,000 feet and a shortening by folding of 250 

feet, or a net widening of 750 feet, slightly less than 3 

percent. It is fully conceded that these computations 

take cognizance only of those structural features that 

are recorded on a structure map having a scale of 1 mile 

to the inch and a 100-foot contour interval. Furthermore, the assumption of maintenance of dip by the fault 

surfaces is probably false. No claim is made that the 

figures even approach precision, but in the absence of 

other evidence they at least suggest that the area has 

been widened by a few percent.

Possibly the faulting resulted in part from deep 

folds arching the stratified rocks. Mesa Lucero and 

the southeast flank of the San Juan Basin may be interpreted as the west flank of a broad arch whose axis is 

in the down-faulted Rio Grande Valley, and the Sandia 

and Manzano Mountains as the east flank. But arching alone of the stratified rocks seems inadequate to 

explain fully the abundant faulting. Back of the westerly forces that produced the Zuni uplift and the deformation beneath Mount Taylor there may have been 

fairly deep tension. However, it is more like^ that an 

appearance of horizontal tension was produced by 

shearing of the surface rocks at a high angle to the 

horizontal.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 18

Horizontal and vertical scale 

5,000 O iO,000 Feet

A. BLOCK DIAGRAM SHOWING STRUCTURAL SURFACE OF PART OF ANTONIO SEDILLO GRANT AT SOUTHWEST CORNER OF THE AREA. 

Isometric projection; vertical scale is not exaggerated. Shading by W. H. Bradley. Ku, Cretaceous shale; Jm, Jurassic sandstone.

Horizontal and vertical scale

B BLOCK DIAGRAM SHOWING STRUCTURAL SURFACE OF MOST OF NORTH HALF OF BASIN AND RANGE PART OF MOUNT TAYLOR

VOLCANIC FIELD. 

Isometric projection. Ku, Cretaceous shale and sandstone; Jm, Jurassic sandstone.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 19

A. FLAT-LYING UNBROKEN SANTA FE FORMATION CROSSING A NORMAL FAULT IN CRETACEOUS SHALE. 

View looking north in Sedillo Grant 2 miles northeast of Suwanee. Tsf, Santa Fe formation; Ku, Cretaceous shale.

B. BASALTIC LAVA IN SANTA FE FORMATION TILTED AND BROKEN BY A SMALL FAULT. 

View along New Mexico State Highway 6 in sec. 2, T. 9 N., R. 1 W. Tsf, Santa Fe formation; Tb, basaltic lava.