Upper Paleozoic Carbonate Bank in 

East-Central Idaho-Snaky Canyon, 

Bluebird Mountain, and Arco Hills Formations, 

and their Paleotectonic Significance 

GEOLOGICAL SURVEY BULLETIN 1486 

Upper Paleozoic Carbonate Bank in 

East-Central Idaho-Snaky C:anyon, 

Bluebird Mountain, and Arco Hills Formations, 

and their Paleotectonic Significance 

By Betty Skipp, R. D. Hoggan, D. L. Schleicher, andR. C. Douglass 

CONTRIBUTIONS TO STRATIGRAPHY 

GEOLOGICAL SURVEY BULLETIN 1486 

Three new formations in east-central Idaho 

and their paleotectonic significance 

~.JNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1979 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Main entry under title: 

Upper Paleozoic carbonate bank in east-central Idaho-Snaky Canyon, Bluebird Mountain, 

and Arco Hills formations and their paleotectonic significance. 

(Contributions to stratigraphy) (Geological Survey Bulletin 1486) 

Bibliography: p. 76 

Supt. of Docs. no.: I 19.3:1486 

1. Geology, Stratigraphic-Paleozoic. 2. Geology, Structural, 3. Geology-Idaho. 

I. Skipp, Betty, 1928- II. Series. III. Series: United States Geological Survey Bulletin 1486. 

QE75.B9 QE654 557.3'08s 551.7'5'0979659 79-607787 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-001-03245-7 
CONTENTS 

Page 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Underlying beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Arco Hills Formation..................................................... 11 

Lithology and thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Age................................................................. 15 

Bluebird Mountain Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

Lithology and thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

Age................................................................. 19 

Snaky Canyon Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Bloom Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

Gallagher Peak Sandstone Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

Juniper Gulch Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

Overlying units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

Paleotectonic significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

Measured sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

Section 1 ................ : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

Section 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 

Section 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 

Section 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 

Section 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 

Section 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

Section 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of east-central Idaho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2. Index map of southern BeavE!rhead Mountains showing type sections of Bluebird Mountain and Snaky Canyon Formations . . . . . 6 

3. Columnar diagram of Bluebird Mountain and Snaky Canyon 

Formations and related rocks in type sections . . . . . . . . . . . . . . . . . 8 

4. Correlation chart showing Bluebird Mountain and Snaky Canyon 

Formations and their relation to associated strata............. 12 

5. Columnar diagrams of Arco Hills and Bluebird Mountain Formations, including the type section of the Arco Hills Formation . . . . 16 

6. Geologic sketch map of northern part of Snaky Canyon 

Quadrangle showing type sedion of upper part of Snaky Canyon 

Formation and associated strata............................. 22 

III 

UPPERPALEOZOICCARBONATEBANK 

IN EAST-CENTRAL IDAHO-SNAKY CANYON, 

BLUEBIRD MOUNTAIN, AND 

ARCO HILLS FORMATIONS, 

AND THEIR PALEOTECTONIC SIGNIFICANCE 

By BETTY SKIPP, R. D. HOGGAN, 1 D. L. SCHLEICHER, and 

R. C. DOUGLASS 

ABSTRACT 

Approximately 1,400 m (meters) (4,600 ft or feet) of interbedded carbonate rocks, 

sandstone, siltstone, and mudstone of latest Mississippian to Early Permian age are 

assigned to the Arco Hills, Bluebird Mountain, and Snaky Canyon Formations in eastcentral Idaho north of the Snake River Plain. The Snaky Canyon Formation (largely 

Permian and Pennsylvanian, locally including latest Mississippian) is divided into three 

members, which are, in ascending order, Bloom, Gallagher Peak Sandstone, and Juniper 

Gulch. The new formations have been recognized, from west to east, in the southern 

Pioneer and White Knob Mountains, the southern Lemhi and Lost River Ranges, and 

the Beaverhead Mountains. The carbonate-bank sequence is overridden on the west by 

the Copper Basin allochthon and extends no farther east than the Medicine Lodge 

thrust. An arbitrary northern limit for the terminology is proposed at about latitude 

44°30'. 

The Arco Hills Formation (uppermost Mississippian) is underlain by the Surrett Canyon Formation of Late Mississippian age. The Bluebird Mountain Formation (uppermost Mississippian) is underlain by the White Knob Limestone in the central White 

Knob Mountains and by the Surrett Canyon Formation in the southern White Knob 

and Pioneer Mountains. From the Lost River Range east to the Blue Dome area of the 

southern Beaverhead Mountains, the Bluebird Mountain Formation is underlain by the 

Arco Hills Formation. In the remainder of the southern Beaverhead Mountains it is 

underlain by the Big Snowy Formation. The Snaky Canyon Formation (Lower Permian 

to uppermost Mississippian) overlies the Bluebird Mountain Formation and is overlain 

by the Phosphoria Formation in the southern Lemhi Range and Beaverhead Mountains, 

and by unnamed Permian beds in the White Knob Mountains. A remnant of the Triassic 

Dinwoody Formation is preserved above the Phosphoria Formation in the southern 

Beaverhead Mountains. 

Sandstones in the Bluebird Mountain and Snaky Canyon Formations generally thin 

westward toward the Copper Basin allochthon and, thus, are not reworked flysch 

deposits. They are composed of craton-derived detritus which spread as far west as the 

1 Ricks College, Rexburg, Idaho 83440. 

1 
2 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

White Knob Mountains f,where the Bluebird Mountain Formation is present above 

orogenic conglomerates i the White Knob Limestone. The presence of shallow-water 

carbonates in the Upper Mississippian part of the Copper Basin Formation, and the 

absence of western-derived detritus above the White Knob Limestone suggests that the 

Copper Basin flysch trough had ceased major subsidence by latest Mississippian time in 

central Idaho. 

The Copper Basin highland, a newly recognized positive element in the approximate 

position of the former Copper Basin trough, separated the Permian-Pennsylvanian 

Wood River basin on the west from the mostly contemporaneous Snaky Canyon carbonate bank on the east during Middle and Late Pennsylvanian and probably Early Permian time. The highland contributed detritus to both areas. The western Wood River 

basin was shorter lived, subsided much more rapidly, and received more than twice as 

much sediment as the eastern Snaky Canyon carbonate-bank area. 

INTRODUCTION 

Recent mapping has confirmed that about 1,400 m (4,600 ft) of carbonate and sandstone beds of largely Permian and Pennsylvanian 

age, which lie above either the Upper Mississippian White Knob 

Limestone or the Surrett Canyon or Big Snowy Formation 

throughout east-central Idaho, can be divided into three formations. 

Two of the formations, which are named for exposures in the 

southern Beaverhead Mountains, are the Bluebird Mountain Formation (uppermost Mississippian) and the overlying Snaky Canyon Formation (largely Permian and Pennsylvanian). The Snaky Canyon Formation is divided into three members which are, in ascending order, 

Bloom, Gallagher Peak Sandstone, and Juniper Gulch. In the 

southern Beaverhead Mountains and the southern Lemhi Range, the 

Snaky Canyon Formation (Juniper Gulch Member) is overlain by the 

Phosphoria Formation. 

Rocks representative of these two formations are present, from 

west to east, in the southern White Knob Mountains, the southern 

Lost River and Lemhi Ranges, and the southern Beaverhead Mountains (fig. 1). 

The Upper Mississippian Arco Hills Formation, named for exposures in the Arco Hills at the southern tip of the Lost River Range, 

occurs between the Surrett Canyon Formation and the Bluebird 

Mountain Formation in the Lost River Range east to the Blue Dome 

area of the southern Beaverhead Mountains (fig. 1). In most of this 

area, the formation is composed largely of thin-bedded argillaceous or 

silty limestone, calcareous mudstone, shale, and siltstone, with a few 

very fine-grained sandstone beds and a few thick-bedded limestone 

beds. Limestone is the major lithology in western exposures, and in 

the White Knob and Pioneer Mountains, beds equivalent in age to the 

Arco Hills Formation are mapped with the Surrett Canyon Formation. 
44°00' 

INTRODUCTION 

. - "1 CRATERS OF THE MOON 1 NATIONAL MONUMENT 

3 

IDAHO 

KILOMETERS 

0 10 20 

I I I I 

0 10' MILES 

FIGURE I.-Index map of east-central Idaho showing the areas in which the Arco Hills, 

Bluebird Mountain, and Snaky Canyon Formations are known to occur (stippled 

areas), outlines of mountain ranges, location of figure 2, locations of measured sections of Arco Hills and Bluebird Mountain Formations of figure 5, generalized 

traces of the Medicine Lodge, Copper Basin, and Wood River thrust systems (Skipp 

and Hait, 1977), locations of quadrangles referred to in text, and generalized locations (roman numerals) of stratigraphic columns used on correlation chart (fig. 4). 

The carbonate-bank facies of the Arco Hills, Bluebird Mountain, 

and Snaky Canyon Formations are cut off on the west by the Copper 

Basin thrust system which brings Mississippian flysch of the Copper 

Basin Formation over upper and lower Paleozoic carbonate rocks 

(Skipp and Hall, 1975a, b; Dover and others, 1976; Skipp and Hait, 

1977; Bond, 1978), and they extend no further east than the trace of 

the Medicine Lodge thrust (Sloss and Moritz, 1951; Scholten and 

others, 1955; Scholten, 1957; Skipp and Hait, 1977; Ruppel, 1978). 

The formations have not been recognized to date on the Medicine 

Lodge allochthon itself (Skipp and Hait, 1977). An arbitrary northern 
4 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

limit for use of the formation names is proposed at about latitude 

44°30' (fig. 1). North of this latitude, other terminology may be more 

appropriate (Scholten and Ramspott, 1968; Ruppel, 1968), though 

the sequences in the central Beaverhead Mountains differ only in being more sandy than those to the south. A natural southern limit is 

provided by the volcanic rocks along the northeast margin of the 

Snake River Plain. 

The need for a formation breakdown such as we have proposed was 

recognized long ago by Shannon (1961) in the southern Lost River 

and Lemhi Ranges where he subdivided the Pennsylvanian and Permian rocks into units C, D and E. The Bluebird Mountain Formation 

largely embraces Shannon's unit C, and the Snaky Canyon Formation is comparable to his combined units D and E. The thin interval 

which makes up the Arco Hills Formation was included in either the 

lower part of unit Cor the upper part of unit B. Shannon's units A 

and B consist of the Upper Mississippian limestones which Huh 

(1967) divided into four formations. These four Upper Mississippian 

formations, with the Surrett Canyon Formation at the top, formed 

the basis for the biostratigraphic study of Mamet, Skipp, Sando, and 

Mapel (1971). Above the four Mississippian formations, Mamet, 

Skipp, Sando, and Mapel (1971) recognized "an unnamed Pennsylvanian sandstone unit" overlain by "an unnamed limestone unit" in the 

central Lost River Range. These unnamed units, which are used as 

map units in the Lost River Range in the Doublespring Quadrangle 

(Mapel and others, 1965) and in the Hawley Mountain Quadrangle 

(Mapel and Shropshire, 1973) (fig. 1), are the Arco Hills, Bluebird 

Mountain and Snaky Canyon Formations of this report. The Arco 

Hills Formation, as interpreted here, is considered to be represented 

by approximately the lower half of the ''unnamed Pennsylvanian 

sandstone" in the Hawley Mountain area (Mamet and others, 1971). 

In the central Beaverhead Mountains, Scholten and Ramspott 

(1968), Ruppel (1968), and Lucchitta (1966) assigned beds to the 

"Pennsylvanian Quadrant Quartzite" and "Pennsylvanian Quadrant 

Limestone" which probably correlate with the Bluebird Mountain 

and Snaky Canyon Formations. 

ACKNOWLEDGMENTS 

Valuable assistance in the measurement of several sections and collection of fossils was given by James Davis, G. F. Embree, B. J. 

France, M. H. Hait, Jr., Craig Hall, Ellen Harrison, Penny Patterson, 

Linda Riglin, W. J. Sando, and E. J. Williams. E. K. Maughan first 

verified the presence of the Phosphoria and Dinwoody Formations. 

Peter Oberlindacher and R. David Hovland measured most of the 
UNDERLYING BEDS 5 

Phosphoria Formation in the southern Beaverhead Mountains. 

Fusulinid Foraminifera were identified by R. C. Douglass, and 

smaller calcareous Foraminifera and algae by Betty Skipp, unless 

otherwise noted. Many paleontologists, in addition to the authors, 

contributed important faunal determinations for this report. These 

include D. S. Crawford, J. T. Dutro, Jr., MacKenzie Gordon, Jr., 0. L. 

Karklins, John Poj eta, John Repetski, W. J. Sando, G. J. Verville, 

B. R. Wardlaw, and E. L. Yochelson, whose individual work is cited 

in the text. Special thanks are extended to Paul Brenckle for the 

many hours of consultation concerning the identification and taxonomy of the Mississippian and lowest Pennsylvanian calcareous 

Foraminifera. The identifications and taxonomy used in this paper, 

however, are the sole responsibility of the senior author. The authors 

would also like to thank M. H. Hait, Jr., and B. R. Wardlaw for their 

helpful criticisms and suggestions. 

UNDERLYING BEDS 

In the southern Lost River and Lemhi Ranges and in the Blue 

Dome area of the southern Beaverhead Mountains (Skipp and Hait, 

1977), the Arco Hills Formation is underlain gradationally by thickbedded pure limestones of the Surrett Canyon Formation of Late 

Mississippian age and is overlain conformably by the Bluebird Mountain Formation. In the southern Pioneer and southern White Knob 

Mountains, limestone beds equivalent in age to the Arco Hills Formation are mapped with the Surrett Canyon Formation. 

In the area of the type section of the White Knob Limestone, in the 

White Knob Mountains (III on fig. 1; Skipp, 1961; Skipp and Mamet, 

1970), the White Knob is overlain gradationally by thin calcareous 

sandstone or sandy limestone beds considered to be correlative with 

the Bluebird Mountain Formation. Faunal studies of this area, including foraminiferal determinations of samples collected by R. A. 

Paull, indicate that the White Knob, which consists of interbedded 

shallow-water limestones and beach or bar conglomerates and sandstones (Skipp, 1961), is entirely of Late Mississippian age. 

In the southern Beaverhead Mountains, exclusive of the Blue 

Dome area, the Bluebird Mountain Formation is underlain gradationally by the Big Snowy Formation also of Late Mississippian age. 

An incomplete section of 60 m (200 ft) of Big Snowy was measured 

below the type section of the Bluebird Mountain Formation on 

Gallagher Peak (figs. 2, 3). At this locality, the Big Snowy consists of 

medium-dark-gray, to dark-gray shale which weathers medium gray 

and contains interbedded medium-gray, fossiliferous, argillaceous, 

and sandy limestone and few thin beds of siltstone and very fine 

grained sandstone. A few thin zones of grayish-green shale are pres-
6 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

FIGURE 2.-Index map of southern Beaverhead Mountains showing location of the 

type sections of the Bluebird Mountain and Snaky Canyon Formations in Clark 

County, Idaho. 2, in the Copper Mountain 7 V2' Quadrangle, is the type section of the 

Bluebird Mountain Formation (Mississippian) and the type section of the Bloom 

Member (Mississippian and Pennsylvanian) of the lower part of the Snaky Canyon 

Formation (Permian, Pennsylvanian, and uppermost Mississippian). 1, in the Snaky 

Canyon Quadrangle, is the type section of the upper part of the Snaky Canyon Formation consisting of the Gallagher Peak Sandstone Member (Pennsylvanian) and 

the overlying Juniper Gulch Member (Permian and Pennsylvanian). 
UNDERLYING BEDS 7 

ent in the uppermost shale zone. Below the measured section of fig. 3, 

the Big Snowy is estimated to be another 107 to 122m (350-400 ft) 

thick with a faulted base, and, in addition to the above lithologies, 

contains lenticular beds of carbonate conglomerate with clasts as 

much as 12.5 em (centimeters) (5 in. or inches) in diameter, abundant 

phosphatic granules ·and pebbles, and lenses of medium-light-gray, 

very fine grained quartzose sandstone. The lowermost dark-gray 

shale beds of the Big Snowy, about 46 m (150ft) thick, contain large 

yellowish-brown-weathering limonitic calcareous concretions as 

much as 0.6 m (2 ft) in diameter. The thickness of the Big Snowy at 

this locality as reported here may be too thick. The section is both 

faulted and folded. Huh (1967) reported a thickness of about 91 m 

(300 ft) for the entire Big Snowy Formation in the vicinity of Copper 

Mountain. Throughout this area, the shales and mudstones of the Big 

Snowy form an incompetent unit which has been squeezed between 

the thick-bedded limestones of the Surrett Canyon Formation and 

the cliff-forming quartzites of the Bluebird Mountain. 

A varied fauna of brachiopods, bryozoans, mollusks, corals, 

ostracods, conodonts, algae, and foraminifers was recovered from 

carbonate lenses in the Big Snowy and indicates a Late Mississippian 

(Chesterian) age for the formation. The inarticulate chitinaphosphatic brachiopod Orbiculoidea is relatively common in both 

shale and argillaceous limestone facies (Huh, 1967). 

Brachiopods present 18 to 41 m (60-135 ft) below the top of the Big 

Snowy in the measured section of figure 3 were identified by J. T. 

Dutro, Jr., as: 

Anthracospirifer shawi shawi Gordon 

lnflatia sp. 

Orbiculoidea cf. 0. wyomingensis Branson and Greger. 

All the forms are similar to those reported from the Carlinia amsdeniana assemblage in the Amsden Formation of Wyoming (Gordon, 

1975), and the interval near the top of the Big Snowy apparently 

represents a late Chesterian equivalent (J. T. Dutro, Jr., written commun., 1976). 

Foraminifera from 18m (60ft) below the top of the Big Snowy (fig. 

3) are representative of Mamet Foraminiferal (or microfossil) Zone (F. 

Z.) 19 (uppermost Chesterian) (Mamet and Skipp, 1970), and the 

fauna includes the following forms: 

Asteroarchaediscus sp. 

Eosigmoilina explicata Ganelina 1956 

"Eosigmoilina" rugosa Brazhnikova 1964 

Hemiarchaediscus sp. 

N eoarchaediscus sp. 
8 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

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number, queried where 

in doubt 

25635-PC Megafauna! collection 

number 

Beaverhead Mountains, sees. 29 and 30 (unsurveyed), T. 10 N., 

R. 31 E. (2), and sec. 21, T. 9 N., R. 32 E. (1), Clark County, 

Idaho. 

Composita sp. 

Diaphragmus? sp. 

I nflatia sp. 

Ovatia sp. 

Reticularina? sp. 

9 
10 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

Bryozoans identified by 0. L. Karklins (written commun., 1975) 

include: 

Rhabdomesids, indet. 

Archimedes sp. 

Conodonts identified by John Repetski (written commun., 1976) 

include: 

Adetognathus? sp. (one broken specimen) 

Cavusgnathus sp. 

Repetski states, ''If this is indeed a specimen of Adetognathus, then 

I favor a late Chesterian assignment for this sample." 

Mollusks identified by J. T. Dutro, Jr., written commun., 1975) 

include: 

Pectinoid pelecypods, undet. 

Platycerotid gastropod, indet. 

Faunas from phosphatic limestone conglomerates near the middle 

of the formation include the brachiopods, Orbiculoidea sp. (large) and 

Composita sp. (J. T. Dutro, Jr., written commun., 1975), and the conodonts Adetognathus unicornis (Rexroad and Burton), 

Cavusgnathus unicornis Youngquist and Miller, and Cavusgnathus 

cf. C. regularis, (John Repetski, written commun, 1976). 

Samples of sandy limestone collected at an estimated 46 m (150 ft) 

and 52 m ( 170 ft) above the faulted base of the Big Snowy and a few 

meters below the limestone conglomerates contain abundant green 

(codiacean) algae, some red algae, and the following Foraminifera: 

Asteroarchaediscus spp. 

Endothyra excellens (D. Zeller) 1953 

''Eosigmoilina '' rugosa Brazhnikova 1964 

N eoarchaediscus spp. 

Planospirodiscus sp. 

Pseudoglomospira sp. 

Zellerina sp. 

This is a F. Z. 19, possibly upper 18, fauna of late Chesterian age 

(Mamet and Skipp, 1970). Reexamination of a foraminiferal fauna 

from a limestone bed about 23 m (75 ft) above the base of the Big 

Snowy in the vicinity of Copper Mountain reported by Crawford 

(1976) indicates that the fauna is representative of F. Z. 17 or 

younger. 

Small silicified ostracods from about 30m (100ft) above the base of 

the Big Snowy in the same section (Crawford, 1976) include: 

Healdia spinosa 

?Paraparchites sp. 

Though no faunas were found in the lowermost shale beds, physical 

stratigraphy and faunal evidence support a Late Mississippian 

(Chesterian) age for the entire Big Snowy Formation in the southern 
ARCO HILLS FORMATION 11 

Beaverhead Mountains. In much of the outcrop area of the Big 

Snowy Formation in the Snaky Canyon quadrangle (fig. 2), 

mudstones of the Big Snowy rest directly on limestones of the Scott 

Peak Formation with no intervening South Creek or Surrett Canyon 

Formation present. In these areas, it is possible that the lower part of 

the Big Snowy Formation includes beds as old as F. Z. 16s of early 

Chesterian age (Sando and others, 1975; fig. 4). 

ARCO HILLS FORMATION 

LITHOLOGY AND THICKNESS 

The Arco Hills Formation is a slope-forming unit between the 

thick-bedded limestones of the Surrett Canyon Formation below and 

the ledge-forming sandstones of the Bluebird Mountain Formation 

above. It ranges in thickness from 76 m (249ft) in the Arco Hills at 

the type section to 52 m (170 ft) in the southern Lemhi Range to 

about 93 m (307 ft) in the southern Beaverhead Mountains (fig. 5, 

sees. 3, 5, and 7). In three of the measured sections of figure 5, a thin, 

covered interval separates the Arco Hills from the underlying Surrett 

Canyon Formation. The formation in measured-section 5, in the 

southern Lemhi Range (figs. 1, and 5, sec. 5), is anomalously thin (52 

m, 170ft) and pervasively sheared. Another section measured about 

2.7 km (kilometers) or (1.7 mi or miles) north of measured-section 5 

along the ridge east of East Canyon (Tyler Peak Quadrangle) (fig. 1, 

sec. 6) above the type section of the underlying Surrett Canyon Formation is even thinner (42. 7 m, 140ft), and has no covered interval at 

the base. It, too, is sheared in places, but the local lateral continuity 

of beds suggests the thinness of the interval is due primarily to 

deposition, not to postdepositional tectonic effects. Beds equivalent 

to the Arco Hills Formation in the southern White Knob Mountains 

(fig. 5, sec. 4) are about 34m (110ft) thick. 

The Arco Hills consists of interbedded medium-gray, olive-gray, 

yellowish-brown, and grayish-red thin-bedded, argillaceous, and silty 

or sandy limestone, calcareous mudstone or shale, siltstone and 

minor sandstone, and medium- to thick-bedded pure limestone. The 

limestones are variably cherty in the western sections and some 

phosphatic limestone was noted in the Lemhi Range section 

(measured section 5). The proportion of shale and (or) mudstone to 

limestone increases from west to east. No shale or mudstone is present in this part of the section in the southern Pioneer Mountains, (fig. 

5, sec. 4), whereas mudstone and siltstone make up 35 percent of the 

exposed part of the type section (fig. 5, sec. 3) and 53 percent of the 

exposed part of the section in the Blue Dome area of the southern 

Beaverhead Mountains (fig. 5, sec. 7). Section 5 in the southern 

Lemhi Range (fig. 5, sec. 5) contains 28 percent shale and siltstone. 
12 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

::!: (/) ]!(I) w w OG> 1- iE .5 ·c::: (/) >Q) 

> w ew (/) (/) a.. 

I 

a: Q. 

w :::J 

a.. ..J <Z a.. Col( 

::J c-

:::J ~ 0 

c( 

z ~ Z< 

~ §~ a: 

w a: ~ a.. w z 

3: < 

0 ii: 

:::!: ...J < 

0 

u.. 

..J 

0 

3: 

z 

< ::::; 

a: a w a: 

a.. > a.. f--;-- ::J a: 

:::lz 

g~ 

rn 

z ~ :i c( i------1 

z z 

< c( Ci) 

~ w 

z ~ > w 0 a: (/) ...J :::!: c( z c rn 

~ z c w w ~ 0 

(/) a.. --z- ::J < 0 ~ 

a: 0 w 1-

u. 

z ~ ~ 'z 0 w a:c ID 3: 03: a: ::to c( g a: 0 

z 

c( z a: a: < a.. w a: 

Ci5 a.. w 

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z 

-? 

21 

1---

20 

?-

19 

f---

18 

f---

17 

f---

16s 

f---

16i 

II Ill IV 

WOOD RIVER SOUTHERN WHITE KNOB SOUTHERN 

AREA PIONEER MOUNTAINS WHITE KNOB 

MOUNTAINS MOUNTAINS 

Tertiary and/ or Cretaceous conglomerate at base 

:11i 

UNIT 

7 

Ill~ g~·!t./A: 

¥192 

UNIT 4 

..---?-

UNIT 

3 

of Challis Volcanics (Eocene) 

? 

Snaky 

Canyon 

Formation 

*J'IL~: I equivalent 

IIIII 

FAULT 

.............. 

............. 

Snaky 

Canyon 

Formation 

:{.!t!rf<>\\=/l)):~r~~t>(r~nnw.;i;.f:r:r~\\\;::~~A 

White Knob ~lA/.f/}/:}\?~2] Limestone Arco Hills I 

:::::::::?.:::::::: (part) equivalent I 

::::::::::::::::::: L1 Conglomerate Surrett 

~coppar··aasiri~ ~-=~===~ lenses I 

:::Formation::: •··• 

::::::(part):::::: ;;;:::... South : 

~~~~~~~~~~~~~~~~~~~ Scott : 

FIGURE 4.-Correlation chart showing ages of the Bluebird Mountain Formation and 

the Snaky Canyon Formation and its members in south-central Idaho, 

stratigraphic relations of the formations and members and associated strata with 

the Wood River Formation to the west, and inferred stratigraphic relations with 

Pennsylvanian and Permian sequences in the central Beaverhead Mountains. 
:::t 

w 

~ 

Cl) 

>- Cl) 

z 

c( 

~ a: 

w 

a.. 

ARCO HILLS FORMATION 13 

v VI VII VIII 

e1 SOUTHERN SOUTHERN SOUTHERN CENTRAL iii LOST RIVER Cl) ·-II) .2 c: RANGE- LEMHI BEAVERHEAD BEAVERHEAD w ()CD E2 RANGE MOUNTAINS MOUNTAINS a: .E ·;:: >CD ARCO HILLS w eCI) Cii= Cl) a.. fi: Tertiary (Eocene) Tertiary Triassic Tertiary (Eocene) 

;::t..2 Challis Volcanics fan gravels Dinwoody Fm. Challis Volcanics 

Middle 

unit 

16i 

Hiatus due to erosion or nondeposition shown by vertical lines. Orogenic 

sediments shown by dot pattern. Mostly craton derived sandstone and mudstone 

shown by stipple (includes Phosphoria Formation). Mostly carbonate-bank and 

related sediments shown by blank areas. Mamet microfossil zones (F. Z.) defined 

in Mamet and Skipp (1970). Sources for data in columns I through VIII are on 

next page. 
14 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

FIGURE 4.-Continued 

I. Hall, Batchelder, and Douglass, (1974); Skipp and Hall (1975a); Hall, Rye, and 

Doe, (1978). 

II. Skipp and Hall (1975a); Skipp and Hait (1977). 

III. Modified from Skipp and Mamet (1970) on the basis of foraminiferal determinations made on samples collected by R. A. Paull 

IV. Ross (1962); Nelson and Ross (1969a, b); this paper. 

V. Shannon (1961); Mamet, Skipp, Sando, and Mapel, (1971); this paper. 

VI. Blackstone (1954); Ross (1961); Shannon (1961); Huh (1967); Buetner (1972); 

Skipp and Hait (1977); this paper. 

VII. Huh (1967); Embree, Hoggan, Williams, and Skipp (1975); Skipp and Hait 

(1977); this paper. 

VIII. Lucchitta (1966); Ruppel (1968); Scholten and Ramspott (1968); Skipp, 

unpublished data. 

The Arco Hills Formation in many outcrop areas is buried beneath 

sandstone debris from the overlying Bluebird Mountain Formation 

and has not been recognized previously as a lithologic unit separate 

from the Bluebird Mountain or underlying Surrett Canyon Formation. Shannon (1961) apparently included it in the upper part of his 

unit Bin the southern Lost River Range, and, possibly, in the lower 

part of his unit C in the southern Lemhi Range. Approximately the 

lower half of the "unnamed Pennsylvanian sandstone" of Mamet, 

Skipp, Sando, and Mapel (1971) probably is the Arco Hills Formation 

in the Hawley Mountain area (Mapel and Shropshire, 1973). This part 

of the section is poorly exposed and contains much thin-bedded 

limestone and some phosphorite pebbles (Mamet and others, 1971, 

p. 22, locs. 118 and 125). These sections have not been revisited, 

however, and at this time the correlation is somewhat speculative. 

Beutner (1972) reported questionable Big Snowy Formation in the 

southern Lemhi Range, and it was this report which started the 

search for the shale unit which is here named the Arco Hills Formation. The Arco Hills undoubtedly is a western facies of the shales and 

limestones of the upper part of the Big Snowy Formation in the 

Beaverhead Mountains, and the fine-grained terrigenous detritus 

probably had a northeastern cratonic-platform source, indicating, 

perhaps, early pulses of the cratonic events which produced the sand 

of the overlying Bluebird Mountain Formation. In Arco Hills time, 

Late Mississippian carbonate-bank buildup was slowed by the influx 

of fine-grained terrigenous material. 

Brachiopods, including Orbiculoidea sp., bryozoans, crinoid debris, 

and calcareous Foraminifera are abundant in the Arco Hills Formation. Corals and trilobites are sparse. Conodonts are probably present 

but were not looked for. 

The upper contact of the Arco Hills Formation is sharp and is placed 

at the base of the stratigraphically lowest thick (4 m or more) sandstone or siltstone bed of the overlying Bluebird Mountain Formation. 
ARCO HILLS FORMATION 15 

AGE 

The Arco Hills Formation is of Late Mississippian, latest 

Chesterian, age based both on fossil determinations and 

stratigraphic position. The upper part of the Surrett Canyon Formation which lies beneath the Arco Hills has been determined to be of 

latest Mississippian age (Mamet and others, 1971), and the overlying 

Bluebird Mountain also is of latest Mississippian age as presented in 

this paper. 

Calcareous foraminifers representative of Mamet Foraminiferal 

Zone (F. Z.) 19 or F. Z. 18-19 have been recovered from several 

stratigraphic positions within all of the measured sections of figure 5. 

Faunas in individual samples are listed in the measured sections. 

Diagnostic Foraminifera present in most of the collections include: 

Asteroarchaediscus sp. 

Biseriella spp. 

"Eosigmoilina" rugosa Ganelina 1956 

Eosigmoilina explicata Brazhnikova 1964 

H emiarchaediscus sp. 

N eoarchaediscus sp. 

Planospirodiscus spp. 

Pseudoammodiscus 

Pseudoglomospira sp. 

Zellerina spp. 

Foraminiferal Zone 19 faunas also have been identified in samples 

from the lower 20m of the "unnamed Pennsylvanian sandstone" of 

locality 118 in the Hawley Mountains (Mamet and others, 1971) since 

publication of that report. 

Several specimens of the small horn coral Amplexizaphrentis sp., 

collected from a calcareous mudstone near the middle of the type section of the Arco Hills Formation, were identified by W. J. Sando 

(written commun., 1977), who states that "it is one of the few coral 

genera found in Mamet F. Z. 19." 

Extensive brachiopod faunas recovered from the measured sections were identified by J. T. Dutro, Jr., (written c:ommun., 1978) who 

considers the faunas compatible with a latest Mississippian (F. Z. 19) 

age. Individual collections are listed with the measured sections, but 

the forms identified include: 

Anthracospirifer aff. A. occiduus (Sadlick), as used by Gordon 

(1975) 

Anthracospirifer cf. A. shawi Gordon 

Anthracospirifer aff. A. welleri (Branson and Gregor) 

Antiquatonia? sp. 

Carlinia? sp. 

Composita cf. C. poposiensis Gordon 
16 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

4 

I

= 0 = '9'0"17<!>'• 

"V'0'\7""'•(20) 

= = = 

Snaky 

I I Vl70""il&#• (20) 

I I """' .. d'>e&h•(20l 

Canyon 

i~i~o Formation · v<;ro~(g)' • (20?) 

24918-PC X (lower part) X o~•v •(20?) 

Surrett 

Canyon 

,ii~~ Arco Hills ~ ... o If equivalent 

.. 1 I 1 V0<2S>'i?.IJ•(19)1 

ll·~· il. -v'\7°~·( 191 

""'i7 0 • ( 1 91 Formation 

·. J fold,break in section 

~~~~:§v 24t~~c~~pe~s approximate 

~IL--._,.-.l....-.j I {upper part) 

~-..-~.l _ _J'\Jv0~Z»<'!?7• (19) 

3 

FAULT 

Bluebird 

Mountain 

Formation 

Olli>V' '17• ( 19) .. VO® 

V0®'\1~•(19) 

b-_-:=!~c-=~_=-=-=--.-~:.:_~-""". r'. Ot1liV/J• ( 1 9) 

A reo 

QV0a!>'l!'•(19) 

g.:::;~26880-PC 

Hills 

Formation 

FIGURE 5.-Lithologic columnar sections and correlation of measured sections 

of the Arco Hills and Bluebird Mountain Formations and their equivalents: 

4, southern White Knob Mountains (NWV4 sec. 29 and NY2 sec. 30, T. 5 N., R. 

25 E.), Custer County; 3, southern Lost River Range-Arco Hills (NY2NEV4 
ARCO HILLS FORMATION 

5 7 

I - l 

.·~·. 

EXPLANATION 

Covered interval 

Siltstone 

Claystone 

Limestone 

Dolomite 

Limy dolomite and 

dolomitic limestone 

17 

Snaky Canyon .... Sandy 

Ouartzitic 

Calcareous 

Dolomitic 

... v 

Formaton 

(lower part) 

Silty 

Argillaceous 

Chert lenses or nodules I ~ I J jv01l!l.'l7•(19?J Incipient chert lenses ,[>( Bluebird 

" Mountain ~- It 

0 

or nodules Phosphorite 

Algae 

Crinoids 

Brachiopods 

Corals 

Bryozoans 

Gastropods 

Ostracods 

Pelecypods 

Echinoids 

Trilobites 

Spicule 

I 

-: .v ·.· ·.·.v .· 

- =-...o ~-=,~:J 

. . . . I· ... I· . Formation h~ -~ ,.<::,... ~<21>•(19) 

I I 

I 

I I 

.. ·: <!P<ll>qe(19) 

A reo 

Hills 

·ll!l.,...-v•( 191 Formation 

v 

Surrett Canyon 

Formation 

(upper part) 

~=~-~~=~-~~--~..:.~~:::=~-r.~'i?ll!l.• ( 1 9) 

·---.. - ... ---

-------

.... ·J. va..'\1 

>--

l19) s~:~~%~~~~r'::~~t~s zonal number, queried 

where in doubt 

25635-PC Megafauna! collection 

number 

METERS 

50 

f-.L.. _:...:._=_r-ci; . .:.J.·..:...:...:.· .:..::._:::;.··.] -'1,);.1&-(• (19) 

~~~~~~~~?~";1. ( 1 9) 

~ 

40 

30 100 

20 

50 

I I 

I J 10 

sec. 31, T. 4 N., R. 27 E.), Butte County (type section of the Arco Hills Formation); 5, southern Lemhi Range (SE% sec. 6, T. 6. N., R. 30 E.) Butte County; 

7, Blue Dome area of the southern Beaverhead Mountains (NW%NW% sec. 

28 and NEV4NE% sec. 29, T. 10 N., R. 30 E.), Clark County. 
18 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

Diaphragmus sp. 

Eolisochonetes pseudoliratus (Easton) 

Flexaria? sp. 

I nflatia sp. 

Leiorhynchoidea sp. 

Orbiculoidea cf. 0. wyomingensis Branson and Gregor 

Ovatia sp. 

These faunas resemble those recovered from the Big Snowy Formation at Gallagher Peak (figs. 2 and 3) and suggest the Arco Hills 

Formation is a contemporaneous western facies of at least the upper 

part of the Big Snowy Formation west of the Medicine Lodge thrust 

(fig. 1). 

BLUEBIRD MOUNTAIN FORMATION 

LITHOLOGY AND THICKNESS 

The Bluebird Mountain Formation is named for Bluebird Mountain 

in the Copper Mountain quadrangle (fig. 2). The formation is present 

along the west side of Bluebird Mountain just off the summit. The 

type section, however, is about 5.8 km (3.6 mi) south of Bluebird 

Mountain on the west flank of Gallagher Peak where exposures are 

good (fig. 2). There, the formation, 105 m (345 ft) thick (fig. 3), consists of medium-gray to medium-light-gray, very fine grained, quartzose, generally quartzitic, mostly thin-bedded (5 cm-0.6 m or 2 in.-2 

ft), cliff-forming sandstone which weathers light brown, interbedded 

with minor beds (1.5 m or 5 ft thick) of gray dolomite and limestone 

(fig. 3). Siliceous nodules as much as 12.5 em (5 in.) in diameter are 

present in several beds, and small-scale crossbedding is present locally. The contact with the overlying Snaky Canyon Formation is gradational. The top of the Bluebird Mountain in the type section was chosen at the base of the stratigraphically lowest (7 .6 m or 25 ft) thick sequence of limestone beds of the overlying Snaky Canyon Formation. 

The lower contact with the Big Snowy Formation in the type section is placed at the base of the lowest thick, calcareous, very fine 

grained ledge-forming quartzose sandstone (fig. 3). 

In the ranges to the west, the Bluebird Mountain Formation gradationally overlies the Arco Hills Formation or its equivalents. In most 

sections however, the contact is sharp and is placed at the base of the 

lowest thick (5 m, 15ft, or more) ledge-forming sandstone or quartzite 

beds (fig. 5). In some places, the calcareous sandstone beds form 

swales rather than ledges, as in measured section 5 in the southern 

Lemhi Range (fig. 5, sec. 5). In these areas, the brown weathering colors of the sandstones indicate their presence. In many places colluvium from the sandstones and quartzites covers the less resistant 

limestones and mudstones of the underlying Arco Hills Formation. 
BLUEBIRD MOUNTAIN FORMATION 19 

The upper contact of the Bluebird Mountain Formation, which is 

gradational with the overlying Snaky Canyon Formation, is placed, 

in general, at a position in the section above which limestones 

dominate the sequence. Covered intervals and variations in proportion of sand to carbonate in individual beds (fig. 5) make this a rather 

subjective contact, a major difficulty in determining age limits for 

the formation. 

The lithologic character of the Bluebird Mountain varies from 

range to range. In the Pioneer and White Knob Mountains, it mostly 

consists of calcareous siltstone, very fine grained sandstone, and interbedded sandy limestone. Where it lies above the White Knob 

Limestone, the Bluebird Mountain is largely sandy limestone. In the 

central Lost River Range it consists of "cross-laminated, very fine 

grained calcareous sandstone and quartzite, interbedded with limestone containing- minor amounts of sand and silt" (Mamet and 

others, 1971, p. 29). 

In the southern Lost River Range, the Bluebird Mountain is made 

up of light-olive-gray, pale-yellowish-brown, and medium-light-gray, 

calcareous, very fine-grained, thin- to medium-bedded sandstone interbedded with medium-dark-gray sandy limestone. Some limestone 

beds are as much as 5 m (15ft) thick. The quartzite and sandstone 

weather moderate brown, pale yellowish brown, light olive gray, and 

grayish orange. 

In the southern Lemhi Range, the Bluebird Mountain consists of 

interbedded medium-gray to brownish-gray quartzite, pale-yellowishbrown, very fine-grained, calcareous sandstone, and medium-gray 

sandy limestone. 

The Bluebird Mountain Formation ranges in thickness from about 

105 m (345 ft) in the type section in the Beaverhead Mountains to 

about 7.6 m (25ft) in the southern White Knob Mountains. Average 

thickness in the southern Lost River and Lemhi Ranges appears to be 

about 40-50 m (130-165 ft) (fig. 5). A thin section (30.9 m or 102 ft 

thick) was measured on the ridge east of East Canyon in the southern 

Lemhi Range (fig. 1, sec. 6). 

In the central Lost River Range, Mamet, Skipp, Sando, and Mapel 

(1971) report 91 m (300ft) of "unnamed Pennsylvanian sandstone," 

some of which probably correlates with the Arco Hills Formation. 

AGE 

The age of the Bluebird Mountain Formation appears to be latest 

Mississippian (latest Chesterian) in most places. Calcareous 

Foraminifera of F. Z. 19 are found above, within, and below the formation in several of the measured sections of figures 3 and 5, including the type section. Mamet, Skipp, Sando, and Mapel ( 1971, 

p. 29) report F. Z. 19 faunas immediately below beds equivalent to the 
20 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

Bluebird Mountain Formation and F. Z. 20 (Lower Pennsylvanian) 

above, and tentatively placed the base of the Pennsylvanian System 

at the base of the sandstone unit. Foraminiferal faunas of F. Z. 19 

have been identified recently within the lower 20 m of the ''unnamed 

Pennsylvanian sandstone unit" of locality 118 (Mamet and others, 

1971, p. 22) in beds which we tentatively correlate with the Arco Hills 

Formation. No new faunas have been identified from the sandstones 

above, and the upper contact drawn there may be higher and include 

more limestone in the upper part than would be included in the present definition of the Bluebird Mountain Formation. 

Foraminiferal faunas, therefore, seem to indicate that the 

Mississippian-Pennsylvanian boundary is above the gradational top 

of the Bluebird Mountain Formation as defined in this paper. 

The conodonts Gnathodus bilineatus and Cauusgnathus sp., identified by D. S. Crawford (written commun., 1978) from the upper part 

of the Bluebird Mountain Formation and the lowermost beds of the 

overlying Snaky Canyon Formation in the southern Lemhi Range (fig. 

1, sec. 6), verify a Late Mississippian age for the Bluebird Mountain. 

Except for the microfossils mentioned above, the unit is largely unfossiliferous. Orbiculoidea sp. is common in some of the calcareous 

siltstones, and brachiopods and associated encrinite debris are found 

locally in limestones interbedded with the sandstones and siltstones 

in the upper part of the formation. 

SNAKY CANYON FORMATION 

The Snaky Canyon Formation is named for Snaky Canyon in the 

Snaky Canyon quadrangle (fig. 2) where the entire formation and its 

three members, the Bloom, Gallagher Peak Sandstone, and Juniper 

Gulch, are present as distinctive map units shown on the geologic 

sketch map of the northwest part of the quadrangle (fig 6). In the 

Snaky Canyon quadrangle, the formation is about 1,200 m (3,940 ft) 

thick and is underlain gradationally by the Bluebird Mountain Formation and overlain conformably by the Phosphoria Formation of 

Permian age. 

The formation largely consists of carbonate rocks interbedded with 

thin calcareous sandstones. The carbonates are medium- to lightgray, medium- to thin-bedded, microcrystalline to medium-grained 

limestone and dolomite which are sandy and silty through much of 

the interval. They are variably fossiliferous and contain numerous 

stromatolite mounds in the lower part and hydrozoan(?) and phylloid 

algal buildups in the upper part; they locally contain oolites and as 

much as 40 percent chert in nodules, layers, and complex networks. 

The thin calcareous sandstones are medium gray, pale grayish red, 

and grayish orange, weather light olive gray, light gray and light 

brown, and are very fine to fine grained. 
SNAKY CANYON FORMATION 21 

A prominent thin sandstone unit about 59 m (195ft) thick, which 

divides the formation approximately in half, is present locally in the 

southern Lost River and Lemhi Ranges, and the southern 

Beaverhead Mountains. This unit is named the Gallagher Peak Sandstone Member. 

The Snaky Canyon Formation is present in the southern Pioneer 

and White Knob Mountains, the Lost River and Lemhi Ranges, and 

the Beaverhead Mountains. Incomplete thicknesses reported by 

Shannon (1961) are 3,403+ ft (1,037+ m) in the southern Lemhi 

Range, and 3,389+ ft (1,033+ m) in the Arco Hills area. An incomplete and faulted sequence at Timbered Dome in the southern 

White Knob Mountains is about 870 m (2,855 ft) thick and contains 

interbedded dark-gray argillaceous limestone, phosphatic in places , 

calcareous, shaly mudstone and siltstone, very fine grained 

calcareous, quartzose sandstone, medium- to coarse-grained 

calcareous sandstone or sandy limestone, some conglomeratic containing grains of quartz and chert, and a bed of chert- and quartzitepebble and cobble conglomerate 14m (46ft) thick, in addition to impure limestone typical of the formation in the ranges to the east. 

The age of the formation is largely Early Pennsylvanian to Early 

Permian, based on fusulinids and stratigraphic position, though 

some uppermost Mississippian beds are included in the base (fig. 4). 

The Phosphoria Formation (Lower Permian) conformably overlies 

the Snaky Canyon in the Snaky Canyon area and in the southern 

Lemhi Range (Skipp and Hait, 1977); beds equivalent to the 

Phosphoria are faulted against the Snaky Canyon Formation in the 

southern Pioneer Mountains (fig 4, column IV; Skipp and Hait, 1977), 

indicating that throughout its outcrop area, the Snaky Canyon is 

nowhere younger than Early Permian. 

Lower Permian fusulinid faunas reported years ago from the 

southern Lemhi Range (Blackstone, 1954; Ross, 1961) came from 

beds now assigned to the upper part of the Snaky Canyon Formation. 

BLOOM MEMBER 

The Bloom Member makes up approximately one-half of the Snaky 

Canyon Formation. In the type section of the member on Gallagher 

Peak, 647 m (2,125 ft) were measured below the Gallagher P:~ak 

Member (fig. 3). This thickness may include some minor repetition of 

beds along faults. Estimated average map thickness in the Snaky 

Canyon Quadrangle is about 550 m (1,800 ft). The Bloom Member is 

named for a triangulatiEstation named Bloom in sec. 32 (unsurveyed), T. 10 N., R. 31 E. about 2.7 km (1.7 mi) south of Gallagher 

Peak in the Copper Mountain Quadrangle (fig. 2). The member strikes 
22 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

112°45'00" 

44° 

07' 

30' 

112°42'30" 

0 MILE 

FIGURE 6.-Geologic sketch map of northwest part of Snaky Canyon Quadrangle showing location and distribution of type section of upper part of 

Snaky Canyon Formation and adjacent overlying and underlying upper 
SNAKY CANYON FORMATION 

Qat Ql 1~ Qat, alluvium and fan gravels 

Ql, landslide deposits 

UNCONFORMITY 

Rhyolite flow1 

UNCONFORMITY 

Volcanics and gravels 

QTvg, basalt flows and rhyolitic 

welded tuffs interbedded 

with fan gravels, some 

cemented, 

QTg, tan gravels, some 

cemented 

UNCONFORMITY 

Dinwoody • Formation 

UNCONFORMITY 

Phosphoria ~ Formation 

I.Q 0 II 

I~ 

r~ I~ Ia. 

Q) 

c: 

Q) 

0 

.Q 

0:::: 

(i) 

0. 

0. 

::::> 

EXPLANATION 

>-

a: 

< z 

a: 

w 

1-

< 

::::> 

0 

>-

a: 

< 

i= 

a: 

w 

1-

Contact 

-.-------- High-angle fault -Bar and ball 

on downthrown side; dashed 

where approximate or inferred; 

dotted where concealed 

A A A ...L.i. 

Thrust fault-Sawteeth on 

upthrown side; dashed where . 

approximate or inferred 

---+--?-

Axial trace of anticline, 

queried where inferred 

Axial trace of overturned anticline, 

showing direction of dip limbs 

Axial trace of syncline 

E9 __c_o 

Horizontal Inclined 

--&0 ~ 

Overturned Crumpled 

Strike and dip of beds 

Line of measured 

~ section of 

Dinwoody Formation, 

Phosphoria Formation, and 

Juniper Gulch Member and 

Gallagher Peak Sandstone 

Member of Snaky Canyon 

Formation 

Paleozoic and Triassic formations. Geology by Betty Skipp, D. L. Schleicher, 

and Raymond Jeanloz, 1974, and by M. H. Hait, Jr.,l977; assisted by Linda 

Riglin, 197 4. 

23 
24 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

generally north-south in this area and is well exposed on both 

Gallagher Peak and in the area of the Bloom triangulation station 

where it dips approximately 30 o to the east. 

The unit consists of medium-gray to medium-dark-gray limestone 

which weathers light gray to medium dark gray in beds 5 em (2 in.) to 

0. 7 m (2 ft) thick, interbedded with thin beds of medium-gray, very 

fine grained quartzose sandstone or siltstone which weather pale 

yellowish brown and light olive gray (fig. 3). The limestone ranges in 

texture from aphanitic to coarse grained, is sandy or silty through 

much of the interval, and contains numerous gray chert and brownweathering incipient chert nodules. The incipient chert nodules have 

rims of partly silicified material on the outside but are limestone on 

the inside. Brown-weathering, generally concentrically laminated, 

stromatolitic mounds are common and have both lenticular shapes 

with long axes parallel to bedding and moundlike forms with long 

axes perpendicular to bedding. Medium- to coarse-grained chert and 

quartz sandstone, shaly mudstone and siltstone, and minor conglomerate are also present in beds correlated with the Bloom Member 

in the southern White Knob Mountains. 

The member in most areas forms a series of ledges or smooth slopes 

with scattered ledges, in contrast to the underlying Mississippian 

limestones which are commonly cliff forming. An exception to this 

ledge form is present in the southern Pioneer Mountains where the 

lower beds of the Snaky Canyon Formation make striking cliffs. Here 

the Pennsylvanian rocks are distinguished from the Mississippian 

rocks by an abundance of impurities,' notably sand and silt. In most 

of the region, the Pennsylvanian limestones are lighter colored than 

the underlying Mississippian rocks and show more variation in color 

on weathered surfaces, probably a result of the sand, silt, and clay impurities. Impure wackestones, packstones, and lesser spiculitic 

mudstones are the chief carbonate lithologies. 

Fossils are fairly common in the unit, and, in addition to the 

stromatolites, include fusulinids and other Foraminifera, algae, 

brachiopods, corals, bryozoans, ostracodes, trilobites, mollusks, 

crinoid columnals, abundant encrinitic debris, and a few conularids. 

Fusulinids and smaller calcareous Foraminifera from about 215 to 

550 m (705-1,805 ft) above the base of the type section of the member 

include: 

Beedeina sp. 

Fusulinella sp. 

Globivalvulina sp. 

Millerella? sp. 

Pseudostaffella sp. 

Staffella sp. 

Textulariid 
SNAKY CANYON FORMATION 25 

The forms are diagnostic of a Middle Pennsylvanian (Atokan and 

DesMoinesian) age. Smaller calcareous Foraminifera from the lower 

15m (50ft) of the Bloom Member in the type section are representative of zone 19 of latest Mississippian age (fig. 3 and measuredsection 2). A zone 20 fauna is reported from the base of beds correlative with the lower part of the Bloom Member in the central Lost 

River Range (Mamet and others, 1971). Zone 20 faunas are present 

about 91 m (300ft) above the base of the Snaky Canyon Formation in 

the southern White Knob Mountains (fig. 5, sec 4). 

Fusulinids from near the top of the member in the Arco Hills include "a problematical fusulinid [that] meets the description of 

Eowaeringella sp. of Late Pennsylvanian (Missourian) age * * *. 

Latest DesMoinesian is also a possibility." (R. C. Douglass, written 

commun., 1976). 

Fusulinids and smaller calcareous Foraminifera from a collection in 

the southern Lemhi Range interpreted to be from the upper 10m (33 

ft) of the Bloom Member include Lunucammina? sp., Schubertella 

sp., Globivalvulina sp., Millerella sp., and Triticites sp. The fauna is 

interpreted to be of post-Middle Pennsylvanian age. 

Corals recovered from the Arco Hills collection that contained the 

fusulinid, Eowaeringella sp., were identified by W. J. Sando (written 

commun., 1976) as Bradyphyllum? sp. and Multithecopora sp. Sando 

states that they are compatible with a latest Middle to early Late 

Pennsylvanian age. Corals from the top of the Bloom Member just 

below the type section of the Gallagher Peak Sandstone Member (fig. 

3, coHn. 25635-PC, measured-section 1) include Multithecopora cf., 

M hypatitae Wilson, Caninia sp., and Dibunophyllum? sp., identified 

by W. J. Sando (written commun., 1975) who assigns the fauna to the 

"Pennsylvanian, probably Middle or Upper." Brachiopods from the 

same collection identified by J. T. Dutro, Jr., (written commun., 1975) 

include W aagenoconcha sp. and productoid and punctate spiriferoid 

fragments. Dutro assigns a ''Permian, or possibly latest Pennsylvanian" age to the collection. Bryozoans, also from the same collection, 

identified by 0. L. Karklins (written commun., 1975), include 

Ascopora? sp., Rhabdomeson sp., and Rhombotrypella sp .. Karklins 

notes that the Rhombotrypella specimen is identical to a form which 

occurs in the lower Oquirrh or Wells Formation in southeastern 

Idaho, and that unpublished data suggest that the genus ranges 

across the boundary between the Pennsylvanian and Permian 

Systems. 

As Late Pennsylvanian fusulinid faunas (fig. 3, colln.f13895) have 

been recovered from the measured section stratigraphically above 

these collections, a Late Pennsylvanian age for the uppermost part of 

the Bloom Member seems to best fit available faunal evidence. Thus, 

the member ranges in age from latest Mississippian to Late Pennsylvanian. 
26 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

Other corals identified by W. J. Sando (written communs., 1975, 

1976) from various stratigraphic positions within the member in the 

Arco Hills, southern Lemhi Range, and southern Beaverhead Mountains include Caninia sp., Ca"f1,inia cf., C. torquia (Owen), and 

Lophophyllidium sp. 

Brachiopods identified by J. T. Dutro, Jr. (written commun., 1975) 

from the lower part of the member in the Arco Hills and southern 

Beaverhead Mountains include Crurithyris? sp., Linoproductus sp., 

Punctospirifer? sp., Schizophoria? sp., and dictyoclostid and productoid fragments. 

Bryozoans including Archimedes sp., Ascopora sp., Rhombopora 

sp., Rhombotrypella sp., and streblotyrpid and sulcoreteporid forms 

from the Bloom Member in the southern Lemhi Range and southern 

Beaverhead Mountains wer~ identified by 0. L. Karklins (written 

communs., 1975, 1977). The corals, brachiopods, and bryozoans are 

all compatible with a Pennsylvanian age. 

Extensive Pennsylvanian faunas have been reported from 

limestone and silicified limestone beds in the southern White Knob 

and Pioneer Mountains (Nelson and Ross, 1969a, b) that are thought 

to be correlative with the Bloom Member of the Snaky Canyon Formation. More recent collections contain the algae, brachiopods, bryozoans, conularids, corals, fusulinids, and other foraminifers, 

mollusks, and trilobites listed below. 

Brachiopods identified by J. T. Dutro, Jr., (written communs., 

1971, 1973, 1974, 1978) include: 

Anthracospirifer cf. A. occiduus (Sadlick) 

Anthracospirifer sp. 

Antiquatonia aff. A. coloradoensis (Girty) 

Antiquatonia sp. 

Cleiothyridina cf. C. orbicularis (McChesney) 

Cleiothyridina sp. (large) 

Composita cf. C. subtilita (Hall) 

Composita sp. 

Derbyia sp. 

Dictyoclostus sp. 

Hustedia sp. 

Kozlowskia sp. 

Leiorhynchoidea rockymontana (Marcou) 

Linoproductus sp. 

Martinia? sp. 

Meekella sp. 

Neospirifer aff. N. alatus Dunbar and Condra 

Orbiculoidea sp. 

Orthotetacean form 
SNAKY CANYON FORMATION 

Punctospirifer sp. 

Roemerella patula Girty 

Rugoclostus sp. 

Schizophoria sp. 

W aagenoconcha? sp. 

W ellerella? sp. 

27 

Bryozoans identified by 0. L. Karklins (written commun., 1974) 

and J. T. Dutro, Jr. (written commun., 1971) include: 

Fenestellid forms, indet. 

Penniretepora? sp. 

Rhabdomeson sp. 

Rhombopora sp. 

Rhombotrypellid forms, indet. 

Stenocladia? sp. 

Tabulipora sp. 

The conularid, Calloconularia? cf. C.? crustula (White), was identified by J. T. Dutro, Jr. (written commun., 1978). 

Corals identified by W. J. Sando (written communs., 1971, 1974) 

and J. T. Dutro, Jr. (written commun., 1978) include: 

? Bradyphyllum sp. 

?Caninia sp. 

?Chaetetes sp. 

Dibunophylloid coral 

Michelinia sp. 

Multithecopora? reptant sp. 

Syringoporoid coral 

Foraminifers and algae include: 

A primitive form of Fusulinella sp. 

Archaediscus sp. 

Asphaltina sp. 

Asteroarchaediscus spp. 

Beresellid algae 

Biseriella spp. 

Primitive bradyinid 

Earlandia spp. 

Endothyra of the group E. bowmani Phillips, emend Brady 

Endothyra excellens (D. E. N. Zeller) 

Endothyranella sp. 

"Eosigmoilina" rugosa Brazhnikova 

Eostaffella spp. 

Eos taffellina sp. 

H emiarchaediscus sp. 

Millerella spp. 

Monotaxinoides sp. 

N eoarchaediscus spp. 
28 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

Planospirodiscus sp. 

Pseudoammodiscus sp. 

Pseudoendothyra spp. 

Pseudoglomospira sp. 

Stacheoides sp. 

Ungdarella sp. 

Volvotextularia sp. 

Zellerina spp. 

Mollusks identified by John Pojeta (written commun., 1978), E. L. 

Yochelson (written communs., 1974, 1978), and MacKenzie Gordon, 

Jr. (written commun., 1978) include: 

Cephalopods: 

Liroceras? sp. 

Orthoconic nautiloid cephalopod, indet. 

Gastropods: 

Glabrocingulum (Glabrocingulum) sp. 

Glabrocingulum (Ananias) sp. 

Glabrocingulum aff. G. nevadensis (Walcott) 

Trepospira sp. 

Pelecypods: 

Aviculopecten sp. 

Bellerophontacean form 

Nuculopsis cf. N. girtyi: Schenck 

Phestia cf. P. bellistriata (Stevens) 

Trilobites identified by MacKenzie Gordon, Jr., (written commun., 

1978) and J. T. Dutro, Jr. (written commun., 1974) include: 

Proetid trilobite fragment, indet. 

Sevillea trinucleata (Herrick) 

These faunas support a latest Mississippian to Late Pennsylvanian 

age for the Snaky Canyon Formation (Bloom Member) in the 

southern Pioneer and White Knob Mountains. 

The contact of the Bloom Member with the overlying Gallagher 

Peak Sandstone Member is gradational everywhere. 

GALLAGHER PEAK SANDSTONE MEMBER 

Medium-gray and light-brownish-gray, very fine grained, thinbedded, calcareous, quartzose sandstone makes up the bulk of the 

Gallagher Peak Sandstone Member which is 59 m (195ft) thick in the 

type section near Snaky Canyon and makes up the summit of 

Gallagher Peak, for which it is named. The section on Gallagher 

Peak, however, is incomplete. 

The type section which is overturned near Snaky Canyon (fig. 6) 

is overlain gradationally by the lower beds of the Juniper Gulch 

Member. 
SNAKY CANYON FORMATION 29 

The Gallagher Peak Sandstone Member has been mapped locally in 

the Beaverhead Mountains and in the Lemhi and Lost River Ranges 

to the west. It is less than 30m (100ft) thick in exposures in the Arco 

Hills, and, like the sandstone of the Bluebird Mountain Formation, 

appears to thin to the west. It has not been recognized with certainty 

west of the southern Lost River Range, but it may be present. Reconnaissance mapping also suggests that the sandstone may not be 

laterally continuous everywhere. It may constitute a series of extensive lenses rather than one continuous bed. 

Beds at or near the base of the member locally are fossiliferous and 

contain large brachiopods, pelecypods, both solitary and colonial corals, and bryozoans. J. T. Dutro, Jr. (writte~n commun., 1975) identified Thamnosia? sp., Kochiproductus? sp. (very large specimens), 

and a pectenoid pelecypod fragment from the base of the member on 

Gallagher Peak (fig. 3, colin. 25634-PC). Dutro considers the forms to 

be of "Permian, or possibly latest Pennsylvanian" age, the same age 

determination made on colin. 25635-PC (fig. 3) from the uppermost 

beds of the Bloom Member, immediately below the type section of 

the Gallagher Peak Sandstone. 

Fusulinids from within and just above the Gallagher Peak Sandstone Member in the type section include staffellid forms and small 

forms with a fusulinellid wall that suggest Fusulinella sp. A Middle 

Pennsylvanian age is suggested in the absence of any fragments suggesting a younger age. A fauna 51.8 m (170ft) above the Gallagher 

Peak Member in the type section of the overlying Juniper Gulch 

Member, however, contains rare Triticites sp., which are assigned to 

the Upper Pennsylvanian (Missourian). In the Arco Hills, fusulinids 

collected about 65 m (213ft) above the base of the Gallagher Peak are 

identified as Eowaeringella sp. of probable Missourian, but possibly 

latest DesMoinesian, age. 

Faunal and stratigraphic evidence point to a Late Pennsylvanian 

age for the Gallagher Peak Sandstone Member, as Upper Pennsylvanian faunas are present both above and below the member. 

JUNIPER GULCH MEMBl~R 

The carbonate rocks of the Juniper Gulch Member gradationally 

overlie the Gallagher Peak Sandstone Member in the type section 

which is overturned and dips to the west 45 o to 60 o near Snaky Canyon (fig. 6). The member is named for Juniper Gulch which lies about 

1 km (less than 1 mi) south of the type section (fig. 2). 

The Phosphoria Formation conformably overlies the Juniper Gulch 

Member in the type area. 

Reconnaissance mapping indicates that the member probably is 

present only in the southernmost parts of the Beaverhead Mountains 
30 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

and the southern Lost River and Lemhi Ranges. Rocks correlative 

with the Juniper Gulch have not been recognized in the southern 

Pioneer and White Knob Mountains. 

The member is 597 m (1,960 ft) thick in the type section and consists of interbedded sandy and cherty, generally light-grayweathering, thin- to thick-bedded limestone and dolomite. Light-gray 

to grayish-black chert in the form of nodules, beds, and complex networks forms as much as 40 percent of the interval in places. In the 

line of the measured section, dolomite makes up about two-thirds of 

the unit and is more abundant in the upper half. Some beds of sandy, 

ripple-marked dolomite, dolomite breccia, and conglomeratic 

(granule) dolomite are present. A medium-dark-gray, oolitic, ledgeforming, dolomite marker bed, 9 m (30ft) thick, is present about 374 

m (1,227 ft) above the base of the member in the type section. This 

lithology has been found as far west as the Arco Hills (fig. 2). 

The base of the member contains some sandy beds just above the 

Gallagher Peak Sandstone Member, and the upper 141m (463ft) of 

the member below the Phosphoria is interbedded sandy limestone 

and dolomite and very fine grained calcareous or dolomitic sandstone. 

The member is characterized by its generally light-colored weathering habit-medium to light gray, light olive gray, and light brown. It 

forms steep slopes or a series of alternating ledges and steep slopes. 

The lower part of the unit is thick to medium bedded, the upper part 

is thin to medium bedded. 

The Juniper Gulch Member, in general, is much less fossiliferous 

than the Bloom Member. Beds rich in crinoidal debris are common in 

the lower part of the type section along with a few beds composed 

largely of crinoid stems, some of which are as much as 2 em (0.8 in.) in 

diameter. Scattered fusulinids, gastropods, brachiopods, bryozoans, 

and both solitary and colonial corals are present throughout the 

member. Hydrozoans(?), calcareous phylloid algae, and encrusting 

foraminifers described from correlative strata in the Lemhi Range 

and Arco Hills to the west (Breuninger, 1976) are characteristic of the 

middle part of this member, and the algal-foraminiferal facies are 

present also in the type section in the southern Beaverhead Mountains in both the middle and upper parts. 

Fusulinids were recovered from only the lower 58 m (190ft) of the 

type section (fig. 3 and measured-section 1). Fusulinella sp. is present 

in the lower 18.3 m (60 ft) and Triticites sp. of Late Pennsylvanian 

(Missourian) age is present about 55 m (180 ft) above the base. 

Eowaeringella sp. is present in lower beds of the member in the Arco 

Hills area as discussed under the Gallagher Peak Sandstone Member. 

Fusulinid faunas of Late Pennsylvanian and Early Permian age 

were collected from the Juniper Gulch Member in several areas. 
SNAKY CANYON FORMATION 31 

Triticites sp. of Late Pennsylvanian (Virgilian) age is present within 

the Juniper Gulch Member in both the Arco Hills and the southern 

Lemhi Range. Several species of Triticites including T. cf. meeki, T. 

cf. ventricasus? and T. sp. were identified from beds correlative with 

the Juniper Gulch in the Arco Hills by the late L. D. Holcomb (in 

Breuninger, 1976, p. 585 and 587). Lower Permian (Wolfcampian) 

fusulinid faunas from the upper part of the Juniper Gulch include: (1) 

Pseudafusulina sp. identified by Douglass from the Arco Hills and 

near Bloom triangulation station in the Copper Mountain 

Quadrangle (figs. 1 and 2); (2) Schwagerina sp. from the Lemhi Range 

(Blackstone, 1954; Ross, 1961; Shannon, 1961) and the Arco Hills 

(Shannon, 1961; L. D. Holcomb in Breuninger, 1976; G. J. Verville, 

written commun., 1977), and (3) Pseudaschwagerina cf. P. gerantica 

Thompson collected and identified by G. J. Verville (written commun., 1977) from about 230m (755ft) above the highest hydrozoan(?) 

mound reported by Breuninger (1976) in his North Howe section in 

the Arco Hills area. 

The age range of the member based on fusulinid faunas is, 

therefore, Late Pennsylvanian to Early Perrnian. 

Brachiopod, coral, and gastropod faunas from the Juniper Gulch 

Member support a Late Pennsylvanian and. Early Permian age for 

the member. Corals from the Pennsylvanian part of the member in 

the Arco Hills identified by W. J. Sando (written commun., 1976) 

include: 

Caninia sp. 

Caninia cf. C. tarquia (Owen) 

Multithecapara? sp. 

Syringoporoid coral, undet. 

The brachiopod Linapraductus sp., identified by J. T. Dutro, Jr. 

(written commun., 1976) from an Arco Hills collection, is "of [a] size 

and form [that] indicate an Upper Pennsylvanian (post-Des Moinesian) to Lower Permian (Wolfcampian) range.'' 

Lower Permian coral faunas from the upper part of the Juniper 

Gulch in the Arco Hills and southern Lemhi Range identified by W. 

J. Sando (written communs., 1976, 1978) are Durhamia sp., 

Heritschiella sp., Heritschaides sp., Multithecapara sp., and Syringapara cf., S. multattenuata McChesney. A coral tentatively identified 

as Heintzella? sp. by W. J. Sando (writt€1n commun., 1978) was 

recovered from the middle part of the membE~r in the southern Lemhi 

Range and may be of either Late Pennsylvanian or Early Permian 

age. The gastropod Omphalatrachus sp. which may belong to the 

species 0. whitneyi Meek was identified by E. L. Yochelson (written 

commun., 1976) in an Arco Hills collection and assigned to the Lower 

Permian. 

Carbonate buildups in the Juniper Gulch Member are composed 
32 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

largely of the hydrozoan(?) Palaeoaplysina, the calcareous phylloid 

alga Eugonophyllum, other phylloid algae, bryozoans, and less common gastropods, crinoids, brachiopods, and encrusting foraminifers. 

The mounds, dated by associated fusulinid faunas, span the 

Pennsylvanian-Permian boundary and consist, in part, of lenticular 

buildups which "stood topographically above the adjacent sea floor 

while they formed" (Breuninger, 1976, p. 584). The algal facies 

studied by Breuninger (1976) in the Lemhi Range and Arco H,iUs are 

also present in the southern Beaverhead Mountains, though not 

studied there. 

The upper contact of the Juniper Gulch Member of the Snaky Canyon Formation with the overlying Phosphoria Formation was not 

observed in the southern Beaverhead Mountains in the area of the 

measured section (fig. 3), but is exposed in the southern Lemhi Range 

(Skipp and Hait, 1977) where it is abrupt and conformable. 

OVERLYING UNITS 

The Snaky Canyon Formation is overlain conformably by the 

Phosphoria Formation of Early Permian age. In the line of the 

measured section (figs. 2, 3, measured-section 1), the Phosphoria is 

about 86 m (282 ft) thick. The base is covered at this locality, but E. 

K. Maughan (written commun., 1975) identified 5 to 7 m (16-23 ft) of 

siltstone which he assigns to the Meade Peak Member of the 

Phosphoria in Snaky Canyon itself 1.6 km (1 mi) to the northwest. A 

thin Rex Chert Member of the Phosphoria may also be present there 

as it is in the Lemhi Range. Above the basal covered interval of the 

measured section, 17 m (56 ft) of interbedded limestone and dolomite, 

some of it cherty or sandy, and minor chert are assigned to the Franson Tongue (of the Park City Formation). Above the Franson is a 

covered interval, 35 m (115 ft) thick, with abundant subcrop of 

phosphorite, dolomite, and siltstone of the Retort Phosphatic Shale 

Member. The phosphorite is medium to dark gray or brownish gray, 

fine grained, oolitic, and sandy and silty in places. The dolomite is 

brownish gray, silty or sandy, and has oolitic phosphorite laminations and nodules. The siltstone is moderate yellowish brown to light 

reddish brown, sandy or muddy, laminated to thin bedded, with 

phosphatic laminations and lenses. Above the covered interval of the 

Retort is 27.4 m (90 ft) of the well-exposed ledge-forming Tosi Chert 

Member. The chert is grayish black to medium dark gray, mostly fine 

grained, thin to medium bedded (0.15-1.0 m, 0.5-3 ft), and is interbedded in the basal 3 m (10 ft) with dark-brownish-gray and 

medium-gray, oolitic, phosphatic limestone. 

Incomplete sequences of the Phosphoria Formation are preserved 

in a graben in the southern Lemhi Range (Skipp and Hait, 1977). In 
OVERLYING UNITS 33 

this area,about 4.5 m (15 ft) of interbedded grayish-black chert and 

light-gray to medium-gray dolomite conformably overlie light-grayweathering dolomite of the Snaky Canyon Formation. The chert is 

medium bedded (1m, 3ft) to irregularly (nodular) bedded and forms 

conspicuous ledges. The dolomite is fine grained with a few lenses of 

bioclastic material and contains nodules and lenses of phosphorite. 

The brachiopod Orbiculoidea sp. is present in all facies. The chert 

unit tentatively is assigned to the Rex Chert Member of the 

Phosphoria. The Rex(?) Chert Member is overlain by a minimum of 25 

m (82 ft) of very dark gray organic-rich mudstone, coarse-grained 

oolitic phosphorite, and phosphatic siltstone (E. K. Maughan, written commun., 1979) similar to that described for the Retort 

Phosphatic Shale Member in the measured section near Snaky Canyon in the southern Beaverhead Mountains. The phosphorite and 

siltstone contain the brachiopods Orbiculoidea cf. 0. missouriensis 

(Shumard) and Cancrinella cf C. phosphatica (Girty), "two species 

[that] occur commonly in the Meade Park Member of the Phosphoria 

Formation in southeast Idaho and western Wyoming" (J. T. Dutro, 

Jr., written commun, 1978). 

Along Antelope Creek just west of measured-section 4 (fig. 1; 

Skipp and Hait, 1977), a minimum of 50 m (164ft) of dark-gray thinbedded argillaceous and silty limestone, some of it phosphatic, is 

present in a small graben. The contained fauna consists of 

brachiopods, bryozoans, and mollusks that indicate an Early Permian (probably Word equivalent) age for the sequence which is correlative with part of the Phosphoria Formation (Nelson and Ross, 

1969a, b). In addition to the extensive faunal list in Nelson and Ross 

(p. 84-85), the following brachiopods were identified by B. R. 

Wardlaw (written commun., 1978) from the same collection: 

Dielasmid form 

Linoproductus? sp. 

Orbiculoidea sp. 

Peniculauris sp. 

Waagenoconcha montpelierensis (Girty) 

Additional brachiopod collections from the same locality identified 

by J. T. Dutro, Jr. (written communs., 1971, 1974) and B. R. Wardlaw 

(written commun., 1978) contain: 

Cancrinella cf. C. phosphatica (Girty) 

Echinoconchus cf. E. inexpectatus (Cooper) 

Orbiculoidea sp. 

Peniculauris sp. 

Pterinopectinella? sp. 

Spiriferinid form 

Waagenoconcha cf. W. montpelierensis (Girty) 
34 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

According to Wardlaw (written commun., 1978), both of these 

faunas indicate a latest Leonardian age for the sequence and 

equivalence to the Meade Peak Member of the Phosphoria in 

southwestern Montana and southeastern Idaho. 

In the Snaky Canyon quadrangle (fig. 2), the Phosphoria Formation 

is overlain, presumably conformably (the contact is not exposed), by 

54.9 m (180 ft) of poorly exposed interbedded light- to moderatebrown calcareous siltstone and pale-olive-gray and yellowish-brown 

shale, micaceous in places, of the Triassic Dinwoody Formation (figs. 

3 and 6). This is the westernmost recognized outcrop of Triassic rocks 

west of the Medicine Lodge thrust zone (Embree and others, 197 5; 

Skipp and Hait, 1977; Ruppel, 1978) north of the Snake River Plain in 

Idaho. Fragments of Dinwoody lithologies, however, have been found 

by Skipp in Tertiary gravels in the southern Lemhi Range. 

Gently tilted rhyolitic welded tuffs of Pliocene(?) age unconformably overlap overturned Triassic beds in the southern Beaverhead 

Mountains (H. J. Prostka, written commun., 1976). 

PALEOTECTONIC SIGNIFICANCE 

The present distribution of the formations discussed in the 

preceding sections has been determined by post-Paleozoic tectonic 

events, particularly eastward thrusting of late Mesozoic and early 

Tertiary age and major block faulting (basin-and-range) of later Tertiary and Quaternary age. The units are all allochthonous, having 

been transported eastward and internally deformed along several major thrusts (Scholten and others, 1955; Scholten and Ramspott, 1968; 

Skipp and Hait, 1977; Ruppel, 1978). They are assumed to be preserved in their relative geographic depositional positions, even though 

much of the record has been destroyed or is unavailable. 

The Arco Hills and Big Snowy Formations record an influx of finegrained cratonic terrigenous material, largely clay and silt, into 

south-central Idaho which, in the Beaverhead Mountains, interrupted Late Mississippian carbonate-bank buildup (Rose, 1976) 

possibly as early as early Chesterian time. West of the area of Big 

Snowy deposition, carbonate buildup recorded in the thick- bedded 

pure limestones of the Surrett Canyon Formation and the White 

Knob Limestone continued into late Chesterian time. In latest 

Chesterian time, fine-grained terrigenous material present in the Arco Hills Formation spread as far west as the Lost River Range. Carbonate deposition continued without major interruption west of the 

Lost River Range in the White Knob Mountains. 

Upper Mississippian strata preserved in the western overlying 

Copper Basin allochthon (fig. 1, Skipp and Hait, 1977) include a few 

shallow-water limestone and dolomite lenses, and coaly beds which 
PALEOTECTONIC SIGNIFICANCE 35 

indicate that by middle to late Chesterian time, flysch sedimentation 

had ceased in much of the area of the former deepwater flysch trough 

(Nilsen, 1977; Skipp and Hait, 1977; Skipp and others, 1979). 

The uppermost Mississippian Bluebird Mountain Formation which 

overlies the Big Snowy and Arco Hills Formations also overlies the 

White Knob Limestone (fig. 4). The very fine grained craton-derived 

sandstones of the Bluebird Mountain thin westward, and though the 

evidence has been removed by erosion and thrusting, the sandstones 

of the Bluebird Mountain Formation may have overlapped some of 

the Copper Basin terrane, completing the demise of the flysch trough 

and marking the end of the Antler orogenic highland described by 

Poole (1974) and Poole and Sandberg (1977) in this part of Idaho. 

As the Antler orogenic belt subsided and deposition ceased in the 

area of the flysch trough, the thick wedge of flysch deposits of the 

Copper Basin Formation began to rise, probably due to thrusting or 

block faulting (Skipp and others, 1979). By Middle Pennsylvanian 

time, parts of the Copper Basin terrane were emergent and being 

eroded. Westward-thinning proximal turbidites composed of reworked Copper Basin orogenic detritus are present in easternmost 

allochthonous exposures of the Wood River Formation (Skipp and 

Hall, 197 5a). To the east, the presence of the Copper Basin highland 

(Skipp and others, 1979) is recorded in the medium- to coarse-grained 

chert and quartzite sandstones and conglomerates in the Pennsylvanian parts of the Snaky Canyon Formation in the White Knob Mountains area. Orogenic material of this kind is not present in the Snaky 

Canyon Formation east of the White Knob Mountains. 

More indirect evidence for the Copper Basin highland is present in 

the Palaeoaplysina-algal mounds in the Pennsylvanian and Lower 

Permian Juniper Gulch Member of the Snaky Canyon Formation according to Breuninger (1976, p. 604), who hypothesizes a protective 

''local tectonic uplift directly west and northwest of the buildup 

area." Because beds correlative with the Juniper Gulch Member of 

the Snaky Canyon Formation have not been identified west of the 

Lost River Range-Arco Hills area, it may be that the area of the 

White Knob Mountains itself was high during latest Pennsylvanian 

and part of Early Permian time and formed the eastern part of an 

uplift immediately west of the Palaeoaplysina-algal-mound buildups. 

Evidence of the Copper Basin highland is also present south of the 

Snake River Plain in the northern Sublett Range where southwardthinning chert-pebble and granule conglomerates are present in the 

Pennsylvanian part of the Oquirrh Formation (R. L. Armstrong, written commun., 1977; Skipp and others, 1979). 

The Copper Basin highland remained a source for detritus in the 

Wood River Formation in the Fish Creek Reservoir area (fig. 1) at 

least as late as Late Pennsylvanian time (Skipp and Hall, 1975a). The 
36 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

highland probably was the major physiographic feature which 

separated the two Permian-Pennsylvanian depocenters of southcentral Idaho-the Wood River basin on the west, and the Snaky 

Canyon carbonate bank on the east (Skipp and others, 1979). 

The Wood River basin developed slightly later (Des Moinesian, 

possibly Atokan time) and received more than twice as much sediment (more than 3,000 m or 9,850 ft vs. 1,300 m or 4,265 ft) as the 

Snaky Canyon area during Pennsylvanian and Permian time. The 

bulk of the terrigenous detritus, however, in both areas consists of 

very fine grained quartz sand and silt that probably had northeastern 

and (or) southern cratonic sources. 

The lower three units of the Wood River Formation (fig. 4), excluding the basal conglomerate facies, are largely sandy and silty 

limestone with interbedded very fine grained calcareous sandstone 

quite similar to the lithologies of the upper part of the Bloom 

Member and the Gallagher Peak Sandstone Member of the Snaky 

Canyon Formation and approximately the same age, though Upper 

Pennsylvanian (Missourian) faunas have not been found in the Wood 

River Formation. The Juniper Gulch Member of the Snaky Canyon 

Formation is about the same age as units 4 through 6 of the Wood 

River Formation (fig. 4), but there is little lithologic resemblance. 

Thick sequences of very fine grained sandstone and sandy limestone 

with abundant fusulinid faunas characterize the Wood River 

(Umpleby and others, 1930; Bostwick, 1955; Hall and others, 1974), 

whereas sandy and cherty carbonate-bank facies including hydrozoan(?) and phylloid algal buildups, oolitic carbonate sand, and 

laminated dolomites make up much of the Juniper Gulch Member. 

Stevens (1971) in a study of Middle Pennsylvanian rocks in Colorado 

concluded that phylloid algae thrived in water depths of 5 to 7 m 

(16-23 ft), whereas fusulinid foraminifers preferred water depths of 

15 m (50 ft) or slightly more. The hydrozoan(?) Palaeoaplysina 

buildups probably formed in the uppermost subtidal zone (Breuninger, 1976). Laminated dolomites and sandstones and dolomite breccias may represent stromatolitic deposits formed in intertidal environments. In general, therefore, the Wood River basin was 

characterized by deeper water environments and greater subsidence 

than the Snaky Canyon carbonate-bank area. 

Facies of the Phosphoria equivalent rocks in the southern White 

Knob Mountains suggest that the area of the Copper Basin highland 

may have continued to have an influence on sedimentation patterns 

into latest Early Permian time. Thin-bedded silty and argillaceous 

limestones containing abundant mollusks and brachiopods indicate 

deposition within a normal marine basin of shallow or moderate 

water depths. There is no evidence of westward-derived orogenic 

detritus in these rocks, however. 
MEASURED SECTIONS 37 

Correlative strata in the Wood River Formation appear to have had 

deeper water origins; they include unit 7 and possibly, unit 8. Unit 7 

consists of interbedded banded dark-gray chert and gray sandy 

limestone (Hall and others, 1974, p. 94), and unit 8 is a thick sequence 

(possibly 1, 770 m or 5,800 ft) of interbedded gray and brown 

siltstone, siltite, quartzite, and dark-gray silicious argillite that contains locally abundant worm tubes, and minor silty and sandy limestone beds (Hall and others, 1978). The units are poorly fossiliferous 

with sparse trace fossils, radiolarians, and foraminifers. 

The Copper Basin highland in the relative position of the present 

Copper Basin allochthon, therefore, contributed minor amounts of 

detritus to depocenters in central and southern Idaho through Middle and Late Pennsylvanian time. Its role in Early Permian time is 

less well defined, but hydrozoan and algal mounds in the Lower Permian part of the Juniper Gulch Member in the southern Lost River 

and Lemhi Ranges require a protective highland on the west and 

northwest (Breuninger, 1976) in the general position of the Copper 

Basin highland. Relatively shallow-water facies of Phosphoria 

equivalent rocks in the White Knob Mountains contrasted with the 

deeper water sediments of correlative parts of the Wood River Formation suggest that the highland may have remained a 

physiographic barrier in central Idaho as late as latest Early Permian 

time. 

MEASURED SECTIONS 

SECTION 1.-Type section of the Gallagher Peak Sandstone and Juniper Gulch 

Members of the Snaky Canyon Formation 

[All of the strata including the overlying Phosphoria and Park City Formations and an incomplete section of the 

Dinwoody Formation (figs. 2 and 3, sec. 1) were measured in sec. 21, T. 9 N., R. 32 E., Clark County, Idaho. 

Members of the Snaky Canyon Formation and the Dinwoody Formation were measured by Betty Skipp, Linda 

Riglin, and James Davis in August 1974 and June 1976. The Phosphoria and Park City Formations were 

measured by Peter Oberlindacher and R. D. Hovland in August 1976]. 

Welded tuffs (Pliocene?): contact with Dinwoody Formation is 

angular unconformity ................................ . 

Total incomplete welded tuff section ................. . 

Dinwoody Formation (Triassic) (incomplete): 

Siltstone and shale, poorly exposed. 

Siltstone, light-brown, medium-brown, and dark yellowish 

orange, calcareous, weathers dark-yellowish orange, thin 

bedded. Shale, pale olive, yellowish brown, some micaceous. 

Forms slope ........................................... . 

Covered interval ......................................... . 

Total thickness incomplete Dinwoody Formation ..... . 

Thickness 

in meters feet 

5.0+ 

5.0+ 

36.6 

18.3 

54.9 

16+ 

16+ 

120 

60 

180 
38 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 1.-Type section of the Gallagher Peak Sandstone and Juniper Gulch 

Members of the Snaky Canyon Formation-Continued 

Phosphoria Formation (Permian): 

Tosi Chert Member: 

Chert and limestone. Chert, grayish-black to mediumdark-gray, fine- to coarse grained, thin- to mediumbedded (0.15-1 m or 0.5-3 ft). Limestone in basal3 m (10 

ft), dark-brownish-gray, medium-gray, phosphatic, 

oolitic. Member forms low ledge ..................... . 

Total thickness Tosi Chert Member .................. . 

Retort Phosphatic Shale Member: 

Phosphorite, siltstone and dolomite, mostly covered. 

Phosphorite in float, brownish gray, medium-gray to 

dark-gray, fine grained, oolitic, pelletal and nodular, 

sandy in places, fetid odor, weathers light reddish brown 

with bluish-white bloom on surface. Siltstone, moderate 

yellowish-brown, light-reddish-brown, pale-reddish-gray, 

sandy in places, argillaceous in places, laminated to thinbedded, calcareous in places, phosphatic laminations 

and lenses. Dolomite, brownish-gray to pale-brown, 

sandy and silty, oolitic phosphatic laminations and reworked fragments and nodules, weathers very pale 

brown ............................................. . 

Total thickness Retort Phosphatic Shale Member ..... . 

Park City Formation (Permian): 

Franson(?) Tongue: 

Limestone, chert, and dolomite. 

Limestone, medium-gray, sandy in places, fossiliferous, 

weathers to light brownish gray, thick-bedded. Chert, 

dark-gray, sandy in places, in nodules, lenses, and beds 

(5-29 em or 2-9.5 in. in thickness), present only in upper 

and lower parts. Dolomite, medium-gray, sandy, interbedded with chert. Unit forms irregular ledges ..... . 

Total thickness Franson(?) Tongue .................. . 

Covered interval (Meade Peak? Member of Phosphoria Formation) ............................................... . 

Total thickness covered interval ..................... . 

Total thickness Phosphoria and Park City Formations . 

Snaky Canyon Formation (Permian to Mississippian): 

Juniper Gulch Member (Lower Permian to Upper Pennsylvanian): 

24. Covered interval. Float limestone and dolomite ...... . 

23. Dolomite and sandstone. Dolomite, medium light-gray, 

sandy, (fine-grained quartz), fossiliferous (phylloid 

algae, gastropods, tubular foraminiferas), porous, 

medium-bedded. Sandstone, dark-gray, quartzose, 

calcareous, very fine grained. Forms ledges ....... . 

Thickness 

in meters 

27.4 

27.4 

35.0 

35.0 

17.0 

17.0 

7.0 

7.0 

86.4 

14.0 

15.2 

feet 

90 

90 

115 

115 

56 

56 

23 

23 

284 

46 

50 
MEASURED SECTIONS 39 

SECTION 1.-Type section of the Gallagher Peak Sandstone and Juniper Gulch 

Members of the Snaky Canyon Formation-Continued 

22. Sandstone, quartzose, calcareous, and sandy limestone. Sandstone, pale yellowish-brown, light-olivegray, very fine grained, weathers light brown and 

orange, thin-bedded to laminated. Sandy limestone, 

light gray. Forms slope .......................... . 

21. Limestone and sandstone. Limestone, medium-gray, 

light-gray, medium grained, sandy streaks, phylloid 

algae, weathers medium light gray, medium-bedded 

(0.3 m or I ft). Sandstone, calcareous, quartzose, 

medium-gray, very fine grained, forms thin beds. 

Forms ledges ................................... . 

20. Limestone, dolomite, dolomitic limestone, and sandstone. Limestone and sandstone in upper 10.3 m (34 

ft) similar to unit 21. Lower 12.2 m (40 ft) partly 

covered interval with ledges of dolomite, light-gray 

to medium gray, fine-grained, and medium gray dolomitic limestone. Forms upper ledge and lower slope . 

19. Dolomite, dolomite breccia, and dolomitic sandstone, 

50 percent exposed. Dolomite, medium-gray, 

moderate-yellowish-brown, pale-red, mottled 

yellowish-brown and medium-gray near top, finegrained to conglomeratic (granule), sandy in places 

(very fine to medium-grained quartz), weathers 

medium light gray, moderate yellowish brown, and 

pale red, thin-to medium-bedded (0.03-0.6 m or 1-2 

ft). Dolomite breccia, fragments of laminated 

dolomite in sandstone matrix. Dolomitic sandstone, 

medium gray, quartzose, very fine grained. Unit 

forms low ledges ................................ . 

18. Sandstone, dolomitic, light-gray, very pale yellowish 

brown, light-brown, medium-olive-gray, quartzose, 

very fine to fine-grained, laminated in part, ripplemarked, weathers light gray to pale red, thin- to 

medium bedded (0.15-0.3 m or 0.5-1 ft), forms slope 

17. Dolomite, sandstone, and sandy limestone. Dolomite 

at top, medium-gray, sandy, very fine grained. Sandstone, light gray to yellowish-orange, quartzose, 

calcareous, fine-grained to very fine grained, friable. 

Limestone, sandy, light gray, grayish-orange-pink, 

yellowish-gray, in part mottled light gray and 

yellowish brown. Unit thin to medium bedded, forms 

slope .......................................... . 

16. Covered interval. Subcrop is dolomitic limestone, 

medium-dark-gray with boxwork chert, weathers 

medium gray. Doubly terminated quartz crystals in 

float ........................................... . 

15. Dolomite, medium-dark-gray, oolitic, medium- to very 

coarse grained, calcareous in places, weathers 

medium light gray and light olive gray, thickbedded, forms cliff .............................. . 

Thickness 

in meters 

21.3 

11.0 

22.5 

34.1 

10.7 

12.2 

73.2 

9.1 

feet 

70 

36 

74 

112 

35 

40 

240 

30 
40 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 1.-Type section of the Gallagher Peak Sandstone and Juniper Gulch 

Members of the Snaky Canyon Formation-Continued 

14. Limestone, dolomitic, and dolomite, light gray to darkgray, 30-50 percent exposed, fine-grained, cherty 

zones; chert, medium-gray, weathers yellowish 

brown, as much as 20 percent of interval in places, in 

beds 0.07 to 0.14 m (0.25-0.5 ft) thick; weathers 

medium light gray, medium-to thick bedded (as 

much as 3m or 10ft), forms alternating ledges and 

slopes ......................................... . 

13. Covered interval. Subcrop is calcareous sandstone or 

dolomitic limestone, light-gray and light-brownishgray, weathers light gray, thin-bedded, forms slope. 

12. Dolomite, medium-gray to medium-dark gray, fine- to 

medium-grained, scattered grayish-black chert 

nodules which weather yellowish brown, fossiliferous 

in upper one-half (crinoid columnals, corals, algae, 

bryozoans), crinoidal debris common throughout, 

thick-bedded, forms ledge or cliff ................. . 

11. Dolomite, calcareous, medium-gray to medium-lightgray, coarse-grained, some fine-grained, few lightbrown weathering chert nodules, small silicified corals, weathers medium light gray, thick-bedded to 

massive (average 3m or 10ft), forms ledge ........ . 

10. Dolomite, calcareous, medium-gray to light-gray, 

coarse- to fine-grained, light-brown-weathering chert 

nodules in upper part, weathers. medium light gray, 

thick-bedded, forms ledge ....................... . 

9. Dolomite, calcareous,medium-gray to light-gray, 

coarse-grained, fossiliferous (crinoid columnals, 

hydrozoans(?), phylloid algae, gastropods), weathers 

medium light gray, thick-bedded, forms ledges .... . 

8. Dolomite similar to unit 10 ........................ . 

7. Dolomite, calcareous, medium-gray to light-gray, 

coarse-grained, fossiliferous (phylloid algae, 

foraminifers), weathers medium light gray, thickbedded, forms ledges ............................ . 

6. Limestone, light-gray to medium-gray, medium-grained, some crinoidal debris present, breccia near top, 

vuggy-weathering, weathers yellowish gray, light 

gray, and pale reddish gray, thick-bedded, 50 percent covered, forms swale ....................... . 

5. Dolomite, calcareous, and dolomitic limestone, 

medium-light-gray to dark-gray, fine- to mediumgrained, fossiliferous (much crinoid detritus, one coral in float, bryozoans), scattered medium-gray to 

medium light-gray chert in blebs, nodules, and irregular layers which weather pinkish or yellowish 

brown; weathers medium light gray with rough surface, thick-bedded (1.5-1.8 m or 5-6 ft), 80 percent 

exposed, forms series of irregular steps ........... . 

Thickness 

in meters feet 

53.3 

13.4 

35.3 

7.6 

13.0 

21.0 

13.7 

7.2 

47.2 

46.6 

175 

44 

116 

25 

43 

68 

45 

24 

155 

153 
MEASURED SECTIONS 41 

SECTION 1.-Type section of the Gallagher Peak Sandstone and Juniper Gulch 

Members of the Snaky Canyon Formation-Continued 

4. Covered interval. Subcrop is sandy limestone, calcareous sandstone, and minor breccia. Calcareous 

sandstone weathers light brown, forms slope ...... . 

3. Dolomite, calcareous in places, and chert. Dolomite, 

medium-gray to medium-light-gray, weathers 

yellowish gray or light gray, medium-bedded 

(0.06-0.3 m or 0.2-1 ft). Chert, light-gray to mediumgray, weathers light gray, thin-bedded (0.07-0.14 m 

or 0.25-0.5 ft), forms low ledge ................... . 

2. Limestone, medium-gray, fine-grained with some fossil 

hash, fossiliferous (phylloid algae, bryozoans, 

fusulinid and other Foraminifera, and ostracodes), 

sandy streaks present, weathers medium gray, 

forms ledge. Fusulinid and other Foraminifera are 

Triticites sp., globivalvulinid and endothyrid forms 

(USGS colin. £13895) ............................ . 

1. Covered interval. Float is sandy limestone and very 

fine grained calcareous sandstone. Float from lower 

18.3 m (60 ft) contains fragments of fusulinids that 

suggest Fusulinella sp. (USGS colln. £13723), crinoid 

debris, brachiopods, and bryozoans .............. . 

Total thickness Juniper Gulch Member .............. . 

Gallagher Peak Sandstone Member (Upper Pennsylvanian): 

Sandstone and limestone (poorly exposed). Sandstone, 

medium-gray, medium-light-gray, pale-grayish red, 

grayish-orange, very fine grained to fine-grained, 

weathers medium light gray, light olive gray and light 

brown, thin- to medium bedded. Limestone, sandy, 

medium gray, medium-bedded. Microfaunas from near 

the middle of member contain a small form with 

fusulinellid wall (USGS colin. £13933) Earlandia spp., 

Globivalvulina sp., Eotuberitina sp., and fragments of 

echinoderms, trilobites, and mollusks. A collection from 

5 m (15ft) above base contains staffellid forms and small 

forms with fusulinellid wall (USGS colin. £13894) ?Climmacammina sp. and pelmatazoan, bryozoan, and 

trilobite debris ..................................... . 

Total thickness Gallagher Peak Sandstone Member ... . 

Bloom Member, part (Upper Pennsylvanian to uppermost 

Mississippian): 

Limestone, medium-dark-gray, sandy, fine grained. Corals, 

brachiopods, and bryozoans collected from this locality 

(USGS colin. 25635-PC). Corals identified by W. J. Sando (written commun., 1975) are Multithecopora cf. M. 

hypatiae Wilson, Caninia sp., and Dibunophyllum? sp. 

Bryozoans identified by 0. L. Karklins (written comThickness 

in meters feet 

29.0 

28.6 

6.0 

51.8 

597.0 

59.4 

59.4 

95 

94 

20 

170 

1960 

195 

195 
42 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION I.-Type section of the Gallagher Peak Sandstone and Juniper Gulch 

Members of the Snaky Canyon Formation-Continued 

mun., 1975) are Ascopora? sp., Rhabdomeson sp., Rhombotrypella sp., and stenoporid, indet. Brachiopods identified by J. T. Dutro, Jr., (written commun., 1975) are 

Waagenoconcha sp., productoid fragment, and punctate 

spiriferoid, and also indeterminate echinoderm debris .. 

Incomplete thickness of Bloom Member .............. . 

Thickness 

in meters feet 

3.0 

3.0 

10 

10 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation 

[Measured in sees. 29 and 30 (unsurveyedl, T. 10 N., R. 31 E., Clark County, Idaho, on east side of saddle between 

Copper Mountain and Gallagher Peak (figs. 2 and 3, sec. 21. Base of section at approximately 2,865 m (9,400 ftl 

elevation. Part of section may be repeated by local faulting. Includes a partial section of the Big Snowy Formation. Measured by R. D. Hoggan, G. F. Elllbree, and E. J. Williams in June 1974] 

Snaky Canyon Formation, part (Upper Pennsylvanian to uppermost Mississippian): 

Gallagher Peak Sandstone Member (Upper Pennsylvanian) 

(incomplete): 

Sandstone and sandy limestone. Sandstone, light brownish-gray, light-olive-gray, quartzose, calcareous, very 

fine grained, weathers pale yellowish brown, thinbedded. Sandy limestone in lower part like unit 67 below. 

Forms talus slope interspersed with thin ledges on summit of Gallagher Peak. Corals present near base. 

Brachiopods and pelecypods from base of unit include 

Thamnosia? sp., Kochiproductus? sp. (very large 

specimens), and pectenoid pelecypod fragments (USGS 

coHn. 25634-PC, identified by J. T. Dutro, Jr., written 

Thickness 

in meters feet 

commun., 1975) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26.1 85 

Total thickness incomplete Gallagher Peak Sandstone 

Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26.1 85 

Gradational contact. 

Bloom Member (Upper Pennsylvanian to uppermost 

Mississippian): 

68. Limestone, gray, medium-grained, weathers gray, 

medium-bedded (0.3-0.6 m or 1-2ft), incipient chert 

nodules, forms cliff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 5 

67. Limestone, medium-dark-gray, sandy (as much as 50 

percent fine-grained quartz in some units), finegrained, weathers medium dark gray, thin- to 

medium bedded (0.05-0.3 m or 2 in.-1 ft), stylolites, 

forms cliffs. The coral Caninia? sp. (USGS coHn. 

25633-PC) was identified by W. J. Sando (written 

commun., 1975) from lower part of unit............ 24.4 80 

66. Limestone, gray, microcrystalline, weathers gray, 

mediul!l·bedded (0.3-0.6 m or 1-2ft), forms cliff.... 6.1 20 
MEASURED SECTIONS 43 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

65. Limestone, gray, medium-grained, weathers gray, incipient chert nodules present, medium-bedded 

Thickness 

in meters feet 

(0.3-0.6 m or 1-2ft) forms cliff.................... 9.1 30 

64. Limestone, gray, microcrystalline, sparry fragments, 

weathers gray, medium bedded (0.3-0.6 m or 1-2ft), 

forms cliff....................................... 3.0 10 

63. Limestone, gray, medium-grained, incipient chert nodules present, weathers gray, medium-bedded (0.3-0.6 

m or 1-2ft), forms cliff........................... 9.1 30 

62. Sandstone, gray, quartzose, calcareous, very fine 

grained, grains subrounded, well sorted, weathers 

pale yellowish brown, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft), forms ledge.............. 4.6 15 

61. Limestone, gray, medium-grained, incipient chert nodules present, weathers gray, medium-bedded (0.3-0.6 

m or 1-2ft), forms cliff........................... 9.1 30 

60. Sandstone, gray, quartzose, calcareous, very fine 

grained, grains subrounded, well sorted, weathers 

pale yellowish brown, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft), forms cliff............... 10.7 35 

59. Limestone, gray, microcrystalline, weathers light gray, 

medium-bedded (0.3-0.6 m or 1-2ft), forms ledge... 1.5 5 

58. Limestone, gray, medium-grained, incipient chert nodules present, weathers gray, medium-bedded (0.3 

-0.6 m or 1-2ft), forms cliff....................... 7.6 25 

57. Sandstone, medium-light-gray, quartzose, calcareous, 

very fine grained, grains subrounded, well-sorted, 

thin-to medium-bedded (0.05-0.3 m or 2 in.-1 ft), 

forms ledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 5 

56. Limestone, gray, medium-grained, incipient chert nodules present, weathers gray, medium-bedded (0.3-0.6 

m or 1-2ft), forms cliff........................... 7.6 25 

55. Limestone, gray, fine grained, fossiliferous, weathers 

light gray, contains chert nodules 2-3 em (1 in.) in 

diameter, thin to medium-bedded (0.05-0.3 m or 2 

in.-1 ft), forms slope. Fusulinids are Fusulinella sp. 

(USGS colin. £13725). Bryozoans include Archimedes 

sp., "probable new species that is somewhat similar 

to A. gillulyi Condra and Elias, 1944," (USGS colin. 

26315-PC) identified by 0. L. Karklins (written commun., 1976) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 5 

54. Limestone, gray, microcrystalline, brecciated, 

weathers gray, forms ledge....................... 1.5 5 

53. Limestone, gray, medium-grained, incipient chert nodules present, weathers gray, medium-bedded (0.3-0.6 

m or 1-2ft), forms cliff........................... 9.1 30 

52. Limestone, gray, microcrystalline, brecciated, 

weathers light gray, medium-bedded (0.3-0.6 m or 

1-2 ft), forms ledge.............................. 4.6 15 
44 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

51. Limestone, sandy, gray, medium-grained, weathers 

gray, medium-bedded (0.3-0.6 m or 1-2 ft), forms 

cliff. Lower part pelletal with oncolites. Fusulinids in 

lower part identified as early Beedeina sp. (USGS 

Thickness 

in meters 

colin. f13724) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.8 

50. Sandstone, gray, quartzose, calcareous, very fine 

grained, grains subrounded, well-sorted, weathers 

pale yellowish brown, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft), forms ledge.............. 1.5 

49. Limestone, gray, medium- to coarse-grained, crinoid 

detritus, incipient chert nodules present, weathers 

gray, medium bedded (0.3-0.6 m or 1-2 ft), forms 

cliff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 

48. Limestone, gray, fine grained, contains chert nodules 

2-3 em (1 in.) in diameter, contains twiggy bryozoans, weathers light gray, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft), forms slope.............. 3.0 

47. Limestone, gray, medium-grained, contains incipient 

chert nodules, weathers gray, medium-bedded 

(0.3-0.6 m or 1-2ft) forms cliff._ ........ -........... 24.4 

46. Limestone, gray, slightly sandy (very fine grained 

quartz), microcrystalline, brecciated, fossiliferous, 

weathers gray, thin- to medium-bedded (0.05-0.3 m 

or 2 in.-1 ft), forms cliff. Microfauna includes 

pseudostaffellid? and fusulinellid? forms, Millerella? 

sp., and textularid forms (USGS colin. f13892) and 

Globivalvulina sp. Pelmatazoan and bryozoan 

fragments common.............................. 4.6 

45. Limestone, gray, medium grained, much crinoid debris, contains incipient chert nodules, weathers gray, 

medium bedded (0.3-0.6 m or 1-2ft), forms cliff.... 19.8 

44. Sandstone, gray, quartzose, calcareous, very fine 

grained, well-sorted, weathers pale yellowish brown, 

thin to medium-bedded (0.05-0.3 m or 2 in.-1 ft) 

forms ledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 

43. Limestone, gray, medium-grained, sandy (very fine 

grained quartz), much pelmatazoan and bryozoan debris, incipient chert nodules in upper part and chert 

nodules in lower part, medium bedded (0.3-0.6 m or 

1-2 ft) forms cliff. Foraminifera from upper part 

(USGS colin. f13891) include Millerella?, textularids 

(?Climmacammina sp.), and small forms with 

fusulinellid wall, some fairly straight septa, heavy 

chomata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 

42. Sandstone, gray, quartzose, quartzitic, very fine grained, well-sorted, weathers gray and pale yellowish 

brown, microcrossbedding, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft) forms ledge............... 6.1 

feet 

65 

5 

30 

10 

80 

15 

65 

5 

50 

20 
MEASURED SECTIONS 45 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

41. Limestone, gray, medium-grained, contains chert nodules 5 em, (2 in.) in diameter, weathers gray, mediumThickness 

in meters feet 

bedded (0.3-0.6 m or 1-2ft), forms cliff............ 10.7 35 

40. Sandstone and thin interbedded limestone. Sandstone, 

gray, quartzose, quartzitic, very fine grained, wellsorted, weathers gray to pale yellowish brown, 

microcrossbedding, slabby-bedded, forms slope. 

Limestone interbed, gray, thin, contains syringoporoid corals (USGS colin. 26313-PC) identified 

by W. J. Sando (written commun., 1976) as 

Multithecopora cf. M. hypatiae Wilson............ 7.6 25 

39. Limestone, gray, medium-grained, contains incipient 

chert nodules, weathers gray, medium-bedded 

(0.3-0.6 m or 1-2ft) forms ledge................... 3.0 10 

38. Sandstone, gray, quartzose, quartzitic, very fine grained, well-sorted, weathers pale yellowish brown, 

slabby-bedded, forms slope . . . . . . . . . . . . . . . . . . . . . . . 3.0 10 

37. Limestone, gray, medium-grained, contains incipient 

chert nodules, much pelmatazoan, some bryozoan 

and arthropod debris, syringoporid coral heads common, weathers light gray, medium-bedded (0.3-0.6 m 

or 1-2 ft) forms cliff. Microfauna (USGS colin. 

f13890) includes stacheiid algae, tubular foraminifers, and small Fusulinella sp., and 

Globivalvulina sp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 30 

36. Sandstone, gray, quartzose, calcareous, very fine 

grained, very well sorted, contains incipient chert 

nodules, weathers gray and pale yellowish brown, 

laminated, thin-bedded, forms ledge............... 3.0 10 

35. Limestone, gray, medium-grained, contains incipient 

chert nodules, some with long axis perpendicular to 

bedding, weathers gray, medium bedded (0.3-0.6 m 

or 1-2ft) forms cliff.............................. 36.6 .120 

34. Sandstone, gray, quartzose, calcareous, fine grained, 

well sorted, weathers gray, laminated with cross 

laminations, forms ledge. Foraminifera (USGS colin. 

f13889) include Tuberitina sp., endothyrid forms and 

small forms with profusullinid-fusulinellid wall..... 3.0 10 

33. Limestone, gray, medium grained, weathers gray, contains minor incipient chert nodules in lower part, 

weathers gray, medium bedded (0.3-0.6 m or 1-2ft) 

forms cliff. Possible fault with minor displacement in 

lower beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 70 

32. Limestone, gray, medium grained, incipient chert nodules with vertical annular rings as much as 20 em (8 

in.) in length, weathers gray, medium-bedded 

(0.3-0.6 m or 1-2ft) forms cliff.................... 12.2 40 

31. Limestone, gray, medium-grained, incipient chert nodules, weathers gray, medium bedded (0.3-0.6 m or 

1-2ft) forms cliff................................ 12.2 40 
46 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

30. Limestone, gray, sandy (very fine grained quartz) and 

silty, medium-grained, large productid and spiriferid 

brachiopods in lower part, weathers gray, mediumbedded (0.3-0.6 m or 1-2ft) forms cliff. Microfauna 

(USGS f13888) includes small forms with profusulinellid-fusulinellid wall and pelmatazoan, bryozoan 

Thickness 

in meters feet 

and brachiopod debris . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 70 

29. Limestone, gray, fine grained, weathers gray, slabby 

bedded, forms slope.............................. 1.5 5 

28. Limestone, gray, medium-grained, fossiliferous (crinoid columnals, tetracorals, and bryozoans), 

weathers gray, medium-bedded (0.3-0.6 m or 1-2ft) 

forms cliff. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.7 55 

27. Limestone, gray, medium-grained, incipient chert nodules, basal beds sandy, weathers gray, mediumbedded (0.3-0.6 m or 1-2ft) forms cliff............. 13.7 45 

26. Sandstone, gray, quartzose, calcareous, very fine 

grained, well sorted, black and brown chert associated with incipient chert nodules, small brachiopods 

in some units, weathers light gray, thin-bedded, 

forms slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 30 

25. Limestone, gray, medium-grained, black chert and 

incipient chert nodules, stylolites, weathers gray, 

medium bedded (0.3-0.6 m or 1-2ft), forms cliff.... 13.7 45 

24. Sandstone, gray, quartzose, calcareous, mediumgrained, subrounded, moderate sorting, weathers 

gray and light brown, crossbeds with 7.6-cm (3 in.) 

sets, thin to medium-bedded (0.05-0.3 m or 2 in.-1 

ft), forms slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.0 10 

23. Limestone, medium-gray to medium-dark gray, 

medium-grained, silty and sandy (very fine grained 

to fine-grained quartz), pelmatazoan, bryozoan, 

ostracod debris, some foraminifers, chert in beds and 

nodules, incipient chert nodules, weathers medium 

light gray, medium bedded (0.3-0.6 m or 1-2 ft), 

forms cliff. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.9 75 

22. Sandstone, gray, quartzose, calcareous, very fine 

grained, well-sorted, 30-percent light-gray chert 

nodules, weathers pale yellowish brown, thinbedded, forms ledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 5 

21. Limestone, medium-gray to medium-light gray, 

medium-grained, chert and incipient chert nodules, 

some 3-cm-(about l in.) thick sandstone stringers, 

stromatolites, and large (1.3 em or 0.5 in. in 

diameter) crinoid columnals, weathers medium light 

gray and pale yellowish brown, medium-bedded 

(0.3-0.6 m or 1-2ft), forms cliff................... 16.8 55 
MEASURED SECTIONS 47 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

20. Sandstone, gray, quartzose, calcareous, very fine 

grained, subrounded, well sorted, weathers pale 

yellowish brown, medium-bedded (0.3-0.6 m or 1-2 

Thickness 

in meters feet 

ft) forms ledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 5 

19. Limestone, gray, medium-grained, chert and incipient 

chert nodules, weathers gray, medium-bedded 

(0.3-0.6 m or 1-2ft), forms cliff................... 7.6 25 

18. Siltstone, gray, weathers light brown, thin- to mediumbedded (0.05-0.3 m or 2 in.-1 ft) forms ledge........ 1.5 5 

17. Limestone, gray, medium grained, chert and incipient 

chert nodules, syringoporid corals in middle of unit, 

weathers gray, medium-bedded (0.3-0.6 m or 1-2ft), 

forms cliff. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.9 75 

16. Sandstone, gray, quartzose, calcareous, very fine 

grained, subrounded, well sorted, weathers pale 

yellowish brown, laminated, medium-bedded (0.3-0.6 

m or 1-2ft), forms ledge.......................... 3.0 10 

15. Limestone, light-gray, fine-grained, incipient chert 

nodules, scattered fossil fragments, weathers light 

gray and medium light gray, medium bedded 

(0.3-0.6 m or 1-2ft), forms cliff................... 15.2 50 

14. Sandstone, gray, quartzose, calcareous, very fine 

grained, subrounded, well sorted, weathers light 

yellowish brown, laminated, medium-bedded (0.3-0.6 

m or 1-2ft), forms ledge.......................... 1.5 5 

13. Limestone, medium-dark-gray, medium grained, sandy 

(very fine grained quartz), chert and incipient chert 

nodules, large brachiopods, pelmatazoan and bryozoan debris, stromatolites, Foraminifera (Earlandia 

sp., Endothyra sp., and Climmacammina cf. C. 

moelleri), F. Z. 21 ?, weathers medium gray, mediumbedded (0.3-0.6 m or 1-2ft), forms cliff............ 16.8 55 

12. Covered interval................................... 6.1 20 

11. Limestone, gray, very fine grained, incipient chert nodules and 5-cm (2 in.) thick chert beds, fossiliferous 

(crinoid columnals, bryozoan debris, stromatolites), 

weathers gray, thin- to medium-bedded (0.05-0.3 m 

or 2 in.-1 ft), forms cliff.......................... 35.0 115 

10. Limestone, medium-light-gray, fine grained, sandy (as 

much as 30 percent very fine grained quartz), 

pelmatazoan and bryozoan debris, stromatolites, 

?Eostaffella sp., weathers medium light gray, 

medium-bedded (0.3-0.6 m or 1-2ft) forms cliff..... 4.6 15 

9. Limestone, gray, medium-grained, incipient chert nodules, weathers gray, medium bedded (0.3-0.6 m or 

1-2ft), forms cliff................................ 18.3 60 
48 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

8. Sandstone, gray, quartzose, calcareous, very fine 

grained, well-sorted, incipient chert nodules and 

beds, stromatolites, weathers pale yellowish brown, 

thin-bedded, forms ledge ........................ . 

7. Limestone, medium-dark-gray, medium grained, sandy 

(as much as 15 percent medium-grained quartz with 

overgrowths), stromatolites, algae (Osagia sp., 

stacheiids), foraminifers (Biseriella sp., Endothyra 

sp., Zellerina sp.), weathers medium gray, mediumbedded (0.3-0.6 m or 1-2ft), forms ledge .. , ....... . 

6. Sandstone, gray, quartzose, calcareous, very fine 

grained, subrounded, well sorted, weathers pale 

yellowish brown, thin- to medium-bedded (0.05-0.3 

m or 2 in.-1 ft) forms ledge ...................... . 

5. Limestone, medium gray, fine grained, sandstone 

stringers (silt to medium grained quartz), incipient 

chert nodules, pelmatazoan, brachiopod, gastropod, 

ostracod, and bryozoan debris, foraminifers from 

base of unit (Asteroarchaediscus sp., encrusting 

tubular foraminifers, Neoarchaediscus sp., ?Planospirodiscus sp.), F. Z. 20?, algae (Asphaltina sp., 

Osagia sp.) from the lower 6.1 m (20 ft), weathers 

medium gray, medium-bedded (0.3-0.6 m or 1-2ft), 

forms cliff ...................................... . 

4. Sandstone, gray, quartzose, calcareous, very fine 

grained, well-sorted, weathers pale yellowish brown, 

thin bedded, forms ledge ........................ . 

3. Limestone, medium-gray, fine-grained, sandy, some in 

stringers (very fine grained quartz, well sorted), 

pelmatazoan and brachiopod debris, algae (Osagia 

sp.), foraminifers representative of F. Z. 19 

("Eosigmoilina" rugosa Ganelina, Zellerina sp.), 

weathers medium gray, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft) forms cliff ............... . 

2. Sandstone, gray, quartzose, calcareous, very fine 

grained, well-sorted, weathers pale yellowish brown, 

medium-bedded (0.3-0.6 m or 1-2ft) forms ledge ... 

1. Limestone, gray, microcrystalline, chert nodules and 

thin (5 em or 2 in.) beds, bryozoans and shell debris, 

weathers gray, thin- to medium-bedded (0.05-0.3 m 

or 2 in.-1 ft), forms cliff ......................... . 

Total thickness Bloom Member ..................... . 

Total composite thickness of three members Snaky CanThickness 

in meters feet 

3.0 10 

1.5 5 

1.5 5 

24.4 80 

1.5 5 

12.2 40 

1.5 5 

7.6 25 

646.6 2,125 

yon Formation .................................... 1,303.0 4,280 

Gradational contact. 

Bluebird Mountain Formation (uppermost Mississippian): 

16. Sandstone, medium-light-gray, quartzose, calcareous, 

very fine grained, well-sorted, weathers grayish 

orange, thin-bedded (0.05-0.3 m or 2 in.-1 ft), forms 
MEASURED SECTIONS 49 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

slope. Thin gray limestone interbed .............. . 

15. Limestone, medium-dark-gray, fine to coarse-grained, 

dark-gray chert nodules 10 em (4 in.) in diameter, 

slightly sandy (very fine grained quartz), bryozoan, 

brachiopod, and crinoid debris, Foraminifera (Biseriella sp., Endothyra sp., ?Hemiarchaediscus sp., Zellerina of the group Z. discoidea (Girty, 1915), 

weathers medium dark gray, medium-bedded 

(0.3-0.6 m or 1-2ft), forms ledge ................. . 

14. Covered interval .................................. . 

13. Sandstone, light-gray, quartzose, quartzitic, very fine 

grained, well sorted, upper 3 m (1 ft) contains siliceous nodules 10-13 em (4-5 in.) in diameter, 

weathers light gray, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft), forms ledge ............. . 

12. Covered interval .................................. . 

11. Sandstone, light-gray, quartzose, quartzitic, very fine 

grained, well-sorted, weathers light brown, thin to 

medium bedded (0.05-0.3 m or 2 in.-1 ft), forms 

ledge .......................................... . 

10. Covered interval .................................. . 

.9. Limestone, medium-gray, fine-grained, medium-darkgray chert nodules 2-3 em (1 in.) in diameter, 

weathers medium gray, thin- to medium-bedded 

(0.05-0.3 m or 2 in.-1 ft), forms ledge ............. . 

8. Sandstone, medium-gray, quartzose, quartzitic, some 

calcareous, fine grained, subrounded, very well 

sorted, siliceous nodules, weathers light olive gray, 

crossbedded with 15-cm (6-in.) sets, medium-bedded 

(0.3-0.6 m or 1-2ft) forms cliff ................... . 

7. Sandstone, light-gray, quartzose, quartzitic, very fine 

grained, well sorted, weathers light brown, thin- to 

medium-bedded (0.05-0.3 m or 2 in.-1 ft), forms cliff 

6. Siltstone, gray, quartzose, quartzitic, thin chert beds, 

weathers brown, medium bedded, forms cliff ...... . 

5. Sandstone, light-gray, quartzose, quartzitic, very fine 

grained, well sorted, weathers light brown, thin- to 

medium-bedded (0.5-0.3 m or 2 in.-1 ft) forms cliff .. 

4. Dolomite, gray, coarsely crystalline, weathers light 

brown, thin bedded, forms cliff ................... . 

3. Sandstone, medium-light-gray, quartzose, quartzitic, 

very fine grained, well sorted, weathers medium 

olive gray, thin- to medium-bedded (0.05-0.3 m or 2 

in.-1 ft), forms cliff._. ........................... . 

2. Covered interval .................................. . 

1. Sandstone, medium-dark-gray, quartzose, calcareous, 

very fine grained, angular to subangular, wellsorted, as much ~s 30-percent bitumin, weathers pale 

yellowish brown, thin-bedded, forms cliff ......... . 

Total Bluebird Mountain Formation ................. . 

Thickness 

in meters feet 

4.6 

1.5 

16.8 

7.6 

9.1 

1.5 

9.1 

1.5 

3.0 

9.1 

1.5 

15.2 

1.5 

4.6 

7.6 

10.7 

104.9 

15 

5 

55 

25 

30 

5 

30 

5 

10 

30 

5 

50 

5 

15 

25 

35 

345 
50 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

Gradational contact. 

Big Snowy Formation, upper part (Upper Mississippian): 

14. Siltstone, dark-gray, quartzose, calcareous, 

bituminous, weathers pale yellowish brown, thickbedded, forms cliff .. ." ........................... . 

13. Shale, medium-dark-gray to dark-gray, weathers 

medium dark gray, laminated, forms slope ........ . 

12. Limestone, dark-olive-gray, very fine to coarsegrained, sandy (very fine grained quartz), abundant 

limonite, pelmatazoan, brachiopod, and bryozoan 

debris, foraminifers of F. Z. 19 (Eosigmoilina explicata Ganelina, "Eosigmoilina" rugosa 

Brazhnikova), weathers light olive gray, thick bedded, forms ledge ................................ . 

11. Shale, medium-gray, noncalcareous, weathers medium 

gray and light olive gray, laminated, forms slope ... 

10. Limestone, mediu'm-dark-gray, very fine grained, 

slightly silty, limonitic, minor pelmatazoan and 

brachiopod debris, the brachiopod Inflatia sp. was 

identified by J. T. Dutro, Jr., (written commun., 

1976), Weathers medium gray and light olive gray, 

thin bedded, forms ledge ........................ . 

9. Shale, dark-gray to grayish-black, calcareous, 

weathers medium gray, laminated, forms slope ..... 

8. Limestone, medium-gray, coarse-grained, sandy (very 

fine grained quartz), abundant limonite, pelmatazoan, brachiopod, and bryozoan debris, the 

brachiopod Anthracospirifer shawi shawi Gordon 

was identified by J. T. Dutro, Jr., (written commun., 

1976), foraminifers of F. Z. 18 or younger (Hemiarchaediscus sp.), weathers medium light gray and 

light olive gray, very thin bedded, forms ledge ..... . 

7. Shale, argillaceous limestone, and siltstone, mediumgray to dark gray, calcareous, fine-grained, limonitic, 

contains brachiopods in thin (3-5 em or 1-2 in.) 

limestone units, Orbiculoidea cf. 0. wyomingensis 

Branson and Greger identified by J. T. Dutro, Jr., 

(written commun., 1976) from lower part, weathers 

dark gray to pale yellowish brown, laminated, forms 

slope .......................................... . 

6. Siltstone, medium-gray, calcareous, limonitic, contains 

brachiopod and other shell debris, weathers grayish 

orange to pale yellowish brown, very thin bedded, 

forms slope ... , ................................ . 

5. Limestone, medium-dark-gray, very fine grained, silty, 

fossiliferous (brachiopods, corals, and bryozoans), 

Inflatia? sp. identified by J. T. Dutro, Jr., (written 

commun., 1976), weathers very pale orange and 

medium dark gray, thin-bedded, forms ledge ...... . 

4. Covered interval .................................. . 

Thickness 

in meters 

4.5 

6.1 

1.5 

3.0 

3.0 

3.0 

1.5 

13.7 

1.5 

3.0 

4.5 

feet 

15 

20 

5 

10 

10 

10 

5 

45 

5 

10 

15 
MEASURED SECTIONS 51 

SECTION 2.-Type sections of the Bloom Member of the Snaky Canyon Formation and 

of the Bluebird Mountain Formation-Continued 

3. Limestone, medium-dark-gray, fine- to coarse-grained, 

silty, limonitic, scattered pelmatazoan, brachiopod, 

and bryozoan debris, crinoid columnals, brachiopods, weathers light olive gray, thin-bedded, forms 

ledge .......................................... . 

2. Covered interval .................................. . 

1. Sandstone, medium-light-gray, light olive-gray, quartzose, calcareous, very fine grained, well-sorted, 

brachiopods, weathers very pale orange, pale yellowish brown, very thin bedded, forms slope .......... . 

Total thickness incomplete Big Snowy Formation ..... . 

Thickness 

in meters feet 

1.5 

13.7 

1.5 

62.0 

5 

45 

5 

205 

SECTION 3.-Type section of the Arco Hills Formation and reference section of the 

Bluebird Mountain Formation 

[Measured in the Arco Hills (figs. 1 and 5, sec. 3), about 1 km (0.6 mi) east of the town of Arco in the N!!.NEV.. sec. 

31, T. 4 N., R. 27 E., Butte County, Idaho, by Betty Skipp and Penny Patterson, July 1976] 

Snaky Canyon Formation: 

Bloom Member lower part incomplete 

(Pennsylvanian and uppermost Mississippian?): 

40. Limestone, medium-dark-gray, fine- to coarse-grained, 

sandy (medium grained quartz), fossiliferous (bryozoans, brachiopods, algae, and foraminifers), 

weathers medium dark gray, thick-bedded (1-2m or 

3-6 ft), forms low ledge. The productoid brachiopod 

Ovatia sp. was identified by J. T. Dutro, Jr., (written 

commun., 1978). Algae include: Calcisphaera sp., Girvanella sp., and stacheiid forms. Foraminifera include Asteroarchaediscus sp., Biseriella sp., Endothyra bowman~ Eostaffella spp., Eotuberitina sp., 

?Millerella sp., Pseudoglomospira sp., and Zellerina 

sp., representative offoraminiferal F. Z. 20? ....... . 

39. Sandstone, quartzose, calcareous, and sandy 

limestone, medium-light-gray, very fine grained, 

fossiliferous (brachiopods and bryozoans), weathers 

pale brown and moderate yellowish brown, mediumbedded (0.3-0.5 m or 1-1.5 ft), forms low ledge ..... . 

38. Sandstone, quartzose, calcareous, as in unit 36 ...... . 

37. Limestone, medium-dark-gray to medium-gray, fine- to 

coarse-grained, sandy (fine grained quartz, some 

with overgrowths), fossiliferous (abundant pelmatazoan debris, brachiopods, algae, and foraminifers), 

weathers medium gray and light olive gray, scattered lenses of incipient chert, thick-bedded (0.3-1.4 

m or 1-4.5 ft), forms cliff. Productoid brachiopod 

fragment identifed by J. T. Dutro, Jr., (written commun., 1978). The algae Aoujgalia sp. and Asphaltina 

cordilleriensis are present. Foraminifers represen3.0 

2.1 

3.0 

10 

7 

10 
52 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 3.-Type section of the Arco Hills Formation and reference section of the 

Bluebird Mountain Formation-Continued · 

tative of F. Z. 19? include: Asteroarchaediscus sp., 

Biseriella moderata, Endothyra bowmani, 

Eosigmoilina explicata?, Hemiarchaediscus sp., 

Neoarchaediscus sp., Zellerina of the group Z. 

discoidea, and Zellerina sp ....................... . 

Total thickness incomplete Snaky Canyon Formation .. 

Gradational contact 

Bluebird Mountain Formation (uppermost Mississippian): 

36. Sandstone, calcareous, and sandy limestone. Sandstone, medium light-gray, quartzose, very fine grained, weathers moderate brown to pale brown, blocky, 

thick-bedded (0.6-2.0 m or 2-6.6 ft), forms ledge. 

Limestone, medium light-gray, very fine grained, 

sandy (very fine grained quartz), weathers grayish 

orange, forms pods near top of unit ............... . 

35. Covered interval. Float is calcareous very fine grained 

sandstone ...................................... . 

34. Limestone, medium-dark-gray, fine- to coarse-grained, 

sandy (very fine grained quartz), some sandstone 

stringers, fossiliferous (pelmatazoan, bryozoan, and 

brachiopod debris, abundant algae, and a few foraminifers), weathers medium light gray, sand stringers weather moderate brown, thick-bedded, forms 

cliff with unit 33. Algae from lower part include 

abundant Stacheoides spp. Foraminifera representative of F. Z. 19? include apterrinellids, Asteroarchaediscus of the group A. rugosus, Biseriella 

moderata, calcivertellids, Endothyra sp., Neoarchaediscus sp., Zellerina of the group Z. discoidea, 

and Zellerina sp ................................. . 

33. Limestone, medium-dark-gray, some brownish-gray, 

coarse-grained, slightly silty, some stringers of 

moderate brown incipient chert, fossiliferous (abundant crinoidal, bryozoan, trilobite, and mollusk 

debris, brachiopods, algae, and foraminifers), 

weathers medium gray, thick-bedded (0.4-1.3 m or 

1.5-4 ft), forms lower step part of cliff with unit 34). 

Large fragment of indeterminate productoid brachiopod identified by J. T. Dutro, Jr., (written commun., 1978). Foraminifera and algae of F. Z. 19? include Asphaltina cordilleriensis, Asteroarchaediscus 

sp., Biseriella parva, Earlandia sp., Endothyra sp. 

Globoendothyra sp., Hemiarchaediscus sp., Neoarchaediscus sp., Pseudoglomospira sp., Zellerina of 

the group Z. discoidea, and Zellerina sp ........... . 

Thickness 

in meters feet 

4.6 15 

12.7 42 

4.6 15.0 

1.8 6.0 

4.4 14.5 

3.0 10.0 
MEASURED SECTIONS 53 

SECTION 3.-Type section of the Arco Hills Formation and reference section of the 

Bluebird Mountain Formation-Continued 

32. Covered interval. Float and one ledge in middle of unit 

are calcareous quartzose sandstone, medium-lightgray, very fine grained, weathers moderate brown, 

forms steep slope ............................... . 

31. Sandstone, light-olive-gray, medium-light gray, 

grayish-orange, pale-yellowish brown, quartzose, 

calcareous, very fine grained, weathers moderate 

brown and pale yellowish brown, weathered rinds as 

much as 2 em (0.75 in.) thick, liesegang banding, 

more than 50 percent exposed, thin- to mediumbedded (0.15-0.6 m or 0.5-2 ft), forms steep slope ... 

30. Sandstone, quartzose, slightly calcareous, very fine 

grained. Upper part light olive gray and grayish 

orange, weathers pale yellowish brown and light 

olive gray. Lower beds light olive gray and pale 

yellowish brown, weather moderate brown. Some 

limonite staining.· More than 50 percent exposed. 

Thin to medium bedded (0.15-0.6 m or 0.5-2 ft), 

forms steep slope ............................... . 

29. Limestone, very sandy, and calcareous sandstone, 

medium-gray, quartzose, very fine to mediumgrained, scattered phosporite nodules, bryozoan and 

crinoidal debris, crossbedded, weathers medium 

gray and moderate brown, forms ledge. Foraminifers 

of F. Z. 19 include Asteroarchaediscus rugosus, 

Eosigmoilina explicata, "Eosigmoilina" rugosa, and 

Planospirodiscus sp ............................. . 

Total thickness Bluebird Mountain Formation ........ . 

Gradational contact. 

Arco Hills Formation (uppermost Mississippian): 

28. Sandstone and limestone. Sandstone, olive gray, 

quartzose, calcareous, very fine grained, weathers 

moderate brown, yellowish gray, and medium light 

gray. Limestone, medium-gray, medium-grained, 

crinoidal, weathers medium gray, forms lens near 

top which grades into sandstone above and below. 

Unit thin to medium bedded, forms slope ......... . 

27. Limestone and chert. Limestone, dark gray, mediumdark-gray, olive-gray, fine- to medium-grained, siliceous, sandy (very fine grained quartz) with sand 

stringers in upper part, fossiliferous (crinoid columnals, brachiopods, bryozoans), crossbedded, thinbedded (0.08-0.3 m or 0.25 in.-1 ft). Chert, grayishblack, olive-gray, and medium light-gray, some laminated, forms blebs 2.5 to 10 em (1-4 in.) across and 

irregular layers. Unit forms cliff. The brachiopod Orbiculoidea sp. (large) occurs near top .............. . 

Thickness 

in meters feet 

7.0 23.0 

14.0 45.8 

13.1 43.0 

.7 2.4 

48.6 159.7 

1.6 5.1 

4.3 14.0 
54 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 3.-Type section of the Arco Hills Formation and reference section of the 

Bluebird Mountain-Continued 

26. Limestone, medium-dark-gray, dark-gray, brownishgray, fine-grained, silty, sandy, and cherty, sand 

laminations common in upper part, grayish-black 

and olive-gray chert in elongate or round nodules, 

weathers medium light gray, grayish orange, and 

grayish red, thin-bedded to platy (0.8-15 em or 

Thickness 

in meters 

0.25-6 in.), forms slope with small ledges........ 7.9 

25. Mudstone, pale-yellow-brown to grayish orange, 

calcareous, not bedded, forms pale-yellowish-brown 

slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 

24. Limestone, medium-brownish-gray, coarse grained, 

slightly sandy (medium grained quartz), porous, 

partly silicified, fossiliferous (crinoidal and bryozoan 

debris, brachiopods, foraminifers), thin-bedded (7 .5 

em or 3 in.), forms ledge between mudstone units. 

Foraminifera of F. Z. 19 include Asteroarchaediscus 

sp., Eosigmoilina explicata, "Eosigmoilina" rugosa, 

Globoendothyra sp., and Hemiarchaediscus sp. . . . . . .3 

23. Covered interval. Float is mudstone like unit 25 . . . . . . 2.0 

22. Limestone and calcareous sandstone. Limestone, 

medium-dark-gray with flecks of dark-yellow 

limonite, coarse-grained, porous, sand-streaked 

(medium-grained quartz) in places, fossiliferous 

(crinoidal, bryozoan, and trilobite debris, silicified 

brachiopods, and Foraminifera), weathers medium 

gray with sandy zones moderate brown. Sandstone, 

dusky-yellow or moderate yellowish-brown, quartzose, calcareous, very fine grained, fossiliferous 

(brachiopods), weathers moderate yellowish brown. 

Unit is medium bedded and forms cliff. Foraminifera 

of F. Z. 19 include Eosigmoilina explicata and 

"Eosigmoilina"rugosa .......................... . 

21. Mudstone, dark-yellowish-brown, pale yellowishbrown, silty, calcareous, carbonaceous, forms slope 

(trenched) ...................................... . 

20. Mudstone and limestone. Limestone, medium-darkgray to dark-gray, coarse-grained, sandy (very fine 

grained quartz), fossiliferous (bryozoans, 

echinoderm debris, brachiopods, foraminifers), 

weathers medium gray and moderate brown, forms 

ledge (0.3 m or 1 ft) at top of unit. Mudstone, lightolive-gray, calcareous, forms slope. Brachiopods 

identified by J. T. Dutro, Jr., (written commun., 

1978) are Carlinia? sp. and Composita? sp. 

Foraminifera of F. Z. 19 include Archaediscus sp., 

?Asteroarchaediscus sp., Eosigmoilina explicata, 

"Eosigmoilina" rugosa . ......................... . 

19. Limestone, medium-gray, fine- to coarse grained, 

sandy (very fine to fine grained quartz), nodules of 

black chert at base, fossiliferous (brachiopods, 

5.3 

4.6 

1.6 

feet 

26.0 

7.0 

1.0 

6.5 

17.4 

15.2 

5.1 
MEASURED SECTIONS 55 

SECTION 3.-Type section of the Arco Hills Formation and reference section of the 

Bluebird Mountain-Continued 

pelmatazoan debris), weathers medium light gray 

and light olive gray, medium-bedded (0.15-0.45 m or 

0.5-1.5 ft), forms ledge with unit 18. Brachiopods 

identified by J. T. Dutro, Jr., (written commun., 

1978), include Anthracospirifer sp., Diaphragmus 

sp., and Inflatia? sp ............................. . 

18. Sandstone, pale-yellowish-brown, light-olive gray, 

quartzose, calcareous, very fine grained, liesegang 

banding, weathers moderate brown, forms ledge 

with unit 19 .................................... . 

17. Mudstone, light-olive-gray, calcareous, with abundant 

limestone nodules, forms slope ................... . 

16. Limestone, medium-gray to medium-dark-gray, fine- to 

coarse-grained, slightly sandy (fine-grained quartz), 

fossiliferous (brachiopods, echinoderm, bryozoan 

and ostracod debis, foraminifers), weathers medium 

gray and greenish gray, thin- to medium-bedded, 

thinner bedded and finer grained at top, forms ledge. 

Foraminifers of F. Z. 19 include Eosigmoilina explicata and "E. " rugosa ......................... . 

15. Mudstone, very pale orange, calcareous, forms slope .. 

14. Limestone, medium-dark-gray, coarse grained, slightly 

sandy (very fine grained quartz), fossiliferous 

(echinoderm and bryozoan debris, brachiopods, some 

phosphatic, algae, and foraminifers), limonitic, 

weathers medium gray, forms slope. The brachiopod 

lnflatia? sp. (corroded specimen) identified by J. T. 

Dutro, Jr., (written comun., 1978). Foraminifera and 

algae of F. Z. 19 include Asteroarchaediscus sp., 

Asphaltina sp., and "Eosigmoilina"rugosa ....... . 

13. Sandstone, light-olive-gray and brownish gray, quartzose, calcareous, very fine grained, some friable, 

weathers medium light gray and moderate brown, 

mostly thick-bedded, forms cliff .................. . 

12. Mudstone, yellowish-gray, calcareous at base, grading 

upward into grayish-orange and pale-olive silty 

calcareous mudstone or argillaceous siltstone, forms 

slope .......................................... . 

11. Limestone, medium-dark-gray, coarse-grained, silty, 

fossiliferous (silicified corals and brachiopods, 

echinoderm and bryozoan debris, foraminifers), 

weathers medium gray with dark yellowish-brown 

staining on surface, medium-bedded (0.3 m or 1 ft), 

forms ledge. Brachiopods Anthracospirifer sp. and 

Inflatia sp., identified by J. T. Dutro, Jr., (written 

commun., 1978). Foraminifera of F. Z. 19 include Archaediscus sp., Endothyra sp., "Eosigmoilina" 

rugosa, and Eotuberitina sp ...................... . 

Thickness 

in meters feet 

.6 

.6 

1.2 

2.7 

1.7 

.1 

1.8 

2.1 

1.2 

2.0 

2.1 

4.0 

9 

5.5 

0.3 

6.0 

7 

4 
56 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 3.-Type section of the Arco Hills Formation and reference section of the 

Bluebird Mountain-Continued 

10. Mudstone, light-olive-gray and yellowish gray, 

calcareous, carbonaceous, limestone nodules, 

fossiliferous (corals and brachiopods), weathers light 

brownish gray and yellowish gray, forms slope. The 

coral Amplexizaphrentis sp. (USGS colin. 26880-PC) 

identified by W. J. Sando (written commun., 1977). 

Brachiopods, lnflatia sp. (small), and Anthracospirifer aff. A. welleri (Branson and Greger) 

identified by J. T. Dutro, Jr., (written commun., 

1978) .......................................... . 

9. Limestone, dark-gray to medium-dark-gray, coarsegrained, slightly silty, fossiliferous (corals, 

brachiopods, echinoderm and bryozoan debris), partly silicified, weathers medium gray and pale 

yellowish brown, medium bedded (15-35 em or 6-14 

in.), forms ledge. The coral, Amplexizaphrentis? sp. 

and productoid brachiopod fragments identified by 

J. T. Dutro, Jr., (written commun., 1978) .......... . 

8. Mudstone, yellowish-gray, calcareous, weathers 

yellowish gray, forms slope ...................... . 

7. Limestone, medium-gray, coarse-grained, silty, fossiliferous (corals, brachiopods, echinoderm and bryozoan debris, foraminifers), partly silicified, weathers 

medium olive gray, thin-to medium-bedded (5 cm-1 

m or 2 in.-3 ft), thin beds at top, forms ledge. 

Foraminifera of F. Z. 19 include Archaediscus sp., 

Endothyra bowmani, ?Eosigmoilina sp ........... . 

6. Mudstone, light-greenish-gray and yellowish gray, 

calcareous, weathers to yellowish gray slope ...... . 

5. Limestone, medium-gray, coarse-grained, silty and 

sandy (very fine grained quartz), some limonitic 

spots, breccia at top, silicified in places, fossiliferous 

(echinoderm, brachiopod, and bryozoan debris, foraminifers), weathers medium gray, some very pale 

orange, one thick bed, forms ledge. Foraminifera of 

F. Z. 19 include Asteroarchaediscus sp., 

"Eosigmoilina" rugosa, Hemiarchaediscus sp., 

Neoarchaediscus sp., Planospirodiscus sp., Priscella 

sp ............................................. . 

4. Limestone, silicified, and chert, interbedded, paleyellowish-brown, medium-gray, olive gray, weathers 

dark yellowish orange, grayish orange, and olive 

gray, some medium-gray bioclastic limestone 

nodules in upper part, thin-bedded, forms low ledge. 

3. Limestone, medium-gray, aphanitic, argillaceous, 

small n;wdium-gray chert nodules and lenses, large 

black ofbiculoid brachiopods in float, weathers pale 

yellowish orange and medium gray, thin-bedded 

(0.05-0.3 m or 2 in.-1 ft) forms slope or series of low 

steps .......................................... . 

Thickness 

in meters 

1.2 

1.5 

.8 

1.2 

4.3 

6.1 

2.4 

4.6 

feet 

4 

5 

2.5 

3.8 

14 

20 

8 

15 
MEASURED SECTIONS 57 

SECTION 3.-Type section of the Arco Hills Formation and reference section of the 

Bluebird Mountain-Continued 

2. Limestone and mudstone. Limestone, medium lightgray to light-gray, fine grained, argillaceous, 

weathers light olive gray. Mudstone, yellowish-gray 

and light-olive-gray, calcareous, silty, scattered 

dark-gray chert nodules, weathers yellowish gray. 

Unit partly covered, forms slope ................. . 

1. Covered interval. Subcrop from upper part like unit 2 . 

Total thickness Arco Hills Formation ................ . 

Abrupt conformable contact. 

Surrett Canyon Formation, upper part (Upper Mississippian): 

Limestone, medium-dark-gray and medium gray, fine- to 

coarse-grained, slightly sandy (fine-grained quartz with 

overgrowths), grayish-black chert in nodules and thin beds, 

fossiliferous (brachiopods, corals, echinoderm, bryozoan and 

ostracode debris, foraminifers and algae), weathers medium 

gray, upper part thick-bedded, lower part thin- to mediumbedded, forms ledge. Sample from top contains the coral 

Caninia sp. and the Foraminifera and algae of F. Z. 19: 

"Eosigmoilina" rugosa, Globoendothyra sp., and 

Stacheoides sp. Sample from base contains the phosphatic 

brachiopod Orbiculoidea cf. 0. wyomingensis Branson and 

Grego~, identified by J. T. Dutro, Jr., (written commun., 

1978) and Foraminifera of F. Z. 18 or 19, ?"Eosigmoilina" 

sp., Hemiarchaediscus sp., and Planospirodiscus sp ....... . 

Total thickness incomplete Surrett Canyon Formation . 

Thickness 

in meters 

4.0 

7.9 

75.7 

12.2 

12.2 

feet 

13 

26 

248.5 

40 

40 

SECTION 4.-Reference section of the Bluebird Mountain Formation with associated 

partial sections of the overlying Snaky Canyon Formation and the underlying Surrett Canyon Formation 

[Measured in the White Knob Mountains, (figs. 1 and 5, sec. 4), just north of the junction of Wood Canyon and Antelope Creek in the NWV. sec. 29 and N'/• sec. 30, T. 5 N., R. 25 E., Custer County, Idaho, by Betty Skipp and 

Ellen Harrison in August 1970] 

Snaky Canyon Formation, lower part (Permian to uppermost 

Mississippian): 

16. Limestone and incipient chert. Limestone, dark-gray 

to medium gray, mostly fine-grained, slightly silty, 

fossiliferous (brachiopods, crinoid, ostracod, and 

bryozoan debris, and foraminifers), weathers light 

gray to light medium gray, thin bedded. Incipient 

chert, dark-gray to medium-gray, forms as much as 

30 percent of unit, weathers reddish brown, forms 

layers as much as 15 em (6 in.) thick. Unit forms cliff 

with unit 15. Foraminifera of F. Z. 20 from base of 

unit include archaediscids, Endothyra of group E. 

excellens, and Millerella sp ....................... . 

Thickness 

in meters feet 

6.1 20 
58 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 4.-Reference section of the Bluebird Mountain Formation with associated 

partial sections of the overlying Snaky Canyon Formation and the underlying Surrett Canyon Formation-Continued 

15. Limestone and chert as in unit 16, forms cliff. 

Foraminifera and algae of F. Z. 20 from top of unit 

include Asphaltina sp:, Asteroarchaediscus sp., 

beresellid algae, Biseriella sp., Calcispheara sp., 

Earlandia sp., encrusting tubular foraminifers, Endothyra excellens, Endothyra sp., Eostaffella sp., 

Eotuberitina sp., ?Globivalvulina sp., Endothyranella sp., Millerella sp., Millerella of group M 

pressa, Neoarchaediscus sp., Pseudoammodiscus 

Thickness 

in meters 

sp., and Zellerina? of group Z? cooperi . . . . . . . . . . . . . 15.2 

14. Limestone. Upper part--limestone, medium gray, fineto medium-grained, slightly sandy (fine-grained 

quartz), fossiliferous (brachiopods, bryozoans, 

ostracods, gastropods, echinoderm debris, algae, and 

Foraminifera), weathe:r:s medium gray, medium- to 

thick bedded (0.3-1.5 m or 1-5ft), forms cliff. Algae 

and Foraminifera (F. Z. 20) from top of unit include 

Asphaltina sp., Asteroarchaediscus sp., Biseriella 

sp., Eostaffella sp., ?Globivalvulina sp., Millerella 

sp., Neoarchaediscus sp., Stacheoides spp., 

Ungdarella sp., and Zellerina spp. Foraminifera and 

algae (F. Z. 20?) from middle of upper part include 

Asphaltina sp., Asteroarchaediscus sp., Biseriella 

sp., Eostaffella sp., ?Globivalvulina sp., Neoarchaediscus sp., Planospirodiscus sp., Stacheoides 

meandriformis, Ungdarella sp., Zellerina sp., and 

Zellerina of group Z. discoidea. Lower 

part-limestone, dark-gray, very fine grained, very 

sandy in lower part (very fine grained quartz), minor 

echinoderm debris, weathers dark gray, thin- to 

medium- bedded, forms steep slope................ 16.8 

13. Limestone, medium-dark-gray, fine-grained, very 

slightly sandy (very fine grained quartz), 

fossiliferous (brachiopods, bryozoan, echinoderm, 

ostracode debris, Foraminifera), weathers light gray, 

thin-bedded (0.15-0.3 m or 6 in.-1 ft), forms steep 

slope. The brachiopod Antiquatonia? sp. (USGS colln. 24918-PC) identified by J. T. Dutro, Jr., (written 

commun., 1973). Foraminifera of F. Z 20? include 

Asteroarchaediscus rugosus, Eotuberitina sp., 

Neoarchaediscus sp., Planospirodiscus sp., 

?Zellerina sp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 

12. Covered interval. Float is thin-bedded light-gray and 

medium-gray limestone and some sandstone....... 10.7 

11. Mostly covered interval. Subcrop is limestone, 

medium-gray, fine-grained, sandy or silty, minor 

chert, fossiliferous (echinoderm, bryozoan, and 

brachiopod debris), thin- to medium bedded (0.15-0.3 

m or 6 in.-1 ft), forms slope. Sample from small ledge 

feet 

50 

55 

15 

35 
MEASURED SECTIONS 59 

SECTION 4.-Reference section of the Bluebird Mountain Formation with associated 

partial sections of the overlying Snaky Canyon Formation and the underlying Surrett Canyon Formation-Continued 

Thickness 

in meters feet 

at top of unit contains Foraminifera of F. Z 20?: 

Asteroarchaediscus sp., Endothyra sp., Eostaffella 

sp., Neoarchaediscus sp., Zellerina sp. . . . . . . . . . . . . . 12.2 40 

10. Limestone and sandstone. Upper 6.1 m (20 

ft)-limestone, dark-brownish-gray to dark gray, 

medium- to coarse-grained, sandy (medium-grained 

quartz), minor grayish-black chert nodules, fossiliferous (brachiopods, bryozoan and echinoderm 

debris), weathers dark gray, thin-bedded. Sandstone, 

medium gray, very fine grained, quartzose, calcareous, forms one 1-m (3 ft)-thick bed 1 m (3 ft) from 

top. USGS colin. 24917-PC from upper beds contains fenestrate bryozoans, Linoproductus sp., and 

productoid fragments identified by J. T. Dutro, Jr., 

(written commun., 1973). Lower 9.1 m (30 ft)-limestone, medium gray, fine-grained, minor grayish 

black and olive-gray chert nodules, fossiliferous 

(bryozoans), weathers medium gray, medium-bedded 

(0.15-0.6 m or 6 in.-2 ft), forms steep slope . . . . . . . . . 15.2 50 

9. Limestone and sandstone. Upper 7.6 m (25ft) interbedded limestone and sandstone. Limestone, mediumgray to dark-gray, fine-grained, sandy (fine-grained 

to very fine grained quartz), black chert nodules 

common, fossiliferous (crinoidal and bryozoan 

debris, brachiopods, some phosphatic, ostracodes, 

foraminifers), weathers olive gray to medium gray, 

medium-bedded. Sandstone, light-yellow gray and 

orange-gray, quartzose, calcareous, fine-grained, 

weathers brownish yellow, medium-bedded (0.15-

0.45 m or 6-18 in.). Foraminifera from top of upper 

part representative of F. Z. 20?. include Archaediscus sp., Asteroarchaediscus sp., Biseriella 

moderata, Biseriella parva, Eostaffella sp., 

Eotuberitina sp., Neoarchaediscus sp., Planoendothyra sp., Zellerina of group Z. discoidea, and 

Zellerina sp. Lower 7.6 m (25 ft) limestone, mediumdark-gray, coarse-grained, crinoidal, weathers 

medium gray, medium-bedded (0.15-0.6 m or 6 in.-2 

ft), forms steplike ledges . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 50 

8. Limestone, medium-gray, fine- to medium grained, silty and sandy (very fine grained quartz) in places, 

scattered olive-gray to grayish-black chert nodules, 

fossiliferous (brachiopods, some phosphatic, echinoderm, bryozoan, and ostracode debris, algae, and 

Foraminifera), weathers light gray to medium gray, 

thin-to medium-bedded (0.15-0.3 m or 6 in.-1 ft), 

forms steep slope of steplike ledges. Interval one 

third covered, but subcrop indicates character of 

beds. Brachiopods from middle of unit (USGS colin. 
60 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 4.-Reference section of the Bluebird Mountain Formation with associated 

partial sections of the overlying Snaky Canyon Formation and the underlying Surrett Canyon Formation-Continued 

24916-PC) include Wellerella sp. and Composita sp., 

identified by J. T. Dutro, Jr., (written commun., 

1973). Foraminifera and algae (F. Z. 20?) from top of 

unit include Archaediscus sp., Archaediscus pauxilles, Asphaltina cordilleriensis, Asteroarchaediscus 

sp., Biseriella moderata, Biseriella sp., Calcisphaera 

sp., Hemiarchaediscus sp., Planospirodiscus sp., and 

Zellerina spp. Foraminifera and algae (F. Z. 20?) from 

middle of unit include archaediscids, Asteroarchaediscus spp., Biseriella sp., Earlandia spp., encrusting tubular foraminifers, Girvanella sp., 

?Hemiarchaediscus sp., Monotaxinoides sp., Neoarchaediscus sp., Stacheoides sp., and Zellerina sp .. 

7. pmestone, medium-gray, medium- to coarse grained, 

sandy (fine-grained quartz), scattered nodules of 

black chert, fossiliferous (brachiopods, crinoidal, 

echinoderm, and bryozoan debris, algae and Foraminifera), weathers medium gray, medium-bedded 

(0.3 m, 1 ft) forms steplike ledges. Linoproductid and 

productoid brachiopod fragments (USGS colln. 

24915-PC) identified by J. T. Dutro, Jr., (written 

commun., 1973). Foraminifera and algae of F. Z. 20? 

at top of unit are Biseriella spp., encrusting tubular 

foraminifers, ?Globivalvulina sp., and Stacheoides 

sp. Foraminifera of F. Z. 19 in middle of unit include 

"Eosigmoilina" rugosa and Biseriella sp .......... . 

Total incomplete Snaky Canyon Formation .......... . 

Gradational contact. 

Bluebird Mountain Formation (uppermost Mississippian): 

6. Siltstone, sandstone, and limestone. Siltstone and 

sandstone, gray, very fine grained, calcareous, 

weathers yellowish brown, thin-bedded. Limestone, 

olive-gray and medium gray, fine- to mediumgrained, silty in places, fossiliferous (bryozoan and 

echinoderm debris) in part, weathers medium gray, 

thin-bedded, forms low ledge .................... . 

5. Siltstone and sandstone, calcareous, and silty 

limestone, light-olive-gray, yellowish-brown, lightbrownish gray, quartzose, very fine grained, 

weathers yellowish brown and reddish brown, platy 

to thin-bedded (0.5-10 em or 0.12-4 in.), forms ledge 

Total thickness Bluebird Mountain Formation ........ . 

Thickness 

in meters feet 

15.2 50 

6.7 22 

117.9 387 

3.0 10 

4.6 15 

7.6 25 
MEASURED SECTIONS 61 

SECTION 4.-Reference section of the Bluebird Mountain Formation with associated 

partial sections of the overlying Snaky Canyon Formation and the underlying Surrett Canyon Formation-Continued 

Gradational contact. 

Surrett Canyon Formation, part: 

Arco Hills equivalent (Upper Mississippian): 

4. Limestone, crinoidal, and silty and sandy limestone, 

interbedded. Crinoidallimestone, medium-light-gray 

and light-olive-gray, medium- to coarse grained, fossiliferous (corals, brachiopods, some phosphatic, 

pelecypods, algae, foraminifers), weathers medium 

light gray and olive gray, thick-bedded (0.6-1 m or 

2-3 ft), forms ledges. Silty and sandy limestone, 

dark-gray and medium-olive-gray, very fine grained, 

fossiliferous (scattered brachiopods, bryozoans, and 

corals), weathers with tints of yellow, orange, and 

red, thin-bedded (0.5-5.0 em or 0.25-2 in.), forms 

slopes. Foraminifera from top (F. Z. 19) include 

Eosigmoilina explicata and Neoarchaediscus sp.; 

USGS coHn. 24914-PC from the upper limestone 

ledge includes: a horn coral, Anthracospirifer aff. A. 

occiduus (Sadlick), lnflatia sp., and a pelecypod, indet., identified by J. T. Dutro, Jr. (written commun., 

1973). Foraminifera and algae (F. Z. 19) from the 

basal limestone ledge include Asteroarchaediscus 

sp., encrusting tubular foraminifers, "Eosigmoilina" 

Thickness 

in meters feet 

rugosa, Eotuberitina sp., and Stacheoides sp....... 22.9 75 

3. Limestone, dark-gray and medium-olive-gray, finegrained, silty and sandy (very fine grained quartz), 

abundant chert and incipient chert nodules, fossiliferous (brachiopods, some phosphatic, bryozoans, 

echinoderm and crinoid debris, foraminifers), 

weathers light gray with tints of yellow orange and 

red, thin-bedded to platy (0.5 em or 0.25 in.), forms 

partly covered slope. Foraminifera (F. Z. 19) from top 

include "Eosigmoilina" rugosa, Eostaffellina sp., 

Neoarchaediscus sp., and Zellerina sp. Foraminifera 

of F. Z. 19 from middle of unit include ?Asteroarchaediscus sp., "Eosigmoilina" rugosa, and Neoarchaediscus sp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7 35 

Total thickness Arco Hills equivalent (upper part of Surrett Canyon Formation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.6 110 

Small fold 

2. Limestone and interbedded chert. Limestone, mediumgray and medium-dark-gray, very fine grained, 
62 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 4.-Reference section of the Bluebird Mountain Formation with associated 

partial sections of the overlying Snaky Canyon Formation and the underlying Surrett Canyon Formation-Continued 

slightly silty in places, a few brachiopods present, 

weathers light gray and light bluish gray, thinbedded (5-15 em or 2-6 in.), forms ledge. Chert, gray, 

lavender, olive-gray, grayish-black, weathers yellow 

orange or shades of gray, thin bedded (5-15 em or 

2-6 in.), forms 25 percent of interval, forms ledge. 

USGS colin. 24913-PC from float in lower half of 

unit contains brachiopods; Composita sp., Leiorhynchoidea? sp., and Ovatia sp., identified by J. T. 

Dutro, Jr., (written commun., 1973) .............. . 

1. Limestone, medium-gray, coarse-grained, fossiliferous 

(bryozoan, brachiopod, echinoderm, crinoid, and 

ostracod debris, foraminifers), weathers medium 

gray, thick-bedded (3 m or 1 ft), forms cliff. 

Foraminifera (F. Z. 19) from base of unit include 

Asteroarchaediscus sp., Earlandia spp., Endothyra 

sp., "Eosigmoilina" rugosa, Neoarchaediscus sp., 

Pseudoendothyra sp., and Zellerina sp ............ . 

Total thickness incomplete Surrett Canyon Formation. 

Thickness 

in meters feet 

9.1 

6.1 

48.8 

30 

20 

160 

SECTION 5.-Reference section of the Arco Hills and Bluebird Mountain Formations 

[Measured in the southern Lemhi Range (figs. 1 and 5, sec. 5), SEV. sec. 6, T. 6 N., R. 30 E., Butte County, Idaho, by 

Betty Skipp, Penny Patterson, and W. J. Sando in July 1976] 

Snaky Canyon Formation, lower part (Permian to Pennsylvanian): 

54. Limestone, medium-light-gray to medium-gray, coarsegrained, sandy (fine-grained quartz), fossiliferous 

(brachiopod, echinoderm, and bryozoan debris, foraminifers), weathers medium light gray with darkyellowish brown chert layers, thick-bedded, forms 

ledge. Foraminifera of F. Z. 19? include Biseriella sp. 

and Zellerina sp ................................ . 

53. Covered interval. Float mostly limestone as in unit 54, 

but also some yellowish-brown weathering very fine 

grained quartzose sandstone .................... . 

Total thickness incomplete Snaky Canyon Formation .. 

Gradational contact. 

Bluebird Mountain Formation (uppermost Mississippian): 

52. Sandstone, light-gray to medium-light gray, very fine 

grained, quartzose, quartzitic, weathers light brown, 

cross bedded in lower part, forms ledge ............ . 

Thickness 

in meters feet 

4.6 

10.4 

15.0 

1.7 

15 

34 

49 

5.5 
MEASURED SECTIONS 63 

SECTION 5.-Reference section of the Arco Hills and Bluebird Mountain Formations 

-Continued Thickness 

51. Limestone, medium-light-gray, some grayish orange at 

base, fine-grained, some very sandy (very fine to finegrained quartz), weathers medium gray, grayish 

orange, and olive gray, medium- to thick-bedded (1-2 

m or 3-6ft), forms ledges, more than 50 percent outcrop ........................................... . 

50. Limestone, medium-gray, fine-grained, sandy (very 

fine to fine-grained quartz), fossiliferous (ostracod 

and echinoderm debris, algae, many foraminifers), 

weathers medium gray, thick-bedded, forms ledge. 

Foraminifera and algae of F. Z. 19 include Archaediscus sp., Asteroarchaediscus sp., Biseriella sp., 

Calcisphaera sp., Earlandia sp., "Eosigmoilina" 

rugosa, Eotuberitina sp., Hemiarchaediscus sp., 

Neoarchaediscus sp., Planospirodiscus sp., 

Pseudoglomospira sp., and Zellerina sp ........... . 

49. Mostly covered interval. Float 90-percent very fine 

grained calcareous quartzose sandstone. Two meters 

of sandstone exposed at base, greenish-gray, quartzose, quartzitic, very fine grained, weathers light 

olive gray with 1-cm (0.4 in.)- thick weathering rind, 

forms slope .................................... . 

48. Sandstone, brownish-gray to medium-gray, quartzose, 

quartzitic, very fine grained, weathers brownish 

gray and pale brown, thin- to medium-bedded 

(0.15-0.3 m or 6 in.-1 ft), forms ledges ............ . 

47. Limestone lens in sandstone. Limestone, medium-gray, 

medium-grained, very sandy (fine- to mediumgrained), fossiliferous (brachiopod, some phosphatic, 

echinoderm, gastropod debris, algae, and Foraminifera), weathers medium gray, thick-bedded. Foraminifera and algae of F. Z. 19 include Asteroarchaediscus sp., Biseriella sp., "Eosigmoilina" rugosa, 

Eostaffella sp., Neoarchaediscus sp., Planospirodiscus sp., Stacheoides sp., and Zellerina sp ....... . 

46. Sandstone, pale-yellowish-brown, dark yellowishbrown, quartzose, calcareous, very fine grained, 

weathers pale yellowish brown and brownish gray, 

thin- to medium-bedded (0.1-0.45 m or 0.3-1.5 ft) 

forms ledges ................................... . 

45. Sandstone, dark-yellowish-orange, quartzose, calcareous, very fine grained, friable, weathers medium 

orange, thin-bedded, forms slope ................. . 

Total thickness Bluebird Mountain Formation ........ . 

Gradational contact. 

Arco Hills Formation (Upper Mississippian): 

44. Limestone, medium-gray, light-olive-gray, aphanitic, 

silty, some siltstone wispy zones, fossiliferous 

in meters feet 

7.9 26.0 

4.6 15.0 

13.7 45.0 

6.5 21.4 

.9 3.0 

5.3 17.5 

1.2 3.9 

41.8 137.3 
64 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 5.-Reference section of the Arco Hills and Bluebird Mountain Formations 

-Continued 

(brachiopods), weathers medium gray and light 

brown, thick-bedded, forms ledge. Brachiopods from 

base include: Orbiculoidea cf. 0. wyomingensis 

Branson and Gregor, Leiorhynchoidea sp., Anthracospirifer sp., identified by J. T. Dutro, Jr., (written 

commun., 1978) ................................. . 

43. Siltstone, olive-gray and light-olive-gray, slightly 

calcareous, weathers light olive gray, shaly, forms 

slope ........................................... . 

42. Siltstone, dark-yellowish-brown, calcareous, weathers 

pale brown and light brown, thin-bedded (0.07-0.15 

m or 3-6 in.), forms slope ........................ . 

41. Mudstone and siltstone. Mudstone, light brownishgray, slightly calcareous, light-brown ironstone concretions, weathers light brownish gray, platy, forms 

slope. Siltstone at base, grayish-orange, slightly 

calcareous, friable, 0.15 m (6 in.) thick, forms slope .. 

40. Limestone, sandy, and calcareous sandstone, mediumgray and medium dark-gray, very fine grained, 

weathers pale yellowish brown and medium gray, 

single bed, forms ledge with unit 39 .............. . 

39. Limestone, medium-dark-gray, fine-grained, sandy 

(very fine grained quartz), weathers medium gray 

and moderate brown, forms ledge with unit 40 ..... . 

38. Limestone, medium-dark-gray, fine-grained, silty or 

argillaceous, weathers light olive gray, thin-bedded, 

forms swale between limestone ledges ............ . 

37. Limestone, medium-gray with grayish-orange limonite 

specks, fine-grained, slightly sandy (very fine grained quartz), fossiliferous (brachiopods, bryozoan, 

echinoderm, tribolite, and ostracod debris, Foraminifera), weathers medium gray, single bed, forms 

ledge. Brachiopods identified by J. T. Dutro, Jr., 

(written commun., 1978) include Anthracospirifer 

aff. A. occiduus (Sadlick), Composita sp., orthotetids, and productoids. Foraminifera of F. Z. 19 

include Asteroarchaediscus sp., "Eosigmoilina" 

rugosa, and silcified Pseudoglomospira sp ......... . 

36. Siltstone, grayish-olive, argillaceous, slightly 

calcareous, thin-bedded to platy, forms slope with 

units 34 and 35 ................................. . 

35. Siltstone, light-brown and grayish-olive, argillaceous, 

noncalcareous, weathers grayish orange or light 

brown, laminated, thin-bedded, forms slope ....... . 

34. Mudstone, grayish-olive and dusky-yellow, slightly 

calcareous, forms slope .......................... . 

33. Limestone, medium-gray with specks and bands of 

grayish-orange limonite, fine to coarse-grained, very 

sandy (very fine grained quartz), fossiliferous 

(brachiopods, echinoderm, ostracod, and bryozoan 

Thickness 

in meters feet 

3.4 11.0 

.3 1.0 

.5 1.7 

.8 2.6 

.6 2.0 

.2 0.5 

.9 3.0 

.5 1.6 

1.0 3.3 

.3 1.1 

.5 1.5 
MEASURED SECTIONS 65 

SECTION 5.-Reference section of the Arco Hills and Bluebird Mountain Formations 

-Continued 

debris), weathers medium gray with moderate 

yellowish-brown banding, medium- to thick-bedded 

(as much as 1 m or 3 ft), forms ledge with unit 32. 

Brachiopods identified by J. T. Dutro, Jr., (written 

commun., 1978) include Anthracospirifer sp. and productoid fragments .............................. . 

32. Limestone as unit 33. Brachiopods same as 33. 

Foraminifera (F. Z. 19) include ?Pseudoglomospira 

sp. and Zellerina sp .............................. . 

31. Limestone, medium-light-gray, very argillaceous, 

weathers medium light gray, platy, forms slope .... 

30. Limestone, dark-gray,. fine- to coarse-grained, silty, 

fossil fragments (crinoid and echinoderm debris), 

weathers light olive gray and pale yellowish brown, 

medium-bedded, forms slope ..................... . 

29. Siltstone, medium-light-gray and yellowish brown, 

noncalcareous to slightly calcareous, weathers 

yellowish brown, medium gray, and grayish brown, 

thin bedded, forms slope ........................ . 

28. Mudstone, varicolored--medium-gray, light gray, lightgreenish-gray, dark yellowish-orange, shaly, forms 

slope .......................................... . 

27. Siltstone and very fine grained sandstone, mediumdark-gray and olive-gray, slightly calcareous, carbonaceous, weathers gray, greenish gray, and 

yellowish brown, banded, thin-bedded, forms slope . 

26. Limestone, medium-gray with some grayish orange 

limonitic streaks, medium- to coarse-grained, slightly silty and sandy (very fine grained quartz), 

fossiliferous, silicified in part (brachiopods, 

echinoderm and bryozoan debris, foraminifers), 

weathers medium gray with grayish-orange streaks, 

medium-bedded (0.6 m or 2 ft), forms slope. The 

brachiopod Anthracospirifer? sp. identified by J. T. 

Dutro, Jr., (written commun., 1978). Foraminifera (F. 

Z. 19) include Neoarchaediscus sp. and silicified 

Pseudoglomospira sp ............................ . 

25. Limestone as unit 24, forms ledge .................. . 

24. Limestone, banded medium-gray and yellowish orange 

or moderate-yellowish-brown, aphanitic, slightly 

sandy (very fine grained quartz), poorly fossiliferous 

(echboderm, sponge, and brachiopod debris, foraminifers), weathers same as fresh surfaces, thinbedded (0.15-0.3 m or 6 in.-1 ft), highly fractured, 

forms ledge. Poorly preserved foraminifers are 

Asteroarchaediscus sp. and Earlandia sp .......... . 

23. Limestone, light-brownish-gray, light-olive gray, some 

medium-dark-gray, very fine grained, argillaceous 

and silty, fossiijferous (phosphatic brachiopods), 

weathers light brownish gray, light brown, and 

Thickness 

in meters feet 

1.4 

.7 

1.3 

.6 

1.2 

1.2 

3.0 

1.1 

6.6 

2.1 

4.6 

1.7 

4.3 

2.0 

3.8 

4.0 

10.0 

3.7 

21.8 

7.0 
66 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 5.-Reference section of the Arco Hills and Bluebird Mountain Formations 

-Continued 

yellowish brown, thin-bedded (8-20 em or 3-8 in.), 

breaks into small chips, forms slope .............. . 

22. Mudstone, calcareous, and argillaceous limestone, 

pale-yellowish-brown on both fresh and weathered 

surfaces, shaly, forms slope ...................... . 

21. Shale, grayish-black with bluish-white phosphate 

bloom ......................................... . 

20. Limestone, argillaceous as in unit 18 ............... . 

19. Limestone, medium-gray, medium-dark-gray, finegrained, sandy (very fine grained quartz), very 

fossiliferous (echinoderm debris, brachiopods), 

weathers medium dark gray and yellowish orange, 

single bed, forms small ledge. Brachiopods identified 

by J. T. Dutro, Jr., (written commun., 1978) are Composita sp. (exfoliated specimens) and Orbiculoidea? 

sp ............................................. . 

18. Limestone, medium-gray, argillaceous, weathers pale 

brown, very thin bedded, forms slope ............. . 

17. Limestone, light-olive-gray, medium-grained to conglomeratic, sandy, phosphatic, weathers light olive 

gray, banded light olive gray and grayish orange, 

thin-to medium-bedded (7.5-10 em or 3-4 in.), forms 

slope .......................................... . 

16. Siltstone, pale-brown, light-brown, calcareous, 

weathers grayish orange, forms slope ............. . 

15. Limestone, medium- dark-gray, very fine grained, 

slightly silty, much iron oxide, spicular mudstone, 

weathers medium dark gray and pale yellowish 

brown, thin-bedded (7.5-10 em or 3-6 in.), forms 

slope .......................................... . 

14. Mudstone, light-olive-gray, calcareous, weathers light 

olive gray, shaly, forms slope .................... . 

13. Limestone, medium-gray, fine-grained, silty, fossiliferous (echinoderm, tribolite debris, brachiopods), 

weathers medium gray and grayish orange, thinbedded, sheared, forms slope. The brachiopod 

Anthracospirifer aff. A. occiduus (Sadlick) identified 

by J. T. Dutro, Jr., (written commun., 1978) ....... . 

12. Limestone, argillaceous, and calcareous silty claystone, olive-gray, fine-grained, fossiliferous (brachiopod fragments), weathers medium gray and pale 

yellowish brown, thin-bedded, forms slope ........ . 

11. Limestone, medium-gray, medium- to coarse grained, 

phosphatic, fossiliferous (brachiopods and echinoderm debris), weathers dark yellowish brown and 

medium gray, form:s ledge ....................... . 

10. Limestone, argillaceous, and calcareous mudstone, 

olive-gray, medium-gray, dark-yellowish-brown, very 

fine grained, sparsely fossiliferous (brachiopods), 

weathers olive gray and grayish red, thin-bedded, 

Thickness 

in meters feet 

4.0 13.1 

.5 1.5 

.01 OJ 

.3 1.0 

.14 0.5 

.1 0.3 

.6 1.9 

.1 0.3 

.9 2.9 

.2 0.8 

.6 1.8 

.6 2.1 

.6 1.6 
MEASURED SECTIONS 67 

SECTION 5.-Reference section of the Arco Hills and Bluebird Mountain Formations 

-Continued 

sheared, breaks into small chips or thin pencillike 

fragments, forms slope .......................... . 

9. Limestone, medium-gray, fine-grained, silty, weathers 

medium gray and grayish orange, medium-bedded, 

sheared, forms ledge ............................ . 

8. Limestone, medium-gray to brownish-gray, finegrained, silty, weathers medium brownish gray and 

yellowish gray, thin-bedded, forms slope .......... . 

7. Limestone, medium-gray, fine- to very coarse grained, 

slightly sandy (very fine grained quartz), fossiliferous (brachiopods, some phosphatic, echinoderm and 

bryozoan debris, foraminifers), weathers medium 

gray, single bed, forms ledge. Brachiopods identified 

by J. T. Dutro, Jr., (written commun., 1978) include 

an indeterminate orthotetid, Anthracospirifer? sp. 

and Inflatia? sp. Foraminifera of F. Z. 19 include 

Archaediscus sp., Earlandia sp., Endothyra sp., 

"Eosigmoilina" rugosa, Hemiarchaediscus sp., and 

Neoarchaediscus sp ............................. . 

6. Limestone, medium-gray and pale-red, fine grained, silty, weathers grayish red, forms slope ............. . 

5. Limestone, medium-gray, fine- to medium grained, 

sandy (very fine grained quartz), fossiliferous 

(brachiopods, echinoderm and trilobite debris, Foraminifera), weathers medium gray, single bed, forms 

low ledge. Brachiopods identified by J. T. Dutro, Jr., 

(written commun., 1978) include Composita sp. and 

productoid fragments. Foraminifera of Mississippian age include calcitornellids, Earlandia sp., Endothyra of group E. bowmani, Globoendothyra sp., and 

Priscella sp ..................................... . 

4. Limestone as in unit 6 ............................. . 

3. Limestone as in unit 5 ............................. . 

2. Limestone as in unit 6 ............................. . 

1. Covered interval, thickness estimated .............. . 

Total thickness Arco Hills Formation ................ . 

Thickness 

in meters feet 

2.1 6.8 

.6 2.0 

.9 3.0 

.5 1.5 

.4 1.2 

.5 1.5 

.2 0.7 

.3 1.0 

.5 1.5 

9.1 30.0 

52.0 169.3 

SECTION 6.-Reference section of the Arco Hills and Bluebird Mountain Formations 

[Measured in the southern Lemhi Range (fig. 1, sec. 6), on the ridge east of East Canyon in the SEt,~, sec. 36 (unsurveyed), T. 7 N., R. 29 E., Butte County, Idaho, by Betty Skipp, Becky Watson, and D. S. Crawford in July, 

1978] 

Snaky Canyon Formation, lower part (Lower Permian to uppermost Mississippian): 

20. Limestone, medium-gray, fine-grained, sandy (very 

fine to fine grained quartz), abundant algae 

(Spongiostromata), some brachiopod, pelmatazoan, 

Thickness 

in meters feet 
68 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 6.-Reference section of the Arco Hills and Bluebird Mountain FormationsContinued 

and ostracod debris, conodonts and foraminifers, 

weathers medium light gray, medium- to thick-bedded, forms cliff. Foraminifera and algae of F. Z. 19? 

include Asteroarchaediscus sp., Biseriella moderata, 

Earlandia sp., Endothyra cf. E. excellens, "Eosigmoilina" rugosa?, Eotuberitina sp., Hemiarchaediscus sp., Neoarchaediscus sp., Pseudoglomospira sp., and Stacheoides sp. Conodonts of 

Late Mississippian age include Gnathodus bilineatus 

and Cavusgnathus sp. (identified by D. S. Crawford, 

written commun., 1978) ......................... . 

Total thickness incomplete Snaky Canyon Formation .. 

Bluebird Mountain Formation (uppermost Mississippian): 

19. Sandstone, light-brown, very fine grained, quartzose 

with as much as 1 percent opaque iron oxide, calcareous in upper part, siliceous in lower, weathers light 

brown and dark yellowish brown, medium- to thickbedded (0.3-1 m or 1-3ft), forms cliff ............. . 

18. Sandstone, light-brown, very fine grained, quartzose 

with siliceous or argillaceous cement, weathers dark 

yellowish brown, shaly, forms slope .............. . 

17. Sandstone as in unit 19, one bed, forms cliff ......... . 

16. Dolomite, sandy, and dolomitic sandstone, mediumlight-gray and light-brown, sandy dolomite makes 

up upper part, dolomitic sandstone lower part, very 

fine grained carbonate mosaic, fine grained quartz 

sand weathers orange gray and dark yellowish 

brown, thick-bedded, forms slope ................ . 

15. Limestone, medium-gray, medium- to coarse-grained, 

slightly silty, a few black chert nodules, fossiliferous 

(brachiopods, some phosphatic, ostracods, conodonts, pelmatazoan and bryozoan debris, and abundant algae and foraminifers), weathers medium dark 

gray, medium-bedded (approximately 0.3 m or 1 ft), 

forms cliff. Foraminifera and algae ofF. Z. 19 include 

Archaediscus sp., Asteroarchaediscus sp., Biseriella 

sp., Earlandia spp., Endothyra of group E. bowmani, 

Eosigmoilina explicata, "Eosigmoilina" rugosa, 

Eostaffella spp., Eostaffellina sp., Eotuberitina sp., 

Hemiarchaediscus sp., Neoarchaediscus sp., Planoendothyra sp., Planospirodiscus sp., Pseudoglomospira sp., Stacheoides sp., Tetrataxis spp., Zellerina 

sp., and Zellerina discoidea. Conodonts of Late 

Mississippian age identified by D. S. Crawford (written commun., 1978) include Gnathodus bilineatus 

and Cavusgnathus sp ........................... . 

Thickness 

in meters feet 

1.2 

1.2 

3.7 

.6 

1.5 

2.1 

1.5 

4 

4 

12 

2 

5 

7 

5 
MEASURED SECTIONS 69 

SECTION 6.-Reference section of the Arco Hills and Bluebird Mountain FormationsContinued 

14. Sandstone, light-brown, very fine grained, quartzose, 

part calcareous and part siliceous cement, weathers 

dark yellowish brown and olive gray, medium-bedded (0.3-0.6 m or 1-2ft), forms cliff ............... . 

13. Sandstone with pods of sandy limestone, light-gray 

and light-olive-gray, very fine grained, quartzose, 

calcareous, weathers very light brown, medium bedded (0.3-0.6 m or 1-2ft), forms slope ............. . 

12. Sandstone, light-gray and light-brown, very fine grained, quartzose, quartzitic, weathers light brown and 

dark yellowish brown, medium-bedded (0.3-0.6 m or 

1-2ft), forms cliff ............................... . 

11. Limestone, medium-gray, medium-grained, sandy 

(very fine grained quartz), fossiliferous (brachiopods 

and foraminifers), weathers medium gray, shaly to 

medium-bedded, forms slope. Foraminifers of F. Z. 

19 include Archaediscus spp., ?Asteroarchaediscus 

sp., "Eosigmoilina" rugosa, Eostaffella sp., Neoarchaediscus sp., Planospirodiscus sp., Tetrataxis sp., 

and Zellerina sp ................................. . 

10. Sandstone, light-brown and medium light-gray, very 

fine grained, quartzose, calcareous in part, siliceous 

in part, weathers light brown, olive gray, and dark 

yellowish brown, medium-bedded (0.3-0.6 m or 1-2 

ft), forms cliff .................................. . 

9. Limestone, medium-light-gray, fine grained, very 

sandy (very fine grained to fine-grained quartz), 

phosphatic, fossiliferous (productoid brachiopods, 

pelmatazoan and bryozoan debris, foraminifers), 

weathers medium light gray and light brown, one 

bed, forms slope. Foraminifers of F. Z. 19 include 

Archaediscus sp., "Eosigmoilina" rugosa, and 

?Hemiarchaediscus sp ........................... . 

8. Dolomite, medium-gray, very fine grained, sandy (finegrained quartz), calcareous, scattered medium-gray 

chert nodules, fossiliferous (phosphatic brachiopods, 

Orbiculoidea sp.), weathers grayish orange and 

moderate yellowish brown with thin weathered rind, 

thick-bedded, forms ledge ....................... . 

Total thickness Bluebird Mountain Formation ........ . 

Gradational contact. 

Arco Hills Formation (Upper Mississippian): 

7. Siltstone, dark-gray, noncalcareous, weathers dark 

gray, platy, forms slope ......................... . 

6. Limestone, light-olive-gray, very fine grained, sandy 

(very fine grained quartz), common opaque iron oxide, platy, forms slope ........................... . 

5. Siltstone, dark-reddish-brown, shaly, forms slope .... . 

Thickness 

in meters feet 

1.8 

.6 

2.1 

.6 

14.0 

.6 

1.8 

30.9 

.6 

.6 

1.8 

6 

2 

7 

2 

46 

2 

6 

102 

2 

2 

6 
70 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 6.-Reference section of the Arco Hills and Bluebird Mountain FormationsContinued 

4. Limestone, medium-gray, medium-grained, sandy 

(very fine grained quartz), common dark-yellowishorange limonite specks, fossiliferous (brachiopods, 

bryozoan, pelmatazoan and trilobite debris, foraminifers), weathers medium gray, medium-bedded, 

forms slope. Foraminifera of F. Z. 19 include Asteroarchaediscus sp., Eosigmoilina explicata (one 

specimen), "Eosigmoilina" rugosa, Neoarchaediscus 

sp., and Planospirodiscus sp ..................... . 

3. Mudstone and siltstone, dark gray, noncalcareous, 

weathers dark gray, shaly, forms slope ........... . 

2. Limestone, medium-gray, fine-to medium-grained, 

sandy (very fine grained quartz), fossiliferous (brachiopods, pelmatazoan, bryozoan, and ostracod 

debris), graded bedding, medium bedded, forms low 

ledge on slope .................................. . 

1. Siltstone, mudstone, and sandy limestone as in units 2 

through 7 ...................................... . 

Total thickness Arco Hills Formation ................ . 

Abrupt contact with thick-bedded pure limestones of Surrett 

Canyon Formation (Upper Mississippian) 

Thickness 

in meters feet 

0.6 

2.2 

1.5 

35.4 

42.7 

2 

7 

5 

116 

140 

SECTION 7.-Reference section of the Arco Hills and Bluebird Mountain Formations 

[Measured in the Blue Dome area of the southern Beaverhead Mountains (figs. 1 and 5, sec. 7), on the north side of 

Skull Canyon in NW%NW% sec. 28 and NE%NE% sec. 29, T. 10 N., R. 30 E., Clark County, Idaho, by Betty 

Skipp, Penny Patterson, and W. J. Sando in July 1976] 

Snaky Canyon Formation, lower part (Lower Permian to Lower 

Pennsylvanian): 

45. Limestone, medium-light-gray, coarse grained, recrystallized, scattered medium-gray chert nodules, 

weathers medium light gray and yellowish brown, 

Thickness 

in meters feet 

thick-bedded, forms cliff......................... 6.1 20 

44. Covered interval. One bed, 2m (6ft) thick in center of 

interval, is limestone, grayish-orange-pink, sandy, 

medium to coarse-grained, weathers light gray, 

forms ledge. Unit forms slope..................... 20.1 66 

Total thickness incomplete Snaky Canyon Formation.. 26.2 86 

Gradational contact. 

Bluebird Mountain Formation (uppermost Mississippian): 

43. Sandstone, medium-gray, quartzose, calcareous, very 

fine grained, weathers pale yellowish brown, thickbedded (1.2 m or 4ft), forms cliff. . . . . . . . . . . . . . . . . . 2.4 8 

42. Sandstone, grayish-orange-pink, quartzose, calcareous, very fine grained, weathers pale yellowish 

brown and moderate brown, thick-bedded, forms cliff 4.3 14 
MEASURED SECTIONS 71 

SECTION 7.-Reference section of the Arco Hill and Bluebird Mountain FormationsContinued 

41. Limestone, dolomitic, medium-gray, medium to coarsegrained, recrystallized, silty, scattered medium-gray 

chert nodules, a few remnants of crinoidal and phosphatic brachiopod debris, weathers light gray and 

Thickness 

in meters feet 

olive gray, one bed, forms cliff. . . . . . . . . . . . . . . . . . . . 1.8 6 

40. Sandstone, light-olive-gray, quartzose, calcareous, 

very fine grained, weathers brownish gray, medium 

bedded, forms cliff............................... 1.5 5 

39. Sandstone, pale-reddish-brown and light olive-gray, 

quartzose, calcareous, very fine grained, weathers 

light olive gray and pale yellowish brown, thick bedded, forms steep slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 11 

38. Sandstone, light-gray, quartzose, quartzitic, very fine 

grained, weathers light olive gray, forms steep slope 1.1 3.5 

37. Limestone, light-brownish-gray and moderate 

yellowish-brown, coarse-grained, recrystallized, 

slightly silty, weathers medium light gray, medium 

bedded (0.5 m or 1.6 ft), forms steep slope . . . . . . . . . . 1.0 3.2 

36. Sandstone, light-brown, light-olive-gray, medium-gray, 

quartzose, quartzitic, very fine grained, weathers 

mostly light brown and light olive gray, some laminations of medium-gray quartzitic sandstone with 

pink spots, medium- to thick-bedded (as much as 1 m 

or 3 ft), 80 percent exposed, forms steep slope with 

ledges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.0 36 

35. Limestone, light-olive-gray, medium-light gray, 

medium- to coarse-grained, recrystallized, very 

slightly sandy (very fine grained quartz), some 

crinoidal debris, weathers light gray and light olive 

gray, medium bedded, forms ledge . . . . . . . . . . . . . . . . .9 2.8 

34. Partly covered interval, 30 percent exposed. Sandstone, very fine grained, and siltstone, olive-gray, 

medium-dark gray, medium-gray, slightly calcareous .. 

in places, largely quartzitic, weathers olive gray and 

medium gray, medium bedded, forms steep slope 

with ledges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 27 

33. Sandstone, olive-gray, brownish-gray, mottled 

medium-gray and light brownish-gray, pale-reddishbrown, quartzose, largely quartzitic, very fine to 

medium-grained, some slightly calcareous, iron oxide 

present, minor light-gray chert nodules near base, 

weathers mostly moderate brown, minor crossbedding and laminations, medium-bedded (0.3-1.3 m or 

1-4ft), forms ledges............................. 19.8 65 

Total thickness Bluebird Mountain Formation . . . . . . . . . 55.4 181.5 
72 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 7.-Reference section of the Arco Hill and Bluebird Mountain FormationsContinued 

Gradational contact. 

Arco Hills Formatipn (Upper Mississippian): 

32. Mudstone, shaly, and siltstone, grayish-black, 

laminated with olive-gray streaks, noncalcareous, 

weathers to grayish black and dark-gray chips, 

forms slope .................................... . 

31. Limestone, medium-dark-gray, medium- to coarsegrained, sandy (very fine grained quartz), fossiliferous (brachiopods, some phosphatic, echinoderm 

debris, foraminifers), weathers medium gray and 

brownish gray, thin-bedded, forms small ledge. 

Brachiopods identified by J. T. Dutro, Jr., (written 

commun., 1978) include Anthracospirifer sp., 

Diaphragmus? sp., Inflatia sp., and indeterminate 

orthotetids. Foraminifera of F. Z. 19 are 

?"Eosigmoilina" rugosa and silicified Pseudoglomospira sp. . ................................ . 

30. Mudstone and siltstone, dark-yellowish brown, 

calcareous, mudstone is silty in upper part, thin 

siltstone at base, fossiliferous (productoid brachiopods), weathers into dark-gray chips, forms slope . 

29. Siltstone, medium-gray and light-olive-gray, noncalcareous, irregular laminations, organic material, 

weathers medium olive gray, medium gray, and 

grayish orange, forms ledge ..................... . 

28. Siltstone, dark-gray, noncalcareous, sheared, weathers 

dark gray, breaks into pencil-shaped· chips, forms. 

slope .......................................... . 

27. Limestone, medium-gray, medium- to coarse grained, 

coquina in part, silty to sandy (very fine grained 

quartz), limonitic, fossiliferous (brachiopods, bryozoan and echinoderm debris, Foraminifera), weathers 

with subdued medium-dark-gray and moderateyellowish brown mottling, thick-bedded (1-1.3 m or 

3-4ft), forms ledge. Brachiopods identified by J. T. 

Dutro, Jr., (written commun., 1978) include Anthracospirifer aff. A. occiduus (Sadlick), Antiquatonia? sp., and Inflatia sp. Foraminifera (F. Z. 19) 

include Archaediscus sp., Asteroarchaediscus sp., 

"Eosigmoilina" rugosa, and Neoarchaediscus sp .... 

26. Mudstone and argillaceous limestone. Argillaceous 

limestone in upper part, medium-gray, fine-grained, 

fossiliferous (brachiopods, see unit 25), thin-bedded, 

forms low ledge. Mudstone in lower part, brownishgray, calcareous, laminated with limonitic zones, 

shaly, weathers into tiny grayish-black shale chips, 

forms slope .................................... . 

Thickness 

in meters feet 

3.0 10.0 

.2 0.5 

1.7 5.7 

1.0 3.4 

1.6 5.1 

2.2 7.1 

2.1 7.0 
MEASURED SECTIONS 73 

SECTION 7.-Reference section of the Arco Hill and Bluebird Mountain FormationsContinued Thickness 

25. Mudstone, dark-gray, silty, calcareous, platy, breaks in meters 

into flat chips about 1 em (0.5 in.) in diameter, forms 

slope. Brachiopods collected from float derived from 

limestone of units 26 and 27 include Anthracospirifer 

aff. A occiduus (Sadlick), Flexaria? sp., Inflatia? sp., 

and orthotetid fragments (identified by J. T. Dutro, 

Jr., written commun., 1978) ...................... . 

24. Limestone, medium-dark-gray, fine- to mediumgrained, matrix recrystallized, slightly silty or sandy 

(very fine grained quartz), limonitic, very fossiliferous (brachiopods, some phosphatic, see unit· 23, 

echinoderm, bryozoan, and trilobite debris), 

weathers medium gray and moderate yellowish 

brown, one bed, forms ledge ..................... . 

23. Mudstone, brownish-gray, silty, upper beds 

calcareous, fossiliferous (brachiopods weather 

white), weathers to small (less than 0.6 em, 0.25 in.) 

grayish-black to dark-gray chips, forms slope. Indigenous brachiopod fauna is composed largely of 

Composita sp. Float collection derived from overlying limestone of unit 24 contains the following 

brachiopods identified by J. T. Dutro, Jr., (written 

commun., 1978) Anthracospirifer sp., Anthracospirifer aff. A occiduus (Sadlick), Composita sp., 

Composita cf. C. poposiensis Gordon, Diaphragmus? 

sp., Inflatia sp., and orthotetid and terebratuloid 

forms .......................................... . 

22. Limestone, medium-gray to dark-gray, fine grained, 

silty in upper part, fossiliferous (brachiopods) in 

lower part, weathers medium dark gray and light 

olive gray, thin-bedded (0.1 m or 4 in.), forms ledge. 

Brachiopods from lower part identified by J. T. 

Dutro, Jr., (written commun., 1978) include Anthracospirifer sp., Carlinia? sp., Composita sp., Inflatia 

sp., Ovatia sp., and orthotetid forms ............. . 

21. Mudstone, light-brownish-gray, silty, calcareous, 

fossiliferous (brachiopods weather white), weathers 

to small chips as unit 23, forms slope ............. . 

20. Limestone, medium-gray and dark-gray, fine to coarse 

grained, silty bands, sheared, limonitic in places, 

some richly fossiliferous zones (brachiopods, some 

phosphatic, echinoderm debris, sparse 

Foraminifera), weathers medium gray, dark 

yellowish orange, and grayish orange in irregular 

bands, medium-bedded (0.6-0.9 m or 2-3 ft), forms 

steplike ledges. Brachiopods from the base identified 

by J. T. Dutro, Jr., (written commun., 1978) include 

Anthracospirifer sp., Carlinia? sp., Cleiothyridina 

sp., Composita sp., and orthotetid forms. The foraminifer Asteroarchaediscus sp. of Late Mississippian age also is present at the base ............... . 

3.7 

.6 

4.6 

.9 

1.8 

5.2 

feet 

12.0 

2.1 

15.0 

3.1 

6.0 

17 
74 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

SECTION 7 .-Reference section of the A reo Hill and Bluebird Mountain FormationsContinued 

19. Mudstone. Upper part, light olive gray and pale 

yellowish brown, noncalcareous, weathers medium 

gray. Lower part, grayish black and brownish gray, 

silty, calcareous, weathers grayish black and 

brownish gray, forms slope ...................... . 

18. Siltstone, medium-dark-gray, calcareous, sheared, 

weathers pale yellowish brown, forms low ledge between mudstones ............................... . 

17. Mudstone, mostly brownish-gray and yellowish brown, 

calcareous. Grayish-black silty noncalcareous mudstone at base. Unit forms slope .................. . 

16. Siltstone, medium-dark-gray, calcareous, laminated, 

weathers light brown and moderate yellowish brown, 

medium bedded (0.3-1 m or 1-3 ft) forms small ledge 

15. Limestone, silty, and calcareous siltstone, dark-gray 

and brownish-gray, slightly · fossiliferous (brachiopods and sponge spicules) weathers medium gray 

and pale yellowish brown, thick-bedded (0.6-2.0 m or 

2-6 ft), forms ledge ............................. . 

14. Limestone, grayish-black (base) to brownish-gray (upper part), fine grained, silty, sheared, fossiliferous 

(brachiopods), weathers medium dark gray, mediumbedded (0.2-0.6 m or 8 in.-2 ft), forms low ledge, 75 

percent exposed. Brachiopods from the middle include Eolissochonetes pseudoliratus (Easton), and 

Anthracospirifer sp. identified by J. T. Dutro, Jr., 

(written commun., 1978) ......................... . 

13. Mudstone, brownish-gray, moderate yellowish-brown, 

noncalcareous, much limonite, weathers moderate 

yellowish brown, forms slope .................... . 

12. Mudstone, grayish-black, noncalcareous, silty, 

weathers dark gray and pale yellowish brown, breaks 

into thin pencillike fragments, forms slope ........ . 

11. Siltstone, brownish-gray, medium-dark gray, 

yellowish-brown, largely noncalcareous, weathers 

light brown and yellowish brown, shaly to thin bedded, breaks into equidimensional fragments, forms 

low ledge ...................................... . 

10. Mudstone as in unit 12 ............................ . 

9. Mudstone, dark-gray, slightly calcareous, laminated, 

weathers dark gray and reddish brown (iron staining), thin bedded (8-10 em or 3-4 in.), forms slope ... 

8. Mudstone and limestone. Mudstone, grayish black, silty, weathers grayish black and pale yellowish brown, 

breaks into pencillike fragments. Thin bed (7.5 em or 

3 in.) of grayish-black argillaceous limestone in middle of unit. Unit forms slope ..................... . 

7. Limestone, medium-gray to medium-dark-gray, finegrained, slightly silty and sandy (very fine grained 

Thickness 

in meters feet 

1.8 

1.7 

1.5 

0.9 

5.5 

7.3 

2.0 

1.2 

1.7 

3.6 

.8 

1.5 

6 

5.5 

5.0 

3.0 

18.0 

24.0 

6.5 

4.0 

5.8 

12.0 

2.8 

5.0 
MEASURED SECTIONS 75 

SECTION 7.-Reference section of the Arco Hill and Bluebird Mountain FonnationsContinued 

quartz), fossiliferous (brachiopods, bryozoans, 

sponge spicules, echinoderm debris, sparse Foraminifera), weathers with alternating bands of 

medium gray and moderate yellowish brown, thickbedded (1-1.6 m or 3-5 ft), forms cliff. Brachiopods 

from the upper and lower parts identified by J. T. 

Dutro, Jr., (written commun., 1978) include: upper 

part, Anthracospirifer aff. A occiduus (Sadlick), 

Anthracospirifer cf. A shawi Gordon, Carlinia? sp., 

Composita sp.; lower part, Carlinia? sp. and lnflatia 

spp. Foraminifera (F. Z. 19) from the middle part are 

Earlandia sp., Eosigmoilina aff. E. explicata, (see 

also unit 6) ..................................... . 

6. Limestone, medium-gray, fine-grained, slightly silty 

and sandy (very fine grained quartz), sheared, limonitic in part, fossiliferous (brachiopods, ostracods, 

and sponge spicules), weathers medium gray and 

light olive gray, thick-bedded, forms slope, 50 percent exposed. Brachiopods collected from float 

represent debris from unit 7 and include Anthracospirifer sp. (large), Anthracospirifer cf., A shawi 

Gordon, Composita sp., Inflatia sp., Orbiculoidea 

sp., orthotetid forms, and Ovatia? sp., identified by 

J. T. Dutro, Jr., (written commun., 1978) .......... . 

5. Limestone, dark-gray, fine-grained, partly dolomitized, 

limonitic, sheared, fossiliferous (brachiopods, bryozoans, sponge spicules, and Foraminifera), weathers 

yellowish orange and medium gray, thick-bedded 

(0.45-1.2 m or 1.5-3.5 ft), forms ledge. Foraminifera 

(F. Z. 19) include: Eosigmoilina explicata, Planospirodiscus sp .................................. . 

4. Limestone, dark-gray, argillaceous, fossiliferous (a few 

orbiculoid brachiopods), weathers dark gray and 

light olive gray, thin-bedded (10-30 em or 2-6 in.), 

forms slope .................................... . 

3. Mudstone, dark-yellowish-brown, calcareous, silty and 

limonitic in lower part, weathers dark yellowish 

brown, forms slope ............................. . 

2. Mudstone, silty, and argillaceous siltstone, mediumdark-gray, mostly noncalcareous, weathers dark 

gray and olive gray, breaks into pencillike 

fragments, forms slope .......................... . 

1. Sandstone, medium-gray, quartzose, calcareous, very 

fine grained, cleaved, contains moderate-brown 

limonite nodules 1 to 2 em in diameter, weathers 

dark yellow brown, thin-bedded (0.2-0.3 m or 0.7-1 

ft), forms low ledge ............................. . 

0. Covered interval. Thickness estimated ............. . 

Total thickness Arco Hills Formation ............... . 

Thickness 

in meters feet 

5.5 

2.4 

1.5 

1.8 

4.0 

3.6 

1.2 

15.2 

93.3 

18.0 

8.0 

5.0 

6.0 

13.0 

11.0 

4.0 

50.0 

306.6 
76 UPPER PALEOZOIC CARBONATE BANK IN EAST-CENTRAL IDAHO 

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-t< U.S. GOVERNMENT PRINTING OFFICE, 1979-677·129 /11