The Jurassic Wanakah and Morrison 

Formations in the Telluride-OurayWestern Black Canyon Area of 

Southwestern Colorado 

U.S. GEOLOGICAL SURVEY BULLETIN 1927 
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Ave. 
The Jurassic Wanakah and Morrison 

Formations in the Telluride-OurayWestern Black Canyon Area of 

Southern Colorado 

By ROBERT B. O'SULLIVAN 

Revision of the Wanakah Formation and basal Morrison Formation 

in part of the Black Canyon of the Gunnison River 

U.S. GEOLOGICAL SURVEY BULLETIN 1927 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

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Library of Congress Cataloging-in-Publication Data 

O'Sullivan, Robert Brett, 1923-

The Jurassic Wanakah and Morrison Formations in the Telluride-Ouray-western 

Black Canyon area of southwestern Colorado I by Robert B. O'Sullivan. 

p. em. - (U.S. Geological Survey bulletin; 1927) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1927 

1. Geology, Stratigraphy-Jurassic. 2. Geology-Colorado. 3. Wanakah 

Formation. 4. Morrison Formation. I. Title II. Series. 

QE75.B9 no. 1927 

[QE681] 

557.3 s-dc20 91-14039 

[551.7'66'09788] CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Stratigraphy 3 

Entrada Sandstone 3 

Wanakah Formation 7 

Pony Express Limestone Member 9 

Pony Express equivalent 10 

Bilk Creek Sandstone Member 10 

Beds at Sawpit 11 

Morrison Formation 12 

Tidwell Member 12 

Gypsum in Morrison Formation 13 

Previous correlations 15 

Reference section 19 

References cited 23 

FIGURES 

1. Map showing Telluride-Ouray-western Black Canyon area, southwestern 

Colorado 2 

2. Chart showing some Jurassic nomenclature in the San Rafael Swell and study 

area 3 

3. Restored sections showing correlation of some Jurassic rocks 6 

4. Photograph showing Wanakah Formation near Bilk Creek 8 

5. Map showing areal extent of basal limestone of Wanakah Formation 10 

6. Photograph showing Bilk Creek Sandstone Member on west side of Leopard 

Creek 11 

7. Map showing paleogeography of western United States in Kimmeridgian 

time 14 

8. Chart showing nomenclature of Jurassic rocks in southwestern Colorado 16 

9. Map showing distribution of Junction Creek Sandstone in southwestern 

Colorado 18 

10. Photographs showing Wanakah Formation and related rocks at Chukar 

trail 20 

11. Map showing location of Chukar trail reference section 22 

TABLES 

1. Number, name, and location of measured sections and thicknesses of the Entrada 

Sandstone and parts of the Wanakah.Formation and Tidwell Member of the Morrison Formation 4 

2. Analytical data for gypsum samples from the bed at Chukar trail 12 

Contents Ill 

The Jurassic Wanakah and Morrison Formations in the 

Telluride-Ouray-Western Black Canyon Area of 

Southwestern Colorado 

By Robert B. O'Sullivan 

Abstract 

Rocks exposed in deep canyons along the San Miguel 

River in the Telluride area, along the Uncompahgre River in 

the Ouray area, and in the western part of the Black Canyon of 

the Gunnison River include the Middle Jurassic Entrada 

Sandstone and Wanakah Formation and the Upper Jurassic 

Morrison Formation. The Entrada Sandstone, 45-95 ft thick, 

and the Wanakah Formation, 45-125 ft thick, are present 

throughout the area. The Wanakah Formation consists, in 

ascending order, of the Pony Express Limestone Member and 

Bilk Creek Sandstone Member, named in the Telluride-Ouray 

area, and an upper red-bed sequence termed the beds at 

Sawpit. The Bilk Creek Sandstone Member and the beds at 

Sawpit are here correlated into western Black Canyon where 

they had not previously been described. The Tidwell Member, 

about 50-210 ft thick, a basal member of the Morrison 

Formation, is here recognized in the Ouray and western Black 

Canyon areas. Locally in western Black Canyon, an eolian 

sandstone 73ft thick and 90ft above the base of the Morrison 

Formation is termed the bed at Smith Fork; previously it was 

correlated with the Junction Creek Sandstone. Gypsum is 

found (1) at the top of the Pony Express Limestone Member 

near Ouray and in western Black Canyon, (2) in the beds at 

Sawpit in western Black Canyon, and (3) in the Tidwell 

Member in western Black Canyon. The gypsum in the Tidwell 

Member is similar to gypsum beds in the Tidwell of Utah and 

probably in basal beds of the Morrison in eastern Colorado. 

Previously, all gypsum-bearing beds were assigned to the 

Wanakah Formation. The Junction Creek Sandstone is now 

recognized along the Gunnison River, east of western Black 

Canyon, but its relationship to the Junction Creek of the 

Durango area is uncertain. 

INTRODUCTION 

Jurassic rocks discussed herein crop out along three 

deep canyons in southwestern Colorado. In the San Juan 

Manuscript approved for publication March 15, 1991. 

Mountains the strata are exposed in the Telluride and Ouray 

areas near the heads of the San Miguel and Uncompahgre 

Rivers, respectively, and north of Montrose the rocks are 

exposed in the western part of Black Canyon of the 

Gunnison River. Western Black Canyon, as used in this 

study, refers to the area along the Gunnison River from 

Chukar trail to Smith Fork (fig. 1). The rocks consist of the 

Middle Jurassic San Rafael Group and the overlying Upper 

Jurassic Morrison Formation. The San Rafael Group is 

underlain by the Upper Triassic Dolores Formation in the 

San Juan Mountains and by Precambrian rocks in western 

Black Canyon. Formations of Cretaceous or younger age 

overlie the Morrison Formation. 

In the San Rafael Swell in Utah, about 160 mi west of 

Telluride, the San Rafael Group consists of five formations, 

in ascending order, Page Sandstone, Carmel Formation, 

Entrada Sandstone, and Curtis and Summerville 

Formations. In the study area along the San Miguel and 

Uncompahgre Rivers and in western Black Canyon, the San 

Rafael Group consists of Entrada Sandstone overlain by 

Wanakah Formation. Eastward from the San Rafael Swell, 

the Page Sandstone, Carmel Formation, and the lower part 

of the Entrada Sandstone lap out on to a west-sloping 

surface of erosion truncating the underlying rocks. The 

Wanakah Formation in the study area is equivalent to the 

upper part of the Entrada and lower part of the Curtis in the 

San Rafael Swell. In the study area, any equivalents of the 

upper part of the Curtis and the Summerville Formation (fig. 

2) were removed before deposition of the Morrison 

Formation. 

Several aspects of the Wanakah Formation and the 

basal part of the Morrison Formation are discussed herein. 

The Wanakah Formation and its subdivisions are correlated 

from the Wanakah mine area near Telluride and Ouray into 

western Black Canyon. As a consequence the Wanakah 

Formation in western Black Canyon is revised to agree with 

the formation in the Wanakah mine area. The relationship 

between the lower part of the Morrison Formation and the 

Introduction 
Naturita 

0

Basin 

38°00' 

0 10 

0 Escalante 

Forks 

~ ~<?/ 

Norwood 0 

20 

Salt Lake 0 City 

~ ~ 

Pea Green 

Corner 

0 

Hoovers 0 Corner 

0 

Moab 

Grand 

0 Junction 

Ophir 0 

COLORADO 

Denver 

0 

Study 0 Colorado 

Area Springs 0 

Canyon 

City 

0 Durango 

Cimarron 0 

WEST 

ELK 

MOUNTAINS 

Lake City 0 

Figure 1. Telluride-Ouray-western Black Canyon area, southwestern Colorado. Numbers and triangles refer to locations of 

stratigraphic sections used to construct restored section shown in figure 3 and described in table 1. 

2 Wanakah and Morrison Formations in Southwestern Colorado 
Series Stage San Rafael ~ Swell Study area 

Younger rocks 

u Tithonian 'iii 

en Morrison Morrison ~ Formation Formation ::::J 

""') Kimmeridgian 

Cii 

c.. 

111 1mnn IJl c.. 

::) Oxfordian 

Summerville 

llTmnr~ Formation 

Callovian c.. Curtis ::::J 

u e Formation Ql Wanakah 'iii ~0.. 

en (.!) ro:J Formation a:o 

::::J 

~ a; 

~ 

Entrada ~(5 Entrada Sandstone Sandstone ""') Ctl (/) 

~ a: 

Jl '0 c '0 Carmel Bathonian Ctl 

~ (/) Formation 

Page unconformlt 

Bajocian Sandstone (part) 

;Ill 11111111 

Older rocks 

Figure 2. Some jurassic nomenclature in the San Rafael Swell 

and study area. Fission-track dating (Kowallis and Heaton, 

1987) indicates that upper part of the Morrison Formation may 

be of Cretaceous age. 

Wanakah Formation is described as is the areal distribution 

of gypsum in the two formations. The stratigraphic 

nomenclature and previous correlations of Jurassic rocks in 

the study area are reviewed. Finally, a reference section is 

established in western Black Canyon for the Wanakah 

Formation and the lower part of the Morrison Formation. 

Acknowledgment.-Thanks are due G.N. Pipiringos 

for sharing with me his extensive knowledge of Jurassic 

rocks in the Western Interior of the United States. His visits 

to western Colorado on several occasions materially aided 

my understanding of the stratigraphic relations described 

herein. 

STRATIGRAPHY 

Jurassic rocks described herein are exposed around 

Telluride and Ouray. At Ouray, Jurassic rocks dip north 

beneath Cretaceous rocks and reappear in western Black 

Canyon. A distance of about 35 mi separates exposures 

around Ouray from those in western Black Canyon. A 

restored section (fig. 3) shows the correlation of the Entrada 

Sandstone, Wanakah Formation, and lower part of the 

Morrison Formation through the study area. The restored 

section extends from Bilk Creek on the San Miguel River 

through Ouray northward to western Black Canyon and is 

based on detailed sections measured in the field. The section 

at Cutler Creek (sec. 4, fig. 3) was measured by Steele 

(1985), and the section at Cedar Hill 6 mi north of Ouray 

(sec. 5, fig. 3) was measured by Burbank (1930). The rest 

were measured by the writer. The locations of the measured 

sections and the thicknesses of individual units are shown 

on table 1. The correlation of Jurassic rocks is complicated 

somewhat by the distribution of gypsum at different 

stratigraphic levels. The correlation benefits from 

recognition of the subdivisions of the Wanakah Formation 

and of several stratigraphic marker beds in the Wanakah and 

Morrison Formations. 

Two Jurassic unconformities in the Telluride-Ouraywestern Black Canyon area are recognized throughout much 

of the Western Interior of the United Sates. These 

unconformities form the boundaries of the San Rafael 

Group and are termed J-2 at the base of the group and J-5 

at the top. Other Jurassic unconformities, such as J-1, J-3, 

and J-4, are not present in the study area. The 

unconformities, their distribution, and their regional 

stratigraphic significance are discussed more completely in 

a report by Pipiringos and O'Sullivan (1978). 

The J-5 unconformity is a widespread break that 

marks the top of the San Rafael Group and consequently the 

base of the Morrison Formation. At some places, 

particularly in the San Rafael Swell area of Utah, the J-5 

surface is angular on underlying rocks. Generally beds 

above and below the unconformity are parallel or almost so, 

and the J-5 surface shows only very slight relief. 

Regionally, however, the unconformity bevels across 

progressively older rocks from the San Rafael Swell 

eastward into Colorado. From east-central Utah into the 

study area all of the Summerville Formation and the upper 

part of the Curtis Formation have been eroded from beneath 

the J-5 unconformity. 

In the study area, the J-5 unconformity is the sharp 

contact between the Wanakah and Morrison Formations. 

The surface represents a hiatus that probably includes all of 

the Oxfordian and much of the upper Callovian (fig. 2). 

Beveling of individual beds was not noted, although the 

marked thinning of the upper part of the Wanakah 

Formation (fig. 3) from the Telluride-Ouray area to western 

Black Canyon is possibly due to truncation beneath the J-5 

surface. About 20 mi east of Chukar trail the surface of 

unconformity bevels out all of the underlying San Rafael 

Group, and the Morrison Formation directly overlies Precambrian rocks. 

Entrada Sandstone 

In the study area the Entrada Sandstone is the basal 

formation of the San Rafael Group. The Entrada ranges in 

thickness from 45 ft at the Smith Fork measured section 

Stratigraphy 3 
olio 

~ = Table 1. Number, name, and location of measured sections and thicknesses of the Entrada Sandstone and parts of the Wanakah Formation and Tidwell Member of the Ill 

~ 

Ill 

=- Morrison Formation Ill 

= [Thickness in feet, totals rounded; N, not recognized; e, estimated. At section 4 thickness of Entrada Sandstone estimated by Steele (1985, p. 29); at section 5 thickness of Entrada and Tidwell a. 

~ Member of Morrison Formation estimated by the writer judged from thicknesses at adjacent sections) 0 

.. .. 

g· Wanakah Formation 

., 

= Bilk Creek Lower eart of Morrison Formation 0 

.. Sandstone Member Beds at Saweit Tidwell Member 

3 Pony Express Carnelian Lower Bed at Lower ~. 0 Entrada Total Umestone Total sandstone Total marker Chukar Upper Total gypslferous Upper (I) = Number, name, and location Sandstone thickness Member thickness marker bed thickness bed Trail beds thickness Bed A bedS bedS 

s· 

Cll 1. Bilk Creek-Measured northeast from 0 

c lat 37"57'05"N., long 107"55 '28"W., to ;. 

! lat 37"57'08"N., long 107"55 '24"W ....................... 53 91 11 27 2 53 N N N 67 16 N N 

.. 2. Telluride-Measured northwest from 

= n lat 37"56 '22"N., long 107"48 '03"W., to 0 

0 lat 37"56 '30"N., long 107"48 '14"W ....................... 47 92 20 22 2 50 N N N 51 20 N N .. 

!. 

0 3. South Ouray-Measured north from 

lat 38"01 '26"N., long 107"39 '13"W., to 

lat 38"01 '28"N., long 107"39'12"W ....................... 49 84 21 21 1 42 N N N 56 27 N N 

4. Cutler Creek-Measured in S~SE~ 

sec. 1, T. 44 N., R 8 W., by Steele (1985) ............ 80e 111 22 27 2 62 N N N 54 33 N N 

5. Cedar Hill-Measured 6 mi north of Ouray, 

approximately in NE~SE~ sec. 35, T. 45 N., 

R 8 W., by Burbank (1930, p.174) ...................... 77e 125 59 20 1 46 N N N 54e 27e N N 

6. North Ouray-Measured in SW~NE~~ 

sec. 35, T. 45 N., R 8 W ......................................... 73 87 9 25 1 53 N N N 53 21 N N 

7. Chukar trail-Measured west from 

tat 380J6'40"N., long 107"50'33"W., to 

lat 380J6'42"N., long 107"50'39"W ....................... 95 59 2 33 14 24 3 6 15 164 14 79 71 

8. Pleasant Park-Measured northeast from 

lat 38°38 '09"N., long 107"48 '56"W., to 

lat 38°38'1S"N., long 107"48'48"W ....................... 64 56 2 31 9 23 2 7 14 209 15 82 112 
Table 1. Number, name, and location of measured sections and thicknesses of the Entrada Sandstone and parts of the Wanakah Formation and Tidwell Member of the 

Morrison Formation-Continued 

Wanakah Formation 

BilK CreeK Lower eart of Morrison Formation 

Sandstone Member Beds at Saweit Tidwell Member 

Pony Express carnelian Lower Bed at Lower 

Entrada Total Limestone Total sandstone Total marKer Chukar Upper Total gypsiferous Upper 

Number, name, and location Sandstone thicKness Member thicKness marKer bed thicKness bed Trail beds thicKness Bed A beds beds 

9. Red Canyon-Measured northeast from 

tat 38°40 'OS"N., long 10r48 '29"W., to 

tat 38°40'14"N., long 10r48'21"W ....................... 70 50 3 29 7 18 1 6 11 189 19 62 108 

10. Bobcat trail-Measured northeast from 

lat 38°38'05"N., long 10r51 '41"W., to 

tat 38°38 '08"N., long 10r51 '48"W ....................... 78 52 *3 27 7 22 2 8 12 104 16 70 18 

11. Duncan trail-Measured northwest from 

tat 38°39'40"N., long torst '47"W., to 

tat 38°39'44"N., long 10r51 '52"W ....................... 85 46 *2 29 8 15 1 6 8 163 14 57 92 

12. Ute trail-Measured northwest from 

tat 38°41 '14"N., long 10r51 '04"W., to 

tat 38°41 '18"N., long 10r51 '09"W ...............•....... 71 57 *2 32 11 23 2 8 13 193 18 64 111 

13. South Smith Fork-Measured north from 

tat 38°42'53"N.,long 10r50'18"W., to 

tat 38°43 '09"N., tong torso '17"W ....................... 71 58 *1 33 11 24 2 9 13 195 16 105 74 

14. Smith Fork-Measured north from 

tat 38°44 '06"N., long 10r49 '03"W., to 

tat 38°44 'lS"N., long 10r49 '07"W. ...................... 45 57 *5 31 11 21 1 7 13 190 15 61 **114 

*Thickness of Pony Express equivalent. 

.. Includes bed at Smith Fork 73 ft. thick; the upper beds below and above the bed at Smith Fork are 15 and 26 ft. thick, respectively. 

~ .. 

a 

o'Q' 

.. I» 

~ 

:::r 

-< 

<.n 
FEET G) 10.6 mi 9.8mi 4.7 mi 1.6 mi @ 0.7 mi ® 35mi 

:-:-:-:·:·:· :-:-:-:-:-: :-:-:-:-:-:-:·:·:·:-:-:-: :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:·:·:·:-:-:·:· -:-:-:-:·:·:·:-:-:-:-:-:- -:-:-:-:-:-:·:·:-:-:·:·:· 

·:-:-:-:-:-: :-:-:-:-:-:· -:-:·:-:-:-:-:·:·:·:-:-:· -:-:-:-:-:-:-:-:-:·:·:-:-:·:·:·:·:-:-:-:-:-:-:-:-: :-:-:-:·:·:·:·:-:-:-:-:-: :-:-:-:-:-:·:·:·:-:-:·:·: 

50 .......................................................................... 

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

100 

150 

200 Tidwell 1 beds 

~--~~---~----Member BedA 

r---~~----r-----~•----r-----------+-----J-5~---4------------r---------~ 250 

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

· · · · ·: ·: ·: ·: ·: ·: ·: ·: ·:·:·:·:Sandstone:·:·: : ·: ·: ·: ·: ·: ·: ·: ·: ·: ·: ·: ·: : ·: ·: ·: ·: ·: ·: ·: ·: ·: ·: · · · · ................................ 

FEET 

10 

20 

30 

40 

50 

A 

B 

Morrison 

Wanakah 

I 

Formation 

. . . . . . . . ..... . ---.._.~~J-2~~_._..-

Formation 

Beds at Sawpit 

Mernber 

Figure 3 (above and facing page). Restored sections showing correlation of some Jurassic rocks. Line of section 

extends from Telluride-Ouray area to western Black Canyon and is shown on figure 1. Measured sections are 

described in table 1. A, Correlation of Entrada Sandstone, Wanakah Formation, and Morrison Formation. 8, 

Correlation of Wanakah Formation and its subdivisions. 

(sec. 14, fig. 3) to 95ft at the Chukar trail measured section 

(sec. 7, fig. 3). At the type locality at Entrada Point in the 

San Rafael Swell, the Entrada is 312ft thick (Gilluly, 1929, 

p. 104-1 05). The Entrada is thinner in the study area partly 

because of internal thinning, partly because of lateral 

gradation into the Wanakah Formation, and partly because 

basal parts of the Entrada lap on to the J-2 surface of 

unconformity. The Entrada wedges out completely along 

the north rim of the Black Canyon about 11.5 mi southeast 

of Chukar trail (Hansen, 1971 ). 

The Entrada Sandstone generally forms a bold cliff 

throughout the area. In the Telluride-Ouray area the Entrada 

6 Wanakah and Morrison Formations in Southwestern Colorado 

is light gray, in contrast to the underlying red Triassic and 

Permian rocks. In western Black Canyon the Entrada is gray 

in most places, but varied amounts are red or banded red and 

gray. At the Pleasant Park section (sec. 8, fig. 3) only the 

basal 3 ft is red, the rest is gray. At Bobcat trail most of the 

Entrada (68 of 78 ft) is banded red and gray. The red and 

gray of the Entrada stand out from the underlying dark 

Precambrian rocks. The Entrada is mainly a fine to very fine 

grained sandstone, but it contains disseminated, sparse, 

well-rounded, medium grains of clear quartz. The coarser 

grains of the Entrada, which are informally termed "Entrada 

berries," contrast markedly with the finer grained matrix. 
2.3 mi ® 2.3 mi ® 3.9 mi 1.9 mi 1.9 mi 2.1 mi 1.7 mi 

FEET 

... .... . . . . . . .. • • • • • • • • 0 • :·:·:· .Salt: :·:·: ·Wash: ·:·:· Member :·: . . . . 

. . . . . . . . . . . ... .. .... .. 50 

Upper 100 

Morrison 

Formation 150 

Lower gypsiferous beds 200 

:-: <Bed _A-: -:-:-: .. .. . J-s_:_:_:_:-t-:-....:__:_: >..:......:.... : ·_:__ ·_:__>_:_>_:_> :

4

· ~· :~<-.:-.:....:.Be....:.d....:.A~·_:-_: ·-:--+: ~~~· ·~ ~·~·"----'--1 

Wanakah 250 Formation 

·. ·-:-: ~ntrada: > . .. ... . .. ·. ·. ·. ·. · ·. ·. : ·~;;~~d.sto.ri.e: : .. ·. ·. · .. . :-: -: . :-:-: : : :-: .: .. .. . . . . . . . . . . . . • • • • • • • :.....:. · ·_.i....:....... · ·__;_·--'---' · ·c__:_·_;_· .'----'-.__,_ • .:___:.· ....t._:___:-.:......:..._:~:..:_:_:..:..:_·.~ ··· ··· _;__:_; ·.· ·· ·~ ~{<<<< :-: .· .·. ·. ·.·.·.·.· . . 

A 

The Entrada was deposited in a sequence of alternating 

crossbedded and flatbedded units. The crossbedded units 

are eolian deposits; the flatbedded units are interpreted as 

interdune deposits. 

Wanakah Formation 

The Wanakah Formation conformably overlies the 

Entrada Sandstone in the study area and tends to form a 

rough slope (fig. 4) beneath the Morrison Formation. The 

Wanakah was named by Burbank (1930, p. 172) for 

exposures at the Wanakah mine about a mile north of 

Ouray. The name Pony Express, derived from the Pony 

Express mine 4 mi north of Ouray, was introduced for basal 

beds of the Wanakah. 

The following section, measured by Burbank (1930, 

p. 17 4 ), shows the thickness and arrangement of the 

Wanakah at Cedar Hill 6 mi north of Ouray. The Wanakah 

was assigned as a member of the Morrison Formation but 

has since been raised to formational status. The "Pony 

FEET 

0 

10 

20 

30 

40 

50 

Express beds" are now the Pony Express Limestone 

Member of the Wanakah Formation. The "sandstone bed" 

or unit 5 and unit 6 now constitutes the Bilk Creek 

Sandstone Member. The "upper shale beds" less unit 6 are 

now recognized as the beds at Sawpit. The entire section is 

quoted directly from Burbank (1930, p. 174). 

Section of the Wanakah member on the east side of the 

Uncompahgre Valley 6 miles north of Ouray. 

Wanakah member of the Morrison formation: 

Thickness 

UPPER SHALE BEDS: (feet) 

19. Shale or mudstone; weathers to small angular 

fragments; color brown; weathered surface 

yellowish .................................................. .......... 1.5 

18. Limestone, crystalline; contains a few shale 

pellets; color brownish; weathers rusty brown.. 0.5 

17. Shale or mudstone; breaks into angular fragments; 

color brown ........................................................ 1.3 

Stratigraphy 7 
Figure 4. Wanakah Formation near Bilk Creek. View looking northeast 1h mi east of Deep Creek and north of junction of Bilk 

Creek with San Miguel River. Dolores Formation (D); Entrada Sandstone (E); Wanakah Formation consists of Pony Express 

Limestone Member (PX), Bilk Creek Sandstone Member (BC), and beds at Sawpit (SP); Morrison Formation consists of Tidwell 

Member (TM) with bed A (A) at base, Salt Wash Member (SW), and Brushy Basin Member (BB). Cretaceous sandstones of Burro 

Canyon Formation overlie Morrison Formation. Entrada Sandstone is about 50ft thick. Photograph by A.L. Bush. 

Thickness 

UPPER SHALE BEDS:-Continued (feet) 

16. Limestone; lenticular bed; white and 

crystalline, weathering brown ...................... 0.3 

15. Shale; like 17 above ......................................... 0.7 

14. Limestone; nodular and crystalline; color 

brown............................................................. 0.3 

13. Shale; limy and sandy layers; breaks to angular 

fragments; color brown, somewhat reddish 

or yellowish; it contains brown and gray 

crystalline lime nodules and lenses and a few 

limy layers are persistent enough to form 

thin limestone beds; near the top some of the 

calcareous nodules are partly altered to red 

chert and quartz ............................................ 26.0 

8 Wanakah and Morrison Formations in Southwestern Colorado 

12. Limestone; gray; dense ..................................... . 

11. Clay shale; greenish gray ................................ . 

10. Shale; sandy and brown at top grading down 

into fine blocky shale of greenish color at 

base .............................................................. . 

9. Shale, brown .................................................... . 

8. Shale, sandy, olive gray; impregnated with 

gypsum in cracks ......................................... . 

7. Sandstones and sandy shales, greenish and 

brownish colors, partly slumped ................. . 

6. Sandstone; hard; greenish gray; consists of 

clean quartz grains and grains of bright red 

chert; appears glauconite ............................. . 

Total thickness shale beds ................................ . 

Thickness 

(feet) 

0.6 

1.8 

6.5 

0.5 

2.4 

4.0 

1.0 

47.4 
SANDSTONE BED: 

5. Sandstone; soft and friable; clayey layers near 

top; color somewhat olive gray near top; near 

base color gray weathering buff ................ .. 

"PONY EXPRESS BEDS": 

4. Gypsum; banded; white with black shaly layers 

3. Gypsum; nodular; roughly bedded; with 

subordinate black interstitial shale and 

irregular shale partings ................................ . 

2. Shale and limestone breccia; small angular 

fragments of limestone and shale partly 

cemented with calcite; porous .................... . 

1. Shale and limestone; thin alternations of black 

shale and dark limestone; some beds have a 

bituminous odor when struck ...................... .. 

Thickness "Pony Express beds" ............................... . 

Total thickness of Wanakah member .............. . 

Gray cross-bedded sandstone of Jurassic age, upon 

which the Wanakah shale rests conformably. 

Pony Express Limestone Member 

Thickness 

(feet) 

19.0 

1.7 

50.0 

1.8 

5.0 

58.5 -==== 

124.9 

The Pony Express Limestone Member forms a 

distinctive lithologic unit within the San Rafael Group. The 

member is at the base of the Wanakah Formation between 

the Bilk Creek (fig. 3, sec. 1) and Red Canyon (fig. 3, sec. 

9) measured sections. The Pony Express is 2-3 ft thick in 

western Black Canyon, 11-20 ft thick near Telluride, and as 

thick as 59 ft near Ouray. 

The Pony Express Limestone Member throughout its 

extent from Bilk Creek to western Black Canyon is 

dominantly limestone; near Ouray it also includes other 

rock types. The limestone is dark gray, and fresh surfaces 

have a petroliferous odor. The member is characterized by 

wavy bedding of low amplitude in beds as thick as half a 

foot. Near Ouray, thin beds of dark-gray shale and 

sandstone are interbedded with the limestone. 

Also near Ouray the limestone just described is 

overlain by a sedimentary breccia, which in tum is overlain 

locally by a thick lens of gypsum. The breccia consists of 

small bits of limestone, shale, and siltstone generally set in 

a dense limestone matrix. The breccia is present at the South 

Ouray, Cutler Creek, Cedar Hill, and North Ouray measured 

sections (sees., 3, 4, 5, and 6, respectively, fig. 3). The 

gypsum, about 50ft thick, is present above the breccia only 

at the Cedar Hill section 6 mi north of Ouray (sec. 5, fig .. 3). 

The limestone breccia and the thick gypsum bed are 

related. According to Burbank (1930, p. 173), the "breccia 

is from a few feet to 20 feet in thickness, shows an indistinct 

stratification at places, and commonly has been cemented 

by calcareous material. The limestone and shale fragmt~nts 

composing the breccia are small, rarely exceeding 2 inches 

in diameter and usually much smaller, and are sharply 

angular. At some places particularly where the formation 

[Pony Express] is unaltered a minimum of cementing matter 

is present and the breccia is consequently extremely 

porous." Northward from the Cedar Hill section, where 

gypsum is present, Burbank (1930, p. 175) found "that the 

gypsum beds rapidly wedge out and entirely disappear 

within a few hundred feet, while the breccia within the same 

range increases in thickness from less than 2 feet to about 8 

feet. It is apparent from the local relations that the breccia 

has been formed by the dissolving of the gypsum and the 

gradual accumulation in place of the interstitial fragments 

and layers of less soluble limy shale. The shape and nature 

of the interstitial shale masses in the gypsum beds are 

identical with the fragments composing the breccia." 

The limestone breccia indicates that gypsum was 

once more widespread around Ouray than is indicated by 

the single occurrence of gypsum at the Cedar Hill location. 

Neither gypsum nor limestone breccia is at the top of the 

Pony Express Limestone Member in the Bilk Creek-Telluride area or in western Black Canyon. Along Cow Creek 

(location A, fig. 5), 6 mi northeast of Ouray, the Pony 

Express Limestone Member is 1.2 ft thick, and gypsum and 

limestone breccia are absent. Gypsum is at least locally 

present near the rim of the Black Canyon east of the area 

studied. J.L. Ridgley (U.S. Geological Survey, oral commun., 1989) reported that a bed of gypsum 5-8 ft thick 

directly overlies the Pony Express Limestone Member 8.7 

mi southeast of the Chukar trail section (fig. 3) and 1 mi 

northwest of the North Rim Ranger Station of the Black 

Canyon of the Gunnison National Monument (location B, 

fig. 5). 

Gypsum associated with the basal limestone member 

of the Wanakah Formation is much more widespread south 

of the study area. In New Mexico gypsum is widespread at 

the top of the Todilto Limestone Member of the Wanakah, 

an equivalent of the Pony Express (fig. 5), and is present as 

far north as the Piedra area in southern Colorado, south of 

the San Juan Mountains. Tertiary volcanic rocks and postJurassic erosion in the San Juan Mountains obscure the 

extent of the gypsum in the area between Piedra and Ouray. 

However, the limestone breccia and isolated lenses of 

gypsum indicate that calcium-sulfate-laden waters extended 

from New Mexico into the study area near the end of Pony 

Express deposition. 

The environment of deposition of the Pony ExpressTodilto Limestone is uncertain; both unusual marine and 

unusual lacustrine origins have been proposed. Sulfurisotope (Adler, 1974, p. 630) and whole-rock carbon, 

oxygen, and sulfur isotope studies (Ridgley and Goldhaber, 

1983, p. 414) suggest a marine rather than a lacustrine 

origin. 

Stratigraphy 9 
I 

Grand Junction 0 

i Montrose~B o Gunnison 

: (Ax\ \)-~ S I Telluride o ~ ~~-<\~~ s'r 0~ Limit of Pony Express 

~ . Limestone Member 

103' 

----- ~~~~go---

0 

I 

50 

I 

1 DO MILES 

I 

Figure 5. Areal extent of basal limestone of Wanakah 

Formation showing distribution of Pony Express Limestone 

Member in Colorado and equivalent Todilto Limestone Member 

in New Mexico. Stippled area indicates distribution of gypsum. 

A and B are locations discussed in text. Modified from Wright 

(195'9); eastern limits of Todilto Limestone Member in New 

Mexico from J.L. Ridgley (written commun., 1989). 

Pony Express Equivalent 

The Pony Express equivalent is recognized beyond 

the limits of the Pony Express Limestone Member. The 

Pony Express wedges out between the Red Canyon (sec. 9, 

fig. 3) and Bobcat trail (sec. 10) measured sections. From 

Bobcat trail north to Smith Fork (sec. 14), the Entrada 

Sandstone is overlain by a sandstone termed, for 

convenience, the "Pony Express equivalent." The Pony 

Express equivalent is 1.4-4.5 ft thick and yellowish gray or 

light brown except at Bobcat trail where it is reddish tan. It 

is fine grained and flatbedded and overhangs the underlying 

Entrada Sandstone. Sparse, rounded, medium grains are 

present at the Duncan trail (sec. 11) and Ute trail (sec. 12) 

measured sections. The Pony Express equivalent is a 

water-laid sandstone deposited at the periphery of the Pony 

Express Limestone Member. 

Bilk Creek Sandstone Member 

The Bilk Creek Sandstone Member overlies either the 

Pony Express Limestone Member or the Pony Express 

equivalent. It was named by Goldman and Spencer (1941, p. 

1750) for Bilk Creek, which drains into the San Miguel 

1 0 Wanakah and Morrison Formations in Southwestern Colorado 

River near section 1 (fig. 3). The Bilk Creek extends beyond 

the limits of the Pony Express Limestone Member along the 

Gunnison River (fig. 3) and along the San Miguel River 

(O'Sullivan, 1986). The Bilk Creek is light gray, greenish 

gray, and yellowish gray; the upper part of the member 

locally shows reddish-brown streaks or bands. The member 

is 20-33 ft thick and is mostly fine grained, although sparse 

medium grains were noted at Pleasant Park (sec. 8). Near 

Sawpit on the San Miguel River, Steele (1985, p. 9) found 

ostracodes and ooliths in the Bilk Creek. Bedding, wherever 

observed by the writer or by Steele ( 1985), is flat or not 

apparent; however, in the Placerville area on the San Miguel 

River, Bush and others (1959, p. 326) described some faint, 

small-scale, low-angle crossbedding. In general, the Bilk 

Creek Member is set back on underlying rocks and is poorly 

exposed, although it locally forms a small cliff (fig. 6). 

The Bilk Creek Sandstone Member is capped by a 

very distinctive unit termed the "carnelian sandstone marker 

bed." The name is derived from the presence at many places 

of "autochthonous red chert, scattered through it to some 

extent but more characteristically on its upper surface" 

(Goldman and Spencer, 1941, p. 1749). The carnelian 

sandstone is poorly sorted and fine grained. It contains 

sparse to abundant, rounded coarse quartz grains as much as 

about 1 mm across. Its bedding is flat or not apparent. In 

southwestern Colorado, Bush and others (1960, p. 441) 

noted that the carnelian marker bed "is remarkably 

persistent over an area covering perhaps several thousand 

square miles." 

The carnelian sandstone marker bed is thickest in 

western Black Canyon and pinches out westward near 

Norwood. In the Telluride-Ouray area the marker bed is 

only 1-2 ft thick; in western Black Canyon it is 7-14 ft 

thick. Both the thicker carnelian sandstone in western Black 

Canyon and the well-rounded grains disseminated 

throughout the sandstone suggest a nearby source. The 

coarse, well-rounded Entrada berries, a nearby potential 

source, could not have been the source because at the 

pinchout east of Chukar trail the Entrada Sandstone is 

directly and conformably overlain by the Pony Express 

Limestone Member (W.R. Hansen, U.S. Geological Survey, 

written commun., 1990). Perhaps the quartz-rich Precambrian metamorphic rocks were the source of the coarse 

grains during deposition of the carnelian sandstone. 

The carnelian sandstone marker bed is undoubtedly a 

reworked zone closely related to overlying strata. As a 

reworked zone it should be assigned as the basal part of the 

overlying beds at Sawpit of the Wanakah Formation. The 

base of the ledge-forming carnelian sandstone is sharp, 

possibly a scour surface (Steele, 1985, p. 31, 52), and the 

bed commonly overhangs (fig. 6) the underlying lower parts 

of the Bilk Creek Member. In addition, coarse-grained 

sandstone beds similar to the carnelian marker locally 

present in equivalents of the beds at Sawpit indicate a 

continuation of carnelian depositional conditions. The 
Figure 6. Bilk Creek Sandstone Member on west side of Leopard Creek, about 3 mi north of confluence of Leopard Creek and 

San Miguel River. Only top of Entrada Sandstone (E) expos1::!d. Wanakah Formation consists of Pony Express Limestone Member 

(PX), Bilk Creek Sandstone Member (BC) with carnelian sandstone marker bed (ern) at top, and slope-forming beds at Sawpit (SP); 

bed A (A) at base of Tidwell Member of Morrison Formation overlies Wanakah Formation. Bilk Creek Member is about 25ft thick. 

coarse, rounded quartz grains in the marker bed are 

probably a lag accumulation resulting from the winnowing 

action of currents. As interpreted by Bush and others (1959, 

p. 327), the "extremely persistent carnelian sandstone***probably was deposited entirely in a seaway, 

within reach of wave base, and its sediments were 

constantly reworked and spread over the sea floor as an 

even uniform blanket." 

Beds at Sawpit 

The beds at Sawpit overlie the Bilk Creek Sandstone 

Member and are mostly a red sequence of sandstone and 

siltstone in strata that are generally less than 3 ft thick. The 

strata are mostly reddish brown to pale red and locally 

mottled or banded gray and commonly form a slope, parts of 

which cover the underlying bedrock. Gray and red ledgeforming sandstone beds, commonly less than 1 ft thick, and 

thin red clay beds are present at most locations. 

The beds at Sawpit differ somewhat from the 

Telluride-Ouray area to western Black Canyon. From the 

Telluride-Ouray area to western Black Canyon, they 

diminish about one-half in thickness, owing possibly to 

truncation beneath the J-5 unconformity. East of Chukar 

trail, possibly 10--15 mi, the beds at Sawpit lap out, and 

younger strata rest on Precambrian rocks. Additionally, in 

western Black Canyon the beds at Sawpit are characterized 

by beds, seams, and nodules of gypsum. 

The beds at Sawpit in western Black Canyon consist 

of three units, in ascending stratigraphic order, the lower 

marker bed, the bed at Chukar trail, and the upper beds. The 

lower marker bed is 1-3 ft thick and forms a conspicuous 

re-entrant under the bed at Chukar trail. The lower marker 

is made up of greenish-gray, locally gypsiferous siltstone 

and shale. The upper beds, 8-15 ft thick, are mostly 

reddish-brown siltstone and minor gray and greenish-gray 

bands and mottles. Locally, the upper beds contain nodules 

and thin seams of gypsum. 

Stratigraphy 11 
Table 2. Analytical data for gypsum samples from the bed at Chukar trail 

[Sample localities described in text by sample number. All results in percent. Leaders (-) indicate below detection limit. All samples are grab 

samples from gypsum outcrops. Gypsum content and purity determined according to American Society for Testing and Materials standards; 

data from Brown (1988, p. 20)) 

Sample Free CaS04 + Si02+ 

no. CaO so3 H20 H20* H20* Si02 insoluble 

1 28.4 39.4 0.30 17.4 85.5 10.0 11.4 

2 31.0 43.7 .40 18.9 93.6 3.1 4.6 

3a 30.5 42.7 .40 18.1 91.3 5.2 6.7 

3b 33.0 46.2 .50 19.8 99.0 .4 1.0 

*Combined. 

The bed at Chukar trail consists of gypsum, 6-9 ft 

thick, that forms a prominent ledge (see fig. 10) throughout 

western Black Canyon. Prospect pits in the bed at Chukar 

trail near Duncan trail and near Smith Fork indicate that 

attempts were made to evaluate or mine the gypsum. Grab 

samples (table 2) of the bed at Chukar trail from four 

localities contain 85-99 percent calcium sulfate (hydrated). 

Sample 1 is from an area of many prospects pits and 

trenches 1 mi south of the junction of Smith Fork with the 

Gunnison River and 0.5 mi north of the South Smith Fork 

measured section (sec. 13, fig. 3). Sample 2 was collected 

500ft south of the Ute trail measured section (sec. 12, fig. 

3). Samples 3a and 3b are from Duncan trail: 3b is from a 

prospect pit at the end of the access road to Duncan trail, 3a 

was collected 300 ft to the north. 

Morrison Formation 

The Morrison Formation overlies the Wanakah 

Formation. Throughout the area it is made up of three parts: 

( 1) the Tidwell Member at the base, (2) the Salt Wash 

Member, and (3) the Brushy Basin Member. The Tidwell 

Member rests on the J-5 unconformity and is discussed 

below. In the study area the Salt Wash Member is 300-400 

ft thick and the Brushy Basin Member is 275-350 ft thick 

(Craig and others, 1955, figs. 21, 29). East of Chukar trail 

the Salt Wash Member thins to 250ft (Hansen, 1965, p. 51, 

written commun., 1990). The Salt Wash is a sequence of 

light-colored, thick sandstone ledges interbedded with red 

and gray siltstone and shale. The overlying slope-forming 

Brushy Basin Member is mostly gray, green, and red 

siltstone containing minor amounts of limestone and conglomeratic sandstone. Only the lower part of the Morrison 

Formation is plotted on the restored section (fig. 3) and 

described in the text. 

Tidwell Member 

The Tidwell Member mostly forms an unbroken slope 

between the overlying thick sandstone ledges of the Salt 

Wash and the underlying Wanakah Formation. In the 

12 Wanakah and Morrison Formations in Southwestern Colorado 

C02 Fe20 3 MgO Al203 ~0 Na20 NaCI MnO Ti02 

0.73 0.19 0.13 1.80 0.38 0.06 0.03 

.29 .13 .11 .49 .13 .08 .003 .13 

.15 .14 .29 1.00 .18 .06 .03 

.22 .02 .01 .05 .04 .02 

Telluride-Ouray area the Tidwell is 50-70ft thick, whereas 

in western Black Canyon the member is much thicker, 

105-210 ft thick. Several subdivisions are recognized. Bed 

A is present at the base of the member throughout the area, 

although the sloping sequence above bed A is unnamed in 

the Telluride-Ouray area. In western Black Canyon, where 

the Tidwell is thicker, the interval above bed A is divided 

into the lower gypsiferous beds and the upper beds. Along 

parts of Smith Fork, the upper beds contain a thick, 

crossbedded eolian sandstone that is informally named the 

bed at Smith Fork. 

The conspicuous basal marker bed, bed A, is a 

yellowish-gray to white, fine-grained sandstone that is 

mostly coarse to very coarse grained. Bed A is 14-33 ft 

thick. Bedding is flat and commonly has well-formed 

current ripple marks. Some minor low-angle cross beds were 

observed at the Bobcat trail, Duncan trail, and South Smith 

Fork measured sections (sees. 10, 11, 13, respectively, fig. 

3). In the Telluride-Ouray area bed A contains many thin 

gray shale beds, whereas in western Black Canyon bed A 

contains gypsum in thin seams parallel with bedding or in 

coatings on the faces of joints. In the Telluride-Ouray area 

bed A forms a prominent and persistent ledge; its base is 

sharp, and the bed generally overhangs the underlying 

Wanakah Formation. In western Black Canyon bed A tends 

to form a slope covered with colluvium and cemented with 

gypsum. 

In the Telluride-Ouray area the Tidwell Member 

above bed A has varied aspects of color; lithology, and 

thickness. The slope-forming sequence between bed A and 

the Salt Wash Member is about 20-50 ft thick and consists 

primarily of red or gray siltstone. Thin, dense, gray 

limestone beds and gray, ledge-forming sandstones in 

which bedding is flat or not apparent crop out at many 

locations. Thin beds of red claystone and sandstone are 

distributed throughout the sequence above bed A. 

In western Black Canyon the lower gypsiferous beds 

overlie bed A and are about 55-105 ft thick. They form a 

gray slope except for the basal 12-20 ft, which is red. The 

lower beds are thinly bedded siltstone and minor sandstone 

and shale, mostly in beds less than 1 ft thick. Some 

sandstone beds, however, are as thick as 12 ft. Gypsum 
throughout the unit forms beds, very thin crosscutting 

seams, nodules, and disseminations. Locally, the lower 

gypsiferous beds contain as many as 22 distinct beds of 

gypsum, mostly less than 1 ft thick, but some as thick as 

7ft. 

The upper beds at the top of the Tidwell Member 

form a gray slope interrupted by several ledge-forming beds 

of sandstone and limestone. Because the top of the upper 

beds is at the base of the lowest thick channel sandstone of 

the Salt Wash Member, the upper beds are from about 115 

ft thick at Smith Fork (sec. 14) to as little as 18ft at Bobcat 

trail (sec. 10, fig. 3). The upper beds lack gypsum; they 

consist mostly of siltstone, limestone, and minor sandstone. 

Sandstone beds are gray to yellowish gray, flatbedded, 

locally crossbedded at low angles, and mostly less than 3 ft 

thick. Limestone beds are gray and form ledges 1-3ft high. 

Some sections contain as many as 13 limestone beds. 

Several limestone beds at the Duncan trail section (sec. 11, 

fig. 3) and elsewhere contain numerous charophytes on 

bedding planes. A limestone bed near the top of the upper 

beds contains Echinochara spinosa, indicating a 

Kimmeridgian age (R.M. Forester, U.S. Geological Survey, 

oral commun., 1986). 

The bed at Smith Fork is in the upper beds of the 

Tidwell Member in the Smith Fork area (sec. 14, fig. 3), 90 

ft above the J-5 unconformity. It is 73ft thick, light gray to 

white, fine grained, and highly cross bedded. The upper 10 

ft, however, is yellowish gray, and its bedding is flat or not 

apparent. The bed at Smith Fork is considered to be of 

eolian origin. It pinches out southward and is not present at 

the South Smith Fork measured section (sec. 13, fig. 3), less 

than 2 mi to the south. Less than 1 mi east of the Smith Fork 

measured section, the bed at Smith Fork dips beneath 

younger rocks. 

The bed at Smith Fork is similar to other eolian 

sandstones in the Morrison Formation in nearby areas. At 

Bridgeport, about 30 mi northwest of Smith Fork, a crossbedded sandstone near the middle of the Tidwell Member is 

interpreted as an eolian lens because of bedding and 

weathering characteristics. The bed at Bridgeport is as thick 

as 45 ft thick and lies 70 ft above the base of the Morrison 

Formation, but it is present only locally. A similar crossbedded eolian sandstone is locally present along Escalante 

Creek about 25 mi west of Smith Fork; it is 65 ft thick and 

lies about 35 ft above the J-5 unconformity at the base of 

the Morrison Formation. Near Placerville on the San Miguel 

River a sandstone 47 ft thick lies 79 ft above the J-5 

unconformity. It is described as "abundantly cross bedded 

on large scale. Forms smooth rounded cliff like those in 

Entrada sandstone; not characteristic of Salt Wash" (Bush 

and others, 1959, p. 335). The sandstone at Smith Fork, the 

sandstone at Bridgeport, the sandstone at Escalante Creek, 

and the sandstone at Placerville all are eolian and of limited 

extent (see fig. 9). They are similar to other eolian sandstone 

beds in the lower part of the Morrison Formation at 

scattered locations in western Colorado. 

Gypsum in Morrison Formation 

The flora and fauna of the Morrison, together with a 

multitude of bedding features, support a terrestrial origin for 

most of the Morrison. Locally, however, some of the basal 

part of the Morrison may be marine or quasi-marine. Imlay 

(1980, p. 95), for instance, stated that "the presence of 

massive gypsum at the base of the Morrison Formation in 

the San Rafael Swell***suggests deposition in lagoons at 

the margin of a shallow sea***." Similarly Holt (1940) 

noted that near Grand Junction "basal beds of the Morrison 

formation***were deposited in lagoons in the borders of the 

Upper Jurassic***sea." These marine or marginal-marine 

beds in the basal part of the Morrison correlate with the 

quasi-marine Windy Hill Sandstone Member of the Sundance Formation overlying the J-5 unconformity in northcentral Colorado and adjacent parts of Wyoming. 

The Windy Hill Sandstone Member is assigned to the 

Sundance Formation partly because it is too thin to map 

separately and partly because its marine or brackish-water 

fauna is thought to be more closely related to fauna of 

underlying parts of the Sundance than to the terrestrial fauna 

of the Morrison Formation (Pipiringos and O'Sullivan, 

1976). The Windy Hill interfingers with and grades upward 

into the Morrison Formation, however, and is therefore 

closely related to the Morrison. Furthermore, the Windy 

Hill is everywhere separated from the middle and lower 

parts of the Sundance by the widespread regional J-5 

unconformity. 

The gypsum in the Morrison Formation is 

undoubtedly a product of deposition within the Windy Hill 

sea, which covered parts of northern Colorado and much of 

Wyoming and adjacent areas. The gypsum was deposited in 

narrow embayments that extended southward from the 

Windy Hill sea. One such embayment in Utah is termed the 

"San Rafael embayment" (fig. 7). In Colorado there are two 

similar features: the Black Canyon embayment in the study 

area and the Front Range embayment in eastern Colorado. 

The gypsum in the San Rafael and Black Canyon embayments is in the Tidwell Member of the Morrison, which lies 

beneath the Salt Wash Member. The gypsum in the Front 

Range embayment is probably in the basal part of the 

Morrison Formation in an area where the Salt Wash is not 

recognized; however, the gypsum and associated strata in 

the lower part of the Morrison Formation in the Front Range 

embayment probably correlate with the Tidwell Member in 

western Colorado. 

Stratigraphy 13 
Figure 7. Paleogeography of western United States in 

Kimmeridgian time showing distribution of features associated 

with deposition of lower part of the Morrison Formation or 

equivalents during early Kimmeridgian. Heavy stipple 

indicates three gypsum-bearing embayments in Utah and 

Colorado. C, Canon City; K, Kassler; S, Colorado Springs; M, 

Morrison. Modified from Peterson (1988a, fig. 17) and Imlay 

(1980, fig. 1 0). 

The San Rafael embayment in Utah lies east and 

south of the San Rafael Swell but also covers parts of it. The 

gypsum beds in the Tidwell Member were deposited in an 

evaporite basin, as described by Peterson and TurnerPeterson (1987, p. 6). 

Gypsum in units as much as 60 ft (18 m) thick, occurs at 

the base of the Tidwell throughout much of the San Rafael 

Swell and northern Henry basin. The gypsum is 

noteworthy because it has carbon isotope values that 

suggest a marine origin (Northrop, 1982). [Actually, the 

carbon is from I imestone beds associated with the gypsum 

(F. Peterson, U.S. Geological Survey, oral commun., 

1990).] The isotopic signature plus tentative correlation of 

lowermost Tidwell beds with the shallow marine Windy 

Hill Member of the Sundance Formation of Wyoming and 

nearby areas suggests that the gypsum was derived from 

evaporation of marine waters in the Windy Hill seaway. 

Correlation of basal Tidwell and Windy Hill is based on 

stratigraphic position of both members just above the J-5 

unconformity and interfingering of both units with other 

14 Wanakah and Morrison Formations in Southwestern Colorado 

beds in the Morrison Formation (Pipiringos, 1968, fig. 3). 

An alternative interpretation of the gypsum beds, here 

considered less likely, is that they were deposited in saline 

lakes under highly evaporative conditions. By this 

interpretation, the marine isotopes would have been 

inherited from eroded older marine evaporite deposits 

such as those in the underlying Summerville Formation. 

The Black Canyon embayment in western Colorado is 

narrow and restricted and is defined by gypsum beds in the 

lower part of the Tidwell Member of the Morrison 

Formation. These beds extend from western Black Canyon 

into the Ridgway area. In western Black Canyon gypsum or 

gypsiferous beds were deposited throughout a stratigraphic 

interval as thick as 120 ft, and individual beds of gypsum 

are as thick as 7 ft. Gypsum, however, is absent from the 

Tidwell Member in outcrops along the northeastern margin 

of the Uncompahgre Plateau to the west of the study area 

and in the Black Canyon of the Gunnison National 

Monument to the east. 

On the north side of Dallas Creek, about 2 mi north of 

Ridgway and about 30 mi south of Chukar trail, the upper 

70 ft of the Tidwell Member crops out in cliffs. At a 

measured section in the NW1

.4SE1.4SE1.4, sec. 5, T. 45 N., R. 

8 W., the upper 45ft of the lower gypsiferous beds includes 

four thin gypsum beds and a gypsum pod about 25 ft thick. 

The gypsum beds are absent about 350 ft to the west. The 

Tidwell Member reappears at the surface along the 

Uncompahgre River about 5 mi south of the location just 

described and again lacks gypsum. 

The Front Range embayment of the Windy Hill 

seaway is also probably narrow and restricted, although its 

limits are incompletely known. Thick beds of Jurassic 

gypsum that define the embayment crop out at several 

locations along the Front Range. Near Colorado Springs a 

gypsum unit as thick as 1 00 ft is placed at the base of the 

Morrison Formation (Noblett and others, 1987, p. 336). In 

the Canon City area gypsum as thick as 36 ft (Frederickson 

and others, 1956, p. 2144) is assigned to the Ralston Creek 

Formation (formerly the Ralston Formation). Gypsum beds 

to the north and south of the Kassler area are assigned to the 

Ralston Creek Formation (Scott, 1963, p. 91-92). A bed of 

gypsum 50ft thick (LeRoy, 1946, p. 53) is at the base of the 

Ralston Creek Formation 2.5 mi south of Morrison . 

The Ralston Creek Formation is closely related to the 

Morrison Formation. In discussing the Ralston Creek 

Formation, Scott (1963, p. 91) noted its "lithologic 

similarity to parts of the Morrison Formation." LeRoy 

(1946, p. 55), in describing the Ralston Creek Formation of 

Jurassic age and underlying Lykins Formation of Triassic 

and Permian age, stated that "Lithologically the sediments 

are more closely related to the overlying basal Morrison 

strata than to the underlying Lykins. At Ralston Creek, 

Aclistochara was noted in the uppermost part of the 

formation. This algal form is identical to that persistently 

present in the lower 40 or 50 feet of the Morrison." 
Furthermore, fossils indicate that the Ralston Creek 

Formation is Kimmeridgian in age (Scott, 1963, p. 92), the 

same age as the Morrison Formation. 

The gypsum-bearing beds in the Front Range embayment probably should be assigned to the Morrison 

Formation as a lower member. The term "Ralston Creek 

Formation" seems inappropriate and unnecessary because it 

is treated in different ways. At the type section a sandstone 

bed 17 ft thick at the base of the Ralston Creek Formation 

is equivalent to the Canyon Springs Sandstone Member of 

the Jurassic Sundance Formation, which is an equivalent of 

the Entrada Sandstone. The beds above the Canyon Springs, 

at the type section, are equivalent only to the Morrison 

Formation (see Pipiringos and O'Sullivan, 1976). Scott 

(1963, p. 91), however, excluded a possible Entrada 

Sandstone (or Canyon Springs equivalent) from the strata he 

assigned to the Ralston Creek Formation. The gypsum beds, 

whether assigned to the Ralston Creek or Morrison, 

probably are the same age as gypsum beds in the Black 

Canyon and San Rafael embayments. They represent an 

episode of evaporation in restricted embayments at the 

beginning of Morrison time-the final Jurassic marine 

invasion-an event related to the Windy Hill sea. 

Previous Correlations 

The stratigraphic arrangement of Jurassic rocks in 

southwestern Colorado was set forth by Burbank (1930). 

The formation at the base of the Jurassic is the Entrada 

Sandstone (fig. 8). Although Burbank (1930, p. 171) 

acknowledged the correlation of the sandstone at the base of 

the Jurassic with the Entrada because of a personal 

communication from J.B. Reeside, Jr., he did not use the 

name because the correlation was then unpublished. The 

distinctive Pony Express Limestone Member of the Wanakah above the Entrada forms an easily recognized marker 

wherever it is present. The Brushy Basin Member of the 

Morrison Formation at the top of Jurassic rocks also forms 

a distinctive stratigraphic unit. 

The strata between the Pony Express Limestone and 

the Brushy Basin Members have been referred to in various 

ways. The Wanakah was originally defined as a member of 

the Morrison Formation. Later, the Wanakah was raised to 

formational rank and removed from the Morrison Formation 

(Eckel, 1949, p. 27-28). The Wanakah is commonly applied 

to the strata that Burbank selected for the unit (fig. 8). Near 

Bilk Creek, however, the term was restricted (Goldman and 

Spencer, 1941, p. 1753) to a part of the Wanakah as defined 

by Burbank; in the Black Canyon of the Gunnison the 

Wanakah as used by Hansen ( 1971) includes more than 

Burbank assigned to it, chiefly because beds now assigned 

to the Tidwell have a lithologic character more like the 

Wanakah than the overlying Salt Wash and because the 

Tidwell-Salt Wash contact is an easily mapped horizon. 

The Bilk Creek Sandstone Member with the carnelian 

sandstone marker bed at its top overlies the Pony Express 

Limestone Member. At the Cedar Hill section of the 

Wanakah, measured 6 mi north of Ouray, a bed described as 

a hard, greenish-gray sandstone 1 ft thick and containing 

grains of bright-red chert (Burbank, 1930, p. 174, unit 

number 6) is the carnelian sandstone. The carnelian 

sandstone was assigned as the basal unit of the upper shale 

beds, which are equivalent mainly to the beds at Sawpit as 

used herein. Later the carnelian sandstone was included 

with the Bilk Creek Sandstone Member by Goldman and 

Spencer (1941, p. 1750, 1752) at their measured type 

section opposite the mouth of Bilk Creek. 

The beds at Sawpit have previously been designated 

the marl member or upper unit. Near Ouray, equivalent beds 

were first termed the upper shale beds of the Wanakah 

Member of the Morrison Formation (Burbank, 1930, p. 

176-177). Later Goldman and Spencer (1941, p. 1750) 

noted that because of "the high carbonate content of the 

argillaceous beds of this member, marl seems the 

appropriate term to apply to them." Bush and others (1959, 

p. 328) believed that the term marl was somewhat 

misleading and that "limy siltstone appears to be a far more 

accurate descriptive term." Marl, for instance, has been 

defined by Crandell (1958, p. 8) as "a partly consolidated 

sedimentary rock consisting of calcium carbonate and clay. 

If the material is more than 75 percent calcium carbonate, 

the rock is an argillaceous variety of limestone; if it is more 

than 75 percent clay, the rock is calcareous claystone or 

shale." Inasmuch as the descriptive term marl seems 

inappropriate, the unit is now referred to as the beds at 

Sawpit. 

The Morrison Formation has been interpreted in 

different ways in the study area. It has included all strata 

between the Entrada and Cretaceous rocks (fig. 8) or all the 

strata or a part of the strata between the Wanakah, as 

defined by Burbank, and Cretaceous rocks. As used in this 

study the base of the Morrison is at the J-5 unconformity, a 

useful, convenient, and widespread break in Jurassic rocks 

(see O'Sullivan, 1984.) Gilluly (1929, p. 111) placed the 

base of the Morrison in the San Rafael Swell at an 

unconformity that is "both angular and erosional" when he 

extended the formation into Utah. Moreover, the 

unconformity recognized by Gilluly (1929, p. 11 0) is the 

J-5 surface at the upper boundary of the San Rafael Group. 

In the Telluride-Ouray area the J-5 unconformity coincides 

with the base of the Morrison as used near Placerville by 

Bush and others (1959) and Steele (1985). The J-5 surface 

also is the same as the basal Morrison contact mapped by 

Burbank and Luedke (1966) in the Telluride area. Near 

Ouray the base of the Morrison as mapped by Luedke and 

Burbank (1962) and the top of the Wanakah as defined by 

Burbank (1930, p. 172-179) coincide with the J-5 

unconformity. 

Stratigraphy 15 
-

~ 

f 

l 

! 

c. 

f 

., 

~· 

= 3' 

3 

~ 

a· 

a 

s· 

~ 

$. 

::r 

I ., 

= 

n 

Q 

C5" 

i 

Q 

Goldman and Spencer Kelly Burbank (1930) (1941, Bilk Creek area, This report (1957) table 1) 

Cretaceous 

rocks 

Brushy Basin Brushy Basin Shale member Member shale member 

I ~I' Salt Wash c: 

0 Member -;;:::; 

co 

Not E Upper 

0 discussed 

; Bed at Junction u.. Creek Sandstone c: Smith 0 

member !/) Qj Fork sandstone ·;: ..c member 0 E beds c: :2: c: Q) c: 0 0 :2: 0 ... ·.;::; Lower ·.;::; co co a; co E E s: gypsiferous E .E 0 "0 beds .E .,_ F c: c: -------- c: 0 0 Junction Creek 0 !/) !/) !/) Lower quartzite Lower quartzite ·;: sandstone Bed A ·;: 

0 0 member (absent) 0 of m~ners 

:2: 1j( :2: :2: 

Wanakah marl Upper Beds at Upper Units member c: Qj shale 0 Saw pit Qj unit 7-19 (restricted) ·.;::; ..c beds a.. ..c E co t--- :::1 E Q) E Unit 6 0 Bilk Q) 

E Bilk Creek a: 0 Creek E Bilk .c: sandstone (!) u.. .c: co Sandstone ...J ~ 

.c: Sandstone co Creek(?) bed member w co Member ~ 

co ~ ~ co c: co c: co c: 

~ Pony Express Pony Express ~ <( co a: Pony Express Pony ~ beds limestone z Limestone Express 

member <( Member beds (/) 

Upper(?) Jurassic Entrada 

sandstone sandstone 

Triassic 

Figure 8. Nomenclature of jurassic rocks in southwestern Colorado. 

Bush and others Chapin and others Steele ( 1985, 

(1959) (1968, p.78) Hansen (1971) Cutler Creek, p. 26-30) 

Cretaceous rocks 

Brushy Basin Brushy Basin Brushy 

c: c: Basin Not discussed shale member Member o.Q Member !/) ... 

·- co ~I' t E 0 .... c: Salt Wash :2:0 0 

·.;::; Member u.. 

c: co 0 E ·.;::; 0 Upper c: co 0 E Salt Wash u.. Salt \:unct;on ·.;::; 

.E c: co 

sandstone 0 Wash Creek E !/) 

c: member ·;: Member Member 0 Salt 0 0 !/) u.. Wash :2: 

part ·;: c: 

0 Member 0 !/) 

·;: :2: 0 

:2: 

c: 

0 

Junction Creek ... Even-bedded co 

E u~t Sandstone 0 Middle u.. 

.c: part co Marl "Marl" ~ Marl c: co 0 member c: member c: c: member ·.;::; 0 co 0 ·.;::; ~ ·.;::; co co co E E E 0 Bilk Creek 0 Bilk Creek 0 Bilk Creek .,_ sandstone u.. Sandstone .c: u.. Sandstone co member ~ .c: .c: Member co Member co co ~ ~ 

c: co co 

~ c: c: Pony Express ~ Pony Express Pony Express ~ Pony Express 

limestone Limestone Limestone Limestone 

member Member Member Member 

Entrada Sandstone 

rocks 
Different terms have been applied to the basal 

sandstone, where recognized, of the Morrison Formation. 

The "lower quartzite" (Burbank, 1930, p. 177) overlying the 

Wanakah Formation near Ouray, the "even-bedded unit" 

(Bush and others, 1959, plate 6) near Placerville, and bed A 

of this study are the same. Bed A is also the same as the 

"lower quartzite of miners" of the "Junction Creek 

sandstone member of Morrison formation" near Ouray 

(Kelly, 1957, p. 203). Bed A was not separately recognized 

in the Black Canyon of the Gunnison by Hansen (1971) or 

in the Placerville area by Steele (1985). Near Bilk Creek the 

Junction Creek Sandstone, as used by Chapin and others 

(1968, p. 78), is described as an "even-bedded sandstone 

forming [a] smooth vertical cliff' that is 26ft high, and it is 

the same as bed A as used herein. 

The Tidwell Member has not heretofore been 

identified in the area, mostly because the Tidwell was only 

recently named and described (Peterson, 1988b, p. 35-42). 

Instead, the slope-forming Tidwell has generally been 

incorporated in the Salt Wash Member, despite its distinctly 

different lithology. On the Uncompahgre Plateau near 

Grand iunction the Tidwell was formerly correlated 

erroneously with the Summerville Formation (Holt, 1940); 

however, the Tidwell has been included within the Junction 

Creek Sandstone Member near Ouray (Kelly, 1957, p. 203) 

and within the Wanakah Formation in the Black Canyon of 

the Gunnison (Hansen, 1971), although W.R. Hansen 

(written commun., 1990) recognized its temporal 

equivalence with the lower part of the Morrison. The Salt 

Wash was originally named by Lupton (1914, p. 127) for a 

series of ledgy sandstones that form high cliffs near Green 

River, Utah. The Salt Wash as used by Lupton (1914) near 

Green River, by Hansen (1965, 1968, 1971) in the Black 

Canyon, and in this study is the same. 

The name Junction Creek Sandstone has been applied 

to different stratigraphic intervals in the study area. Chapin 

and others (1968, p. 78) used the name for a sandstone, 26 

ft thick, overlying the Wanakah Formation and below their 

Morrison Formation. Kelley (1957, p. 203) applied the 

name Junction Creek Sandstone Member of the Morrison to 

250-300 ft of beds between the Wanakah Formation and the 

Brushy Basin Member of the Morrison. In western Black 

Canyon, a thick eolian sandstone along Smith Fork and 

farther east on the north rim of the Black Canyon was 

termed the Junction Creek Sandstone Member of the Wanakah Formation (Hansen, 1968, 1971). 

The name Junction Creek, derived from a creek near 

Durango, was applied to a thick, crossbedded sandstone in 

the Durango-La Plata Mountains area. As described by 

Goldman and Spencer (1941, p. 1749-1750), the Junction 

Creek Sandstone "is divided conspicuously into two parts, a 

lower more horizontally banded part with diagonal bedding 

within the horizontal layers, and an upper part with diagonal 

bedding on a large scale, that is, a single direction of 

diagonal bedding extending across a horizontal layer as 

much as 60 feet thick and continuing parallel with itself 

over long stretches, suggesting big decapitated dunes. This 

sandstone attains thicknesses as great as 500 feet." Goldman 

and Spencer (1941, p. 1750-1751) treated the sandstone as 

the Junction Creek Sandstone Member of the Morrison 

Formation. Near Durango the Junction Creek Sandstone is 

mainly of early Morrison age, although its base may be 

older. 

The Junction Creek Sandstone is distributed over a 

part of southwestern Colorado (fig. 9). It forms a very thick 

and extensive curved lens in the La Plata MountainsDolores area, where it is 300-500 ft thick (Goldman and 

Spencer, 1941, fig. 1). From the thick lens the Junction 

Creek thins to the north and south. Along the Dolores River, 

about 20 mi southeast of Dove Creek, the sandstone was 

mapped to a northward pinchout by Haynes and others 

(1972). The Junction Creek was not recognized at Dunton 

(Goldman and Spencer, 1941, fig. 1). Along the Piedra 

River, east of Durango, the Junction Creek thins northward 

(Bush and others, 1983) from 120 to 25ft, 4 mi and 15 mi, 

respectively, north of the settlement of Piedra (fig. 9, 

locations A and B). The rate of thinning along the Piedra 

River suggests, but does not prove, that the Junction Creek 

may pinch out not very far north of location B. A line 

connecting the pinchout along the Dolores River with a 

point south of Dunton (shown by an X in fig. 9) and a point 

near location B on the Piedra River forms a convenient, 

although partly arbitrary, northern limit of Junction Creek 

Sandstone. Extension of the name Junction Creek north of 

this line is unnecessary and misleading. 

The name Junction Creek has been used, however, 

north of the line just described. At Bilk Creek, Junction 

Creek Sandstone was applied (Chapin and others, 1968) to 

bed A of the Tidwell Member as used in this study. In the 

Placerville area, Bush and others (1959) used "even-bedded 

unit" for bed A. In discussing the unit, Bush and others 

(1959, p. 331-332) stated that "The relationship of the unit 

to the Junction Creek sandstone of Goldman and Spencer 

( 1941) has not been determined, but the unit appears to be 

absent where typical Junction Creek is present. The unit 

may be a facies of the Junction Creek, with the sediments 

deposited in, and reworked by, the waters of a waning Late 

Jurassic seaway." Bed A most likely correlates with the 

lowermost part of the Junction Creek Sandstone; however, 

the even-bedded sandstone of bed A is best assigned to the 

Tidwell Member and can easily be separated from the thick, 

crossbedded, eolian Junction Creek Sandstone. 

The name Junction Creek has been used at Ouray and 

in western Black Canyon. At Ouray the Junction Creek 

Sandstone used by Kelley (1957) as a member of the 

Morrison Formation is clearly equivalent to the Tidwell and 

Salt Wash Members and is unlike typical Junction Creek 

south of Dunton. In western Black Canyon a thick eolian 

sandstone in Smith Fork has been called Junction Creek 

Member of the Wanakah Formation (Hansen, 1968). In this 

Stratigraphy 17 
NEW MEXICO 0 

I 

10 

I 

20 

I 

30 

I 

40 MILES 

I 

EXPLANATION 

I Volcanic rocks 

' Alluvial fill 

Junction Creek Sandstone 

Figure 9. Distribution of Junction Creek Sandstone in southwestern Colorado. A and B along Piedra River are 

locations discussed in text. Modified partly from L.C. Craig (U.S. Geological Survey, written commun., 1973). 

Distribution of volcanic rocks and alluvial fill from King and Beikman (1974). 

study the sandstone is termed "bed at Smith Fork" and is 

considered an isolated eolian sandstone of limited extent 

(fig. 9) positioned about 90 ft above the base of the Tidwell 

Member of the Morrison Formation. The bed at Smith Fork 

passes below drainage eastward along Smith Fork, so its 

true extent is unknown; however, it is probably a restricted 

local body of eolian sandstone because similar lenses of 

eolian sandstone-the bed at Bridgeport, the bed at Escalante Creek, and the bed at Placerville-are of limited extent 

(fig. 9). 

The Junction Creek has also been recognized along 

other parts of the Gunnison River. From near Curecanti 

Creek (fig. 9) eastward to Gunnison a sandstone generally 

18 Wanakah and Morrison Formations in Southwestern Colorado 

overlying Precambrian rocks has been termed Junction 

Creek Sandstone. Along Curecanti Creek the sandstone is 

6--10 ft thick and is flat bedded, but a thicker eolian 

sandstone at about the same stratigraphic level crops out 

near Blue Mesa Dam and at many places upstream from the 

dam. The relationship between the sandstone along Curecanti Creek and the bed at Smith Fork is uncertain, but the 

physical appearance is similar. The bed does not crop out 

anywhere along the Gunnison River from a point about 3 mi 

west of Curecanti Creek (where the bed is in a faulted 

window between Tertiary volcanic rocks and Precambrian 

gneiss) to Smith Fork (Hansen, 1971). West of Curecanti 

Creek, however, the entire Jurassic section has been 
stripped off for several miles by early Tertiary, pre-volcanic 

truncation. Near Cebolla Creek, the bed mapped as Junction 

Creek Sandstone is 50-100 ft thick, generally has largescale crossbedding, and probably is of eolian origin (Hedlund and Olson, 1973). Near Almont, north of Gunnison, the 

eolian crossbedded sandstone is 120 ft thick (Craig and 

others, 1959, measured section 2) and is referred to as 

Junction Creek Sandstone (Tweto, 1983). The sandstone 

along the Gunnison River east of Curecanti Creek is far to 

the north of the known distribution (fig. ·9) of typical 

Junction Creek Sandstone in south westernmost Colorado. 

The geologic record between the two areas is either 

obscured by Cenozoic volcanic rocks of the San Juan 

Mountains or has been removed by post-Jurassic erosion. A 

sandstone has been identified as Junction Creek (Gries, 

1989, p. 72) in a drill hole through the volcanic cover in the 

eastern San Juan Mountains near Del Norte; thus, the total 

extent of the Junction Creek must be very broad. The 

extension of the name Junction Creek Sandstone into the 

Gunnison area seems tenuous. The sandstone might better 

be recognized as an extensive lentil of Morrison age of, as 

yet, uncertain correlation. 

Reference Section 

A reference section for the Wanakah Formation and 

the Tidwell Member of Morrison Formation in western 

Black Canyon is described below to show the thickness and 

arrangement of the various parts of the San Rafael Group 

and the lower part of the Morrison Formation. The section 

is well exposed (fig. 10) along the east-facing cliffs 

bordering the Gunnison River. The section is the nearest 

exposure of the Wanakah and related rocks to the north of 

the Wanakah mine near Ouray. The section (fig. 11) is 

accessible along a dirt road that ends in a parking area at the 

head of the Chukar trail, which leads down to the Gunnison 

River. The dirt road, impassable in wet weather, connects 

with paved Falcon Road, which in tum connects, about 10 

mi north of Montrose, with U.S. Highway 50. 

Black Canyon reference section for the Wanakah Formation 

and its Pony Express Limestone and Bilk Creek Members and 

the Tidwell Member of Morrison Formation 

[Measured about 1/2 mi west of Chukar trail] 

Morrison Formation (part): 

Salt Wash Member (part): 

Thickness 

(feet) 

82. Sandstone, gray, stained red by wash from above; 

fine grained, faintly crossbedded; contains 

flattened greenish-gray clay galls as much as 2 

in. across; base is a scour surface; forms cliff.. 15.0+ 

Total thickness measured Salt Wash Member................ 15.0+ 

Tidwell Member: 

Thickness 

(feet) 

Upper beds: 

81. Siltstone, greenish-gray; forms re-entrant ............. 1.3 

80. Limestone, medium-gray; dense; forms ledge ...... 3.1 

79. Siltstone, greenish-gray; forms re-entrant ............. 0.9 

78. Limestone, medium-gray; dense; forms ledge ...... 1. 7 

77. Siltstone, greenish-gray; weathers nodular in 

vertical cliff; two thin sandstones in upper part. 4.4 

76. Sandstone, gray; fine grained; flatbedded; forms 

ledge ................................................................... 0.6 

75. Siltstone, greenish-gray; weathers nodular in a 

vertical cliff ........................................................ 2.9 

7 4. Sandstone, yellowish-gray, weathers brown; very 

fine grained; flatbedded; forms hard ledge ...... 1.3 

73. Siltstone, banded reddish-brown and mediumgray; forms a well-exposed slope ..................... 11.7 

72. Sandstone, yellowish-gray; very fine grained; flatbedded; forms a ledge ....................................... 0.4 

71. Siltstone, medium-gray; forms a well-exposed 

slope ................................................................... 5.5 

70. Limestone, light-gray; forms a ledge .................... 0.7 

69. Siltstone, medium-gray; forms well-exposed slope. 6.0 

68. Sandstone, medium-gray; very fine grained; flatbedded; forms a hard ledge ............................... 0.6 

67. Siltstone, medium-gray; forms poorly exposed 

slope ................................................................... 2.4 

66. Limestone, medium-gray; dense; forms ledge ...... 1.2 

65. Siltstone, medium-gray and reddish-brown; forms 

a poorly exposed slope ...................................... 14.0 

64. Sandstone, yellowish-gray; very fine grained; flatbedded; forms hard ledge . .. .. .. .. .. .. ... . .. .. ... ....... ... 0.5 

63. Siltstone, medium-gray, forms poorly exposed 

slope ................................................................... 4.0 

62. Sandstone, yellowish-gray; flatbedded; forms a 

ledge ................................................................... 0.4 

61. Siltstone, medium-gray, and sandstone, yellowishgray; interbedded in a poorly exposed slope .... 5.0 

60. Sandstone, yellowish-gray; irregularly bedded, 

some poorly defined, very low angle crossbeds; 

forms a ledge ..................................................... 2.0 

Thickness of upper beds ......................................... 70.6 

Lower gypsiferous beds: 

59. Siltstone, mostly medium-gray and minor reddishbrown; clayey; gypsum nodules 3-4 in. across 

in lower half; poorly exposed in a slope ......... . 

58. Interbedded greenish-gray and medium-gray siltstone, white sandstone, and gypsum in thin beds 

0.1-0.4 ft thick; forms conspicuous ledge ........ 

57. Sandstone, white: poorly defined, very low angle 

crossbeds; interstitial gypsum; forms hard ledge 

56. Siltstone, medium-gray; contains gypsum nodules . 

55. Sandstone, white; very fine grained; forms slight 

ledge .................................................................. . 

54. Clay, medium-gray; fissile; contains nodules of 

gypsum as much as 2 in. across ..................... .. 

53. Sandstone, white; very fine grained; forms slight 

ledge .................................................................. . 

52. Siltstone, medium-gray; contains seams and thin 

beds of gypsum ................................................. . 

51. Gypsum, white; contains some interstitial gray 

siltstone ............................................................. . 

15.1 

17.8 

4.1 

0.5 

1.1 

2.3 

0.6 

2.5 

0.5 

Stratigraphy 19 
Figure 10 (above and facing page). Wanakah Formation and related rocks at Chukar trail. View looking north. Entrada Sandstone 

(E). Wanakah Formation comprises Pony Express Limestone Member (PX), Bilk Creek Sandstone Member (BC) with carnelian 

sandstone marker bed (ern) at top, and beds at Sawpit (SP) including gypsum layer termed bed at Chukar trail (Ct). Morrison 

Formation in view consists of Tidwell Member with bed A (A), lower gypsiferous beds (GB), and upper beds (UB) and Salt Wash 

Member (SW). 

Lower gypsiferous beds-Continued 

Thickness 

(feet) 

50. Siltstone, medium-gray and streaks of dark yellow 

gray; contains vertical seams of gypsum .......... 0.9 

49. Gypsum, white ....................................................... 0.5 

48. Siltstone, medium-gray; contains seams and thin 

beds of gypsum; forms slope ............................ 3.8 

47. Gypsum, white; contains thin seams of gray siltstone ................................................................... 2.3 

46. Siltstone, medium-gray, stained and streaked 

yellowish-gray; contains gypsum seams; forms 

slope ................................................................... 0.8 

45. Gypsum, white ....................................................... 0.4 

44. Siltstone, medium-gray, streaked yellowish-gray; 

forms slope ........................................................ 0.8 

43. Gypsum, white ....................................................... 0.2 

42. Siltstone, medium-gray, spotted, dark-yellowishgray; contains gypsum seams; forms slope ...... 1.1 

41. Gypsum, white; forms a slight ledge .................... 0.6 

40. Siltstone, medium-gray; clayey; contains gypsum 

nodules and seams; forms a slope...................... 1.2 

39. Gypsum, white; contains a thin gray clay seam 

near the middle; forms a slight ledge ............... 0.9 

20 Wanakah and Morrison Formations in Southwestern Colorado 

38. 

37. 

36. 

35. 

34. 

33. 

32. 

31. 

30. 

29. 

28. 

Shale, gray, fissile; minor thin reddish-brown and 

yellowish-gray siltstone; contains some thin 

gypsum seams and nodules; near middle is 

highest continuous red-bed in lower part of 

Tidwell Member; forms slope .......................... . 

Gypsum, white;· minor interbedded gray siltstone 

forms slight ledge ............................................. . 

Siltstone, banded reddish-brown, medium-gray, 

and yellowish-gray; shaley; forms slope ......... . 

Gypsum, white; forms slight ledge ...................... . 

Siltstone, banded reddish-brown, medium-gray, 

and dark-yellowish-gray; forms slope ............. . 

Sandstone, white; very fine grained; very hard; 

although hard forms a slope ............................. . 

Siltstone, streaked medium-gray and reddishbrown; thin white gypsum seam near middle ... . 

Gypsum, white; forms slope ................................. . 

Siltstone, medium-gray; forms slope ................... . 

Siltstone, reddish-brown; forms slope .................. . 

Siltstone, reddish-brown; contains nodules of 

gypsum and a thin bed of gypsum in upper 0.3 

ft ........................................................................ . 

Thickness 

(feet) 

2.6 

0.5 

1.0 

0.9 

0.6 

0.3 

1.0 

0.4 

1.1 
Lower gypsiferous beds-Continued 

Thickness 

(feet) 

27. 

26. 

25. 

24. 

23. 

22. 

21. 

20. 

Siltstone, reddish-brown; forms slope ................. .. 

Sandstone, reddish-brown; some interstitial 

gypsum and some gypsum in thin seams; forms 

slight ledge ......................................................... . 

Siltstone, reddish-brown; forms slope .................. . 

Gypsum, white; forms soft slope ......................... . 

Siltstone, reddish-brown; forms slope .................. . 

Gypsum, white; nodular; contains some brown 

siltstone ............................................................... . 

Siltstone, medium-gray, greenish-gray, and 

yellowish-gray; very fine grained sandy; foims 

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

Siltstone, reddish-brown; contains gypsum in thin 

seams and as nodules as much as 2 in. across; 

forms slope ......................................................... . 

19. Sandstone, yellowish-gray, weathers reddishorange; very fine grained; forms hard ledge ... . 

18. Siltstone, reddish-brown; forms slope .................. . 

Thickness of lower gypsiferous beds .................... . 

Bed A 

17. Sandstone, white; fine grained; abundant very 

coarse grains both scattered and clustered in thin 

stringers; base is rippled marked; two thin seams 

of gypsum less than 1 in. thick in lower 5 ft; 

forms ledge ......................................................... . 

1.3 

0.4 

1.4 

1.0 

0.9 

0.4 

1.8 

1.8 

0.3 

1.6 

78.8 

14.4 

Thickness of bed A ................................................. . 

Total thickness of Tidwell Member .............................. . 

Wanakah Formation: 

Beds at Sawpit: 

Upper beds: 

16. Siltstone, reddish-brown; small gypsum nodules 

scattered in lower 7.5 ft; a red, very fine grained 

sandstone 0.4 ft thick makes slight ledge 2.5 ft 

above base; a gray clay 1 in. thick 5.0 ft above 

base; a gray clay 0.3 ft thick 6.0 ft above base; 

forms slope ......................................................... . 

Bed at Chukar trail: 

15. Gypsum, white; forms conspicuous ledge ............. . 

Lower marker bed: 

14. Siltstone, greenish-gray; contains flattened gypsum 

nodules parallel with bedding and several feet 

long; makes re-entrant ....................................... . 

Total thickness of beds at Sawpit ..........•......................... 

Thickness 

(feet) 

14.4 

163.8 

14.5 

6.4 

3.0 

23.9 

Stratigraphy 21 
1 MILE 

1 KILOMETER 

Figure 11. Location of Chukar trail reference section. Base is from Red Rock 

Canyon 71f2-minute topographic quadrangle (1955); contour interval 20 ft. 

Bilk Creek Sandstone Member: 

Thickness 

(feet) 

Carnelian sandstone marker bed: 

13. Sandstone, gray; fine grained with abundant 

medium rounded grains; sandstone in beds at 

base, middle, and top separated by reddishbrown siltstone about 0.5 ft thick; siltstone 

contains nodules and seams of gypsum; 

sandstone forms ledges, siHstone forms reentrant ................................................................. . 4.7 

22 Wanakah and Morrison Formations in Southwestern Colorado 

12. Sandstone, greenish-gray with minor reddishbrown bands near middle; fine grained; sparse 

rounded medium grains; sparse pink limestone 

nodules as much as V2 in. across; silty and 

shaley .................................................................. . 

11. Sandstone, gray to brownish-gray; abundant 

rounded medium grains; forms slope ................ . 

Thickness of carnelian sandstone marker bed ...... . 

Thickness 

(feet) 

8.6 

1.1 

14.4 
Bilk Creek Sandstone Member-Continued 

Thickness 

(feet) 

10. Sandstone, yellowish-gray to greenish-gray; fine 

grained; flatbedded; a 0.3-ft-thick reddishbrown to brown shaley, silty, sandstone bed 

18.9 ft above base; gypsum nodules lh-lh in. 

across in lower 10 ft; forms slope ........ .... .... ..... 19.4 

Total thickness of Bilk Creek Sandstone Member ........ 33.8 

Pony Express Limestone Member: 

9. Limestone, black; petroliferous; thinly laminated 

and in beds as thick as 4 in. ............................. 1.9 

Total thickness of Pony Express Limestone Member ... 1.9 

Total thickness of Wanakah Formation ......................... 59.6 

Entrada Sandstone: 

8. Sandstone, yellowish-gray; fine grained; abundant 

rounded medium grains in upper part; lower 0.5 

ft contains flattened limestone nodules as much 

as 1.25 in. across; flatbedded .. .... .... .... .... ......... .. 1.5 

7. Sandstone, yellowish-gray, banded reddishbrown; upper part finely cross laminated.......... 3.4 

6. Sandstone, yellowish-gray; flatbedded; base is a 

nick...................................................................... 12.4 

5. Sandstone, yellowish-gray; flatbedded; base is a 

nick...................................................................... 4.5 

4. Sandstone, yellowish-gray; flatbedded .................. 6.0 

3. Sandstone, very light gray; cross bedded................ 1.5 

2. Sandstone, yellowish-gray with minor light-gray 

bands; flatbedded; unit in sharp color break with 

underlying sandstone.......................................... 20.0 

1. Sandstone, reddish-tan; a 0.7-ft-thick gray band 

31.6 ft above base; flatbedded; lower 7 ft 

contains black mica schist pebbles as much as 

10 in. across and pebbles of quartz as much as 

21/z in. across....................................................... 45.9 

Total thickness of Entrada Sandstone ............................ 95.2 

Precambrian: Dark-gray quartzitic gneiss. 

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24 Wanakah and Morrison Formations in Southwestern Colorado 

___ 1986, Stratigraphic sections of Middle Jurassic San Rafael 

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southwestern Colorado: U.S. Geological Survey Oil and 

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421-B, p. 71-125. 

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p. 61-66. 

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