Petrology and Depositional Setting of 

Mississippian Rocks Associated 

with an Anoxic Event at Samak, 

Western Uinta Mountains, Utah 

Petrology and Significance of a Mississippian 

(Osagean-Meramecian) Anoxic Event, 

Lakeside Mountains, Northwestern Utah 

U.S. GEOLOGICAL SURVEY BULLETIN 1787-S, T 
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Ave. 
Petrology and Depositional Setting of 

Mississippian Rocks Associated 

with an Anoxic Event at Saniak, 

Western Uinta Mountains, Utah 

By K.M. NICHOLS and N.J. SILBERLING 

Petrology and Significance of a Mississippian 

(Osagean-Meramecian) Anoxic Event, 

Lakeside Mountains, Northwestern Utah 

By K.M. NICHOLS and N.J. SILBERLING 

Chapters SandT are published together in a single volume and 

are not available separately. 

U.S. GEOLOGICAL SURVEY BULLETIN 1787-S, T 

EVOLUTION OF SEDIMENTARY BASINS-UINTA AND PICEANCE BASINS 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

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Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Nichols, K. M. (Kathryn Marion), 1946-

Petrology and depositional setting of Mississippian rocks associated with an 

anoxic event at Samak, western Uinta Mountains, Utah. Petrology and 

significance of a Mississippian (Osagean-Meramecian) anoxic event, Lakeside 

Mountains, northwestern Utah I by K.M. Nichols and N.J. Silberling. 

p. em. - (U.S. Geological Survey bulletin ; 1787-S, n 

"Chapters S and T are published together in a single volume and are not 

available separately." 

Includes bibliographical references. 

Supt. of Docs. no.: 119.3:1787>-T 

1. Geology, Stratigraphic-Mississippian. 2. Geology-Uinta Mountains 

(Utah and Wyo.) 4. Sedimentation and deposition-Uinta Mountains (Utah and 

Wyo.). I. Silberling, N. J. (Norman John), 1928- . II. Title. Ill. Series. 

QE75.89 no. 1787->-T 

[QE672] 

551.7'51'0979214-dc20 91'-12493 

CIP 
VOLUME CONTENTS 

[Letters designate the chapters] 

(S) Petrology and depositional setting of Mississippian rocks associated with an 

anoxic event at Samak, western Uinta Mountains, Utah, by K.M. Nichols and 

N.J. Silberling. 

(T) Petrology and significance of a Mississippian (Osagean-Meramecian) anoxic 

event, Lakeside Mountains, northwestern Utah, by K.M. Nichols and 

N.J. Silberling. 

Volume Contents Ill 

ChapterS 

Petrology and Depositional Setting of 

Mississippian Rocks Associated 

with an Anoxic Event at Samak, 

Western Uinta Mountains, Utah 

By K.M. NICHOLS and N'.J. SILBERLING 

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern 

U.S. GEOLOGICAL SURVEY BULLETIN 1787 

EVOLUTION OF SEDIMENTARY BASINS-UINTA AND PICEANCE BASINS 

CONTENTS 

Abstract 81 

Introduction 81 

Stratigraphic petrology 83 

Lithic unit 1-0olitic grainstone 84 

Lithic unit 2-Laminated pelletal limestone 84 

Lithic unit 3-Lower crinoidal grainstone-packstone 85 

Lithic unit 4--Crinoidal wackestone-packstone 86 

Lithic unit 5-0stracode lime mudstone 88 

Lithic unit 6--Spiculitic bryozoan-crinoidal packstone 88 

Lithic unit 7-Upper crinoidal wackestone-packstone 810 

Lithic unit 8-Dolostone 810 

Discussion 811 

References cited 812 

FIGURES 

1. Map showing location of Samak, western Uinta Mountains, Utah, and of 

2. 

3-23. 

studied stratigraphic section 82 

Charts showing stratigraphy of Mississippian rocks at Samak 

Photomicrographs of: 

3. Dolomitized oolitic grainstone, lithic unit 1 84 

4. Fecal-pellet bioclastic grainstone, lithic unit 2 84 

5. Fecal-pellet bioclastic packstone, lithic unit 2 85 

83 

6. Finely bioclastic fecal-pellet wackestone, lithic unit 2 85 

7. Fecal-pellet bioclastic grainstone laminae, lithic unit 2 85 

8. Solution-compacted mass of crinoid columnals and their rim cements, 

lithic unit 4 86 

9. Extremely dissolved, solution-compacted bioclastic grainstone or wackestone grainstone, unit 4 86 

10. Extremely dissolved, solution-compacted phosphatized mass of crinoid 

cc;>lumnals, lithic unit 4 87 

11. Extremely solution compacted, leached phosphatized mass of crinoid 

columnals, lithic unit 5 87 

12. Ostracode lime mudstone, lithic unit 5 88 

13. Ostracode lime mudstone, lithic unit 5 88 

14. Bioclastic wackestone, lithic unit 5 89 

15. Black chert, lithic unit 5 89 

16. Partly silicified spiculitic bioclast packstone, lithic unit 6 89 

17. Neomorphosed, solution-compacted packstone, lithic unit 6 810 

18. Neomorphosed, partly silicified, solution-compacted, peloidal packstone, 

lithic unit 6 810 

19. Solution-compacted, bioclastic, peloidal wackestone, lithic unit 6 810 

20. Solution-compacted wackestone-packstone, lithic unit 7 810 

21. Solution-compacted crinoidal and bryozoan wackestone-packstone, lithic 

unit 7 811 

22. Solution-compacted crinoidal and bryozoan bioclasts in original packstone or grainstone, lithic unit 7 811 

23. Coarsely crystalline secondary dolomite, lithic unit 8 811 

Contents m 
, 

EVOLUTION OF SEDIMENTARY BASI~S-UINTA AND PICEANCE BASINS 

Petrology and Depositional Setting of 

Mississippian Rocks ·Associated with an 

Anoxic Event at Samak, Western Uinta Mountains, Utah 

By K.M. Nichols and N.j. Silberling 

Abstract 

Shelf carbonate deposition in the Uinta and Piceance 

basins study area of the U.S. Geological Survey Evolution of 

Sedimentary Basins program was interrupted by an anoxic 

event during Mississippian time. Rocks influenced by this 

event, assigned by previous workers to the upper part of the 

Lodgepole Limestone and lower part of the overlying Brazer 

Dolomite, are especially well exposed and easily accessible 

near Samak, Utah, at the west end of the Uinta Mountains. In 

the Samak section, the upper few meters of normal marineshelf limestone of the Lodgepole, originally rich in echinoderm 

ossicles and other open-marine, normal-salinity bioclasts, 

has undergone partial dissolution, solution corrosion, and 

concentration of insoluble components such as conodonts, 

and replacement by phosphate, culminating in a 2-3-cm-thick 

layer of pelletal phosphate at the top of the Lodgepole. Above 

this layer, comprising the lowermost part of the Brazer, 

deposits of the anoxic event are distinctive organic-rich ostracode I ime mudstone that grades upward through about 20 m of 

spiculitic, bioclastic wackestone and packstone and then into 

normal shelf limestone and dolostone. 

Mississippian rocks associated with the anoxic event at 

Samak reflect the incursion of upwelled, nutrient-rich marine 

water far onto the shelf. The stratigraphy and petrology of these 

rocks indicate the existence of a broad carbonate shelf having 

little topographic relief; they do not support the postulated 

"Deseret deep starved basin" model of previous workers. 

INTRODUCTION 

Mississippian phosphatic rocks in northwestern Utah 

are anomalous with respect to the thick, laterally variable 

sequence of subtidal to peritidal shelf carbonate rocks that 

Manuscript approved for publication January 28, 1991. 

stratigraphically underlies them as well as the thick 

sequence of peritidal carbonate and (or) craton-derived 

siliciclastic rocks that overlies them. These phosphatic 

rocks were included in the Delle Phosphatic Member by 

Sandberg and Gutschick (1984), who established this unit 

for exposures in the southern Lakeside Mountains (location 

LSM, fig. 1), Utah, where it was defined as the basal 

member of the Woodman Formation. These authors 

extended the Delle as the basal member of several different 

Mississippian formations in western Utah, northeastern 

Nevada, and southeastern and central Idaho and regarded 

the Delle Phosphatic Member as characteristic of their 

postulated "Deseret deep starved basin." 

In the Samak area, at the west end of the Uinta 

Mountains, Mississippian strata were assigned, in ascending 

order, to the Lodgepole Limestone and Brazer Dolomite by 

Sandberg and Gutschick (1980). Because revision of this 

stratigraphic nomenclature was beyond the scope of a study 

in the U.S. Geological Survey Evolution of Sedimentary 

Basins-Uinta and Piceance Basins program, these names 

are retained herein, but their usage is not endorsed by us. 

The lowermost 17 m of the Brazer was termed the 

phosphatic member by Sandberg and Gutschick ( 1980) and 

presumably is the same as rocks included by these authors 

in the Delle Phosphatic Member when they extended usage 

of this name to the Samak area in 1984. We prefer not to use 

the member name here, however, because placement of its 

upper limit is essentially arbitrary, as explained below, and 

its thickness as a rock unit is trivial. Instead, we previously 

informally named this episode of anomalous anoxic 

deposition the "Delle phosphatic event" (Silberling and 

Nichols, 1990, 1991) because this event initiated anoxic, 

phosphatic deposition in the type section of the Delle 

Phosphatic Member. This concept is referred to herein 

simply as the anoxic event. Regionally, the Delle 

Mississippian Rocks, Samak, Western Uinta Mountains, Utah S1 
L ____ _ 

1 KILOMETER 

25 KILOMETERS 

Figure 1. Location of Samak, western Uinta Mountains, Utah, and of studied stratigraphic section (A-A') shown in figure 2. LSM, 

Lakeside Mountains; ETM, East Tintic Mountains; CM, Crawford Mountains. Topographic contours in feet, interval 200ft. 

Phosphatic Member is better regarded as the result of an 

anoxic geochemical event rather than as a formal rock unit 

because it cannot be recognized away from the vicinity of 

its type section using a consistent set of lithic criteria (see 

also Nichols and Silberling, this volume). 

Although rocks of the anoxic event are relatively thin, 

they are of particular interest within the Evolution of 

Sedimentary Basins-Uinta and Piceance Basins study area 

(lat 38°30'-42° N., long 107°-112°30' W.) (referred to 

herein as the study area) because they have an unusual 

depositional and diagenetic history and because they form 

the basis for inferences about economically important 

paleogeographic features such as the nature and location of 

the Mississippian shelf margin (Lane and others, 1980; 

~andberg and Gutschick, 1980, 1984 ). Furthermore, 

assumptions about depositional environments during the 

Delle event and the postulated "Deseret deep starved basin" 

have significantly influenced interpretations of Mississippian facies patterns, eustatic sea-level history, diagenetic 

features, conodont and ostracode biofacies, and the 

Cord.illeran paleotectonic history of Utah, Nevada, and 

Idaho (Rose, 1976; Sando, Dutro, and others, 1976; Poole 

and Sandberg, 1977; Sandberg and Gutschick, 1977, 1979, 

1980, 1984; Gutschick and others, 1980; Lane and others, 

1980; Sandberg, Poole, and Gutschick, 1980; Sando, Sandberg, and Gutschick, 1981; Sandberg, Gutschick, and 

others, 1982; DeCelles and Gutschick, 1983; Gutschick and 

S2 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

Sandberg, 1983; Poole and Claypool, 1984; Gutschick, 

1987; Sohn and Sando, 1987; Hintze, 1988. 

Throughout the study area rocks influenced by the 

anoxic event are characterized by the abrupt disappearance 

of normal-marine shelf benthic fossils, the occurrence of 

pelletal or oolitic phosphorite, stratigraphically isolated 

massive beds or intervals of megascopically unfossiliferous 

lime mudstone, the rare occurrence of pelagic fossils such as 

radiolarians and goniatites, and the pervasive dissolution of 

limestone and replacement by silica. In the western part of 

the study area rocks affected by the anoxic event are as thick 

as 60 m. The rocks immediately older than those deposited 

during the anoxic event are thought by Sandberg and 

Gutschick (1984) to lie within the Gnathodus typicus 

Conodont Zone and are the same age everywhere that the 

effects of the event are recognized. 

The easily accessible Mississippian section near 

Samak at the west end of the Uinta Mountains (fig. 1) was 

chosen for detailed sedimentologic and petrographic study 

because it includes the best expression of the anoxic event 

east of the Wasatch Mountains. This section, which is 

autochthonous with respect to Sevier-Laramide thrust 

faulting, provides an excellent tie-point between the 

Mississippian lithic units in the Uinta Mountains (Dockal, 

1980) and those farther to the west in Utah. In the Samak 

section, rocks most affected by the anoxic event comprise a 

thickness of approximately 30 m within about 260 m of 
--------

120 

8 

~ 

~ ------_SQ.. 0 

...J 

0 -------- 0 

a: 60 

~ 7 

m ------ -30 

6 

-------- 5 

4 

w -------- z 30 0 

1- 3 en 

w 

:E 

::J 60 w 

...J 

0 

D.. 

w 2 (.!) 90 0 

0 

...J 

-------- 1 120 -------- Lithic METERS 

A unit 

Levels 7-21 shown in figure 28 

5 

Level 

EXPlANATION 

Evaporite solution breccia 

Primary dolomite 

Ostracode lime mudstone 

Crinoidai wackestone-packstone 

Crossbedded crinoidal grainstone-packstone 

I Secondary dolomite 

Laminated pelletal limestone 

Oolitic limestone 

Figure 2. Stratigraphy of Mississippian rocks at Samak, 

western Uinta Mountains, Utah. A, Generalized stratigraphy of 

Mississippian rocks. Numbers indicate stratigraphic levels 

discussed in text. 8, Stratigraphy of rocks associated with the 

anoxic event. Numbers indicate lithic units and stratigraphic 

levels discussed in text. 

reasonably well exposed Mississippian marine carbonate 

rocks (fig. 2) assigned by Sandberg and Gutschick (1980) to 

the Lodgepole Limestone and Brazer Dolomite. 

Our observations and interpretation of depositional 

environments at Samak agree closely with those in the 

comprehensive, unpublished doctoral thesis by Docka1 

(1980) and lead to a much different Mississippian (Osagean 

and Meramecian) paleogeography and depositional history 

for the eastern Great Basin and the western part of the study 

area than that proposed by Sandberg and Gutschick ( 1980, 

1984) and Lane and others ( 1980). 

STRATIGRAPHIC. PETROLOGY 

In the Samak section, the anoxic event altered the 

upper few meters of the Lodgepole Limestone and is 

represented by anomalous carbonate-rock deposits that 

represent the lower part of the Brazer Dolomite. Neither the 

base of the Lodgepole nor the uppermost part of the Brazer 

is exposed. The partial section at Samak is divided here into 

eight distinct lithic units (fig. 2A) that represent distinct 

depositional environments which have actual or potential 

regional significance. In ascending order, these units are (1) 

w 

I-

~ 0 

..J 

0 

Q 

a: 

w 

~ a: 

Ill 

w 

z 

0 

Ien 

w 

~ 

:::::; 

w 

..J 

0 

a.. 

w 

(!) 

Q 

0 

..J 

B 

0 

10 

4 

20 

3 

Lithic 

unit 

oolitic grainstone, (2) laminated pelletal limestone, (3) 

crinoidal grainstone-packstone, ( 4) lower crinoidal 

wackestone-packstone, (5) ostracode lime mudstone, (6) 

spiculitic bryozoan-crinoidal packstone, (7) upper crinoidal 

wackestone-packstone, and (8) dolostone. The LodgepoleBrazer contact lies between units 4 and 5. Because strata in 

the Samak section are very gently dipping, stratigraphic 

thicknesses of units l through 3 were estimated using an 

altimeter and are precise only to several meters. Units 4 

through 6, including the strata influenced by the anoxic 

event, were measured using a Jacob's staff. 

Mississippian Rocks, Samak, Western Uinta Mountains, Utah S3 
Lithic Unit 1-0olitic Grainstone 

The stratigraphically lowest outcrops in the Samak 

section, beginning about 5 m uphill from the highway grade 

(fig. 1), are 2-3 m of massive, cliff-forming, sugary, 

secondary dolostone containing scattered ghosts of echinoderm ossicles. Although assigned to the Devonian Fitchville 

Formation by Spreng (1979), this dolostone is regarded here 

as part of the overlying, partly dolomitized, oolitic rocks 

and thus part of the Mississippian, following Dockal ( 1980). 

Above the massive secondary dolostone is about 5 m of 

thick-bedded, partly dolomitic, oolitic grainstone (level 1, 

fig. 2A), which in favorable exposure exhibits conspicuous 

large-scale, planar crossbedding. Ooids predominate as 

allochems in the stratigraphically lower part of the grainstones (fig. 3), whereas bioclasts and peloids (probably 

micritized ooids and rounded bioclasts) are more abundant 

in higher beds. Bioclasts in this unit are mainly crinoid 

ossicles. Large fragments of colonial algae are present along 

with ossicles in the lower oolitic grainstone beds. 

By tracing this unit eastward in the Uinta Mountains, 

Dockal (1980) demonstrated that it is in the basal part of the 

Mississippian section. It may occupy the same stratigraphic 

position as the oolitic rocks that characterize the base of the 

type Gardison Limestone in the East Tintic Mountains 

(location ETM, fig. 1) (unpublished data). 

Lithic Unit 2-Laminated Pelletal Limestone 

Thick beds of cross-stratified oolitic grainstone unit 

are abruptly overlain by regularly thin and medium parted, 

fine-grained limestone of unit 2 (levels 2-5, fig. 2A), much 

of which exhibits pronounced planar lamination. 

Subordinate, distinct, medium-thick interbeds are coarse 

bioclast-intraclast packstone and grainstone. 

Figure 3. Photomicrograph of dolomitized oolitic grainstone. 

Level 1, lithic unit 1 (fig. 2). Cores of oolites are mainly fossil 

fragments; sparry cement mainly replaced by secondary 

dolomite, whereas ooids and bioclasts are still calcitic. Bar 

scale, 0.2 mm. 

S4 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

Figure 4. Photomicrographs of fecal-pellet bioclastic grainstone from a tempestite bed. Level 2, lithic unit 2 (fig. 2). Bar 

scales, 0.2 mm. A, Finer grained part of tempestite bed 

showing fringing cement (f), micritic envelope (m), rim cement 

around echinoid grain (r), and endothyrid in lower center of 

view. 8, Coarser grained part of tempestite bed showing 

intraclast (i), coarse sparry void-filling calcite cement (c), 

fringing cement (f), and cross section of whorl of low-spired 

grazing gastropod (g). 
Figure 5. Photomicrograph of fecal-pellet bioclastic packstone grading up into wackestone lamina at top of view. Level 

3, lithic unit 2 (fig. 2). Bar scale, 0.2 mm. 

The laminated fine-grained limestone is 

predominantly composed of fecal pellets mixed with 

varying proportions of finely comminuted bioclasts and 

lime mud. The pellets are generally uniform in size, and the 

planar lamination reflects an inverse variation in bioclast 

size and lime mud content. These fine-grained, evenly 

bedded rocks are wackestone to grainstone in texture (figs. 

5-7). The coarse, intraclastic, shelly interbeds are 

discontinuous and as thick as 10 em. They occur at 0.2-2 m 

intervals in the laminated pelletal limestone, generally have 

sharp contacts with the enclosing fine-grained limestone, 

and tend to be either normally or inversely graded. Intraclasts in these beds are composed of the enclosing finegrained limestone, and bioclasts are commonly whole 

fossils of brachiopods, corals, endothyrids, and gastropods 

(fig. 4). Large, low-spired gastropods dominate some beds. 

Rocks of this distinctive unit, which is about 50 m 

thick, are interpreted as deposits formed on a current-swept 

subtidal flat, the coarse, shelly interbeds being storm 

deposits or tempestites. 

Figure 6. Photomicrograph of partly dolomitized, finely bioclastic fecal-pellet wackestone in which layering is produced 

by oriented thin shell fragments; cloudy dolomite subhedra 

replace much of original mud matrix. Level 4, lithic unit 2 (fig. 

2). Bar scale, 0.2 mm. 

Figure 7. Photomicrograph of fecal-pellet bioclastic grainstone laminae in a grainstone-packstone laminated rock. Level 

5, lithic unit 2 (fig. 2). Bar scale, 0.2 mm. 

This same lithic unit forms the major part of the type 

Gardison Limestone in the East Tintic Mountains and the 

rock unit commonly referred to the Lodgepole Limestone at 

locations in northwestern Utah such the Lakeside 

Mountains (Nichols and Silberling, 1990) and Crawford 

Mountains (unpublished data). This widespread lithologic 

unit has been recognized by Dockal (1980, fig. 2) in 

sections in the Uinta Mountains as much as 100 km east of 

the Samak section. 

Lithic Unit 3-Lower Crinoidal 

Grainstone-Packstone 

Rocks of this unit, which is about 25 m thick, are 

massive and cliff forming, partly owing to secondary 

Mississippian Rocks, Samak, Western Uinta Mountains, Utah S5 
dolomitization. Peloidal, ossicle-rich, coarse bioclastic 

grainstone and packstone (for example, level 6, fig. 2A) are 

gradationally intercalated with massive, sugary, secondary 

dolostone. Remnant bedding characteristics within the 

massive units include irregular current layering and largescale, high-angle crossbedding. 

Dockal ( 1980, fig. 8) traced this part of the section 

into microbial-mat primary dolostone and related rocks 

farther to the east in the Uinta Mountains. Thus, secondary 

dolomitization within this unit most likely was an eogenetic, 

perhaps permeability controlled, effect of evaporitic 

environments farther inland on the shelf. 

High-energy, shoal-water, crossbedded, crinoidal 

limestone, resembling that of unit 3 in the Samak section, 

occupies a similar stratigraphic setting in rocks assigned by 

previous workers to the Lodgepole Limestone in the Crawford Mountains where it forms a conspicuous 3-m-thick unit 

near the top of a thick interval corresponding lithologically 

to unit 2 of the Samak section (unpublished data). 

Lithic Unit 4-Crinoidal 

Wackestone-Packstone 

The uppermost unit of the Lodgepole Limestone is 

about 25-30 m thick and is massive, pelleted crinoidallime 

wackestone that grades upward into bioturbated, diffusely 

and irregularly interstratified, fecal-pelleted lime wackestone, packstone, and, more rarely, grainstone. Pelmatozoan 

columnals and other bioclasts, including a few solitary 

corals and brachiopod shells, are abundant in the packstone 

and grainstone. The base of unit 4 is not necessarily a 

primary lithic boundary because the protolith of the 

underlying secondary dolostone is unknown. 

Figure 8. Photomicrograph of solution-compacted mass of 

crinoid columnals (cloudy large grains) and their rim cements, 

which are probably remnants of an original grainstone fabric; 

minor components are fecal pellets, spiney brachiopod shell 

fragments, and endothyrids. Level 7, lithic unit 4, (fig. 2). Bar 

scale, 0.2 mm. 

S6 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

Figure 9. Photomicrographs of extremely dissolved, 

solution-compacted bioclastic grainstone or wackestone grainstone. Level 8, lithic unit 4. Bar scales, 0.2 mm. A, Grain 

boundaries are dissolution surfaces. 8, Magnification of 

different view than A showing solution suturing of grains and 

thin-walled bioclasts crushed between crinoid columnals; 

white areas are voids. 

Rocks having the character of unit 4 in the Samak 

section are present at the top of the type Gardison 

Limestone in the East Tintic Mountains and virtually 

throughout northwestern Utah where they form the highest 

part of rock units referred to the Gardison, Lodgepole, or 

Joana Limestones. Nowhere in Utah is this regionally 

extensive unit of subtidal platform deposits more than a few 

tens of meters thick. 

In the Samak section the uppermost 10-15 m of unit 

4, as seen in thin section, is increasingly solution 

compacted, so that original textures and bedding 

characteristics are increasingly obscured upward (figs. 8, 9). 

The more resistant allochems, such as coarse shell 

fragments and crinoid ossicles, are compacted, as indicated 

by highly sutured grain boundaries and collapsed remnants 

of more delicate bioclasts such as bryozoan fragments (fig. 

9). Phosphate occurs in the uppermost 1 m of the Lodgepole 

Limestone as scattered peloids and pore fillings within 
Figure 10. Photomicrographs of extremely dissolved, 

solution-compacted phosphatized mass of crinoid columnals; 

grain boundaries are dissolution surfaces. Level 9, lithic unit 4 

(fig. 2). Bar scales, 0.2 mm. A, Phosphate fills pores in crinoid 

grains and is replacive following solution sutures of grains (p); 

conodont (c). 8, Lower magnification of different view of A 

showing extreme degree of volume compaction and total loss 

of matrix and cement; three of the grains in this view are 

fragments of conodonts and illustrate the abundance of conodonts in these highly dissolved limestones; white spaces are 

voids. 

crinoid ossicles (fig. 1 0). This interval mainly consists of 

ossicle remnants, the original outlines of which have been 

totally destroyed and whose boundaries are entirely solution 

seams separating one from another. In the highest few 

centimeters of the interval, phosphate replaces the ossicles 

and other remaining allochems following these solution 

seams, so that the grains first are rimmed with phosphate 

and then ultimately are transformed into phosphate peloids 

(fig. 11). The phosphatizing solutions probably followed the 

same pathways as the carbonate-dissolving solutions and 

thus may be related to them. In outcrop the top of the 

crinoidal wackestone-packstone unit is an irregular layer of 

phosphate pellets 2-3 em thick that grades sharply down 

into the underlying partly phosphatic limestone. 

Figure 11. Photomicrographs of extremely solution 

compacted, leached phosphatized mass of crinoid columnals. 

Level 10, lithic unit 5 (fig. 2), and KAM-1 of Lane and others 

(1980, p. 125). Bar scales, 0.2 mm. A, White spaces are mostly 

voids; conodonts are very abundant (estimated 1 percent of 

grains) in this rock and at this level (Lane and others, 1980, p. 

125, table 4), although none are in this view. 8, Magnification 

of left center of A, plane I ight; phosphatic pore fillings in 

cloudy single crystals of calcite (c), phosphatic replacement 

rim (p), calcite rim cement (r). C, Magnification of left center of 

A, crossed nicols; black areas are phosphate and voids, which 

can be differentiated in B. 

Mississippian Rocks, Samak, Western Uinta Mountains, Utah S7 
Figure 12. Photomicrograph of ostracode lime mudstone 

showing the finely fragmental "micropackstone" texture of the 

matrix formed mainly of 1 0-20-micron bioclasts; ostracode 

shell in center of view, thin brachiopod shells are present in 

other areas out of view; large white spots are voids. Level 11, 

lithic unit 5, (fig. 2). Bar scale, 0.2 mm. 

Figure 13. Photomicrograph of ostracode lime mudstone 

showing the finely fragmental "micropackstone" texture of the 

matrix; the only identifiable larger bioclasts are ostracode 

shells. Level 12, lithic unit 5 (fig. 2), and KAM-1 A of Lane and 

others (1980, p. 125). Bar scale, 0.2 mm. 

Lithic Unit 5-0stracode Lime Mudstone 

The ostracode lime mudstone unit, lithic unit 5, is 

recessive and poorly exposed, especially in the lower 

two-thirds of its total thickness of 17 m. In shallow 

excavations, dense, black, laminated, platy lime mudstone 

of this unit lies with sharp contact on the underlying 

phosphate-pellet ·layer of lithic unit 4. X-ray diffraction 

analysis of the impure-looking lime mudstone from level 

11, as well as from level 14, reveals no detectable mineral 

phases other than calcite, and thus the pigment and impurity 

in these limestones are probably entirely organic matter. 

58 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

Sparse outcrops of the ostracode lime mudstone unit at level 

12 and higher levels also are laminated, platy weathering, 

and dark on fresh surfaces but weather brownish or pinkish 

gray. 

The ostracode lime mudstone unit generally can be 

described as sparsely bioclastic, more or less laminated, 

organic lime mudstone to wackestone; however, rather than 

being micritic, the matrix consists of discrete, subequidimensional, 10-30-micron grains that appear to be bioclasts. 

Scattered among these grains in most samples are thin, flat 

shell fragments as long as 100 microns and oriented parallel 

with stratification, giving a layering to the rock (figs. 12, 

13). The fabric of this matrix could be termed micropackstone rather than lime mudstone. Larger bioclasts in this 

matrix are mainly ostracodes that are commonly thick 

shelled (figs. 12, 14B, C). Some laminae several meters 

above the base of the unit and higher are wackestones that, 

in addition to the ubiquitous ostracodes, contain abundant 

fragments of thin-shelled brachiopod shells (fig. 14A) and 

rare solitary corals (fig. 14B). Near the top of the unit, 

bioclasts include bryozoan fragments and some highly 

corroded echinoderm debris. Thus, within the upper part of 

unit 5 ostracode lime mudstone grades upward into wackestone that has a more normal-marine assemblage of bioclasts. 

Chert replacement in some beds of the ostracode lime 

mudstone and bioclastic wackestone high in the unit has 

preserved the original matrix and bioclasts (fig. 15). No 

radiolarians were observed by us in unit 5, but they are 

reported by Dockal (1980) from this unit in the Samak 

section. 

Petrographically, the ostracode lime mudstone at 

Samak is similar to that of units 13 and 16 of the type Delle 

Phosphatic Member in the Lakeside Mountains (Nichols 

and Silberling, 1990, fig. 2; this volume), which are 

interpreted as restricted, possibly hypersaline deposits. 

Ostracode lime mudstone is a distinctive part of the rocks 

influenced by the anoxic event at most of their occurrences 

in western Utah (unpublished data). 

Unit 5 corresponds to the "phosphatic member" of the 

Brazer Dolomite of Sandberg and Gutschick (1980), which 

in 1984 was incorporated into their Delle Phosphatic 

Member. As explained above, we do not recognize this 

formal member here because placement of its upper 

boundary within the lithologically transitional sequence of 

unit 5 is arbitrary, using the lithic criteria that define the top 

of the type Delle section in the Lakeside Mountains. 

Lithic Unit 6-Spiculitic BryozoanCrinoidal Packstone 

The contrast between the massive, 15-m-thick, cliffforming rocks of lithic unit 6 and the underlying, recessive 

strata of unit 5 reflects in part an upward transition from 
Figure 14. Photomicrographs of bioclastic wackestone in the 

ostracode lime mudstone unit. Level 13, lithic unit 5 (fig. 2), 

and KAM-2 of Lane and others (1980, p. 125). Bar scales, 0.2 

mm. A, Exceptionally shelly laminae containing thick-shelled 

ostracode grains (o), very thin shelled brachiopod grains (b), 

and indeterminate thin, flat shell fragments. 8, Transverse view 

of rare solitary coral showing two partly silicified septa; 

interseptal space is filled with ostracode shell in pelleted 

fine-grained packstone matrix. Fine-grained packstone matrix 

outside of coral calyx is not pelletal as in A. C, Thick-shelled 

ostracode and unidentifiable thin (brachiopod?) shell 

fragments in bioclastic wackestone. 

Figure 15. Photomicrograph of black chert that is silicified 

and dolomitized lime wackestone having "micropackstone" 

texture of the matrix and containing larger shells of ostracodes 

and brachiopods; scattered white spaces are dolomite euhedra; patches of dark pigment are most likely organic material. 

Level 14, lithic unit 5 (fig. 2) ,and KAM-3 of Lane and others 

(1980, p. 125). Bar scale, 0.2 mm. 

Figure 16. Photomicrograph of partly silicified spiculitic bioclast packstone; large bioclasts are solution-corroded crinoids 

and bryozoans; white spaces are voids. Level 16, lithic unit 6 

(fig. 2), and KAM-5 of Lane and others (1980, p. 125). Bar 

scale, 0.2 mm. 

matrix -supported to allochem-supported limestone (fig. 16). 

More importantly, this contrast is also the result of partial 

silicification and solution compaction related to the 

development of stylolitic bedding. The boundary between 

units 5 and 6 is conspicuous in outcrop mainly because of 

these secondary effects. In unit 6, secondary chert is present 

as pervasive webbing and as irregular thin layers at about 

medium-bedded intervals. Much of the matrix in the rocks 

comprising unit 6 resembles that of the underlying ostracode lime mudstone and bioclastic wackestone of unit 5 in 

that it has a finely fragmental micropackstone fabric (figs. 

17-19). Allochems are mainly fine grained and include 

abundant calcareous spicules and peloids. Bryozoan and 

Mississippian Rocks, Samak, Western Uinta Mountains, Utah S9 
Figure 17. Photomicrograph of neomorphosed, solutioncompacted packstone of poorly sorted crinoids, ostracodes, 

bryozoans, peloids, and sponge spicules. Level 17, lithic unit 

6 (fig. 2). Bar scale, 0.2 mm. 

Figure 18. Photomicrograph of neomorphosed, partly 

silicified, solution-compacted, peloidal packstone containing 

poorly sorted crinoid, bryozoan, and brachiopod debris. Level 

18, lithic unit 6 (fig. 2). Bar scale, 0.2 mm. 

crinoidal bioclasts predominate, but, as noted by Dockal 

(1980), ossicles generally are more abundant upward within 

the unit. Thus, the spiculitic packstone unit represents a 

transition from the underlying anomalous lime mudstone 

and wackestone of unit 5 to the more normal marine shelf 

limestone of the overlying unit 7. 

Lithic Unit 7-Upper Crinoidal 

Wackestone-Packstone 

Rocks of lithic unit 7 are diffusely interstratified, 

conspicuously crinoidal, bioclast-peloid lime mudstone, 

wackestone, and packstone in irregular, medium-thick beds 

comprising a monotonous interval of 25-30 m, the top of 

the unit being unexposed. Solution compaction blurs the 

distinction between wackestone and packstone fabrics (figs. 

S1 0 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

Figure 19. Photomicrograph of solution-compacted, bioclastic, peloidal wackestone containing debris of echinoderms, ostracodes (o), and unknown thin shell fragments. Level 

19, lithic unit 6 (fig. 2), and KAM-6 of Lane and others (1980, 

p. 125). Bar scale, 0.2 mm. 

Figure 20. Photomicrograph of solution-compacted 

wackestone-packstone containing corroded echinoid and 

bryozoan debris and peloids that may originally have been 

micritic pore fillings in bryozoans. Level 20, lithic unit 7 (fig. 

2), and KAM-7 of Lane and others (1980, p. 125). Bar scale, 

0.2 mm. 

20-22). Secondary chert nodules are present sparsely 

throughout the unit, and some have poorly developed 

wood-grained texture. Planar lamination is present in finegrained packstone that is interstratified with lime mudstone 

low in the unit. 

Lithic Unit 8-Dolostone 

An unexposed interval of about 25 m separates units 

7 and 8. Near the base of lithic unit 8 is a conspicuous thick 

bed of solution-compacted, partly dolomitized crinoidalpackstone that is overlain by 25 m of exposure formed of 

dolostone. The dolostone interval consists of alternating 
Figure 21. Photomicrograph of solution-compacted crinoidal 

and bryozoan wackestone-packstone; dark peloids may have 

originally been micritic pore fillings in bryozoa, two micritefilled skeletal fragments of which are in middle part of view. 

Level 21, lithic unit 7 (fig. 2), and KAM-8 of Lane and others 

(1980, p. 125). Bar scale, 0.2 mm. 

Figure 22. Photomicrograph of solution-compacted crinoidal 

and bryozoan bioclasts in an original packstone or grainstone. 

Level 22, lithic unit 7 (fig. 2), and KAM-14 of Lane and others 

(1980, p. 125). Bar scale, 0.2 mm. 

1-2-m-thick packages of brownish-gray, vaguely bioclastic, 

massive, sugary secondary dolostone (fig. 23) and finely 

crystalline, partly laminated, light-colored dolostone. Some 

light-colored dolostone contains birds-eyes, and in places 

fragments of light-colored dolomite are reworked as rip-ups 

into overlying, coarsely crystalline, secondary dolostone 

beds. This interstratified primary and shelly secondary 

dolostone indicates episodic exposure and evaporitic 

conditions during deposition, and it resembles much of the 

type Brazer Dolomite in the Crawford Mountains ( location 

CM, fig. 1). 

The highest outcrop shown on figure 2A, about 10 m 

above the more or less continuous exposures of dolostone, 

is a massive, coarse dolostone breccia that most likely 

Figure 23. Photomicrograph of coarsely crystalline 

secondary dolomite. Level 23, lithic unit 8 (fig. 2). Bar scale, 

0.2 mm. 

resulted from evaporite solution. From here to the top of the 

ridge (fig. 1) are only scattered outcrops of brownish-gray 

sugary dolostone. 

DISCUSSION 

The section at Samak, in which the rocks affected by 

the anoxic event occur, is part of the North American 

Mississippian carbonate shelf and comprises lagoonal, tidalflat, shoal, and evaporitic deposits (Dockal, 1980). Some of 

the lithic units representing shelf depositional environments 

are widespread, extending from the Uinta Mountains across 

western Utah. The anoxic event thus affected a broad shelf 

area of little topographic relief. 

An alternative interpretation of the Mississippian 

depositional system in the area of the Uinta and Piceance 

basins is based on the concept of the "Deseret deep starved 

basin," a concept that requires assigning some of the 

Mississippian carbonate rocks at Samak to a variety of 

deep-water, basinal, slope, and platform-margin facies 

(Lane and others, 1980; Sandberg and Gutschick, 1980). 

Unit 5, for example, the ostracode lime mudstone, was 

assigned to the "basinal and lower slope facies" on the east 

side of the "Deseret deep starved basin" by Lane and others 

(1980, table 4) and Sandberg and Gutschick (1980, fig. 3). 

In our interpretation, the ostracode lime mudstone, the rock 

type that is the hallmark for deposits of the anoxic event 

throughout northwestern Utah, has none of the important 

characteristics required of a slope deposit. The anoxic event 

may represent an abrupt increase in water depth on the shelf, 

but it does not involve the development of a discrete deep 

basin delimited by distinct slope and platform-margin 

environments. 

In the Samak section the anoxic event resulted in 

carbonate dissolution and limestone replacement by 

phosphate in the uppermost part of unit 4 and in deposition 

Mississippian Rocks, Samak, Western Uinta Mountains, Utah 511 
of the peculiar organic-rich limestone of unit 5. The 

bioclast-producing, normal-marine biota was abruptly 

exterminated and recovered only gradually during 

deposition of the higher parts of unit 5 and on though unit 

6 (Dockal, 1980). 

The abrupt biotic event at the base of unit 5 and the 

extensive solution compaction in the subjacent limestone 

raises the possibility of exposure and the influx of meteoric 

water at this time. Nonetheless, occurrence data for conodonts in this part of the Samak section provided by Lane and 

others ( 1980, table 4) indicate that no hiatus took place 

between units 4 and 5 within the resolution of their 

zonation. Eleven conodont species and variants are reported 

by Lane and others from their sample KAM-1 , which, 

judging from the location of the sample number painted on 

the outcrop, represents the uppermost few centimeters of 

limestone in unit 4. The stratigraphic ranges of all of these 

kinds of conodonts, as given by Lane and others (1980, 

table 2), overlap those of conodonts reported from various 

levels through unit 5. Both the uppermost part of unit 4 and 

all of unit 5 were thus deposited within the time span of the 

Upper Typicus Zone (Lane and others, 1980), without 

biostratigraphically perceptible interruption. 

Unusually large recoveries of conodont elements are 

reported by Lane and others (1980, table 4) from sample 

KAM-1, which yielded an amazing extrapolation of 1, 194 

elements per kilogram. Dockal ( 1980, p. 80) also reported a 

high yield of conodonts from limestone immediately below 

unit 5, about 30 km east of the Samak section. This high 

yield corroborates the relative overabundance of conodonts 

in thin sections of the limestones in the upper few meters of 

unit 4 (fig. I 0). According to Lane and others (1980, p. 

125), high abundances of conodonts, such as those reported 

in the Samak section, represent foreslope sedimentary 

environments. In this case, however, an alternative 

explanation is simply that the conodonts are concentrated as 

insolubles in this extremely solution compacted limestone. 

In general, the interpretation of absolute water depth on 

conodont abundances (Sandberg and Gutschick, 1979; Lane 

and others, 1980) is imprudent in the absence of other kinds 

of data and without consideration of other variables. 

Rocks influenced by the anoxic event at Samak, as 

well as those elsewhere in western Utah, show evidence of 

the effects of upwelling marine water, such as relatively 

high concentrations of phosphate and silica, high organic 

productivity, and the subsequent development of anoxia in 

bottom waters and in the shallow subsurface. The model 

proposed by Heckel ( 1977), applied by Dockal ( 1980) to 

Mississippian rocks in the Uinta Mountains area, provides 

insight as to how upwelled, deep-marine water could be 

introduced far onto the shelf and how anoxia in the lower 

water column on the shelf could be maintained. In this 

model, generation of a mixing barrier, such as a thermocline 

related to an increase in water depth across the shelf, acting 

in concert with oceanward wind-driven surface currents, 

S12 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

would establish the circulation pattern in shelf waters of a 

quasi-estuarine cell that would bring cool, oxygen-deficient, 

nutrient-rich upwelling water landward across the shelf 

beneath oxygenated surface waters. The waters of the 

quasi-estuarine cell that would replace the normal, fully 

oxygenated waters of the normal anti-estuarine circulation 

cell on the shelf. Broad topographic features on the shelf, 

such as a silled margin, could affect the maintenance and 

nature of quasi-estuarine circulation, but the generally 

broad, flat configuration of the shelf surface would be 

preserved. Therefore, the anomalous shelfal rocks 

representing the anoxic event in the study area need not be 

related to an ephemeral deep basin such as the postulated 

"Deseret deep starved basin." Platform-margin carbonate 

buildups that might rim such a deep basin and form 

important hydrocarbon reservoirs in this part of Utah, as 

predicted by Rose (1976), have neither been recognized nor 

are they expected in the rock record. 

REFERENCES CITED 

DeCelles, P.G., and Gutschick, R.C., 1983, Mississippian woodgrained chert and its significance in the western interior 

United States: Journal of Sedimentary Petrology, v. 53, 

p. 1175-1191. 

Dockal, J.A., 1980, Petrology and sedimentary facies of Redwall 

Limestone (Mississippian) of Uinta Mountains, Utah and 

Colorado: Iowa City, University of Iowa, Ph.D. thesis, 

423 p. 

Gutschick, R.C., 1987, Letter: Geology, v. 15, p. 684. 

Gutschick, R.C., and Sandberg, C.A., 1983, Mississippian 

continental margins of the conterminous United States, in 

Stanley, D.J., and Moore, G.T., eds., The shelf-break; 

critical interface on continental margins: Society of 

Economic Paleontologists and Mineralogists Special 

Publication 33, p. 79-96. 

Gutschick, R.C., Sandberg, C.A., and Sando, W.J., 1980, 

Mississippian shelf margin and carbonate platform from 

Montana to Nevada, in Fouch, T.D., and Magathan, E.R., 

eds., Paleozoic paleogeography of the west-central United 

States: Society of Economic Paleontologists and 

Mineralogists, Rocky Mountain Section, Rocky Mountain 

Paleogeography Symposium 1, p. 111-128. 

Heckel, P.H., 1977, Origin of phosphatic black shale facies in 

Pennsylvanian cyclothems of Mid-Continent North 

America: American Association of Petroleum Geologists 

Bulletin,v. 61, p. 1045-1068. 

Hintze, L.F., 1988, Geologic history of Utah: Brigham Young 

University Geology Studies, Special Publication 7, 202 p. 

Lane, H.R., Sandberg, C. A., and Ziegler, W., 1980, Taxonomy and 

phylogeny of some Lower Carboniferous conodonts and 

preliminary post-Siphonodella zonation: Geologica et 

Palaeontologica, v. 14, p. 117-164. 

Nichols, K.M., and Silberling, N.J., 1990, The Delle Phosphatic 

Member-An anomalous part of the Mississippian 

(Osagean-Meramecian) shelfal sequence of central Utah: 

Geology, v. 18, p. 46-49. 
Poole, F.G., and Claypool, G.E., 1984, Petroleum source-rock 

potential and crude-oil correlation in the Great Basin, in 

Woodward, Jane, Meissner, F.F., and Clayton, J.L., eds., 

Hydrocarbon source rocks in the greater Rocky Mountain 

Region: Rocky Mountain Association of Geologists, 

Denver, Colorado, p. 179-229. 

Poole, F.G., and Sandberg, .C.A., 1977, Mississippian paleogeography and tectonics of the western United States, in 

Stewart, J.H., Stevens, C.H., and Fritsche, A.E., eds., 

Paleozoic paleogeography in the western United States: 

Society of Economic Paleontologists and Mineralogists, 

Pacific Section, Pacific Coast Paleogeography Symposium 

1, p. 67-85. 

Rose, P.R., 1976, Mississippian carbonate shelf margins, western 

United States: U.S. Geological Survey Journal of Research, 

v. 4, p. 449-466. 

Sandberg, C.A., and Gutschick, R.C., 1977, Paleotectonic, biostratigraphic, and economic significance of Osagean to early 

Meramecian starved basin in Utah: U.S. Geological Survey 

Open-File Report 77-121, 16 p. 

___ 1979, Guide to conodont biostratigraphy of Upper Devonian and Mississippian rocks along the Wasatch front and 

Cordilleran hingeline, Utah, in Sandberg, C.A., and Clark, 

D.L., eds., Conodont biostratigraphy of the Great Basin and 

Rocky Mountains: Brigham Young University Geology 

Studies, v. 26, pt. 3, p. 107-134. 

___ 1980, Sedimentation and biostratigraphy of Osagean and 

Meramecian starved basin and foreslope, western United 

States, in Fouch, T.D., and Magathan, E.R., eds., Paleozoic 

paleogeography of the west-central United States: Society 

of Economic Paleontologists and Mineralogists, Rocky 

Mountain Section, Rocky Mountain Paleogeography 

Symposium 1, p. 128-147. 

____ .1984, Distribution, microfauna, and source-rock potential 

of Mississippian Delle Phosphatic Member of Woodman 

. Formation and equivalents, Utah and adjacent states, in 

Woodward, Jane, Meissner, F.F., and Clayton, J.L., eds., 

Hydrocarbon source rocks of the greater Rocky Mountain 

Region: Rocky Mountain Association of Geologists, 

Denver, Colorado, p. 135-178. 

Sandberg, C.A., Gutschick, R.C., Johnson, J.G., Poole, F.G., and 

Sando, W.J., 1982, Middle Devonian to Late Mississippian 

geologic history overthrust belt region, western United 

States, in Powers, R.B., ed., Geologic studies of the 

Cordilleran thrust belt, v. 2: Rocky Mountain Association of 

. Geologists, Denver, Colorado, p. 691-719. 

Sandberg, C.A., Poole, F.G., and Gutschick, R.C., 1980, Devonian 

and Mississippian stratigraphy and conodont zonation of 

Pilot and Chainman Shales, Confusion Range, Utah, in 

Fouch, T.D., and Magathan, E.R., eds., Paleozoic paleogeography of the west-central United States: Society of 

Economic Paleontologists and Mineralogists, Rocky 

Mountain Section, Rocky Mountain Paleogeography 

Symposium 1, p. 71-79. 

Sando, W.J., Dutro, J.T., Jr~. Sandberg, C.A., and Mamet, B.A., 

1976, Revision of Mississippian stratigraphy, eastern Idaho 

and northeastern Utah: U.S. Geological Survey Journal of 

Research, v. 4, p. 467-479. 

Sando, W.J., Sandberg, C.A., and Gutschick, R.C., 1981, 

Stratigraphic and economic significance of Mississippian 

sequence at North Georgetown Canyon, Idaho: American 

Association of Petroleum Geologists Bulletin, v. 65, 

p. 1433-1443. 

Si1berling, N.J., and Nichols, K.M., 1990, Tectonic implications of 

widespread anoxia on the Mississippian shelf, western and 

northern Utah: Geological Society of America Abstracts 

with Programs, v. 22, p. 83-84. 

___ 1991, Petrology and regional significance of the 

Mississippian Delle Phosphatic Member, Lakeside 

Mountains, northwestern Utah, in Cooper, J.D., and 

Stevens, C.H., eds., Paleozoic paleogeography of the 

western United States-11: Society of Economic 

Paleontologists and Mineralogists, Pacific Section, v. 67, p. 

425-438. 

Sohn, I.G., and Sando, W.J., 1987, Paleobathymetric significance 

of ostracodes from the Paine Member of the Lodgepole 

Limestone (Early Mississippian), northeast Utah: U.S. 

Geological Survey Bulletin 1690-E, 7 p . 

Spreng, W.C., 1979, Upper Devonian and Lower Mississippian 

strata on the flanks of the western Uinta Mountains, Utah: 

Brigham Young University Geology Studies, v. 26, no. 2, 

p. 67-79. 

Mississippian Rocks, Samak, Western Uinta Mountains, Utah S13 
Chapter T 

Petrology and Significance of a Mississippian 

(Osagean-Meramecian) Anoxic Event, 

Lakeside Mountains, Northwestern Utah 

By K.M. NICHOLS and N.J. SILBERLING 

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern 

U.S. GEOLOGICAL SURVEY BULLETIN 1787 

EVOLUTION OF SEDIMENTARY BASINS-UINTA AND PICEANCE BASINS 

CONTENTS 

Abstract Tl 

Introduction T:n. 

Petrologic observations T2 

Lodgepole Limestone T2 

Delle Phosphatic Member of the Woodman Formation T4 

Younger rocks T9 

Discussion TlO 

Regional geologic implications Tll 

References cited Tll 

FIGURES 

1. Map showing geology of study area in southern Lakeside Mountains, 

Utah T3 

2. Chart showing generalized stratigraphy of type Delle Phosphatic Member of 

the Woodman Formation and adjoining strata, Lakeside Mountains T4 

3. Photographs showing deposits in the Lodgepole Limestone T4 

4-5. Photomicrographs of deposits in the Lodgepole Limestone showins: 

4. Fecal-pellet crinoidal packstone 8 m below top TS 

5. Fecal-pellet crinoidal wackestone immediately underlying type section of 

Delle Phosphatic Member TS 

6. Photograph and sketch of burrows in silicified limestone filled with pelletal 

phosphorite T6 

7. Photograph of slabbed cross-sectional surface showing contact between 

rocks of anoxic event and underlying limestone, central Wasatch 

Mountains T6 

8-14. Photomicrographs of Delle Phosphatic Member showing: 

8. Flattened phosphate pellets in solution-compacted secondary chert, 

level 14 T7 

9. Dolomitized calcareous mudstone, level 9 T7 

10. Pelletal phosphate crust, level 11 T7 

11. Oolitic and pisolitic phosphorite, level 11 T8 

12. Detrital oolitic and pelletal phosphorite, level 8 T8 

13. Bored phosphate grain, level 8 T9 

14. Endolithic algal borings in oolitic phosphate grains, level 11 T9 

15. Photograph showing limestone pinch-and-swell structure, Delle Phosphatic 

Member, level 15 T9 

16. Photomicrograph of neomorphosed limestone composing a pinch-and-swell 

dissolution structure, Delle Phosphatic Member, level 15 TlO 

17. Photograph of megascopically featureless, ostracode lime mudstone that 

defines top of type Delle Phosphatic Member, level 16 TlO 

18. Photomicrograph of ostracode lime mudstone at top of Delle Phosphatic 

Member, level 16 Tll 

Contents · Ill 
EVOLUTION OF SEDIMENTARY BASINS-UINTA AND PICEANCE BASINS 

Petrology and Significance of a Mississippian 

(Osagean-Meramecian) Anoxic Event, 

Lakeside Mountains, Northwestern Utah 

By K.M. Nichols and N.J. Silberling 

Abstract 

Phosphatic rocks are a distinctive part of the Lower and 

middle parts of the Mississippian rock record in the western 

part of the study area of the U.S. Geological Survey Evolution 

of Sedimentary Basins-Uinta and Piceance Basins program 

and are particularly significant because of their influence on 

interpretations about the paleogeography and depositional 

framework of this part of the Paleozoic section. These rocks 

have been included in the Delle Phosphatic Member, which at 

its type section in the Lakeside Mountains, northwestern Utah, 

is a member of the Woodman Formation. Deposition of the 

Delle Phosphatic Member was initiated by an abrupt anoxic 

·event. Throughout western Utah, rocks influenced by this 

event are characterized by the absence of normal marine-shelf 

benthic fossils, evidence for limestone dissolution, pelletal, 

pisolitic and oolitic phosphorite, secondary chert, and megascopically unfossiliferous, massive lime mudstone sandwiched 

within thick sequences of normal-marine subtidal and peritidal 

Mississippian shelf-carbonate and craton-derived siliciclastic 

rocks. Features of the Delle phosphatic rocks such as shelf 

sedimentary structures and evidence of endolithic algal boring 

of phosphate layers, coupled with the stratigraphic context of 

the anoxic event, indicate a generally shallow depositional 

environment for rocks influenced by this event and are not 

compatible with the concept of a deep, sediment-starved 

depositional basin, the "Deseret deep starved basin" of 

previous authors. The peculiar nature of this event most likely 

resulted from anoxic bottom water and strongly reducing 

in-sediment conditions alternating with oxygenated, possibly 

subaerial, conditions. These conditions probably were 

produced by incursion far onto the Early Mississippian shelf of 

nutrient-rich, organically productive water that was possibly 

produced by upwelling in the Antler foreland trough in central 

Nevada. This interpretation places the Early Mississippian 

Manuscript approved for publication January 28, 1991. 

carbonate shelf-margin to the west of the Lakeside Mountains 

rather than in central Utah as required by the "Deseret deep 

starved basin" model. 

INTRODUCTION 

Phosphatic rocks have long been known within the 

Lower Mississippian at various locations in northwestern 

Utah (Cheney, 1957; Schell and Moore, 1970; Gutschick, 

1976). These and associated rocks that reflect anoxic 

deposition are anomalous with respect to the thick, laterally 

variable sequence of normal subtidal to peritidal shelf 

carbonate rocks that stratigraphically underlies them as well 

as to the thick sequence of peritidal carbonate and (or) 

. craton-derived siliciclastic rocks that overlies them. The 

Delle Phosphatic Member was proposed by Sandberg and 

Gutschick ( 1984) to include these phosphatic rocks, and the 

type section of this member in the Lakeside Mountains, 

Utah, was treated as the basal member of the Woodman 

Formation. Sandberg and Gutschick (1984) regarded the 

Delle Phosphatic Member as characteristic of their "Deseret 

deep starved basin," and the unit has been extended by these 

authors throughout a wide area of western Utah, 

northeastern Nevada, and southeastern and central Idaho as 

the basal member of several different Mississippian 

formations. Away from the vicinity of its type section, 

however, we cannot recognize the top of the Delle 

Phosphatic Member using a consistent set of lithologic 

criteria. Consequently, for the purpose of regional 

discussion we find it more useful to treat the rocks in 

question as those affected by an abrupt, widespread anoxic 

depositional event that initiated deposition of the Delle 

Phosphatic Member at its type section. The anoxic event 

caused diagenetic alteration of pre-existing carbonate rocks, 

Mississippian Anoxic Event, Lakeside Mountains, Utah T1 
and its effect on deposition diminishes upsection in varying 

ways at different places. The term "Delle phosphatic event 

(DPE)" has been applied informally by Silberling and 

Nichols (1990, 1991) to this particular phenomenon, and it 

is used herein in the same sense and referred to as · "the 

anoxic event." 

Although relatively thin, rocks influenced by the 

anoxic event are of particular interest within the UintaPiceance study area (lat 38°30'-42° N., long 107°-112°30' 

W.), not only because of their peculiar depositional and 

diagenetic history, but also because they form the basis for 

inferences about economically important paleogeographic 

features such as the nature and location of the Mississippian 

shelf margin. Furthermore, assumptions about the 

depositional environment during the anoxic event and about 

the postulated "Deseret deep starved basin" have 

significantly influenced interpretations of regional 

Mississippian facies patterns in the Cordillera, eustatic 

sea-level history, diagenetic features, conodont and ostracode biofacies, and the paleotectonic history of Utah, 

Nevada, and Idaho (Rose, 1976; Sando and others, 1976; 

Poole and Sandberg, 1977; Sandberg and Gutschick, 1977, 

1979, 1980, 1984; Gutschick and others, 1980; Lane and 

others, 1980; Sandberg and others, 1980; Sando and others, 

1981; Sandberg and others, 1982; Gutschick and Sandberg, 

1983; DeCelles and Gutschick, 1983; Poole and Claypool, 

1984; Gutschick, 1987; Sohn and Sando, 1987; Hintze, 

1988). 

Throughout the study area rocks deposited during the 

anoxic event are characterized by the absence of normal 

marine shelf benthic fossils, the occurrence of pelletal or 

oolitic phosphorite and stratigraphically isolated massive 

beds or units of megascopically unfossiliferous lime mudstone, the rare occurrence of pelagic goniatites and 

radiolarians, and pervasive limestone dissolution and 

replacement by silica. In the western part of the study area 

rocks most strongly affected by the anoxic event are a few 

meters to several tens of meters thick. According to Sandberg and Gutschick (1984), the rocks immediately older 

than the anoxic event lie within the Gnathodus typicus 

Conodont Zone and are permissibly the same age 

everywhere the rock record of the event is recognized. 

Thicker manifestations of the event, such as the section in 

the southern Lakeside Mountains in which the Delle 

Phosphatic Member was typified, may represent a 

condensed sequence of most of the Osagean and the lowest 

Meramecian Provincial Series (Sandberg and Gutschick, 

1984 ). Our studies suggest, however, that abbreviation of 

the section resulted from carbonate dissolution and resulting 

compaction of stratigraphic section as well as from slow 

accumulation of sediment. 

Outcrops in the Lakeside Mountains that include the 

type section of the Delle Phosphatic Member were chosen 

for detailed sedimentologic and petrographic study, the 

results of which serve to constrain depositional and paleoT2 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

tectonic interpretations of the anomalous Delle interval. In 

the type section, which is easily accessible, pelletal, 

pisolitic, and oolitic phosphorite and associated rock types 

are present at more stratigraphic levels and through a greater 

thickness and variety of rocks than in related sections 

elsewhere in the study area and yet are characteristic in kind 

and stratigraphic setting of other expressions of the anoxic 

event in the region. In the Lakeside Mountains (fig. 1), 

rocks beneath the type section of the Delle Phosphatic 

Member were assigned by Sandberg and Gutschick ( 1984) 

to the Lodgepole Limestone, and the Delle was established 

as the lower member of the Woodman Formation (fig. 2). 

Strata immediately overlying the Delle Phosphatic Member 

in the Lakeside Mountains were treated by them as the 

Needle Siltstone Member of the Woodman Formation. This 

usage of Lodgepole Limestone and Needle Siltstone 

Member is not endorsed by us, but, because revision of 

stratigraphic nomenclature is beyond the scope of this 

paper, this part of the nomenclature used by SanJt>crg and 

Gutschick (1984) in the Lakeside Mountains is retained 

herein. Our interpretation of the nature, depositional 

environments, and paleotectonic setting of these rocks in 

this expanded and more fully documented version of 

Nichols and Silberling (1990) differs from that proposed by 

Sandberg and Gutschick ( 1984) and indicates a much 

different Mississippian (Osagean-Meramecian) paleogeography than that proposed by them for the western part 

of the study area. 

PETROLOGIC OBSERVATIONS 

Lodgepole Limestone 

The type section of the Delle Phosphatic Member 

(line A-A', fig. 1) overlies about 100m of carbonate rocks 

assigned to the Lodgepole Limestone by Sandberg and 

Gutschick (1984). About 30m below the base of the Delle 

the Lodgepole is cut by low-angle bedding-plane faults on 

which some strata may have been omitted. Massive, 

secondary dolomite obliterates 10-12 m of the. limestone 

above this fault zone. Below the fault zone is a thick 

sequence of regularly thin to medium parted, fecal-pelleted 

lime packstone, much of which exhibits pronounced planar 

lamination (fig. 3A). Only the uppermost 10 m of this 

interval is shown on the columnar section. Partly graded, 

discontinuous interbeds as thick as 10 em and composed of 

crinoidal, coralline, and shelly debris are present at 0.2-2-m 

intervals in the laminated lime packstone. and are interpreted 

as storm deposits or tempestites (fig. 3B). Both the pelleted 

fabric and pervasive current lamination of this lime packstone unit are features characteristic of mid-shelf 

environments (for example, Enos, 1983). The uppermost 

unit of the Lodgepole, beginning above the secondary 

dolomite of the fault zone and about 18 m below the 
' 

' 

UTAH 

' ' 

1 KILOMETER 

Figure 1. Geology of study area (T. 1 N., R. 8 W.) in southern Lakeside Mountains, Utah. Qu, Quaternary deposits, 

undifferentiated; QTg, Quaternary or Tertiary gravel; Po, Permian and Pennsylvanian Oquirrh Formation; Mississippian-Mgb, 

Great Blue Limestone; Mh, Humbug Formation; Mwn, Needle Siltstone Member of Woodman Formation (as used by Sandberg 

and Gutschick, 1984); Mwd, Delle Phosphatic Member of Woodman Formation; Ml, Lodgepole Limestone (as used by Sandberg 

and Gutschick, 1984); Du, Devonian, undifferentiated; heavy line, fault; dotted line, concealed fault; dashed line, concealed 

contact. Elevations in feet. Line A-A', Type section of Delle Phosphatic Member. 8, Location of trenched section. 

projected top of the Lodgepole, is massive, chert-nodulerich, pelleted, crinoidal lime wackestone grading upward 

into bioturbated, diffusely interstratified, fecal-pelleted lime 

wackestone and packstone (fig. 4) containing abundant 

crinoid columnals and other bioclasts including a few 

solitary corals and brachiopod shells. At the type section, 

which is along a ridge crest, the upper 7-8 m of this 

wackestone-packstone unit is not exposed, but it can be 

observed in the gully leading downhill to the south, as at 

location B (fig. 1 ). 

The highest beds of limestone assigned to the Lodgepole are well_exposed in the gully leading south from the 

base of the Delle type section, but trenching is required, as 

at location B, to expose higher beds and the base of the 

Delle. Here, the uppermost 1 Q-..:20 em of the Lodgepole 

Limestone (level 1, fig. 2) is blotched, red-stained, solutioncompacted, fecal-pelleted, bioclastic wackestone (fig. 5). 

Phosphate is present in this rock both as replacement bodies 

within the remaining limestone matrix and as void fillings 

within the pores of crinoid columnals, bryozoans, and other 

bioclasts. This partly dissolved, compacted limestone is the 

"deep-water winnowed encrinite" of Sandberg and Gutschick (1984, p. 140, 142). Immediately above the highest 

Lodgepole Limestone (level 1) and exposed by trenching at 

location B is 0.5 m (level 2) of punky, red, dissolutioncompacted, microcrystalline quartz and illitic clay and 

scattered partially dissolved calcitic bioclasts similar in kind 

to those of the immediately underlying limestone. Above 

this residuum is a 40-cm-thick resistant level (level 3) 

composed of silicified limestone nodules that probably are 

dissolution remnants in a matrix of laminated siliceous 

mudstone and pelleted phosphorite. Features interpreted as 

burrow cavities as wide as several millimeters in the 

secondarily silicified nodules are infilled by phosphorite 

pellets (fig. 6). The stratigraphically lowest occurrence of 

phosphorite pellets (level 3, fig. 2) and the base of the Delle 

Phosphatic Member are thus associated with carbonate 

dissolution and apparent burrowing. The pelletal phosphorite that marks the beginning of the anoxic event elsewhere 

in central and western Utah either commonly fills 

complexly pinnacled solution cavities in the underlying 

limestone (fig. ?)(Sandberg and Gutschick, 1984, plate lB) 

-or is associated with a layer of insoluble material as thick as 

a meter that was produced by dissolution of the underlying 

Mississippian Anoxic Event, Lakeside Mountains, Utah T3 
w wza: 

0 

z 5~~ 

~ WUJ::2: Wt-w 

2::::!::2: ::2: Ul a: 

0 

LL 

z 

<t: 

::2: 

0 

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0 

s: a: 

w 

00 

::2: 

w 40 

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u 

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0 

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METERS 

9 10 8 

7 /6 

= 

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= 

Section at location B 

on figure 1 

Section A-A' 

on figure 1 

EXPlANATION 

Dolomitized calcareous slit- stone and sandstone 

Lime mudstone 

Secondary chert nodules 

Limestone pinch-and- swell structures 

Mudstone 

Phosphorite 

Chert 

Limestone 

Secondary dolomite 

Figure 2. Generalized stratigraphy of type Delle Phosphatic 

Member of the Woodman Formation and adjoining strata, 

Lakeside Mountains. Numbers label stratigraphic levels 

discussed in text; stratigraphic names are those applied by 

Sandberg and Gutschick (1984). 

limestone. Along with other insoluble materials, conodonts 

have been concentrated by carbonate dissolution in the 

limestone immediately underlying rocks of the anoxic 

event. The high yield of conodonts from this level, as 

reported from, outcrops farther east in the study area and 

elsewhere in Utah, has been interpreted as indicating 

absolute water depth (Lane and others, 1980, p. 125; 

Sandberg and Gutschick, 1979); however, it is probably the 

result of concentration by limestone dissolution and 

compaction. 

Delle Phosphatic Member of the 

Woodman Formation 

The lower 20 m (below ievel 13, fig. 2) of the type 

Delle Phosphatic Member in the Lakeside Mountains is 

poorly exposed but consists of a heterogeneous succession 

T4 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

,fi 

I) 

"'""' 

J ~ 't:.r r lJ 

) .~J 

'¢ 

q; 

./-' ~7 ~'11. ~ 

Figure 3. Lodgepole Limestone, east side of Lakeside 

Mountains. A, Laminated fecal-pellet packstone from same 

part of section as B. 8, Tidal-flat storm deposit or tempestite 

within lime mudstone in the upper part of the Lodgepole 

Limestone (of Sandberg and Gutschick, 1984) underlying 

type section of the Delle Phosphatic Member. 

of diagenetically reconstituted rocks characterized by 

phosphorite and by replacement silica and dolomite. 

Above level 3, in the highest 1.0 m of trenched 

section at location B (fig. 1 ), four thin, indurated layers of 

pale-yellowish-brown phosphatic, siliceous mudstone or 

impure chert are in the otherwise punky, deeply weathered 

material that characterizes the basal part of the Delle. In a 

particularly cherty layer (level 4) bands of scattered, round, 

or elliptical voids as much as 200 microns in diameter may 

represent the former presence of calcified radiolaria. 

Phosphorite in these layers is present as flattened shreds that 

are interpreted to have been originally pellets, as explained 

below. 
Figure 4. Photomicrograph of fecal-pellet crinoidal packstone 8 m below the top of the Lodgepole Limestone {of 

Sandberg and Gutschick, 1984) underlying type section of the 

Delle Phosphatic Member, Lakeside Mountains. Scale bar, 0.2 

mm. 

The stratigraphically lowest beds exposed in the cut 

banks of the road leading to the radio towers on top of Black 

Mountain in the type section of the Delle (fig. 1) are about 

0.5 m higher than level4. In the road cut at levelS, pale- and 

moderate-yellowish-brown siliceous mudstone and impure 

chert similar to that of level 4 at location B grades into 

5-1 0-cm-thick interbeds of black phosphatic chert (level 6) 

bearing scattered radiolarians visible on weathered surfaces. 

The black phosphatic cherts at this level and higher in the 

section at levels 7 and 14 have a distinctive solutioncompacted fabric of laterally discontinuous, lenticular and 

sinuously flattened grains of internally featureless 

phosphate, along with sparse grains of quartz silt enveloped 

in microcrystalline silica (fig. 8). These rocks are 

interpreted to have been relatively pure limestone beds that 

contained scattered replacement pellets of phosphate and 

underwent solution compaction prior to or during 

silicification. Laminae of uncompacted, phosphate pellets in 

grain-to-grain contact are present in some of these cherts 

Figure 5. Photomicrograph of strongly solution compacted, 

partly phosphatic, fecal-pellet crinoidal wackestone from the 

top of the Lodgepole Limestone (of Sandberg and Gutschick, 

1984) immediately underlying type section of the Delle 

Phosphatic Member at locality B (fig. 1 ), Lakeside Mountains. 

Scale bar, 0.2 mm. 

(fig. 8) and support the interpretation that the flattened 

fabric was produced by dissolution of a limestone matrix in 

parts of the rocks where scattered pellets were originally 

matrix supported. Thus, rather than being primary biogenic 

siliceous rocks, or "lydites" as they are characterized by 

Sandberg and Gutschick (1984, p. 143 ), we interpret these 

black phosphatic cherts to be secondary replacements of 

beds that were originally limestone. 

Above the road-cut exposures in the lower part of the 

type Delle, the next 17 m of the Delle is poorly exposed up 

to level 13 (fig. 2). Outcrops of relatively resistant rock 

types within this poorly exposed lower Delle interval 

represent less than 20 percent of the section and are (I) 

black, phosphatic-chert replacement of limestone (levels 

5-7), (2) black pelletal, pisolitic, and oolitic phosphorite in 

conspicuous beds as thick as 10 em (levels 8 and 11 ), (3) 

yellowish-brown, illitic siliceous mudstone and impure 

secondary chert (for example, at levels 4 and 10), and (4) 

Mississippian Anoxic Event, Lakeside Mountains, Utah TS 
Figure 6. Sketch (A) showing laminated, pelletal phosphorite 

and siliceous mudstone (a), dense, silicified limestone (b), 

burrows in silicified limestone filled with pelletal phosphorite 

(c); and red quartz-illite limestone-solution residuum (d). 8, 

Photograph of cross sectional surface of remnant silicified 

limestone nodule from level 3 (fig. 2), a sketch of which is 

shown in A. Natural size. 

dolomitized calcareous mudstone and silty limestone (levels 

9 and 12). More than a decimeter of vivid, red-weathering 

mudstone or shale overlies the phosphorite bed of level 11. 

Authigenic ammonium feldspar, or buddingtonite (fig. 

9) (X-ray diffraction identification by R.M. Pollastro, U.S. 

Geological Survey, oral commun., 1988), is in phosphatic, 

·cherty, and dolomitized beds at several levels (levels 3-5, 

9)(fig. 9) and is associated with pseudomorphs after 

framboidal iron sulfide in level 9. Both buddingtonite and 

framboidal iron sulfide indicate strongly reducing insediment conditions. In contrast to these diagenetically 

anomalous rocks, a conspicuous and laterally persistent 

0.6-m-thick unit of evenly bedded, partly dolomitic, megascopically unfossiliferous lime mudstone is at level 13. 

Mudstone immediately underlying this limestone also 

weathers vivid red. 

Conspicuous, well-indurated, laterally discontinuous 

beds of pelletal and oolitic phosphorite are present at levels 

8 and 11 (fig. 2) in the lower part of the Delle. Phosphate 

grains are closely packed in these rocks, which have little or 

no matrix; the pore space between the phosphate grains is 

T6 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

Figure 7. Slabbed cross-sectional surface showing contact 

between rocks formed during the anoxic event and underlying 

limestone assigned to Gardison Limestone by Bromfield and 

others (1970), central Wasatch Mountains, Utah, about 120 

km east-southeast of the study area in the Lakeside Mountains. 

Detrital phosphate ooids, peloids, and bioclast (mainly ossicle) 

replacements fill solution cavities having a complex pinnacle 

geometry in the underlying partly phosphatic, crinoidal packstone. Natural size. 

filled with large, commonly cloudy, poikilitic calcite 

crystal,s that are in optical continuity from void to void. 

Laterali:f:: within single beds and from bed to bed the 

character of the phosphate grains and the manner in which 

they are aggregated changes among three different ~ypes: 

(1) pelletal phosphatic crust composed of closely packed 

aggregates of internally featureless phosphate pellets 

cemented by an isopachous, delicately lamellar, phosphate 

cement (fig. 1 0); (2) pisolitic phosphorite composed of 

poorly size sorted aggregates of concentrically coated 

phosphate grains (fig. 11); and (3) detrital aggregates of 

ooidal and other kinds of phosphate grains most likely 

formed by erosion and reworking of the pisolitic phosphorite (fig. 12). Any one of these three types can constitute an 

entire bed as thick as 10 em. 

Within the pisolitic phosphorite of level 11 the grain 

size of pisoids tends to decrease both stratigraphically 

upward and downward, and the pisolitic phosphorite is 

bordered by irregular layers a few millimeters thick of 

dense, black phosphorite. These layers are reworked into 

oolitic detrital beds as large solid lumps. The pisolitic and 

pelletal-crust phosphorite in the lower Delle resembles that 

described by Southgate (1986) from the Cambrian of 

Australia and interpreted by him as phoscrete formed in the 

surficial sediment on semiemergent flats associated with an · 

epeiric marine basin. In further support of a phoscrete origin 

for some of the phosphorites in the Delle-particularly that 

at level 11-are well-developed endolithic algal borings 

(figs. 13, 14) in both the pisolitic and detrital oolitic 
Figure 8. Photomicrograph showing flattened phosphate 

pellets in solution-compacted secondary chert (light areas), 

Delle Phosphatic Member, level 14 (fig. 2). Note less 

compacted fabric of phosphate pellets in more grain supported 

layer in lower part of view. Scale bar, 0.2 mm. 

Figure 9. Photomicrograph of dolomitized calcareous mudstone, Delle Phosphatic Member, level 9 (fig. 2). Framboidal 

ferric oxides after pyrite (A); authigenic ammonium feldspar or 

buddingtonite (B). Scale bar, 0.2 mm. 

Figure 10. Photomicrographs of pelletal phosphate crust, 

Delle Phosphatic Member, level 11 (fig. 2). Scale bars, 0.2 

mm. A, Poikilitic, single-crystal late calcite cement (c); 

authigenic quartz euhedra (q). 8, Closer view of A; laminated, 

isopachous, phosphate cement (1). 

phosphorite. These borings are 10-20 microns in diameter 

and as long as a few hundred microns. In size and 

configuration they are virtually identical to borings in 

carbonate substrates by modem blue-green algae such as 

Hyel/a (Golubic, 1969; Buddand Perkins, 1980). They are 

both too large and of the wrong shape to be fungal borings, 

and they lack the verrucrose surface texture indicative of 

sponge borings. In modem marine environments, living 

blue-green algae, such as Hyella, are restricted to the upper 

photic zone (intertidal to about 20 m water depth) (Budd 

and Perkins, 1980). Furthermore, algal borings-especially 

of an encrusted surface-in older sedimentary rocks are 

generally strong evidence for deposition at shallow depths. 

Algal boring in the phosphorite of the Delle took place after 

authigenic quartz euhedra had grown in some grain interiors 

(figs. 13, 14), and diagenesis of the phosphorite and 

possibly the dissolution of its probable original lime-mud 

matrix most likely preceded exposure at the sediment 

surface and algal infestation. The complexly interwoven 

Mississippian Anoxic Event, Lakeside Mountains, Utah T7 
Figure 11. Photomicrographs of oolitic and pisolitic 

phosphorite, Delle Phosphatic Member, level 11 (fig. 2). Scale 

bars, 0.2 mm. 

pattern of algal borings in the Delle phosphorite are filled 

with the same single-crystal poikilitic calcite cement that 

fills the intergranular areas. 

The upper two-thirds of the Delle type section, 

between levels 13 and 16, is mainly yellowish-brownweathering, very sparsely fossiliferous, partly laminated, 

secondarily dolomitized calcareous siltstone, mudstone, 

fine-grained sandstone, and silty secondary dolomite. 

Marine benthic fossils, such as bryozoans, are sparsely 

present at a few levels. In addition to quartz, detrital mica, 

potassium feldspar, and plagioclase indicate derivation of 

these clastic rocks from the craton, not from the Roberts 

Mountains allochthon, as argued by previous authors (Sandberg and Gutschick, 1984, p. 143 ). Other rock types in the 

lower 25 m of this interval are black, phosphatic, 

replacement chert (at and near level 14) and laminae of 

phosphate pellets associated with limestone pinch-and-swell 

structures (as at level 15). 

The limestone pinch-and-swell structures were 

interpreted as concretions by Sandberg and Gutschick 

(1984) but probably instead resulted from partial dissolution 

of once continuous limestone beds (fig. 15). Phosphate 

T8 Evolution of Sedimentary Basins-Uinta and Piceance Basins 

Figure 12. Photomicrographs of detrital oolitic and pelletal 

phosphorite; Delle Phosphatic Member, level 8 (fig. 2). Scale 

bars, 0.2 mm. Poikilitic late calcite cement (c); grain-interior 

aggregates of authigenic euhedral quartz (q). 

pellets are scattered within these pinch-and-swell structures, 

are in thin discontinuous selvages or crusts on their top 

surfaces, and are present as partings within the surrounding 

siltstone and mudstone. Even though radiolaria tend to he 

well preserved in these limestone bodies, mostly as ferricoxide replacements, the original limestone fabric is coarsely 

neomorphosed (fig. 16). The presence in these pinchand-swell structures of radiolarians and of large numbers of 

conodonts, which are concentrated as naturally occurring 

insoluble residues, was described at length by Sandberg and 

Gutschick (1984). 

The conspicuous unit of dense, medium-bedded lime 

mudstone (fig. 17) that defines the top of the type Delle 

(level 16) is megascopically featureless except for many 

nodules of wood-grained secondary chert (DeCelles and 

Gutschick, 1983). Cross sections of ostracodes are 

conspicuous in thin section (fig. 18); fossils of marine-shelf 

benthic organisms generally are absent. This ostracode-rich 

limestone and the similar, but dolomitized limestone lower 

in the section at level 13 are believed to represent deposition 
Figure 13. Photomicrographs of a bored phosphate grain, 

Delle Phosphatic Member, level 8 (fig. 2). Scale bars, 0.2 mm. 

Calcite fillings of borings are in optical continuity with intergranular cement and do not penetrate cement. A, Plane light. 

8, Crossed nicols. 

in a restricted, possibly hypersaline, low-energy 

environment, and they are a distinctive, although minor, 

component of the rock record influenced by the anoxic 

event throughout central and western Utah. Both these rocks 

and those of the same character that formed during the 

anoxic event elsewhere in Utah were interpreted as 

regionally significant gravity and slope deposits by Sandberg and Gutschick (1984, p. 138), but nowhere do they 

bear evidence of gravity-flow deposition, slump folding, or 

any other features characteristic of such deposits. 

Younger Rocks 

. Immediately overlying the Delle Phosphatic Member 

in its type section are dolomitized calcareous siltstone and 

·fine-grained sandstone similar in composition and bedding 

characteristics to those that form much of the upper part of 

Figure 14. Photomicrographs of endolithic algal borings in 

oolitic phosphate grains, Delle Phosphatic Member, level 11 

(fig. 2). Scale bars, 0.2 mm. Aggregate of authigenic euhedral 

quartz in 8 (q). 

Figure 15. Limestone pinch-and-swell structure enveloped1 

by stratification in enclosing dolomitic mudstone, Delle 

Phosphatic Member, level 15 (fig. 2). Maximum thickness of 

structure 25 em . 

the type Delle. These are about 75 m thick and are referred 

on figures 1 and· 2 to the Needle Siltstone Member of the 

Woodman Formation, following the terminology applied by 

Mississippian Anoxic Event, Lakeside Mountains, Utah T9 
Figure 16. Photomicrograph of neomorphosed limestone 

composing a pinch-and-swell dissolution structure, Delle 

Phosphatic Member, level 15 (fig. 2). radiolaria (r). Scale bar, 

0.2 mm. 

Sandberg and Gutschick (1984, p. 141) to the strata 

immediately above the type Delle. The next higher map unit 

(Mh, fig. 1) is coarser and more conspicuously cross bedded 

quartzose sandstone to which the name Humbug Formation 

is applied. This unit is lithologically unlike either the Needle 

Siltstone Member of the Chainman Formation or the Woodman Formation, and in assigning it to the Humbug 

Formation we are following Hintze (1988), who applied the 

name Deseret Limestone to much or all of the underlying 

interval termed Needle Siltstone Member of the Woodman 

Formation by Sandberg and Gutschick (1984). The siltstones and sandstones assigned here to the upper part of the 

Delle and to the Woodman and Humbug show only 

tractional bedding characteristics indicative of peritidal 

depositional environments. 

DISCUSSION 

The heterogeneous rocks in the lower part of the type 

Delle Phosphatic Member record fluctuating depositional 

and diagenetic environments, the effects of which have been 

complexly superimposed on one another. The abrupt 

disappearance of normal marine shelf benthic bioclasts and 

shelly fossils during deposition of these rocks can be 

explained by anoxia in bottom water related to the 

accumulation of organic matter. Reducing, organic-rich 

conditions within the sediment are indicated by the presence 

of phosphorite and ammonium feldspar, the development of 

iron sulfide minerals, and possibly the presence of some of 

the secondary dolomite, the formation of which may have 

been enhanced by sulfate reduction. Episodes of oxidation, 

on the other hand, are indicated by the low (mostly less than 

0.5 percent) organic carbon content (Sandberg and Gutschick, 1984, p. 177), the general lack of black organic 

pigments in much of the section and infestations of algal 

T10 Evolution of Sedimentary Basins-Uinta and Piceance. Basins 

Figure 17. Megascopically featureless, regularly thin to 

medium parted ostracode lime mudstone containing 

secondary chert nodules. This rock type defines the top of the 

type Delle Phosphatic Member, Lakeside Mountains (level 16) 

(fig. 2). Hammer shown for scale. 

endoliths. Episodic penecontemporaneous oxidation of iron 

sulfide minerals most likely produced the layers of vivid red 

mudstone associated with some phosphorite beds. 

Although similar in some respects to basinal deposits, 

a deep-water origin for the anomalous rocks produced by 

the anoxic event is precluded by the sedimentary features 

and biotic content of the rocks and is especially implausible 

by stratigraphic context. The approximately 30-m-thick, 

partly phosphatic section in the Lakeside Mountains is 

sandwiched between normal marine mid- to inner-platform 

shelf deposits. Elsewhere in the study area the depositionally and diagenetically peculiar phosphatic rocks of the 

anoxic event are substantially thinner. Evidence is also 

lacking for regionally significant slope environments in the 

form of features such as turbidites, significant gravity-flow 

deposits, or soft-sediment slump folds. Within strata 

influenced by the anoxic event, the restriction of clastic 
Figure 18. Photomicrograph of ostracode lime mudstone at 

top of Delle Phosphatic Member, level 16 (fig. 2). Scale bar, 

0.2 mm. 

sedimentary structures to only those that would have been 

produced above storm-wave base is evidence for shelf 

deposition, as are the endolithic algal borings in some 

phosphatic layers. 

The nature and stratigraphic setting of rocks of the 

anoxic event can be explained by an incursion onto the shelf 

of nutrient-rich, organically productive, open-marine water. 

Such water would have favored the presence of pelagic 

organisms, such as radiolarians and goniatites, and the 

extermination of the normal marine shallow, shelly benthos. 

The apparent compaction of rocks affected by the anoxic 

event relative to that of shelf carbonate rocks of supposed 

equivalent age (Sandberg and Gutschick, 1980) can be 

explained by the absence as rock-forming constituents of 

the usual calcareous bioclasts, such as crinoid columnals, by 

the penecontemporaneous dissolution of calcareous 

material, for which there is abundant evidence as described 

above, and perhaps by one or more appreciable erosional 

disconformities. 

REGIONAL GEOLOGIC IMPLICATIONS 

The regionally extensive phosphat~c system reflected 

by the anoxic event could not have been controlled by the 

geochemistry of local depressions such as lagoons and 

" estuaries that surely existed on the Early Mississippian 

shelf. It is not coincidental that the anoxic event began at or 

just after final deformation in central Nevada of the Antler 

allochthon (Jansma and Speed, 1985). According to modelling by Speed and Sleep ( 1982), tectonic emplacement of 

the Antler allochthon onto the continental margin would 

have produced an elongate, asymmetrically deep foreland 

basin. Upwelling within this ephemeral foreland basin, as 

previously hypothesized by Roberts (1979), may have 

produced the nutrient-rich waters that so dramatically 

affected depositional and diagenetic environments on the 

broad Mississippian shelf on the continental side of the 

foreland basin (Nichols and others, 1988). 

Interpretation of the Delle Phosphatic Member, and 

rocks influenced by the anoxic event in general, as part of 

the shelf succession has significant paleogeographic 

implications. The Early Mississippian shelf margin, instead 

of being east of the Lakeside Mountains, as required by the 

"Deseret deep starved basin model" (Rose,1976; Poole and 

Sandberg, 1977; Gutschick and others, 1980; Sandberg and 

others, 1982; Gutschick and Sandberg, 1983), must have 

been much farther to the west, well to the west of the study 

area. 

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