RICE UNIVERSITY

DEPOSITION AND DIAGENESIS OF THE MISSISSIPPIAN LODGEPOLE FORMATION, CENTRAL MONTANA

by

Susan E. jenks

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

Master of Arts

Thesis Director's signature

Houston, Texas

May, 1972

3 1272 00197 2320 Deposition and Diagenesis of the

Hississippian Lodgepole Formation,

Central Montana

Susan Jenks

ABSTRACT

The lower Mississippian Lodgepole Formation is exposed in central

Montana in the anticlines which form the Big Snowy and Little Belt

Mountains. Four sections averaging 130 feet in length were measured at the base of the Woodhurst Limestone, the uppermost member of the Lodge¬ pole. Three of the sections were located in the vestern end of the Big

Snowy Mountains. These were composed of two major bioclastic and ooid grainstone units, and a succession of mudstones, wackestones, packstones and argillaceous dolomites and pellet grainstones and pelleted mudstones.

Field, faunal, and petrographic evidence indicate these rocks were deposited in very shallow water, the grainstones in the form of carbonate sand shoals, the remaining rock types in a broad lagoon behind the shoals.

One section was measured 70 miles to the west in the Little Belt moun¬ tains. Rocks here consist of crinoid grainstones and packstones, skeletal and ooid grainstones, mudstones, bryozoan packstones and wackestones, and calcareous shales. Evidence suggests these rocks formed down paleoslope from those in the Big Snowys, some of the sediments being deposited in deeper water in a normal marine shelf environment.

A number of diagenetic processes affected the sediments after deposition. Morphology and distribution of cements and evidence of tim¬ ing relative to other diagenetic events indicate cementation of the carbonate sands took place in the intertidal or shallow subtidal environment soon after deposition. Dolomitization of crinoid debris with magnesium derived from the high magnesian calclte of the crinoid skeletons themselves also took place very early in the history of the sediment. This was followed by silicification and pyritization of skeletal debris and ooids. Another, more extensive period of dolomitiza¬

tion occurred which affected micrite and, to a lesser extent, ooids and tended to avoid skeletal material and spar. The last diagenetic event to affect these rocks was compaction and stylolitization filled remaining pore space as coarse blocky calcite spar. ACKNOWLEDGEMENTS

I would first like to thank Prof. James Lee Wilson who suggested

the project and supervised all aspects of the work. I would also like

to thank Profs. John Warme and Lewis Nettleton who read the manuscript and made many helpful suggestions for improvement.

Field and laboratory expenses for the project were supported by

National Science Foundation Grant 23-583 awarded to J. L. Wilson for the study of carbonate shelf margins. My work at Rice was supported by a National Science Foundation traineeship and National Science Foundation fellowships.

There are many others whose help is deeply appreciated:

Mrs. J. L. Wilson, who accompanied us in the field.

D. L. Smitht who showed me the Rock Creek section, and shared his own work on the Lodgepole with me.

Karen Gaffey, who helped with the field work.

Mr. and Mrs. Paul Gaffey, who provided a field vehicle.

Prof. Owen Bricker of the Johns Hopkins University, who made helpful criticisms of the manuscript.

I would also like to thank Dr. William Rohrbacher, whose excellent medical care during a long and difficult illness enabled me to continue my work in school.

Very special thanks go to my mother, Mrs. John W. Jenks, for her support, both moral and financial, and for her constant encouragement.

And last, but not least, I wish to thank Michael Gaffey for his help in the field, for his unwavering moral support, and most especially for his belief that "women are people, too." TABLE OF CONTENTS

INTRODUCTION 1

STRATIGRAPHY AND GEOLOGIC SETTING 2

PROCEDURE 4

PETROLOGY 5

ENVIRONMENT OF DEPOSITION 12

PREVIOUS WORK 12

INTERPRETATION 18

DIAGENESIS 26

CEMENTATION 26

DOLOMITE 31

CHERT 38

COMPACTION 41

CONCLUSIONS 44

REFERENCES CITED 46 i

INTRODUCTION

In central Montana the Mississippiar. Lodgepoie Formation is composed of a series of shales, micritic limestones, and coarse grainstone layers.

The stratigraphy of the Lodgepoie in this area was discussed by Sloas and

Hamblin (1942) and Andrichuk (1955), and in recent years much attention has been focussed on the environment of deposition of these and similar rocks. Smith (1972) made a detailed facies study of the Lodgepoie rocks in this area, and proposed a model for environmental interpretation, and

Cotter (1965, 1966) and Stone (1971) studied the environments of deposi¬ tion and diagenesis of the Waulsortian-type bioherms which occur in the middle (Paine Shale) member of the Lodgepoie. However, little work has been done on the diagenesis of the rest of the Lodgepoie rocks, and it was to fill this gap that this study was undertaken.

About fifteen years ago J. L. Wilson (personal communication, 1969) made a facies study of the Lodgepoie in Montana and concluded that these rocks were cyclically deposited in an offshore marine environment, each cycle concluding with buildup of sediments to sea level and a period of subaerial exposure. There have been many studies of Recent and Pleistocene carbonates in the Bahamas, Florida, Yucatan, the Persian Gulf, and else¬ where. This study was to be a detailed look at these rocks, to see if criteria developed in Recent and Pleistocene studies could be applied in interpreting environments of deposition and diagenesis of ancient carbon¬ ates. Particular attention was focussed on the grainstones in an effort to determine if diagenesis took place in the vadose, phreatic, or sub¬ marine environment. 2

STRATIGRAPHY AND GEOLOGIC SETTING

The Mlssissippian Lodgepole Formation is exposed in central Montana

in east-west trending anticlines which are elliptical in plan and form

the Big Snowy and Little Belt Mountains (see fig. 1).

The Lodgepole was defined by Collier and Cathcart (1922) for strata

in the area of Lodgepole Creek in the Little Rocky Mountains. It is con¬

formably overlain by the Mission Canyon Limestone, and these two forma¬

tions, together with the Charles Formation comprise the Madison Group

(see fig. 2). The Lodgepole unconformably overlies a number of older

formations ranging in age from Devonian to Cambrian. In the Big Snovys and Little Belts it overlies the Devonian Three Forks Formation (Montana

Geological Society, 1969).

The Lodgepole is subdivided into three members, the Woodhurst Limestone and Paine Shale Members, as defined by Slcss and Hamblin (1942) and the

Cottonwood Canyon Member, defined by Sandberg and Klapper (1967). The

Cottonwood Canyon Member occurs at the base of the Lodgepole in central and southern Montana, is ten to twenty feet thick, and is composed of a western shale and siltstone facies and an eastern dolomite facies. The

Paine Shale Member is composed of about 200 feet of shales and dark gray to brown fine-grained limestones. The Woodhurst Limestone Member, about

300 feet thick, is composed of fine-grained limestones and thick layers of coarse grained, fossiliferous limestone.

The Lodgepole sediments were deposited on a broad shallow Mlssissippian shelf which sloped from southeast to northwest across Montana. The base of the Lodgepole Formation rests on successively older strata to the south¬ east, and at its extreme edge in Nebraska lies on rocks of Precambrian age Fig. 1 Map showing location of measured sections. e. Heath Formation o o u o Otter Formation or c Caesterian CD CO •HCD Kibbey Formation c CO Charles Formation a, a,

U1 (A Meramecian

(A a Mission Canyon Formation CA § o a Osagian o T3 CO S Woodhurst Limestone Member V C r-4 O aoO *rl Paine Shale Member 0) <0 •§>6 Cottonwood Canyon Member Kinderhookian o o

c •H c Upper Three Forks Formation o o

Fig. 2 Stratigraphic column for tae Middle Paleozoic in central Montana (Andrichuk, 1955). Wilson (1969* p. 15) evolved naleotectonic picture shown In fig* 3 for the Love Mississippiar ised on facies studies of Lod^epole rocks. by isopach lines) of the "deeper water" facies of the Lodgepole (from Wilson, 1969, p. 14, 15) zlpatone plus pattern marks basin areas. Black circles indicate control points. 4

PROCEDURE

Two weeks were spent in the field in the summer of 1970 measuring

and describing sections and collecting samples. Four sections averaging

130 feet in length were measured and sampled, three in the Big Snowy

Mountains (two in Timber Creek canyon, one near Crystal Lake along Rock

Creek) to study local variations in rock characteristics, and one in the

Little Belt Mountains along Belt Creek near Monarch to give some idea of

regional trends (see map, fig. 1; see appendix I for precise locations).

The interval sampled lies at the base of the Woodhurst Limestone Member of the Lodgepole Fromation and is Kinderhook to Osage in age (Sando eit al., 1969). Samples were taken at two foot intervals in the sections at

Timber and Rock Creeks, and at three foot intervals in the Belt Creek section, Additional samples were taken at important lithologic contacts.

Samples were studied in thin section with both petrographic and binocular microscopes. Thin sections were stained for calcite with alizarin red-S following the method outlined by Friedman (1959), and for ferrous iron with potassium ferricyaniae using the procedure proposed by

Evamy (1969). Estimates of percentages of various constituents were made by means of visual comparison with the charts of Terry and Chilingar

(1955). 5

PETROLOGY

The measured sections were subdivided into five basic rock types.

In the sections in the Big Snowys these are 1) bioclastic grainstone,

2) dolomite, 3) interbedded mudstone, waekestone, and packstone, 4)

argillaceous dolomite and pellet grainstone and pelleted mudstones, and 5) ooid grainstone. The limestone classification used is that of

Dunham (1962). The Rock Creek section is the most complete of the

three measured in the Big Snowys. All three sections are very similar

(see fig, 12 and appendix II) and the Rock Creek section was selected

to illustrate the basic characteristics of the rocks in this area

(figs. 4 and 5).

The bioclastic grainstone occurs at the base of the measured sections and is about twenty feet thick. Both grainstone (ooid and bioclastic) units are resistant, and form massive cliffs above the west

fork of Timber Creek. The light gray lime sands of the bioclastic grain¬ stone are festoon cross bedded in cross sets six inches to one and one half feet in thickness. In thin section the sand can be seen to be composed of ten to fifteen per cent colds, ten to fifteen per cent pellets, scattered lithoclasts, and about 75 per cent bioclasts. The skeletal portion includes abundant crinoid material, biyozoans, oncoids, brachio- pods, Endothyrid foraminifera* and shell fragments (perhaps aragonitic pelecypods and gastropods) in which original shell structure has been lost due to neomorphism. The centra] canals of crinoid ossicles, hryozoan zooecia, etc. are generally filled with miciite. Les& common types of skeletal debris include ostracodes, trilobites, gastropods, and echinoid spines. Average grain diameter is about ,5 to .6 mm, while grains range Per cent 0 20 40 60 80 100 0 20 40 60 80 100 1 1 1 1 1 1 I 1 1 1 1 1

ooid gramstone dolomite argillaceous dolomite, T~si biociastic pellet gramstone ana grainstone pel letted mudstone rz^j interbedded mudstone, chert wackestone and packstone

Fig. 4 ROCK CREEK SECTION Fig. 5 RCCK CHEEK RCCK TYPES 6 in size from .1 millimeter to some elongate or tabular skeletal grains up to seven millimeters long.

Minor amounts of micrite (less than five per cent) occur in laminae scattered throughout the section. However, the rocks are largely micrite-

free and voids are filled with clear calcite cement. At Rock Creek the contact between the grainstone and the overlying dolomites is sharp; at

Timber Creek the bioclastic grainstone becomes finer grained upward and grades into a packstone over a distance of two or three feet.

The dolomite occurs immediately above the bioclastic grainstone in

the section. It is buff in color and occurs in beds four inches to two feet thick.

Staining and examination in thin section show the rock to consist of

50 to 100 per cent dolomite. The only undolomitized portions of the rock are crinoid debris and spar,although scattered bryczoan and brachiopod fragments occur. Scattered fossil debris preserved in this way indicates the original texture of the rock was that of a packstone or wackestone.

The dolomite rhombs appear to be of two basic types, a small (average crvs- stal size .035 millimeter), cloudy variety which makes up the majority of any sample, and a coarser (average crystal size .11 millimeter) clearer variety which occurs in small patches a few tenths of a millimeter across.

This coarser variety often occurs lining vugs or associated with what¬ ever calcite remain in the section. In samples containing somewhat less dolomite and somewhat more skeletal material it often impinges on calcite spar and skeletal material and sometimes preserves relict features such as grain boundaries in the form of inclusions. In places it shows zoning, as described by Murray (1964). 7

In places the rock also contains small patches of microcrystalline quartz, but chert is uncommon in this part of the section.

The interbedded mudstone, packstone, and wackestone occur in two or three layers nine to twenty five feet thick in the Timber Creek and

Rock Creek sections. Beds average four to six inches in thickness.

These dark gray limestones show wavy, irregular bedding, and beds thin and pinch out over distances of a few tens of feet. The limestones are

interbedded with buff shale partings and argillaceous layers about half an inch thick. Abundant chert occurs in nodules and discontinuous

layers one half to two inches thick. These are frequently disposed on either side of the shaley and argillaceous layers. Scattered layers

and lenses of bioclastic, pellet, and ooid grainstone also occur in these

intervals.

The mudstones are pelleted and occasionally show evidence of burrowing. Pellets are ellipsoidal and are quite uniform in size,

ranging from .02 to .25 millimeter in length and averaging about .08 millimeter. Faunal elements include the ubiquitous crinoid and brvozoan

debris, as well as solitary corals, brachiopods, and gastropods. In

places the biota is restricted, being dominated by ostracodes and calci-

pheres (see fig. 5).

The upper layer of these rocks (between 53 and 67 feet, fig. 3), as well as the overlying upper grainstone contain small amounts of opague

minerals. These opaques are concentrated within grains or at their

boundaries, and are sometimes associated with chert replacements. The

opaques occur in crystalline masses, as single euhedral crystals, and in

finely disseminated form, (see fig. 20) are gold with metallic luster in

reflected light, or black, red and yellow in reflected light, the former

being pyrite, the latter various iron oxides. 8

Layers of argillaceous yellow gray and buff weathering dolomite

and pellet packstone and grainstone three to ten feet thick occur in the

Timber and Rock Creek sections. They are yellow gray in color, and

weather to buff. The rocks display horizontal millimeter laminations and

ripple cross lamination. Some samples appear burrow mottled. In places

these rocks occur in thin layers interbedded with the mudstones, wacke-

stones, and packstones described above.

A few discontinuous chert layers and nodules one to two inches thick

occur in these intervals. Fossils are rare. The rock is composed of

clay, quartz silt, calcite and dolomite. Some thin sections consist of

even grained dolomite; others, those showing millimeter lamination, consist

of laminae of coarser dolomite (average rhomb size .03 to .04 millimeter)

alternating with finer grained laminae (average rhomb size .01 millimeter

or less). The finer grained laminae are those in which the clay minerals

are concentrated. In slides, original textures of the rock are preserved

in unaltered laminae, and in chert nodules. These show that the rock

was composed largely of pellets and silt size skeletal material.

The ooid grainstone is 60 to 70 per cent ooids. It contains minor

amounts of pellets and lithoclasts. Crinoid debris dominates the

bioclastic fraction, but oncoids and brachiopods are also common, and

there is some fenestrate bryozoan and ostracod material present. The sedi¬

ments are essentially mud free and are cemented by clear calcite spar.

In outcrop the rocks are red in color, owing to opaque minerals

(pyrite and iron oxides) concentrated in the ooid coatings, pellets, j lithoclasts, and micrite coatings of bioclastic debris. The Rock Creek

section is the freshest of the four in the Big Snowys, being exposed in a 9

fairly recent road cut rather than along a stream as at Timber Creek.

Here many of the oolitic strata are predominantly black in color, and are red only along bedding planes and joint surfaces where they have been affected by weathering.

The grainstones are cross bedded in sets six inches to a foot thick

(fig. 6). The limited cross bedding data available are shown in fig. 7.

Grains become coarser upward in the section and increasing amounts of

lithoclasts and whole brachiopod shells occur. At the top of the Rock

Creek section is a dune-like feature with a relief of about two feet. In

all the sections in the Big Snowys, the ooid grainstone has a sharp upper

contact with a layer of encrinite, which is interbedded upward with mud¬ stone, wackestone, and argillaceous layers.

Five basic rock types also occur in the Belt Creek section: 1)

crinoidal grainstone and packstone, 2) skeletal grainstone, 3) bryozoan packstone and wackestone, and mudstone, 4) argillaceous wackestone and

fossiliferous shale, and 5) ooid grainstone and packstone (see figs. 8 and 9).

In outcrop the crinoidal packstone and grainstone are resistant

cliff forming units. They are light orange-gray in color, weathering tc gray and buff. Beds are six inches to a foot thick. In thin section the

rocks are composed almost entirely of crinoidal debris, with lesser

amounts of fenestrate bryozoans and brachiopods. Small amounts of micrite occur in the intergranular spaces, but grains are largely cement¬ ed by syntaxial overgrowths on the crinoid material. Stylolltes are ex¬

tremely abundant, and apparently the residues which have accumulated

along these surfaces give the rock its faint orange tinge. Fig. 6. Cross bedding in the ooid grainstone exposed along Timber Creek. Hammer is 12-1/2 inches long. N

B.

Fig. 7 Paleocurrent directions. Data from cross be.'.ding in grainstones. A. Rock Creek; 20 measurements B. Timber Creek; 10 measurements 10

The skeletal grainstone overlies the crinoidal packstone, and occurs elsewhere throughout the section, commonly interbedded with the odd grainstones. The skeletal grainstones form resistant layers. Units are generally massive, in places cross bedded. Fresh surfaces are gray, weathered ones are somewhat yellowish. In thin section the rocks are composed of sand sized fragments of fenestrate bryozoans and crinoids.

Minor biotic elements include ostracodes, oncoids, gastropods, and trilobites. Pellets may also be a common constituent. The grainstones contain little micrite and are cemented by clear calcite spar. Grains are moderately sorted, ranging in diameter from .1 to 2.0 millimeter and averaging about 0.4 millimeter.

The bryozoan packstones and wackestones, and mudstones make up a large portion of the section. They are frequently interbedded with the fossiliferous shale and argillaceous wackestones described below. They are cliff forming in outcrop, although they are not as resistant as the grainstones and packstones mentioned above, probably because of the shale interbeds. Bedding thickness varies from four inches to a foot.

Bedding surfaces are wavy and irregular, and in places beds are discon¬ tinuous and can only be traced for a few tens of feet (see figs. 10 and

11). The rocks are brownish gray when fresh and weather to yellow gray or buff. Silicified fossil material, including brachiopods, solitary corals, crinoids, and bryozoans, is common. In thin section the fauna is dominated by fronds of feaestrate bryozoans; crinoid debris and brachio¬ pods are also common. Some of the interstitial micrite is pelleted, but much of it appears to be fairly homogeneous. Some samples are mottled, perhaps due to burrowing. In some of the wackeetone and mudstone fossils Per Cent 0 20 40 60 50 100 0 20 40 60 8J 100 1 1 1 1 1 1 I 1 1 1 1 1

I II o Q. o W O 0) 3 3 r+ CL o (/>

23 Ooid grainstone Skeletal and packstone grainstone Argillaceous wackestonc 33T Crinoid grainstone and fossilliferous shale and packstone Bryozoan packstone and Chert wackestone; mudstone

Fig. 8 BELT CREEK SECTION Ooid grainstone and packstone

Argillaceous wackestone and fossiliferous shale

Bryozoan packstone and wackestone, and mudstone

Skeletal grainstone

Crinoid grainstone and packstone

Fig. 9 BELT CREEK ROCK TYPES QJ •H >> c X r-H o O CO u Pi •n (0 C- <0 a. < 0 o u CO o > • a ■o o c Pi • CO U C PI o 0) a *H c o 4-1 o U w c 0) 0) •H 9) > N e •H U o 0) 0) PI PI « c CQ <0 B *H

•H00 fp 11

tend to be broken and fragmented, but in many of the packstones, large unbroken bryzoan fronds form the framework of the rock, the mud forming geopetal fabrics in the interstices.

The argillaceous wackestones and fossiliferous shale form beds a few feet thick below the crinoidal grainstone and packstone, and below the uppermost grainstone unit in the section. These rocks are brownish gray and weather to buff, orange, and in places red. They are thinly bedded (about one half inch). They have little resistance to weathering, form the recessive portions of the section, and probably constitute the dominant rock type in the covered intervals. They also occur in layers a few inches thick interbedded with the packstones, wackestones and mudstones (fig. 11). In thin section they consist of a thinly laminated mixture of raicrite and clay minerals containing scattered fossil debris, predominantly crinoids, bryozoans, and braehiopods.

The ooid grainstones and packstones, so prominent in the sections in the Big Snowys, form only a minor portion of the section at Belt

Creek, and occur interbedded with other types of grainstones and pack¬ stones mentioned above. Some of these ooid grainstones are very similar in appearance to those which occur in the Timber and Rock Creek sections.

In general, however, the oolitic coatings are not so well developed, and a micritic matrix is common. Some oolitic coatings have been mieritized, and the original lamination can barely be seen.

In outcrop these rocks form part of the massive ledges in which the skeletal grainstones occur.

Correlations based on lithology between all the sections measured are shown in fig. 12. Rock Cc Timber Cn 3 Timber Cc ENVIRONMENT OF DEPOSITION

PREVIOUS WORK

A number of models have been proposed for deposition of sediments similar to those which formed the Lodgepole. Criteria from all of these were used in interpreting the rocks described above.

Wilson (1967b) proposed the following typical cycle for the upper

Pennsylvanian of the Sacramento Mountains of New Mexico:

SOUTH WALL MOUTH OF SEEMAN CANYON SW 1/4, SE 1/4 SEC i T I 6 S R 10 E M O UNTA I N S NEW M E XI CO PENNSYLVANIAN (VIRGIL)

OOLITE Su3PHASE (Copping Bed )

'O' £ j Lowe r I Terrigenous ^>CI ostic Phase

2mm iv- *3 o LEGEND

| & | Ooid

SZ3 llndet., m i c r o b i o c I a s t $ (.• Pellets and shelly bioclasts (BC)

Edgewise conglomerates - pieces up to several cm.

\Q o | Llthoclasts of o few mm size

Tubular Foramimfera end chambered Forommifera

TYPICAL CYCLE IX UPPER PENNSYLVANIAN OE SACRAMENTO MOUNTAINS, NEW MEXICO, SOUTH BEEMAN CANYON, CYCLE 2

Fig. 13. From Wilson, 1967b 13

Wilson (1967a) also described sedimentary cycles in the Duperow

Formation of the Williston basin deposited in a broad shallow sea behind the Devonian Leduc reefs. Brown lime muds were deposited during the beginning normal marine phase. These were thoroughly burrowed and pelleted by mud injesting organisms, and little lamination was preserved.

The sparse fauna consisted of brachiopods, crinoids, ostracodes, and gastropods.

This was followed by a period of restricted marine deposition re¬ sulting from increasing salinity. Although there was limited burrowing, lamination was generally preserved in these rocks. The rocks included some beds of pellet grainstones. The fauna was restricted to pelagic forms or those with a tolerance for high salinity (ostracodes, calcispheres, and some segmented spicular forms of uncertain biological affinities). Shoaling and continued increase in salinity resulted in deposition of unfossiliferous lime muds which display lamination, pelleting, mud cracks, and algal stromatolites. Deposition of evaporites ended the cyclic sequence.

Wilson (1969) as a result of facies studies of the Lodgepole

Formation in Montana, concluded that the sediments were deposited in the form of "6 to 8 typical neritic cycles" around and over a Mississippian positive element, the central Montcuna high. Wilson (personal communica¬ tion, 1968, 1969, 1970, 1971, 1972) used the criteria developed in his work in the Sacramento Mountains to interpret the environments of deposi¬ tion of the Lodgepole rocks (see fig. 14). This illustration gives his interpretation of the same portion of the section which I studied.

Wilson believes the top of the bioclastic grainstone (the 280' level in TIMBER CREEK

Oolite subphase \ of clear water shoaling phase

Normal marine phase

Beginning clastic phase

m Ooid _ CH Crtnotd _ Pellets and shelly bioclosts _ E0 Edgewise conglomerates _ pieces up to several cm _ fecal Uthoclosts of o few mm size _ + | Tubular forammifero and chambered forammifera __ Very fm# grained “evaponte* dolomite _ y /Sucrose or fme »o medium grained replacement dolomite _

Fig. 14 Environment of deposition of Lodgepole rocks (Wilson, pers, comm.) 14

his section) to have a subaerial exposure surface. This period of sub¬ aerial exposure was followed by rapid marine transgression and deposi¬ tion of the ’’beginning clastic phase** in a deep water or basinal environ ment. Gradual shoaling occurred, and "normal marine** sediments were deposited. Continued buildup toward sea level resulted in sedimentation in the zone of wave and current action, the **clear water shoaling phase"

A period of subaerial exposure of the top of the oolites of the clear water shoaling phase was followed by resubmergence and another pulse of deep water "clastic phase" sedimentation.

Smith (1972) also made a facies study of the Lodgepole in central

Montana and he divided the Lodgepole rocks into five two-part deposi- tional cycles. Each of his cycles is characterized by a fine grained lower unit dominated by horizontally laminated carbonate mudstones, pellet carbonate grainstones, and finely crystalline dolomites, and a coarser upper unit characterized by cross laminated, medium- to coarse¬ grained bioclastic and oolitic carbonate grainstones. He interprets the rocks of the fine-grained lower unit as quiet water deposits of a deeper water transgressive phase, and the upper coarse grained unit as accumula tion of the shallow water regressive phase of the Lodgepole sea.

Armstrong and Mamet (1970, p. Nl) used the criteria established by

Wilson (1967a,b; 1969) as well as those established by other workers

(Ball, 1967; Murray and Lucia, 1967; and others) for interpretation of carbonate environments in a study of the Carboniferous Lisburne Group of the northeastern Alaska. They developed the model shown in fig. 15. §■ o K4 o «A .M 4 « • <4-1 vA O . .. •H « < —. *-l jB O •«WBN ectj M o O' •H O O h P-4 6 C w BACK aw> 15

They state (p. N4) "apparently vast gardens of crinoids and bryozoans existed in the vicinity of the oolitic banks. They are believed to have lived in shallow water.... Behind the oolitic banks was a realm of invertebrate remains again dominated by crinoids and bryozoans, but as the distance from the shelf edge increased, the diversity of invertebrates decreased and the percentage of lime mud increased.... The slope of the carbonate shelf is composed of lime mudstone and bioclastic material deposited in a down-slope or basinal position. These rocks are thin bedded, are characterized by millimeter laminations, and, upon close examination, are micropelletoid lime grainstones. They may contain radiolarians and may have dark nodular to bedded chert (Wilson, 1969)."

Edie (1958) studied the sediments of the Mississippian Bakken,

Lodgepole, Mission Canyon, and Charles Formations in southeastern

Saskatchewan. These sediments were deposited on the northeast side of the

Williston basin. Edie (1958, p. 102. 103) proposed the interpretation and classification of the environments of deposition and the sediments formed in them shown in fig. 16.

Illing (1959) studied the cyclically deposited Mississippian sedi¬ ments at Moose Dome, southwestern Alberta, and developed the following

"general carbonate sedimentation cycle": Fig. 16. Interpretation of environments of depostion of the Bakken, Lodgepole, Mission Canyon, and Charles Formation in south¬ eastern Saskatchewan. From Edie, 1958, p. 102, 103. 16

- ~ I * L*J nil n to i . . „ — « ! ft rNViRONMLNT Sf DlMENr 1; * 1 l *«>■< t mu H ‘ I G |l< jl<4 •M lll't Hjij f r - , t' “ • i f — ... . — e 5 i; eufcei'.J-

1 i>n fir-r.M . / D • ,n*trttSJ0

Fig. 17. General carbonate sedimentation cycle. From Illing, 1959, p. 39.

According to Illing (1959) the sediments were deposited on a broad shallow epicontinental shelf which deepened to the west and southwest away from the Canadian Shield. Environmental belts formed, controlled by depth of water and degree of contamination by terrigenous sediment.

These belts (argillaceous pasty limestones, bioclastic lime sands, oolites, lagoonal lime muds, and evaporites) moved back and forth across the shelf in response to changes in sea level and gave the sediments a "marked cyclicity".

Macqueen and Bamber (1967,1968) developed the following model for the Mississippian Banff Formation (equivalent to the Lodgepole) and the overlying Rundle Group of southwestern Alberta: 17

Fig. 18. Diagraomatic interpretation of Mississippian depositional and early diagenetic environments, southwestern Alberta (see text/for explanation). From Macqueen and Baaber, 1967, 1968.

Their classification is based on studies of recent carbonates, especially those in the Persian Gulf, and work by Douglas (1958) and by Illing

(1959). Each depositional gnalnnaatt is closely dependent on water depth and distance from shore and is characterized by distinct assemblages of sediments. These occur as subparallel sedimentary facies perhaps tens of miles wide fringing the craton to the east, and seaward of islands.

They believe that modern easiMneats analogous to these Mississippian ones occur along the southern and southeastern shore of the Persian Gulf. 18

Malek-Aslani (1971, p. 351) found a similar sequence of depositional environments in the Mississippian Mission Canyon carbonates in north- central North Dakota. He states "the deepest part of the Williston basin during Mission Canyon deposition was the northwestern corner of

North Dakota. A broad shelf extended eastward, where marine water circu¬ lation was sufficient to support bottom-dwelling and skeletal producing organic communities. A trend of oolitic bars separated the open shelf from a restricted lagoon farther east. The lagoon was an area of non- skeletal carbonate sedimentation which changed facies eastward to tidal flats and supratidal 'sabkhas9 where evaporitic deposition occurred."

The summary of the sedimentary and faunal characteristics of each en¬ vironment shown in fig. 19 was gleaned from his lectures given in 1971 at

Rice University and at the annual meeting of the Geological Society of

America in Milwaukee.

INTERPRETATION

Most of the models discussed above are basically similar, and criteria from all of them were used in interpreting the environments of deposition of the Lodgepole sediments.

Wilson (1969), as a result of facies studies of the Lodgepole, be¬ lieves the rocks in the Big Snowy Mountains to have been deposited in the form of "typical neritic cycles" around a Mississippian positive element, the central Montana high. He believes the sediments farther to the west, in the Little Belt Mountains, which consist largely of "dark siliceous argillaceous thin-bedded lime mudstone whose fauna is almost exclusively siliceous spicules and calcareous fenestrate bryozoan fronds" (Wilson,

1969, p. 14) to have been deposited in somewhat deeper water. c c VJ P O

P T- c r- O'

C

'r-* P (/) r~* •r*i O' P <. G I - *■'' U C CV r—-f

G G C £ c F, AJ ^ it - L c c

0 • r; c c G ;>, G c ■G c

G c. G u u

•r-* t. 19

Petrographic and faunal evidence tends to substantiate this.

Unlike the Rock Creek and Timber Creek sections there are few grainstones

in the Belt Creek section, the rocks consisting largely of packstones, mudstones, and wackestones. This abundance of micritic rock types in¬

dicates deposition in quieter, presumably deeper water. The scarcity of oolites, which are so common in the section in the Big Snowys, also

indicates a lack of vigorous and continuous current action. There

characteristics, plus the very minor occurrences of dolomite, and the

greater quantity of argillaceous sediments indicates that the majority

of the sediments in the Belt Creek section fall into Macqueen and Bamber's

environments A, B ("offshore") and C ("shallow water open sea") environ¬

ments.

Faunal evidence includes the scarcity of such presumably shallow water forms as Endothyrid foraminifera (Sando et al., 1969; Armstrong

and Mamet, 1970), oncoids, and possibly calcispheres (Baxter, 1960) in

the Belt Creek rocks. All these fossil types are common in the rocks

in the Big Snowys. In the Belt Creek section the abundance of fenestrate

bryozoan fronds showing little evidence of transport or breakage indicates

somewhat deeper water environment. Growth forms of bryozoans appear to

be controlled by degree of turbulence and the nature of the substrate

(Duncan, 1969). According to Gautier (1962) fenestrate bryozoans occur

most abundantly dn unconsolidated sandy or muddy-sandy bottoms subjected

to little or no turbulence (Duncan, 1969, p. 368). In the lower portions

of the Lodgepole bryozoans were important contributors to "Waulsortian-

type" carbonate mounds which reached heights of 60 to 150 feet above the

surrounding sea floor (Cotter, 1965, p. 888; Stone, 1971, p. 723). This 20 gives some measure of minimum water depths at the time of deposition, and indicates that the fenestrate bryozoans grew in relatively deep, normal marine water.

The crinoids probably occupied an environment similar to that of the bryozoans. There seems to be general agreement (Murray and Lucia,

1967; Macqueen and Bamber, 1968; Armstrong and Mamet, 1970) that crinoids grew in vast gardens or banks in moderately shallow, normal marine waters

(Macqueen and Bamber's environment C) subject to moderate current action.

They were extremely abundant and were able to form thick accumulations of encrinite such as occur at the base of the Belt Creek section.

The bioclastic and ooid grainstones in the Big Snowys were deposited in the highest energy environment (Wilson's dear-water shoaling en¬ vironment; Macqueen and Bamber's environment D; Malek-Asiani's shelf margin oolite bar; etc.). By analogy with oolite environments in the

Bahamas, Persian Gulf, and elsewhere, the ooids were probably deposited in very shallow, turbulent water. The limited paleocurrent data obtained from cross bedding in these grainstones are shown in fig. 7. These data conform to the pattern described by Ball (1967, p. 583) for marine sand belts. Additional data on the type and orientation of sedimentary structures within these grainstones and on the areal distribution of the grainstones themselves are necessary to clarify their exact mode of deposi¬ tion.

The presence of pyrite in the upper grainstone presents a problem, since one would assume ooids would form in a well-oxygenated environment.

Perhaps the pyrite was deposited after deposition of the sand by waters from the more restricted lagoon which appears to have existed behind the sand shoals. The iron oxides which give much of the upper grainstones Fig. 20. Pyrite replacements. A. The black spots in this picture (arrows) are pyrite, which partially replaces ooids and bioclasts, and occurs disseminated through the micrite. (The ooids to the right, of the picture fill a burrow which penetrates the micritic layer.) B. Large euhedral pyrite crystals associated with a chert replacement (lighter area) of a bryozoan. 21

their red color are probably the product of recent weathering and altera¬

tion of the pyrite. In thin section pyrite is often seen which is par¬

tially altered to red iron oxides, and commonly the iron oxides occur in

crystals with eunedral outlines indicating they are probably pseudoraorphs

after pyrite (fig. 20).

Studies in the Bahamas (Purdy, 1964) indicate that few organisms

are capable of living on the constantly shifting sands of the ooid shoals.

Most of the skeletal material was probably derived from the crinoid and

bryozoan banks growing in slightly deeper water. The common micrite

fillings of the axial canals of crinoid columnals and bryozoan zooecia

may indicate that much of the skeletal material was originally deposited

in the muddy offshore environment or in swales between ooid bars, and

was later reworked, perhaps by storms or strong tides, and delivered to

the sand shoals. Some of the many lithoclasts containing mud and

skeletal material may also be the result of this reworking of material.

The portions of the Timber and Rock Creek sections between the upper

and lower grainstones composed of the interbedded mudstones, wackestones,

and packstones, (including those later dolomitized) as well as the

argillaceous dolomites and pellet grainstones and packstones, were prob¬

ably deposited in a lagoonal environment.

These sediments are composed dominantly of pellets and pelleted muds.

In portions of the section the skeletal material is extremely scarce, or

is dominated by calcispheres, spicules, and ostracodes. Wilson (1967a,

p. 245) described similar sediments in the Duperow ("unfossiliferous

pelleted lime mudstone; pelletal, fine to medium lime grainstone; and lime wackestone with a restricted marine fauna'1) and interpreted them as

restricted marine carbonates deposited during episodes of high salinity. 22

Macqueen and Bamber (1968, p. 269) described similar sediments from the

Carnarvon Member of the Mount Head Formation and interpreted them as

being deposited in "the shallow, lime-saturated waters of an extensive

lagoonal system".

The portions of the section which contain a more normal marine

fauna (some crinoids, bryozoans, and brachiopods) still contain a high

precentage of mud, and the skeletal material is commonly disarticulated, broken, and abraded. These sediments may have been deposited close behind the ooid shoals, where waters were somewhat less restricted and where skeletal material could be washed over the shoal and deposited behind it. Armstrong and Mamet (1970, p. N4) describe a similar situation

(see previous discussion). Further evidence of restriction lies in the presence of pyrite indicating reducing conditions, and the lack of burrowing which permitted the preservation of delicate laminations in the argillaceous dolomites and their precursors, the pelletal grainstones and packstones.

There is no evidence of this restriction in the Belt Creek section, where the sediments are all normal marine types. Thus the restriction was probably a local one, rather than basin-wide, and probably occurred in lagoons separated from the main water body by the oolite and skeletal sand banks and shoals.

Rapid lateral and vertical changes in lithology indicate deposition in shallow water. So does the abundance of Endothyrids. Armstrong and

Mamet (1970, p. N2) state that "most, if not all, Carboniferous small foraminifers are benthonic...and are essentially restricted to the euphotic zone and to the upper part of the platform." 23

Wilson (personal communication) considers the portion of the section between the two thick grainstone layers to have been deposited in a deeper water offshore environment, rather than behind the ooid shoals in a lagoonal system, (see fig. 14) He interprets the tops of the grain- stone layers as subaerial exposure surfaces. His interpretation is based on criteria developed in the above mentioned studies in the Pennsylvanian of New Mexico, as well as on facies studies of the Mississippian of

Montana. He notes that the tidal flat deposits which one would expect to find associated with lagoonal deposits are absent, and that the lagoon¬ al deposits which occur higher in the section in the Mission Canyon do not resemble the rocks in the portion of the Lodgepole covered in this study. He also states that the pellet-silt calcarenites show ripple cross-lamination, and that this would be unlikely to occur in a lagoon where water is shallow and currents are rare. In addition he states that some of the petrographic types in this portion of the section in the Big

Snowys occur interbedded with dark shale and chert in wells in the

Williston basin.

I think it is difficult to apply the model developed by Wilson in his study of the Pennsylvanian in the Sacramento Mountains of New Mexico to the Mississippian of central Montana. First, Wilson (1967b) states the cyclothems in the Sacramento Mountains formed during a period of active movement along a fault whose scarp probably formed the shoreline at certain times in the history of the cycle. Cycles contain conglom¬ eratic channel fills and cross bedded quartz sandstones. In contrast, the

Mississippian rocks contain only fine-grained elastics: clays and minor amounts (never more than five per cent) of quartz silt. Deposition oc¬ curred on a very broad, stable shelf; there is no evidence of faulting. 24

The great disparity in rock types present and in paleotectonic environ¬

ment would indicate the two situations are not analogous.

In the Sacramento Mountains, Wilson (1967b) cites leaching, channel¬

ing, fracturing, and red staining of capping beds as evidence of subaerial

exposure. Of these features, only the red stain is present in the Lodge-

pole rocks in the area covered in the present study, and this is a Recent

weathering phenomenon, the result of alteration of pyrite to iron oxides

(see above). Several authors (Purdy, 1968; Dunham, 1969a,b; Land, 1970;

Ward, 1970) discuss petrographic features of carbonate rocks which result

from diagenesis in the subaerial environment, including meniscus cement,

microstylactitic cement, leaching, vadose silt, and others. Extra samples were collected at likely horizons (the tops of grainstone units), and a

careful search was made for these features in all the Lodgepole rocks, but

none were found. Nor is there evidence of a widespread break in deposi¬

tion at the tops of these grainstone units, as in the Timber Creek sec¬

tions the bioclastic grainstone becomes finer-grained upward, its micrite

content increases upward, and it grades into a bioclastic packstone just below the contact with the dolomite.

In addition, a close study of figure 12 will show that, since there were no large breaks in deposition of the Lodgepole rocks, deposition must have been more or less continuous over this area. Since the grain¬

stone beds in the Big Snowys and the Little Belts are considered correla¬

tive, the material between these two units must have been deposited during essentially the same period of time. (I am not suggesting the grainstones

are time horizons, but merely that there was some overlap in time of depo¬ sition of the intervening rocks.) It is difficult to see how "deep water" sediments (the mudstones, wackestones, packstones, and pellet grainstones and packstones) could have been deposited on the paleo-high in the region 25

of the Big Snowys, while down paleoslope in the present area of the Little

Belts, shallow water sediments were forming, as evidenced by the cross bedded ooid and skeletal grainstone layers which occur in the central part of the section there (see fig. 8). Presumably sea level was somewhat lower during this intervening period, and as the zone of carbonate sand deposition migrated to the northwest, lagoonal sediments accumulated in the region behind them.

The absence of tidal flat deposits in the section does not invali¬ date this picture. If the lagoon were a very broad rather deep one, such as that which occupies the center of the Bahama banks, one would -xpect tidal flat sediments to be absent over large areas.

Most of the vertical lithologic changes within the rocks covered in the present study are gradational, and the different rock types are inter- bedded. The absence of any laterally continuous sharp lithologic breaks or evidence of disconformity in these sections indicate variations in sea level and the attendant shifts in facies were probably fairly continuous

(see fig. 21).

The terrigenous elastics in the Lodgepole are all very fine grained, and may have been carried into the sea by winds. Although they dominate the deepest water sediments, they occur throughout the sections, even forming occasional thin layers through the ooid grainstones (see appendix

II, section TC-1). Quartz silt, although never abundant, occurs in many of the sediments in the Big Snowys, but is very scarce in the Belt Creek rocks, indicating that only the finest sediments were carried into deeper water.

There is no evidence of tidal flat or sabkha sedimentation anywhere in these sections. 9 r 1 c :•!.« . 1 ■ r i a t i on r, i n . r n l o vo 1 an t• a r ( on d r. n t a i * t ^ i n on vi. rorvu^n t* o of do >o s i t. i on ol_ lod ooolo roc . 4- 26

DIAGENESIS

CEMENTATION

Cements in the Lodgepole rocks are of four basic types, all composed of low magnesian calcite:

1) isopachous druse 2) micrite cement 3) syntaxial overgrowths on echinoderm material 4) coarse, blocky spar

The isopachous druse forms crusts .005 to .04 millimeter thick around grains. Crystals making up the crusts are oriented perpendicular to the surface of the grain encrusted and have euhedral terminations into the pore spaces (fig. 22). Cement of this type occurs sporadically throughout the grainstones in all sections measured (see appendix III). It is pres¬ ent in approximately half the grainstone samples. In some cases it occurs in only a portion of a single thin section and is absent from the rest.

In many cases grains are fractured, or ooid and micritic coatings of grains with their overlying coat of druse are fractured and split away from the interior of the grain, apparently as a result of compaction. These frac¬ tures are healed not by the druse, but by coarse blocky calcite (see below). Evidently cementation with isopachous druse preceded compaction.

In addition, highly deformed and compacted grains may be surrounded by a rim of druse separating grains (fig. 23). Coogan (1970, p. 926) notes a similar occurrence in the Mississippian of Tennessee in which "the grains have been pressed closely together, indeed appear to be fitted together but few of the grains touch." He suggests that this might be a result of partial cementation "at or near the surface during or immediately after deposition", and subsequent compaction after burial, during which the early formed cement held the grains apart. Fig. 22. Isopachous Druse - Each grain is surrounded by a rim of calcite crystals. Remaining pore space is filled by coarse blocky

Fig. 23. Compacted and deformed ooids separated by rims of isopachous druse. 27

Land (1970, p. 181) describes similar cement from the Belmont beach-

rocks in Bermuda, and attributes the isopachous nature of the cement to

"uniform radial crystal growth from grain surfaces into sea-water-filled

pores." Uniform fibrous crusts of aragonite and equant crusts of magnesian

calcite have been described from Recent intertidal sands by several workers (Taylor and Illing, 1969; Moore, 1970; Tietz and Muller, 1970).

They have also been described in Recent submarine sediments (Shinn, 1969;

MacIntyre and Milliman, 1970; Marlow, 1970). Ball (1967, p. 563) describes oolite sands in the marine sand belts of the Bahamas which are cemented by a "furry fringe of aragonite cement". He states that this cement occurs in areas where the surface of the sand has been immobilized for some time, often by the binding action of grasses. Broken fragments of this rock sometimes form clasts in the sands.

If the isopachous druse in the Lodgepole rocks formed as high magne¬ sian calcite, it would originally have looked as it does now. However, if it originally formed as fibrous aragonite, we have a morphology problem.

Both Land (1970) and Shinn (1969) describe low magnesian calcite cements which have resulted from alteration of these aragonite cements. Land

(1970, p. 180) describes the result as "a distinct isopachous 'onion-skin' rind of calcite cement around each grain. ... Individual crystals are not acicular, but bladed or bloeky."

The similarity of the isopachous druse to modern intertidal and sub¬ marine cements, the evidence that it formed early in the history of the sediment, probably soon after deposition, and its irregular distribution in the measured sections, all tend to indicate that it formed in the inter¬ tidal or shallow submarine environment. 28

Micrite occurs in two forms in the grainstone layers. Both can be seen in fig. 24. One type is generally pelleted, may contain quartz silt grains and small skeletal fragments, and is generally confined to narrow laminae a few millimeters to a few centimeters thick. It shows shelter effects and forms geopetals, evidence of mechanical deposition.

The other type of micrite lines the void in the top half of the brachio- pod shell above the geopetal in fig. 24. This micrite sometimes has a slightly clotted appearance but is generally homogeneous. It forms irregular coatings on grains, and in places almost entirely fills pore spaces. It does not show shelter effects, but occurs on all sides of pores.

Many of the workers who described aragonite cements from the Recent also describe cements which are composed of microcrystalline high magne¬ sian calcite and look like semiopaque mud. Cements of this type are forming today in both the intertidal and submarine environments.

The micrite cement occurs in some lithoclasts in the upper grainstone layer, indicating that the cement formed quite early, since it was eroded and reincorporated into the sediments. The cement obviously formed in water shallow enough to allow erosion by waves and/or currents. This evidence plus the similarity to Recent intertidal and shallow marine ce¬ ments suggest that these rocks were cemented in the intertidal or shallow subtidal environments.

In Carbonate Cements (Bricker, ed., 1970, p. 47) it is stated "there are virtually no differences between beachrock and submarine cementation as far as morphology and mineralogy are concerned." Thus at present I am unable to determine in which of these environments the isopachous druse and micrite cements formed. Fig. 24. Micrite cement - the lower half of the brachiopod shell in the center of the photograph is filled with mechanically deposited sediment composed largely of pellets. The upper half of the shell is lined with micrite cement. The remainder of the cavity is filled with coarse blocky spar. 29

Shinn (1969) notes that rate of submarine cementation is controlled by rate of sedimentation. It may be that the vertical variations in the occurrence of the micritic and isopachous druse cements reflect variations in rates of sedimentation. Shinn (1969, p. 141) states that in the near¬ shore belt in the Persian Gulf, submarine cementation occurs when ’’sedi¬ mentation rate has become retarded as a result of build-up to sea level or other factors which locally restrict current movement and rapid accumulation." This may have applied to the Lodgepole sands. Or, since ooids are generally deposited in very shallow water, as in the Bahamas, these beds may have been cemented during periods of exposure during low tides.

Calcite cement also occurs as syntaxial overgrowths on crinoid material, being coarse crystals that when viewed under crossed nicols are in optical continuity with the crinoid skeletal material. The quantita¬ tive importance of this cement type depends in part on the abundance of such crinoid material. However, not all such material has these over¬ growths, and crinoid material with and without syntaxial overgrowths mav occur side by side in the same slide. Presumably both types of grains were in the same physico-chemical environments, so the difference must lie in the grains themselves. Those with thick oolitic or micritic coatings lack the overgrowths and are coated by isopachous druse or are cemented by coarse, blocky spar. In general, those which lack coatings or whose coatings are less than .01 to .02 millimeters thick have the syntaxial overgrowths. In most instances when grains with thicker coatings have the overgrowths close examination of the grain reveals breaks in the coating.

In these cases the overgrowth is generally confined to the area of the break. 30

In the lower part of the Belt Creek section, some of the crinoid overgrowths may have formed at the expense of a surrounding micritic matrix. While the overgrowths in the Big Snowy rocks are relatively clear, those in the lower Belt Creek section in places are dusty with minute inclusion. In some cases the inclusions in a former void space in a skeletal grain show a geopetal arrangement in agreement with the orientation of the rock, as in accompanying sketch:

Under crossed nicols all this extinguishes as a unit. These dust inclusions may represent the former existence of lime mud in the void, which has been recrystallized in optical continuity with the surrounding echinoderm particle.

In places crinoid overgrowths show cross-cutting relationships with other grains. This might be the result of recrystallization, but no relict textures are preserved. In some cases the contact between over¬ growth and penetrated grain has the jagged shape and film of insoluble residues characteristic of stylolites. In addition, in some highly com¬ pacted portions of the section the crinoid material with its overgrowths is quite widely spaced, but the rest of the skeletal material is tightly compacted (see fig. 25), Apparently the overgrowths formed before compaction, and during compaction crinoid plus overgrowth behaved as a single unit.

Coarse blocky calcite spar fills all the remaining pore space in the rocks. Each pore is filled by one or a few crystals. Only a few samples of bryozoan packstone in the Belt Creek section have voids which lack this blocky cement or are incompletely filled by it. All the other Lodgepole JUL 72

Fig. 25. Syntaxial overgrowths on crinoids. Note that in lower and upper laminae grains are widely spaced, while in laminae in center of picture where overgrowths are lacking, grains are tightly compacted. 31

rocks in these sections are tightly cemented.

There is generally a sharp discontinuity in size between this cement and the isopachous druse. Crystals of coarse spar range in size from .1 to 3.3 millimeters, while the largest crystals in the druses average about

.01 millimeter in size. In some of the Belt Creek bryozoan packstone, voids are lined by isopachous druse, but are incompletely filled by or completely lack the blocky spar. This is another indication that the two cement types represent two separate stages of pore filling.

The blocky spar is apparently the last formed of the four cement types. It shows no strain or pressure solution effects as a result of compaction, and indeed heals fractures which resulted from compaction and so apparently post-dates it. Considerable grain interpenetration and stylolitization occurred during compaction and the calcium carbonate derived from this pressure solution would certainly be adequate to account for all of the coarse blocky calcite observed. The presence of some un¬ filled pore space in the Belt Creek rocks might be caused by lack of com¬ paction in the muddy lime sediments which dominate the Belt Creek section.

DOLOMITE

Dolomite appears in two forms in the Lodgepoie rocks. The first type formed very early in the history of the rocks and occurs in strata which contain abundant crinoid material. This dolomite occurs as patches in the center of and in optical continuity with crinoid ossicles and plates.

These dolomite patches are often surrounded by mierocrystailine quartz.

In some cases the dolomite has been replaced by the chert and the relict rhombic outlines of the dolomite patches are still visible as inclusions in the chert. The dolomite patches and the chert which replaces them, 32 unlike the surrounding crinoid particle and chert, are quite clear and free of inclusions. The doiomitization process must have resulted in concentration of impurities as the edge of the growing crystal.

Crinoid skeletons are composed of high magnesium calcite (13 to 14

mol per cent MgC03 according to Dodd, 1967, p. 1318). This would seem an obvious source for this type of dolomite. High magnesian calcite is altered to low magnesian calcite very early during diagenesis (Friedman,

1964). This crinoid debris was probably dolomitized during this early alteration process, with magnesium concentrating in certain portions of the skeletons and altering them to dolomite. Later in the sediment's history, the remaining crinoid material had been altered to low magnesian calcite and when the second phase of doiomitization occurred (see below) the large crystal size of the crinoid plates and their relative insolu¬ bility prevented it from being altered. The large quantities of crinoid material in the Belt Creek section probably made this sort of doloraitiza- tion possible. The concentration of crinoid material in the Rock Creek and Timber Greek rocks is less, and made this a much rarer occurrence

(only one example was found, near the base of section TC-3).

The second type of dolomite occurs quite abundantly in the Timber

Creek and Rock Creek sections, less abundantly in the Belt Creek rocks.

It is the main constituent of the beds immediately overlying the bioclastic grainstone and of the argillaceous laminated rocks farther up in the section (see fig. 4).

Little textural detail is preserved in the dolomites which immediate¬ ly overly the bioclastic grainstone, but the broader aspects of the sediments can be seen. In the incompletely altered rocks the finer- grained, dusty dolomite described in the section on petrology replaces the 33

micritic portions of the rock. The coarser dolomite impinges upon and partially replaces spar and skeletal material, and in places preserves some relict textures (fig. 26). Skeletal material shows a preferred order of doloroitization, the bryozoan and brachiopod material being affected more readily than the crinoid material. However, fossil material is rare¬

ly completely dolomitized. The percentage of dolomite a rock contains depends to some extent on the amount of skeletal material it first con¬

tained. Per cent dolomite corresponds roughly to original percentage of mud. Those rocks which are most completely dolomitized were originally wackestones and mudstones, Those only partially altered to dolomite were originally packstones.

Where rock has been thoroughly altered, skeletal material is pre¬ served as vugs lined with coarse dolomite euhedra, fig. 27. Murray (1960) noted the same phenomenon in dolomites from the Mississippian Charles

Formation in Saskatchewan and attributed it to ‘'local source" dolomitlza- tion. Weyl (1960) discussed this process and stated that ground waters commonly contain small concentrations of carbonate relative to concentra¬ tions of calcium and magnesium ions, and the bulk of carbonate for the dolomite is derived from the preexisting calcite and aragonite. This results in a mole-for-mole replacement of calcite by dolomite, and since dolomite is the denser of the two minerals it occupies a smaller volume which results in an increase in porosity in thoroughly dolomitized rocks,

In the argillaceous dolomites essentially no primary textures are preserved, and the only indications of the original character of the sediment occur in rocks which were only partially dolomitized, or which were silicified prior to dolcraitization. These occurrences show that these sediments were pelletal, or more rarely oolitic grainstones and packstones Fig. 26. Dolomitized Snail - Staining shows this sample to be nearly 100 percent dolomite. The out¬ line of the snail in the center of the picture has been preserved in the form of coarse, clear dolomite. The micrite matrix surrounding the snail has been replaced by fine-grained dusty dolomite. Fig. 27. Skeletal material in thoroughly dolomitized rock is preserved as vugs surrounded by coarse dolomite euhedra. Pelleted texture of micritic portion of rock can still be seen in dolomite replacement. Fig. 28. Relict textures preserved in chert nodules. Inclusions within these cherts preserve relict textures which have been destroyed by dolomitiza- tion in the surrounding unsilicified rock and show that these rocks were originally: A. ooid grainstone; B. pellet grainstone. 34

and pelleted mudstones (fig. 28). The chert nodules also show that silicification preceded this type of dolomitization, since primary tex¬ tures are obliterated by dolomite in the surrounding rock. In some of the incompletely altered samples it can be seen that dolomitization showed a preference for oolitic and micritic coatings and pellets (fig.

29).

Lamination in the primary sediments probably reflected differences in grain size and composition. However, in the completely altered rocks, the laminations result from differences in crystal size and clay content, the finer grained laminae being those in which clay minerals are concen¬ trated. Marschner (1968, p.55) notes that the "grain size of early diagenetic carbonate minerals is inversely proportional to the clay min¬ eral content. Probably envelopes of clay minerals prevented formation of larger carbonate crystals during diagenetic recrystallization."

Other minor amounts of dolomite occur throughout the section. These consist of small patches or isolated rhombs in the micrite of the mud¬ stones and wackestones, and as isolated rhombs in the oolitic and micritic grain coatings in some of the grainstones.

Dolomitization in these measured sections has a very uneven distri¬ bution. It shows a preference for replacement of micrite, including pellets, and in some cases, ooid coatings (fig. 29). But it shows rapid variations in distribution, both vertically and laterally. Murray and

Lucia (1967) suggest a number of possible causes for the distribution of dolomite. One of these is permeability control, such as Illing (1939) suggests. The most permeable sediments would be exposed to the greater volume of dolomitizing fluids and would be preferentially altered. As mentioned above, this is not the case in the Lodgepole, since in general Fig. 29. Dolomite preferentially replacing oolitic and micritic coatings of grains. 35

it is the least permeable sediments which are dolomitlzed. However,

permeability differences within the muds caused by cementation may have

been a controlling factor.

Murray and Lucia also suggest variations in solubility. This might

be the reason for the preferential replacement of ooid coatings. Pre¬

sumably they were composed of aragonite and were more soluble than their

nuclei or the other grains. Murray and Lucia also note that Recent lime

muds tend to have a high aragonite content, and suggest this as a possible

reason for preferential alteration of mud.

Particle size was also suggested as a controlling factor. "Since

lime muds have a larger surface area than the lime sands, for example, many more dolomite crystals per unit volume could nucleate in the mud

than in the sand. Thus given the same volume and kind of water the lime muds could be completely dolomitlzed and the lime sands only partially

dolomitlzed simply because of their reactivity difference." (Murray and

Lucia, 1967, p. 29). This might explain not only the preferential re¬ placement of mud, in the Lodgepole rocks, but also the differences in

crystal size between the dolomite replacing micrite, and that smaller portion of the dolomite which replaced spar and skeletal material.

Formation of this second type of dolomite post-dated silicification and early cementation. The former is shown by preservation of primary textures in chert nodules and obliteration of these textures by dolomite

in the surrounding rock. The latter is evidenced by the fact that the dolomite impinges on and partially replaces calcite cements in these rocks.

This second type of dolomite is clearly distinct from the first-described type. The first replaces crinoid skeletons, whereas the second tends to avoid them. The first type is much coarser than the second. In addition 36

the two are separated in time, since the first type preceded the forma¬

tion of chert whereas the second type post-dated it.

The relation in time between doloraitization and compaction is

difficult to determine. The wackestones and packstones in which dolomi-

tization tends to occur are subject to very little compaction, and the

sands, which may be intensely compacted, are little dolomitized.

Several mechanisms for dolomitization of these and similar rocks

have been suggested. Illing (1959) suggests that some of the magnesium

for dolomitization of the Mississippian rocks at Moose Dome was derived

from the high magnesian echinoderm skeletons in that deposit. However

he considers this inadequate to account for all of the dolomite, and be¬

lieves that the rest of the magnesium was derived from connate waters

escaping from more deeply buried sediments during compaction. These

waters would pass through the most permeable sediments, and the sands

would be the most likely to be dolomitized. Neither of these theories

seems applicable to the dolomites in the Big Snowys. The quantity of

echinoderm material present in the dolomitized rocks is much too small to

account for more than a very minor portion of the observed dolomite. And

if this were indeed the source of the magnesium then the bulk of the

dolomite should occur at Belt Creek, where very thick encrinites occur,

rather than in the Big Snowys.

Dolomitization by connate waters prior to cementation seems unlikely,

since rocks representing the least permeable sediment, the packstones

and wackestones and pellet grainstones and packstones, are dolomitized,

rather than the originally more permeable ooid and skeletal sands. Fur-

thur, it is doubtful that cementation produced a reversal in relative

porosity between the muds and the sands. The grainstones are compacted, but the micritic rocks are not, indicating the muddy sediments were the 37

first to be thoroughly cemented and thus remained the relatively less

permeable material.

Cotter (1966) attributes dolomitization of the Waulsortian bioherms

in the Paine Shale Member of the Lodgepole to seepage refluxion during

subaerial exposure of the tops of these banks. The absence of any

evidence of subaerial exposure in the rocks covered by the present study militates against this as a source for the dolomite.

Murray and Lucia (1967) suggest seepage refluxion as outlined by

Oeffeyes et al (1964, 1965) as a mechanism for dolomitization. They

state that if this is to be accepted as a mechanism for dolomitization,

there should be evidence of evaporites and tidal flats in the overlying section. The overlying Mission Canyon and Charles Formations do contain evaporite and tidal flat sequences which could have served as a source of dolomitizing waters.

Hsu and Siegenthaler (1969) propose an alternative to Murray and

Lucia's (1967) hypothesis. Hsu and Siegenthaler (1969, p. 13) state that

"the slight difference in density between a supersaline brine and normal seawater is not an effective hydraulic head to drive the brine through fine-grained lime sediments back to the oceans". They also cite lack of concrete evidence for seepage refluxion from the Recent as an argument against it as a mechanism. They call their alternative mechanism evapora¬ tive pumping. This mechanism calls for concentration of magnesium in

Interstitial brines under sabkhas by evaporation, and movement of these brines under the steepened hydraulic gradient produced by evaporative

loss near the surface. Flow rate is not governed by permeability, but is the same through coarse and fine sediment. 38

Total lack of any evidence for a supratidal environment in these dolomites, plus the very close association of dolomitized and undolomiti ed muds, agrue against evaporative pumping as a mechanism in the Lodge- pole. Presumably in a section dolomitized in this fashion the entire section below the tidal flat would be dolomitized, the depth of dolomitization depending on the length of time the process continued.

In conclusion, none of the above mechanisms is an entirely satis¬ factory explanation for the origin of the Lodgepole dolomites.

CHERT

The Lodgepole rocks contain minor amounts of quartz silt (see appendix II) but most of the quartz in these rocks is secondary. It occurs in several different forms:

1) as overgrowths on detrital quartz silt grains 2) as void filling crystals 3) as nodules 4) as equant granular mosaic of unoriented quartz crystals replacing skeletal material and, rarely, ooids. 5) as spherulites replacing fossil material 6) as large, generally euhedral crystals replacing skeletal material

The overgrowths on detrital quartz silt are in optical continuity with the host grain, but can be distinguished from it by minute calcite inclusions which mark the boundaries of the original grain and are scattered through the overgrowth. These overgrowths commoniy have euhedral crystal faces.

The void filling crystals fill cavities within fossil skeletons, and intergranular voids within silicified grainstones. This void fillin quartz commoniy overlies a layer of calcite cement and chert partially replaces the calcite cement, indicating silicification post-dated early cementation (see fig. 30). Fig. 30. The void which largely fills this picture was first lined with calcite crystals (A). Later quartz filled the remaining void space (B) and partially replaced the earlier formed calcite (C). Chert nodules and layers are very abundant in the Lodgepole, particularly in the packstones and wacket:£ones. They are relatively scarce in the grainstones. The quartz replaces both grains and mud.

This replacement chert is generally uniformly fine-grained under crossed nicols. Under plane light, relict textures can be seen in the form of

inclusions within the chert, as in the sillcified oolite in fig. 28a.

Original voids within the alicified rock are filled with a drusy mosaic of quartz crystals or layers of chalcedony. In partically silicifled

rocks, voids may be partially or totally filled with calcite cement.

Chert in the grainstone most commonly occurs as partial replacements of individual skeletal grains. These quartz replacements occur in three basic forms, equant granular mosaic of unoriented quartz crystals, spherulites, and large, euhedral crystals, numbers 4, 5, and 6 listed

above.

The equant granular mosaic patches (4) and spherulites (5) are generally yellow in plane light, while the large euhedral crystals (6) are clear and colorless. The granular form of chert in places replaces small amounts of mud and spar surrounding the altered fossil material.

The spherulites are composed of radial, fibrous quartz crystals and show a pseudo-uniaxial cross under crossed nicols. They may transect grain boundaries and extend Into intergranular areas.

The large euhedral quartz crystals may also transect grain boundaries.

They frequently contain a concentration of dusty inclusions toward their

centers.

The occurrences of the different quartz types seem to show some rela¬

tion to skeletal grain type. Although there are many exceptions, crinoids

seem to be most frequently replaced by 4), whereas brachiopods are more 40

frequently replaced by 5) and 6) and bryozoans tend to be replaced by 6).

Data in table 1 show the number of slides in which each type of chert replacement occurred.

The source of the chert is uncertain. The frequent close association of chert nodules with argillaceous and shaly layers might indicate that these layers supplied the silica. Or there may have been a biogenic source for the silica, as suggested by Cotter for Lodgepole rocks lower in the section. However, organically derived silica would probably not provide nearly enough silica to account for the volume of chert observed.

The frequent close association of chert and pyrite might indicate that they formed at the same time under the same conditions (fig. 20b).

Silicification probably occurred relatively early. The fact that void filling quartz overlies calcite cement indicates that it probably post dated the earliest calcite cementation. There is little evidence of compaction in the few silicified ooid and pellet grainstones. In thin section it can be seen that stylolites bend around the chert nodules. Thus it would appear that silicification preceded the bulk of compaction. Type of chert replacement

granular spherulitic large crystal

particle replaced section

crinoid TC-3 29 7 10 TC-1 23 L 9 RC 18 3 10 BC 36 9 6 total 106 21 35

brachiopod TC-3 6 12 23 TC-1 2 6 15 RC 1 8 29 BC 6 22 19 total 15 48 86

bryozoan TC-3 3 5 19 TC-1 1 1 14 RC 3 1 14 total 7 7 47

Table 1 - Relationship between particle type and form of chert replace- ment. Values are the number of thin sections in which a given type of replacement occurred. 41

COMPACTION

Compaction in the Lodgepole rocks affects mainly the grainstones, and, to a lesser extent, the packstones. Micritic rocks are largely un¬ affected. This is shown by the lack of deformation or breakage of skele¬

tal material in the wackestones, and by the limited grain interpenetra¬ tion and breakage in the packstones.

Percentage of floating grains (grains lacking contacts with any other grains in the plane of section) was taken as a measure of degree of compaction (see appendix III). Percentages were determined and averaged for from five to ten fields of view per slide. At low power six to eight fields of view covered essentially the entire surface of the slide. One would expect 20 to 30 per cent floating grains in an uncompacted sediment composed of spheres.

Degree of compaction as indicated by per cent of floating grains varies throughout the grainstone portions of the section, and may vary from one portion of a thin section to another.

Indications of compaction include:

1) grain interpenetration by pressure solution 2) grain breakage 3) grain deformation 4) grain reorientation or rotation

Grain interpenetration by means of pressure solution is the dominant sign of mode of compaction in the Lodgepole limestones (fig. 31). Form of grain contacts resulting from pressure solution seemed, in general, to reflect the factors outlined by Trurnit (1969). A variation of pres¬ sure solution is shown in figs. 32 and 23, in which pressure solution occurs not between separate grains, but between the outer laminae of ooids and the inner laminae and ooid nuclei, resulting in collapse of the grains. 42

This phenomenon is especially prominent toward the base of the upper

grainstones in the Big Snowys. The iron oxides which stain the ooids in

this portion of the section accumulate as residues along the microstylo-

11tic seams between laminae and between laminae and nuclei.

Grain breakage affects primarily the platy or tabular grains, such

as bryozoan skeletons or brachiopod shells. To a lesser extent it affects ooids and grains with micritic coatings by splitting or spalling off of outer oolitic or micritic laminae.

Compaction also occurs by apparent plastic deformation of grains.

Ooids and platy grains such as bryozoan skeletons are most frequently affected.

Grain reorientation is generally not obvious, except in a few cases where geopetals have been rotated away from their original horizontal position.

Assuming a primary porosity of 33 to 40 per cent, compaction of the sands has reduced pore space by up to 50 per cent. Grain breakage, deformation, and pressure solution phenomena as well as evidence that compaction occurred relatively late in the history of the sediment indi¬

cate that compaction was the result of increasing pressure due to burial.

There is no evidence of compaction due to solution and leaching in

the vadose zone.

Calcium carbonate derived from stylolitization and grain interpene¬

tration was precipitated in remaining pore space as coarse blocky caicite spar. This process of compaction and accompanying cementation probably began when some critical depth and weight of overburden due to burial was reached, and progressed more or less continuously until the rocks were uplifted and exposed in the present mountain ranges. In most cases this Fig. 31. Grain interpenetration.

Fig. 32. Collapsed ooid - Outer laminae of ooid are com¬ pacted against inner laminae and core of ooid along microstylolitic seams. Insoluble residues along the seams are the iron oxide alterations of the pyrite which partially replaced the ooid. 43

compaction and cementation resulted in a thoroughly cemented impermeable rock, although in a few of the bryozoan packstones in the Belt Creek rocks the process did not go to completion and voids lack or are only partially filled by coarse blocky spar. 44

CONCLUSIONS

The sediments of the lower portion of the Woodhurst Limestone Member

of the Lodgepole Formation in central Montana were deposited in a shallow

shelf sea across and around a Mississippian positive element, the central

Montana high. Table 2 presents a summary of the characteristics of the

rocks from the Little Belt and Big Snowy Mountains. Across the central

Montana high, in the present region of the Big Snowy Mountains, sediments were deposited in very shallow water in ooid shoals and broad lagoonal

areas behind these shoals. To the west where the Little Belt Mountains now are, sediments were deposited in somewhat deeper water in environments

ranging from the ooid shoals to open marine shelf and basin.

The Lodgepole sediments were altered by a number of diagenetic pro¬

cesses. A summary of the processes and the order in which they occurred

is presented in fig. 33. Cementation by isopachous druse, micritic cement, and syntaxial overgrowths on crinoid material took place in the intertidal or submarine environment soon after deposition. Dolomitization of small

amounts of crinoid skeletal debris by magnesium derived from the high magnesian calcite crinoid skeletons themselves also probably occurred

shortly after deposition.

This was followed by silicification and pyritization of skeletal material and ooids. Extensive dolomitization of portions of the fine

grained sediments in the Rock Creek and Timber Creek rocks, and to a

lesser extent in the Belt Creek rocks, next took place.

Compaction of the carbonate sands after burial proceeded by means of

grain breakage, deformation, rotation, and interpenetration. Crain inter¬

penetration was the result of solution compaction, and the calcium °-DO ® C O 0°

N TJ iz Qs> e r -go -8£

OJ rO ti? _c — 4- T3 $ S O Big Snowy Mountains- Little Belt Mountains- sections RC, TC-1, TC-3 section BC

1. Shallow water fauna: Endothyrids, 1. Deep water fauna - crinoids calcispheres, oncoids are abun¬ and bryozoans are abundant. dant. Skeletons of deeper-water Shallow water forms are un¬ types (crinoids, fenestrate common. bryozoans) are broken and abraded, showing they have undergone trans¬ portation.

2. Thick accumulations of cross-bedded 2. Predominance of micritic rock grainstones indicating deposition types and shales, indicating in shallow wave- and current- deposition in deep, quiet water. agitated water.

3. Abundant dolomite 3. Little dolomite

4. Abundant oolites 4. Odlites scarce.

5. Evidence of restriction in rocks 5. No evidence of restriction interpreted as "lagoonar1 - restricted maine fauna, lack of burrowing, abundant pyrite.

Table 2 - Summary of the differences between the rock types found in the Big Snowy and Little Belt Mountains. 45 carbonate thus derived filled most of the remaining pore space in the rocks in the form of coarse» blocky calcite spar. 46

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Ball, M. M., 1967, Carbonate sand bodies of Florida and the Bahamas, Jour. Sed. Pet., v. 37, p. 556-591.

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Cotter, Edward, 1966, Limestone diagenesis and dolomitization in Mississippian carbonate banks in Montana, Jour. Sed. Pet., v. 36, p. 764-774.

Deffeyes, K. S., Lucia, F. J., and Weyl, P. K., 1964, Dolomitization: observations on the island of Bonaire, Netherlands Antilles, Science, v. 143, p. 678-679.

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Dodd, J. R., 1967, Magnesium and strontium in calcareous skeletons: a review, Jour. Paleo., v. 41, p. 1313-1329.

Douglas, R. J. W., 1958, Mount Head map-area, Alberta, Geol. Survey Canada Mem. 291. 47

Duncan, Helen, 1969, Chapter VII Bryozoans, p. 345-433, in History of the Redwall Limestone of northern Arizona, E. D. McKee and R. C. Gutschick, eds., Geol. Soc. Araer. Memoir 114, 726p.

Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, p. 108-121, in Classification of carbonate rocks, W. E. Ham, ed., Amer. Assoc. Pet. Geol. Memoir 1, 279p.

Dunham, R. J., 1969a, Early vadose silt in Townsend mound (reef), New Mexico, p. 139-181, in Depositional environments in carbonate rocks, G.M. Friedman, ed., Soc. Econ. Pal. Min. Spec. Pub. 14, 209p.

Dunham, R. J., 1969b, Vadose pisolite in the Capitan reef (Permian), New Mexico and Texas, P. 182-191, in Depositional environments in car¬ bonate rocks, G. M. Friedman, ed., Soc. Econ. Pal. Min. Spec. Pub. 14, 209p.

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Hsu, K. J. and Siegenthaler, Christoph, 1969, Preliminary experiements on hydrodynamic movement induced by evaporation and their bearing on the dolomite problem, Sedimentology, v. 12, p. 11-25.

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Land, L. S., 1970, Phreatic versus vadose meteoric diagenesis of lime¬ stones: evidence from a fossil water table, Sedimentology, v. 14, p. 175-185.

Macintyre, J. G., and Milliman, J. D., 1971, Limestones from the outer shelf and upper slope continental margin, southeastern U.S., p. 103- 110, in Carbonate Cements, O.P. Bricker, ed., The Johns Hopkins Press, 376p. 48

Macqueen, R. W., and Bamber, E. W., 1967, Stratigraphy of Banff Formation and Lower Rundle Group (Mississippian), southwestern Alberta, Geol. Survey of Canada Paper 67-47, 37p.

Macqueen, R. W., and Bamber, E. W., 1968, Stratigraphy and facies rela¬ tionships of the upper Mississippian Mount Head Formation, Rocky Mountains and Foothills, southwestern Alberta, Bull. Canadian Pet. Geol., v. 16, p. 225-287.

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Wilson, J. L. , 1967b, Cyclic and reciprocal sedimentation in Virgilian strata of southern New Mexico, Geol. Soc. Amer. Bull., v. 78, p. 805-818. 50

Wilson, J. L. 1969, Microfacies and sedimentary structures in "deeper water" lime muds tones, p. 4-19, in Depositional environments in carbonate rocks - A symposium, G. M. Friedman, ed., Soc. Econ. Pal. Min. Spec. Pub. No. 14, 209P. APPENDIX I

LOCATION OP MEASURED SECTIONS

Rook Creek section - designated CL in appendices II and III. Located

in a road cut where the road crossed Rock Creek, about a mile

(as the crow flies) from Crystal Lake. There is a black shale

at the base of the stratigraphic section here, and the interval

measured for this study begins about 210 feet above this shale

at the base of a massive ooid grains tone unit.

Timber Creek sections 1_ and 2 ~ designated 1 and 3 respectively in

appendices II and III. Located in Timber Creek Canyon above the

west fork of Timber Creek on the south side of the stream. The b

base of the section is at the base of the first prominent cliff

above the creek. The limestones form "castles* above the creek,

and TC-3 was measured on the second castle from the west end of

the line of castles. TC-1 is located $ mile to the east.

Belt Creek section - designated BC in appendices II and III. Located

£ mile north of Monarch on highway 89. The base of the section

is opposite the cementary and about 150 feet above the road. It

is a composite section, the bottom 38 feet being measured on the

south side of the gully, and the rest of the section on the north

side. There is an overlap of ten feet in the sections. A massive

bed at 28 feet can be traced across the gully.

APPENDIX II - PETROGRAPHY

KEY

Rock Column

Limestone Abundant Grain Types

© Ooids Dolomite V Brachiopods

Dolomitic Limestone # Crinoids

Bryozoans Shale

Argillaceous Carbonate

Chert

b Bedding is drawn to scale. 3—39 ~ Indicates two

samples taken at the same level.

Rock Types

G Grainstone Dol Dolomitic

P Packstone Sh Shale

W Wackestone Recrystallized limeston Ls original character c^n M Mudstone not be determined

D Dolomite Arg Argillaceous Sedimentary Structures (and Macrofauna)

Fx Festoon cross bedding

tb Beds thin uniformly over short distances

x Cross bedding

rxl Ripple cross lamination

mml Horizontal millimeter lamination

lam Laminated

w b Wavy bedding

xxx Fossil hash

Symbols for megafauna discernible in outcrop - see Skeletal Grain Types.

Skeletal Grain Types

V Brachiopod

^ Colonial coral

^ Solitary coral

^ Bryozoan

A Crinoid

Trilobite

sf Shell fragments in which original skeletal structure has been lost

Percentages

*■ Present, but in very minor quantities

? Rock is too recrystallized or otherwise altered to determine Bioclasts

A > 2/3 of bioclasts

C 1/3 to 2/3 of bioclasts

R ^-1/3 of bioclasts

Present in very minor amounts of bioelasts)

Residue

Number or ,/ followed by c indicates percentage of clay minerals

n 11 11 n »» q B " " quartz silt

Chert - Mode

pod. Nodular

Symbols indicate partial, replacement of individual skeletal grains. See Skeletal Grain types. i i o ! o> _ __cvboiew:QCLO "Soar—per cenTof w Micrite-per centot cu o o Sedi mentary Rock d I icclPjIl^CbruozoafisN L: C C '0 j) o> Grain ..b_w.hal21_.mck ) 1 *a ins-percenrot _;o i- W U) (5 whole rock Structures 'ostra codes Rex id tri lobites unk own shelf fragmenfs O tenesrafc* bn-jo. Do lornite gastropds spicule crinoid calcisoheres echmod soines En dothiijrids XI oncoids other forams \\ 1ia total ~6rr3~ci~iic6ods Productids Ooids o t. Pel let; Lithoc Biocla "Zv ^a.-7d..jru-,-•a rx~ n Oxides id ode Per Ceiit es C) O * T> cr or cn C\J o & o CL CD cn o h- _s C‘, D & \ O \ \ in or cr in o on + CD o m cn q o U J* CL (^0 ID ID o ~o* x~ f> 1 \ \ CL cn in QL ♦ o xID>b\ 'o "o m o cn CL CL r- cn o in a> in 1 u_ 0 in O in |w c\ N CL O 'W CL o co O s f t> \ in o o in O \ o in N N CL O ID ro O f [ u L.U_„ 1 ! ^ • v — :::: l> 5 ”o — ■■■ X \ o CL u CD O o o O cn d) M <\l

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C L—30

0 o t> V t> *

V X \ X X X X X x

00 o m CD J K? O • r>

* — i i l

O'- ^ in r in o in 20 25 30 25 i c\r 20 CO

1 i cr X cr X X cr cr X X X X

< x X \ X X X cr

x \ X \ X X CT cr x cr cr cr cr cr X \ x \ X X X X X X X S \ x X X X X i i 1 cr cr cr cr cr cr cr rr i t X cr cr cr cr cr cr cr X ♦ X cr l ! 01 u cr cr cr cr cr X x x X X X X ♦ < < < U u o u u o

50 CO 8 20 25 80 40 80 100

100 'T X X X X X X X in in O O in o CNJ 20 T— o 20 35 65 45 45

45 ie

in in in in 35 25 70 75 £ x 0 65 K? ! X 0 0 O 0 8 4

X X X 1 #v

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APPENDIX III - DIAGENESIS

KET

For details of rook column see Appendix II.

Diagemetio Features ✓ / / Rare Abundant * X / Common 7 Rock is too recrystallized or otherwise altered to determine

Chert and Micritization - symbols indicate grain type altered

V Brachiopod

** Bryozoan

* Crinoid

Trilobite

Ostracod

O Oncoid

sf Shell fragments in which original structure has been lost due to recrystallization

Ooids

Colonial ooral

s Overgrowths on quartz silt

Iron Oxides and Pyrite

/ Present, often in micrite or along stylolitic seams

m In micritei pellets, lithoclasts, micrite coatings

xl Occurs in euhedral crystals

ch Associated with chert

O Replacing ooids

V * brachiopod

* pellets

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Dolomite

Distribution

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