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 <y> 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).