The American Association of Petroleum Geologists Bulletin V. 70,ND. 11 (November 1986), P. 1651-1673, 17 Figs. Lacustrine and Fluvial-Deltaic Depositional Systems, Fort Union Formation (Paleocene), Powder River Basin, Wyoming and Montana^ W.B.AYERS,JR.' ABSTRACT The Powder River basin is a Laramide foreland basin that was filled by a combination of fluvial, deltaic, palu- dal, and lacustrine sediments. The depositional history of the Fort Union Formation was unraveled in a regional subsurface study using data from approximately 1,400 geophysical well logs. The depositional model developed from the subsurface study was tested by selective field- work. The Powder River basin originated as a structural and depositional basin in earliest middle Paleocene. As a result of rapid subsidence, a lake (Lake Lebo) formed along the basin axis. Lake Lebo, documented in the mudstone of the Lebo Shale Member, spread rapidly to cover an area greater than 10,000 mi^ (25,900 km^). During the middle through late Paleocene, Lake Lebo wasfilledperipherally byfluvial-deltaicsystems that are recorded in the coarser elastics of the Tongue River Member. Primary basin fill was from: (1) the eastern margin by elongate deltas fed by suspended to mixed-load fluvial systems issuing from the ancestral Black Hills, and (2) the southwestern margin by mixed to bed-load streams emanating from the Wind River basin. Secondary fill was from the northwest by an elongate delta system fed by a suspended to mixed-load fluvial systemflowingfrom the Bull Mountain basin. INTRODUCTION The early Tertiary is recognized as a time of persistent lacustrine sedimentation in the western United States (Feth, 1964; Picard and High, 1972, 1981) with significant lacus- trine components recognized in several basins (Bradley, 1964; Keefer, 1965a; Koesoemadinata, 1970; Ryder et al, ©Copyright 1986. The American Association of Petroleum Qeoiogists. All rights reserved. ^Manuscript received, December 17,1984; accepted, May 29,1986. ^Bureau of Economic Geology, University of Texas at Austin, Austin, Texas 78713. Published with permission of the director of the Bureau of Economic Geology, University of Texas at Austin. This paper Is part of my dissertation research under the supervision of W. R. Kaiser and A. J. Scott. Their comments, and suggestions by W. R. Muehlberger, W. A. Wescott, and two anonymous reviewers significantly improved the manuscript. Well logs were provided by M. J. Systems, Wyoming Geological Survey, Cities Sen/ice Oil and Gas Corporation, and American Stratigraphic Company. Fieidwork was supported by the University of Texas at Austin Geology Foundation and by grants-in-aid from the American Associa- tion of Petroleum Geologists and Sigma Xi, the scientific research society. I thank AMAX Coal Company and Big Horn Coal Company for access to their mines, and the many landowners who permitted fieidwork on their ranches. 1976). However, Ethridge et al (1981) and Flores (1979, 1983) proposed that during the late Paleocene the Powder River basin was filled aggradationally by a fluvial system that flowed northward along the basin axis; lacustrine deposits were of secondary importance and were restricted to local flood basins. This regional study contradicts the fluvial model and proposes a lacustrine model character- ized by the existence of a large, persistent laker along the basin axis during the middle to late Paleocene. The basin was filled peripherally by high-constructive deltas that pro- graded basinward from at least three margins. This paper reviews previous studies of the Fort Union Formation in the Powder River basin, discusses the meth- odology used in the study, describes the regional distribu- tion of lithofacies and sand-body geometry, and presents a regional depositional model based on subsurface mapping with supporting fieidwork. Geologic and Structural Setting The Powder River basin (Figure 1) is bounded on the east by the Black Hills and on the west by the Big Horn Moun- tains and Casper arch. The Laramie Mountains and Hart- ville uplift form the southern boundary, and the Miles City arch separates it from the Williston basin to the northeast. More than 13,000 ft (3,960 m) of Phanerozoic sediments overlie the Precambrian basement in the Powder River basin (Sharp and Gibbons, 1964). In the Early Cretaceous, stable marine shelf conditions were dominant, but by Late Cretaceous the Fox Hills and Lance Formations (Figure 2) recorded a shift toward paralic environments (Dunlap, 1958) as the epicontinental sea retreated m response to the Laramide orogeny (Trimble, 1980). Laramide foreland deformation of the North American craton in the Late Cretaceous through late Eocene (80-40 Ma) (Coney, 1972) formed a series of asymmetrical uplifts and basins that extended from southern New Mexico to northern Montana. Structural relief between the basement- cored uplift of the Big Horn Mountains and basement rock in the adjacent Powder River basin is approximately 4 mi (6.4 km) (Bayley and Muehlberger, 1%8). Laramide uplifts provided immediate source areas for more than 5,000 ft (1,500 m) of lacustrine and fluvial-deltaic Fort Union (Paleocene) sedunents that were deposited in the center of the Powder River basin. The Fort Union Formation crops out around most of the margin of the basin (Figiu^e 3); it is overlain by the predominantly fluvial Wasatch Formation (Eocene), which along the basin axis exceeds 1,000 ft (300 m). 1651