Journal of the Geological Society, London, Vol. 145, 1988, pp. 613-620, 12 figs. Printed in Northern Ireland Depositional models. for two Tertiary coal-bearing sequences in the Powder River Basin, Wyoming, USA P. D. WARWICK & R. W. STANTON US Geological Survey, Reston, VA 22092, USA Abstract: Depositional controls on peat-forming environments which produce thick (>l0 m) coal beds canbe inferred from relationships between coal bed geometry, maceral composition and associated lithologies. Study of these relationships within sedimentary sequences associated with the Wyodak-Anderson (Palaeocene) and the Felix (Eocene) sub-bituminous coal beds in the Powder River Basin, Wyoming, USA suggests two modes of fluvially controlled peat accumulation. The Wyodak-Anderson peat is interpreted to have formed in restricted parts of the floodplain that were separated by deposits of contemporaneous, anastomosed channels. The channels and associated sediments maintained their position through time because they were confined by thick deposits of raised Wyodak-Anderson peat. In contrast, the Felix coal bed is interpreted to have formed as a raised but widespread peat on an abandoned platform of meander-belt sands. The purpose of this paper is to compare andcontrast two different fluvial depositional settings that produced anomalously thick (>10m) coal deposits in theintermontane Powder River Basin of Wyoming, USA. These models may be useful as predictive tools for coal exploration and production. This paper was presentedat the Coal and Coal-bearing Strata Formation to represent deposits of a fluvio-deltaic system Symposium in April 1986. that built out into a closed lacustrine basin. Two separate coal-bearing stratigraphic sequences were The Powder River Basin is an asymmetrical structural basin considered in the present study, (1) an approximately 130 m in north-central Wyoming and south-eastern Montana with section that includes inits lower portion the Wyodak- the axis of the Basin located along the western side (Fig. 1) Anderson coal bed of the Tongue River Member of the Fort (Foster etal. 1969; Curry 1971; Blackstone 1981). In Union Formation; and,for comparison, (2) an approxi- Wyoming, the Powder River Basin is bound on the south by mately 100 m thick section that contains the Felix coal bed of the Laramie Range and the Hartville Uplift, on the east by the Wasatch Formation (Fig. 2). Denson & Horn (1973) and the Black Hills and on the west by the Bighorn Mountains. Denson & Keefer (1974) have described the Wyodak- Most of thestrata exposed in the basin rangefrom the Anderson bedas a thick (>30m), north-south-trending, Cretaceous Lance Formation to the Oligocene White River 2000 km2, laterally continuous coal body that is located on Formation (Fig. 2). The Palaeocene Fort Union Formation, the eastern flank of the Powder River Basin. This coal bed which consists of the Tullock, Lebo Shale and Tongue River splits into two or more beds towards the central area of the Members, contains sub-bituminous coal, carbonaceousshale, basin. Within the thick Wyodak-Andersonbed areareas interbedded siltstone andshale, sandstone, conglomerate, where thebed is thin or absent (‘wants’) which most fresh-waterlimestone and ironstone(Flores & Ethridge workers have attributed to post-depositional channel erosion 1985). Natural burning of the coal beds has locally baked (Denson & Horn 1973; Denson & Keefer 1974; Glass 1980). thesesediments toform a red rock called clinker. The Law (1976), however, suggested that the areas of thin or no Eocene Wasatch Formation conformably overlies the Fort coal may haveformed contemporaneously with peat UnionFormation in the central andeastern parts of the accumulation. basin and has lithologies similar tothe underlying unit The Felix bed underlies a 6000 km2 area in the central (Flores & Warwick 1984; Flores & Ethridge 1985). part of the PowderRiver Basin (Kent etal. 1988). The However,Wasatch conglomeratic rocks unconformably northern part of this deposit is exposed in the valley walls of overlie Fort Union strata along the western margin of the the PowderRiver and its associated rocks have been basin (Mapel 1959). Ayers & Kaiser (1984) and Ayers described by Olive (1957) and Warwick & Flores (1987). (1986) have suggested that the sediments of the Fort Union Other studies that describe Felix rocks include those by Formationwere deposited by a fluvial-deltaic prograda- Culbertson & Mapel (1976) andEthridge etal. (1981). tional system that slowly filled a Palaeocene basin-wide lake. Subsurface andoutcrop data indicate thatthe Felix bed These interpretations, however, are based on widely spaced thins and splits laterally from a thick (12 m) central coal bore hole data and generally ignore much of the detailed body that covers a 150 km2 area in the central part of the outcropand subsurface work that has recently been Powder River Basin (Warwick & Flores 1987; Kent etal. reviewed by Flores & Ethridge (1985). Flores & Ethridge 1988). (1985) suggest that the rocks of the Tullock and Tongue RiverMembers of the Fort Union Formation andthe Wasatch Formation represent the deposits of north-south Methods of study oriented meandering and anastomosed trunk streams that Over 450 geophysicallogs, core descriptions and mine highwall were fed by basin margin alluvial fans and tributaries. They observations of the Wyodak-Anderson coal-bearing sequence (Fig. interpret only the Lebo Shale Member of the Fort Union 3) were used to construct regional cross-sections and isopach maps 613 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/613/4889400/gsjgs.145.4.0613.pdf by guest on 01 October 2021 614 P.D. WARWICK & R.W. STANTON 107' 105O .I I I 105' 35' n I . 105'25' .. ... .. .:;. * . 45O km I - 440 25' .... 80 km 4 43O . L 440 20' Fig. 1. Map showing the location of the Powder River Basin. Area A is the study area for the Felix sequence and Area B is the study I. for the Wyodak-Anderson sequence. .. .. '4.. l GILLETTE~ l Fig. 3. Enlarged map of Area B in Fig. 1 showing Wyodak- similar to those that have been prepared for the Felix sequence by Anderson sequence borehole and coal sample locations. Location Warwick & Flores (1987). Maceral compositions of the Felix and for the cross-section in Fig. 6 is indicated. Wyodak-Andersonbeds were determined, using reflected white light andblue light, by pointcount analysis of megascopically differentsubunits of thecoal beds. Felix samples consisted of channelsamples from coal outcrops and the Wyodak-Anderson samples consisted of continuous cores and channel samples taken from mine highwalls. Locations of the coal sample sites are shown on Figs 3 & 4. Maceralsand submacerals, as defined by the InternationalCommittee for CoalPetrology (1971), werepoint counted and data were grouped into three major categories based on cluster analysis of correlation coefficients among variables (Fig. 5). Detaileddescriptions of measuredsections and coalsamples form the Feli sequence are found in Warwick (1985). ++ OG+ WHITERIVER FORMATION \e Ob > a 0 WASATCHFORMATION 44O%5 +oG 000 W c Fig. 4. Enlarged map of Area A in Fig. 1 showing Felix bed thickness, and measured section, drill hole and coal sample locations. West of the split line thecoal bed is divided into two or Fig. 2. Stratigraphic nomenclature ofr the rocks exposed in the more beds. Contours (in m) and data points from Warwick & Flores Powder River Basin which are mentioned in this paper. (1987). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/613/4889400/gsjgs.145.4.0613.pdf by guest on 01 October 2021 TERTIARY COAL-BEARING SEQUENCES, WYOMING 615 Relative distance between variables 0 5 10 15 20 25 1 MACRlNlTE INERTODETRINITE -J .. FUSlNlTE ... ... GROUP A SEMlFUSlNlTE ..... ... .... ... DENSlNlTE ...... SCLEROTINITE . ’ . 1 ULMlNlTE , ........ .EU‘CORPOHUMINITE ..... ... Fig. 5. Dendrogram using single linkage PORlGELlNlTE GROUP B cluster of correlation coefficients among the 18 macerals and submacerals iden- tified in the Felix bed. Macerals and .. SPORINITE submacerals within each group are .. SUBERlNlTE ,. ....... .. .... p c interpretedto haveconcentrated been ALGlNlTE ......... TELOGELINITE ., by similar processes. Lithofacies characteristics and environmental individual coal bodies thatare cumulatively greater than implications 24 m in thickness. These bodies are surrounded by and Lithofacies types of the Wyodak-Anderson and Felix laterally interlinger with detrital lithofacies consisting of sequences consist primarily of sandstone, interbedded interbedded siltstone and shale and stackedsandstone siltstone and shale, and coal. Siltstone and shale, commonly bodies.Subsurface dataand mine highwall observations containing freshwater gastropods and bivalves, range from indicate that the sandstone bodies (generally <l5 m thick grey to dark grey and havegradational to sharp contacts and <OS km wide) commonly have erosive basal contacts with sandstones. Greyto light grey sandstone which is and contain trough and tabular cross-stratification (Fig. 7). commonly cross-stratified ranges from medium to very The sandstone grades upward and laterally into interbedded fine-grained in the Wyodak-Anderson sequence and from siltstone and shale that often is rooted and contains upright conglomeratic to fine-grained in the Felix sequence. tree stumps in growth position. Articulated mollusc fossils Carbonaceousshale grades into massive bright coal beds are also commonin the sandstone bodies. The detrital (composed of woody structures).Banded