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) andthe 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 confinedby thick deposits of raised Wyodak-Anderson peat. In contrast, the Felixcoal 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 thatthe 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

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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).

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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 fossils Carbonaceousshale grades into massive bright coal beds are also commonin the sandstone bodies. The detrital (composed of woody structures).Banded coal beds, lithofacies which are lateral tothe main coalbodies, consisting of alternating bright and dull layers generally less commonly are overlain by a thick (c24 m) coalbed or than 8 cm thick, are interbedded with the massive coal beds ‘rider’ that may connect to the upper portion of the lower and carbonaceous shales. main coal body (Fig. 6). Because of the area1 overlap of the main coal body and ‘rider’ coal beds, mapswere preparedto show total coal Wyodak-Anderson sequence thickness. The variation in coal bed thicknesses as seen on An east-west cross-section (Fig. 6) based on closely spaced these maps indicates that areas of thin or no coal (or ‘want data illustrates thatthe Wyodak-Anderson bed is not areas’) extend across much of the study area. In Fig. 8, the continuous across the study area but consists of a number of ‘want areas’ are best outlined by areas that contain less than

WEST EAST l I I IIII I I Ill Ill l II I Ill

I Drlllhole 1 km Coal Fig. 6. Cross-section through the Wyodak-Anderson 0 Sandstone sequence. Location is shown in Fig. 3. Drill holes in the 0 Sdtstoneand shale western part of the cross-section show only rock or coal Undifferentiatedlithologies lithologies. Vertical exaggeration = 53 X .

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Branching north and south from this largebelt are ‘want areas’ that are less than 0.5 km wide. In the northernmost map area, two of thesesmaller ‘want areas’ trend north-north-east and north-east and are probably intercon- nected. Between these ‘want areas’ is an east-west elliptical coal pod which is less than 4.5 km long and 1.4 km wide, and ranges from 24 to 54 m in thickness. Parts of other coal pods may be observed in otherareas between the ‘want areas’. The thickest parts of these coal pods occur along the edges of the coal body and contain numerous minor rock partings that increase in thickness toward the ‘want areas’. Detailed descriptions of sedimentary structures were not generally available for most of the Wyodak-Anderson logs, however, the areal distribution of the detrital lithofacies that interfingers with the coal bed indicates that these sediments probablyrepresent the deposits of channelsand their overbank detritus. The vertically stacked sandstone bodies Fig. 7. Photograph of a cross-bedded sandstone bodyin the Wyodak-Anderson sequence. Arrows indicate the scour at the base andtheir finer graineddeposits in thedetrital lithofacies probablyrepresent deposits channelsthat maintained of the sandstone. The bar in the upper left-hand corneris 2 m long. of their position through time as peat was contemporaneously 15 m of coal. All of the ‘want areas’consist of stacked, deposited (Fig. 9). The arealdistribution of the small slightly offset sandstonebodies and their associated finer north-south‘want areas’ (Fig. 8) suggeststhat during grainedrocks that may beoverlain by a ‘rider’ bed. The deposition of the Wyodak-Andersonpeat the contem- largest ‘want area’ is located in the central part of the map poraneouschannels may have been interconnected or areaand formsan east-west belt that is 1.5 km wide. anastomosedtributaries of largera east-west oriented

‘ 105O25’

<15m 44O25‘ >15- <30m >30- <40m >40m - 0 3 km

A140 20’ Iy GILLETTE+t##tl L I Fig. 8. Isopach map showing total Wyodak-Anderson bed thickness for the areaof closely spaced data in Fig.3. Lines around the mapped area correspond to mine property boundaries (contours in m).

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EXPLANATION

STRUCTURES IN SANDSTONES

CROSSBEDDING / grained fine ’ Itrn peat redimentr CONVULUTED channelsand

PLANAR #- EPSILON Fig. 9. Block diagram showing the relationship between the channel deposits (‘want areas’) and Wyodak-Anderson peat.

channel system. This large channel appears to join an even larger north-south channel system west of the study area, and whose deposits form a greater than 30 m thick parting Metres separating the discontinuous coal beds of the Wyodak- Andersonsequence (Warwick & Stanton 1986). The dominance of organic- and mollusc-rich floodplain sediments, the vertical stacking of channelsediments, the merging pattern of the channeldeposits and the overall size and distribution of the deposits are similar to modern anastomosed fluvial and lacustrine facies of the Sas- katchewanRiver area in western Canadathat have been described by Smith & Smith (1980) and Smith (1983). Inthe study area,the Wyodak-Anderson peat was probablydeposited in elliptical bodies that formed in restricted interfluves between the contemporaneouschan- Fig. 10. Sedimentary structures in a thick, widespread, cross- nels (Fig. 9). The thickest part of the coal bedcontains bedded sandstone body below the Felix coal bed. minor partings and is located adjacent to the ‘want areas’. The thick part of the bed probably represents relatively less compacted, wood-dominated peat that formed on nutrient- area indicate that the coal bed is uniform in thickness within rich levee sediments along the edges of the channels. This the central area of the deposit (Fig. 4). Outward from the interpretation differs significantly from that of Ayers & thick central part of the bed, toward the north and west, the Kaiser (1984) and Ayers (1986) who suggest that the thick coal bed thins and splits into a number of separate beds; peats of the PowderRiver Basin weredeposited in erosion has removed the interval to the east.Underlying the widespread,sediment-free, delta plain environments. Felix bed, a widespread, fining-upward sandstone complex Simultaneousdeposition of clastic and organicdeposits, containsmultiple scoured surfaces, basal lag deposits, similar tothe Wyodak-Andersondeposits, have been trough and tabular cross-stratification, and lateral accretion described forthe Kland andLangat Rivers of Malaysia deposits (Fig. 10). Warwick & Flores (1987) have suggested (Coleman et al. 1970), Fraser Riverdelta in British this sandstoneand its associated finer grainedsediments Columbia (Styan & Bustin 1983), Baram River of Malaysia represent the deposits of north-north-west-flowing mean- (McCabe 1984) and Saskatchewan River in western Canada dering rivers. (Smith & Smith 1980; Smith 1983). The clastic rocks above The Felix coal bed probably developed as a widespread, the Wyodak-Anderson bed represent an increase in detrital continuous peat deposit that accumulated on this poorly sedimentationalthough the association of the ‘rider’ coal compactible platform of meander-belt sediments. Thin and beds with the ‘want areas’ of the main coal bed may indicate split areas of the coal bed, underlain by more compactible that compaction of the underlying clastic sediments in the fine-grained rocks represent topographically lower parts of ‘want areas’ created topographically higher areas that were thepeat deposit that weresubject to occasional crevasse protected from detritalsedimentation and were more splay sedimentation that originated from contemporaneous suitable for peat accumulation. channels. These channels spread sedimentsover thepeat deposit as it compacted. The rocks above the Felix bed consist of coarsening- Felix sequence upward sequences that often are overlain by small (

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and the coarsening-upward sequences that overlie the Felix tional groups similar to those shown in Fig. 5 can be defined bed were deposited by anastomosed streamsthat often within the coal bed. These groups are dispersed throughout overflowed their banks into adjacent flood plain lakes and the bed and appear to define several zones within the bed post-Felix peat-forming areas. thatare compositionally similar. More work,however, is needed to improveunderstanding of thesevariations and their distribution. Coal characteristics and a raised peat model Variations in the megascopic and microscopic charac- Megascopic characterization of the Wyodak-Anderson and teristics, lateral continuity and associated rock types of the Felix beds indicates thatboth coalbeds contain at least Wyodak-Anderson and Felix coal beds suggest an three major types of laterally persistent facies (or subunits ombrotrophic (rain-fed)bog model as summarized for of the coal bed): (1) stacked, large (0.25-0.5 m diameter) modern peat accumulations by Gore (1963). Similar fragments of roots, stems and stumps of predominantly depositionalmodels have been proposed by Flores (1981) taxodiaceous trees, (2) coarsely laminated woody or bright and Ethridge et al. (1981) for thick coal beds of the Powder lenses (<0.25 m thick)in anattrital groundmass, and (3) River Basin, and Stanton et al. (1986) and Esterle & Ferm finely laminated dull and bright layers (<8 cm thick). The (1986) forcoal beds in the Appalachian Basin. Figure 12 percentage of woody materialgenerally is greatestin the illustrates the relationshipsbetween the characteristics of lower part of the bed, immediately above clay partings and the original peatand sediment types andresulting near 'want areas' in the Wyodak-Anderson bed. Wyodak-Anderson and Felix coalsequences. Although Microscopic analyses of samples taken from megascopi- compaction of the peat would be highly variable, an average cally different facies of the Felix bed indicatedifferent compaction ratio of 7: 1, as suggested forthe Wyodak- maceral compositions of the subunits of the bed (Fig. 11). Anderson peat by White (1986), is assumed for both peat The lower parts of the bed and partsimmediately above clay deposits. In general, the lower part of both peat deposits partingscontain thegreatest percentage of Group C was dominated by taxodiaceousmaterial as indicated by macerals (Figs 5 & 11). The most common Group C maceral abundant identifiable woody parts, Group C macerals (Figs is telogelinite, which consists of preserved cell walls that 5 & 11) and taxodiaceous pollen. In the developing peat were identified to be from taxodiaceouswood (Warwick 1985). Satchel1 (1985) and Pocknall (1986) havereported that the dominantpollen type in the Wyodak-Anderson and the Felix beds is that fromtaxodiaceous trees, probably fromthe genus Glyptostrobus. Taxodiaceous pollen percentages as well astelogelinite percentages generally 2-F om decrease upward within the Felix bed. Group A macerals _L RESULTINGCOAL BED .10 km increase in percentage toward the top of the bed. The most common Group A macerals are (1) densinite, which consists of broken cell fragmentsin a structureless humic groundmass, and (2) fusinite, which consists of oxidized and . carbonized plant parts. Variation among Group B macerals 10 km (Figs 5 & 11) is not consistent but percentages tend to be b) WYODAK-ANDERSON greatest in the central partof the coal bed. PEAT ACCUMULATION Preliminary datafrom petrographic analysis of the Wyodak-Andersonsamples indicate that several composi-

F-3 F-2 RESULTINGCOAL BEDS 1 km F- 1

L 1 km

m Finelylaminated . Plantmaterial m Coal 0-100% ' woody lenses in Sandand sandstone EXPLANATION 0- 100% attritalgroundmass 0 Group A woodypeat (root., a m Finegrained sediments stemsand stumps1 m 0 Group 0 0'l Fig. K!. Hypothetical cross-section showing Wyodak-Anderson and Felix raised peat deposition and the resulting coal beds. These Group C models are based on lithologic variations and coalmegascopic and b i00% m Detrital parting microscopic characteristics (lithologic data for the Felix sequence from Warwick & Flores 1987). An average peat to coal compaction Fig. 11. Maceral group composition of megascopically raio of 7 : 1 (as suggested by White 1986) is assumed for both different subunits of the Felix bed (Dry, mineral matter sequences. Note that wood-dominated peat is expected to compact free). Group components are shon in Fig.5. Sample less than the finely laminated plant material. Theresulting coal bed locations are shown in Fig. 4. thickens near the edge.

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deposits, initial vegetation rooted in a mineral-rich substrate The Wyodak-Anderson peat accumulted onthe eastern which produced large, luxurious arborescent plants. As peat basin margin, east of the main sediment transport comdor accumulated, the depositsprobably developed into an that occupied the central part of the basin (described by ombrogenous, raised peat deposit with aperched water Flores & Ethridge 1985). The Felix sequence, however, was table that was supplied by rainfall. (Ground water has very deposited in the central part of the basin where the amount little ability to move upwards in peat deposits,Romanov of sedimentdeposited in and carried by the anastomosed 1968). The peat associated with the raised system contained streams may have been too great to allow peat to the remains of herbaceous plants that were probably stunted accumulate.This difference in sedimentation rate may be in size because of the very acid and nutrient-poor related to differential subsidence rates across the basin conditions. Similar observationsin modern raised peat which in turn may have controlled the relative position of deposits have been made by Romanov (1968) and Anderson stream base level. The central part of the basin probably & Muller (1975). Thischange from woody peat to mixed subsided more rapidly than the eastern part and therefore shrubby and woody peat indicative of ombrogenous more sediment would beexpected to bedeposited in the conditions is evidenced in the Felix coal bed by the upward area of high subsidence. The location of sites of thick peat decrease in Group C macerals (predominantly telogelinite) accumulation (e.g. Felix bed) in the central part of the basin andthe upward increasein Group A macerals (pre- appears to be associated with the positions of large dominantly densinite and fusinite). meandering rivers with well developed levees that protected The Wyodak-Anderson peat was probably raised. This the flood plains and peat deposits from sediment deposition is suggested by the presence of laterally restrictive clastic (Flores 1981). The river sediments later served as platforms sediments that are bound by thick coal. Peat accumulations for thick peat accumulation. Obernyer (1978) and Flores & near the edge of the deposits were subjected to flooding of Warwick (1984) have described third a depositional adjacent streams which caused water-bornedetrital environment nearthe western basin margin which differs sediments to spread into the peat deposits, and resulted in from that of the Wyodak-Andersonand the Felix rock partings in the coal bed. The abundance of telogelinite sequences. In this area faulting influenced sites of thick peat and woody material above clay partings suggests that when development. Such structural features have not been flood-water levels dropped, woody peat was again able to observed in either the Wyodak-Anderson or the Felix study accumulate owing tothe presence of increased mineral areas. nutrients. The upper part of the well-developed raised peat deposit probably experienced fluctuations in the level of the perched water table owing to variations in the amount of Conclusions rainfall. Duringdrier periods, thepeat deposits were The distribution of rock types and coal bed characteristics in subjected to frequent forest fires, recorded by the presence the Wyodak-Anderson and Felix sequences of the Powder of fusinite and other Group A macerals. River Basin suggests that thick peats in this area The formation of raised peat influenced clastic accumulated in at least two different fluvial settings during sedimentation in both the Wyodak-Anderson and the Felix Tertiarytime. The thick Wyodak-Anderson bed resulted sequences. The raised Wyodak-Anderson peat restricted from peat accumulations in raised bogs betweencontem- the lateral movement of the contemporaneous channels and poraneous, probablyanastomosed tributaries of large their associated sediments now represented in the rock channels. The position of the channels was maintained record by the ‘want areas’ (Fig. 12). Clastic deposition was through time by the raised peat deposits. Later, during Felix confined to the edges of the peat deposit. The Felix peat time, thepattern of sedimentationchanged giving rise to probablydeveloped as a widespread raised peat deposit meanderbelts thattrend north to south. Following (Fig. 12). This is suggested by the continuity of the central abandonment, thesebelts served as platforms for the part of the Felix bed which appears to be uninterrupted by formation of the Felix peat. contemporaneouschannel deposits similar to those of the The location of these peat accumulations within the Wyodak-Anderson bed.The detritalpartings within the Powder River Basin may have influenced the character of Felix bed weredeposited by crevasse-typesedimentation the resulting coaldeposits. The Wyodak-Anderson peat that was able to invade the relatively low outer parts of the accumulated onthe eastern basin margin away from the raised dome. When clastic sedimentationceased, peat main sediment transportpaths in the central part of the formation could then re-establish itself. basin. In contrast the Felix peat formed in the sediment-rich central part of the basin where thick peat usually Tectonic implications accumulated onthe levee-protected flood plains and abandoned channel sandstone deposits. Differences in the character of the Wyodak-Anderson and Felix sequences may be related totheir position in the Study of the Felix sequence was supported by the Branch of Coal Powder River Basin. Thick peats, comparable tothe Geology of the US Geological Survey, Wyoming Geological Wyodak-Anderson peat did not form in the anastomosed Survey, University of Kentucky andthe Coal Section of the sequence above the Felix coal bed (described by Warwick & Geological Society of America while PDW was a student at the Flores 1987). Although the mode of deposition forthe University of Kentucky (Warwick 1985). Study of the Wyodak- anastomosed deposits above the Felix bed and for the rocks Anderson sequence was done while PDW held a National Research associated with the Wyodak-Anderson sequence is similar, Council-US Geological Survey Research Associateship. The the ratio of clastic to organic rocks in the Felix sequence is authors are indebted to the ranchers in the study areas for access to much greater than that in the Wyodak-Anderson sequence. theirproperty and to the mining companies nearGillette, This difference may be related to the difference in relative Wyoming, for coal samples, subsurface data, mine maps and access position within the basin in which the peatsaccumulated. to their properties.

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Received 2 March 1987; revised typescript accepted 11 February 1988.

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