Willwood Formation, Bighorn Basin, Wyoming)

28 RESEARCH REPORTS Sedimentological, Taphonomic, and Climatic Aspects of Eocene Swamp Deposits (Willwood Formation, Bighorn Basin, Wyoming) K. SIAN DAVIES-VOLLUM and SCOTT L. WING Dept. ofPaleobiology, MRC121, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560 PALAIOS, 1998, V. 13, p. 28-40 providing spatially and temporally constrained "snapshots" of the vegetation that originally grew on the alluvi- Alluvially deposited carbonaceous beds are a major source al floodplain (Wing et al., 1995; Wing and DiMichele, of leaf compression fossils, but the accumulation of both the 1995). If this is true, then such fossil assemblages are sim- sediments and the fossils is poorly understood. We have ilar to one another and to ecological samples of living veg- studied the sediments and fossil plant assemblages of three etation from small areas. However, little is really under- laterally extensive carbonaceous beds from the Eocene of the stood about the formation of these backswamp deposits Bighorn Basin, Wyoming. Field, laboratory, and microscop- and the processes that preserve plant fossils within them. ic observations show that carbonaceous beds are highly het- In particular, the deposition and preservation of plant fos- erogeneous, varying in the amount and type of organic mat- sils on a floodplain surface that supports vegetation pre- ter, preservation of primary sedimentary structures, and the sents an apparent paradox, as the preservation potential prevalence ofpaleosol features. Small-scale lateral and ver- for leaves in soils is very low (Retallack, 1984). We need to tical variations indicate a mosaic of conditions in the wet understand the genesis of carbonaceous deposits in order floodplain backswamp. The variables controlling this mo- to explain how plant fossils are preserved and not degrad- saic were primarily rate of clastic input and degree of sub- ed in this setting. strate flooding which, in turn, influenced the amount of The purpose of this paper is to better define the sedi- pedogenesis. mentary setting and environmental conditions required Fossil plant assemblages occur in sediments least modi- for the formation of leaf compression assemblages by fied by pedogenesis, and the best preservation is in the dis- studying laterally extensive carbonaceous beds from the tal, fine-grained portions ofoverbank flood deposits. There lower Eocene of the Bighorn Basin, Wyoming. We also is no evidence for transport of plant material from channel evaluate the amount of time represented in a carbona- to backswamp. Fossil leaves in a single quarry sample (i.e., ceous bed, the probability of spatial mixing of fossil leaves those collected from within 10-20 cm stratigraphically and within such a deposit, and the type of vegetation repre- from an area of 4—10 m2) probably accumulated during one sented by its fossil plant assemblages. In addition, we ex- or several closely-spaced flood events. Quarry samples are plore the implications of carbonaceous bed deposition for thus spatially and temporally constrained "snapshots" of the tectonic and climatic evolution of the Bighorn Basin in the vegetation that grew on the alluvial floodplain. Each the early Eocene. carbonaceous bed is estimated to represent roughly 2,000 years, based on the length of fluvial avulsion cycles. Multi- GEOLOGICAL SETTING ple quarry samples from the same carbonaceous bed can thus be considered coeval for most evolutionary orpaleocli- The Bighorn Basin of northwest Wyoming (Fig. 1) is matic analyses. bounded by several structural areas that were elevated In the central Bighorn Basin, laterally extensive carbo- during the middle and latter parts of the Laramide oroge- naceous beds are abundant in the lower and upper Will- ny (Bown, 1980). The major uplift of the northern Bighorn, wood Formation, but essentially absent in the middle. We Pryor, and Beartooth Mountains took place during the attribute this to tectonic and climatic change, especially to mid-late Paleocene (Bown, 1980; Beck et al., 1988). During higher rates of sediment accumulation that caused greater the late Paleocene, uplift around the southern margin of dispersal of organic material within sediments and climat- the basin formed the southern Bighorn and Owl Creek ic drying that allowed degradation of organic material be- Mountains, which were subsequently thrust southward in fore it could be preserved. the mid-early Eocene. Throughout the Paleocene and early Eocene, the Big- horn Basin accumulated predominantly fluvial sand- INTRODUCTION stones, mudstones, lignites, and freshwater carbonates (Gingerich, 1983). Up to 3,000 m of drab (gray-brown col- Laterally extensive carbonaceous beds are common in ored) and lignitic sediments deposited during the Paleo- lower Cenozoic sequences of the western interior of North cene are assigned to the Fort Union Formation (Bown, America, and are a major source of leaf compression fossils 1980). Variegated, oxidized mudstones showing clear that provide data on floral composition, paleoclimate, and pedogenic features were deposited in the latest Paleocene paleoecology. It has been argued that deposition of fossils and early Eocene, and are assigned to the Willwood For- in these distal alluvial settings is largely autochthonous, mation (Fig. 2; Bown and Kraus, 1981). Climates warmed Copyright © 1998, SEPM (Society for Sedimentary Geology) 0883-1351/98/0013-0028/S3.00 EOCENE SWAMPS 29 109° 108° l ~\\ y & TATMAN FM "— iv%/v\/\ Greybul£| 0rt_9^>"^ \\LI 700—1 Ar/Ar %^ date J^ TATMAN 52.8 ma J FIFTEENMILE y MTN J^C^ uV 600- CREEK BED FifleenmiVOr^ ll jjj'3 3 t J ~V_ 500- o H Worland < ' Carbonaceous bed outcrop el The^/ o c km f Honeycombs 400— Owl Creek J Bridger Mountains / Mountain O O FIGURE 1—Location of Bighorn Basin and the three carbonaceous o C24n 53.2 ma beds studied. 1: South Fork of Elk Creek carbonaceous bed, 2: Hon- 300 — C24r eycombs carbonaceous bed, 3: Fifteenmile Creek carbonaceous bed. globally and locally during the late Paleocene, then appear 200— to have cooled in the Bighorn Basin during the earliest Eo- cene, followed by renewed warming in the middle-late early Eocene, with a thermal maximum around 52-53 Ma ELK CREEK BED (Wing et al., 1991; Wing, in press). The presence of ma- 100- ture, oxidized paleosols in the early Eocene reflects a Carbon warm temperate to subtropical climate with seasonally al- isotope ternating wet and dry conditions (Bown and Kraus, 1981); excursion warm winters are also indicated by physiognomic and flo- 0- 54.8 ma HONEYCOMBS ristic analyses of plant fossils found in the Willwood For- FORT„ BED mation (Wing and Greenwood, 1993). UNION FM K/w\/ Study Sites FIGURE 2—Generalized section of the upper Fort Union Formation and Willwood Formation in the Bighorn Basin showing stratigraphic We have studied three laterally extensive carbonaceous positions of the carbonaceous shales studied with respect to isotopic beds from the southern part of the Bighorn Basin (Figs. stratigraphy, lithostratigraphy and magnetostratigraphy. Black horizon- 3a-c). Beds in all areas are nearly horizontal (less than 3° tal lines in the column represent tabular carbonaceous shale units as dip). These beds are located at: described in this paper. Black lenses represent lenticular carbonaceous shale units as described by Wing (1984). Lithostratigraphy from (1) The Honeycombs (KSDV 94 2.0): This bed is located Schankler (1980) and Wing (1984), Chron 24R/Chron 24N boundary within 20 km of the southeastern margin of the Bighorn from Tauxe et al. (1994), position of carbon isotope anomaly from Basin. Approximately 200 m of Willwood Formation and Koch et al. (1995), radiometric date from Wing et al. (1991). 100 m of the underlying Fort Union Formation are preserved in this area. The carbonaceous unit is stratigraphically about 10 m below the WaO red bed, which contains outcrop area and is lost to erosion by the Elk Creek valley basal Wasatchian faunas (Wing, in press), and is exposed at the north end. for about 3 km in an east-west direction and 1.5 km north- (3) Fifteenmile Creek (KSDV 94 3.0): This bed is locat- south. At its eastern end, the bed becomes coarser grained ed in the central Bighorn Basin along the south side of Fif- and eventually disappears into the subsurface. At the teenmile Creek, in a drab interval of the upper Willwood western end of the outcrop, the unit is cut out by channel Formation. It occurs at the 621-m level of the Elk Creek sandstones and is preserved only as a series of lenses. section, and is associated with early Lostcabinian mam- (2) Elk Creek (KSDV 94 1.0): This bed is located in the mals (Schankler, 1980; Bown et al., 1994). The Fifteenmile east-central part of the Big Horn Basin, in a relatively Creek bed is 13 m stratigraphically beneath a bentonitic drab section of the lower Willwood Formation exposed tuff dated at 52.8 Ma (Wing et al., 1991), making it approx- along the south fork of Elk Creek. It is at the 112-m level imately 3 m.y. younger than the Honeycombs bed; it is the of the Elk Creek section, and is associated with early same bed as WCS 15 of Wing (1984). The Fifteenmile Graybullian mammalian faunas (Schankler, 1980; Bown Creek bed is exposed east-west for a distance of 18 km and et al., 1994). This bed is approximately 0.7 m.y. younger north-south for a distance of 3 km. At its westward end, than the Honeycombs bed, and was designated WCS 7 by the bed thins, becomes less carbonaceous, and is replaced Wing (1984). The bed is exposed for about 3 km in a north- by mudstones and siltier shales. Eastward it is intermit- south direction and about 1 km east-west. The bed dips be- tently cut out by a thick sheet sand, but is laterally contin- low the surface at the southern and western ends of the uous with a thin organic mudstone lying on top of a thick, 30 DAVIES-VOLLUM & WING 1 • 1 O SAMPLE SITES 1 'si 2.93 Carbonaceous bed outcrop KEY TO LITHOLOGIC SECTIONS Brown slickensided f~_ — rz.

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