Sedimentological, taphonomic, and climatic aspects of Eocene swamp deposits (Willwood Formation, Bighorn Basin, Wyoming)

Abstract

Alluvially deposited carbonaceous beds are a major source of leaf compression fossils, but the accumulation of both the sediments and the fossils is poorly understood. We have studied the sediments and fossil plant assemblages of three laterally extensive carbonaceous beds from the Eocene of the Bighorn Basin, Wyoming. Field, laboratory, and microscopic observations show that carbonaceous beds are highly heterogeneous, varying in the amount and type of organic matter, preservation of primary sedimentary structures, and the prevalence of paleosol features. Small-scale lateral and vertical variations indicate a mosaic of conditions in the wet floodplain backswamp. The variables controlling this mosaic were primarily rate of clastic input and degree of substrate flooding which, in turn, influenced the amount of pedogenesis. Fossil plant assemblages occur in sediments least modified by pedogenesis, and the best preservation is in the distal, fine-grained portions of overbank flood deposits. There is no evidence for transport of plant material from channel to backswamp. Fossil leaves in a single quarry sample (i.e., those collected from within 10-20 cm stratigraphically and from an area of 4-10 m 2 ) probably accumulated during one or several closely-spaced flood events. Quarry samples are thus spatially and temporally constrained snapshots of the vegetation that grew on the alluvial floodplain. Each carbonaceous bed is estimated to represent roughly 2,000 years, based on the length of fluvial avulsion cycles. Multiple quarry samples from the same carbonaceous bed can thus be considered coeval for most evolutionary or paleoclimatic analyses. In the central Bighorn Basin, laterally extensive carbonaceous beds are abundant in the lower and upper Willwood Formation, but essentially absent in the middle. We attribute this to tectonic and climatic change, especially to higher rates of sediment accumulation that caused greater dispersal of organic material within sediments and climatic drying that allowed degradation of organic material before it could be preserved.