FLUVIAL SEDIMENTOLOGY AND PLANT TAPHONOMY OF A DINOSAUR SITE AND ASSOCIATED STRATA, UPPER JURASSIC MORRISON AND LOWER CRETACEOUS CLOVERLY FORMATIONS, BIGHORN BASIN, WY

We investigated the fluvial sedimentology and plant taphonomy of a portion of the Morrison and Cloverly Formations in the Bighorn Basin north of Shell, WY in order to develop paleoenvironmental interpretations. Working in and around two adjacent dinosaur excavation sites in the upper part of the Morrison Formation and lower part of the Cloverly Formation, we used measured sections, photomosaics and paleocurrent analysis to describe the architecture of several sandstone bodies. We correlated facies to provide a clearer understanding of depositional packages. The major sandstone (informally termed the “scorpion sandstone”), the upper part of which is stratigraphically equivalent to the lower dinosaur quarry, is well-sorted, fine-medium grained, and is characterized by medium-scale trough cross-beds, iron-rich concretions, and bone and plant fossil fragments. The architecture of the “scorpion sandstone” and its relationship to surrounding facies suggests it was deposited by a fluvial system in a paleotopographic low, and well-constrained paleocurrent data indicate a southeast flow direction. Rose diagrams of in situ logs and preferred orientations of small plant fragments in associated siltstone facies reinforce this paleocurrent direction. Well preserved plant remains found on the site include a possible Coniopteris fern frond and permineralized logs including Xenoxylon and Mesembrioxylon. Additional plant cuticle and seed fragments (Jensensispermum) were recovered from the carbonate-cemented siltstone matrix using acid dissolution. Taphograms comparing the “scorpion sandstone” in the Morrion Formation and overbank mudstone deposits (site of permineralized logs) in the overlying Cloverly Formation highlight differences in the preservation of plant fragments between these two sites. Larger fragments are preserved in situ in the lower energy mudstone depositional environment and small, diverse fragments are concentrated, and in some cases oriented, by the high energy stream environment. Thus plant taphonomy can be used to reinforce sedimentological interpretations of paleoenvironment. These data will serve as a preliminary basis for understanding the paleoenvironment of the site.