Paleoecology, taphonomy and paleoenvironments of a unique littoral ecosystem within the Eocene Green River Formation of Wyoming

A unique nearshore sedimentary facies occurs in the Laney Member of the Green River Formation. It consists of well-preserved laminae and an exquisite assemblage of fossils including fish fry, juvenile and adult fish, leaves, pollen and spores, algal cells and cysts, gastropods, ostracodes and a variety of insects.; Calcareous fine to medium grained, thin to thick bedded sandstone alternates with siltstone and mudstone. Some nearshore facies are associated with stromatolites, tufa-coated logs and caddisfly larval bioherms. Massive claystone beds contain thin (<10cm) shaly interbeds rich with plant fragments. These fluvial units typically contain plant macrofossils and are dominated by a regional pollen and spore assemblage. Shale with varying amounts of calcite, dolomite, quartz and clay minerals, forms thin fossiliferous beds. The clayshale facies contains abundant fossil fish, ostracodes, insects, and plant fragments and is characterized by a local and extralocal palynological assemblage. Unique sedimentologic and taphonomic characteristics of this facies provide insights into depositional processes and environments.; Palynological, macrofloral and macrofaunal evidence suggests a lacustrine nearshore shallow water depositional environment. The fauna and flora were part of a littoral community. At this stage the lake was wide and shallow, the lake water was fresh and weakly alkaline, and intermittently highly alkaline and/or saline. Sedimentation rate was high and controlled by fluvial inflow. A combination of background and episodic sedimentation resulted in various finely laminated lithofacies that contain a well-preserved fossil assemblage. Within this sedimentary regime periodic mass mortality events and transport processes contributed a variety of biotic remains to an oxic, to intermittently dysoxic, lake bottom. The exceptional preservation of this rich fossil assemblage resulted primarily from a taphonomically high burial rate that enhanced early diagenetic mineralization.; The nearshore sedimentology and paleontology suggest that deposition of finely laminated sediments (typically characterized as “varves”) and the excellent preservation of delicate organisms are not necessarily dependent on deep, stratified, anoxic lake conditions.