Lacustrine to fluvial floodplain deposition in the Eocene Bridger Formation

The depositional systems and sedimentology of lithofacies sequences in the Bridger Formation (unit B) are described. A lithofacies association is determined and includes from base to top: limestone, claystone, thin bedded sandstone and siltstone, and a cross-bedded sandstone facies. The limestone facies is generally composed of a dense resistant limestone that may contain fossil plant impressions, tufa and/or stromatolites, gastropods, bivalves, and other fossils. The claystone facies contains abundant fossil turtles, gar, crocodiles, gastropods and plants. Desiccation cracks, plant roots and soil features are lacking. Extremely abundant turtles may occur in this facies, sometimes associated with abundant coalified plant fragments. The thin bedded sandstone and siltstone facies is composed of silty clay to medium sand. At a few localities it grades laterally into medium- to course-grained sandstone channels up to 3 m thick. Fossil turtles are present, but are much less abundant than in the claystone facies. The cross-bedded sandstone facies is composed of thick, laterally extensive channel sandstones associated laterally with silty claystone beds. The lithofacies association is interpreted as a shallowing upward sequence. It started with a basal limestone facies. deposited in a widespread, shallow, carbonate-precipitating lake. The lake was rapidly filled by episodic volcaniclastic deposition, delivered via air-fall and/or prograding fluvial-deltaic systems. Deltaic deposition may have been similar to the infilling of the northern portion of Lake Turkana in Africa by the Omo River Delta. The claystone facies is interpreted as a short-lived fluvial-lacustrine system that prograded out over the limestone facies and/or was deposited as air-fall ash into the entire lake. The sharp upper contact of the limestone facies with the claystone facies suggests that an abrupt paleoenvironmental change may have taken place with a large influx of volcaniclastics. An organic-rich unit, the organic turtle bed within the Black Mountain turtle layer (a claystone facies), contains abundant plant material and turtles and is interpreted as representing an episodic depositional event, probably storm related. As volcaniclastic deposition continued to dominate the system, the thin bedded sandstone and siltstone facies sequence was deposited. It is interpreted as distributary and crevasse splay sheet sands uniformly deposited (laterally correlative) over large areas. This depositional system was ultimately replaced by a well-established, fluvial floodplain environment with large fluvial channels representing major meandering rivers. Development of the succeeding lake system occurred when siliciclastic deposition significantly slowed relative to the rate of basin subsidence. Deposition in the Bridger Formation, unit B, suggests a delicate balance between a closely interacting lacustrine and fluvial system. Although fluvial processes and elastic sedimentation dominated depositional patterns, lacustrine depositional regimes frequently returned. (C) 2000 Elsevier Science B.V. All rights reserved.