ABSTRACT

The Bear Gulch Bed (Upper Mississippian) of the Heath Formation is a limestone lens up to 100 feet thick by 4 miles north-south by 8 miles in east-west extent composed predominantly of sparsely fossiliferous laminated to thinly bedded silty micritic limestone. The limestone grades laterally and downward into black shale. Shelly fossils and stromatolites, which increase in abundance near the top as well as along much of the eastern and the northern edges of the limestone, are evidence of shallowing. Within the Bear Gulch several regionally concentrated fossil assemblages can be recognized. The first is a sparsely distributed nektonic assemblage composed principally of small cephalopods, shrimp, fish, and soft-bodied organisms. The second is a diverse shelly benthic assemblage composed primarily of branching sponges, brachiopods, bivalves, algae, and conulariids, that probably thrived in Sargassum-like floating mats. The third is an assemblage of productid brachiopods and filamentous algae, with associated vertebrates. A fourth assemblage consists of terrestrial plants.

Fossil preservation ranges from flawless specimens with exquisite details of soft anatomy to completely disarticulated specimens. Disarticulated specimens show little evidence of current scattering, but abundant evidence of scavenging. Other evidence of biotic or abiotic disturbances of the sea floor is sparse: most shelly fossils probably did not live on the sea floor and there is little evidence of burrowing. The high quality of preservation of many fish may have resulted from burial by sediment raining down from detached turbidity currents that moved along a density gradient The high diversity of fish (108 species) and wide range of body forms is evidence of a complex ecosystem perhaps most similar to modern bay or estuarine communities.