The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition

摘要

The Bear Gulch Limestone (Heath Formation, Big Snowy Group, Fergus County, Montana, USA) is a Serpukhovian (upper Mississippian, Namurian E 2 b) Konservat lagerst?¤tte, deposited in the Central Montana Trough, at about 12?° North latitude. It contains fossils from a productive Paleozoic marine bay including a diverse biota of fishes, invertebrates, and algae. We describe several new biofacies: an Arborispongia-productid, a filamentous algal and a shallow facies. The previously named central basin facies and uppermost zone are redefined. We address the issue of fossil preservation, superbly detailed for some of the fish and soft-bodied invertebrates, which cannot be accounted for by persistent anoxic bottom conditions. Select features of the fossils implicate environmental conditions causing simultaneous asphyxiation and burial of organisms. The organic-rich sediments throughout the central basin facies are rhythmically alternating microturbidites. Our analyses suggest that these microturbidites were principally generated during summer monsoonal storms by carrying sheetwash-eroded and/or resuspended sediments over a pycnocline. The cascading organic-charged sediments of the detached turbidity flows would absorb oxygen as they descended, thereby suffocating and burying animals situated below the pycnocline. Seasonal climatic variability would have provided the cycling between vertically mixed to density-stratified water column. These dynamics are likely to have promoted the high biodiversity of the bay, would have produced the rhythmic repetition of microturbidites that characterize the Bear Gulch Limestone, and provide a compelling explanation for the detailed preservation of its fossils.