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Summary

limestone outcropStratigraphic data present the Bear Gulch Limestone as a small, narrow, shallow lens of limestone deposited in the narrow Central Montana Trough and surrounded by littoral and supralittoral sediments.

The paleontologic data from the Bear Gulch Limestone present a high-diversity shallow marine vertebrate record. There is a low to moderate diversity invertebrate record with conspicuous absences and highly localized occurrences among the more typical invertebrates of the Upper Mississippian. There are virtually no terrestrial macrofossils or indicators of fluviatile input from the nearby shores. Persistent anoxia or hypoxia of the bottom waters or sediments is absolutely contraindicated by the ubiquitous benthic and burrowing fishes.

The sedimentologic data present classical indicators of arid to semi-arid climatic conditions. These conditions could partially explain the dearth of fluviatile or terrestrial indicators. More strikingly, the rhythmically recurring sedimentary units conform in detail (although on a small scale) to the parts of a Bouma sequence of turbidites. Explaining the deposition of rhythmically repeated shallow water microturbidite sequences requires a concordance of peculiar climatic circumstances.

The paleogeography of the Bear Gulch Limestone locates the bay at a latitude between 10 and 12° north of the Namurian equator, with the bay oriented almost north to south. The paleoclimate in the area would most probably have been semi-arid to arid as well as tropical, corresponding to the African Sahel region, and monsoonal atmospheric circulation patterns would have prevailed. Climatic data strongly suggests that the turbidites were principally generated during summer monsoonal storms. Rains would carry sheetwash-eroded sediments as well as resuspended sediments and organics in a hyposaline upper water layer, while dry monsoonal storms would resuspend accumulated sediments over more hypersaline bottom waters. Under either set of conditions the result would be turbidite deposition in the central bay.

The preservation of the fishes and soft-bodied invertebrates of the Bear Gulch Limestone requires that death and burial were simultaneous in real time while a persistent high-diversity fish fauna continued to exist within the confines of the bay. Paleopathologic examination indicates suffocation as the cause of death for several fish. The zone of best preservation is limited to the central basin, and all lines of evidence point to the deposition of organic-charged microturbidite flows as the agents of both death and immediate burial.

The nature of the sedimentology, the diversity of the biota, and the extraordinary preservation of the fossils of the Bear Gulch Limestone require the development of regular, probably annual, periods of microturbidite deposition. Seasonal and intraseasonal variability in the Sahel-like climatic and wind conditions would have provided the necessary regularity of change from density-stratified conditions to water column turnover. These would furnish the mechanism necessary to produce the repetitive microturbidites that characterize the Bear Gulch Limestone and its fossils.

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