PALEOECOLOGY AND SEDIMENTOLOGY OF THE BEAR GULCH LIMESTONE, CENTRAL MONTANA: HOW DEPOSITION AND TAPHONOMY MAY BE DRIVING DIVERSITY AND ABUNDANCE

The Bear Gulch Limestone (BGL) provides a rare glimpse into Late Mississippian life. The BGL is a finely laminated micritic limestone best known for soft-tissue preservation of fish and a lesser known diversity of invertebrates such as sponges, annelids, decapods, and lingulid brachiopods. Alternating resistant flinz and weathered fäule bedsets may record tidal, seasonal, or climatic cycles and detailed paleoecological study reveals how invertebrate benthic macrofauna responded to environmental change. Recent excavations in 2012 and 2013, conducted with systematic collecting techniques, have revealed an even more abundant invertebrate fauna than previously recorded. Six excavations were performed in 2012 in Miller Canyon targeting the base, middle, and top of the BGL. Two excavations in 2013 were conducted approximately 2 km east of Miller Canyon in Book Coulee and targeted the middle of the BGL. Miller Canyon is fossil poor with fossils occurring almost exclusively in the flinz bedsets. Flinz bedsets from the Book Coulee excavation have the greatest diversity and often include dense assemblages of productid brachiopods, whereas fäule bedsets have a paucity of fossils restricted to few productids and rare orthiconic cephalopods. Preliminary results suggest that flinz beds may represent episodic events such as turbidites or storms, whereas fäule beds indicate normal or continuous sedimentation. Increasing fossil abundance from Miller Canyon to Book Coulee may represent a depth gradient, as the depo-center is reconstructed to the east of both Miller Canyon and Book Coulee. The processes controlling the alternating bedsets are not known but may result partly from differential diagenesis. Mollusk shells are often dissolved, but internal structures such as cephalopod mandibles and molds/casts of bivalves and gastropods suggest that the fossil signature is not completely lost and paleoecological trends can still be elucidated. Current stratigraphic models place the BGL within or very close to the Serpukhovian stage, globally a time of great biotic and climatic fluctuation. The BGL likely preserves unique snapshots of benthic life during a critical time in Earth history. Future study may greatly increase understanding of processes underway during the Serpukhovian Biodiversity Crisis.