Origin of the Bear Gulch Beds (Namurian, Montana, USA)

Abstract

The Bear Gulch Beds of the Heath Formation are well known for their diverse and well-preserved assemblage offish, arthropods, and soft-bodied animals (they constitute a Lagerstätte). The Bear Gulch is a lens of lithographic limestone (approximately 12 km in lateral extent and up to 30 m thick) surrounded by black, platy shale. The lens is composed primarily of alternating massive beds (up to 25 cm thick) and argillaceous platy beds (up to 30 cm thick). Platy and massive beds are both composed primarily of normally graded laminations (1–15 mm thick). Laminations typically have sharp bases and grade upward from microspar to micrite. Lateral continuity of individual beds (at least 1 km) and laminations (at least 500 m), lack of evidence of bottom currents, and paucity of erosional features all suggest a quiet-water environment. Fossils are generally rare in the Bear Gulch Beds. The most common fossils in most beds are cephalopods, shrimp, fish, and soft-bodied organisms. Rare beds contain abundant benthic fossils including brachiopods, sponges, bivalves, conulariids, and crinoids. Fish preservation ranges from completely articulated with traces of internal organs and preserved color patterns to completely disarticulated, however most fish are well preserved. Fish and shrimp occur at the boundaries between laminations. Preservational quality of fossils and presence of abundant dewatering structures suggest rapid deposition. Lack of normal-marine shelly fossils in most beds may indicate stressful conditions at the seafloor, however some fish and shrimp were apparently well adapted for a benthic habitat. No evidence of current-modified fish debris has been observed and only rarely are fish aligned on a single bedding plane. Conditions at the seafloor were calm, possibly inhospitable, but frequently disturbed by rapid depositional events. The mechanism of deposition of the laminations remains enigmatic.