FORMS OF DOLOMITE AND SILICA AS PROXIES FOR PALEOCLIMATE, PALEOGEOGRAPHY, AND PALEOCEANOGRAPHY OF UPPER DEVONIAN BLACK SHALES IN NORTH AMERICA

Dolomite and silica phases are common minerals in Upper Devonian black shale from the southern Midcontinent of North America. Dolomite occurs as abraded silt-sized anhedral and subhedral grains floating in organic-rich shale matrix or concentrated in thin laminae and beds. In thin beds, abraded dolomite grains comprise graded layers and Bouma sequences and may be randomly mixed with varying amounts of silt-sized quartz. Distribution and sedimentary structures indicate deposition from bottom flows. Locally, in proximal settings or on intrabasinal highs, pristine, euhedral dolomite rhombs float in organic-rich black shale matrix. Dolomite is monocrystalline and monotonously uniform in texture. Notably absent are carbonate rock fragments and the wide variety of textures common in older carbonate rocks that underlie black shale throughout the region. These observations suggest that dolomite is not terrigenous detritus but formed contemporaneously and was resedimented episodically by bottom flows.

Silica phases include detrital silt-sized quartz, biogenic (radiolarian) chert, and authigenic quartz, chert, and chalcedony (both length slow and length fast). Detrital quartz is most abundant in proximal settings and in deep basin depocenters where it frequently occurs in graded beds with Bouma divisions. Radiolaria are common on distal highs and slopes, forming chert layers on some intrabasinal highs and novaculite formations along continental margins. Anomalously high concentrations of biogenic silica are testament to upwelling in ancient seas. Authigenic silica occurs as replacements and fillings of body and trace fossils, as fracture fillings, and as replacement of earlier authigenic minerals. Quartz, chert, and chalcedony locally replace anhydrite in burrows and syneresis cracks.

These features support the interpretation that black shale formed in an arid climate where evaporation locally favored dolomite and anhydrite formation. The abundance of organic matter is explained by both high biologic productivity in surface waters and stagnant bottom conditions. High biologic productivity persisted due to the nutrients in upwelled ocean water and bottom stagnation arose due to strong density stratification enhanced by the hypersaline brine that sank to the sea floor and prevented deep circulation.