--> Abstract: The Forms of Quartz and Dolomite in Woodford Shale of the southern Midcontinent, USA: Indicators of Paleoclimate, Paleogeography, Paleoceanography, and Depositional Processes, John Comer, Article #90097 (2009)

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The Forms of Quartz and Dolomite in Woodford Shale of the southern Midcontinent, USA: Indicators of Paleoclimate, Paleogeography, Paleoceanography, and Depositional Processes

John Comer1

1 Indiana Geological Survey

Quartz and dolomite are common minerals in Woodford Shale from the southern Midcontinent of North America. Quartz types 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 in distal settings, forming bedded novaculite along continental margins. Anomalously high concentrations of biogenic silica are testament to upwelling in ancient seas. Authigenic quartz, chert, and chalcedony locally replace anhydrite in burrows and syneresis cracks.

Dolomite mostly occurs as abraded silt-sized grains concentrated in thin layers where they frequently comprise graded layers and Bouma sequences and may be randomly mixed with varying amounts of silt-sized quartz. 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. These observations suggest that dolomite is not terrigenous detritus but formed contemporaneously and was resedimented episodically by turbid bottom flows.

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 hypersaline brine that sank to the sea floor and prevented deep circulation.

 

 

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