Permian Carbonate Precipitation in the Magnesian Reef, Northeastern England

James W. Bishop
UC Davis, Geology Dept.
Davis, CA 95616
[email protected]

Middle and Late Permian reefs are distinct during Phanerozoic time for their abundance of microbial and abiotic precipitates (e.g., Grotzinger & Knoll, 1995). These anachronistic reef textures may reflect unusual oceanographic conditions that persisted for tens of millions of years prior to and after the Guadelupian-Lopingian and Permo-Triassic extinction events. The goal of this investigation is to unravel the controls on carbonate precipitation in Middle and Late Permian reefs—beginning with the Magnesian Reef. Just completed fieldwork will be complemented by petrographic and geochemical studies of collected samples to understand the genesis of microbial and abiotic precipitates. Isolated quarries around Sunderland, England, contain reef, forereef, and reef-base coquina sediments with voluminous microbial? encrustations and botryoidal precipitates. The reef margin was episodically coated with up to 30cm thick laminated mats; columnar stromatolites coated the upper forereef; and reef framework was commonly encrusted by microbial? laminae. Calcite pseudomorphs of botryoidal aragonite? are common between skeletal grains in the reef base coquina, and in the reef, large dm-scale accumulations of botryoidal cements either precipitated directly on the seafloor or in m-scale crypts. In both cases, botryoids likely precipitated from seawater and reflect unusual Permian ocean chemistry. Petrographic and geochemical description will help constrain the genesis of these microbial? and abiotic precipitates. A thorough understanding of the controls on carbonate precipitation is the first step in understanding biogeochemical cycling and oceanographic conditions during the Middle and Late Permian and will help provide a context to the Phanerozoic’s largest mass extinction.

AAPG Search and Discovery Article #90033©2004 AAPG Foundation Grants-in-Aid