Anoxia along the Western Margin of North America During the Early Triassic

Adam D. Woods and David J. Bottjer

Widespread oceanic anoxia has been suggested in previous studies as one of the possible causes for the Permian-Triassic mass extinction, and reduced oceanic oxygenation may have persisted through the Early Triassic. Depauperate marine faunas, reduced degrees of bioturbation, and a shift in the ratio of burial of organic carbon to pyritic sulfur have all been suggested as evidence of oceanic stagnation during the Early Triassic.

To test this hypothesis, deep-water marginal marine sequences have been examined from throughout the western coast of North America in order to determine Lower Triassic depositional paleoxygenation levels. The sequences examined include the Union Wash Formation of east-central California, the Dinwoody and Thaynes Formation of the Phosphoria Basin (Idaho and Montana), and the Sulphur Mountain Formation of western Alberta and eastern British Columbia. Initial results show all three sequences exhibit little to no bioturbation, contain depauperate to nonexistent infaunal body fossils, and demonstrate organic carbon enrichment. Although the above sequences are still under investigation, these early results are suggestive of widespread deep-water anoxia stretching over almost 30° of la itude along the western coast of North America during the Early Triassic. Such widespread anoxia is suggestive of a shift in oceanic oxygenation conditions during the Early Triassic, and the impingement and persistence of this anoxia could have resulted in both the Permian-Triassic mass extinction as well as the delayed Mesozoic radiation.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California