--> Abstract: Anatomy of Anachronistic Carbonate Platform: The Lower Triassic Moenkopi Platform of the Southwestern United States; #90063 (2007)

Datapages, Inc.Print this page

Anatomy of Anachronistic Carbonate Platform: The Lower Triassic Moenkopi Platform of the Southwestern United States

 

Woods, Adam D.1, Michelle L. Mary2, Scott A. Mata1 (1) CSU Fullerton, Fullerton, CA (2) University of Utah, Salt Lake City,

 

Examination of Lower Triassic carbonate rocks from around the world has revealed the occurrence of a variety of unusual features in the period following the Permian-Triassic mass extinction. Widespread, harsh environmental conditions, particularly anoxia in the global ocean, appears to have affected paleobiologic trends as well as the carbonate chemistry of the oceans, and led to the resurgence of unusual carbonates that had last been common in the Proterozoic (“anachronistic facies”). The marine rocks of the Moenkopi Formation (Virgin Limestone) and laterally equivalent deep-water limestones of the Union Wash Formation provide a means to produce a facies tract for a carbonate platform (in this case, a distally-steeped ramp) from this unusual period. Deep-water facies are typically undisturbed by bioturbation and contain synsedimentary, seafloor calcium carbonate precipitates, that occur as laterally-extensive crusts, up to 3 cm thick (basinal environments), or as interlocking mosaics of hemispheres and fans (with diameters up to 10 cm) (outer shelf to slope). Inner shelf environments contain aggregates of cabbage-head stromatolites that formed 1-2m high “patch reef” bioherms (Pruss et al., 2004). Flat-pebble conglomerates and mud chip facies are also common in the inner shelf environment and likely represent suppression of vertical burrowing (Pruss et al, 2005). Small, fringing stromatolite bioherms occur in shallow subtidal environments, and ooid shoals were prevalent in many shallow subtidal to intertidal areas. The occurrence of microbial and inorganic carbonates likely reflects unusual oceanic conditions, related to widespread anoxia, which led to the enhanced precipitation of microbial and inorganic carbonate.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California