Anatomy of
Anachronistic Carbonate Platform: The Lower Triassic Moenkopi Platform of the
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