Tidal Deposits of the Triassic Moenkopi Formation, Capitol Reef National Park, Utah
The Triassic Moenkopi Formation in Utah’s Capitol Reef National Park is comprised of four members that consist of reddish to chocolate-brown colored siltstones, fine- to very fine-grained sandstones and a yellowish-grey limestone. Deposition occurred on a gentle westward-dipping slope that was part of a trapped shelf. Members display several different tide-dominated depositional environments, and three of the four members are clastic dominated. Evidence to support a tidal setting includes cyclic bedding patterns, as well as sedimentary structures diagnostic of tidal processes, including flaser, lenticular, and wavy bedding. Also present are current and wave ripples, mud cracks, reactivation surfaces, load casts, and mm-scale micaceous lamina on rippled surfaces which helps define spring-neap cyclicity.
The basal member (the Black Dragon) was deposited in the sub-tidal zone, and represents the most distal environment of the three clastic members. This member contains 1-2 meter thick upward-fining successions of interbedded siltstone and fine- to very fine grained sandstone, interpreted as the sub-tidal equivalent of a tidal-flat. Wave ripples and flaser bedding are ubiquitous, and lamina are separated by mm-thick micaceous partings. The middle member (Torrey Member) was deposited in the intertidal to sub-tidal zone. This member contains upward-fining, 2-3 meter thick bedsets in an overall upward coarsening succession. Bedsets include alternating beds of fine to very-fine grained sandstone and siltstone. Both wave and current ripples occur throughout the member, as well as flaser and lenticular bedding, mud cracks, and various sole marks. These successions are interpreted as tidal flat strata, and begin with a 10 cm to .5 meter thick fine grained sandstone bed followed by interbedded siltstone and sandstone. Strata from these two members compare well to the extensive tidal flat deposits of Sapelo Island, Georgia. The uppermost member (Moody Canyon Member) was deposited in the supratidal/sabkha setting. This member is comprised of aggradationally stacked bedsets of siltstone, very-fine grained sandstone and occasionally, interbedded gypsum. Bedsets range from 1 to 1.7 meters in thickness, and are largely structureless, except for a few wispy laminations and thin wave-rippled beds. The vertical succession through the 3 clastic members record the overall transition from sub-tidal to intertidal to supratidal depositional settings.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California