Tidally Influenced Deltaic Deposits, Torrey Member of the Lower Triassic Moenkopi Formation, Capitol Reef National Park, Utah

Tidally influenced deltaic deposits of the Torrey Member of the Lower Triassic Moenkopi Formation are stratigraphically complex, as predicted by the natural variability and complexity associated with modern tidal settings. A detailed examination of Torrey Member outcrops in Capitol Reef National Park, Utah provides a valuable database for tidally influenced delta deposits. The Torrey Member is interpreted as deposits of lower delta plain, distributary channel, and delta front environments. Delta front sandstones are upward coarsening and consist primarily of alternating beds of siltstone and sandstone, with the thickness and frequency of siltstone beds decreasing up-section. Individual sandstone beds contain either wave- or current-ripple laminae. Facies include overbank/floodplain deposits, interdistributary bay fills, crevasse splays, and tidal flats. Overall the beds are laterally continuous, especially in delta front facies. Tidal and wave influence are found throughout each facies. Evidence for tidal influence includes flaser and lenticular bedding in siltstones, mica-rich mud drapes in sandstones, reversing flow indicators, inclined heterolithic strata and compound complex stratification in channel-fill deposits. The system was mud dominated with a maximum grain size of fine lower sand occuring within incised valley fills.

Sequence boundaries are identified by regionally extensive scoured bases containing rip-up clasts, truncation of underlying beds, and increase in grain size. Incised valley fills have well-developed intraformational conglomerates at the base and are identified as nested channel-fill successions, characterized by inclined heterolithic strata and compound complex stratification with abundant climbing ripples and wave-modified current ripples. The channel-fills within the incised valley fills generally are larger than the distributary channels that carried sediment to the deltas.