DELTAIC FACIES OF THE TRIASSIC MOENKOPI FORMATION AT CAPITOL REEF NATIONAL PARK

Outcrops of the Black Dragon and Torrey members of the Triassic Moenkopi Formation exposed in and around Capitol Reef National Park, Utah, are interpreted to represent tide-dominated delta strata. These outcrops provide ideal exposures and contain abundant sedimentary structures needed to refine the facies model for tide-dominated deltas. Study of these outcrops permits the interpretation of deltaic facies within the formation: prodelta, delta front, distributary channels, and mouth bars.

Prodelta deposits (Black Dragon and lowermost Torrey members) are composed of mudstones with centimeter-scale siltstone interbeds, which thicken upward. Prodelta deposits are overlain by delta front deposits (Torrey Member), which coarsen upward, and contain the majority of subenvironments in the delta: tidal flats and tidal channels, both of which contain intertidal and subtidal zones. Tidal flat deposits form tabular, decimeter-scale beds and can be sandstone or siltstone dominated. These beds form bedsets up to 3 m in thickness. Tidal flat strata possess current, wave, or interference ripples, and most contain climbing ripples. Most beds have sharp lower contacts. Tidal channels differ from distributary channels in that they are relatively flat-bottomed, subaqueous, restricted to the delta front, cut into delta front deposits, and facilitate tidal exchange in the delta front facies. Tidal channels are predominantly sandstone, contain internal scours and rip-up clasts, and exhibit lateral variability in thickness. Climbing ripples dominate channel deposits but often contain low-angle foresets, which are interpreted to represent the toes of bars. All subenvironments in the delta front exhibit strong tidal influence, evidenced by the presence of flaser, wavy, and lenticular bedding, multiple reactivation surfaces, double mud drapes, and tidal bundles. Climbing ripples are common alongside wave ripples and soft-sediment deformation features. Distributary channel and mouth bar candidates have been identified. Distributary channels are thicker than tidal channels, and have concave lower contacts and limited lateral extent. Mouth bars exhibit internal scours overlain by planar bedding which transition laterally to climbing ripples, and evidence of decelerated current flow at the river mouth.