SIGMOIDAL CROSS-STRATIFICATION IN CARBONATE-SILICICLASTIC MIXTURES IN THE PERMO-PENNSYLVANIAN HONAKER TRAIL FORMATION, SOUTHEASTERN UTAH

The Permo-Pennsylvanian Honaker Trail formation in southeastern Utah contains sigmoidal cross-beddings of carbonate-siliciclastic mixtures . The sigmoidal cross-bedded sequences are about 3 m in thickness, and are overlain by Skolithos-bearing, thick bedded, calcareous sandstone. The boundary between the sigmoidal cross-bedded unit and the overlying burrowed unit is generally sharp and planar. These bundles overlie thin-bedded grainstone with wave ripple lamination.

The sigmoidal cross strata occur as sets or bundles. The bundles are usually 30 to 40 cm in thickness, and show lateral thickness variation. Each bundle is bounded by centimeter-scale mud drapes. The bundles of sandy foresets bounded by mud drapes are very similar to those described in the Jurassic Curtis Formation (Kreisa and Moiola, 1986) and Holocene examples in the southern part of the Netherlands (Visser, 1980; Allen and Homewood, 1984). The mud drapes form 1 to 2 cm thick layers, which are discontinuous and rarely extend to the top of the foreset boundary. Thin mud layers often drape the small-scale wave ripples that are abundant on the toes of the sigmoidal cross-beds. These mud drapes are interpreted as pause planes in the tidal bundles.

Reactivation surfaces in the bundle sequences are present in between sigmoidal cross-bedding. Each reactivation surface occurs between two sets of cross-bedded strata. The occurrence of reactivation surfaces can also be related to tidal effects. Since tidal currents fluctuate in flow velocities, erosional surfaces can be formed between two migrating megaripples. The resulting reactivation surface is superimposed again by avalanching as the upper megaripple overtakes and advances further. Sigmoidal cross-bedding described above suggests that there had been tidal influences in the Permo-Pennsylvanian time along the study area.