Tidal Channel Deposits in Upper Cretaceous of Northern Kaiparowits Plateau, Utah

J. David Sanchez, Peter J. McCabe

Seven coarsening-upward sequences have been recognized in the 300 to 400-m thick John Henry Member of the Straight Cliffs Formation. These sequences have abundant hummocky cross-stratification and are interpreted as having formed by the progradation of wave-dominated shorelines. A detailed study of these sequences showed that in many cases channel deposits are incised into upper shoreface deposits. These channels are up to 15 m deep. Mudclasts, Ostrea and Inoceramus fragments, and pebbles are present at the base of many channels. Some channel lag deposits also contain logs with Teredolites borings. Thin units of flaser, wavy and lenticular bedding may be present at any position within the channel deposits but are most common higher in the sequences. The channels are, howe er, infilled predominantly with trough cross-bedded, fine to medium-grained sandstones. Some cross-beds show multiple reactivation surfaces and the bimodal nature of the paleocurrents suggests that the cross-beds were deposited by tidal currents. The presence of tidal bundles with double mud drapes in a few cross-beds confirms the interpretation of the sandstones as tidal channel deposits. At least 22 tidal bundles are present in one tidal bundle sequence, suggesting a semi-diurnal tidal cycle.

Although there is convincing evidence of tides within the channel deposits, the shoreface deposits show little evidence of reworking by tidal currents. Possible beach or intertidal mudflat deposits have a maximum thickness of 1.5 m. The Kaiparowits region during the Upper Cretaceous probably experienced, therefore, a microtidal regime with significant tidal currents being restricted to tidal inlets or estuaries.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.