Abstract: Distribution of Sedimentary Structures in Ebb-Tidal Delta Sediments

Dennis K. Hubbard, Miles O. Hayes, John H. Barwis, George F. Oertel

Studies in several South Carolina and New England estuaries have led to the recognition of a distinct zonation of primary sedimentary structures in the vicinity of ebb-tidal deltas. Flat beds and landward-oriented planar cross-beds dominate on channel-margin areas. These commonly are accompanied by ebb-oriented trough cross-beds related to megaripple migration on lower bar surfaces. Adjacent and subjacent to these are planar and trough cross-beds related to marginal flood channels. In the main ebb channel, planar and trough cross-beds that increase in scale with depth are dominant. These may be oriented either landward or seaward. Channel bottoms can display varied forms. Shell and pebble lags are common in many South Carolina inlets where scour by dominant ebb currents has flushed most of the sand from the channel section. Mud deposits have two main modes of origin: armored mud balls and slump blocks derived from marsh sediments eroded by channel migration and flasered muds related to flocculation during extreme rainstorms. Bed forms, where present, vary from small megaripples to sand waves with heights exceeding 2 m, and account for the bidirectional trough cross-bedding in channel-bottom box cor s. Continuous diver observations of bed-form migration through a complete tidal cycle show that in areas where large sand waves dominate, bidirectional trough cross-beds are related to megaripple migration over the sand-wave surfaces and not to migration of the sand wave as a discrete packet.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC