|Katherine A. Kowalski, Department of Geology, University of South Florida, Tampa, FL
Richard A. Davis, Jr., Department of Geology, University of South Florida, Tampa, FL
Ebb-tidal deltas are important factors in controlling morphodynamics of barrier island systems. Big Sarasota pass and adjacent New Pass exchange tidal flux with Sarasota Bay and the Gulf of Mexico but are quite different in their size, shape, and in their relationship with adjacent barriers. Examination of historical morphodynamics and Holocene stratigraphy permits an understanding of how the ebb-tidal deltas have evolved and how they have influenced barrier island behavior.
Numerous techniques were used in order to determine the history and processes affecting the deltas. Aerial photographs aided in understanding historical morphodynamics, cores and high resolution seismic surveys revealed stratigraphy, and current meters measured tidal flux (Figure 1).
Historically, Big Sarasota Pass has had a large ebb-tidal delta with pronounced asymmetry that is directly updrift of the bulbous end of a drumstick barrier that dates to 3,000 YPB (Stapor et al, 1991). Comparatively, New Pass was formed by a hurricane in 1848 and the ebb-tidal delta initially displayed a wave dominated morphology. Big Sarasota Pass is also much larger than New Pass; the tidal prism and aerial extent is three times larger. Numerous historical events have occurred that have affected the hydraulics of the area. Lido Key was formed in the 1920s by the artificial filling of mangrove islands at which time regular dredging operations of the channel and ebb-tidal delta of New Pass commenced. This intervention resulted in the ebb-tidal delta changing to a tide dominated morphology and a change in sediment transport patterns; currently the only means of sediment bypassing the inlet is by dredging (Davis and Gibeaut, 1990). Other numerous human acts have affected the patterns of tidal flow around the inlets as well as the tidal prism of the inlets; dredge and fill construction of the flood-tidal delta of Big Sarasota Pass and surrounding area, dredging of the Intercoastal Waterway in the 1950s and 60s, and the closure of Midnight Pass, the immediate inlet to the south, in 1983.
Although the history, size, and tidal flux of these two ebb-tidal deltas are extremely different, they have similar sedimentary units and similar patterns to their internal stratigraphic geometry. Swash platforms are dominated by find sand with an extremely small gravel component, less than 10%. These sand deposits can reach over 2 meters thick and are planar laminated with scattered thin shell hash lamina (Figure 2). Flood channels with high velocities, which are closest to the adjacent islands, produce shell beds that are laminated and range from 15 to 120 cm thick. The beds consist of greater than 25% gravel, which is composed of poorly sorted, whole shells. Flood channels with low velocities produce shell beds that are laminated and thin; less than 25 cm thick. The beds consist of less than 25% gravel, which is composed of moderately sorted, fragmented shells.
Both ebb-tidal deltas produce the same three lithic elements; thick sand units, thin sell beds, and thick shell beds. The difference in the stratigraphy of the deltas is the internal organization of these units. The sand units and shell beds are found on both sides of the main ebb channel of New Pass due to its symmetrical morphology. The dramatic asymmetry of Big Sarasota Pass only allows sedimentation of the sand and shell units on only one side of the main ebb channel.
The stratigraphy of these two ebb-tidal deltas coupled with the recent work on both flood and ebb deltas by Cuffe (1991) and Shock (1994) show patterns to the stratigraphy of these important sediment bodies. In all cases the sand-dominated tidal delta rests on muddy sand and is typically less than 2-3 m thick. Shell concentrations are at the base of and along the channel margins reflecting aggradation and migration of the channels. This is particularly common on ebb deltas. Flood deltas tend to display greater contrast in lithologies at the base of the sequence where they overly muddy sands.