Abstract: Physical and Sedimentologic Processes in Mesotidal Salt Marsh

Larry G. Ward

Salt marshes are accreting at various rates depending on their stage of development and geographic location. The processes responsible for the transport and deposition of the sediment accumulating within the marshes are not understood. To determine the sedimentologic processes in a marsh at Kiawah Island, South Carolina, detailed sediment budgets were done during fortnightly periods in March and August 1977. Current velocity, discharge, suspended load, temperature, and salinity were measured during different tidal and meteorologic conditions in a channel which normally transmits the entire flood and ebb discharge to a 3-sq km area. In total, over 2,000 water samples were taken during 24 thirteen-hour tidal cycles. Results from a detailed experimental error analysis indica e that discharge and suspended load were determined within ±10%.

Tidal flow in the marsh system has a pronounced velocity asymmetry with the ebb-current velocities 20 to 30% higher than flood currents. Maximum currents during spring tides, occurring 1 to 2 hours after high slack water, reach velocities of 100 cm/second under normal conditions. Suspended sediment budgets indicate that, during spring tides, there is an export of organic material and a balance or import of inorganic material. High winds and/or heavy rains can cause significant import or export of fine-grained sediments and are extremely important to marsh sedimentary processes. Northeast winds cause elevated water levels, which result in longer flood-tide duration, abnormally high current velocities, and higher suspended loads. Rainstorms, wash large amounts of sediment from the marsh surface into the channels, also increasing suspended load. Depending on the tidal stage, the increased suspended load can be transported farther landward and deposited or transported seaward and flushed from the marsh system.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma