Impact of Sea Level Rise on the Sedimentology and Stratigraphy of Estuarine Systems

Larry G. Ward, Michael S. Kearney

Drowned-river valley estuaries are characteristic features of trailing-edge continental margins, as exemplified by the United States Atlantic Coast. During marine transgressions, the classic cycle of estuarine development is one of initial submergence and subsequent infilling, with the latter stages marked by extensive accumulations of fine-grained sediments in expanding marshes, deltas, and floodplains. Seismic surveys, vibracoring, and radiocarbon dating in the estuarine tributaries in middle Chesapeake Bay (the largest estuary along the Atlantic Coast) indicate that thick accumulations (> 25 m) of organic-rich fine-grained sediments have been deposited since the middle Holocene. However, studies of recent accretion rates (based on pollen and radionuclide analyses) s ggest the marshes, which represent a near-end member of the estuarine depositional sequence, may no longer be accumulating significant volumes of sediment. Relatively rapid crustal subsidence plus eustatic sea level rise produces a local submergence of ^approx 4 mm/yr. Although marsh accretion rates in the upper estuarine tributaries approach 1 cm/yr, marsh accretion rates in the middle and lower reaches are significantly less (< 2 mm/yr) than submergence. Here, numerous marshes are converting to open water as they become increasingly flooded by the tides. This change in depositional regime is also reflected in the carbon content (decreasing) and grain size (coarsening) of the marsh sediments and tidal channel migrations. In the coming decades, the rate of the world sea level rise is rojected to increase significantly. This acceleration in the global eustatic trend together with lower sediment inputs from surrounding watersheds may reverse the historic trend of estuarine infilling.

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