A MODEL FOR THE EVOLUTION OF ESTUARINE MARGIN DRAINAGES AND COASTAL STRATIGRAPHIC SEQUENCES -- SEA LEVEL CYCLES AND FLUVIAL DISPLACEMENT

Quaternary deposits of the Delaware Bay estuarine margin comprise a complex stratigraphic section that is in part a product of drainage rearrangement along the bay margin. Drainage pattern changes result from bay margin geomorphic evolution through successive cycles of sea level. Bay margin stratigraphy reflects the episodes of valley displacement that accompanied these cycles. We identify several allostratigraphic units in the bay margin, including highstand deposits from marine oxygen isotope stages (OIS) 13, 11, 9, 7, 5, and 1, and three subunits of OIS 5. Highstand deposits are thin, sandy, and lack dateable material. In contrast, paleovalleys contain thick, organic-rich transgressive and highstand deposits. To clarify Quaternary sea level events recorded in the bay margin we must find and sample paleovalley fills.

A model for Quaternary evolution of the estuary margin enables us to predict the location of buried paleovalleys. We suggest that Pleistocene highstand conditions on the bay margin were sand dominated (today tidal wetlands and mud flats dominate) and that a wide, shallow platform developed. The platform was capped by sandy barriers and bars that were driven up-bay by longshore current. Upon initial sea-level fall these sand bodies diverted streams, resulting in fluvial valley offset. The subsurface record of valley displacement is characterized by numerous paleovalleys of various ages that have inset and cross-cutting relations, and that are often successively offset through time.

Our model is similar to that of Oertel and Foyle (1995) for the displacement of the Susquehanna River by the evolution of the southern Delmarva Peninsula, but 1) is different in scale, and 2) relates to bay margin rather than bay mouth systems. Using our model we have successfully predicted the location of buried paleovalleys. An important result is that we now have access to the most complete sea level record in the bay margin and we will use this to refine our understanding of sea level events of the Quaternary.