About This Item
Share This Item
Along the Atlantic coast of Delaware lies a lagoon-barrier, headland, and spit complex which has transgressed landward during the Holocene Epoch. The shoreface is relatively steep with major erosion occurring from 0 to -10 m ( -33 ft). In addition, erosional recession of the lagoon barrier and headland coast varies from 1 to 3 m (3 to 10 ft) per year averaged over the past 150 years. The inner shelf from -10 to -30 m ( -33 to -100 ft) is also undergoing modification as the transgression continues. The net result is a sequence of Holocene depositional lithosomes in the valleys of the pre-Holocene (late Wisconsinan age) land surface. These valleys were cut by the ancestral Delaware River and its tributaries. Extensive surveys of the Delaware inner shelf with high-resolution seismic profiling tied in with vibracoring have allowed delineation of the three-dimensional character of the Quaternary erosional and depositional events controlling the Holocene coastal units. The separate and distinct sedimentary environmental lithosomes of the various coastal environments form irregularly shaped depositional units that are in some places presently undergoing erosion in the shoreface and in other places are being buried by thin transgressive sands on the inner shelf. Erosion during storm periods occurs to depths of greater than 30 m (100 ft), as evidenced by outcrop of Pleistocene sedimentary units along the former pre-Holocene interfluves that have been transgressed and are now exposed on the inner shelf.
Forty-two vibracores were drilled in the shoreface in water depths of 2 to 10 m (6.6 to 33 ft). These cores encountered a great variety of stratigraphic units including Holocene Epoch barrier sands, lagoons, spits, and marshes, as well as Pleistocene sediments of the pre-Holocene headlands, originally deposited in similar coastal sedimentary environments. The present depositional shoreface sands range in thickness from 20 cm to 1.5 m (8 in. to 5 ft). In some areas, slumping or soft sediment flow is indicated.
Thus, the ravinement surface along the Atlantic coast of Delaware underlies extremely thin depositional sands in the shoreface and adjacent inner shelf. Potential preservation of coastal lithosomes in this setting is highly variable, dependent on pre-Holocene topography in many places. The transgressive sequence studied is the result of migration of coastal environments from a position near the axis of the Baltimore Canyon trough geosyncline approximately 50 km (31 mi) to the east at a
time of lower sea stand 12,000 to 14,000 years b.p. Because of higher rates of eustatic sea level rise in the early Holocene and possible subsidence of the main part of the geosyncline, it is possible that greater thicknesses of Holocene coastal sediments may be deposited in the middle and outer continental shelf. On the inner shelf and present coastal area, the upper portions of the Holocene sediments tend to be destroyed by erosion at the shoreface and move to new depositional loci within the transgressing system. An understanding of the lateral and vertical facies relationships as compared with time-depositional planes within this transgressive sequence of coastal stratigraphic units is critical in the formation of modern analogs for interpretation of ancient coastal systems. This tudy also demonstrates that rates and especially volumes of sediment eroded in the shoreface may play a major role in the economics of human occupation of the coastal zone.
End_of_Article - Last_Page 498------------