David W. Folger

Deep Sea Drilling Project Site 612 bolide event: New evidence of a late Eocene impact-wave deposit and a possible impact site, U.S. east coast

A remarkable >60-m-thick, upward-fining, polymictic, marine boulder bed is distributed over >15,000 km[sup 2] beneath Chesapeake Bay and the surrounding Middle Atlantic Coastal Plain and inner continental shelf. The wide varieties of clast lithologies and microfossil assemblages were derived from at least seven known Cretaceous, Paleocene, and Eocene stratigraphic units. The supporting pebbly matrix contains variably mixed assemblages of microfossils from the same seven stratigraphic units, along with trace quantities of impact ejecta (tektite glass and shocked quartz). The youngest microfossils in the boulder bed are of early-late Eocene age. On the basis of its unusual characteristics and its stratigraphic equivalence to a layer of impact ejecta at Deep Sea Drilling Project (DSDP) Site 612 (New Jersey continental slope), the authors postulate that this boulder bed was formed by a powerful bolide-generated wave train that scoured the ancient inner shelf and coastal plain of southeastern Virginia. The most promising candidate for the bolide impact site (identified on seismic reflection profiles) is 40 km north-northwest of DSDP Site 612 on the New Jersey outer continental shelf.

Delaware River: Evidence for its former extension to Wilmington Submarine Canyon

Seismic-reflection profiles indicate that during the Pleistocene the Delaware River flowed across the continental shelf east of Delaware Bay and emptied into Wilmington Submarine Canyon. The ancestral valley (width, 3 to 8 kilometers; relief, 10 to 30 meters) is buried, is not reflected in the surface topography, and probably predates the formation of the present canyon head

Large sand waves on the Atlantic Outer Continental Shelf around Wilmington Canyon, off Eastern United States

Abstract New seismic-reflection data show that large sand waves near the head of Wilmington Canyon on the Atlantic Outer Continental Shelf have a spacing of 100–650 m and a relief of 2–9 m. The bedforms trend northwest and are asymmetrical, the steeper slopes being toward the south or west. Vibracore sediments indicate that the waves apparently have formed on a substrate of relict nearshore sediments. Although the age of the original bedforms is unknown, the asymmetry is consistent with the dominant westerly to southerly drift in this area which has been determined by other methods; the asymmetry, therefore, is probably modern. Observations in the sand-wave area from a submersible during August 1975, revealed weak bottom currents, sediment bioturbation, unrippled microtopography, and lack of scour. Thus, the asymmetry may be maintained by periodic water motion, possibly associated with storms or perhaps with flow in the canyon head.

The geologic framework of southern Lake Michigan

This part of the project was carried out with geophysical and geological sampling techniques to determine in more detail the geology of southwestern Lake Michigan and thereby provide the essential framework on which to base subsequent lake level, process, and sediment budget studies. The bathymetry of southwestern Lake Michigan is controlled by the underlying bedrock, which dips northeast toward the center of the Michigan basin. Bedrock comprises Silurian dolomite and Devonian limestone and shale. Quaternary sediment, 10 to 40 m thick, overlies bedrock. From Waukegan, Illinois, south to Indiana Harbor, the bottom is floored by till, sand, pebbles, and cobbles. Sand, more common within 1 to 2 km of shore, thins lakeward to a patchy veneer. The lake floor is erosional or nondepositional where till or gravel-cobble pavement is exposed. In contrast, north of Waukegan and east of Indiana Harbor, fine sand covers much of the bottom and grades offshore to muddy sand, which is part of the modern, lacustrine Lake Michigan Formation. The complex surficial bottom sediment distribution between Waukegan and Michigan City, Indiana, could be mapped in detail only where we have sidescan sonar mosaics. In those areas, the till, or coarse lag sand-gravel surface, is covered intermittently with a layer of fine sand most often about 0.5 to 1.0 m thick. The sand appears to be mobile, covering and uncovering the substrate in response to storm-driven waves and currents. Thus, sand, important for protecting the substrate from erosion and for maintaining beaches, is not abundant throughout much of the area.