Robert B. Mixon

Meteoroid mayhem in Ole Virginny: Source of the North American tektite strewn field

New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A polymictic, late Eocene impact breccia, composed mainly of locally derived sedimentary debris (determined from four continuous cores), surrounds and partly fills the crater. Structural and sedimentary characteristics of the Chesapeake Bay crater closely resemble those of the Miocene Ries peakring crater in southern Germany. We speculate that the Chesapeake Bay crater is the source of the North American tektite strewn field.

The record of major quaternary sea-level changes in a large coastal plain estuary, Chesapeake Bay, Eastern United States

Abstract Seismic-reflection surveys of the Chesapeake Bay, combined with geologic mapping and analysis of boreholes on the Delmarva Peninsula, provide evidence of at least three generations of the Susquehanna River system and three generations of the Chesapeake Bay. The evidence for ancient courses of the Susquehanna River is preserved as three distinct paleochannels, and evidence for ancient versions of the Chesapeake Bay is preserved as three sets of paleochannel fill beneath the bay and three generations of barrier-spit deposits on the southern Delmarva Peninsula. The paleochannels represent relative sea-level minima and the channel-fill and barrier-spit deposits represents relative sea-level maxima. A history of three major marine transgressions is recorded in the stratigraphy preserved in the filled paleochannels and in the overlying barrier-spit complexes: three systematic progressions from fluvial to estuarine to bay or nearshore marine environments. This sea-level record seems to be compatible with the saw-toothed pattern of the marine oxygen-isotope record and with the concept of glacial-interglacial terminations. It also seems to have a climax character in which most of the preserved evidence is related to the largest terminations and to the extreme sea-level positions that bound those terminations. The three paleochannel-fill and barrier-spit complexes appear to correspond to oxygen-isotope stages 1,5, and either 7 or 11; the three related paleochannels correspond to stages 2, 6, and either 8 or 12.

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.

Stafford fault system: Structures documenting Cretaceous and Tertiary deformation along the Fall Line in northeastern Virginia

Four en echelon northeast-trending structures, including southeast-dipping monoclines and northwest-dipping, high-angle reverse faults, have been mapped along the inner edge of the Coastal Plain in northeastern Virginia–an area generally considered to be undeformed. Although displacements are small (15 to 60 m), the structures markedly affect the present distribution and thickness of Coastal Plain sediments. Structure-contour maps on Cretaceous and Paleocene lithostratigraphic units show that the amount of displacement on the structures increases downward, indicating recurrent movement. The major deformation took place in the Cretaceous and the middle(?) Tertiary, but some latest Tertiary or Quaternary movement is possible. The structures, herein named the Stafford fault system, extend for at least 56 km parallel to the Fall Line and the northeast-trending reach of the Potomac estuary. This relationship supports the hypothesis that the Fall Line and major river deflections along it have been tectonically influenced.

Ancient channels of the Susquehanna River beneath Chesapeake Bay and the Delmarva Peninsula

Three generations of the ancestral Susquehanna River system have been mapped beneath Chesapeake Bay and the southern Delmarva Peninsula. Closely spaced seismic reflection profiles in the bay and boreholes in the bay and on the southern Delmarva Peninsula allow detailed reconstruction of each paleochannel system. The channel systems were formed during glacial low sea-level stands, and each contains a channel-fill sequence that records the subsequent transgression. The trunk channels of each system are 2 to 4 km wide and are incised 30 to 50 m into underlying strata; they have irregular longitudinal profiles and very low gradients within the Chesapeake Bay area. The three main-stem channels diverge from the head of the bay toward the southeast. The channels are rarely coincident, although they commonly intersect. All three main channels pass beneath the southern Delmarva Peninsula, forming an age progression from north (oldest) to south (youngest) beneath the Peninsula, and from east (oldest) to west (youngest) beneath Chesapeake Bay. Southward progradation of the tip of the Delmarva Peninsula during interglacial high sea-level stands caused southward migration of the mouth of the bay, so that the next generation of channels were incised progressively further towards the southwest. The youngest paleochannel is clearly of late Wisconsinan age, about 18 ka, and the intermediate one appears to be late Illinoian in age, or about 150 ka. The age of the oldest paleochannel is not well constrained, but it is in the range of about 200 to 400 ka. The three paleochannel systems imply a dynamic coastal-plain environment and at least two previous generations of the Chesapeake Bay. Both the Chesapeake Bay and the southern Delmarva Peninsula have changed considerably in the past half million years.

E-3 Southwestern Pennsylvania to Baltimore Canyon Trough

DNAG Transect E-3. Part of GSA’s DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.