As part of an evaluation of the acoustic response of relatively unconsolidated sediments of Cretaceous and younger age beneath the Atlantic Coastal Plain, a seismic-reflection survey was conducted in Northumberland County, Smith Point, Virginia. The seismic data were acquired by the VP1&SU VIBROSEIS crew using a single Y-1100 vibrator. The survey area is on the flank of a negative gravity anomaly centered east of Smith Point and covering much of Chesapeake Bay. The purpose of the study was to confirm the depth to, and the nature of, acoustic basement and to investigate the continuity of reflectors in the overlying sedimentary sequence. Seismic and drill-hole data from many sites in the northern Virginia and Maryland area provided a generalized stratigraphy from which to interpret the seismic data.

Seismic data indicate an average basement depth of 950 m. Data quality is sufficient to identify two normal basement faults with displacements of 40 m and 15 m bounding a tilted basement fault block. The presence of a large negative gravity anomaly and the known high heat flow (64.9 mW/m2) at this location suggest that the basement rock type is granite.

Analysis of velocity spectra reveals the presence of an indurated unit 90 m thick of Triassic(?) age overlying crystalline basement.

Shallow reflections marking regional Eocene and Cretaceous unconformities can be correlated with lithostratigraphy at three drill sites in Northumberland County.

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