Submarine Sand Resources, Southeastern Virginia-Contributions from Year 9 and Year 10 of Virginia's Continental Margins Program

摘要

Virginia's year 9 and year 10 funds from the Continental Shelf Program were used to supplement other work funded by the Minerals Management Service in an ongoing Cooperatice Agreement focused on the area offshore of southeastern Virginia. Year 9 and year 10 funds facilitated interpretation of subbottom profiles and the analysis of sediment samples from cores and grabs.On Virginia's sediment-starved continental shelf deposits of material potentially suitable for use as beach nourishment or, perhaps, as construction aggregate occur in three stratigraphic settings, each with specific characteristics of morphology, grain-size gradients, likelihood of discovery, and physical ease of exploitation. All must be verified with a careful program of coring. Modern shoals generally are easier to identify, prove, and access than either filled channels or lenticular facies. Shoals usually are identifiable on nautical charts and characteristically have a definite lower boundary that can be seen in subbottom profiles. In most cases, the base of the shoal coincides with the level of the surrounding sea floor. Filled channels are readily identifiable on subbottom profiles but may have a narrow, sinuous form and steep lateral gradients in sediment properties. Buried lenticular facies of good-quality sand usually are found only fortuitously. As the lateral and often vertical gradients in geotechnical properties usually are low, the lenticular facies can be mined with a lesser concern for the consequences of violating the deposit's limits than with the other two types of deposit.There are three types of filled paleochannels in the study area. (1) Relatively near-surface, generally small roughly shore normal channels most likely mark the migration of tidal inlets across the shelf during the most recent transgression. (2) Small relatively wide and relatively shallow generally shore parallel channels may be filled back-barrier or lagoonal channels. (3) Larger channels trending across the shelf probably result from riverine flow.The complexity of the seismostratigraphy of the Quaternary deposits on southeastern Virginia's inner continental shelf is a result of series of high-frequency (fifth-order, 10-20,000 y), low-amplitude (20-30 m) variations in sea level that occurred during the last highstand, roughly 80,000 to 130,000 BP. The evidence of the small oscillations in sea level is best seen in the regions that were between the shoreline and wave base, today's inner shelf however; the very low rates of deposition on the shelf make it difficult to correlate specific reflectors or beds or, at times, to distinguish between fifth- and fourth-order changes.Results for the continuing studies already have been used in the determination to mine several hundred thousand cubic meters of sand from Sandbridge Shoal for use on a Navy-owned facility and in consideration of mining greater quantities of sand from Sandbridge and other shoals for use in local beach nourishment and hurricane protection efforts.