Maryland Department of Natural Resources

Reports

Non-Energy Resources and Shallow Geological Framework of the Inner Continental Margin Off Ocean City, Maryland


1994, Wells, Darlene V.

Open-File Report 16


Summary

As part of the seventh year of the Minerals Management Service ))Association of American State Geologists Continental Margin Program, the Maryland Geological Survey examined over 300 kilometers of high resolution seismic profile records and lithological logs and textural data from 162 vibracores to further delineate the shallow geologic framework of Maryland's inner continental shelf.. The seismic profiles and vibracores were originally collected by the U.S. Army Corps of Engineers (COE) to locate and assess beach fill borrow areas for the Ocean City Beach Replenishment Project.

The textural data from sediment samples taken from the vibracores show that the shallow shelf sediments consist primarily of medium to fine sand. Gravel is not a major component. Sediments become coarser in the northerly and offshore direction.

Vibracores penetrated at least two distinct depositional units in addition to modern shoal sands. The oldest unit penetrated is Pleistocene in age, interpreted to be equivalent to oxygenisotope stage 5 deposits (~128 - 80 ka). This Pleistocene unit is heterogeneous in texture, ranging from sequences of interbedded, green to gray, muddy sands to gravelly sands. This unit extends throughout the study area and is exposed along the sea floor in the inter-shoal trough areas.

Vibracores also penetrated a broad, shallow paleochannel feature that cuts into the underlying Pleistocene unit and extends under a shore-attached shoal within the shoreface zone. The associated fill deposits contained peat that yielded a radiocarbon date of 5,570±70 yr. B.P. The geometry of shallow paleochannel feature suggests that it is an extension of Roy Creek which drains into Assawoman Bay. Reconstruction of the paleodrainage off Ocean City suggests that the paleointerfluve corresponding to the Wisconsin drainage divide separating Delaware River system from the St. Martin River system, and perhaps the Susquehanna River system, is located along the Maryland/Delaware state line.

Of the three depositional units sampled within the study area, modern shoal deposits represent the most viable sand source for beach fill. The quality and quantity of shoal sand vary depending on whether the shoal is detached or shore-attached. Detached shoals generally contain larger volumes of coarser sand as opposed to the shore-attached shoals.

The shore-attached shoals within the study area contain very limited volumes of sand suitable for beach fill. These shoals were eliminated by the COE as borrow areas for the Ocean City Beach Replenishment project. Three detached shoals (shoal 2, 3, and 9) were found to contain sufficient volumes of suitable sand and, thus, were selected as potential borrow areas. Two of these shoals were recently dredged for beach fill for the Ocean City Beach Replenishment Project. Over7 million cubic meters of sand were excavated from shoals 2 and 3, essentially removing large portions of the shoals themselves and exhausting the borrow areas of suitable sand. The third detached shoal (shoal 9) contains approximately 5 million cubic meters of suitable sand. This shoal will be dredged for beach fill for maintenance replenishment at Ocean City.