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| OFR 16 Abstract | contact: D.V. Wells (dwells@mgs.md.gov) |
by
Darlene V. Wells
(1994)
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. Over 7 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.
(Updated 1/29/04 )
