OERTEL, GEORGE F., and HAN J. WOO, Department of Oceanography, Old Dominion University, Norfolk, VA, MICHAEL S. KEARNEY, Department of Geography, University of Maryland, College Park, MD, and ANTHONY M. FOYLE, Department of Oceanography, Old Dominion University, Norfolk, VA

ABSTRACT: Regressive to Transgressive Quaternary Deposits in Delmarva Coastal Lagoons

Shallow sediments in coastal barrier lagoons of the southern Delmarva Peninsula contain sandy mud sequences 3-8 m thick. The sequences originally were believed to be the result of lagoonal fill during the late Holocene; however, it is now believed that the sequences, in part, were produced during regression, transgressive-reworking, and transgression. Textural distinction alone is not possible because sandy muds and muddy sands of the regressive coastal deposits are texturally similar to Holocene transgressive deposits. Correlation of subaerially exposed regressive units with buried regressive equivalents under the Holocene sediments was made possible through detailed microfossil (primarily foraminifera and pollen) analysis of Vibracore(TM) samples coupled with very high-resolution se smic reflection data.

During the late Wisconsin regression, strand-plain deposits (of the Wachapreague Formation) were deposited along the Mappsburg shoreline and above the shoreface sands of the Nassawadox spit. The sandy ridges (Bell Neck and Upshur Neck among others) and larger islands (the Mockhorn Islands) of the Wachapreague Formation stepped offshore as the late Wisconsinan sea began receding toward the shelf edge. Sandy ridges contain several prominent basal gravel layers and are closely spaced immediately south of the "highstand" mouth of the ancestral Machipongo River near Fowling Point, Virginia. During regression, the swales between ridges were partially filled with mud containing modest amounts of the pollen of spruce (2-6%) and birch (2-6%). As the late Wisconsinan sea continued to recede tow rd the shelf edge, the Machipongo River mouth and regressive strand-plain deposits migrated seaward. The strand-plain deposits gently dipped seaward above the relict Mappsburg shoreface and shelf surface. During the late Wisconsinan and early Holocene, these features were subaerially exposed and stood high above sea level. During the middle to late Holocene, the ancestral Machipongo valley was inundated by rising sea level forming an estuary with muddy fringe marshes along its margins. The topographically higher and adjacent portions of the relict strand plain became a brackish swamp, where the pollen of hickory, pondweed, cattail, and other grasses accumulated with organic muds. As rising water in the estuary spilled over the banks of the Machipongo River, it formed a marsh lagoon above the relict stand-plain surface and between the mainland and the outer barriers. The formation of the lagoon appears to be relatively recent, because sediments rich in ragweed pollen (4-9%) commonly were found immediately above Wachapreague sands or muds containing modest amounts of spruce and birch. Commonly, contacts were not sharp, but contained reworked beds with mixtures of both cool- and warm-climate pollen. Continued inundation to the present highstand elevation has submerged marshes in the middle to outer parts of the lagoon and restricted subaerial exposure of the strand-plain ridges to the inner and middle parts of the modern lagoon. Wave agitation in exposed bays has winnowed away much of the marsh-mud surface, leaving a coarsening-upward layer on the lagoon floor.

AAPG Search and Discovery Article #90995©1993 AAPG Eastern Section Meeting, Williamsburg, Virginia, September 19-21, 1993.