Geologic units in Accomack county, Virginia

Additional scientific data in this geographic area

Omar Formation - Accomack Member (Quaternary) at surface, covers 31 % of this area

Accomack Member of Omar Formation (middle Pleistocene, Mixon, 1985). Light-to dark-gray, light-yellowish-gray, brownish-gray, and yellowish-orange sand, gravel, silt, clay, and peat of south west-trending central upland (altitude 38-50 feet) in Accomack County. Upper part of unit is bounded on east and west by ocean- and bay-facing scarps; lower part present in subsurface of adjacent lowland areas where it is overlain unconformably by Upper Pleistocene and Holocene deposits. In northern part of county, unit is a barrier-back barrier sequence of clean, cross-bedded, gravelly sand (above) and peat, clayey silt, and muddy sand (below); mollusks include Crassostrea, Mercenaria, and Noetia. In southern part of county, fine to coarse, trough cross-bedded sands of barrier-spit origin overlie fine- to very-fine-grained, muddy, nearshore-shelf sand containing Spisula, Ensis, Anomia, and Mulinia. At base of unit, pebbly to bouldery, medium- to very-coarse-grained sand and thick, compact clay-silts constitute the fluvial-estuarine fill of a paleochannel of the Susquehanna River system. Accomack Member and underlying channel fill are as much as 200 feet, or more, in thickness.

Marsh and Intertidal Mud Deposits (Quaternary) at surface, covers 24 % of this area

Medium to dark-gray soft mud, and grayish-brown peat, comprising sediment of marshes in coastal areas and Chesapeake Bay; thickness is 0 to 10 feet. Also, sandy mud and muddy fine sand, light- to dark-gray. Locally, contains abundant shell material characterized by Crassostrea virginica and Mercenaria mercenaria. Comprises sediments of shallow bays and flats in area of Atlantic coastal lagoons of the Eastern Shore.

Kent Island Formation (Quaternary) at surface, covers 23 % of this area

Kent Island Formation (upper Pleistocene, Owens and Denny, 1979). Pale-gray to yellowish-gray, medium to coarse sand and sandy gravel grading upward into poorly to wellsorted, fine to medium sand, in part clayey and silty. Unit is a surficial deposit of broad, bayward-sloping lowland (altitude ranges from sea level to about 20 feet) bordering east side of Chesapeake Bay. Thickness ranges from a feather-edge at scarp along eastern edge of lowland to about 40 feet in downdip areas.

Joynes Neck Sand (Quaternary) at surface, covers 6 % of this area

Joynes Neck Sand (upper Pleistocene, Mixon, 1985)). Yellowish-gray, fine to coarse sand coarsening downward to gravelly sand and sandy gravel. Cross-lamination in finer grained sands accentuated by black, heavy minerals. Unit was deposited in nearshore-shelf depositional environment; constitutes surficial deposit of coast-parallel terrace (altitude 23-26 feet) on eastern side of upland in Accomack County. Thickness ranges from 0 to 30 feet.

Beach Sand and Dune Sand Deposits (Quaternary) at surface, covers 6 % of this area

Pale-gray to light-yellowish gray, fine to coarse, poorly sorted to well-sorted, shelly in part; contains angular to rounded fragments and whole valves of mollusks. Comprises deposits of coastal barrier islands and narrow beach dune ridges bordering brackish-water marshes of Chesapeake Bay. Thickness is as much as 40 feet.

Wachapreague Formation (Quaternary) at surface, covers 5 % of this area

Wachapreague Formation (upper Pleistocene, Mixon, 1985). Coarsening upward sequence, includes a lower member of clayey and silty, fine to very-fine, gray sand interbedded with clay-silt and an upper member of medium to coarse, gravelly sand. Mollusks, including Mesodesma arctatum and Siliqua costata, and ostracode assemblages dominated by Elofsonella concinna and Muellerina canadensis indicate cooling ocean temperatures during deposition of unit. Pollen assemblage dominated by pine, spruce, birch, and alder suggests cool- to cold-temperate conditions in nearby land areas. Unit is surficial deposit of narrow, arcuate coastal lowland ranging in altitiude from sea level, at eastern border with Holocene barrier-lagoon complex, to about 15 feet at toe of ocean-facing scarp forming western boundary. Thickness is 0 to 40 feet.

Nassawadox Formation: Butlers Bluff Member (Quaternary) at surface, covers 2 % of this area

Butlers Bluff Member (Mixon, 1985). Pale-gray to light yellowish-gray, fine to coarse, cross-bedded, pebbly sand and sandy gravel comprising surficial deposits of upland (altitude 35-40 feet.). Diverse molluscan assemblage in lower part of unit, including Marginella, Mulinia, Nassarius, Spisula, Pleuromeris, and Olivella, indicates a shallow, nearshore-shelf depositional environment. Unit was deposited as a southward-building complex of spit-platform sands and shallow shoals and is as much as 60 feet in thickness. In subsurface, unit overlies 140 feet, or more, of pebbly to cobbly sand, clay silt, and muddy fine-grained sand of the Stumptown Memberof the Nassawadox Formation, which fills a late Pleistocene paleochannel of the Susquehanna River system.

Nassawadox Formation: Occohannock Member (Quaternary) at surface, covers 2 % of this area

Occohannock Member (Mixon, 1985). Light-yellowish-gray, fine to medium sand underlying southwest-sloping terrace (altitude 30-18 feet) on west side of upland. Sand is dominantly massive to horizontally bedded, but shows some small-scale cross-bedding; locally contains clay and silt as matrix and thin beds. Unit was deposited in a low-energy, open-bay environment. Thickness ranges from a featheredge near bay-facing scarp along western margin of upland to 20 feet in downdip areas near present bay.

Quaternary Deposits Undivided (Quaternary) at surface, covers 0.5 % of this area

Undifferentiated gray to buff sand and gravel, gray to brown lignitic silt and clay, occasional boulders, and rare shell beds. Surficial deposits occur as intercalated fluvial sands and marsh muds (e.g. in upstream floodplain of the Wicomico and Nanticoke Rivers), well-sorted, stablized dune sands (e.g. eastern Wicomico County), shell-bearing estuarine clays and silts (e.g. lower Dorchester County) and Pocomoke River basin of Worcester County), and beach zone sands (e.g. Fenwick and Assateague Islands). Wisconsin to Holocene in age. Subsurface deposits of pre-Wisconsin age consist of buff to reddish-brown sand and gravel locally incised into Miocene sediments (e.g. Salisbury area), estuarine to marine white to gray sands, and gray to blue, shell-bearing clays (e.g. Worcester County).