Geologic units in Northumberland county, Virginia

Additional scientific data in this geographic area

Windsor Formation (Tertiary-Quaternary) at surface, covers 49 % of this area

Windsor Formation (lower Pleistocene or upper Pliocene, Coch, 1968). Gray and yellowish- to reddish-brown sand, gravel, silt, and clay. Constitutes surficial deposits of extensive plain (altitude 85-95 feet.) seaward of Surry scarp and of coeval, fluvial-estuarine terraces west of scarp. Fining upward sequence beneath plain consists of a basal pebbly sand grading upward into cross-bedded, quartzose sand and massive, clayey silt and silty clay; lower and upper parts of sequence were deposited, respectively, in shallow-marine or open-bay and restricted-bay or lagoonal environments. In terraces west of Surry scarp, fluvial-estuarine deposits comprise muddy, coarse, trough cross-bedded sand and gravel grading upward to sandy silt and clay. Thickness is 0 to 40 feet.

Tabb Formation: Lynnhaven and Poquoson Members (Quaternary) at surface, covers 23 % of this area

Undifferentiated Qtl, Qts.

Chesapeake Group (Tertiary) at surface, covers 15 % of this area

May include the following fomations - Chowan River, Yorktown, Eastover, St. Marys, Choptank, and Calvert. Chesapeake Group (upper Pliocene to lower Miocene,Darton, 1891). Fine-to coarse-grained, quartzose sand, silt, and clay; variably shelly and diatomaceous, deposited mainly in shallow, inner- and middle-shelf waters. Ages of units based on studies of foraminiferal, nannofossil, diatom, and moluscan assemblages in Virginia and adjacent states (Andrews, 1988; Gibson, 1983; Gibson and others, 1980; Poag, 1989; Ward and Blackwelder, 1980; Ward and Krafft, 1984). Includes the following formations, from youngest to oldest: Chowan River Formation (upper Pliocene, Blackwelder, 1981). Gray to dusky-blue-green sand, fine- to medium-grained, clayey and silty, commonly very shelly; grades laterally into laminated, silty clay and upward into cross-bedded, biofragmental sand, clayey silt, and silty clay. Discontinuous pebbly to bouldery sand at very irregular base of unit. Mollusks include Glycymeris hummi, Noetia carolinensis, and Carolinapecten eboreus bertiensis. Thickness is 0 to 50 feet. Recognized only in southeasternmost Virginia and North Carolina. Yorktown Formation (lower upper Pliocene to lower Pliocene, Clark and Miller, 1906). Bluish-gray and brownish-yellow sand, fine- to coarse-grained, in part glauconitic and phosphatic, commonly very shelly, interbedded with sandy and silty blue-gray clay. In lower York and James River basins, unit includes cross-bedded shell hash. Mollusks in clude Glycymeris subovata, Chesapecten jeffersonius, Chesapecten madisonius, Mercenaria tridacnoides, Panopea reflexa. Coarse-grained sand and gravel facies of the Yorktown in updip areas is mapped separately as unit psg. Thickness is 0 to 150 feet. Eastover Formation (upper Miocene, Ward and Blackwelder, 1980). Dark-gray to bluish-gray, muddy sand, very fine to fine, micaceous, interbedded with sandy silt and clay. Lower part of unit is dominantly medium- to very-thin-bedded and laminated silt and clay interbedded with very-fine sand, lenticular and wavy bedding common; upper part is mainly very-fine- to fine-grained sand containing abundant clay laminae. Typical mol lusks include Chesapecten middlesexensis, Marvacrassatella surryensis, Glossus fraterna. Thickness is 0 to 270 feet. St. Marys Formation (upper and middle Miocene, Shattuck, 1902). Bluish- to pinkish-gray, muddy, very-fine sand and sandy clay-silt, locally abundantly shelly. Chesapecten santamaria, Buccinofusus parilis, and Ecphora gardnerae are characteristic mollusks. Occurs northeast of Mattaponi River. Thickness is 0 to 40 feet. Choptank Formation (middle Miocene, Shattuck, 1902). Olive-gray sand, fine to very-fine, clayey and silty, shelly, and diatomaceous clay-silt; common y forms fining-upward sequences. Mollusks include Chesapecten nefrens, Mercenaria cuneata, Ecphora meganae. Thickness is 0 to 50 feet. Calvert Formation (middle and lower Miocene, Shattuck, 1902). Commonly consists of 2 to 7 fining-upward sequences. Each sequence includes a light- to dark-olive-gray basal sand, very fine to fine, clayey and silty, very sparsely to abundantly shelly; grades upward to sandy, diatomaceous clay-silt and diatomite. Typical molluscs include Chesapecten coccymelus, Crassatella melinus, Ecphora tricostata. Thickness is 0 to 600 feet.

Tabb Formation; Sedgefield Member (Quaternary) at surface, covers 9 % of this area

Sedgefield Member (Johnson, 1976). Pebbly to bouldery, clayey sand and fine to medium, shelly sand that grades upward into sandy and clayey silt; locally channel fill at base of unit includes as much as 50 feet of fine to coarse, cross-bedded sand and clayey silt and peat containing in-situ tree stumps. Sandy bay facies commonly contains Crassostrea biostromes, Mercenaria, Anadara, Polynices, Ensis, and other mollusks. Specimens of the coral Astrangia have yielded estimated uranium-series ages averaging 71,000 ± 7,000 yrs B.P. (Mixon and others, 1982). Unit constitutes surficial deposit to river and coast-parallel plains (altitude 20-30 feet) bounded on landward side by Suffolk and Harpersville scarps. Thickness is 0 to 50 feet.

Moorings Unit of Oaks and Coch (1973) (Tertiary) at surface, covers 2 % of this area

White, light-gray, and grayish-yellow quartzose sand and gray to grayish-brown clayey silt and silty clay. Constitutes discontinuous linear body along and just west of the Surry scarp; depositional surfaces range in altitiude from 130 feet along slightly higher, ridge-like topography at scarp to about 110 feet west of scarp. Eastern facies of unit is unfossiliferous, massive to cross-laminated, moderately well-sorted, fine sand believed to have been deposited in beach and near-shore environments. Upper part of fine sand facies interfingers westward with massive, bioturbated clay and silt deposited in a lagoon or shallow bay. Thickness is as much as 30 feet.

Bacons Castle Formation (Tertiary) at surface, covers 1 % of this area

Tb1/Tb2 Bacons Castle Formation (upper Pliocene, Coch, 1965). Gray, yellowish-orange, and reddish-brown sand, gravel, silt, and clay; constitutes surficial deposits of high plain extending from Richmond, eastward to the Surry scarp. Unit is subdivided into two members: Tb1, massive to thick-bedded pebble and cobble gravel grading upward into cross-bedded, pebbly sand and sandy and clayey silt, and Tb2, predominantly thin-bedded and laminated clayey silt and silty fi ne-grained sand. Tb2 is characterized by flaser, wavy, and lenticular bedding and rare to common clay-lined burrows including Ophiomorpha nodosa. Thickness is 0 to 70 feet.

Beach Sand and Dune Sand Deposits (Quaternary) at surface, covers 0.4 % 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.

Shirley Formation (Quaternary) at surface, covers 0.2 % of this area

Shirley Formation (middle Pleistocene, Johnson and Berquist, 1989). Light- to dark-gray, bluish-gray and brown sand, gravel, silt, clay, and peat. Constitutes surficial deposits of riverine terraces and relict baymouth barriers and bay-floor plains (altitude 35-45 feet) inset below depositional surfaces of the Chuckatuck Formation (Johnson and Peebles, 1984). Upper part of unit is truncated on the east by the Suffolk and Harpersville scarps; locally, lower part occurs east and west of scarps. Fluvial-estuarine facies comprises (1) a lower pebble to boulder sand overlain by (2) fine to coarse sand interbedded with peat and clayey silt rich in organic material, including in-situ tree stumps and leaves and seeds of cypress, oak, and hickory, which grades upward to (3) medium- to thick-bedded, clayey and sandy silt and silty clay. Marginal-matrix facies in lower James River and lowermost Rappahannock River areas is silty, fine-grained sand and sandy silt containing Crassostrea virginica, Mulinia, Noetia, Mercenaria, and other mollusks. Astrangia from lower Rappahannock River area has yielded a uranium-series age of 184,000 ± 20,000 yrs B.P. (Mixon and others, 1982). Thickness is 0 to 80 feet.

Marsh and Intertidal Mud Deposits (Quaternary) at surface, covers 0.1 % 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.