Geologic units in Cumberland county, New Jersey

Cohansey Formation (Middle Miocene, Serravallian) at surface, covers 77 % of this area

Sand, white to yellow with local gravel and clay. Locally stained red or orange brown by iron oxides and (or) cemented into large blocks of ironstone. Unweathered clay is typically dark gray, but commonly weathers white where interbedded with thin beds of ironstone. Unit is a complex of interfingering marine and nonmarine facies. Sand is typically medium grained and moderately sorted although it ranges from fine to very coarse grained and from poorly to well sorted. Sand consists of quartz and siliceous rock fragments. Some beds are locally micaceous, and in the Lakehurst area, Ocean County, some beds have high concentrations of "black" sand (pseudorutile) that was once extensively mined. In general, the sand is crossbedded, although the style of crossbedding varies significantly with the paleoenvironment. Trough crossbedding predominates, especially in the nonmarine channel fill deposits, and the scale of the crossbeds varies from small to large. In some areas, planar bedding is well developed in sections that have abundant marine burrows (mostly the clay-lined trace fossil Ophiomorpha nodosa). Such marine-influenced beds (largely foreshore deposits) occur on the central sheet west of Asbury Park, near Adelphia, Monmouth County, north of the Lakehurst Naval Air Station, Ocean County, and at Juliustown, Burlington County (Owens and Sohl, 1969), and on the southern sheet as far north as Salem, Salem County. Gravel beds occur locally, especially in updip areas such as near New Egypt, Ocean County, in the Atlantic Highlands and in the highlands west of Barnegat, Ocean County, in the southern part of the central sheet and in mixed marine and nonmarine facies in the northeastern part of the southern sheet where gravel occurs in well-defined channels. Most of the gravel is 1.3 to 2.5 cm (0.5-1.0 in) in diameter, but pieces as long as 10 cm (4 in) are present. The gravel is composed of quartz with small amounts of black chert and quartzite. Clay commonly occurs as discrete, thin, discontinuous beds, is dark gray where unweathered, white or red where weathered. Lesser, thin laminated clay strata also are present. Locally, as near Lakehurst, thick, dark-gray, very lignitic clay was uncovered during the mining of ilmenite and is informally called the Legler lignite (Rachele, 1976). An extensive, well-preserved leaf flora was collected from a thick clay lens in a pit near Millville, Cumberland County. The leaf flora was dominated by Alangium sp., a tree no longer growing in eastern North America (J.A. Wolfe, written commun., 1992). Maximum thickness in the map area is about 60 m (197 ft); however, thickness is difficult to determine because of the irregular basal contact and extensive post-depositional erosion. There is as much as 18 m (59 ft) of relief along the basal contact. The basal contact is sharp, undulatory, and directly overlain by a thin gravel bed. The Cohansey Formation unconformably overlies the Kirkwood Formation and is found in channels cut down into the Kirkwood. Where the Kirkwood consists of sandy, light-colored sediments, the basal contact of the Cohansey is drawn below crossbedded sediments. Where the Kirkwood consists of dark-colored silty beds, the basal contact is drawn between light-colored Cohansey sediments and the underlying dark-colored sediments. The Cohansey was markedly thinned because of erosion prior to deposition of overlying units in the western and southern parts of the southern sheet (Owens and Minard, 1975). The unit has been extensively eroded and stripped from large areas of the New Jersey Coastal Plain, particularly in the central sheet where outliers are common. In spite of its widespread nature, the Cohansey is poorly exposed because of its loose sandy composition, which causes it to erode easily (Newell and others, in press). Because of this same sandy nature, the Cohansey has been widely mined for sand, and manmade exposures are common in many areas. The age of the Cohansey is controversial because no calcareous microfauna or macrofauna have been found in this formation. The best indication of age comes from pollen and spores obtained from dark carbonaceous clay. Rachele (1976) analyzed the microflora from the Legler site and noted that the Cohansey had a rich and varied assemblage including several genera labeled "exotics" which no longer occur in the northeastern United States: Engelhardia, Pterocarya, Podocarpus, and Cyathea. Greller and Rachele (1984) estimated a middle Miocene age. Ager's (in Owens and others, 1988) analysis of the Cohansey from a corehole at Mays Landing also suggests a middle Miocene (Serravallian) age.

Belleplain Member of the Kirkwood Formation (middle Miocene, Serravallian) at surface, covers 11 % of this area

Clay to silty clay at the base and sand at the top. Clay, massive to laminated, gray-brown, locally contains abundant diatoms and scattered small shell fragments. Sand, fine- to medium-grained, pale-gray to white, somewhat micaceous and woody with scattered shell fragments. Most Belleplain sand is quartz with lesser amounts of feldspar and mica. Pyrite is common in clayey strata. The Belleplain subcrops beneath surficial deposits where the overlying Cohansey Formation was eroded away. Along the Atlantic Ocean, the member is exposed on the southern sheet between Beach Haven Terrace, Ocean County, and Brigantine, Atlantic County, where the unit is overlain by thin to thick deposits of alluvium. The member is as much as 15 m (49 ft) thick. The basal contact with the underlying Wildwood Member is sharp and unconformable with a thin bed of reworked coarse-grained quartz sand at the base. A middle Miocene age for the Belleplain was determined from diatoms. Specifics of the diatom biostratigraphy are discussed in the Description of Subsurface Units. Shells from this unit had a strontium-isotope age estimate of 13.2 Ma (Sugarman and others, 1993).

Wildwood Member of the Kirkwood Formation (middle and lower Miocene, Langhian and Burdigalian) at surface, covers 7 % of this area

Clay, silty, massive to finely bedded, dark-gray to olive-gray, locally interbedded with thin beds of light-colored sand. Contains small shell fragments primarily at base. Upper beds are more sandy than lower beds but also contain many thin to thick beds of clay. The sand is fine to medium grained, light gray, and commonly has dispersed wood fragments. Shell fragments are locally present in this facies. Basal beds are micaceous, locally diatomaceous, and contain wood fragments. Quartz is the major sand constituent with minor amounts of siliceous rock fragments and feldspar. The Wildwood subcrops beneath surficial deposits where the Belleplain Member and Cohansey Formation were stripped away. Along Delaware Bay, the Wildwood subcrops from near the Cohansey River to Fortescue, Cumberland County. Along the Atlantic Coast, the unit subcrops from Bay Shore Park to near Beach Haven Park, Ocean County. The maximum thickness of the unit is about 18 m (59 ft). The contact with the underlying Shiloh Marl Member is sharp and unconformable. Diatoms from the Wildwood Member are from the East Coast Diatom Zone (ECDZ) 2 of Andrews (1988) indicating a latest Burdigalian and Langhian age (late early and early middle Miocene).

Shiloh Marl Member of the Kirkwood Formation (lower Miocene, lower Burdigalian) at surface, covers 5 % of this area

Clay, massive, dark-gray with abundant large mollusks. Unit subcrops near the Delaware River where the overlying Wildwood Member and Cohansey Formation have been stripped away. The Shiloh Marl Member averages about 30 m (98 ft) thick. Diatoms recovered from the Shiloh from the ACGS-4 corehole 5.3 km (3.2 mi) northwest of Mays Landing, Atlantic County, contain the diagnostic diatom Actinoptychus heliopelta (Andrews, 1987) indicating an early Miocene age. Strontium-isotope analysis of shells yielded an age of 20 Ma (Burdigalian) (Sugarman and others, 1993).

Lower Member of the Kirkwood Formation (lower Miocene, Burdigalian and Aquitanian) at surface, covers 0.2 % of this area

Sand and clay. Upper sand facies: sand, typically fine- to medium-grained, massive to thick-bedded, locally crossbedded, light-yellow to white, locally very micaceous and extensively stained by iron oxides in near-surface beds. The thick-bedded strata commonly consist of interbedded fine-grained, micaceous sand and gravelly, coarse- to fine-grained sand. Some beds are intensely burrowed. Trough crossbedded strata with high concentrations of ilmenite and a few burrows are most commonly seen in the Lakewood quadrangle. Lower clay facies: clay and clay-silt, massive to thin-bedded, dark-gray, micaceous, contains wood fragments, flattened lignitized twigs, and other plant debris. Locally, the clay has irregularly shaped sand pockets, which may represent some type of burrow. In the least weathered beds, the sand of the upper sand facies is principally quartz and muscovite with lesser amounts of feldspar. The light-mineral fraction of the dark-colored clay has significantly more feldspar (10-15 percent) and rock fragments (10-15 percent) than the upper sand facies, where the feldspar was probably leached during weathering. The basal beds have a reworked zone 0.3 to 1.2 m (1-4 ft) thick that contains fine- to very coarse grained sand and, locally, gravel. These beds are very glauconitic and less commonly contain wood fragments. Reworked zones are present throughout the lower member. The lower member consists of a lower finegrained, clayey, dark-colored, micaceous sand (transgressive) and an upper massive or thick-bedded to crossbedded, light-colored sand (regressive). The lower, dark clayey unit was formerly called the Asbury Park Member. The clay-silt was previously called the Asbury Clay by Kmmel and Knapp (1904). The upper sand facies has been observed only in pits and roadcuts. It is poorly exposed because of its sandy nature. In the central sheet, the lower clay facies is exposed in pits north of Farmingdale, Monmouth County; in a few cuts along the Manasquan River, north of Farmingdale; and along the Shark River, northeast of Farmingdale. In the southern sheet, the lower clay facies is exposed only where the Coastal Plain was deeply entrenched and stripped away. In the southwesternmost part of the southern sheet, for example, the Cohansey Formation and much of the upper sand facies were stripped away by successive entrenchments of the Delaware River. On the central sheet, the lower member ranges in thickness from 20 to 30 m (66-98 ft) along strike, but thickens to over 60 m (197 ft) to the southeast. On the southern sheet, the unit ranges in thickness from 15 to 25 m (49-82 ft). The age of the lower member is based on the presence of the diatom Actinoptychus heliopelta, which was recovered from an exposure southwest of Farmingdale near Oak Glen, Monmouth County (Goldstein, 1974). This diatom places the lower member in the lower part of the ECDZ 1 of Andrews (1987), indicative of an early Miocene (Burdigalian) age (Andrews, 1988). Sugarman and others (1993) report strontium-isotope ages of 22.6 to 20.8 Ma, thereby extending the age of the unit to Aquitanian.