Cohansey Formation

Cohansey Formation - Sand, fine- to coarse-grained, locally gravelly, massive to crossbedded, gray-brown or dark-gray; weathers yellow to white. Typically, the weathered sand is nearly all quartz or rock fragments of orthoquartzite. Where less weathered, small amounts (5-10 percent) of potassium feldspar are present. Interbedded with discrete beds of clay or silty clay, thin- to thick-bedded, massive to finely laminated, dark-gray; weathers white, yellow, or red. Darkgray beds commonly contain carbonized wood fragments, some of which are log size. The thicker clay beds occur in lenses that commonly have small to very large pieces of lignitized wood. An extensive, well-preserved leaf flora was collected from a very thick clay lens in the Cohansey near Millville, Cumberland County. The leaf flora were dominated by Alangium sp., a tree no longer growing in eastern North America (J.A. Wolfe, written commun., 1992). Locally, formation consists of several thin- to thickbedded, upward-coarsening sections (clay to sand). The depth of weathering ranges from 24 m (79 ft) in the ACGS-4 corehole near Mays Landing, Atlantic County (Owens and others, 1988), to 70 m (230 ft) in the Atlantic City corehole (F-F'). In the southern part of the southern sheet, in the Belleplain State Forest, Cape May County (G-G'), the formation consists of thin to thick beds of fine- to medium-grained, micaceous quartz (both colorless and green) sand and dark-gray to grayish-brown, woody clay. The sand is locally coarsely stratified (typically small-amplitude crossbeds) and locally highly bioturbated. The clay is extensively bioturbated. These beds represent the deepest marine beds found in the Cohansey in the New Jersey Coastal Plain. The basal contact with underlying units has considerable relief. The contact is sharp and commonly consists of a thin bed of fine gravelly sand. The original thickness of the Cohansey is difficult to ascertain because of extensive erosion. The formation lies in a broad channel and is thickest in the thalweg near Atlantic City where it is nearly 107 m (351 ft) thick. The base of the formation rises rapidly to the south and north of this channel axis. In downdip areas near Belleplain State Forest, the Cohansey contains marginal marine and shelfal facies. The shelfal facies is composed of interbedded, highly bioturbated, micaceous, slightly glauconitic quartz sand and massive clay. Most of the sand in the Cohansey is medium grained and moderately sorted although coarse and fine sandy beds also are common. Beds that have gravel as a major component are locally common in the mixed marine-nonmarine facies in the northeastern corner of the southern sheet. Here, the gravel occurs in well-defined channels. Most of the gravel is 2.5 cm (1 in) or less in diameter, although clasts up to 13 cm (5 in) in diameter have been locally observed. The gravel is mostly quartz or quartzite with lesser amounts of white and black chert. Previously, the age of the Cohansey was postulated from its stratigraphic position, its perceived contact relations with the underlying Kirkwood Formation (conformable or unconformable), and its macro- and microflora. The palynology of upper Tertiary formations in the northeastern United States is, however, only generally understood. Commonly, Pliocene beds have less exotic species than Oligocene or Miocene beds. If this is the case, then the Cohansey, which has a large number of exotics of some species, has more Miocene affinities than Pliocene, an age some have assigned to this formation. Ager (in Owens and others, 1988) discusses the microflora in the Cohansey near Mays Landing. He notes that the Cohansey has a large number of exotics similar to those in the underlying Wildwood Member of the Kirkwood, and because of this, thought the Cohansey to be Miocene. Pollen from the Cohansey at Belleplain also has a large variety of exotics in a warm temperate to subtropical pollen assemblage (Les Sirkin, Adelphi University, oral commun., 1991), which includes Clethra, Cyathea, Cyrilla, Engelhardia, Epilobium, Gordonia, Planera, Podocarpus, Pterocarya, and Symplocos. The major sources of tree pollen at Belleplain are pine, oak, and hickory. The contained dinocyst flora from marine beds at Belleplain can be correlated with the known dinocyst assemblages from the Choptank and the lower part of the St. Marys Formation of the Chesapeake Bay region and therefore is middle Miocene in age (Laurent de Verteuil, University of Toronto, written commun., 1991). These dinoflagellate data therefore confirm the Miocene rather than Pliocene age for the Cohansey. The strontium-isotope age from shells at the base of the Cohansey in an offshore well (ACOW-1) was approximately 11 Ma or latest middle Miocene or late Serravallian