Belleplain Member of the Kirkwood Formation

Belleplain Member - New member named for a corehole at Belleplain State Forest headquarters (Belleplain II). A thin gravel bed, containing gravel up to 1 cm (0.4 in) in diameter, is present along the contact with the underlying Wildwood Member. The gravel is mainly quartz with small amounts of phosphatized vertebrate remains and sharks teeth. The lower 10.5 m (34 ft) is massive to horizontally laminated, very diatomaceous, dark-gray clay or silty clay with common, small, thin-walled mollusks. This basal clayey unit is overlain by as much as 23 m (75 ft) of mostly sand. The lower 1.2 m (4 ft) of fine- to medium-grained, dark-gray, woody sand is interbedded with clay. These grade up into a fine- to medium-grained, massive, rarely crossbedded, medium- to dark-gray, micaceous, bioturbated, quartz sand. The sand in the upper 10.5 m (34 ft) of this interval becomes coarser grained and is extensively stained gray brown by humates. Sand in the Belleplain is mostly quartz with a minor amount of siliceous rock fragments. Potassic feldspar is a common constituent but typically is less than 10 percent of the sand fraction. The upper 10 m (33 ft) is finely laminated, dark-gray clay with common, thin interbeds of fine- to medium-grained, micaceous quartz sand. Flaser bedding is common in this upper clayey unit. Gamma-ray values are high for the clayey unit at the base (transgressive deposits) and low for the sandy unit above (regressive deposits). This high-low couplet is a distinctive gamma-ray pattern that is typical of most marine units in the New Jersey Coastal Plain (unconformity-bounded sequences that represent an asymmetric transgressive to regressive cycle of sedimentation). The Belleplain is restricted to the southern bedrock sheet and generally occurs in the subsurface except where younger Pleistocene units have deeply entrenched through the overlying Cohansey Formation and exposed it. The Belleplain is greater than 100 m (338 ft) thick along the coast from Strathmere, Cape May County, to Cape May, Cape May County. The age of the Belleplain was determined by using a combination of different fossil types. Andrews (1988) considers the diatom assemblage of Actinoptychus marylandicus, Coscinodiscus lewisianus, Delphineis angustata, D. novaecaesaraea, D. penelliptica, Rhaphoneis clavata, R. gemmifera, and R. scutula to be characteristic of East Coast Diatom Zone (ECDZ) 6 or Bed 15 (equivalent to the uppermost part of the Calvert Formation of the Chesapeake Bay region). Silicoflagellates recovered from the Belleplain include Corbisema triacantha, Distephanus crux crux, and D. stauracanthus. The co-existence of the diatom Coscinodiscus lewisianus with the silicoflagellate Distephanus stauracanthus indicates an age of 13.2 to 12.3 Ma (David Bukry, written commun., 1990). Strontium-isotope ages of the shells range from 14.7 to 12.3 Ma and confirm the paleontologic middle Miocene age. Pollen assemblages from the base of this formation in the Belleplain I core contain spruce, pine, oak, hickory, and poplar (all abundant) with black gum, sweet gum, maple, birch, and Myrica (all sparse). Exotics include Clethra, Cyrilla, Engelhardia/Momipites, Planera, Podocarpus, and Symplocos. This assemblage is a mixture of cooltemperate forms (spruce) and warm-temperate forms (oak, hickory, and exotics; a lowland assemblage). The pollen assemblage in the upper part of the formation lacks the cool-temperate elements and is, overall, a warm-temperate microflora, thus indicating a general warming of the climate during the time of deposition