Lower Member of the Kirkwood Formation

Lower Member - Complex unit in which facies changes are common. In outcrop, some of the lower member is a light-colored quartz sand (Grenloch facies of Isphording, 1970). The facies pinches out rapidly in the subsurface, and the unit is primarily a massive to finely laminated, dark-gray clay (Alloway clay of Kummel and Knapp, 1904). This clay facies occurs as far south as Clayton, Gloucester County, where the lower part changes to a fossiliferous clayey silt. The lower clayey facies thickens to over 30 m (98 ft) in the coastal region. Here, the lower facies is mostly dark-gray clayey silt that is locally very fossiliferous. The unit varies in thickness but is thickest, as much as 80 m (262 ft), in the southeastern part of the central sheet. South of Bridgeton, Cumberland County, sand is present at the top of the member. This sand thickens to over 23 m (75 ft) at the coast where it is part of the "800 foot" sand, the principal aquifer in the coastal region. Quartz and siliceous rock fragments are the major sand minerals. Feldspars typically constitute less than 10 percent of the sand fraction except in the reworked beds where they make up as much as 25 percent of the sand. Mica and wood fragments are minor constituents. The basal contact with the underlying unit is sharp and unconformable. Commonly there is a 0.3- to 1-m (1- to 3-ft)-thick zone along this contact in which coarse-grained quartz sand and some gravel are present. Because of the quartz sand in the lower member in outcrop, some considered the lower member and the Cohansey Formation to be transitional (Isphording, 1970). Obviously this is not the case as there are three other members (Shiloh Marl, Wildwood, and Belleplain) lying between the lower member and the Cohansey. The age of the lower member was determined from planktic foraminifera and diatoms, and from strontium-isotope age estimates. The foraminiferal assemblage includes Globigerina praebulloides, Globigerinoides trilobus, G. altiapertura, and Globorotalia siakensis. These species are characteristic of the lower Miocene Zone 5 of Blow (1969) of Burdigalian age. The diatom assemblage is characterized by Actinoptychus heliopelta, Aulacodiscus rogersii, Coscinodiscus lewisianus, Sceptroneis caduceus, and Triceratium acutum. These diatoms are characteristic of ECDZ 1 of Andrews (1988), which is considered to be early Miocene (Burdigalian) in age. The strontium-isotope analyses of shells from this member indicate ages from 23 to 20.2 Ma (Aquitanian), thereby extending the age of this member (Sugarman and others, 1993). The pollen in the clay facies of the Kirkwood, as determined from the ACGS-4 drillhole, are oak, hickory, and pine with smaller amounts of beech, black gum, sweet gum, alder, elm, linden, and birch. The sandy facies has essentially the same assemblage but has unusually high concentrations of beech. Exotic species such as Podocarpus and Momipites are common in the unit. The cool floral elements found in the overlying Belleplain and Wildwood Members were not found in the lower member.