ABSTRACT

The late Tertiary Kirkwood Formation is exposed over an area of nearly 600 square miles in the Coastal Plain Province of New Jersey and is made up of sands, silts and clays deposited in marine and transitional marine environments.

Detailed field investigation and sediment analysis revealed that the Kirkwood Formation can be subdivided into three members: (1) The Alloway Clay Member, exposed in the southern part of the State. (2) The Asbury Park Member, found in the northeastern portion of the Coastal Plain, in Monmouth County and (3) the Grenloch Sand Member, which grades laterally into both of the other members and also overlies them and is best exposed in the central and northern portion of the Coastal Plain.

The Alloway Clay Member consists of marine clays, with some interbedded sands and sandy silts that were deposited, chiefly in the middle neritic (sub-littoral) zone. Fossil evidence indicated that deposition may have been initiated as early as the late Oligocene and continued, uninterrupted, through the lower middle Miocene. Diagenetic changes resulting from the upward leaching of groundwaters have caused part of the Alloway Clay to become greatly enriched in Kaolinite and have also resulted in the formation of a bed of exceptionally large kaolinite grains, some of which exceed 0.2 mm in size. In addition, a highly fossiliferous portion of the Alloway Clay, known locally as the "Shiloh Marl," has been transformed into an opal-cemented orthoquartizite by precipitation of dissolved sili a in groundwaters.

The Asbury Park Member is the smallest of the three members that make up the Kirkwood Formation and is, for the most part, non-marine in origin. The finely laminated, organic-rich, silty sands are thought to be the product of deposition in swamps, lagoons, and estuaries that were open to the sea and reflect this origin both in texture and mineralogy.

The pale yellow, orange, and pink silts and fine sands of the Grenloch Sand Member are found cropping out from Delaware Bay to Raritan Bay and form the largest member of the Kirkwood Formation. Sediments in the lower portion of this unit were deposited in the middle-neritic environment during the final transgressions of the late Tertiary seas and grade upward into coarser grained sediments. These coarser sediments were deposited in shallow marine and transitional marine environments during the late middle Miocene when the seas began to withdraw from the present Coastal Plain. Deposition and withdrawal of the sea is thought to have continued throughout the upper Miocene with the result that an essentially conformable relationship exists between the Kirkwood Formation and overlying Coha sey Sand.

Heavy minerals were not found to be useful for stratigraphic correlation within the formation but the extremely limited suite coupled with the light minerals and clay minerals present furnished strong evidence for humid, sub-tropical climatic conditions in northeastern United States during the middle Miocene.