ORIGIN AND AGE OF FLUVIAL SEDIMENTS, CHARLESTON, STATEN ISLAND

The southern part of Staten Island comprises sedimentary units from the Atlantic Coastal Plain that are Cretaceous in age, overlain by more recent surface deposits of outwash and till. This research focuses on a small outcrop of cross-bedded sands and massive gravels that is present in a small road cutting in Charleston in the southwest of Staten Island. In previous studies, the origin and age of this unit has been disputed (Pensauken Formation or Quaternary glacial outwash). A detailed analysis of the sedimentary structures, grain size distributions and the nature of the clasts suggests that this outcrop was deposited on an outwash plain during the Wisconsinian deglaciation.

There is some lateral variation within the outcrop, so two sections were logged and samples collected for analysis. The sediments consist of interbedded gravels and sands, with one partially lithified silt/clay layer. The gravels are poorly to very poorly sorted and show a bimodal grain size distribution with coarse pebbles and medium sand predominating. These layers are 20 – 80 cm thick and most likely represent crevasse splay deposits. The sand layers comprise moderately to poorly sorted medium sands, some of which display cross bedding containing fine gravels. They are not all laterally continuous, and represent migrating stream channel deposits. The silt/clay layer has been disrupted, probably by a crevasse splay, and is overlain by a gravel that includes rip up clasts of this mud.

Although there was no pollen or other organic matter found in the sediments collected, the age can be deduced from the composition of the clasts in the gravels. They include numerous red sandstones and siltstones, which are not normally associated with the Pensauken Formation, but are associated with similar Quaternary outwash deposits in neighboring parts of New Jersey. It is assumed that these sediments were deposited after the initial retreat of the ice sheet due to their location north of the terminal moraine. They were most likely deposited on an outwash plain as the ice continued to retreat, close to the shores of glacial Lake Bayonne.

Support for this project was provided by a PSC-CUNY Award, jointly funded by The Professional Staff Congress and The City University of New York.