Geological History of the Northern Atlantic Coastal Plain
The upper Cretaceous and lower Tertiary of the New Jersey Coastal plain is a fascinating and widely studied region. In the late Cretaceous (93-65 mya), many continents were covered by shallow seaways, including North America. Continental flooding was prevalent, and the continents were starting to move into positions reminiscent of modern geography. By the end of the Cretaceous period, North America made contact with South America, and the seas were largely withdrawn from the continents.
New Jersey is part of the Atlantic Coastal Plain. In the northern sector of New Jersey, the Cretaceous-Tertiary boundary strikes out to sea, while in the southern sector the Upper Cretaceous and Lower Tertiary layers finger out and are overlapped and covered by younger strata. The K/T boundary of New Jersey represents a classic column, which is often used in comparison with surrounding areas such as the Atlantic and Gulf Coastal Plains. The entire complex may be migrating inland in response to a Holocene eustatic sea level rise. In comparison with more dynamic regions of the world, this area was not heavily affected by tectonics in the Mesozoic. The change in sea level had a greater impact on this passive region than tectonics.
Being at more southern latitudes, the temperatures were warmer. In addition, the Atlantic Ocean was smaller, both leading to lower mixing rates of water masses, and to widespread anoxia. A warm Tethyan current controlled the ocean circulation patterns. Also, plate tectonics could be responsible for the fluctuations in sea level. The rate at which seafloor spreading occurred would affect how much water was displaced.
The stratigraphic sequence of this area, from the most basal formation to the most recent one, is Navesink Formation (oldest), the Hornerstown Formation [K-T Boundary/ MFL], the Vincentown Formation, the Kirkwood Formation, and the Pennsauken Gravel layer (youngest).
The condensed K-T boundary section in New Jersey is characterized by a succession of claystone, sandstone, and glauconitic units. Glauconite is a type of clay, formed in low oxygen and marine-like conditions. A relatively thin layer, these sediments were laid down in the sediment-starved environments along a passive continental margin. During the time period, transgressive and regressive cycles of glauconite, clays and sand result from changes in sea level. Even lower in the strata (before the K-T: the Campanian stage) there is evidence of storms and hurricanes, which produced mixed scrap assemblages. Most interestingly, there were a lot of biological changes displaying some type of selective extinction among marine fauna.
A pattern of selective extinction and survival across the Cretaceous/Tertiary boundary in this area is revealed by especially fossiliferous deposits from the Campian, Maastrichtian, Danian, and Thanetian stages that yield a succession of marine faunas. The Navesink deposits yield marine fauna that consist of benthic invertebrates such as oysters (shell bed at base of formation) and other semi-infaunal and infaunal mollusks. These organisms exhibit planktotrophic larval stages. Ammonites thrive in this layer, as do nautilids and bivalves.
The most productive fossiliferous area is the Main Fossiliferous Layer (MFL), which is between the Hornerstown Formation (above) and the Navesink Formation (below). The MFL appears to be a condensed section shell bed that preserves the remains of the last Cretaceous fauna in the region. Ammonites, turtles, crocodiles, mosasaurs, and shark teeth are abundant. The MFL has the last of the planktotrophic organisms. Non-planktotrophic marine organisms such as brachiopods and other epifaunal benthos became dominant in the Tertiary. Corals, sponges, some bivalves, and dwarfed fauna were abundant. The ammonites of the Cretaceous became extinct, and the nautilids prevailed with their alternative non-planktotrophic reproductive strategies. A crash in the plankton population is attributed to possible changes in the marine environment including sea level fluctuations, climate changes, tectonic activity, and bolide impacts (extraterrestrial activity).
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