VERTICAL DISTRIBUTION OF FOSSILS IN THE LOWER HORNERSTOWN AND UPPER NAVESINK FORMATIONS AT EDELMAN FOSSIL PARK, NJ

The Navesink and Hornerstown formations record Cretaceous-Paleocene shallow marine depositional environments. The latter contains a well known, diverse, and regionally extensive fossil horizon known as the Main Fossiliferous Layer (MFL). The stratigraphic thickness of the MFL has been variously reported as 10cm to 40cm thick at what is now the Jean and Ric Edelman Fossil Park, formerly the Inversand Company quarry, the best exposure of this bonebed. However, the precise vertical position of fossils in relation to the Navesink-Hornerstown contact was not well constrained in previous reports. We mapped the precise spatial relationship of every fossil within a one square meter area, including the vertical position above or below the Navesink-Hornerstown contact. Data were collected for the upper six centimeters of the Navesink Formation and the lower 55cm of the Hornerstown Formation. In this span, roughly 1,000 fossils were mapped in three dimensions. When graphed by vertical position in one centimeter bins, there are two stratigraphic regions exhibiting high fossil abundance. Each region is approximately 10cm thick, and there is a roughly five-centimeter gap of reduced fossil abundance between them. Within these two stratigraphic zones of high fossil abundance, faunal composition differs such that the upper assemblage yields a Species Diversity Index (at the genus level) roughly twice that of the lower assemblage. The lower assemblage is dominated (80-90%) by the oyster Pycnodonte dissimilaris, largely accounting for its reduced diversity. These patterns match empirical observations from a separate, systematic excavation of over 180 square meters of these beds at Edelman Fossil Park.

Previously, some reports lumped together both fossil layers as components of the MFL, while other reports excluded the lower oyster bed. Our high-resolution data indicate that the upper fossil layer comprises a distinct unit, representing a distinct event, which can be recognized by high diversity and greater abundance of large fossil bones and wood. It represents what previous authors have described as the MFL, with the top of this assemblage approximately 30-35 cm above the formational contact. The lower, oyster-rich layer represents a stratigraphically, faunally, and ecologically separate fossil assemblage.