DANIAN? AMMONITE JAWS AT THE BASE OF THE HORNERSTOWN FORMATION, MONMOUTH COUNTY, NEW JERSEY

Recent investigations have revealed the presence of the K/P boundary in the Manasquan River Basin in Monmouth County, New Jersey. The section includes the topmost Tinton Formation represented by a richly fossiliferous unit that Landman et. al (2007) referred to as the Pinna Layer. The Pinna Layer is overlain by the Hornerstown Formation. In down dip sites, an additional unit occurs between the Pinna Layer and the rest of the Hornerstown Formation referred to as the Burrowed Unit. Although biostratigraphically the Pinna Layer and Burrowed Unit are Maastrichtian in age, an Ir anomaly (~700ppt) occurs at the base of the Pinna Layer suggesting a lowermost Danian age. The current investigation focuses on the fossil content of the Burrowed Unit. The Burrowed Unit consists of a light green, muddy, glauconitic sand with sparse large quartz grains. It is heavily burrowed by dark green, glauconitic sand piped down from the overlying Hornerstown Formation. The matrix is relatively unfossiliferous both in macro and micro fossils compared to the Pinna Layer below. The fauna includes 4 species of bivalves (Pecten venustus (Morton 1833) is the most common), 4 gastropods, 3 ammonites, 1 arthropod, and several shark teeth. Of particular interest are 4 specimens of ammonite aptychi which includes the first recorded baculite jaws on the Atlantic Coastal Plain. Within the burrows, the oyster Ostrea pulaskensis (Harris 1894) dominates comprising close to 90% of the fauna. Also common in the burrows are reworked Cretaceous forms preserved as siderite nodules most likely representing a hiatus within the base of the Hornerstown Formation. The preservation of aptychi limits the possibility of rigorous reworking and exhumation as a source of the fossils within the matrix material of the Burrowed Unit. This suggests that the animals lived, died and were buried during the deposition of the Burrowed Unit. If the Ir anomaly is in place, the depauperate fauna of the Burrowed Unit could represent a unique window recording the last gasp of the Cretaceous ecosystem, including ammonites.