DEATH AND RECOVERY AT THE K/T BOUNDARY: EVIDENCE FROM NEW SITES IN NEW JERSEY

The Cretaceous/Tertiary boundary (=K/Pg) was one of the most devastating extinctions in Earth’s history. This has been attributed to an asteroid impact. However, the detailed sequence of events immediately following the impact is still under debate. New sections discovered in the Manasquan River Basin in Monmouth County, New Jersey, expose the K/T boundary interval. They include the uppermost part of the Tinton Formation represented by a richly fossiliferous unit referred to as the Pinna Layer. The Pinna Layer is overlain by the Hornerstown Formation. In downdip sites, an additional unit, referred to as the Burrowed Unit, occurs between the Pinna Layer and the Hornerstown Formation. Biostratigraphically, the Pinna Layer and the Burrowed Unit are late Maastrichtian in age. However, an Ir anomaly (~700 ppt) occurs at the base of the Pinna Layer, suggesting an early Danian age instead. The Burrowed Unit consists of light green, muddy, glauconitic sand, with quartz grains. It is heavily burrowed by dark green, glauconitic sand piped down from the overlying Hornerstown Formation. The burrows contain a total of 19 species: 2 ammonites, 2 gastropods, 9 bivalves, 2 annelids, 1 crustacean, 1 echinoid, 1 foraminiferan, and 1 bryozoan. The assemblage is dominated by a small oyster Ostrea pulaskensis. Some fossils, notably the ammonites, occur in hard siderite nodules, indicating that they have been reworked from the Tinton Formation. The matrix of the Burrowed Unit contains a total of 15 species: 2 ammonites, 2 gastropods, 8 bivalves, 1 sponge, 1 coral, and 1 echinoid. Four specimens of ammonite jaws are present, which are attributed to Eubaculites and Discoscaphites, based on their shape. The presence of these jaws argues against reworking and exhumation as a source of the fossils within the matrix of the Burrowed Unit. In contrast to the Pinna Layer (74 macroinvertebrate species and 37 dinoflagellate species), the fossil assemblage in the Burrowed Unit is much less diverse (15 macroinvertebrate species and 2 dinoflagellate species). If the horizon at which the Ir anomaly occurs marks the K/T boundary, this fauna, including the ammonites, may represent the last gasp of the Cretaceous ecosystem. The fauna in the overlying part of the Hornerstown Formation (= the burrows) may represent the recovery community following the K/T extinction.