ICHNOLOGICAL CONSTRAINTS ON THE ORIGIN OF THE MAIN FOSSILIFEROUS LAYER, NEW JERSEY

The ichnology surrounding the Main Fossiliferous Layer (MFL) in the Hornerstown Formation (HF) provides evidence that the layer contains a thanatocoenosis and rules out a hiatus, whereas articulated specimens and lanthanide signatures argue against a transgressive-lag interpretation. Abundant burrow networks, dominated by Thalassinoides (Th), occur throughout the uppermost portion of the late-Cretaceous Navesink and New Egypt Formations as well as the glauconite-rich K(?)-Pg HF. Based on burrow morphology, fossilized chelae, and modern analogs, the tracemaker was likely a ghost shrimp, which dominated the upper-to-mid shelf endobenthic community. At the base of the MFL, ca. 18 cm above the lithologic contact, the Th diameter decreases abruptly from an average of 16.8 to 11.9 mm. The smaller size persists at least 1.2 m up section. Sizes of both populations are normally distributed and exhibit similar morphology, indicating that the trace is of the same type but smaller in size. We interpret the punctuated reduction in trace diameter to the “Lilliput effect”, dwarfing of the surviving endobenthic population in response to environmental stresses accompanying the K-Pg mass extinction. The abrupt nature of the dwarfing is likely due to adaptation of the rapidly reproducing crustaceans to these perturbations, coupled with extremely slow sedimentation rates of the glauconite-producing environment. Gradual up-section increases in glauconite maturity and ichnofabric index (from 2 to 5) indicate that the sedimentation rate gradually decreased into the Danian. This is not consistent with a hiatus and thus MFL/large burrow contact likely represents the paleo-seafloor at the time of impact. Thus Cretaceous vertebrate fossils occurring directly on the upper boundary of large burrows represent a thanatocoenosis. Since the ejecta layer has potentially been completely bioturbated, but large burrows immediately beneath the horizon have not been fully overprinted, the detritivore population reworked the surface contemporaneous with fallout and prior to being dwarfed. Such a sequence should leave the dispersed shocked minerals within large burrows below the MFL and we suggest sampling the burrow fill exceeding 18 mm in diameter for future shocked mineral searches.