SHRIMP-TRACE ELEMENTS: GEOCHEMISTRY OF "THALASSINOIDES"- DOMINATED MARINE K/PG SECTIONS OF NEW JERSEY

We investigated the chemical composition of Thalassinoides isp. burrow fill and surrounding matrix in the unconsolidated Upper Cretaceous Navesink (NVF), New Egypt (NEF), and overlying Cretaceous-Paleogene (K/Pg) Hornerstown Formation (HF) at the Inversand and Meirs Farm sites, New Jersey, using a Niton XL3t XRF analyzer to determine the stratigraphic origin of burrow fill. The laterally equivalent NVF/NEF and overlying HF are glauconite-rich marls deposited during transgression at the end of the Cretaceous and beginning of Paleogene. Near the base of the HF is a renowned bonebed, the Main Fossiliferous Layer (MFL), that contains isolated to articulated Cretaceous marine reptiles, turtles, birds, and crocodiles that has been interpreted as a thanatocoenosis. These formations are heavily bioturbated (ii = 2-5); and well-defined Thalassinoides burrows filled with green glauconite pellets penetrate downward into the chocolate NVF/NEF marl. Above the MFL, these burrows, though more abundant, are ~30% smaller in diameter than those contained within the underlying strata, which is attributed to reduction in crustacean body size resulting from environmental stress associated with the Chicxulub impact. Elemental trends (K, Ti, Zr, Al, and Fe) in the matrix are consistent with marine transgression and increasing glauconite maturity. Concentrations of diagnostic heavy-mineral indicators in the clay-rich NVF matrix (3,000 < Ti < 4,500 ppm; 40 < Zr < 80 ppm) differ from the enclosed burrow fill (1,000 < Ti < 2,000 ppm; 12 < Zr < 30 ppm) and heavily bioturbated HF, suggesting minimal mixing between burrow fill and matrix. Decreases in K suggest that immature glauconite filled burrows below the zone of glauconite formation. Up-section variations in Fe content are generally consistent with bulk magnetic susceptibility trends, with Cr, Mn, and other trace elements indicative of increasing anoxia, particularly at Inversand. The composition of burrow fill below the MFL is different from the surrounding NVF/NEF matrix and similar to the HF sediment above and including the MFL. Therefore burrows originate above the formational contact, penetrate nearly 2 m, and are passively filled with Danian sediments. Diagnostic Danian microfossils, shocked quartz, and Ir may thus be translocated downward, diffusing the K/Pg boundary.