Chemical Composition of Thalassinoides Boxwork Across the Marine K–PG Boundary of Central New Jersey, U.S.A.

Abstract

Thalassinoides isp. burrow networks filled with green to dark bluish-gray glauconite pellets occur in the Cretaceous to Paleogene Hornerstown Formation and in the uppermost 1–2 m of the underlying Upper Cretaceous Navesink and New Egypt Formations at many central New Jersey Cretaceous–Paleogene (K/Pg) boundary sections. Near the base of the Hornerstown Formation is a renowned bonebed, the Main Fossiliferous Layer (MFL), which contains isolated to articulated Cretaceous marine reptiles, turtles, birds, and crocodiles interpreted as a thanatocoenosis. We investigated the bulk chemical composition of the burrow fill and surrounding matrix in the largely unconsolidated sections of the K–Pg interval at two sites, Inversand and Walnridge (Meirs) Farm, New Jersey, using a Niton XL3t XRF analyzer to place them in proper stratigraphic context. Bulk matrix Zr and Ti concentrations decrease up section as a result of transgression, deepening, and increased distance from source areas. In contrast, an increase in K is attributed to greater amounts and maturity of the glauconite, due to decreasing sedimentation rates. Zr and Ti concentrations in burrow fills below the MFL are virtually the same as in overlying sediments above the MFL and much less than in the surrounding matrix. This suggests that burrows in the Cretaceous strata were passively filled with Paleogene sediments. K concentrations of burrow fills are generally intermediate between the high values encountered in the Paleogene matrix and the lower values of the Cretaceous matrix. Although this may have resulted from removal of glauconite during bioturbation and burrow filling, it is more likely that immature glauconite was sequestered in the burrow fills below the depth of glauconitization before it could attain full maturity. Therefore, burrows originating above the formational contact at or above the MFL penetrate as much as two meters into underlying Cretaceous sediments, and are passively filled with Paleogene sediment. Burrows originating above the MFL may translocate diagnostic Danian microfossils, shocked quartz, and Ir, effectively diffusing the K–Pg boundary. This research demonstrates that care must be taken in sampling pervasively bioturbated strata to avoid incorporating stratigraphically younger burrow-fill sediments.