T116 Impact Cratering: From the Lab to the Field; from the Earth to the Planets (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

NI AND CO IN PYRITE FRAMBOIDS FROM AGONY CREEK SECTION, K/PG BOUNDARY IN NEW JERSEY COASTAL PLAIN

Paper number: T116-3
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Pyrite framboids in Cretaceous/Paleogene (K/Pg boundary) muds from Agony Creek in Monmouth, Co. NJ contain elevated Ni and Co trace metal concentrations in and above, but not below, the same 6 mm layer containing a sharp Ir anomaly (700 ppt). K/Pg boundary deposits from Agony Creek (Manasquan River Basin) record offshore sedimentation in 20-30 m deep water, as a poorly consolidated, glauconite-rich fossiliferous facies, reflecting post-depositional in situ diagenetic alteration of parent minerals and/or bio-chemical reduction. At Agony Creek, the Tinton Formation, including the fossiliferous “Pinna layer” at its top, is unconformably overlain by the Hornerstown Formation, which contains very sparse invertebrate fauna. A sharp Ir anomaly at the base of the Pinna layer occurs ~20 cm below the highest appearance of Upper Cretaceous ammonites and benthic fauna buried in life positions (Landman et al. 2007, AMNH Bull. 303). The chemical and biostratigraphic locations of the K/Pg boundary are therefore inconsistent in this section. At the down-dip Crosswicks Creek section (~50 m paleodepth, ammonite-free) framboid Ni and Co spike at the lithostratigraphic boundary between the New Egypt Formation and the Hornerstown Formation (Ebel et al., AGU 2010). Siderophile enrichment of secondary, low-temperature pyrite is consistent with its formation in situ from primary Ni-rich spinels co-deposited with Ir, all from the Chicxulub impact plume.

An oriented, dewatered, epoxied block ~15 cm wide, sampling ~10 cm of vertical outcrop (~3 cm deep) containing the Ir-rich layer, was sliced into 10x3cmx5mm thick slabs. Two slabs were divided into 2.5 cm round thick sections. Framboids were identified in reflected light, analyzed by microprobe, and then by LA-ICPMS for Fe, Co, Ni, Ir, and Si. In, and up to 8 mm above the Ir-rich layer, Ni reaches 930 ppm and Co reaches 420 ppm, much lower than in the lithostratigraphic boundary at Crosswicks Creek. We will report measurements farther up and down section, and also from the top of the Pinna layer.