DISTRIBUTION OF SHOCKED QUARTZ AND CHANGES IN GLAUCONITE MATURITY IN THE K-T SECTION AT SEWELL, NJ: IMPLICATIONS FOR GENESIS OF THE MAIN FOSSILIFEROUS LAYER

The position of the K-T boundary in southern and central New Jersey has been the subject of extensive research. Some workers have proposed that it is at the contact between the Navesink and overlying Hornerstown Formations (NHC); others have placed the K-T boundary within the Hornerstown Formation, specifically in or above the Main Fossiliferous Layer (MFL), a concentration of disarticulated to partially articulated bones of Mosasaurs and other latest Cretaceous fauna which occurs from ca. 10 to 40 cm above the NHC. We have recovered shocked quartz grains in samples between the NHC and the top of the MFL at the Inversand Quarry in Sewell, NJ. The original distribution of shocked quartz has probably been dispersed by the extensive bioturbation in this part of the section. Shocked quartz and spherules at the K-T boundary in this region have previously been found in a core from Bass River, NJ (Olsson et al., 1997). In an attempt to clarify the genesis of the MFL and to determine the relationship of the NHC and MFL to the K-T boundary, we measured glauconite maturity stages across the NHC. Increased glauconite maturity results from more prolonged chemical interaction along the sediment-water interface as a result of slower sedimentation. Glauconite maturity, as evidenced by higher concentrations of K and octahedral Fe and by more complex and bulbous morphology with healed cracks, gradually increases up section across the NHC and MFL, suggesting progressively slower sedimentation rates, without any indication of an abrupt pulse of increased maturity as would be expected if the MFL was the result of a hiatus or time-averaged attritional accumulation. This agrees with previous REE analyses of fossil bones below, within, and above the MFL, which shows that the bones of the MFL are not reworked, that they are not the result of sediment winnowing and concentration, and that they represent a taphonomic assemblage distinct from either the Navesink or Hornerstown Formations (Staron et al., 2001). The presence of shocked quartz in this interval suggests that the MFL may be a taphonomic manifestation of the K-T impact.