ICHNOLOGY OF K/T SECTIONS OF CENTRAL NEW JERSEY: BURROW GEOMETRY AND MAGNETIC PROPERTIES

Recent investigation of Cretaceous-Tertiary exposures of central New Jersey focused on documenting the dominant ichnofossils within key lithostratigraphic units and on assessing the potential use of low-field magnetic susceptibility (MS) as a means of differentiating the bulk magnetic properties of the burrow fill and surrounding matrix. At a source-proximal Buck Pit locality, exposures of unconsolidated quartz-rich sands of the late Maastrichtian Red Bank Formation contain a typical foreshore-nearshore ichnoassemblage represented by clay-filled Skolithos and Psilonichnus burrows with oxidized rims. In situ MS measurements of the diamagnetic quartz-rich matrix yield low values (<50 μSI), with burrow fill and beige clay horizons attaining higher values of 80-130 μSI. MS increases up-section through this transgressive tract, exceeding 500 μSI in the glauconitic Hornerstown Formation. At four down-dip localities, the “burrowed layer” within Navesink and Hornerstown Formations is dominated by actively and passively filled Thalassinoides burrow networks, with bioturbation exceeding 90%. The burrows exhibit linear, elliptical, and triple-junction cross-sections in vertical and bedding-plane exposures and likely reflect a regional intensification of activity by crustacean-dominated detritivores. Such cross-facies ichnofossils are characteristic of endobenthic ecosystem responses to the end-Cretaceous mass extinction and closely resemble their counterparts in western Europe. The glauconitic burrow fill MS values exceed 600 μSI, reaching more than 900 μSI for traces within the Main Fossiliferous Layer (MFL). A dense MS grid (5 cm spacing) on a bedding surface yielded median values of 640-670 μSI, with elongated anomalies generally corresponding to large burrows. An anomalous section with low MS values captured a bioturbated beige clay clast (220-290 μSI) immediately underlying the surveyed surface. Within the MS grid, an enigmatic sub-horizontal, tapering, clay-lined structure (length>1 m; width: 8-10 cm) cross-cuts the Thalassinoides framework. Similar adjacent and sub-parallel features may represent tool marks or passively filled drag traces produced by large vertebrates (chelonians, crocodilians, sharks), which are known from numerous body fossils within the MFL.