Paper No. 192-6
Presentation Time: 9:45 AM
SEDIMENTOLOGIC AND STABLE ISOTOPIC EVIDENCE FOR RAPID POST-IMPACT SEDIMENTATION IN THE CHICXULUB IMPACT CRATER
WHALEN, Michael T., Department of Geosciences, University of Alaska Fairbanks, Fairbanks, AK 99775, BRALOWER, Timothy, Department of Geosciences, Penn State University, University Park, PA 1682, GULICK, Sean S.P., Institute for Geophysics - Jackson School of Geosciences, University of Texas at Austin, 10100 Burnet Rd., Bldg 196, R2200, Austin, TX 78758-4445, MORGAN, Joanna V., Department of Earth Science and Engineering, Imperial College London, Prince Consort Road, London, SW7 2BP, United Kingdom, LOWERY, Christopher M., Institute for Geophysics, University of Texas, JJ Pickle Research Campus, Bldg 196, 10100 Burnet Rd, Austin, TX 78758, RODRÍGUEZ-TOVAR, Francisco, Departamento de Estratigrafía y Paleontología, Universidad de Granada, Granada, 18002, Spain and IODP EXPEDITION 364, Science Party Members, College Station, mtwhalen@alaska.edu
During spring 2016, International Ocean Discovery Program/International Continental Scientific Drilling Program Expedition 364 sampled facies recording the transition from high-energy, impact-related units to relatively normal marine deposits atop the Chicxulub impact crater peak ring at Site M0077. An 80 cm thick “transitional unit” records this change in a series of 0.5-3 cm thick, cyclic or episodic, light to dark brown, normally graded, silty calcareous laminated couplets with local basal scours. The upper transitional unit is interrupted by a 30 cm thick soft sediment fold overlain by 15 cm of laminae with trace fossils. Normally graded deposits from 4-60 cm thick also occur in the underlying, sand-sized suevite. Petrographic and stable C and N isotopic analyses from bulk organic matter provide insight into post-impact depositional processes and environment. We hypothesize that the normally graded packages record rapid deposition either from currents due to impact and slope failure-related tsunamis and ensuing seiches and/or from post-impact tidal or turbidity currents. The former processes would act on the order of hours to days while the latter might operate over weeks to years, post-impact.
δ13C values fluctuate around -25‰ throughout the transitional unit while δ15N values are mostly between 0 and 5‰ except for a remarkable series of excursions ranging from -5 to -18‰ in the upper portion of the unit. Rapid deposition is supported by the presence of the soft-sediment fold and repetition of the -18‰ δ15N excursion within the fold reinforces interpretations of soft sediment deformation, rapid deposition, and mass wasting.
Comparable negative δ15N excursions are not reported in terrestrial materials but relatively negative excursions were documented during atmospheric disturbances such as lightning or nuclear blasts where nitrogen is fixed, creating NOx. Prior modeling implies that 2% of NOx production is directly due to impact while 98% is related to atmospheric heating by ejecta reentry. The sedimentologic, ichnologic, and isotopic data thus appear to support very rapid deposition (hours to days post-impact) of the transitional unit up through the soft sediment fold. Rates of sedimentation above the soft sediment fold appear to be much lower than the underlying facies.
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