SEISMIC, WELL-LOG AND CORE DATA CHARACTERIZATION OF THE K/PG BOUNDARY, CRETACEOUS CHALK/PALEOGENE SHALE, IN CENTRAL LOUISIANA

The Chicxulub Impact had a major effect on the shelf sediments of the northern Gulf of Mexico. Sanford et al. (2016) document that the impact initiated earthquake caused failure of the shelf resulting in downslope deposition of about 150 ft of mass transport deposits in central Louisiana. Egedahl et al. (2012) and Strong and Kinsland (2014) report and describe 40 – 50 ft (12 – 15 m) high, ¼ - ½ mile (.4 - .8 km) wavelength ripples on the K/Pg surface from the tsunami(s) generated by the impact. Not recognizing the undulatory nature of the K/Pg surface resulted in Justiss Oil Company predicting a core point incorrectly and led to them continuously coring from within the overlying Midway Shale, across the K/Pg and about 120 ft (37 m) into the chalk of the Cretaceous. This “mistake”, and Justiss Oil’s donation of access to this core gives us unprecedented access to very well preserved K/Pg boundary stratigraphy. We are intensively analyzing this core and another short (15 ft) (5 m) section of another core, both cores are from LaSalle Ph. Louisiana. We have described, photographed and run XRF on the core already. We are in the process of cutting thin-sections for analysis and imaging with scanning electron microscopy on both “raw” samples and insoluble residue samples. We will present available results.

The well-log contains two sharp gamma ray peaks each of which correlates with a pebble layer, carbonate pebbles with carbonate rinds, within the core. The upper pebble layer is immediately overlain by about 650 ft (200 m) of Paleogene Midway Shale. The other is within the chalk. In preliminary investigation of the chalk (“raw” and insoluble) we have seen spherules [diameter about 100 microns (.004 in)], resembling the glass spherules often interpreted to be transported Chicxulub Impact debris. Our preliminary interpretations are that the spherules and the pebbles are Chicxulub ejecta material.

We recognize that the Chicxulub Impact affected the stratigraphy in the area with three mechanisms: 1) the earthquake which resulted in the mass transport, 2) the tsunami(s) which formed mega-ripples in/on the chalk and 3) direct ballistic and fallback delivery of Chicxulub target rock and impactor material which altered the composition of the chalk. Our detailed studies should allow us to sort out the timings and interactions of these three.