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Effects of the Chicxulub Impact Found in the Subsurface of Northern Louisiana

Abstract

Approximately 65 Mya the impact of a large bolide (~6 km in diameter) into the water of the Gulf of Mexico’s northern shelf on the Yucatan Peninsula, the Chicxulub Impact, had well-known far-ranging/worldwide effects: 1. an earthquake, 2. tsunami’s, 3. direct ejecta, 4. atmospheric debris fall-back and 5. climatic effects from atmospheric modification. Most of northern Louisiana, within about 1200 km of the impact site, was a marine shelf of the Gulf of Mexico with some lowlands near the northern border at the time of the impact. Tsunami “mega ripples” about 17 m high with about 1 km wavelength have been recognized and analyzed in 3D and 2D seismic data from northern central Louisiana. Several authors have demonstrated that the shelf around the Gulf of Mexico collapsed as a result of the earthquake and have mapped the resulting mass-transport deposits. A conventional core from about 1500 m depth (Justiss Oil Company, LA Central IPNH No. 2) from LaSalle Parish in Louisiana has been obtained and analyzed. This core contains about 10 m of Paleogene Midway Shale, about 13 m of the mass-transport deposit and about 17 m of pre-impact Cretaceous marl. Evidence from surrounding wells indicates this core was retrieved from deep within the trough of the tsunami rippled surface⋯thus it contains only about 13 m of mass-transport material while the thickness of the deposit under a ripple peak is about 30 m. Analyses include: 1) “inch by inch” visual inspection, 2) thin-section description, 3) intact core XRF data, 4) electron microscopic imaging and elemental analysis, 5) insoluble (in 10% HCl) weight percent and 6) XRF and XRD analysis of the insoluble component. The present interpretation of the sequence of events recorded in the core is: 1) relatively undisturbed Upper Cretaceous coccolith rich chalk (marl), 2) a well-developed marine hardground, 3) mass transport deposit mobilized by the Chicxulub Impact earthquake, at least at this locality, material from up-dip was transported over an intact hardground. 4) less-well-developed hardground capping the mass-transport deposit, with some material included which was ballistically/atmospherically transported from the Chicxulub Impact site and 5) Paleogene terrestrially sourced clays of the Midway Shale.