Department of Geology and Geological Engineering Van Tuyl Lecture Series- Spring 2016 4:00-5:00 p.m. in Berthoud Hall Room 241    Thursday, March 10, 2016 Richard Denne Biostratigraphy SME   “Massive Cretaceous-Paleogene Boundary Deposit and Initial Incision of the Ancestral Desoto Canyon: New Evidence for Widespread Chicxulub-induced Slope Failure in the Deep-water Gulf of Mexico” Abstract: The Chicxulub bolide impact on the Yucatan Peninsula at the Cretaceous-Paleogene (KPg) boundary has been postulated as the trigger that caused the collapse of the Yucatan Platform and re-mobilized large quantities of sediment into mass transport flows on the submerged shelf along eastern North and Central America, as well as around the Gulf of Mexico.  Well log and biostratigraphic data from over 40 Cretaceous well penetrations in the north-central deep-water GOM show a distinctive calcareous micritic deposit at the KPg boundary, corresponding to the so-called “MCU” seismic horizon. The deposit contains the Cretaceous-Tertiary boundary “cocktail” of Bralower et al. (1998) associated with the Chicxulub impact, and is predominantly composed of graded, pelagic carbonates, similar in composition to age-equivalent sediments found near the Chicxulub Crater, in DSDP/ODP cores, and in outcrops in Cuba.  This deposit represents the single largest-known mass wasting deposit, with an average thickness of 10–20 m on the upper slope and 90–200 m on the lower slope and basin floor of the GOM.  Underneath the deposit is an unconformity ranging from 9 m.y. to 85 m.y. in extent.  Investigation of seismic data in the north-central GOM shows distinctive sedimentary wedges that exhibit high-amplitude reflections situated at the top of the section interpreted as Cretaceous.  This interval is thought to be the resulting deposit from the mass transport flows and suspension fallout initiated by the impact.  Sediment redistribution caused by the Chicxulub impact from the shelf and slope to the deep-water environment filled in available accommodation space around salt highs, as well as deposited sediments on the highs themselves, and altered the seafloor topography across the north-central GOM.  Examination of seismic profiles in the vicinity of the modern DeSoto Canyon indicate that the initial incision of the canyon can be tied to the KPg boundary reflection in nearby industry wells.  Initial incision of the canyon is visible on reflection seismic profiles as a series of truncated reflections that outline a canyon-shaped feature on the Upper Cretaceous isochore map.  Although much of the early canyon was buried by Cenozoic siliciclastic deposition, it remained a zone of instability, characterized by chaotic seismic facies and common truncation of internal reflections. Smaller than the ancestral canyon, the modern DeSoto Canyon remains within the confines of the initial KPg incision. These new findings substantiate widespread slope failure induced by the Chicxulub impact and provide further evidence of a single impact coincident with the KPg mass extinction.