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Paper No. 246-7
Presentation Time: 8:00 AM-5:30 PM
AMMONITE JAWS ACROSS THE CRETACEOUS PALEOGENE BOUNDARY IN MISSISSIPPI: EVIDENCE FOR AMMONITE ABUNDANCE LEADING UP TO AND AFTER THE CHICXULUB IMPACT
The latest Cretaceous Prairie Bluff Chalk (PBC) and earliest Paleogene Clayton Formation are well exposed at Trim Cane Creek near Starkville, Mississippi. The PBC is represented by a muddy, very fine marl to chalk. Discoscaphites iris has been recorded in this section of the PBC, evidence for the latest Maastrichtian age. It is overlain by the Clayton Formation. The base of the Clayton Formation consists of a clastic unit composed of two main facies. The lowest facies consists of multiple event beds dominated by impact spherules. The upper facies is a heavily bioturbated, fine grain quartz sand with abundant lignite, referred to as the Burrowed Unit (BU). The spherule beds are interpreted as high energy tsunami deposits related to the Chicxulub Impact and evidence of the earliest Danian age. The BU represents post impact deposition. Discoscaphites iris, Eubaculites carinatus, and E. latecarinatus are present in the PBC as occasional internal molds. In addition, jaws of these ammonites (aptychi) are abundant. A total of 13 ammonite aptychi were recovered from the sequence at Trim Cane Creek. This includes nine Eubaculites aptychi that were recovered from the PBC. Two additional aptychi were recovered from the spherule bed; an aptychus of Eubaculites from a PBC rip up clast and an aptychus of Discoscaphites from the spherule bed matrix. We identified additional aptychi of Discoscaphites and Eubaculites in the Burrowed Unit. Most specimens were preserved as recrystallized calcite, although a couple were preserved as molds. There is preservation bias associated with aragonite dissolution and calcite preservation within the PBC. This suggests that the PBC is more fossiliferous (including ammonites) than previously thought. These observations shed light on the abundance of ammonite populations leading up to the impact. The occurrences within the BU suggest ammonites possibly survived for a short time after the impact.