A fossiliferous spherule-rich bed at the Cretaceous–Paleogene (K–Pg) boundary in Mississippi, USA: Implications for the K–Pg mass extinction event in the Mississippi Embayment and Eastern Gulf Coastal Plain

Abstract

We describe an outcrop of the Cretaceous-Paleogene (K-Pg) boundary exposed due to construction near New Albany, Union County, Mississippi. It consists of the Owl Creek Formation and overlying Clayton Formation. The Owl Creek Formation is rich in the ammonites Discoscaphites iris and Eubaculites carinatus, which, along with biostratigraphically important dinoflagellate cysts and calcareous nannofossils, indicate deposition occurred within the last 1 million years, most likely last 500 kyrs, of the Cretaceous. The base of the overlying Clayton Formation marks the K-Pg boundary, and consists of a 15-30 cm thick muddy, poorly sorted quartz sand containing abundant spherules representing ejecta derived from the Chicxulub impact event. Impact spherules range in size from 0.5 mm to 1 mm in diameter and are hollow and well preserved, with details such as smaller vesicular spherules enclosed within. The spherules are altered to clay minerals such as smectite and are typical of those found at K-Pg boundary sites in the Gulf of Mexico and beyond. Spherules are scattered throughout the bed, and surface counts suggest an average of 4 spherules per cm 2 . Macrofossils within the spherule bed represent a rich fauna of ammonites, benthic molluscs (bivalves and gastropods), echinoids, as well as crabs and sharks. Macrofossil preservation ranges from whole to fragmentary, with most fossils preserved as internal moulds. The infill of the fossils is lithologically identical to the matrix of the spherule bed, including impact ejecta preserved within phragmocones and body chambers of ammonites, and differs from the underlying Owl Creek Formation. This suggests that the animals were either alive or loosely scattered on the sea floor at the time of deposition. Grain size changes indicate multiple events were responsible for deposition, and together with taphonomic evidence are consistent with dynamic high energy post-impact processes. Later sea level change during the Paleocene is responsible for a sharp contact at the top of the spherule bed. Geochemical evidence from the Owl Creek and Clayton Formations at this locality indicate numerous local paleoenvironmental changes affected the Mississippi Embayment at the time of the K-Pg boundary and mass extinction event.

A seismically induced onshore surge deposit at the KPg boundary, North Dakota

“…A large-scale inundation such as the one that affected Tanis could have resulted from a variety of impact-related triggering mechanisms. Onshore inundation by a massive tsunami caused by the Chicxulub impact (i.e., propagating directly from the impact site), which has already been documented in marine facies at more proximal localities (9, 10), might have been capable of producing the sedimentary surge deposit preserved at Tanis. However, such a tsunami would have been greatly attenuated in the shallow WIS, even assuming the WIS was an uninterrupted, open corridor at that time, which is not presently known.…”

“…Intimately associated with the third-greatest global extinction, a variety of immediate and protracted results have been proposed for the Chicxulub impact, including atmospheric perturbations and long-term global climatic shifts (3), possible impact-induced volcanism (4), and eventual worldwide ecological collapse (1). More-instantaneous effects, much more poorly resolved, include seismic disturbances (5–7) and the triggering of seiches (harmonic waves that can develop in large bodies of water) and megatsunami (8–10). Some of the most visually apparent disturbances are the tsunami/seiches recorded in high-energy sediment packages up to 9 m thick in marine deposits throughout the Gulf Coastal Plain and Caribbean (8–10).…”

“…More-instantaneous effects, much more poorly resolved, include seismic disturbances (5–7) and the triggering of seiches (harmonic waves that can develop in large bodies of water) and megatsunami (8–10). Some of the most visually apparent disturbances are the tsunami/seiches recorded in high-energy sediment packages up to 9 m thick in marine deposits throughout the Gulf Coastal Plain and Caribbean (8–10). It is problematic, however, to trace their geographic extent in the Western Interior Seaway (WIS) because the terminal-Cretaceous geologic record for that depositional system is not preserved.…”

Cephalopods from the Cretaceous-Paleogene (K-Pg) Boundary Interval on the Brazos River, Texas, and Extinction of the Ammonites

“…Ammonoid cephalopods (ammonites) are among the most prominent victims of this crisis, while closely related nautilids survived into the Cenozoic (Landman et al, 1983;2015). The record of ammonite extinction has been extensively studied in the Atlantic and eastern Gulf Coastal plains of North America (Landman et al, 2007a;Witts et al, 2018). However, important gaps remain in this record, especially in the western part of the Mississippi Embayment.…”

“…This is primarily due to the presence of complex and laterally variable siliciclastic deposits coincident with the K-Pg boundary, ascribed either to rapid depositional processes related to the Chicxulub impact event and its aftermath (Bourgeois et al, 1988;Smit et al, 1996;Yancey and Liu, 2013) or non-catastrophic sea-level change (Gale, 2006;Keller et al, 2007; papers in . Similar discussions have focused on other successions around the Gulf of Mexico, proximal (< 1500 km distance) to the Chicxulub crater (Schulte et al, 2010) where high energy deposition is most likely to have occurred immediately following impact (Sanford et al, 2016;Witts et al, 2018;Gulick et al, 2019). Intense debate has also focused on the precise placement of the K-Pg boundary at Brazos based on lithostratigraphic and biostratigraphic grounds.…”