160-3 Commotion in the Ocean: Tsunamis, Seiche Waves, and Sedimentary Responses to the Chicxulub Impact on the US Mid-Atlantic margin

Abstract:

The mid-Atlantic Coastal Plain preserves an exceptional record of the Chicxulub impact at an intermediate distance of approximately 2,500 km from the crater. Sections found beneath the modern outer shelf and continental slope contain major unconformities, collectively termed the East Coast Sequential Unconformity (Olsson and Wise, 1985), which were likely produced by slumping triggered by an impact-generated mega-earthquake estimated to have approached Mw 11-12 and by subsequent aftershocks. Across the New Jersey Coastal Plain, preservation of the impact record varies systematically with depositional setting. At the downdip Bass River site, deposited in a middle-shelf environment at approximately 75 m paleodepth, an undisturbed spherule layer is interpreted as ballistic ejecta that arrived within hours of impact. More updip sites, including Ancora (~50 m paleodepth), contain reworked spherules and sedimentary deposits attributed to swash-back and seiche activity. These sedimentary observations are consistent with tsunami or seiche waves reaching heights on the order of 15-20 m and disrupting the shallow (<50-75 m) seafloor. Farther updip, sections are commonly diastemic or preserve only weak and diffuse Ir enrichments that have been redistributed by bioturbation. The most proximal updip locality, the Campo-Buck Pit (proposed Millstone Geological Park), records the impact in a lower-shoreface setting at approximately 20-30 m paleodepth. A 50-cm-thick clay layer contains a pronounced Ir peak near its base and multiple thin siderite laminae near its top. These laminae are interpreted as concentrations of reworked impact spherules deposited by earthquake-generated seiche activity during the months to years following the impact. The laminae document repeated episodes of ejecta remobilization and help explain the heterogeneous preservation of K/Pg boundary deposits throughout the region. At the Edelman Fossil Park, deposited in paleodepths of 30-40 m, the Main Fossiliferous Layer (MFL; Obasi et al., 2011) contains abundant in situ Danian fossils mixed with reworked Cretaceous material. The MFL lies approximately 50 cm above the heavily bioturbated K/Pg contact, and neither horizon contains a significant Ir anomaly, suggesting removal of the primary fallout layer. We propose that the MFL at Edelman represents an early post-extinction recovery fauna containing reworked Cretaceous material. This assemblage was later buried by aftershock-induced seiche deposits years to decades after the impact rather than representing a death assemblage generated directly by the extinction event.

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