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12Z6618N#54511273

Abstract:

Deposits of Chicxulub tsunamite have thus far only been recorded from marine strata in the circum-Gulf of Mexico region; none from the Western Interior Seaway (WIS), which lacks a terminal-Cretaceous sedimentological record due to Neogene erosion. We report a new example of Chicxulub tsunamite, from a locality in the continental Hell Creek Formation near the western shoreline of the WIS. The site, named Tanis, constitutes the northernmost record of the Chicxulub tsunami and the first known example of onshore-inundation. The distinct sediment package at Tanis contains multiple features diagnostic of tsunami, including abundant large rip-up clasts within a normally graded sequence deposited rapidly out of suspension, evidence for inland flow from the paleo-direction predicted for the seaway, 180o flow reversal within the event deposit, and an abundance of late Maastrichtian marine palynomorphs and macrofossils atypical for the Hell Creek Formation. The site stratigraphically occupies the uppermost Hell Creek Formation, and is directly overlain by a thin, ~1.5 cm-thick band of impactite that typifies the KPg boundary in North America and has an iridium anomaly of 3.8 ppb. Additionally, ratios of Platinum Group Elements Pt/Pd, Pt/Ir, and Pt/Ru indicate a cosmic rather than crustal source. Impact-derived material from within the impactite and underlying tsunamite is identical, and includes diagenetically altered ejecta spherules, shocked minerals, microkrystites, and rare fragments of unaltered impact glass, the latter two being reported here for the first time from any site in North America. Diagenetically altered spherules and unaltered glass shards were investigated via SEM, Electron Microprobe, INAA, and LA-ICP-MS, and bear near-identical major oxidation state and trace element composition to known examples from the KPg boundary in Mexico, Haiti, Colombia, and elsewhere. Ar/Ar dating of the glass, dated together with the Haitian glass, revealed an age of 66.03 mya, reaffirming a temporal link with the end-Cretaceous impact event. Due to the rapid terrestrial deposition and unprecedented preservation potential, Tanis offers a unique window that is critical to better understanding the dynamics of the KPg impact, its initial effects on North America, and the Hell Creek biota at the close of the Cretaceous.