Possible Mechanisms for Tsunami‐Like Surge Deposits Due To the Chicxulub Impact at the K‐Pg Boundary at the Tanis Site, North Dakota.

At Tanis, a unique bi‐directional sediment package occurs precisely at the Cretaceous‐Paleogene (K‐Pg) boundary, recording the first hours of the Paleogene in uncommonly fine temporal detail. The impact ejecta‐bearing sediment package was rapidly emplaced by two massive, ∼10‐m‐high, potentially impact‐triggered surges, that inundated a steep, deeply incised paleo river valley from the direction of the contemporaneous Western Interior Seaway (WIS). Intermingling of fresh‐ and salt‐water fossils at Tanis, coeval brackish water indicators in the nearby region, and historical tsunami observations, suggest that the WIS paleoshoreline was nearby Tanis at K‐Pg time. The interpreted timing for deposition (including ejecta infall) of ∼1–2‐hr immediately post‐impact precludes a direct tsunami from the Chicxulub impact site, which would have required much more than 10 hr to reach Tanis. Seismic waves from the Mw ∼ 11 Chicxulub earthquake, arriving just minutes post‐impact, might have triggered the surge, for example, via seismic excitation of large water waves in the WIS, as proposed by DePalma et al. (2019, https://doi.org/10.1073/pnas.1817407116). Here, we explore this mechanism via a simple mathematical model of seismic excitation and propagation of a water wave into a shallow river and upstream. Matching the observations implies a relatively long source process time of many minutes, such as generated by Chicxulub crater rebound processes, in order to explain sufficient upriver amplitudes and advective transport. Atmospheric waves due to the expanding Chicxulub ejecta curtain might have provided a smaller, secondary contribution during triggering. Thus, the mechanism(s) for the surges at Tanis are now better‐constrained, yet remain incompletely resolved. Plain Language Summary: A remarkable geological record at the "Tanis" site (North Dakota) shows that two ∼10 m high tsunami‐like surges occurred on a point bar along a river upstream from the then existing Western Interior Seaway (WIS). The surge deposits are interpreted to have occurred within 1–2 hr of the Chicxulub impact associated with the end‐Cretaceous mass extinction including the dinosaurs, and are challenging to explain via traditional tsunami mechanisms because they occurred too soon after the impact to have been tsunami waves traveling from the impact site. Alternative explanations include excitation of large water waves in the WIS due to seismic shaking from the magnitude ∼11 impact‐generated earthquake or atmospheric effects from the expanding impact ejecta curtain. Here, we explore one potential triggering mechanism via a simple mathematical model of direct seismic excitation and propagation of a water wave into a shallow river and upstream. This model shows the difficulty of generating the observed surge deposits unless the timescale for impact‐generated seismic waves is much longer than is commonly presumed. Thus the actual triggering mechanism(s) of the Tanis depositional surges remains incompletely resolved, while the sediment package bears a rich source of information about the global effects of the Chicxulub impact. Key Points: An ∼10 m surge event coincident with the Chicxulub impact transported mixed freshwater/marine organisms up the Tanis river valleyNew modeling confirms limitations on the inland transport of marine fossils and helps to constrain the impact‐related triggering mechanismOther potential triggering mechanisms and strategic avenues of future study are discussed [ABSTRACT FROM AUTHOR]