Tanis (fossil site)
It is part of the heavily studied Hell Creek Formation, a geological region renowned for many significant fossil discoveries from the Upper Cretaceous and lower Paleocene.
Uniquely, Tanis appears to record in detail, extensive evidence of the direct effects of the giant Chicxulub asteroid impact which struck the Gulf of Mexico 66.043 million years ago, and wiped out all non-avian dinosaurs and many other species (the so-called "K–Pg" or "K–T" extinction).
The site was originally discovered in 2008 by University of North Georgia Professor Steve Nicklas and field paleontologist Rob Sula.
Recognizing the unique nature of the site, Nicklas and Sula brought in Robert DePalma, a University of Kansas graduate student, to perform additional excavations.
At Tanis, unlike any other known Lagerstätte site, it appears specific circumstances allowed for the preservation of moment-by-moment details caused by the impact event.
[1]: p.8 The site formed part of a bend in an ancient river on the westward shore of the seaway,[1]: p.8192 [4]: pp.5, 6, 23 and was flooded with great force by these waves, which carried sea, land, freshwater animals and plants, and other debris several miles inland.
The seiche waves exposed and covered the site twice, as millions of tiny microtektite droplets and debris from the impact were arriving on ballistic trajectories from their source in what is now the Yucatán Peninsula.
Proposed by Luis and Walter Alvarez, it is now widely accepted that the extinction was caused by a huge asteroid or bolide that impacted Earth in the shallow seas of the Gulf of Mexico, leaving behind the Chicxulub crater.
With the exception of some ectothermic species such as the ancestors of the modern leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kg (55 lb) survived.
[18] The formation contains a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the Maastrichtian and Danian (respectively, the end of the Cretaceous and the beginning of the Paleogene periods) by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway.
Any water-borne waves would have arrived between 18 and 26 hours later,[1]: p.24 long after the microtektites had already fallen back to earth, and far too late to leave the geological record found at the site.
Notably, the powerful magnitude 9.0 – 9.1 Tōhoku earthquake in 2011, slower secondary waves traveled over 8,000 km (5,000 miles) in less than 30 minutes to cause seiches around 1.5–1.8 m (4.9–5.9 ft) high in Norway.
[20][1]: p.8 The Chicxulub impact is believed to have triggered earthquakes estimated at magnitude 10 – 11.5,[1]: p.8 releasing up to 4000 times the energy of the Tohoku quake.Note 1 Co-author Mark Richards, a professor of earth sciences focusing on dynamic earth crust processes,[21] suggests that the resulting seiche waves would have been approximately 10–100 m (33–328 ft) high in the Western Interior Seaway near Tanis[1]: p.8 and credibly, could have created the 10 – 11 m (33 – 36 feet) high water movements evidenced inland at the site; the time taken by the seismic waves to reach the region and cause earthquakes almost exactly matched the flight time of the microtektites found at the site.
