The K-Pg extinction event . The intermediate
claystone layer contains 1000 times more
iridium than the upper and lower layers (
San Diego Natural History Museum). The
Cretaceous–Paleogene ("K-Pg" or "K-T") extinction event around 65.5 million years ago wiped out almost
dinosaurs and many other species. 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. The impactor tore through the Earth's
crust, creating huge
earthquakes,
giant waves, and a
crater 180 kilometers (112 mi) wide, and blasted aloft trillions of tons of dust, debris, and climate-changing
sulfates from the
gypsum seabed, and it may have created
firestorms worldwide. With the exception of some
ectothermic species such as the ancestors of the modern
leatherback sea turtle and
crocodiles, no
tetrapods weighing more than survived. It marked the end of the
Cretaceous period and the
Mesozoic Era, opening the
Cenozoic Era that continues today. However, because it is rare in any case for animals and plants to be fossilized, the fossil record leaves some major questions unanswered. One of these is whether dinosaurs were already declining at the time of the event due to ongoing
volcanic climate change. Also, there is little evidence on the detailed effects of the event on Earth and its
biosphere. No fossil beds were yet known that could clearly show the details that might resolve these questions. There is considerable detail for times greater than hundreds of thousands of years either side of the event, and for certain kinds of change on either side of the K-Pg boundary layer. But relatively little fossil evidence is available from times nearer the crucial event, a difficulty known as the
Signor–Lipps effect.
Hell Creek Formation The
Hell Creek Formation is a well-known and much-studied fossil-bearing
formation (geological region) of mostly
Upper Cretaceous and some lower
Paleocene rock that stretches across portions of
Montana,
North Dakota,
South Dakota, and
Wyoming in
North America. The formation is named for early studies at Hell Creek, located near
Jordan, Montana, and it was designated as a
National Natural Landmark in 1966. 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. The
iridium-enriched Cretaceous–Paleogene boundary, which separates the Cretaceous from the
Cenozoic, is distinctly visible as a discontinuous thin marker above and occasionally within the formation. Numerous famous fossils of plants and animals, including many types of
dinosaur fossils, have been discovered there. At the time of the Chicxulub impact, the present-day North American continent was still forming. Most of central North America had recently been a large shallow seaway, called the
Western Interior Seaway (also known as the North American Sea or the Western Interior Sea), and parts were still submerged. This had initially been a seaway between separate
continents, but it had narrowed in the late Cretaceous to become, in effect, a large inland extension to the
Gulf of Mexico. The Hell Creek Formation was at this time very low-lying or partly submerged land at the northern end of the seaway, and the Chicxulub impact occurred in the shallow seas at the southern end, approximately from the site. long after the microtektites had already fallen back to earth, and far too late to leave the geological record found at the site. It is not even clear whether the massive waves were able to traverse the entire Interior Seaway. the
1950 Assam-Tibet earthquake (
India/
China) (seiches in
England and
Norway), the
2010 Chile earthquake (seiches in
Louisiana). Notably, after the powerful
Tōhoku earthquake in 2011, slower
secondary waves traveled over in less than 30 minutes to cause seiches around high in
Norway. suggests that the resulting seiche waves would have been approximately high in the
Western Interior Seaway near Tanis This would resolve conflicting evidence that huge water movements had occurred in the Hell Creek region near Tanis much less than an hour after impact, although the first
megatsunamis from the impact zone could not have arrived at the site for almost a full day. ==Site description==