Prehistoric Flora & Fauna

Cretaceous – Paleogene Extinction Event

Saturday, November 19, 2016

The Cretaceous–Paleogene (K–Pgextinction event, also known as the Cretaceous–Tertiary (K–Textinction, was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time approximately 66 million years ago. With the exception of some ectothermic species like the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms (55 lb) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.

In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth’s crust but abundant in asteroids.

As originally proposed in 1980 by a team of scientists led by Luis Alvarez, it is now generally thought that the K–Pg extinction was caused by a massive comet or asteroid impact, estimated to be 10 km (6.2 mi) wide, 66 million years ago and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis. The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K–Pg extinction was caused by the asteroid. It was possibly accelerated by the creation of the Deccan Traps. However, some scientists maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions, climate change, or sea level change, separately or together.

A wide range of species perished in the K–Pg extinction. The best-known victims are the non-avian dinosaurs. However, the extinction also destroyed a plethora of other terrestrial organisms, including certain mammals, pterosaurs, birds, lizards, insects, and plants. In the oceans, the K–Pg extinction killed off plesiosaurs and the giant marine lizards (Mosasauridae) and devastated fish, sharks, mollusks (especially ammonites, which became extinct) and many species of plankton. It is estimated that 75% or more of all species on Earth vanished. Yet the devastation caused by the extinction also provided evolutionary opportunities. In the wake of the extinction, many groups underwent remarkable adaptive radiations—a sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches resulting from the event. Mammals in particular diversified in the Paleogene, producing new forms such as horses, whales, bats, and primates. Birds, fish and perhaps lizards also radiated.

Extinction patterns

The K–Pg extinction event was severe, global, rapid, and selective. In terms of severity, the event eliminated a vast number of species. Based on marine fossils, it is estimated that 75% or more of all species were made extinct by the K–Pg extinction event.

The event appears to have affected all continents at the same time. Non-avian dinosaurs, for example, are known from the Maastrichtian of North America, Europe, Asia, Africa, South America and Antarctica, but are unknown from the Cenozoic anywhere in the world. Similarly, fossil pollen shows devastation of the plant communities in areas as far apart as New Mexico, Alaska, China, and New Zealand.

Even though the boundary event was severe, there was significant variability in the rate of extinction between and within different clades. Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked sunlight and reduced the solar energy reaching the Earth’s surface. This plant extinction caused a major reshuffling of the dominant plant groups. Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. No purely herbivorous or carnivorous mammals seem to have survived. Rather, the surviving mammals and birds fed on insects, worms, and snails, which in turn fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus (non-living organic material).

In stream communities, few animal groups became extinct because such communities rely less directly on food from living plants and more on detritus that washes in from the land, buffering them from extinction. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding. Coccolithophorids and mollusks (including ammonites, rudists, freshwater snails and mussels), and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary. The largest air-breathing survivors of the event, crocodyliforms and champsosaurs, were semi-aquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.

After the K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches.

Radiolaria have left a geological record since at least the Ordovician times, and their mineral fossil skeletons can be tracked across the K–Pg boundary. There is no evidence of mass extinction of these organisms, and there is support for high productivity of these species in southern high latitudes as a result of cooling temperatures in the early Paleocene. Approximately 46% of diatom species survived the transition from the Cretaceous to the Upper Paleocene. This suggests a significant turnover in species, but not a catastrophic extinction of diatoms, across the K–Pg boundary.

Marine invertebrates

There is significant variation in the fossil record as to the extinction rate of marine invertebrates across the K–Pg boundary. The apparent rate is influenced by the lack of fossil records rather than actual extinction.

Ostracods, a class of small crustaceans that were prevalent in the upper Maastrichtian, left fossil deposits in a variety of locations. A review of these fossils shows that ostracod diversity was lower in the Paleocene than any other time in the Cenozoic. However, current research cannot ascertain whether the extinctions occurred prior to or during the boundary interval itself.

Approximately 60% of late-Cretaceous Scleractinia coral genera failed to cross the K–Pg boundary into the Paleocene. Further analysis of the coral extinctions shows that approximately 98% of colonial species, ones that inhabit warm, shallow tropical waters, became extinct. The solitary corals, which generally do not form reefs and inhabit colder and deeper (below the photic zone) areas of the ocean were less impacted by the K–Pg boundary. Colonial coral species rely upon symbiosis with photosynthetic algae, which collapsed due to the events surrounding the K–Pg boundary. However, the use of data from coral fossils to support K–Pg extinction and subsequent Paleocene recovery must be weighed against the changes that occurred in coral ecosystems through the K–Pg boundary.

The numbers of cephalopod, echinoderm, and bivalve genera exhibited significant diminution after the K–Pg boundary. Most species of brachiopods, a small phylum of marine invertebrates, survived the K–Pg extinction event and diversified during the early Paleocene.


There are substantial fossil records of jawed fishes across the K–Pg boundary, which provides good evidence of extinction patterns of these classes of marine vertebrates. While the deep sea realm was able to remain seemingly unaffected, there was an equal loss between the open marine apex predators and the durophagous demersal feeders on the continental shelf.

Within cartilaginous fish, approximately 7 out of the 41 families of neoselachians (modern sharks, skates and rays) disappeared after this event and batoids (skates and rays) lost nearly all the identifiable species, while more than 90% of teleost fish (bony fish) families survived.

In the Maastrichtian age, 28 shark families and 13 batoid families thrived, of which 25 and 9 survived the K-T boundary event, respectively. Forty-seven of all neoselachian genera cross the K/T boundary, 85% being sharks. Batoids display with 15% a comparably low survival rate.

There is evidence of a mass kill of bony fishes at a fossil site immediately above the K–Pg boundary layer on Seymour Island near Antarctica, apparently precipitated by the K–Pg extinction event. However, the marine and freshwater environments of fishes mitigated environmental effects of the extinction event.

Terrestrial invertebrates

Insect damage to the fossilized leaves of flowering plants from fourteen sites in North America were used as a proxy for insect diversity across the K–Pg boundary and analyzed to determine the rate of extinction. Researchers found that Cretaceous sites, prior to the extinction event, had rich plant and insect-feeding diversity. However, during the early Paleocene, flora were relatively diverse with little predation from insects, even 1.7 million years after the extinction event.

Terrestrial plants

There is overwhelming evidence of global disruption of plant communities at the K–Pg boundary. Extinctions are seen both in studies of fossil pollen, and fossil leaves. In North America, the data suggests massive devastation and mass extinction of plants at the K–Pg boundary sections, although there were substantial megafloral changes before the boundary. In North America, approximately 57% of plant species became extinct. In high southern hemisphere latitudes, such as New Zealand and Antarctica, the mass die-off of flora caused no significant turnover in species, but dramatic and short-term changes in the relative abundance of plant groups. In some regions, the Paleocene recovery of plants began with recolonizations by fern species, represented as a fern spike in the geologic record; this same pattern of fern recolonization was observed after the 1980 Mount St. Helens eruption.

Due to the wholesale destruction of plants at the K–Pg boundary, there was a proliferation of saprotrophic organisms, such as fungi, that do not require photosynthesis and use nutrients from decaying vegetation. The dominance of fungal species lasted only a few years while the atmosphere cleared and there was plenty of organic matter to feed on. Once the atmosphere cleared, photosynthetic organisms, like ferns and other plants, returned. Polyploidy appears to have enhanced the ability of flowering plants to survive the extinction, probably because the additional copies of the genome such plants possessed allowed them to more readily adapt to the rapidly changing environmental conditions that followed the impact.


There is limited evidence for extinction of amphibians at the K–Pg boundary. A study of fossil vertebrates across the K–Pg boundary in Montana concluded that no species of amphibian became extinct. Yet there are several species of Maastrichtian amphibian, not included as part of this study, which are unknown from the Paleocene. These include the frog Theatonius lancensis and the albanerpetontid Albanerpeton galaktion; therefore some amphibians do seem to have become extinct at the boundary. The relatively low levels of extinction seen among amphibians probably reflect the low extinction rates seen in freshwater animals.

Non-archosaur reptiles

The two living non-archosaurian reptile taxa, testudines (turtles) and lepidosaurians (lizards and tuataras), along with choristoderes (semi-aquatic archosauromorphs that would die out in the early Miocene), survived across the K–Pg boundary. Over 80% of Cretaceous turtle species passed through the K–Pg boundary. Additionally, all six turtle families in existence at the end of the Cretaceous survived into the Paleogene and are represented by living species. Living lepidosaurs include the tuataras (the only living rhynchocephalians) and the squamates. The rhynchocephalians were a widespread and relatively successful group of lepidosaurians during the early Mesozoic, but began to decline by the mid-Cretaceous, though they were very successful in the Late Cretaceous of South America. They are represented today by a single genus located exclusively in New Zealand.The order Squamata, which is represented today by lizards, including snakes and amphisbaenians (worm lizards), radiated into various ecological niches during the Jurassic and was successful throughout the Cretaceous. They survived through the K–Pg boundary and are currently the most successful and diverse group of living reptiles with more than 6,000 extant species. Many families of terrestrial squamates became extinct at the boundary, such as monstersaurians and polyglyphanodonts, and fossil evidence indicates they suffered very heavy losses in the KT event, only recovering 10 million years after it. Giant non-archosaurian aquatic reptiles such as mosasaurs and plesiosaurs, which were the top marine predators of their time, became extinct by the end of the Cretaceous. The ichthyosaurs had already disappeared before the mass extinction occurred.


The archosaur clade includes two surviving groups, crocodilians and birds, along with the various extinct groups of non-avian dinosaurs and pterosaurs.


Ten families of crocodilians or their close relatives are represented in the Maastrichtian fossil records, of which five died out prior to the K–Pg boundary. Five families have both Maastrichtian and Paleocene fossil representatives. All of the surviving families of crocodyliforms inhabited freshwater and terrestrial environments—except for the Dyrosauridae, which lived in freshwater and marine locations. Approximately 50% of crocodyliform representatives survived across the K–Pg boundary, the only apparent trend being that no large crocodiles survived. Crocodyliform survivability across the boundary may have resulted from their aquatic niche and ability to burrow, which reduced susceptibility to negative environmental effects at the boundary. Jouve and colleagues suggested in 2008 that juvenile marine crocodyliforms lived in freshwater environments like modern marine crocodile juveniles, which would have helped them survive where other marine reptiles became extinct; freshwater environments were not as strongly affected by the K–Pg extinction event as marine environments.


The Choristodera, a generally crocodile-like group of uncertain phylogeny (possibly archosaurian) also survived the event, only to become extinct in the Miocene. Studies on Champsosaurus’ palatal teeth suggest that there were dietary changes among the various species across the KT event.


One family of pterosaurs, Azhdarchidae, was definitely present in the Maastrichtian, and it likely became extinct at the K–Pg boundary. These large pterosaurs were the last representatives of a declining group that contained 10 families during the mid-Cretaceous. Several other pterosaur lineages may have been present during the Maastrichtian, such as the ornithocheirids, pteranodontids and/or nyctosaurids, as well as a possible tapejarid, though they are represented by fragmentary remains that are difficult to assign to any given group. While this was occurring, modern birds were undergoing diversification; traditionally it was thought that they replaced archaic birds and pterosaur groups, possibly due to direct competition, or they simply filled empty niches, but there is no correlation between pterosaur and avian diversities that are conclusive to a competition hypothesis, and small pterosaurs were present in the Late Cretaceous.


Most paleontologists regard birds as the only surviving dinosaurs (see Origin of birds). It is thought that all non-avian theropods became extinct, including then-flourishing groups like enantiornithines and hesperornithiforms. Several analyses of bird fossils show divergence of species prior to the K–Pg boundary, and that duck, chicken and ratite bird relatives coexisted with non-avian dinosaurs. Large collections of bird fossils representing a range of different species provides definitive evidence for the persistence of archaic birds to within 300,000 years of the K–Pg boundary. The absence of these birds in the Paleogene is evidence that a mass extinction of archaic birds took place there. A small fraction of the Cretaceous bird species survived the impact, giving rise to today’s birds. The only bird group known for certain to have survived the K–Pg boundary is the Aves. Avians may have been able to survive the extinction as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of avians can build burrows, or nest in tree holes or termite nests, all of which provided shelter from the environmental effects at the K–Pg boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs.

Non-avian dinosaurs

Excluding a few controversial claims, scientists agree that all non-avian dinosaurs became extinct at the K–Pg boundary. The dinosaur fossil record has been interpreted to show both a decline in diversity and no decline in diversity during the last few million years of the Cretaceous, and it may be that the quality of the dinosaur fossil record is simply not good enough to permit researchers to distinguish between the options. There is no evidence that late Maastrichtian non-avian dinosaurs could burrow, swim or dive, which suggests they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K–Pg boundary. It is possible that small dinosaurs (other than birds) did survive, but they would have been deprived of food, as herbivorous dinosaurs would have found plant material scarce and carnivores would have quickly found prey in short supply.The growing consensus about the endothermy of dinosaurs (see dinosaur physiology) helps to understand their full extinction in contrast with their close relatives, the crocodilians. Ectothermic (“cold-blooded”) crocodiles have very limited needs for food (they can survive several months without eating) while endothermic (“warm-blooded”) animals of similar size need much more food to sustain their faster metabolism. Thus, under the circumstances of food chain disruption previously mentioned, non-avian dinosaurs died, while some crocodiles survived. In this context, the survival of other endothermic animals, such as some birds and mammals, could be due, among other reasons, to their smaller needs for food, related to their small size at the extinction epoch.

Whether the extinction occurred gradually or suddenly has been debated, as both views have support from the fossil record. A study of 29 fossil sites in Catalan Pyrenees of Europe in 2010 supports the view that dinosaurs there had great diversity until the asteroid impact, with over 100 living species. However, more recent research indicates that this figure is obscured by taphonomical biases and the sparsity of the continental fossil record. The results of this study, which were based on estimated real global biodiversity, showed that between 628 and 1078 non-avian dinosaur species were alive at the end of the Cretaceous and underwent sudden extinction after the Cretaceous–Paleogene extinction event. Alternatively, interpretation based on the fossil-bearing rocks along the Red Deer River in Alberta supports the gradual extinction of non-avian dinosaurs; during the last 10 million years of the Cretaceous layers there, the number of dinosaur species seems to have decreased from about 45 to about 12. Other scientists have pointed out the same.

Several researchers support the existence of Paleocene dinosaurs. Evidence of this existence is based on the discovery of dinosaur remains in the Hell Creek Formation up to 1.3 m (4.3 ft) above and 40 thousand years later than the K–Pg boundary. Pollen samples recovered near a fossilized hadrosaur femur recovered in the Ojo Alamo Sandstone at the San Juan River indicate that the animal lived during the Cenozoic, approximately 64.5 Ma (about 1 million years after the K–Pg extinction event). If their existence past the K–Pg boundary can be confirmed, these hadrosaurids would be considered a dead clade walking. Scientific consensus is that these fossils were eroded from their original locations and then re-buried in much later sediments (also known as reworked fossils).


All major Cretaceous mammalian lineages, including monotremes (egg-laying mammals), multituberculates, metatherians, eutherians, dryolestoideans, and gondwanatheres survived the K–Pg extinction event, although they suffered losses. In particular, metatherians largely disappeared from North America, and the Asian deltatheroidans became extinct. In the Hell Creek beds of North America, at least half of the ten known multituberculate species and all eleven metatherians species are not found above the boundary. Multituberculates in Europe and North America survived relatively unscathed and quickly bounced back in the Palaeocene, but Asian forms were decimated, never again to represent a significant component on mammalian faunas.

Mammalian species began diversifying approximately 30 million years prior to the K–Pg boundary. Diversification of mammals stalled across the boundary. Current research indicates that mammals did not explosively diversify across the K–Pg boundary, despite the environment niches made available by the extinction of dinosaurs. Several mammalian orders have been interpreted as diversifying immediately after the K–Pg boundary, including Chiroptera (bats) and Cetartiodactyla (a diverse group that today includes whales and dolphins and even-toed ungulates), although recent research concludes that only marsupial orders diversified after the K–Pg boundary.

K–Pg boundary mammalian species were generally small, comparable in size to rats; this small size would have helped them find shelter in protected environments. In addition, it is postulated that some early monotremes, marsupials, and placentals were semiaquatic or burrowing, as there are multiple mammalian lineages with such habits today. Any burrowing or semiaquatic mammal would have had additional protection from K–Pg boundary environmental stresses.


North American fossils

In North American terrestrial sequences, the extinction event is best represented by the marked discrepancy between the rich and relatively abundant late-Maastrichtian palynomorph record and the post-boundary fern spike.

At present the most informative sequence of dinosaur-bearing rocks in the world from the K–Pg boundary is found in western North America, particularly the late Maastrichtian-age Hell Creek Formation of Montana. This formation, when compared with the older (approximately 75 Ma) Judith River/Dinosaur Park Formations (from Montana and Alberta respectively) provides information on the changes in dinosaur populations over the last 10 million years of the Cretaceous. These fossil beds are geographically limited, covering only part of one continent.

The middle–late Campanian formations show a greater diversity of dinosaurs than any other single group of rocks. The late Maastrichtian rocks contain the largest members of several major clades: TyrannosaurusAnkylosaurusPachycephalosaurusTriceratops and Torosaurus, which suggests food was plentiful immediately prior to the extinction.

In addition to rich dinosaur fossils, there are also plant fossils that illustrate the reduction in plant species across the K–Pg boundary. In the sediments below the K–Pg boundary the dominant plant remains are angiosperm pollen grains, but the actual boundary layer contains little pollen and is dominated by fern spores. More usual pollen levels gradually resume above the boundary layer. This is reminiscent of areas blighted by modern volcanic eruptions, where the recovery is led by ferns, which are later replaced by larger angiosperm plants.

Marine fossils

The mass extinction of marine plankton appears to have been abrupt and right at the K–Pg boundary. Ammonite genera became extinct at or near the K–Pg boundary; however, there was a smaller and slower extinction of ammonite genera prior to the boundary that was associated with a late Cretaceous marine regression. The gradual extinction of most inoceramid bivalves began well before the K–Pg boundary, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Further analysis shows that several processes were in progress in the late Cretaceous seas and partially overlapped in time, then ended with the abrupt mass extinction. The diversity of marine life decreased when the climate near the K-T boundary increased in temperature. The temperature increased about three to four degrees very rapidly between 65.4 and 65.2 million years ago, which is around the time of the extinction event. Not only did the climate temperature increase, but the water temperature decreased causing a drastic decrease in marine diversity.


The scientific consensus is that the asteroid impact at the K–Pg boundary left tsunami deposits and sediments around the area of the Caribbean Sea and Gulf of Mexico. These deposits have been identified in the La Popa basin in northeastern Mexico, platform carbonates in northeastern Brazil, and Atlantic deep-sea sediments. The megatsunami has been estimated to be over 100 metres (330 ft) tall, as the asteroid fell in an area of relatively shallow sea; in deep sea it would have been 4.6 kilometres (2.9 mi) tall.


The length of time taken for the extinction to occur is a controversial issue, because some theories about the extinction’s causes require a rapid extinction over a relatively short period (from a few years to a few thousand years) while others require longer periods. The issue is difficult to resolve because of the Signor–Lipps effect; that is, the fossil record is so incomplete that most extinct species probably died out long after the most recent fossil that has been found. Scientists have also found very few continuous beds of fossil-bearing rock which cover a time range from several million years before the K–Pg extinction to a few million years after it. The sedimentation rate and thickness of K-Pg clay from three sites suggest short duration of event, perhaps less than ten thousand years.

Chicxulub impact

Evidence for impact

In 1980, a team of researchers consisting of Nobel Prize–winning physicist Luis Alvarez, his son geologist Walter Alvarez, and chemists Frank Asaro and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous–Paleogene boundary contain a concentration of iridium many times greater than normal (30, 160 and 20 times in three sections originally studied). Iridium is extremely rare in Earth’s crust because it is a siderophile element, and therefore most of it traveled with the iron as it sank into Earth’s core during planetary differentiation. As iridium remains abundant in most asteroids and comets, the Alvarez team suggested that an asteroid struck the Earth at the time of the K–Pg boundary. There were earlier speculations on the possibility of an impact event, but this was the first hard evidence of an impact.

This shaded relief image of Mexico’s Yucatan Peninsula show a subtle, but unmistakable, indication of the Chicxulub impact crater. Most scientists now agree that this impact was the cause of the Cretatious-Tertiary Extinction, the event approximately 66 million years ago that marked the sudden extinction of the dinosaurs as well as the majority of life then on Earth. Author: NASA/JPL-Caltech


This hypothesis was viewed as radical when first proposed, but additional evidence soon emerged. The boundary clay was found to be full of minute spherules of rock, crystallized from droplets of molten rock formed by the impact. Shocked quartz and other minerals were also identified in the K–Pg boundary. Shocked minerals have their internal structure deformed, and are created by intense pressures such as those associated with nuclear blasts or meteorite impacts. The identification of giant tsunami beds along the Gulf Coast and the Caribbean also provided evidence for impact, and suggested that the impact may have occurred nearby—as did the discovery that the K–Pg boundary became thicker in the southern United States, with meter-thick beds of debris occurring in northern New Mexico.

Further research identified the giant Chicxulub crater, buried under Chicxulub on the coast of Yucatán, as the source of the K–Pg boundary clay. Identified in 1990 based on work by geophysicist Glen Penfield in 1978, the crater is oval, with an average diameter of roughly 180 kilometres (110 mi), about the size calculated by the Alvarez team. The discovery of the crater—a necessary prediction of the impact hypothesis—provided conclusive evidence for a K–Pg impact, and strengthened the hypothesis that the extinction was caused by an impact.

In 2007, a hypothesis was put forth that argued the impactor that killed the dinosaurs belonged to the Baptistina family of asteroids. Concerns have been raised regarding the reputed link, in part because very few solid observational constraints exist of the asteroid or family. Indeed, it was recently discovered that 298 Baptistina does not share the same chemical signature as the source of the K–Pg impact. Although this finding may make the link between the Baptistina family and K–Pg impactor more difficult to substantiate, it does not preclude the possibility. A 2011 WISE study of reflected light from the asteroids of the family estimated the break-up at 80 Ma, giving it insufficient time to shift orbits and impact the Earth by 66 Ma.

In a 2013 paper, Paul Renne of the Berkeley Geochronology Center reported that the date of the asteroid event is 66.043±0.011 million years ago, based on argon–argon dating. He further posits that the mass extinction occurred within 32,000 years of this date.

Effects of impact

In March 2010, an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism. They had determined that a 10-to-15-kilometre (6.2 to 9.3 mi) space rock hurtled into Earth at Chicxulub on Mexico’s Yucatán Peninsula. The collision would have released the same energy as 100 teratonnes of TNT (420 ZJ), over a billion times the energy of the atomic bombings of Hiroshima and Nagasaki.

The consequences of the Chicxulub impact were of global extent. Some of these phenomena were brief occurrences that immediately followed the impact, but there were also long-term geochemical and climatic disruptions that were catastrophic to the ecology.

The reentry of ejecta into Earth’s atmosphere would include a brief (hours long) but intense pulse of infrared radiation, killing exposed organisms. A paper in 2013 by a prominent nuclear winter modeler suggested that the global debris layer deposited by the impact contains enough soot to hint that the entire terrestrial biosphere burned, with an implication of this being that this would have caused a global soot-cloud blocking out the sun, creating the nuclear winter effect. However the suggestion that global firestorms occurred is debated, with opponents arguing that while ferocious fires probably did result locally, ferocious fires do not necessarily equal firestorms, and any such ferocious fires were instead limited to, the immediate American continent. This disagreement between researchers is termed the “Cretaceous-Palaeogene firestorm debate.”

This happened: 65 million years ago
End of the Cretaceous period
Start of the Palaeocene epoch

The Carboniferous Period

Saturday, November 19, 2016

The Carboniferous Period

The Carboniferous Period lasted from about 359.2 to 299 million years ago* during the late Paleozoic Era. The term “Carboniferous” comes from England, in reference to the rich deposits of coal that occur there. These deposits of coal occur throughout northern Europe, Asia, and midwestern and eastern North America. The term “Carboniferous” is used throughout the world to describe this period, although in the United States it has been separated into the Mississippian (early Carboniferous) and the Pennsylvanian (late Carboniferous) Subsystems. This division was established to distinguish the coal-bearing layers of the Pennsylvanian from the mostly limestone Mississippian, and is a result of differing stratigraphy on the different continents. The Mississippian and Pennsylvanian, in turn, are subdivided into a number of internationally recognized stages based on evolutionary successions of fossil groups . These stages are (from early to late) Tournaisian, Visean, and Serpukhovian for the Mississippian — and Bashkirian, Moscovian, Kasimovian, and Gzhelian for the Pennsylvanian.

In addition to having the ideal conditions for the formation of coal, several major biological, geological, and climatic events occurred during this time. Biologically, we see one of the greatest evolutionary innovations of the Carboniferous: the amniote egg, which allowed for the further exploitation of the land by certain tetrapods. It gave the ancestors of birds, mammals, and reptiles the ability to lay their eggs on land without fear of desiccation. Geologically, the Late Carboniferous collision of Laurasia (present-day Europe, Asia, and North America) into Gondwana (present-day Africa, South America, Antarctica, Australia, and India) produced the Appalachian Mountain belt of eastern North America and the Hercynian Mountains in the United Kingdom. A further collision of Siberia and eastern Europe created the Ural Mountains of Russia. And climatically, there was a trend towards mild temperatures during the Carboniferous, as evidenced by the decrease in lycopods and large insects, and an increase in the number of tree ferns.

An artist’s impression of a Carboniferous forest

The stratigraphy of the Mississippian can be easily distinguished from that of the Pennsylvanian. The Mississippian environment of North America was heavily marine, with seas covering parts of the continent. As a result, most Mississippian rocks are limestone, which are composed of the remains of crinoids, lime-encrusted green algae, or calcium carbonate shaped by waves. The North American Pennsylvanian environment was alternately terrestrial and marine, with the transgression and regression of the seas caused by glaciation. These environmental conditions, with the vast amount of plant material provided by the extensive coal forests, allowed for the formation of coal. Plant material did not decay when the seas covered them, and pressure and heat eventually built up over millions of years to transform the plant material to coal.


The beginning of the Carboniferous generally had a more uniform, tropical, and humid climate than exists today. Seasons if any were indistinct. These observations are based on comparisons between fossil and modern-day plant morphology. The Carboniferous plants resemble those that live in tropical and mildly temperate areas today. Many of them lack growth rings, which suggests a uniform climate. This uniformity in climate may have been the result of the large expanse of ocean that covered the entire surface of the globe, except for a localized section where Pangea, the massive supercontinent that existed during the late Paleozoic and early Triassic, was coming together.

Shallow, warm, marine waters often flooded the continents. Attached filter feeders such as bryozoans, particularly fenestellids, were abundant in this environment, and the sea floor was dominated by brachiopods. Trilobites were increasingly scarce while foraminifers were abundant. The heavily armored fish from the Devonian became extinct, being replaced with more modern-looking fish fauna.

Though many spectacular plant forms dominated the Carboniferous, most of them disappeared before the end of the Paleozoic. On the left, Neuropteris, a leaf form associated with the cycad-like seed-ferns.

Uplifting near the end of the Mississippian resulted in increased erosion, with an increase in the number of floodplains and deltas. The deltaic environment supported fewer corals, crinoids, blastoids, cryozoans, and bryzoans, which were abundant earlier in the Carboniferous. Freshwater clams made their first appearance, and there was an increase in gastropod, bony fish, and shark diversity. As the continents moved closer to forming Pangea, there was a net decrease in coastline, which in turn affected the diversity of marine life in those shallow continental waters.

Two large ice sheets at the southern pole locked up large amounts of water as ice. With so much water taken out of the water cycle, sea levels dropped, leading to an increase in terrestrial habitat. Increases and decreases in glaciation during the Pennsylvanian resulted in sea level fluctuations that can be seen in the rocks as striped patterns of alternating shale and coal layers.


The appearance or disappearance of fauna usually marks the boundaries between time periods. The Carboniferous is separated from the earlier Devonian by the appearance of the conodont Siphonodella sulcata or Siphondella duplicata. Conodonts are fossils that resemble the teeth or jaws of primitive eel- or hagfish-like fish. The Carboniferous-Permian boundary is distinguished by the appearance of the fusulinid foram Sphaeroschwagerina fusiformis in Europe and Pseudoschwagerina beedei in North America. Fusulinids are giants among protists and could reach a centimeter in length. They were abundant enough to form sizable deposits known as “rice rock” because of the resemblance between fusulinids and rice grains.

The Mississippian Subsystem is differentiated from the Pennsylvanian by the appearance of the conodont Declinognathodus noduliferus, the ammonoid genus Homoceras, and the foraminifers Millerella pressa and Millerella marblensis, though these markers apply only to marine deposits. The distinction between the Mississippian and Pennsylvanian subsystems may also be illustrated by a break in the flora due to transitional changes from a marine to a more terrestrial environment.

The stratigraphy of the Mississippian is distinguished by shallow-water limestones. Some of these limestones are composed of parts of organisms, primarily the remains of crinoids that thrived in the shallow seas. Other limestones include lime mudstones, composed of the carbonate mud produced by green algae, and oolithic limestones, composed of calcium carbonate in concentric spheres produced by high wave energy. Also found in Mississippian strata, though not as common, are sandstones (sedimentary rock composed of quartz sand and cemented by silica or calcium carbonate) and siltstones (rock composed of hardened silt).

Coal beds, which can be up to 11 to 12 meters thick, characterize the late Carboniferous. The forests of seedless vascular plants that existed in the tropical swamp forests of Europe and North America provided the organic material that became coal. Dead plants did not completely decay and were turned to peat in these swamp forests. When the sea covered the swamps, marine sediments covered the peat. Eventually, heat and pressure transformed these organic remains into coal. Coal balls, pockets of plant debris that were preserved as fossils and not converted to coal, are sometimes found within the coal layers.

Multiple transgressions and regressions of the Pennsylvanian seas across the continent can be seen in the rocks, and even counted, because they leave a telltale sequence of layers. As sea levels rise, the layers may go from sandstone (beach), to silty shale or siltstone (tidal), to freshwater limestone (lagoon), to underclay (terrestrial), to coal (terrestrial swampy forest). Then as sea levels fall, one may see a shale (nearshore tidal) grade to limestone (shallow marine) and finally to black shale (deep marine).

Index fossils are the remains of plants and animals that characterize a well-defined time span and occur over a wide range of geography. Fossils of marine life characterize the Mississippian, as shallow epicontinental seas covered the United States at that time. These fossils include solitary corals and Syringopora, tubular colonial corals. Other fossil colonial corals include Stelechophyllum and Siphonodendron. Because conodont fossils are distributed all over the world, they are utilized internationally to date Mississippian rocks.

Index fossils used for the Pennsylvanian Subsystem are fusulinid foraminifers and the pollen and spores from the coal forests prevalent during that time. The Mississippian-Pennsylvanian boundary is marked by the appearance of the fusulinid Pseudostaffella antiqua. Other fossils used to identify the early Pennsylvanian are the three ammonoid cephalopod genera GastriocerasDaiboloceras, and Paralegoceras, all found in marine deposits.


Joggins, Nova Scotia: This Pennsylvanian UNESCO World Heritage Site was home to early tetrapods such as Dendrerpeton.

Mazon Creek, Illinois: This site has become famous for its iron concretions preserving both plants and marine invertebrates.

Snowball Earth

Saturday, November 19, 2016

The Snowball Earth hypothesis proposes that Earth’s surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes, and other otherwise enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation, the geophysical feasibility of an ice- or slush-covered ocean, and the difficulty of escaping an all-frozen condition. A number of unanswered questions exist, including whether Earth was a full snowball, or a “slushball” with a thin equatorial band of open (or seasonally open) water.

The snowball Earth episodes occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity. Another, much earlier and longer snowball episode, the Huronian glaciation, which occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the “Great Oxygenation Event.”


The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes. According to modelling, an ice-albedo feedback would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25° to 30° of the equator. Therefore, the presence of glacial deposits within the tropics suggests global ice cover.

Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove two things:

  1. that a bed contains sedimentary structures that could have been created only by glacial activity;
  2. that the bed lay within the tropics when it was deposited.

During a period of global glaciation, it must also be demonstrated that glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence.

This last point is very difficult to prove. Before the Ediacaran, the biostratigraphic markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at precisely the same time. The best that can be done is to estimate the age of the rocks using radiometric methods, which are rarely accurate to better than a million years or so.

The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the approximate latitudes of landmasses even as recently as 200 million years ago can be riddled with difficulties.


The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth’s long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism.

When sedimentary rocks form, magnetic minerals within them tend to align themselves with the Earth’s magnetic field. Through the precise measurement of this palaeomagnetism, it is possible to estimate the latitude (but not the longitude) where the rock matrix was formed. Palaeomagnetic measurements have indicated that some sediments of glacial origin in the Neoproterozoic rock record were deposited within 10 degrees of the equator, although the accuracy of this reconstruction is in question. This palaeomagnetic location of apparently glacial sediments (such as dropstones) has been taken to suggest that glaciers extended from land to sea level in tropical latitudes at the time the sediments were deposited. It is not clear whether this implies a global glaciation, or the existence of localized, possibly land-locked, glacial regimes. Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator.

Low-latitude glacial deposits

Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the snowball Earth hypothesis many Neoproterozoic sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, it is worth remembering that many sedimentary features traditionally associated with glaciers can also be formed by other means. Thus the glacial origin of many of the key occurrences for snowball Earth has been contested. As of 2007, there was only one “very reliable” – still challenged – datum point identifying tropical tillites, which makes statements of equatorial ice cover somewhat presumptuous. However evidence of sea-level glaciation in the tropics during the Sturtian is accumulating. Evidence of possible glacial origin of sediment includes:

  • Dropstones (stones dropped into marine sediments), which can be deposited by glaciers or other phenomena.
  • Varves (annual sediment layers in periglacial lakes), which can form at higher temperatures.
  • Glacial striations (formed by embedded rocks scraped against bedrock): similar striations are from time to time formed by mudflows or tectonic movements.
  • Diamictites (poorly sorted conglomerates). Originally described as glacial till, most were in fact formed by debris flows.

Open-water deposits

It appears that some deposits formed during the snowball period could only have formed in the presence of an active hydrological cycle. Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers melted and re-formed repeatedly for tens of millions of years; solid oceans would not permit this scale of deposition. It is considered possible that ice streams such as seen in Antarctica today could have caused these sequences. Further, sedimentary features that could only form in open water (for example: wave-formed ripples, far-traveled ice-rafted debris and indicators of photosynthetic activity) can be found throughout sediments dating from the snowball-Earth periods. While these may represent “oases” of meltwater on a completely frozen Earth, computer modelling suggests that large areas of the ocean must have remained ice-free; arguing that a “hard” snowball is not plausible in terms of energy balance and general circulation models.

Carbon isotope ratios

There are two stable isotopes of carbon in sea water: carbon-12 (12C) and the rare carbon-13 (13C), which makes up about 1.109 percent of carbon atoms.

Biochemical processes, of which photosynthesis is one, tend to preferentially incorporate the lighter 12C isotope. Thus ocean-dwelling photosynthesizers, both protists and algae, tend to be very slightly depleted in 13C, relative to the abundance found in the primary volcanic sources of Earth’s carbon. Therefore, an ocean with photosynthetic life will have a lower 13C/12C ratio within organic remains, and a higher ratio in corresponding ocean water. The organic component of the lithified sediments will forever remain very slightly, but measurably, depleted in 13C.

During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of 13C to 12C. This is consistent with a deep freeze that killed off most or nearly all photosynthetic life – although other mechanisms, such as clathrat release, can also cause such perturbations. Close analysis of the timing of 13C ‘spikes’ in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic.

Banded iron formations

Banded iron formations (BIF) are sedimentary rocks of layered iron oxide and iron-poor chert. In the presence of oxygen, iron naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of the Earth’s atmosphere during the Palaeoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and precipitated out as iron oxide.

The bands were produced at the tipping point between an anoxic and an oxygenated ocean. Since today’s atmosphere is oxygen-rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with Cryogenian glacial deposits.

Changing acidity

Isotopes of the element boron suggest that the pH of the oceans dropped dramatically before and after the Marinoan glaciation. This may indicate a buildup of carbon dioxide in the atmosphere, some of which would dissolve into the oceans to form carbonic acid. Although the boron variations may be evidence of extreme climate change, they need not imply a global glaciation.

Space dust

Earth’s surface is very depleted in the element iridium, which primarily resides in the Earth’s core. The only significant source of the element at the surface is cosmic particles that reach Earth. During a snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An iridium anomaly has been discovered at the base of the cap carbonate formations, and has been used to suggest that the glacial episode lasted for at least 3 million years, but this does not necessarily imply a global extent to the glaciation; indeed, a similar anomaly could be explained by the impact of a large meteorite.

Cyclic climate fluctuations

Using the ratio of mobile cations to those that remain in soils during chemical weathering (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods. This pattern, if a true reflection of events, suggests that the “snowball Earths” bore a stronger resemblance to Pleistocene ice age cycles than to a completely frozen Earth.


A snowball Earth has profound implications in the history of life on Earth. While many refugia have been postulated, global ice cover would certainly have ravaged ecosystems dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials.

The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the Cryogenian period.

Flood Basalt Eruptions

Saturday, November 19, 2016

 Flood Basalt Eruptions

flood basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalt provinces such as the Deccan Traps of India are often called traps, which derives from the characteristic stairstep geomorphology of many associated landscapes. Rampino and Stothers (1988) cite eleven distinct flood basalt episodes occurring in the past 250 million years, creating large volcanic provinces, plateaus, and mountain ranges. However, more have been recognized such as the large Ontong Java Plateau, and the Chilcotin Group, though the latter may be linked to the Columbia River Basalt Group. Large igneous provinces have been connected to five mass extinction events, and may be associated with bolide impacts.



Prehistoric Earth. Computer artwork showing how the surface of the Earth may have appeared beneath its clouds about 500 million years after its birth, during a period known as the Hadean eon. Massive volcanoes and lava fields still dominate the landscape. In a few million years rain will begin falling, further cooling the crust. In about another 200 million years the first living microbes will call the Earth home.

The formation and effects of a flood basalt depend on a range of factors, such as continental configuration, latitude, volume, rate, duration of eruption, style and setting (continental vs. oceanic), the preexisting climate state, and the biota resilience to change.

One proposed explanation for flood basalts is that they are caused by the combination of continental rifting and its associated decompression melting, in conjunction with a mantle plume also undergoing decompression melting, producing vast quantities of a tholeiitic basaltic magma. These have a very low viscosity, which is why they ‘flood’ rather than form taller volcanoes. Another explanation is that they result from the release, over a short time period, of melt that has accumulated in the mantle over a long time period.

The Deccan Traps of central India, the Siberian Traps, and the Columbia River Plateau of western North America are three regions covered by prehistoric flood basalts. The Mesoproterozoic Mackenzie Large Igneous Province in Canada contains the Coppermine River flood basalts related to the Muskox layered intrusion. The maria on the Moon are additional, even more extensive, flood basalts. Flood basalts on the ocean floor produce oceanic plateaus.

The surface covered by one eruption can vary from around 200,000 km² (Karoo) to 1,500,000 km² (Siberian Traps). The thickness can vary from 2000 metres (Deccan Traps) to 12,000 m (Lake Superior). These are smaller than the original volumes due to erosion.


Flood basalts have tholeiite and olivine compositions (according to the classification of Yoder and Tilley). The composition of the basalts from the Paraná is fairly typical of that of flood basalts; it contains phenocrysts occupying around 25% of the volume of rock in a fine-grained matrix. These phenocrysts are pyroxenes (augite and pigeonite), plagioclases, opaque crystals such as titanium rich magnetite or ilmenite, and occasionally some olivine. Sometimes more differentiated volcanic products such as andesites, dacites and rhyodacites have been observed, but only in small quantities at the top of former magma chambers.

Moses Coulee in the US showing multiple flood basalt flows of the Columbia River Basalt Group. The upper basalt is Roza Member, while the lower canyon exposes Frenchmen Springs Member basalt



Subaerial flood basalts can be of two kinds:

  • with a smooth or twisted surface : very compact surface; vesicles (gas bubbles) are rare. Degassing was easy (magma maintained at a high temperature and more fluid in a chamber of a size such that confining pressures did not confine gases to the melt before expulsion). Such lava flows may form underground rivers; when degassing fractures and conduits are present, very large flows may reach the surface.
  • with a chaotic surface : the basalt flood is very rich in bubbles of gas, with an irregular, fragmental surface. Degassing was difficult (less fluid magma expelled from a rift with no chance of progressive expansion in a hot chamber; the degassing took place closer to the surface where the flow forms a crust which cracks under the pressure of the gases in the flow itself and during more rapid cooling).

In the Massif Central in Auvergne, France, there is a good example of chaotic lava flow, produced by eruptions from Puy de la Vache and Puy de Lassolas.


Geochemical analysis of the major oxides reveals a composition close to that of mid-ocean ridge basalts (MORB) but also close to that of ocean island basalts (OIB). These are in fact tholeiites with a silicon dioxide percentage close to 50%.

Two kinds of basaltic flood basalts can be distinguished:

  • those poor in P2O5 and in TiO2, called low phosphorus and titanium
  • those rich in P2O5 and in TiO2, called high phosphorus and titanium

The isotopic ratios 87Sr/86Sr and 206Pb/204Pb are different from that observed in general, which shows that the basalt flood magma was contaminated as it passed through the continental crust. It is this contamination that explains the difference between the two kinds of basalt mentioned above. The low phosphorus and titanium type has an excess of elements from the crust such as potassium and strontium.

The content in incompatible elements of flood basalts is lower than that of ocean island basalts, but higher than that of mid-ocean ridge basalts.

Other occurrences

Basalt floods on the planet Venus are larger than those on Earth.

Source: BBC Earth

Impact Events

Saturday, November 19, 2016

Impact events, proposed as causes of mass extinction, are when the planet is struck by a comet or meteor large enough to create a huge shock wave felt around the globe. Widespread dust and debris rain down, disrupting the climate and causing extinction on a global, rather than local, scale. The demise of the dinosaurs at the end of the Cretaceous has been linked to an impact that left a crater in the seabed off the Yucatan peninsula of Mexico. Impacts have also been blamed for other mass extinctions, but the timing and links between cause and effect for these is still debated by scientists.

Asteroid impact. Illustration of a large asteroid colliding with Earth on the Yucatan Peninsula in Mexico. This impact is believed to have led to the death of the dinosaurs some 65 million years ago. The impact formed the Chicxulub crater, which is around 200 kilometres wide. The impact would have thrown trillions of tons of dust into the atmosphere, cooling the Earth’s climate significantly, which may have been responsible for the mass extinction. A layer of iridium- rich rock, known as the K/T boundary, is thought to be the remnants of the impact debris.


Chicxulub crater

The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named. It was formed by a large asteroid or comet at least 10 kilometres (6 miles) in diameter, the Chicxulub impactor, striking the Earth. The date of the impact coincides precisely with the Cretaceous–Paleogene boundary (K–Pg boundary), around 66 million years ago, and a widely accepted theory is that worldwide climate disruption from the event was the cause of the Cretaceous–Paleogene extinction event, a mass extinction in which 75% of plant and animal species on Earth suddenly became extinct, including the dinosaurs. The crater is more than 180 kilometers (110 miles) in diameter and 20 km (12 mi) in depth, well into the continental crust of the region of about 10–30 km depth. It makes the feature the third of the largest confirmed impact structures on Earth.

Location of Chicxulub crater, Mexico


The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with Alan Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.

Imaging from NASA’s Shuttle Radar Topography Mission STS-99 reveals part of the 180 km (110 mi) diameter ring of the crater. The numerous sinkholes clustered around the trough of the crater suggest a prehistoric oceanic basin in the depression left by the impact.