DEFINITION OF CRETACEOUS-TERTIARY BOUNDARY

The Cenozoic began after a major extinction at the Cretaceous-

Tertiary boundary, marking the boundary between the

Mesozoic and Cenozoic Eras. This extinction event was probably

caused by a large asteroid impact that hit the Yucatán

Peninsula near Chicxulub at 66 million years ago. Dinosaurs,

ammonites, many marine reptile species, and a large number

of marine invertebrates suddenly died off, and the planet lost

about 26 percent of all biological families and numerous

species. Some organisms were dying off slowly before the

dramatic events at the close of the Cretaceous, but a clear

sharp event occurred at the end of this time of environmental

stress and gradual extinction. Iridium anomalies have been

found along most of the clay layers that mark this boundary,

considered by many to be the “smoking gun” indicating an

impact origin for cause of the extinctions. One-half million

tons of iridium are estimated to be in the Cretaceous-Tertiary

boundary clay, equivalent to the amount that would be contained

in a meteorite with a 6-mile (9.5-km) diameter. Some

scientists have argued that volcanic processes within the

Earth can produce iridium, and an impact is not necessary to

explain the iridium anomaly. However, other rare elements

and geochemical anomalies are present along the Cretaceous-

Tertiary boundary, supporting the idea that a huge meteorite

hit the Earth at this time.

Many features found around and associated with an

impact crater on Mexico’s Yucatán Peninsula suggest that it

is the crater associated with the death of the dinosaurs. The

Chicxulub crater is about 66 million years old and lies halfburied

beneath the waters of the Gulf of Mexico and half

on land. Tsunami deposits of the same age are found in

inland Texas, much of the Gulf of Mexico, and the

Caribbean, recording a huge tsunami perhaps several hundred

feet high that was generated by the impact. The crater

is at the center of a huge field of scattered spherules that

extends across Central America and through the southern

United States. It is a large structure, and is the right age to

be the crater that resulted from the impact at the Cretaceous-

Tertiary boundary, recording the extinction of the

dinosaurs and other families.

The 66-million-year-old Deccan flood basalts, also

known as traps, cover a large part of western India and the

Seychelles. They are associated with the breakup of India

from the Seychelles during the opening of the Indian Ocean.

Slightly older flood basalts (90–83 million years old) are

associated with the breaking away of Madagascar from

India. The volume of the Deccan traps is estimated at 5 million

cubic miles (20,841,000 km3), and the volcanics are

thought to have been erupted within about 1 million years,

starting slightly before the great Cretaceous-Tertiary extinction.

Most workers now agree that the gases released during

the flood basalt volcanism stressed the global biosphere to

such an extent that many marine organisms had gone

extinct, and many others were stressed. Then the planet was

hit by the massive Chicxulub impactor, causing the massive

extinction including the end of the dinosaurs. Faunal extinctions

have been correlated with the eruption of the Deccan

flood basalts at the Cretaceous-Tertiary (K-T) boundary.

There is still considerable debate about the relative significance

of flood basalt volcanism and impacts of meteorites

for the Cretaceous-Tertiary boundary. However, most scientists

would now agree that the global environment was

stressed shortly before the K-T boundary by volcanicinduced

climate change, and then a huge meteorite hit the

Yucatán Peninsula, forming the Chicxulub impact crater,

causing the massive K-T boundary extinction and the death

of the dinosaurs.

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