EarthWord – Tertiary

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Definition:

• The Tertiary is a system of rocks, above the Cretaceous and below the Quaternary, that defines the Tertiary Period of geologic time. The Tertiary Period began about 66 million years ago with a mass extinction that ‘clocked’ the dinosaurs and ended when the ice ages of the Quaternary Period began, about 2.6 million years ago. These dates have been adjusted as science advances or when new evidence is found.

Etymology:

• Tertiary comes from the Latin word “tertiarius,” which actually means “the third.” Tertiary was first used to describe the third of only four age classifications for rocks.

Use/Significance in the Earth Science Community:

• The Tertiary Period began abruptly when a meteorite slammed into the earth, leading to a mass extinction that wiped out about 75 percent of all species on Earth, ending the reptile-dominant Cretaceous Period and Mesozoic Era. This event formed the Cretaceous-Tertiary, or K-T, boundary. While the Tertiary began with a biosphere in ruins, it recovered. Mammals became dominant and humanity evolved to the genus of modern humans. The Tertiary began hot and humid and ended in an ice age.
• The current International Geological Timescale uses the terms Paleogene and Neogene instead of Tertiary. It recognizes three periods of geologic time, including the Quaternary, in the Cenozoic Era.

USGS Use:

• Tertiary rocks have been noted on geologic maps produced throughout the earth science community for more than 100 years. The USGS continues to use the term Tertiary on its maps and reports to avoid confusion and to denote a unique and well defined period of Earth history told in rocks from around the world.
• Arrows on the map mark the K-T boundary, just west of Raton, NM. http://ngmdb.usgs.gov/Prodesc/proddesc_10203.htm
• Tertiary rocks sometimes contain energy resources. The USGS has produced a summary of Tertiary coal resources of the Denver Basin, Colorado. http://pubs.usgs.gov/pp/p1625a/Chapters/SD.pdf.
• Also see a USGS study of the Cretaceous-Tertiary petroleum system in Wyoming and Montana. http://pubs.usgs.gov/dds/dds-069/dds-069-v/REPORTS/69_V_CH_3.pdf

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