The Specials of June. The Paleozoic Era(The Cambrian Period)The Cambrian environment

Several regions of Cambrian volcanism have been identified. Australia was especially active; large areas in the northern and central regions were covered by flood basalts during the early parts of the Cambrian with residual activity extending into the middle of the period. Basalts and mafic intrusives (molten rock derived from basic magmas) in southeastern Australia formed in a volcanic island arc setting during the early and middle parts of the Cambrian. Volcanic suites of similar age are also present in New Zealand and in parts of Antarctica (northern Victoria Land, Ellsworth Mountains, and Pensacola Mountains). Other significant Cambrian volcanic deposits occurring during the early and middle parts of the period are found in southern Siberia and western Mongolia (Altai and Sayan mountains), eastern Kazakhstan and northwestern China (Tian Shan), and northeastern China. Cambrian volcanics are scattered along the easternmost margin of the United States, but most are probably island arc deposits that were accreted to Laurentia after the Cambrian. In the southern United States (Oklahoma), granitic intrusives and basaltic and rhyolitic extrusives are associated with a large tectonic trough that was formed by rifting, or crustal extension, throughout the first half of the period.



Minor volcanic deposits, mainly ash beds and thin flows, are widely known. In general, these have received little study, but some are suitable for determination of isotopic ages. Zircons from a lower Lower Cambrian (pretrilobite) volcanic ash bed in New Brunswick, Can., have a uranium-lead age of 531 million years. Volcanic tuffs near inferred Tommotian-Atdabanian boundaries (Russian designations for the pretrilobite and trilobite portions of the Cambrian explosion) in both Morocco and southwestern China have yielded similar dates of 521 million years.