The Absaroka Sequence: Lower Pennsylvanian—Lower Jurassic

Abstract

The Absaroka Sequence was deposited through the longest of Phanerozoic geological intervals to be considered in this book. It took more than three geological periods (Pennsylvanian through Early Jurassic time) for the complete sequence to be laid down and, during these roughly 145 Myr., some fundamental changes occurred on Earth and on the continent. Among these changes were (1) assembly of Pangaea II, completed by the Permian Period, and the initial breakup of the global landmass during Triassic—Jurassic time; (2) the first sustained episode of nonvolcanic tectonic activity within the western craton; (3) the most prolonged interval of subaerial exposure of the craton in Phanerozoic time; (4) uniquely symmetrical and pervasive deposition of coal-bearing cyclical deposits during the Pennsylvanian and Permian Periods (also occurring on other continents); (5) appearance by latest Absarokan time of all known phyla of animals including all classes of vertebrates; (6) along with the above, appearance of a uniquely diverse arborescent flora and the first diverse terrestrial fauna; and (7) the most pervasive mass extinction event in the history of the marine biota, from which, it is claimed by some authors, less than 5% of all species of marine animals may have escaped!