Black shales and other sediments with high organic matter contents in Phanerozoic climatic cycles: Communication 1. Black shales at the Gondwanan stage of biosphere evolution

摘要

The terminal Neoproterozoic was marked by the formation of a supercontinent, which was located during a long period at high latitudes of the Southern Hemisphere. The supercontinent named Gondwana united present-day Africa, Australia, Antarctica, South America, Arabia, Hindustan, and several other small continental blocks. It was separated from other, smaller continental blocks by ocean basins with the thinned continental and oceanic crust with the Panthalassa Ocean being largest among them. The Gondwanan stage, which comprised the Vendian and significant part of the Paleozoic, was one of the longest stages defined in the Neoproterozoic and Phanerozoic geological history lasting over 300-330 Ma. During this stage, the continental sector of the Earth's crust included the Laurentia, Baltica, Siberia, Kazakhstan, and North China (Tarim) blocks, in addition to Gondwana, which were largely located at low latitudes of the Northern Hemisphere. In the Early Paleozoic, main tectonic events were related to the breakup in peripheral parts of Gondwana with the separation of small continental massifs, which moved in the northerly direction. Their collision with other continents was accompanied by the folding and formation of mountainous structures with corresponding foredeeps. For example, the Taconian phase of fold-thrust deformations resulted from the convergence of Laurentia with the Avalon block, which was separated from the northern margin of Gondwana in the Late Cambrian or Early Ordovician. The movement of Gondwana toward the equator and its amalgamation with other continents and smaller continental blocks in the terminal Cambrian resulted in the formation of the Pangea supercontinent, whose location near the equator for more than 150 Ma up to its disintegration at the terminal Jurassic-initial Cretaceous transition was responsible for significant global changes in the floral and faunal communities. Paleozoic epochs of the relatively stable warm climate alternated with shorter periods, which were characterized by global cooling due to expansion of the ice sheet in Gondwana. These periods are reflected in frequent migrations of climatic belts and rapid sea level oscillations, which were accompanied by changes in boundaries of facies zones and extinctions of biotic communities. Distribution patterns of black shales and other organic-rich sediments in sections imply cyclic climatic variations during the entire Paleozoic. Duration of individual cycles, which included a long phase of the dominant warm or even hot climate and shorter cooling phase crowning the cycle, was as long as 47 Ma. Two such cycles terminated with glaciations; others, with a less significant growth of the Gondwanan ice sheet.