Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion

Changes in ocean chemistry promoted during the formation of the Great Unconformity, a stratigraphic surface that separates continental basement rock from younger marine sedimentary deposits, are proposed as the cause of the Cambrian explosion of marine animals. A life-changing geological event The 'Great Unconformity' is a worldwide stratigraphic feature marking a divide between continental crystalline basement rock and younger shallow marine sedimentary deposits. Occasionally — in the Grand Canyon, for example — it is exposed on Earth's surface to dramatic effect. Geologists have been debating the origins and the global impact of the Great Unconformity ever since the term was coined in 1869. Shanan Peters and Robert Gaines now present a new analysis of stratigraphic and lithologic data from 830 locations in North America, together with petrologic and geochemical data. They find evidence that the formation of the Great Unconformity caused enhanced continental weathering and increased oceanic alkalinity and ionic strength in expanding shallow seas, which in turn triggered biomineralization and the Cambrian explosion of marine animals. The transition between the Proterozoic and Phanerozoic eons, beginning 542 million years (Myr) ago, is distinguished by the diversification of multicellular animals and by their acquisition of mineralized skeletons during the Cambrian period 1 . Considerable progress has been made in documenting and more precisely correlating biotic patterns in the Neoproterozoic–Cambrian fossil record with geochemical and physical environmental perturbations 2 , 3 , 4 , 5 , but the mechanisms responsible for those perturbations remain uncertain 1 , 2 . Here we use new stratigraphic and geochemical data to show that early Palaeozoic marine sediments deposited approximately 540–480 Myr ago record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust. These geochemical conditions were caused by a protracted period of widespread continental denudation during the Neoproterozoic followed by extensive physical reworking of soil, regolith and basement rock during the first continental-scale marine transgression of the Phanerozoic. The resultant globally occurring stratigraphic surface, which in most regions separates continental crystalline basement rock from much younger Cambrian shallow ma