A GREAT DIVIDE: THE PHYSICAL STRATIGRAPHIC RECORD OF EARTH SYSTEMS CHANGES AT THE DAWN OF ANIMALS

The Neoproterozoic-Cambrian transition established a new Phanerozoic mode in the Earth-life system. Most studies addressing this transition have rightly focused on paleontological and geochemical proxy data analyzed in a chronostratigraphic context. However, even in the absence of paleontological and biogeochemical proxy records, the Neoproterozoic-Cambrian stands out as the single biggest transition in the physical stratigraphic record. In most regions of Laurentia, the transition is marked by the Great Unconformity, a geomorphologic surface unique in its areal extent, that documents the physical and chemical weathering of crystalline igneous and metamorphic rocks followed by transgressive marine shoreface erosion and sedimentation during the Cambrian. Continental flooding/exposure cycles of similar duration and magnitude to the Cambrian Sauk sequence recurred throughout the Phanerozoic. Thus, the most prominent feature of the Great Unconformity is the nonconformity that juxtaposes Cambrian marine sediments and underlying crystalline basement rocks. Nevertheless, there remain regions with comfortable boundary-spanning sedimentary successions and a significant volume of Precambrian sedimentary rock remains on the continent. Here, we present results from the Macrostrat database that cover the past 3000 Myr in Laurentia, augmented by globally distributed geological map-based data. These results reinforce the marked and relatively abrupt increase in the extent of epicontinental sedimentation that occurred during the final stages in the formation of the Great Unconformity. This boundary, and to a lesser extent the Archean/Proterozoic boundary, divides the sedimentary record into three distinct modes that correspond in time to the three eons. Within each eon there is little or no long-term trend in preserved sediment quantity. Instead, variability principally reflects supercontinent coalescence and breakup cycles. Although the mechanism driving apparent step-wise shifts in the extent and persistence of sedimentation on the continents remains unknown, changes in the nature of continental weathering and rates of long-term sediment cycling on the continents likely played an important role in shaping the Earth-life system at the dawn of animals.