LAUSR

Abstract The Ediacaran–Cambrian transition was a time of marked biological and sedimentary changes, representing a significant evolutionary breakthrough in Earth’s history. Numerous explanations have been posited for these dramatic changes. One view emphasizes an extensive period of widespread continental denudation during the Neoproterozoic followed by extensive reworking of the basement during the early Cambrian, resulting in the formation of what is known as the “Great Unconformity”. Geologic events leading to the formation of this unconformity have been hypothesized as an environmental trigger for the Cambrian explosion. Here we review seventeen key Ediacaran-Cambrian successions around the globe with a focus on their sedimentary facies and sequence-stratigraphic architecture. Based on this information, we explore the relationship of the Ediacaran–Cambrian boundary to this unconformity, underscoring a more nuanced scenario. We consider each section in a chemostratigraphic and chronostratigraphic framework, and consider the hypothesized origin of this unconformity at each locale—for example, whether it was eustatic in origin and therefore of global stratigraphic significance or whether it resulted from local tectonics, implying marked inter-basin diachronism. Our systematic review shows that significant diachronism was involved in the generation of the Great Unconformity, suggesting that the sea-level fall reflects overprint of local tectonics on pure eustasy. In most places, the Great Unconformity is actually a composite sequence boundary resulting from successive eustatic episodes of sea-level rise and fall overprinted by multiple tectonic events of subsidence and uplift. Thus this surface should not be used for inter-basin correlations. Although the general proximity and common co-occurrence of the Ediacaran–Cambrian boundary with a sequence boundary (SB) in places resulting in fluvial valley incision, has proved challenging for biostratigraphic sampling, by mapping this unconformity and its associated facies, there are new opportunities. In particular, paleontological work in terminal Ediacaran deposits located in interfluve positions may be promising because these areas may have preserved thick shallow-marine strata below the SB. Such strata could hold additional clues to help fill in gaps in our understanding of the Ediacaran roots of the Cambrian explosion.