DEVELOPMENT OF THE GREAT UNCONFORMITIES

The Proterozoic-Phanerozoic transition is commonly marked in the geologic record by a substantial time gap across the “Great Unconformity”. Classically, this feature refers to where Cambrian sedimentary units overlie crystalline basement on North America, but it has been globally correlated with unconformities elsewhere. Erosion across this feature has been linked to the Cambrian explosion, Earth’s oxygenation, and the initiation of modern plate tectonics. “The Great Unconformity” has traditionally been referred to in the singular, conjuring up the notion of a synchronous global feature generated by one erosion event, such as Snowball Earth. However, unconformities in different places needn’t have the same history and cause.

Here we summarize work from four locations to document multiple “Great Unconformities”. Contrary to recent assertions that thermochronologic data alone can uniquely resolve the timing of sub-Great Unconformity erosion, we highlight in all examples how holistic integration of thermochronology, geology, and thermal history modeling is required to disentangle exhumation timing. In Colorado, zircon (U-Th)/He data (ZHe) and sandstone injectites support initial exhumation of Precambrian basement to the surface before the 717 Ma Sturtian glaciation. In Grand Canyon, multiple unconformities and spatially variable ZHe data patterns are best explained by multiple erosion events during polyphase Rodinia assembly and breakup. In the central Canadian shield, apatite (U-Th)/He data and the geologic context support several km of exhumation below the Great Unconformity after 650 Ma, later than the western U.S. examples. In the southern Canadian shield, ZHe data and emplacement depths of Ediacaran intrusives require substantial exhumation between 580 and 470 Ma, after Snowball Earth (Peak et al., this meeting). None of our datasets detect substantial erosion during the Snowball glaciations. Rather, the outcomes support multiple Great Unconformities generated by distinct erosion events with differing footprints and likely regionally diachronous tectonic /geodynamic causes.