GSA Connects 2021 in Portland, Oregon

Paper No. 76-16
Presentation Time: 11:45 AM


MORRIS, Dustin and GROTZINGER, John P., Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125

The termination of the Marinoan "Snowball Earth" event constitutes a significant period of climate change in Earth's history. This field study seeks to investigate this termination by characterizing the cap carbonate sequence post-dating the Marinoan glacial deposits exposed in the Naukluft Mountains of Namibia by using structural, sedimentologic, stratigraphic field observations, and chemostratigraphic analyses. Multiple stratigraphic sections and bed/unit tracing allows delineation of stratigraphic stacking patterns and regional facies changes. Immediately overlying the Marinoan glacial diamictite unit are well laminated, fine-grained dolostones. In the more proximal regions of the cap, the laminated dolostone grades vertically into cm-scale stromatolite domes that form decameter-scale buildups, that become strongly elongate upward. More distally the stromatolite facies passes laterally into laminated, fine-grained dolostones. Most distally the cap carbonate appears to pinch out. The cap carbonate facies assemblage grades vertically into laminated dolostone with increasing quantities of siliciclastic mudstone that shows lenses of imbricated, edgewise, intraclast conglomerates associated with cross-stratified fine quartz sandstone. The cap carbonates are overlain by massive mudstone and intraclast conglomerate limestone; regionally the cap facies pass vertically into thick laminated dolostones and intraclast conglomerate dolostone variably mixed with quartz sands. This cap carbonate facies distribution indicates that the more proximal stromatolite-bearing strata were deposited in shallower marine environments, which pass downdip into more distal laminated dolostones, consistent with Precambrian carbonate facies of other ages. Vertical facies trends imply shallowing of depositional environments, coincident with an influx of shallow marine siliciclastics. Together, these conclusions suggest that the Marinoan cap carbonate in this region represents overall marine regression, rather than the glacioeustatic transgression that has been suggested by some studies of Marinoan cap carbonates elsewhere.