NEWLY DISCOVERED STRATIGRAPHIC RECORD OF A SNOWBALL EARTH DEGLACIATION, CARRINGTON ISLAND, UTAH

A cap carbonate, underlain by a distinctive pebbly quartzite unit, was recently discovered and sampled on Carrington Island, which structurally lies within the Willard thrust sheet of the Sevier belt in northern Utah. Diamictite exposed nearby on Hat Island contains basement-gneiss cobbles and pebbles that sit in a lithic wacke matrix. This diamictite is massive and similar lithologically to diamictite exposed on Fremont Island, and partly similar to diamictite exposed on Antelope Island that lies in the footwall of the Willard thrust and preserves glaciogenic sedimentary structures. The top contact of the diamictite on Hat Island is covered and presumed to underlie the quartzite unit on Carrington Island. The quartzite unit is ~40 m thick and comprises: a lower part with fine- to medium-grained quartzite arranged in m-thick beds with trough crossbeds, barforms, and broad channelforms; and an upper part with quartz pebble conglomerate and massive quartzite containing outsize pebble to rare cobble clasts. The contact with the overlying cap carbonate is sharp and undulose, possibly indicating loading during rapid deposition. The cap carbonate is 6 m thick and comprises pinkish-tan to white, massive to laminated dolostone with isopachous cements between laminae that are similar to sheet cracks seen in other cap carbonates. Orientations of laminations vary from horizontal, to low-angle, to high angles, related to soft-sediment deformation and draping over previously deposited carbonate.

Multiple sections of the cap dolostone were measured and samples were acquired at a 25 cm spacing. Samples from different facies within the quartzite were also collected for detrital zircon U-Pb age analysis. Results of stable carbon and oxygen isotope analyses of dolostone, and detrital zircon analysis of quartzite will be presented. The Carrington Island cap carbonate shares similar and different sedimentary characteristics to other cap carbonates in the region, including caps exposed on Promontory Point and Antelope Island. We hypothesize that this and other cap carbonates exposed across northern Utah to southeast Idaho are Marinoan in age. This is testable to some degree with C-isotope curves and detrital zircon maximum depositional ages.