DETRITAL ZIRCON GEOCHRONOLOGY AND C-ISOTOPE STRATIGRAPHY OF THE MARINOAN(?) CAP DOLOSTONE AND ASSOCIATED CLASTICS, GREAT SALT LAKE, UT

Cryogenian strata on islands and the north shore of Great Salt Lake, Utah, provide information about Snowball Earth, particularly a Marinoan(?) cap dolostone and quartzite interval that overlies polymict diamictite. The most recently discovered exposure of cap dolostone (6.9 m thick) resides on Carrington Island. The cap is underlain by a distinctive pebbly quartzite (40 m thick). Diamictite exposed nearby on Hat Island is interpreted to underlie the quartzite unit, similar to relations of diamictite exposed on Fremont Island, overlain by pebbly quartzite and a cap dolostone on Promontory Point, and to relations of diamictite with glaciogenic features and a cap carbonate exposed on Antelope Island.

Detrital zircon (DZ) analyses (LA-ICPMS) were conducted on a sample of diamictite from Hat Island and three quartzite samples from Carrington Island. The lowermost quartzite sample is coarse-grained, moderately sorted and contains trough crossbeds and broad channelforms. The middle quartzite sample is similar but better sorted and finer grained. The uppermost quartzite is medium-grained with rare pebbles, soft sediment deformation, and is sharply overlain by a cap dolostone. DZ spectra display two main age distributions. The diamictite and lower quartzite have large age peaks at 2.45 and 2.5-2.6 Ga. The diamictite also has a few grains between 1.7 and 2.0 Ga and a young population of ca. 680-692 Ma (n=5 grains). The upper two quartzite samples show multimodal peaks at 1.07, 1.15, 1.3, and 1.45 Ga and minor Paleoproterozoic and Archean grains. The diamictite and lower quartzite age distributions are similar to those from polymict diamictite on Antelope Island and Fremont Island that likely record nearby basement sources. The multimodal age distribution of the upper two quartzite samples may reflect mixing from distal basement sources and reworking of lower clastic strata during rapid deglaciation, isostatic rebound, resuming of the hydrologic cycle, and transgression.

A 6.9 m section of the cap dolostone was measured and sampled at decimeter-scale spacing (n=2.8). Carbon isotope values range from -2.57 to -3.53 per mil and Oxygen isotope values range from -8.88 to -12.54 per mil. These values are similar to those from the cap dolostone on Antelope Island and Promontory Point and may reflect a primary Marinoan signature.