GEOLOGIC MAPPING, PETROGRAPHY AND GEOCHEMISTRY OF AN INTERDUNE CARBONATE LENS IN THE LOWER JURASSIC NAVAJO SANDSTONE, SOUTHEASTERN UTAH

A lacustrine, interdune carbonate lens, up to 3 meters (9.8 ft) thick, crops out for a distance of 2.4 km (1.5 mi) along Indian Creek Canyon (0631097mE; 4199722mN, UTM Zone 12N) in southeastern Utah. Giant stromatolites found near the central part of the paleolake, tepee and boxwork sedimentary structures along the basal contact, and northern and southern paleoshoreline boundaries, are well-exposed in outcrop. The carbonate is fossil-poor but possible Chara species impressions were identified along the basal contact.

Carbonate samples were collected from stratigraphic sections through stromatolite-bearing zones and near the paleoshoreline, as well as along the basal contact. Carbonate samples are mostly dolomite as determined from X-Ray Diffractometry. In thin section most of the carbonate was fine-grained micrite in the 10-60 µm range. Occasional dolomite and zoned dolomite rhombs (20-200 µm) occur adjacent to - and within - pores which are larger than grain-supported spaces (i.e., vugs and higher porosity zones). A small percentage of carbonate in vugs has also been recrystallized to spar. The lack of extensive recrystallization and the preservation of micritic dolomite suggest that the primary precipitate was dolomite. ICP-MS analysis of basal carbonate samples yielded elevated Sr values (119-161 ppm) along with minimal MnO. The percentage of Mn usually increases during secondary alteration and Sr values often decrease. The ICP-MS data adds credence to the suggestion that there has not been wholesale recrystallization and that the dolomite was the likely primary precipitate.

Carbon and Oxygen isotope values from both the stromatolite-bearing and basal contact carbonate produced ¹⁸O/¹⁶O ratios between +1.1 and -13.1‰ VPDB and ¹³C/¹²C ratios between -3.63 and -5.98‰ VPDB. The stable isotope ratios indicate carbonate precipitation in non-marine, fresh-water conditions. Basal contact carbonate showed the greatest depletion in ¹⁸O/¹⁶O and may reflect in influence of meteoric waters associated with groundwater discharge zones; ¹⁸O/¹⁶O ratios on carbonate samples from all other sample locations appear to be more evaporatively-enriched.