GEOLOGIC MAPPING, PETROGRAPHY
AND GEOCHEMISTRY OF AN
INTERDUNE CARBONATE LENS IN THE
LOWER JURASSIC NAVAJO
SANDSTONE, SOUTHEASTERN UTAH
Hencmann, Devin and Kenny, Ray
Geosciences Department
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 (119161 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.