GEOCHEMICAL ANALYSES OF SEPTARIAN CONCRETIONS FROM THE RUBY RANCH MEMBER OF THE CEDAR MOUNTAIN FORMATION, GREEN RIVER, UTAH

Septarian concretions were collected from mudstones beneath elevated paleochannels in the Ruby Ranch Member of the Cedar Mountain Formation near Green River, Utah. These concretions display striking variation in appearance, morphology, and fracture mineralization. Geochemical analyses including x-ray diffraction (XRD), C and O stable isotopes, fluid inclusion microthermometry, and SEM-EDS (x-ray spectroscopy) were performed to further investigate these variations and interpret any relation to a large tectonic calcite vein that crosscuts a paleochannel outcrop. Stable isotopic values derived from septarian concretion veins and matrix (concretion body) carbonate display ‰δ¹³C values ranging from -6.24 to -3.50 VPDB and ‰δ¹⁸O values with a greater range from -13.22 to -7.52 VPDB. These values are suggestive of a meteoric fluid source for both matrix and vein precipitation. The fluid inclusion freezing point of 32.1⁰C supports a meteoric source. The tectonic calcite vein also appears to be meteoric, yet its isotopic values of ‰δ¹³C -6.44 and ‰δ¹⁸O -12.45 VPDB indicate a differing fluid source from the septarian concretion veins.

XRD analyses indicate that the varying color in concretion sample matrix (maroon, green, and beige) is not caused by mineralogical differences as micritic calcite, dolomite, and quartz were the only minerals identified. Concretion vein mineralogy was of greater complexity with some maroon concretions bearing multiple generations of calcite, chalcedony, authigenic quartz, and barite. Most beige concretions did not display chalcedony and barite, as their vein width is comparatively smaller indicating fewer fracturing events. Precipitation sequences for beige concretions included a micro-spar calcite rarely followed by chalcedony, and larger spar calcite. Using petrographic microscopy and SEM-EDS elemental mapping, a complete sequence of mineralization was determined for maroon concretion samples. The sequence from first precipitated to last: micro-spar calcite, large spar calcite, barite, chalcedony, followed by macro-crystalline quartz and late calcite. The identification of this sequence provides crucial insight into the dynamic paleohydrology of the Ruby Ranch Member during early diagenesis, which otherwise may not be recognized in sediment cements.