CHEMOSTRATIGRAPHY OF A MIXED CARBONATE-EVAPORITE-CLASTIC SYSTEM: ELEMENTAL AND MINERALOGICAL PATTERNS IN THE MIDDLE JURASSIC GYPSUM SPRING FORMATION, BIGHORN BASIN, WYOMING

Ongoing X-Ray diffraction (XRD) and X-Ray fluorescence (XRF) analyses improve upon existing lithostratigraphic and biostratigraphic correlations of the Middle Jurassic Gypsum Spring Formation in the Bighorn Basin of Wyoming. Examination of elemental and mineralogical components provides additional insight in this traditionally difficult to correlate, mudstone-rich unit.

The Gypsum Spring Formation of the Middle Jurassic is divided into three lithstratigraphic units: (1) a lower unit of gypsum, shale and siltstone, (2) a middle unit of interbedded carbonates and variegated shales, and (3) an upper unit comprised mainly of shales and siltstones. These units were deposited in restricted marine and marginal-marine environments along a cratonic forebulge in a retro-arc foreland basin related with the Nevadan Orogeny.

XRF analyses of 284 samples from nine outcrops recognizes significant elemental changes through time involving the elements Al, Si, K, CA, Ti, FE, V, CR and Zr. These patterns outline ten chemostratigraphic zones displaying two distinct depositional sequences. XRD analyses build upon these elemental data to produce a “mineral stratigraphy”. Mineralogical composition aids in the identification of depositional cycles and diagenetic processes within the mudstones of the Gypsum Spring Formation. The XRD analyses also indicate volcanic mineral components in particular sections of the Gypsum Spring, which, upon further examination, may aid stratigraphic correlations in the Middle Jurassic in Wyoming.