ELEMENTAL CHEMOSTRATIGRAPHIC ZONES IN THE MIDDLE JURASSIC GYPSUM SPRING FORMATION, NORTHERN BIGHORN BASIN, WYOMING

Analyses of major and minor trace elements by portable XRF provide an independent tool for stratigraphic correlation in the Middle Jurassic Gypsum Spring Formation of Wyoming. The shale-rich Gypsum Spring Formation represents deposition on the margin of a retroarc foreland basin contemporaneous with the Nevadan Orogeny. The Gypsum Spring Formation is traditionally divided into a basal unit containing gypsum, red shale and siltstone, a middle unit of interbedded carbonates and variagated shales, and an upper unit largely containing red shale and siltstone. The sedimentary succession reflects a change in depositional environments between widespread sabkha, salina and supratidal sediments to shallow marine mixed carbonate and clastic subtidal to intertidal sediments.

Whole-rock elemental data were acquired using Olympus Delta Premium XRF handheld analyzer. In total, 284 samples from nine outcrops were prepared for XRF analysis. The study finds Al, Si, K, Ca, Ti, Fe, V, Cr and Zr particularly useful in subdividing and correlating stratigraphic intervals. XRF analyses indicate ten chemostratigraphic zones in the Middle Jurassic Gypsum Spring Formation, bounded by flooding surfaces. Parasequences stacking patterns form two depositional sequences, observed by mapping elemental and elemental ratios across each chemostratigraphic zone. These interpretations match lithostratigraphic correlations made from outcrop and subsurface GR and wireline logs. Elemental signatures also provide clues to changes in environmental conditions through time during deposition of the Gypsum Spring Formation. Variations in Ti, Si, K, and Al indicate changing terrestrial input through time from a source area to the east and southeast of the study area. Data also suggest paleorelief in the vicinity of Clark, WY.

Defining elemental chronostratigraphic zones provides an additional approach for stratigraphic analysis in the difficult-to-correlate, shale-rich Middle Jurassic strata of Wyoming. Results from this study provide evidence for paleoenvironmental and paleogeographic change over time during the Bathonian and Bajocian in the Western Interior.