Depositional environments of the Middle Jurassic lower Sundance Formation, Bighorn Basin, Wyoming

Contrary to historical interpretations, the Middle Jurassic lower Sundance Formation in the Bighorn Basin of north---central Wyoming was not the product of solely an open marine environment. This work integrated the influences of tectonics, paleogeography, paleoclimate, rapid sea level fluctuations, and periods of erosion. Nine distinct and laterally extensive facies were recognized through fieldwork and petrography, including shelf, nearshore, lagoon, reworked lagoon, shallow intertidal, tidal flat, supratidal sabkha, eolian, and reworked eolian environments.; Three major results of the study are: (1) Petrologic evidence implies that distinctive large cross-stratified sets of ooids deposited during the late Callovian are of eolian origin caused by the deflation of the emergent ooid shoals after eustatic sea level fall. Primary indications for an eolian origin of the cross-strata are climbing translatent stratification produced by migrating wind ripples, pinstripe lamination in the cross-strata, coarsening-upward sequences, and intergranular micrite of vadose origin. The oolitic dunes are the first Mesozoic eolian carbonates ever documented in the Western Interior. (2) Measurements of finely laminated strata of a prevalent lagoonal facies appear to carry an overprint of lamination thickness variations due to climate fluctuations influenced by the 11 - year sunspot cycle. (3) Integrating high spectral resolution remote sensing data from the ASTER (Advanced Spaceborne Thermal Emission Reflection Radiometer) instrument with high spatial resolution NAPP (National Air Photography Program) images allowed the location of small (10m x 10m) but important outcrops of predictable lithology in advance of fieldwork.; Although sea level was generally retreating during the Callovian, frequent small-scale transgressions and regressions of the Sundance Sea contributed to the depositional environments of the diverse facies. The Callovian's and climate affected deposition of subaerial and lagoonal facies. Active tectonics to the south and reactivation of Precambrian basement lineaments shaped the paleogeography and exposed new source areas for redistribution of sediments. The interplay of these factors contributed to the varied depositional environments of the lower Sundance Formation.