CHRONOLOGY, SOURCES AND TRANSPORT MECHANISM OF MEDIAL JURASSIC - EARLY CRETACEOUS TEPHRA DELIVERED TO AN EVOLVING RETROARC FORELAND BASIN, NORTH CENTRAL WYOMING

The development of the Western Interior foreland basin from mid-Jurassic to early Cretaceous was associated with a complex record of activity in the adjacent Cordillera magmatic arc. Resultant distally deposited tephra are now preserved as tuffaceous or bentonitic sequences throughout much of the basin and are exceptionally exposed today in the Bighorn Basin of Wyoming. Altered tephra in the Sundance, Morrison, Cloverly, Thermopolis, Muddy, Shell Creek, and Mowry formations have been investigated in this study. Petrographic analysis, including a characterization of zircon morphology, as well as K-Ar (biotite) and U-Pb (zircon) dating were carried out to establish a distinctive mineralogical and geochemical signature for representative tephra occurrences.

Typical tephra beds range in thickness from a few centimeters to more than 1 meter, with the frequency of occurrence increasing, from an early appearance in the Early Bajocian, to the limits of this study in the Albian, indicative of increasing volcanic intensity in source areas. The overall composition of the original tephra varies from dacitic to rhyolitic. Typical micro-phenocrysts include sanidine, plagioclase, quartz, biotite, hornblende, pyroxene, apatite, and zircon. Zircons can be classified into two morphologic groups: the prevalent S₁₃ types with well developed {101}{211}{110}{100} faces, and the P₄ types with well developed{101} {100} faces. Typologic evolution trend falls within a calc-alkaline granitoid source terrane with basic xenoliths.

The tephra of north central Wyoming can be correlated with similar occurrences in Utah, Idaho and Colorado. Most appear to represent plinian eruptive products sourced from localities within the present Sierra Nevada of California, Idaho batholith, with several tephra suggestive of Arizona sources. These tephra were erupted into upper troposphere /lower stratosphere, and carried by the southwesterly jet stream before falling over the Sundance/Western Interior Seaway. This results in a northeast-trending taper to the airfall distribution throughout the basin. An unusual green lapilli tuff in the Morrison Formation, and an extensive white siliceous tuff in the Cloverly Formation appear to be local deposits, suggesting volcanic activity from unknown sources in relatively close proximity.