Chronology, sources and dispersal of Middle Jurassic -- early Late Cretaceous tephra in an evolving foreland basin, north central Wyoming.

摘要

This study presents the first extensive tephrochronology of the Middle Jurassic -- lower Upper Cretaceous strata in the Bighorn Basin, Wyoming. These new data provide a reference for stratigraphic correlation, and assessment of the tectonic evolution of a portion of the Western Interior Basin.;Zircon U-Pb and biotite K-Ar ages were determined for 13 altered tephra and bentonitic sandstone beds. Ages of tephra with relatively greater stratigraphic significance are: 1) basal bentonite at Lower Sundance Formation, 166 Ma; 2) basal bentonite at Upper Sundance Formation, 160 Ma; 3) devitrified tuff at middle Cloverly Formation, 111 Ma; 4) bentonite at top Thermopolis Shale and basal Muddy Sandstone, 100 Ma; 5) basal bentonite at Shell Creek Shale, 98 Ma; 6) bentonite at top Mowry Shale, 96 Ma. Detrital zircon ages were obtained from bentonitic sandstones at basal Morrison Formation, with a range from 151 to 2800 Ma.;Zircon morphology analysis was conducted to determine potential magma source characteristics. Most tephra beds are from calc-alkaline granitoid sources that are hybrid continental crustal and mantle origin. The devitrified tuff from the Cloverly Formation is from apparently intrusive aluminous granites influenced by a significant continental crust component. One of the bentonite beds from top Mowry is from alkaline granites with a significant mantle input.;The tephra occurrences of the Bighorn Basin correlate well with known magmatic activity of the Cordillera arc at that time, as well as with similar-aged tephra occurrences from the Colorado Plateau to southern Canada. The major tephra source was the Sierra Nevada during Jurassic time, but switched to the Idaho Batholith in Late Cretaceous. The devitrified tuff in the Cloverly Formation appears to be a more local deposit, suggesting volcanic activity from unknown sources in relatively close proximity.;Detrital zircon provenance analysis for the Morrison Formation indicates that certain clastics of the Morrison of the Bighorn Basin might have been sourced from areas as far west as the Blue Mountains terranes of eastern Oregon. However, local highlands, possibly nascent Sevier thrust sheets, blocked most of these sediments delivered from the Cordillera arc to Colorado Plateau and northwestern Montana.