Abstract:
Cosmogenic 26Al/10Be burial dating and tephrochronology of cave deposits provide minimum estimates for the timing of cave development in the Bighorn Basin of Wyoming. Spence Cave is a linear phreatic passage formed along the fold axis of the Sheep Mountain anticline and subsequently truncated by 119 m of Bighorn River incision. A fine-grained eolian (windblown) sand deposit just inside the entrance yields a 26Al/10Be burial age of 0.31 ± 0.19 million years (Ma). This represents a minimum age for the development of Spence Cave, and provides a maximum incision rate for the Bighorn River of 0.38 ± 0.19 mm/yr. Horsethief Cave is a complex phreatic cave system located 43 km north of Spence Cave on a plateau surface ~340 m above the Bighorn River. Electron microprobe analyses of white, fine-grained sediment in the Powder Mountain section of Horsethief Cave confirm that this deposit is Lava Creek B fallout ash, erupted from the Yellowstone Plateau volcanic field ca. 0.64 Ma. Assuming this as a minimum age for the development of Horsethief Cave, extrapolation of the cave profile gradient westward to the Bighorn River gorge suggests a maximum incision rate of 0.35 ± 0.19 mm/yr. Incision rates from both caves match well, and are broadly similar to other estimates of regional incision, suggesting that they record lowering of the Bighorn Basin during the late Pleistocene. However, we caution that deposition of both the Spence Cave sand and the Horsethief Cave volcanic ash may postdate the actual timing of cave development. Thus, these ages place upper limits on landscape evolution rates in the Bighorn Basin