Abstract

The Squaw Peaks are two isolated, high-standing landforms in the south-central Big Horn Basin about 20 km ESE of Meeteetse, WY. The lower portion of each peak is composed of flat-lying Eocene Tatman Formation lacustrine and fluvial clastic sediment (~300 m) and the peaks are capped by allochthonous, steeply dipping Eocene volcanic and volcaniclastic rocks. U/Pb dating methods on detrital zircons found in both the Tatman (77 zircons analyzed) and the overlying allochthonous volcanic rocks (49 zircons analyzed) further constrain the depositional ages, provenance, and genesis of each of these rock units. The TuffZirc depositional age of the Tatman Formation is 50.84 +1.29/−0.78 Ma, which makes it Bridgerian in age, and is correlative to the Aycross Formation. The detrital zircon age spectrum of the Tatman is dominated by Cretaceous and Archean ages, and ultimate sediment source areas for the Tatman Formation is interpreted to be the Laramide Beartooth Uplift of Montana to the north and the Sevier Highlands of central Idaho to the northwest. The TuffZirc maximum depositional age of allochthonous volcanic rocks is 50.53 + 1.59/− 0.78 Ma, which makes it part of the Cathedral Cliffs or Lamar River Formation from the northern Absaroka Range rather than the more proximal but younger Teepee Trails or Wiggins formations rocks of similar lithology. The detrital zircon’s age spectrum for these volcanic rocks also is diverse where 70% of the zircons are older than Eocene in age. Mesozoic, Proterozoic, and Archean zircons within these allochthonous rocks suggest that the Sevier Highlands and Beartooth Uplift remained important sediment sources during the early episodes of Absaroka volcanism. As no vent-facies rocks of this age occur in the southern Absaroka Range, the 50.53 Ma age of the allochthonous volcanic rocks also indicates that these rocks were not likely emplaced as part of the late Cenozoic Enos Creek-Owl Creek debris-avalanche deposit. These volcanic rocks were more likely part of the 48.87 Ma massive Heart Mountain slide. The Squaw Peaks outliers extend the domain of the Heart Mountain slide to more than 5000 km² and increases minimum transport distances of the slide mass to more than 85 km.