Low-temperature Thermochronology of the Beartooth Conglomerate

Abstract

During the Laramide Orogeny, the crystalline core of the Beartooth Plateau in northwest Wyoming and south-central Montana was thrust over Mesozoic and Paleozoic rock in the adjacent Big Horn Basin. Omar et al. (1994) reported apatite fission-track data of 30 samples from ~4 km of vertical section through the Red Lodge corner of the Beartooth overthrust. These authors presented two hypotheses to explain these data, both of which describe rapid uplift in the Paleocene with a second uplift event during the late Miocene or early Pliocene. The period between these two uplift events was characterized by Oligocene and Miocene sedimentation (Hypothesis A), or by tectonic quiescence (Hypothesis B). In an attempt to test the hypotheses of Omar et al. (1994), apatites and zircons from Precambrian crystalline clasts within the synorogenic Beartooth Conglomerate were analyzed by (U-Th)/He methods. In all, 25 apatite and 15 zircon aliquots from 5 basement clasts were measured. Two clasts presented average zircon ages older than average apatite ages, which I interpret to be the result of natural radiation damage (raising the closure temperature of apatite and lowering the closure temperature of zircon), the possible presence of zircons with uranium-rich rims, or zircon inclusions within apatite grains. Two apatite aliquots gave ages younger than the depositional age of the conglomerate (~55 Ma), suggesting low helium closure temperatures or an incorrect assessment of the age of deposition. All remaining apatite apparent ages (~60 Ma to ~190 Ma) and zircon apparent ages (~100 Ma to ~800 Ma) suggest a period throughout the Phanerozoic characterized by slow burial since Cambrian exposure, which continued until rapid uplift initiation of the Beartooth Range (~60 Ma).