THE PALEOCENE-EOCENE THERMAL MAXIMUM ENHANCED EROSION OF LARAMIDE RANGES

The Paleocene-Eocene Thermal Maximum (PETM) at ca. 56 was a pronounced period of global warming and has been linked to increased erosion in the intermountain west. Temporally, the PETM overlapped with the Laramide orogeny in North America. The Laramide orogeny was a formative orogenic event in the development of the North American Cordillera. Typified by intraforeland basement uplifts and basins, the Laramide orogeny is hypothesized to have occurred in response to increased coupling of the upper North American plate with the down-going Farallon plate. This period of enhanced deformation had both a broad physiographic and temporal fingerprint across the intermountain west, extending from Mexico in the south to Montana in the North and initiated in the Late Cretaceous and lasted until the Paleogene. Although tectonic processes associated with the evolution of the slab have played a key role on the erosional and depositional history of the Laramide, extreme climate changes, such as the PETM, have likely impacted the erosional record. Published low-temperature thermochronology data from Laramide structures across the western USA yield a bimodal distribution of exhumation ages with an initial peak at ~ 80 Ma and a secondary peak at ~55 Ma. When considering the timing and distribution of exhumation it is important to take into account mechanisms for erosion as well as rock-uplift. We present apatite fission-track analysis from four detrital samples which straddle a key PETM locality (two above and two below) in the Big Horn Basin, Wyoming. Below the PETM, we find that samples record two Cretaceous single-age peaks, one at 143 ± 11 Ma and another at 95 ± 3 Ma. Above the PETM samples record a Late Cretaceous age of 76 ± 4 Ma and a Paleocene-Eocene age of 57 ± 3 Ma. The near-zero lag time grains above the onset of the PETM supports rapid exhumation and subsequent deposition. Here, we suggest that the PETM played a major role in driving Laramide exhumation. Increased erosion facilitated by the warming during the PETM removed overburden produced during the initial Laramide deformation (~80 Ma) and caused major unroofing at ca. 56 Ma.