Polecat Bench (44.76889N, 108.88833W)
Polecat Bench is the best documented continental record of a hyperthermal known in the world. It is the site where the oldest North American Artiodactyla, Perissodactyla, and true Primates defining the Paleocene-Eocene boundary were first found, the site where conspicuous dwarfing of mammals was first documented, the site where the PETM carbon isotope anomaly was first found on land, the site where the first high-resolution continental CIE sections were studied, and the site where the CIE in organic carbon was first compared to that in paleosol carbonate. Paleosols here have been studied in detail.
Superposed wash sites show that onset of the PETM is found in a Clarkforkian mammalian faunal interval several meters below the first Wasatchian Wa-M and Wa-0 faunas. In recent years, Polecat Bench is the site where the first attempt was made to recognize orbital cyclicity in a continental Paleocene-Eocene section. This was used to infer that the main body of the CIE spans ~5.5 precession cycles, or ~115 k.y., and the recovery tail of the CIE spans about 2 precession cycles, or ~42 k.y. (total ca. 157 k.y.).
The south end of Polecat Bench, where all of the above studies were carried out, is an ideal target for coring because:
- the PETM-CIE interval of interest is exposed in outcrop on three sides (north, west, and south);
- key strata have been mapped in detail;
- 1500 identifiable mammalian fossils representing four successive faunas spanning the PETM are known from this small area;
- the PETM-CIE interval of interest occurs in an easily reachable interval between about 70 and 130 meters below the surface of the bench top; and
- the bench top is flat and easily accessible for coring equipment.
We expect that many of the same marker beds mapped on the surface will be recognizable in the core section, permitting new information about the pre-PETM, PETM, and post-PETM conditions in this core to be related to mammalian faunal change documented in surface localities.
From July 18 - 25, 2011 two overlapping cores were successfully drilled at Polecat Bench. One core (BBCP-PCB11-2A) is 130.0 meters and the second core (BBCP-PCB11-2B) is 245.1 meters long. The first core was drilled using only municipal water and the second was drilled with bentonite and polymer additives to maximize hole depth and recovery. Downhole geophysical logs (magnetic susceptibility, resistivity, density etc) were recovered for the entire 245 meters of the second hole.
Drill site on top of Polecat Bench showing partially covered strata being targeted by the coring (photo by S. Wing).