REFINING INTRA- AND INTER-BASINAL CHRONOSTRATIGRAPHIC CORRELATIONS OF PALEOCENE MAMMAL-BEARING LOCALITIES IN THE CRAZY MOUNTAINS BASIN, MONTANA: INITIAL PALEOMAGNETIC RESULTS

The sediments of the Bear, Lebo and Melville formations (Fort Union Group) of the eastern Crazy Mountains Basin (CMB) represent one of the thickest (>1500m) fossiliferous, Paleocene-aged sections in the world. They provide important localities representing Torrejonian (To)2 [Gidley Quarry (Q)], Tiffanian (Ti)1 (Douglass Q), Ti2 (Scarritt Q) and Ti3 (Locality 13) land mammal age zones. The magnetic polarity of sediments from these localities has previously been determined, allowing for more refined correlation with basins outside of the eastern CMB. Furthermore, a number of new fossiliferous localities spanning the To-Ti boundary have been discovered in the eastern CMB in the last 20 years.

Only three fossiliferous sites in the western CMB have been referenced in published works. However, the exquisitely preserved, articulated and associated skeletons at one of these sites [the Bangtail (BT) site] have provided data with major implications for both mammalian and avian paleontology. Although BT appears to represent early Ti, correlation with eastern CMB localities is vague because of the transitional nature of faunas from this time, and a lack of magnetostratigraphic data from the western CMB. Furthermore, correlations at the To-Ti boundary in other basins have remained vague for the same reasons. Fortunately, the low frequency of geomagnetic reversals through this interval provides potential for refinement in chronostratigraphic correlation.

The To-Ti boundary is currently thought to correspond to the C27n-C26r boundary. We sampled siltstones, sandstones, limestones and mudstones from BT in the western CMB and Bingo Q in the eastern CMB. Like BT, Bingo Q has a fauna that suggests an early Ti age, and further, Bingo Q is lithostratigraphically intermediate between To2 and Ti1 sites. Paleomagnetic data had the potential to refute the hypothesis that the faunas from BT and Bingo Q are of equivalent age. However, sediments at both BT and Bingo Q show dominantly reverse polarity, thus placing them in C26r. Further sampling down section from these sites will eventually lead to documentation of the C27n-C26r boundary, which will serve as a tie point between the two sub-basins and to other basins in the Western Interior yielding Paleocene mammals.