Laramide Structural Evolution of the Stillwater Complex Portion of Beartooth Mountains Front, South Central Montana: Integration of Surface and Underground Mapping and Drilling

Geraghty, Ennis P. ¹ (1) Stillwater Mining Company, 59061 Nye, MT

The Stillwater Complex, a late Archean (2.705 Ga) layered, mafic-to-ultramafic intrusive-rock body, provides marvelous marker horizons with which to decipher Laramide (latest Cretaceous to Paleocene) structural relationships within the crystalline Beartooth uplift. The complex is exposed for 48 km (30 mi.) along the northern margin of the uplift. Mining company investigations of the surface and subsurface extent of mineralized horizons (J-M Reef Pt-Pd zone, chromite layers, and Cu-Ni zones) have been integrated with surface and subsurface mapping by government and academic groups. Scaled, regional, south-to-north cross sections display a complex interplay between WNW-trending, south-side-up, reverse and thrust faults (forethrusts that mimic the northeast thrust of the uplift) and WNW-trending, imbricate backthrusts (north side up). The alongstrike disappearance, or lessening in offset, of forethrusts at surface is compensated by increased prevalence and intensity of backthrusts, and vice-versa. The backthrusts and all pre-Laramide structural features have been rotated and tilted northward 25 to 100 degrees. A blind forethrust (WNW-trending, gently to moderately south-dipping) has been intersected in 4 of the deeper, continuously cored drillholes. This thrust and other forethrusts and reverse faults (south side up) have not been rotated from their original formation position. Where offset amounts can be estimated (assumed pure dip slip, unbalanced), shortening is on the order of 6 to 7.5 km (3.9 to 4.7 mi.) on observed faults near the Stillwater Pt-Pd mine.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana