Triangle Zones in Front of Basement-Cored Uplifts in the Rocky Mountain Region

Charles F. Kluth¹, Edward J. Sterne³, Jason Lillegrave², and Aurthur W. Snoke^2
1Geology and Geological Engineering, Colorado School of MInes, Littleton, CO
²Geology, University of Wyoming, Larimie, WY
³Petrohunt, Denver, CO

Frontal triangle zones, formed by a frontal thrust, a back thrust, linked by a basal detachment are common features at the front of thrust belts around the world. Recent field mapping, seismic and well data have shown that they are also present in some areas at the front of Laramide age basement-involved faults in the Rocky Mountain Foreland. These areas include at least the eastern margin of the Beartooth Mountains, the eastern Absaroka Front, the western flank of Casper Arch, the southern Owl Creek Mountains, the southern Gros Ventre Mountains, the southern margin of the Granite Mountains, and the east flank of Front Range Uplift. These structures are identified by divergence of reflections in seismic data, abrupt decreasing dip and parallel thrust faults that dip in opposite directions, and often have unexpected and complicated fault displacement relationships. The triangle zones for which we have the best data and that form in the basement-cored structures develop early in the structural history of the uplift and basin. They form by wedging of deformed strata into the basin stratigraphy on a low angle thrust that soles beneath the mountain front. This wedging delaminates and elevates the undeformed strata above the wedge. The structure is in places then cut off by the main bounding fault between the uplift and the basin. Dipping strata above the wedge yield hydrocarbons from combination stratigraphic-structural traps, but to date, the deformed wedge in the core of the triangle zones is underexplored.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado