Triangle Zones from the Front of Basement-Cored Uplifts in the Rocky Mountains: Implications for New Exploration
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 Casper Arch, the Owl Creek Mountians, the Gros Ventre Mountains, the Granite Mountains, the Beartooth Mountains and the Front Range Uplift. These structures are identified by divergence of reflections in seismic data, abrupt decreasing dip and thrust faults that are parallel in map view but 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 sediments into the basin stratigraphy on a low angle thrust that soles beneath the mountain front. This wedging delaminates and uplifts the undeformed sediments above the wedge. The early triangle zone structure is in places then abandoned and cut off by the main bounding fault between the uplift and the basin. The triangle zones are not continuous along the entire mountain front. The development of the early triangle zones can change the predicted history of hydrocarbon system risk factors such as maturation and migration timing, trap development, location, and integrity. Dipping strata above the wedge produce 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 #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009