STRUCTURAL CONTROLS on SUBSURFACE FLUID MIGRATION THROUGH THRUST SHEETS OF THE STEWART PEAK CULMINATION, NORTHERN SALT RIVER RANGE, WYOMING

The Stewart Peak Culmination (SPC) is an exposed duplex thrust zone in the Northern Salt River Range, WY that may have been a large structural trap for hydrocarbons and CO2 prior to Neogene Basin-and-Range extension. The SPC lies east and structurally up-dip of known subsurface accumulations of natural CO2 and methane in SE Idaho that are associated with unusually high down-hole temperatures. The SPC exposes analogous traps, reservoir rocks and seal rocks, thus allowing for outcrop studies of fluid migration pathways and relative timing of faulting, fracturing and structurally controlled diagenesis.

The SPC exposes highly deformed Paleozoic-Mesozoic rocks in the Absaroka thrust sheet and its associated imbricate thrusts. Faulting has led to extensive fracturing and brecciation. Brecciated fault zones are up to 4 m thick and highly fractured damage zones extend for 10s of meters into the hanging walls of large-displacement thrust faults. Fracturing facilitated fluid flow throughout the allochthon, locally enhancing porosity and permeability. Fractures often contain multiple generations of calcite and dolomite cement indicating a complex diagenetic history. For example, sub-vertical fracture swarms dissect fault breccia and associated damage zones in the Cambrian Gros Ventre Formation in the Stewart Creek recess, post-dating faulting.

Several discrete breccia zones locally cross-cut stratigraphy in the hanging wall of the Absaroka thrust. These breccia zones form “pipes” that are normal or oblique to bedding with anastomosing splays (some that follow bedding – “breccia sills”). Secondary mineralization (saddle dolomite and disseminated sulfides), brecciation and related rock fabrics suggest a low temperature hydrothermal origin for these zones. Hydrothermal brines may have enhanced fluid migration pathways through interrelated processes of boiling-induced brecciation and dolomitization. Late-stage calcite cements found within these pipes sometimes cross-cut cataclasites and fault breccia indicating that the pipes post-date faulting. We propose that late-stage brecciation was associated with migration of hydrothermal brine solutions and CO2 through the allochthon following thrusting and that fluid migration pathways were initially controlled by the geometry of faults and fractures.