Abstract

Gilsonite, a solid, brittle hydrocarbon, forms scores of large, northwest-trending dikes that cut Tertiary sedimentary rocks of the eastern Uinta basin. Previous studies established the source of the gilsonite as the kerogen-rich beds of the upper part of the Green River Formation, of middle Eocene age. New structural work reported here indicates that the dikes originated as large hydraulic extension fractures whose formation was prompted both by high pore-fluid pressures in the source beds and by decreasing minimum horizontal compressive stress during the initial phases of regional northeast-southwest post-Laramide (<40 Ma) tectonic extension. Failure of the previously unbroken strata above the overpressured source beds allowed escape both of large amounts of formation water and of the viscous precursor to gilsonite into the vertically and laterally extensive system of hydraulic fractures. Early passage of aqueous fluids through these fractures is recorded by coatings of limonite on dike walls and by the presence of continuous reduction rinds along dike fractures discontinuously occupied by gilsonite. Evidence for forceful rather than passive injection is common; the widespread presence of gilsonite sills, in particular, shows that fluid pressures during intrusion frequently exceeded lithostatic load. Depths to the source beds during dike emplacement are estimated at 3,800-8,000 ft (1,160-2,440 m), increasing generally northwestward. Continuing northeast-southwest tectonic extension following dike emplacement later gave rise to an early regional set of joints nearly parallel to the dikes and ultimately to scattered normal faults of small to moderate throw.