A structural model for brittle detachment faulting in the central Grant Range and the Grant Canyon and Bacon Flat oil fields in Railroad Valley, eastern Nevada

Abstract

Characterization of the geometry and kinematics of normal faulting is critical for hydrocarbon exploration within extensional settings such as the Basin and Range Province. Here, we integrate geologic mapping, field relationships, well data, and deformed and restored cross sections to present a model for the kinematic development of a system of west-vergent detachment faults in the Grant Range and adjacent Railroad Valley, which contains Nevada's most productive oil field. The detachment faults deform a 10 km-thick section of Paleozoic-Paleogene sedimentary and volcanic rocks, and exhibit brecciation and stratigraphic cutoff angles of 5-15˚ at all structural levels. The detachment system is folded across an anticlinal culmination, which grew during extension, as indicated by interlimb angles in the axial zone that increase from 130˚ to 160˚ in progressively younger faults. The eastern limb consists of an imbricate stack of faults that were emplaced from bottom-to-top. In the western limb, several faults exhibit apparent thrust relationships. Several of the structurally-highest detachment faults in the eastern part of the Grant Range cut volcanic rocks that are as young as ~32 Ma. However, the oldest, structurally-deepest detachment faults in the western part of the range are cut by a ~29 Ma dacite dike, indicating that extension had to have begun by the late Oligocene. This indicates that detachment faulting was a distinctly older extensional event, which pre-dates the inception of Late Miocene and younger high-angle normal faulting associated with formation of the Railroad Valley structural basin. Retro-deformation of folding and minimal angularity across a Paleogene unconformity indicate that the detachment faults were initially active at ≤15˚ angles. Retro-deformation of offset indicates ≥49 km (98%) of extension, and stratigraphic omission in the axial zone is as high as 5-6 km. We propose a model of stationary, sustained isostatic uplift and incision at the culmination axis (a ‘fixed hinge’), with back-rotation and excision up-dip of the axis producing bottom-to-top growth of an imbricate stack of detachment faults, and excision down-dip of the axis yielding apparent thrust relationships. Culmination growth rotated faults from primary dip angles of 5-15˚W to as high as 20˚E. The lack of hinge migration in the down-transport direction was likely aided by excision down-dip of the culmination axis, which resulted in local stratigraphic repetition (i.e., apparent thrust relationships), and therefore less denudation than the axial zone. This structural model yields a novel interpretation for the subsurface structural geometry of the Grant Canyon and Bacon Flat oil fields in Railroad Valley, which is supported by well data. An older-over-younger relationship (Devonian rocks over Mississippian-Pennsylvanian rocks) previously interpreted as either a thrust fault or slide block is here interpreted as an extensional detachment fault, and is correlated with the set of geometrically-similar detachment faults exposed in the Grant Range. This indicates the potential for subsurface projection of highly-fractured reservoir carbonates that were originally emplaced as ~200-500 m-thick, laterally-continuous, detachment-bound sheets westward under Railroad Valley, which were subsequently deformed by high-angle, Miocene-Pliocene normal faults. This subsurface geometry should be considered in future exploration strategies.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016