--> Unraveling the Geometry and Origin of a Northeast-Southwest Striking Linked Fault Array at Marshall Mesa, Western Denver Basin: A Solution Through Integrated Digital Mapping

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Unraveling the Geometry and Origin of a Northeast-Southwest Striking Linked Fault Array at Marshall Mesa, Western Denver Basin: A Solution Through Integrated Digital Mapping

Abstract

An array of faults mapped at Marshall Mesa represent the southwestern exposure of a fault system interpreted as the result of either “listric growth faulting” or “decollement high-angle reverse faulting”. The faults crop out within the Late Cretaceous Fox Hills Sandstone and Laramie Formation in part of the Boulder-Weld coal field and have economic and environmental implications for sub-surface gas and coal reserves and the Laramie-Fox Hills aquifer in the Denver Basin. The listric growth faulting model interprets high-angle normal faults at the surface as the products of growth faulting developed in the delta-front environment that deposited the upper Pierre Shale through the lower Laramie Formation. Listric normal faults observed in seismic data within the uppermost Cretaceous are also interpreted to have spacial a relationship with NE-SW striking deep basement-controlled faults on the northeast projection of the Ralston shear zone. NE-SW striking faults also compartmentalize reservoirs within the nearby Wattenberg gas field. The decollement high-angle reverse faulting model interprets sub-surface faults along strike to the northeast of the mapped array as representing the leading edge of deformation of the upper Cretaceous sedimentary rocks along a bedding plane decollement within the upper Pierre Shale. This deformation has uplifted the Laramie-Fox Hills aquifer and repeated sections within the aquifer, locally increasing its thickness by nearly 75 meters. Detailed, integrated digital mapping of the exposed fault array at Marshall Mesa reveals a series of complex structural relationships that indicate both normal faulting and subsequent shortening, marked by folding and locally reverse faulting, plus inversion of earlier normal faults. A history of extension and growth faulting linked to down-dip contraction, with localized inversion reconciles the competing earlier interpretations and illustrates the advantages of integrating digital data collection with detailed field mapping and cross section restorations.