MICROGRAVITY MAPPING OF COMPLEX LARAMIDE FORELAND STRUCTURE AND ABANDONED COAL MINES NEAR MARSHALL, COLORADO

Laramide faulting east of the Rocky Mountain front took advantage of inherited weaknesses but also generated new structures in several phases. Pre-orogenic subsidence caused listric normal faulting rooted in decollements in the Pierre Shale, some of which may had developed earlier as growth faults during deposition. Subsequent hinterland contraction inverted some of these normal faults and broke new thrusts throughout the more competent—and coal-bearing—uppermost Cretaceous units. In some places, strike-slip faults compartmentalize hangingwall deformation, effectively tearing E–W across thrust sheets to create discrete blocks. For instance, this compartmentalization resulted in a complex network of coal mines near Marshall, Colorado, that was poorly mapped 150 years ago and remains poorly delineated, despite increasing development atop the abandoned mine workings. Using more than 1,500 microGal-precision relative gravity measurements and in-situ constraints from ~20 geotechnical borings, we identify numerous blind or cryptic faults including oblique to strike-slip tear faults that demonstrably cut across major thrusts and associated thrust sheets. Doing so allows us to isolate the signals of unmapped, abandoned coal mines to delineate and characterize workings. These hybrid geophysical/structural geological investigations are an efficient approach to set the stage for geotechnical confirmation drilling and then backfilling, grouting, or other remediation of abandoned workings.