THE EFFECTS OF FAULT-INDUCED STRESS ANISOTROPY ON FRACTURING, FOLDING AND SILL EMPLACEMENT: A STUDY OF THE BOWIE NO. 2 COAL MINE, SOUTHERN PICEANCE BASIN, WESTERN COLORADO

A reverse-reactivated, penecontemporaneous fault is associated with several unusual geological phenomena, including rotated cleat (fracture) orientations, changing fault mode, and a footwall kink band fold. The fault caused an abrupt revision in long-term mine plans, the closure of two major entries, and several roof and rib failures. Mapping of >600 face cleat orientations show that cleat formation was affected by fault-induced stress anisotropy within 60 m of the fault. Several non-regional cleat domains in the fault-perturbed stress zone may have formed during distinct paleoslip events, with areas of scatter occurring where slip events overlapped. Reverse reactivation of the fault appears to be localized adjacent to a synclinal footwall kink band fold. An apparent correlation between areas of reverse offset (where reactivated reverse slip > initial normal slip) and the footwall fold suggests that either the fold was generated by a sharp fault slip gradient, or fold propagation was the dominant mechanism in fault reactivation. In addition, igneous sills intruded into three Bowie coal seams display irregular isopach contours that steepen near and subparallel preexisting normal faults. Where fault stress shadows and extensive fault-related fracture sets favored sill propagation, reaching a thickness of 6 m in one fault zone.