FAULT-PROPAGATION ORIGIN FOR FOLDING IN THE WESTERN BARABOO RANGES, WISCONSIN, USA, BASED ON NEW GEOLOGIC MAPPING

Recent 1:24,000 scale field mapping across the Baraboo Ranges, south-central Wisconsin, USA, has established an informal stratigraphy within the Proterozoic Baraboo Quartzite, and identified previously unrecognized thrust and normal faults. The Baraboo Quartzite, a roughly 2-kilometer-thick package of <1700 Ma Precambrian quartzite, metaconglomerate, and phyllite, was folded within a poorly understood Precambrian fold-thrust belt. Past kinematic models that explain the causes of folding have been hindered by a lack of detailed field mapping, few internal marker horizons within the Precambrian section, and a lack of mapped faults. Many of the folds, particularly in the more complexly deformed western Baraboo Ranges, have uncertain origins, and may have formed as detachment folds, fault-propagation folds, or fault-bend folds. The newly recognized faults may help explain the origins of some of the folds. One such fault, the South Range Fault, is exposed in the southwestern Baraboo Ranges. It strikes N60E, and dips approximately 30 degrees NW. It places basal Baraboo conglomerate and quartzite over younger Baraboo quartzite and phyllite, and is interpreted to represent a hanging wall ramp on footwall flat. The fault contains several <50 cm thick quartzite shear zones within a heavily fractured and veined quartzite host rock. Quartz grains in the shear zones are dynamically recrystallized by bulging recrystallization. Slip on the fault decays northeastward, where the fault transitions into the previously recognized Skillet Creek anticline and Otter Creek syncline. This relationship suggests the Skillet Creek anticline-Otter Creek syncline pair formed as a fault-propagation fold in advance of the tip line of the South Range fault. The recognition of new marker horizons within the Baraboo Quartzite provides new opportunities to interpret the origins of many of the folds in the Baraboo Ranges, and construct more detailed and accurate cross-sections.