Abstract

Some of the minor structures in the Baraboo Quartzite on the southern limb of the Baraboo syncline, including the well-known fracture cleavage and the drag folds exposed near Devil's Lake, dip northward at angles greater than bedding and are “normal” in their orientation with respect to the major fold. Other minor structures, including small folds, slip cleavage, and shears, dip southward and are “reverse” with respect to the major fold.

The normal minor structures are restricted to thin silty argillite layers interbedded with quartzite. The reverse minor structures are confined to thick argillite beds. The two sets of structures occur together in interbedded argillite-quartzite, where reverse structures are superposed on normal structures. The evidence suggests that gravity tectonics was responsible for the reverse structures. Reverse folds are asymmetric with the steep limb down-dip; they intensify down-dip from relatively open folds to tight folds with slip cleavage, and pile up on the updip side of vein quartz fragments. Stratigraphically higher layers within any given fold outrun the lower layers in the down-dip direction.

The authors believe that the reverse structures formed sometime after development of the major fold by plastic flow in the thick, northward-dipping argillite. Little, if any, readjustment of the major structure was necessary to accommodate the second generation reverse structures in the argillite.

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