ANALYSIS OF FAULTING AND FOLDING IN A MAP SCALE, PLUNGING FOLD PAIR ALONG THE SOUTHERN BOUNDARY OF THE HELENA STRUCTRUAL SALIENT, MONTANA

The Cave anticline-Greer Gulch syncline is a northeast-plunging, southeast-verging map scale fold pair that is part of a fold train in Middle Proterozoic and Paleozoic sedimentary rocks located along the southern margin of a large oroclinal bend (the Helena salient) of the Late Cretaceous – Early Tertiary Cordilleran fold and thrust belt. The fold pair is on the hanging wall of the principal, Lombard, thrust sheet of this salient and is bounded on the south by the Jefferson Canyon-Cave fault system, thought to be the southern lateral ramp boundary of the salient. It is located, and easily accessible off of Montana State Highway 2, within the boundaries of the Lewis and Clark Caverns State Park, where we have constructed panoramic views with overlays, geologic interpretations, and trail guides.

The fold pair has a vertical-to-slightly overturned mutual limb, with a trough-to-trough wavelength of ~3km. Fold plunge varies progressively from 57ºNE in the lower Cambrian section to 31ºNE in the Devonian section. The down-plunge profile was constructed using a mean fold axis orientation of 41, N48E. The fold pair is well-exposed within the Paleozoic section due to a moderate northeast plunge related to the northerly dip of the hanging wall homocline of the Jefferson canyon fault. The fold pair has a fault that cuts mostly along the synclinal hinge and looses displacement upward. In cross section, based largely on the down plunge section, the Cave anticline resembles a fault-propagation fold with a somewhat attenuated (30%) and overturned forelimb, and the structure has a geometry that is compatible with the existing models of fault-propagation folding. However, interpretation of the balanced cross section indicates a different history than that which is inferred from application of analytical fold-fault models. It suggests that the fold pair initially developed as a detachment fold above the underlying Jefferson Canyon fault. Folding continued as the Jefferson Canyon fault itself was folded. The fault which breaks along the synclinal hinge actually developed as a minor splay and propagated into the fold after most of the folding was complete.