THE JEFFERSON CANYON AND CAVE FAULTS REVISITED

The Jefferson Canyon and Cave faults are the principal structures of the southern lateral ramp boundary of the Late Cretaceous Lombard thrust sheet in southwestern Montana. Their position is controlled by a buried north-dipping Middle Proterozoic fault (Willow Creek fault) or set of normal faults in Precambrian basement rocks. The Jefferson Canyon and Cave faults have right hand (oblique) thrust displacement and place Proterozoic turbidites (La Hood Formation) on rocks as young as Late Cretaceous. These two faults are now interpreted to be segments of the same fault that was folded when movement became “locked”. The Cave fault is the folded western segment. The Jefferson Canyon segment is the eastern segment. It contains a horse block of Precambrian basement that was “picked off” of the uplifted footwall of the Willow Creek fault and attained its final position before folding of the Cave fault. During the early movement on the Cave –Jefferson Canyon fault the Phanerozoic hanging wall rocks were folded into tight NNE - trending folds interpreted to be early detachment folds with detachment in the La Hood Formation. When fault movement “locked”, the Cave fault segment was tightly folded by regional right-hand simple shear and the Phanerozoic hanging wall section ahead (east) of it was also folded into a southeast-verging, overturned fold pair. A minor thrust that splays from the hanging wall of the Cave fault and across the hinge of the syncline is a late structure that developed after the fold pair. A simple fault-fold model cannot be applied to this structure. It probably started as a detachment fold that was modified by later simple shear and thrust faulting. Based on fold trends and cleavage attitude the early shortening direction immediately north of the Jefferson Canyon fault system was ESE. Lock up of fault movement rotated the shortening direction to the SSE.

The Jefferson Canyon fault system truncates earlier thrusts and a basement-cored anticline south of it and is therefore out-of-sequence with respect to those features. It is interpreted to divide near Willow Creek, with the northern splay becoming the Lombard thrust. The other splay continues beneath the Tertiary sediments to the Pass fault in the Bridger Range, where it becomes a younger (Proterozoic sedimentary rocks) on older (Precambrian basement) detachment.