Abstract

Over 30,000 feet of Lower Cambrian through Upper Pennsylvanian rocks are exposed in the southern Egan and Schell Creek Ranges. Thicknesses are: Cambrian, 13,426 feet; Ordovician, 4,609 feet; Silurian, 1,013 feet; Devonian, 4,342 to 5,341 feet; Mississippian, 2,091 feet; Pennsylvanian, 3,525 feet. Unconformities are present at the base of the Mississippian and possibly also at the base of the Devonian and at the base of the Upper Pennsylvanian.

There was no local structural deformation in the Paleozoic. By the Cenozoic, beds were cut by a few steep normal and reverse faults and an east- and northeast-trending fault through Shingle Pass had moved slightly. Deposition of Eocene lacustrine beds (up to 3,176 feet thick) was locallized north of the Shingle Pass fault; large exotic (slide) blocks of Pennsylvanian limestone both underlie and overlie the Eocene.

Middle Tertiary (Oligocene) rocks begin with a basal conglomerate, grade upward into welded tuffs and tuffaceous sediments with interbedded volcanic flows, and are capped by a persistent welded tuff series. Conformably overlying the volcanics are fluviatile sediments of Miocene and possibly Early Pliocene age.

Normal faults dipping 42° to 58° west or northwest were active in the north of the area from Early Oligocene through Miocene. One of these had movement of about 3.9 miles. The present north-, northeast-, and west-trending faults, which define the ranges, were formed in the Late Tertiary (Pliocene?); subsequent movement on these faults was accompanied by about 31° east tilting of the Egan Range, decreasing the dip of earlier normal faults to 20° to 42° west. Basalt flows and fluviatile sediments deposited unconformably on Miocene and older rocks were tilted about 9° to the east during the latest stages of movement on the north-trending faults. Faulting continues, as a Recent fault is present along the west front of the southern Egan Range.