ABSENCE OF LOWER PALEOZOIC STRATIGRAPHIC SECTION AS EVIDENCE FOR TERTIARY EXTENSION IN SOUTHWEST MONTANA

On three hills in Alder Gulch, south of Virginia City, Montana, Mississippian Madison limestone lies in direct contact above Precambrian crystalline basement rock. The basement rocks consist mostly of quartzofeldspathic and amphibole gneisses of the northern Wyoming province. The Madison limestone in the study area is dominantly massive and crystalline, but locally contains thin chert horizons and bedding. Tertiary olivine-pyroxene basalt is in contact with both the basement rock and the limestone in this area. Based on nearby, complete stratigraphic sequences, there should be a section of lower Paleozoic clastic and carbonate rocks several hundred meters thick between the Madison limestone and the Precambrian gneisses. Field study of the area at 1:12,000 scale has constrained the contact between the limestone and the gneiss, and establishes that the contact dips shallowly (less than 20 degrees), with variable dip directions. The contact truncates both the bedding in the Madison limestone (which dips southwest at over 35 degrees) and gneiss foliation (generally dipping south at greater than 50 degrees). The absence of the lower Paleozoic stratigraphic section is, therefore, most likely due to a low-angle normal fault, probably of Tertiary age. Samples from both above and below the contact, and both close to the contact and at a distance from it, aid in the assessment of the direction and conditions of faulting. Geochemical analysis investigates any geochemical changes between rocks close to the contact, as compared to rocks farther away, which might be due to metasomatism by fluids introduced along the contact surface.