Preliminary Report of 1986 University of Arkansas Geophysical Field Exercise: Geophysical Investigation at Intersection of Two Neogene Fault-Bounded Basins, Southwestern Montana

M. K. Kaplinski, M. Ahlert, B. Hecker, J. Spellman

Neogene range bounding normal faults in southwestern Montana form a mosaic of northeast and northwest trends. The 1986 Geophysical Field Exercise collected gravity, magnetic, and seismic data from the zone of intersection of the northeast-trending Ruby Range boundary fault and the northwest-trending Blacktail fault in an attempt to delineate the structures covered by Tertiary valley fill.

The Blacktail fault bounds a Tertiary half-graben (Blacktail Deer Valley) on the southwest. A 10-mgal anomaly across the basin suggests a maximum Tertiary sediment thickness of 4,700 ft and shows the presence of a southwest-dipping fault of lesser magnitude within the valley, interpreted to be antithetic to the main fault. Previous gravity surveying north of the Axes canyon area indicates a strong northeast-trending gradient along the front of the Ruby Range that has been interpreted as the position of the Ruby front fault. This strong gradient is not evident south of Axes canyon in the study area. One possible interpretation is that the Ruby front fault is displaced approximately on mile to the southeast into the Blacktail Deer Valley along a minor northwest-trending fault. Integrati n of NURE aeromagnetic data and new ground level data indicates that dikes have intruded along the faults. Three-dimensional gravity modeling and the location of magnetic linears will be important in determining the position of the Ruby front fault south of Axes canyon. Gravity contours indicate that the Blacktail fault truncates the Ruby front fault.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.