ABSTRACT: Using Surface Geology of Folded Thrusts to Explore Hidden, Subsurface Thrust Packages; Three Examples from the Montana Thrust Belt
Edward J. Sterne
In areas with complex structure, rugged topography, and poor seismic data, interpreting surface geological relationships remains the strongest tool available to the explorationist. Three examples of folded surface thrusts from the Northern Big Belt Mountains, Jefferson Canyon, and McKnight Canyon of western Montana show how surface geology can predict the presence of hidden, subsurface thrust packages.
Two of these features have been drilled and have proven the existence of subsurface structural culminations, and the third remains untested. The well in the Northern Big Belt Mountains spudded in a window of a folded thrust and drilled several hidden duplex packages of prospective Paleozoic rocks before reaching total depth. At McKnight Canyon, the well penetrated a previously unknown thrust package as predicted by the surface geological analysis of the overlying, folded thrust. In the Jefferson Canyon example, an exposed duplex of Paleozoic rocks folds the overlying Precambrian Belt-carrying thrust and provides an exploration analog for possible hidden subthrust packages coring the huge Devil's Fence anticline along strike to the north.
Initial identification of the folded thrusts was made using surface geological maps. The map relationships and thrust attitudes were field checked and photographed. Thrust plane contour maps were constructed using aerial photographs and surface geological maps and were found to be very useful in analyzing and presenting the prospects. Even though production was not established in the two wells discussed here, the techniques are readily applicable to areas of untested potential.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990