Approximately 23 square miles of the east-central part
of the Tendoy Range are included in the thesis area which
lies three miles west of Dell, Montana.
Rocks ranging in age from Late Mississippian to Recent
are exposed in the study area. The upper Paleozoic section
includes approximately 4,200 feet (1,281 m.) of strata which
are divided into four formations. The nine formations of
Mesozoic aae total approximately 2,960 feet (903 m.) plus part
of the Cretaceous-Tertiary Beaverhead Formation. Cenozoic
strata in the thesis area include Beaverhead conglomerates,
basaltic andesite flows, tuffs, and breccias, and unconsolidated
gravels, alluvium, and alluvial fan gravels.
The thesis area was examined in detail to define formation
distribution and structural complications. The
Triassic and Jurassic (Ellis Group) rocks were examined
specifically in an effort to interpret their depositional
environments. Sedimentary structures, fossils, and petrographic
evidence indicate a shallow subtidal and intertidal
setting for the deposition of Triassic rocks. Algal
laminated strata within the Woodside Formation have not
previously been described. Much of the Ellis Grout:, was
deposited under shallow subtidal and locally shoaling conditions.
Glauconite calcarenites in the Swift Formation may
represent deposition in sand waves.
Folding and faulting within the thesis area represent
the overlapping influence of geosynclinal and cratonic
tectonism in the hinge line setting of the Tendoy Range.
The imposition of a southward-plunging anticline on a
southwestward-plunging syncline has resulted in the overturning
of the northern limb of the syncline. A low-angle
thrust with eastward vergence has been developed in the
Dry Canyon area. The thrust has been cut by a later high-angle
reverse fault. White Knob strata (Mississippian)
have been carried into the western margin of the thesis area
on the Medicine Lodge thrust.
The economic possibilities are not encouraging. The
phosphate deposits of the thesis area have been analyzed and
deemed uneconomical. Distance from intrusions precludes the
chance of significant mineralization in the area. The petroleum
potential for the area is doubtful. Late Cretaceous
folding and erosion may have allowed escape of hydrocarbons
from the potential source and reservoir rocks of the region.
However, the Medicine Lodge thrust sheet may conceal structural
and stratigraphic configurations capable of trapping oil