About This Item
Share This Item
Detailed mapping of Mt. Putnam in the Portneuf Range (Fort Hall Indian Reservation) has revealed the presence of previously unrecognized large-scale overturned folds and thrust faults characteristic of the Idaho-Wyoming overthrust belt. The structure of Mt. Putnam is controlled primarily by a northwest-trending overturned anticline-syncline system that is responsible for the inverted Precambrian Z-Cambrian stratigraphy of the area. Parts of the upper limb of the overturned anticline were sheared off and thrust over more gently dipping strata to the east. This "Bear Canyon thrust" places Precambrian Z-Cambrian Camelback Mountain Quartzite over the Cambrian-Ordovician carbonate sequence.
Mt. Putnam, as well as the rest of the Portneuf Range, is in the upper plate of the Putnam thrust that is exposed 7 km (4 mi) northeast of Mt. Putnam. The thrust is then displaced 5 km (3 mi) to the east by east-trending normal faults interpreted as reactivated tear faults. These faults have created windows through the upper plate Pennsylvanian-Permian Wells Formation, exposing the Ordovician Garden City Formation of the lower plate. South of this offset, the Putnam thrust resumes its southeasterly trend toward the Chesterfield Range and a possible juncture with the Paris thrust.
Ten kilometers (6 mi) west of Mt. Putnam in the Bannock Range are younger over older low-angle faults characteristic of the hinterland of the thrust belt. The area of transition between the two structural styles lies in the Pocatello Range north of Inkom, Idaho, and is presently being remapped in detail.
End_of_Article - Last_Page 945------------