Chapter 3: Tertiary paleogeologic maps of the western Idaho-Wyoming-Montana thrust belt
Published:January 01, 1992
David W. Rodgers, Susanne U. Janecke, 1992. "Chapter 3: Tertiary paleogeologic maps of the western Idaho-Wyoming-Montana thrust belt", Regional Geology of Eastern Idaho and Western Wyoming, Paul Karl Link, Mel A. Kuntz, Lucian B. Piatt
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Maps of the distribution of Middle Proterozoic through Cretaceous rocks beneath the Tertiary unconformities in eastern Idaho effectively remove differential uplift associated with Basin and Range tectonism and reveal preextensional structural relief associated with folds and thrusts in the western part of the Idaho-Wyoming-Montana thrust belt. North of the Snake River Plain, the paleogeologic map shows that regionally extensive Middle Proterozoic to Triassic strata and the Cretaceous Idaho batholith were variably uplifted and exposed prior to formation of the Eocene Challis volcanic field. In the Beaverhead Mountains, Ordovician and Triassic strata are juxtaposed along the Hawley Creek thrust. In the northern Lemhi and Lost River ranges, west- and south-dipping homoclines beneath the Eocene unconformity are interpreted to reflect folding above frontal and lateral ramps in the footwall of the Hawley Creek thrust system. In the southern Lemhi and Lost River ranges, the White Knob Mountains, and eastern Pioneer Mountains, Mississippian to Permian strata underlie most of the unconformity. Stratigraphic offset is evident along the exposed Pioneer and Copper Basin thrust faults, but along the exposed Glide Mountain thrust as well as the concealed White Knob, Grouse, and Lost River-Arco Hills thrusts uplift was insufficient to juxtapose rocks of different systems. To the west, the Idaho batholith and Pennsylvanian-Permian rocks are juxtaposed beneath the Eocene unconformity. Several kilometers of overburden covered both the batholith and Permian rocks in Cretaceous time and were regionally eroded prior to extrusion of Eocene volcanic rocks.
South of the Snake River Plain and west of the exposed traces of the Paris and Putnam thrust faults, Late Proterozoic to Permian strata unconformably underlie Miocene sedimentary and volcanic rocks. The Paris thrust separates Late Proterozoic through Mississippian strata in its hanging wall from Pennsylvanian through Triassic rocks in its footwall, and the Putnam thrust separates Late Proterozoic to Ordovician strata from Pennsylvanian to Triassic strata. Beneath the unconformity, stratigraphic displacement appears to diminish northwestward along the Paris thrust and southeastward along the Putnam thrust, supporting the interpretation that displacement is progressively transferred from one thrust to the other. Lower Paleozoic strata generally underlie the unconformity between the emergent thrust system and the Arbon and Malad valleys. West of these valleys, Pennsylvanian-Permian strata everywhere underlie the unconformity. A simple flat-ramp-flat-thrust fault geometry of the Paris-Putnam thrust system may explain the outcrop pattern, with hanging-wall and footwall flats beneath the regions of low structural relief, separated by a footwall ramp located beneath the modern Malad and Arbon valleys. Paleogeologic maps show that the Paris-Putnam thrust sheet was probably not an important source of quartzose clasts in the Cretaceous-Tertiary Harebell and Pinyon formations of northwest Wyoming.