Full Title: Geologic Map, Cross Section, and Meso-structures Across the Idaho-Wyoming Fold-and-thrust Belt at Latitude 42°45'

Compiler: John P. Craddock

One sheet, 40" x 33". From text: Detailed geologic mapping has disclosed meso-scale structures that are important for understanding the development of the Idaho-Wyoming fold-and-thrust belt (Paris to Prospect thrust sheets). Interpretation of these structures and their deformation mechanisms yields thrust kinematic information, intermediate to the regional "wedge," and local ramp models of fold-and-thrust belt development. Deformation in the eastern, frontal Prospect thrust sheet is characterized by an asymmetric, overturned anticline, younger crosscutting thrust faults and no regional cleavage or veins. The Darby sheet has the most topographic relief along its thrust contact, and is associated with the folded Lander Peak klippe and a previously unrecognized Darby synorogenic conglomerate. Synorogenic clastic dikes were found only along this fault contact, and both veins and solution cleavage are common in the hanging wall. Tight, disharmonic and broad concentric folds, the latter frequently cored by east-dipping "contraction" structures, are common in the Absaroka sheet. The Meade sheet is complexly folded and is characterized by a thick, regionally extensive folded fault gouge zone. Fibrous veins, solution cleavages, and widespread "breccia bodies" are found within the Meade sheet. Folds are dominantly of the chevron type. The eastern edge of the Paris sheet is composed predominantly of cleaved Precambrian-Cambrian Brigham quartzite, and is overprinted by Miocene normal faulting. In the overlying Cambrian shales and limestones, east-west-trending and north-dipping thrust faults offset concentric east-west-trending, west-plunging folds which are superimposed on a regional north-south-trending ramp-syncline. In general, vein sets are more common in these rocks of the western thrust sheets. Out-of-sequence, out-of-transport and "piggyback" meso-scale deformations in nearly all of the major thrust sheets place restrictions on the commonly used rules of cross section balancing and assumptions of fold-and-thrust belt modeling. These rules and assumptions include plane strain, constant rock volume and rheology during deformation, and an undeformed footwall, all of which disagree with field observations.

Published: 4/01/1988

Pages: 18 plus 1 sheet

Product Category: EBooks