Kinematic history of the Meade Thrust based on provenance of the Bechler conglomerate at Red Mountain, Idaho, Sevier Thrust Belt

Abstract

With up to ∼45 km of slip, the Meade thrust fault in southeastern Idaho is one of the major thrusts in the Sevier thrust belt, yet its age of displacement has remained enigmatic because of the lack of a derivative synorogenic deposit. We propose that the synorogenic conglomerate produced by initial Meade thrusting crops out on Red Mountain, in southeastern Idaho, ∼1.5 km east of the present trace of the Meade thrust. This conglomerate historically has been considered part of the Ephraim Formation of the Lower Cretaceous Gannett Group, but we suggest that it is a conglomeratic facies of the Bechler Formation localized to the area of Red Mountain. Regional stratigraphic considerations and paleontological dates from underlying and overlying strata indicate that the Bechler conglomerate facies (BCF) is Aptian in age. The BCF is 850 m thick and consists of pebble‐ to boulder‐conglomerate, sandstone, and mudrock deposited by fluvial and mass flow processes on medial to distal parts of an alluvial fan. Paleocurrent data indicate an eastward, fan‐shaped dispersal pattern. The BCF contains clasts of micritic limestone, chert‐pebble conglomerate, and cherty litharenite that were derived from the Ephraim Formation. In addition, the conglomerate contains abundant clasts of Ordovician, Carboniferous, and lower Mesozoic rocks that crop out on both the Meade thrust sheet and the Paris thrust sheet ∼25–30 km to the west. The base of the BCF is marked by a local 27° angular unconformity on top of the Ephraim Formation. The BCF contains intraformational, progressively rotated angular unconformities, internal growth folds, and minor dip discontinuities that were produced by simultaneous folding and sediment accumulation on the proximal footwall of the Meade thrust. Provenance modeling indicates that the BCF was derived from Mesozoic strata that were exposed along the frontal part of the Meade thrust sheet and from Paleozoic strata that were coevally exposed in the hindward located Paris thrust sheet. Topography on the Paris sheet was rejuvenated as it was carried passively over a major ramp in the Meade thrust. Provenance and structural data are combined to produce an incremental, bulk‐rock retrodeformation of initial Meade displacement. Clasts of Jurassic Twin Creek Limestone with pressure‐solution cleavage in the lower part of the BCF indicate that an episode of layer‐parallel shortening occurred in the Meade hanging wall prior to Meade displacement; this may have been related to emplacement of the Paris thrust sheet. The BCF recorded ∼6 km of initial Meade‐related shortening by thrust slip and fault‐propagation folding during Aptian time. Illite crystallinity, systematic fracture sets in the Ephraim Formation and BCF, finite strain in Meade footwall and hanging wall rocks, and thermal models by previous workers indicate that the Meade thrust sheet ultimately overrode the BCF and other footwall rocks, probably during Albian‐Cenomanian time. The BCF and the Meade thrust were folded during slip on thrust faults related to the northern Crawford thrust during Coniacian time. A revised interpretation of Gannett Group provenance throughout the thrust belt suggests that the Ephraim Formation was derived from both the Paris thrust sheet and an older, hindward located thrust sheet. The Bechler was derived from the Meade and Paris sheets.