The structural geology of the Shadow Mountains area, San Bernardino County, California
Wilson, Raymond Carl
Burchfiel, B. Clark
Master of Arts
Within the Shadow Mountains area, located in the northeast corner of San Bernadino County, California, a series of allochthonous blocks of Precambrian gneisses rest on Late Tertiary non-marine sediments. The Precambrian gneisses consist of dioritic sills and granitic country rock. The Tertiary sediments consist of Pliocene (?) non-marine gravels, sands, tuffs, bentonitic playa clay, and lenses of massive, recemented dolomite megabreccia up to 500 feet thick. During the Sevier Orogeny, the Mesquite and the Winters thrusts developed in the Mesquite Range and Winters Pass area to the east of the Shadow Mountains, and the Halloran and the Kingston Peak batholiths were intruded south and north of the area respectively. In the early Tertiary (?), the Riggs thrust was emplaced in the Silurian Hills west of the Shadow Mountains, and the Kingston Valley graben formed between the Silurian Hills, the Kingston Range, and the Mesquite Range. During middle and late Tertiary (?), a thick sequence of non-marine sediments were deposited in the Kingston Valley graben. Interbedded with these sediments are a series of thick lenses of dolomite megabreccia which were emplaced by a combination of sedimentation and incoherent gravity sliding. The Goodsprirgs Dolomite exposed in the Riggs thrust plate in the Silurian Hills appears to have been the source of the megabreccia. The allochthonous Precambrian gneiss blocks of the Shadow Vountains area may have been emplaced originally as a single original sheet, 6 miles by 10 miles in area and more than 600 feet thick. The contact surface beneath the gneiss blocks is sharp, smooth, planar, and has a dip of less than one degree. A layer of bentonitic clay 3 inches to 4 feet thick is found immediately under the soles of the gneiss blocks and apparently served as a lubricating layer for the movement of the gneiss sheet. Although they are incompetent, the Tertiary sediments underlying the thin clay layer are virtually undeformed, but the Tertiary beds are overturned at Spring Peak, the one locality where the clay layer is missing. The source area for the allochthonous gneiss sheet was in the vicinity of Winters Pass. Because the underlying Tertiary rocks are undeformed where the clay layer is present, most of the shear stress needed to emplace the gneiss sheet was concentrated upon the gneiss itself. In a gravity slide, this stress would be distributed over the area of the plate, rather than on the thin rear edge as in a compressional thrust. For this and other reasons, the gneiss blocks of the Shadow Mountains area are interpreted as the remnants of a gravity slide block. The theoretical mechanics of gravity slide blocks are also discussed. In general, a gravity slide requires a plane of weak-ness with a dip steeper than its angle of repose, and a horizontal stress less than the overburden stress. Formulas are derived for the necessary horizontal (0-3) / overburden (0-1) stress ratios.