Abstract: Stratal Reconstructions of Northwest Great Valley Forearc Basin: Implications for Thrust-Wedge Tectonics

CONSTENIUS, KURT N., ROY A. JOHNSON, and WILLIAM R. DICKINSON

Interpretation of reflection seismic data and restoration of the stratal geometry of the Cretaceous forearc basin in the northwest Sacramento basin provide important controls on structural reconstructions of the basin. Monoclinal east dips in Great Valley Group strata, which previous models have proposed as a hanging wall ramp sequence above a blind west-dipping Great Valley-Franciscan sole fault, are expressions of bedding architecture that results from folded angular unconformities and depositional onlap. Convergence of these unconformities above a west-dipping ophiolitic basement gives rise to fork-shaped reflection patterns - "fork structures". In the vicinity of the major "fork structure" a prominent subsurface discordance is found at the base of the Lower Cretaceous Leesville Sandstone (Lodoga Formation). Strata above this unconformity dip to the east and comprise part of the Great Valley monocline; beneath this unconformity, rocks of the Jurassic-Lower Cretaceous Stony Creek Formation are in marked angular divergence with the supracrop. Correlation of the angular seismic reflection discontinuity to basinward facies now exposed along the outcrop belt shows conformable supracrop-subcrop beds, a relationship seen in extant forearc basins. Thus, the dip divergence found locally between the Lodoga and Stony Creek Formations is not the product of blind thrusts but, rather, is a manifestation of a fundamental change in the pattern of forearc basin sedimentation. West-dipping divergent events imaged beneath Great Valley Group rocks compare favorably with seaward-dipping reflections imaged in basement rocks along so-called volcanic passive margins.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California