EVOLUTION OF A MIOCENE HALF-GRABEN BASIN, COLORADO RIVER EXTENSIONAL CORRIDOR, SOUTHEASTERN CALIFORNIA

The < 1-km-thick, middle Miocene, predominantly sedimentary succession exposed within the northern Sacramento Mountains contains many charac teristics of sedimentation in an extension-generated half-graben basin . These deposits constitute the uppermost plate of a three-plate detac hment-fault system. Three facies associations are recognized and are i nferred to represent (1) small, high-gradient, gravity-dominated alluv ial fans; (2) lake and lake margin; and (3) large, low-gradient, fluvi ally influenced alluvial fans. The small, high-gradient fans consist m ainly of red and green, matrix-supported, boulder breccia and conglome rate, interpreted as debris flows, and less abundant gravity-glide blo cks and clast-supported boulder breccia. Lacustrine deposits consist o f green mudstone and sandstone, tan sandstones organized into fining-u pward sequences (< 2 m thick), and red, cross-bedded sandstones. The l arge, low-gradient fans consist of interbedded, red, crudely stratifie d gravelly sandstone and matrix-supported cobble conglomerate interpre ted as mixed fluvial and debris-flow sedimentation. Lacustrine deposit s are intimately mixed with the small, high-gradient fans in the south west end of the basin. The large, low-gradient fans are extensive in t he northeast end of the basin. This permits the interpretation that th e basin opened as an asymmetric half graben prior to 14.6 +/- 0.9 Ma ( tilted dated lavas) and after 18.5 +/- 0.2 Ma (age of older, middle-pl ate, probable Peach Springs Tuff equivalent), along a gently northeast -dipping detachment fault. Following deposition, extension segmented t he basin into numerous west-southwest tilted blocks, which were then c overed by a thin veneer of gravels and finally by the lavas of Flattop Mountain at 14.6 +/- 0.2 Ma. Such overlapping of dates between dippin g and flat-lying strata indicates that a significant amount of sedimen tation and tilting can occur rapidly during crustal extension.