The <1-km-thick, middle Miocene, pre-dominantly sedimentary succession exposed within the northern Sacramento Mountains contains many characteristics of sedimentation in an extension-generated half-graben basin. These deposits constitute the upper-most plate of a three-plate detachment-fault system. Three facies associations are recognized and are inferred to represent (1) small, high-gradient, gravity-dominated alluvial fans; (2) lake and lake margin; and (3) large, low-gradient, fluvially influenced alluvial fans. The small, high-gradient fans consist mainly of red and green, matrix-supported, boulder breccia and conglomerate, interpreted as debris flows, and less abundant gravity-glide blocks and clast-supported boulder breccia. Lacustrine deposits consist of green mudstone and sandstone, tan sandstones organized into fining-upward sequences (<2 m thick), and red, cross-bedded sandstones. The large, low-gradient fans consist of interbedded, red, crudely stratified gravelly sandstone and matrix-supported cobble conglomerate interpreted as mixed fluvial and debris-flow sedimentation. Lacustrine deposits are intimately mixed with the small, high-gradient fans in the southwest end of the basin. The large, low-gradient fans are extensive in the northeast end of the basin. This permits the interpretation that the 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-plate, probable Peach Springs Tuff equivalent), along a gently northeast-dipping detachment fault. Following deposition, extension segmented the basin into numerous west-south-west tilted blocks, which were then covered 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 dipping and flat-lying strata indicates that a significant amount of sedimentation and tilting can occur rapidly during crustal extension.

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