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Late Cenozoic Sediment Provenance and Tectonic Evolution of the Northernmost Coast Ranges, California
Marine sandstones of the Lower-Middle Miocene Bear River beds, Upper Miocene-Lower Pleistocene Wildcat Group and Pliocene lower Falor Formation contain an average of 6 % K-spar. 40Ar/39Ar laser probe analyses of Lower Pleistocene Wildcat Group micas yield ages of approximately. 52–57 Ma. Analyses of Pliocene Wildcat Group micas yield three groups of ages: approximately 66–75, 128.5, and 299–303 Ma. A logical source for the three younger groups are Cretaceous-Eocene plutonic rocks in Idaho, a source proposed for subjacent Eocene Yager Formation sandstones in the Coast Ranges by Underwood & Bachman (1986). The source of the older micas remains enigmatic, but may be reset Devonian Abrams Schist of the central Klamath Mountains.
Fluvial conglomerates and sandstones of the Upper Oligocene-Lower Miocene Weaverville Formation and Lower Pliocene Wimer Formation in the Klamath Mountains, the Upper Pleistocene upper Falor and Carlotta Formations in the Coast Ranges, and marine sandstones of the Lower Pliocene Wimer and St. George Formations in the Klamaths and Coast Ranges are derived largely from nearby basement rocks. Imbrication of western Weaverville Formation fluvial clasts indicates westerly flow of a major braided river. However, the deltaic facies of this formation is missing, due either to Miocene erosional stripping or removal by faulting. The eastern Weaverville Formation accumulated syntectonically in upper plate grabens above the regional LaGrange detachment fault. Initiation of LaGrange detachment faulting, Weaverville sedimentation and, possibly, Klamaths uplift is coeval with increased oblique convergence of oceanic and continental plates ca. 30 Ma, and with alkalic intrusive activity in the southwest Oregon and northwest California Coast Ranges. The intrusions may reflect an unrecognized rifting event which might have resulted from clockwise rotation of the Klamath Mountains and Coast Ranges provinces.
The Bear River beds accumulated at bathyal depths. Basal Wildcat Group sediments are fluvial to littoral. However, rapid submergence of the Coast Ranges region to bathyal depths (2–3 km) occurred following the initiation of Wildcat sedimentation. This abrupt bathymetric changes may relate to greater normal plate convergence ca. 9 Ma. Ca. 5 Ma the northwest Klamath Mountains were at sea level, as is indicated by presence of Wimer marine sediments on the present-day elevated erosion surface. Poorly dated, coarse fluvial conglomerates of the eastern Wimer facies may reflect local uplift and warping of this surface about a northwest-trending axis. Sediment shoaling, and onset of fluvial sedimentation in the Coast Ranges, may relate to the decreased plate convergence rate, lessening of angle of subduction and increased normal convergence ca. 1 Ma.
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