Paleochannels, stream incision, erosion, topographic evolution, and alternative explanations of paleoaltimetry, Sierra Nevada, California

Abstract

Geologic relationships in the Sierra Nevada, California, show negligible stream incision between Eocene and Late Miocene-Pliocene time. Stream incision of up to ~1 km began at (from south to north) ca. 20 Ma in the Kern to Kings River drainages, between 6 and 10 Ma in the San Joaquin River drainage, 3.6-4 Ma in the Stanislaus and Mokelumne River drainages, and ca. 3 Ma in the American and Feather River drainages. These differences in incision timing greatly exceed the time of knickpoint retreat, based on the example of the North Fork Feather River, where the knickpoint may have retreated over 100 km in less than 300 k.y. based on ages of interfl uve-capping andesites and an inset basalt fl ow. The knickpoint in the Stanislaus River may have retreated over 50 km in less than 400 k.y. based on somewhat looser constraints. Eocene paleochannels show lowest gradients parallel to the range axis, steepest ones perpendicular, and reaches with signifi cant "uphill" gradients that rise in the paleo-downstream direction. Modern Sierran rivers lack this relationship. The azimuth-gradient relationships of paleochannels, especially the uphill gradients, require late Cenozoic tilting and uplift. Incision began in spite of decreasing discharge and increasing sediment load and must have resulted from steepening associated with tilting and uplift. Stable-isotope paleoaltimetry apparently records a profi le similar to that of the modern range and areas east of it, in spite of signifi cant vertical deformation that postdates the age of the sampled deposits, suggesting fairly recent reequilibration, in contrast to the published interpretations of closed-system behavior since the Oligocene or Eocene. Such apparent opensystem behavior agrees with studies showing progressive hydration of volcanic glass and the correspondence between weathering and erosion rates. Northward-younging initiation of late Cenozoic uplift and stream incision suggests a relationship with triple-junction migration, possibly associated with slab window development, with a second uplift pulse related to delamination and limited to the southern Sierra (San Joaquin River drainage and southward). Basement features may have signifi cantly infl uenced along-and across-strike differences in Cenozoic tectonics and geomorphic response. GENERAL GEOLOGIC FRAMEWORK AND TECTONIC HISTORY The 600-km-long Sierra Nevada is the most prominent mountain range in California. The Sierra Nevada and the Central Valley belong to the Sierra Nevada microplate, an element of

Climate and topography control the size and flux of sediment produced on steep mountain slopes

“…1). In that case, the steepest slopes associated with the fastest erosion rates and coarsest sizes could have reached the modern catchment divide at Lone Pine Peak in just 2-4 My (SI Appendix), similar to the time elapsed since movement on the Sierra Nevada Frontal Fault accelerated base-level lowering of streams draining the range (34). This raises the possibility that the catchment itself (Fig.…”

Mechanics of relative and absolute displacements across normal faults, and implications for uplift and subsidence along the eastern escarpment of the Sierra Nevada, California

“…Based on reconstructed topographic surfaces across Owens Valley, Jayko (2009) estimated the onset of faulting as ca.9 Ma. Geomorphic relationships in the Sierra Nevada indicate that accelerated stream incision and subsequent knickpoint creation occurred between 3 and 10 Ma (Wakabayashi and Sawyer, 2001;Stock et al, 2004Stock et al, , 2005Clark et al, 2005;Wakabayashi, 2013). The geologic data collectively point to a period of enhanced faulting and volcanism along the eastern margin of the Sierra Nevada between ca.…”

“…(e.g., Christensen, 1966;Huber, 1981;Unruh, 1991;Wakabayashi and Sawyer, 2001;Jones et al, 2004;Stock et al, 2004Stock et al, , 2005McPhillips and Brandon, 2012;Wakabayashi, 2013). This view is based on a variety of stratigraphic and geomorphic data, including inclinations of bedding, analyses of rigid-body rotations (tilts), stream channel gradients, and incision rates, and exhumation rates derived from thermochronometric data.…”

Designing forward with an eye to the past: Morphogenesis of the lower Yuba River

“…At that time, the river flowed across an eastern plateau similar to the Altiplano of South America or the Tibetan Plateau in southern Asia (Wakabayashi, 2013). Sierra erosion and Sacramento Valley sedimentation rates were rapid during the Cretaceous, slow through the Eocene and Oligocene periods, and increased during the Plio-Pleistocene (Wakabayashi, 2013). Tilting caused by uplift to the east and subsidence to the west generated deep alluviation, channel avulsions, and fan extension in the Sacramento Valley to the west.…”

“…The ancestral Yuba channel system had formed by the Eocene Epoch with drainage divides well to the east of the present divide along the Sierra Nevada crest. At that time, the river flowed across an eastern plateau similar to the Altiplano of South America or the Tibetan Plateau in southern Asia (Wakabayashi, 2013). Sierra erosion and Sacramento Valley sedimentation rates were rapid during the Cretaceous, slow through the Eocene and Oligocene periods, and increased during the Plio-Pleistocene (Wakabayashi, 2013).…”