The Hornbrook Formation of southern Oregon and northern California preserves an important record of regional mid- to Late Cretaceous tectonic events that improves our understanding of paleogeography and regional tectonics for this critical time in the development of the North American Cordillera. Initial deposition in the Hornbrook basin likely began in a hanging-wall basin as footwall-block uplift exhumed the Klamath Mountains and generated a proximal sediment source for the lower members of the Hornbrook Formation. Kathleen Surpless and Emily Beverly suggest that changing plate motions along the U.S. Cordilleran margin during the Late Cretaceous shifted Hornbrook sediment sources to newly uplifted regions east and southeast of the basin, as continued subsidence occurred in the Klamath Mountains. Upper members of the Hornbrook Formation received sediment largely derived from the main Cretaceous Sierra batholith, with little to no sediment derived from the Klamath Mountains. A similar shift in sediment sources occurs in the Upper Cretaceous Great Valley Group of California, which likely formed the southern continuation of the Hornbrook basin during the Late Cretaceous. Thus, the record of changing sediment sources preserved within the Hornbrook Formation provides evidence of significant regional tectonic events in the mid- to Late Cretaceous Cordillera.
Fault-zone controls on the spatial distribution of slow-moving landslides
Joel S. Scheingross et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Posted online 11 Jan. 201
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