EVIDENCE FOR LATE CRETACEOUS-PALEOGENE BASIN INVERSION, UPLIFT, AND EROSIONAL STRIPPING IN THE KLAMATH MOUNTAINS, PRIOR TO EOCENE COLLISION OF SILETZIA

The regional Late Cretaceous forearc basin in western California likely continued north to the Ochoco basin in central Oregon and blanketed the intervening Klamath Mountains Province (KMP). Sometime during latest Cretaceous to Paleogene time, exhumation and erosion of the KMP portion of the forearc basin initiated, but the timing and mechanisms of this process are not well understood. The sparse Cretaceous-Paleogene sedimentary record in the KMP consists of small Cretaceous outliers and Late Cretaceous to Eocene marine and terrestrial sedimentary rocks preserved around its margins. In this study, we compile published and new detrital U-Pb zircon age, εHf isotopic, and vitrinite reflectance data from the Dothan Formation (Fm), Montgomery Creek Fm, and several Cretaceous outliers mapped as Great Valley Sequence (Big Bar, Readings Creek) and Hornbrook Fm (O’Brien) in the KMP, integrated with bedrock thermochronometry (zircon (U-Th)/He, ZHe; apatite fission track, AFT; apatite (U-Th)/He, AHe) to chronicle Cretaceous to Miocene burial, exhumation, erosion, and erosion in the in the KMP.

The Early Cretaceous Great Valley Sequence (Readings Creek outlier; this study) yields detrital U-Pb ages and εHf geochemistry consistent with sources in the KMP, indicating some topographic relief in the Klamath Mts at that time. Vitrinite reflectance values, geologic mapping, provenance data, and bedrock thermochronometry record subsequent subsidence and deposition of ~4-10 km of Late Cretaceous forearc basin sediments. Preliminary thermal models using thermochronometers from Klamath plutons require burial and thus reheating to reset AFT and AHe systems, followed by cooling, exhumation, and erosional removal of the Late Cretaceous section sometime during ~75-55 Ma, prior to early Eocene collision of the Siletzia terrane in the northern KMP. The following observations are consistent with this timing of basin inversion: (i) Klamath plutonic clasts in the early Eocene Umpqua Group indicate at least some of the KMP was at the surface by early Eocene time; (ii) a major unconformity in the Montgomery Creek Fm records erosion or nondeposition between 85 and 45 Ma; and (iii) evidence for recycling of Late Cretaceous sediments into the Eocene upper Montgomery Creek Fm suggests basin inversion between 85 and 45 Ma.