EARLY PALEOGENE SEDIMENT DISPERSAL AND PALEOGEOGRAPHY OF THE NORTHERN CALIFORNIA COAST RANGES: NEW INSIGHTS FROM DEPTH-PROFILED DETRITAL ZIRCON U-PB AGES

The role of large-scale (>1000 km) northward translation of western North America terranes during the Late Cretaceous and Paleogene remains controversial. Although recent studies focus on rocks in southern California and the Pacific Northwest, intervening regions have received less attention despite their relevance to this issue. Recent geologic mapping in northern California west of Ukiah reveals previously unrecognized marine clastic rocks in fault contact with the underlying Central belt of the Franciscan Complex that provide important constraints on Late Cretaceous-Paleogene paleogeography. The newly mapped rocks include a lower, locally sheared and boudined turbidite unit of Santonian-Campanian age and an upper, little deformed massive arkosic sandstone unit (15-30% detrital K-feldspar) of unknown age. These two units share affinities with Great Valley forearc basin strata to the east, and the lower unit is correlative in age with the Franciscan Novato Quarry terrane.

New depth-profiled detrital zircon (DZ) U-Pb ages (726 analyses from 3 samples) provide novel constraints on the maximum depositional age (MDA) and provenance of the upper, arkosic unit. Seventeen grains yielded latest Cretaceous to Paleocene ages, 15 of which overlap within 2σ uncertainty to yield a weighted mean age of 64.4 ± 1.2 Ma (±2σ), representing the MDA of the unit. The majority of these young ages are interpreted to be metamorphic rims on Cretaceous (ca. 85-115 Ma) or Proterozoic (ca. 1.3-1.4 Ga) cores. An additional group of grains has 125-145 Ma igneous rims on 165-200 Ma cores. The DZ U-Pb ages suggest their derivation from central Idaho with additional sources in the Klamath Mountains and (or) northern Sierra Nevada. Approximately two thirds of analyses are Mesozoic in age with major age peaks at 87 Ma, 109 Ma, and 162 Ma. The majority of remaining analyses are Proterozoic in age with major peaks at 1.36 Ga and 1.71 Ga. These samples may provide the first evidence that Idahoan detritus was delivered to northern California between previously documented episodes in Campanian and Eocene time. Existence of a long-lived sediment routing system from Idaho to northern California casts doubt on large-scale dextral displacement inboard of the subduction zone in northern California during Late Cretaceous-Paleogene time.