Abstract

Samples collected from the northern meander of Monterey Canyon, California, and the adjacent Soquel Canyon include Cretaceous granodiorites and middle Tertiary basaltic andesites and sandstones. Plagioclase separated from the granodiorite basement rocks from Soquel Canyon yielded an age of 79 ± 0.8 Ma and are isotopically similar to Salinia-terrane granitoids exposed on the Monterey Peninsula. The Soquel Canyon granodiorite is crosscut by mafic dikes that are basaltic andesite in composition. Plagioclase separated from one mafic rock has been dated at 23.7 ± 0.5 Ma, consistent with the middle Tertiary pulse of volcanism characteristic of this region. This mafic unit also intrudes an overlying sandstone unit forming a “peperite” texture resulting from contemporaneous volcanism and sedimentation. The peperite constrains the lithic-rich sandstone, which we propose to have been deposited in a sedimentary basin associated with local tectonic extension, to a late Oligocene and (or) early Miocene age. The artifacts of the sedimentary basin are truncated (and deformed) on the south by the Monterey Bay fault zone, and exposed within the northern meander of the Monterey Canyon. These new lithologies require a revision of the Neogene lithostratigraphy of Monterey Bay and may also be useful in linking the local volcanic, tectonic and sedimentary history to the complex tectonic development of central California during the middle Tertiary. We suggest that strike-slip or transtensional movement along the Monterey Bay Fault Zone opened a basin in late Oligocene and (or) early Miocene into which was deposited a coarse, lithic-rich (Vaqueros?) sandstone. The contemporaneous volcanism of basaltic andesite is alkalic in character and may have been a result of mantle upwelling within a slab window or local transtension along the major faults active during this period.