PETROGRAPHY AND SEDIMENTARY ANALYSIS OF THE ANOMALOUS OFFSHORE COARSE, YELLOW-BROWN SANDS BLANKETING THE MID TO OUTER SAN PEDRO/LONG BEACH SHELF, SOUTHERN CALIFORNIA, USA

The mid to outer San Pedro Shelf (SPS) southeast of the Palos Verdes Peninsula is the benthic gateway to the bustling Port of Los Angeles (one of the largest US ports). Benthic grab and sediment core samples collected from the SPS have largely focused on sediment texture and associated benthic fauna. A substantial portion of the mid to outer SPS is thinly mantled with an anomalously coarse, well sorted, yellow-brown sand that has eluded detailed petrographic examination. The SPS sands are located around a possible bedrock exposure, based on dredging and geophysical surveys conducted over the past 50 years.

The siliciclastic component of SPS is archetypical coarse sand, with about 75% by weight ranging between -1 and 0 phi. Pebbles and cobbles are not evenly distributed in the sand, but are congregated in places. The sand is highly polished and grains are coated with a thin veneer of iron oxide, giving the sand is anomalous color. The iron oxide veneer obscures the grains and was removed with HCl for analysis. Additionally, the HCl dissolved carbonate bioclasts in the sand, concentrating the siliciclastics and facilitating grain identification.

Compositionally, the sand is primarily quartzofeldspathic arenite, with generally over 50% feldspar. Quartz grains are typically polycrystalline, and quartz is often frequently interlocking with feldspar. Collectively, feldspar and quartz comprises around 3/4 of identified grains. Monocrystalline quartz is not that common. Many of the feldspar and quartz grains have chlorite patches in fractures and along grain edges. Other lithics include fine “dirty” sand-siltstones with well rounded grains and matrix; rare dark cherts; quartz-muscovite phyllite/schists; epidote-chlorite-quartz-feldspar grains; and greenschist fragments.

The SPS sand may have been derived from older Pleistocene deposits mantling the shelf, and may be correlative with the Palos Verdes Sand (PVS) of Woodring, Bramlette, and Kew (1946). Reports of ecologic and stratigraphic admixtures of Recent and Pleistocene forams in the SPS sand (Crouch, 1954) suggest that a possible source may be the PVS or equivalent.

A single partially exposed stratigraphic section in northeastern San Pedro reveals that the basal, fossiliferous yellow-brown (PVS) fills interstices in bioeroded cobbles/boulders of Miocene Monterey and is somewhat comparable to the SPS.