Abstract

Basaltic rocks, hyaloclastites and fossil fragments incorporated in volcanic material were dredged from the flanks of Northeast Bank (lat 32°20′ N., long. 119°40′ W.) on the Southern California Borderland. Unaltered, fine-grained alkali basalt has 2.6 percent Ne and 10.8 percent Ol in the norm. Trace element abundances are typical of eastern Pacific basin alkali basalts but Ba is enriched (880 ppm). The fossil fragments, incorporated in breccia, hyaloclastite and agglomerate include a fauna which lived in less than 50 m of water, as well as forms from the intertidal zone. Bathymetry of the bank and the depth range of the dredged zone indicate that there has been at least 300 or as much as 500 m of subsidence since the volcanism which incorporated the fauna in volcanic material. Isostatic adjustments due to crustal load of the bank can account for this subsidence.

A fission track date of unaltered basaltic glass gives an age of 4.5 ± 0.5 m.y.; a K/Ar date on unaltered holocrystalline basalt gave an age of 4.36 ± 0.8 m.y.; the faunal assemblage is Plio-Pleistocene and manganese crust thicknesses are 0.2 to 1 cm. This is the youngest reported volcanism for the borderland. The magnetic anomaly due to the bank lies on the projected trend of anomaly 5c of the sea floor. It seems clear that this is fortuitous; there is no genetic relationship between these anomalies and these data do not support the suggestions that Mason-Raff lineations can be traced through the Borderland.

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