Southern Continental Borderland, Baja California: Its Tectonic and Environmental Development

Abstract

A study was made of the Continental Margin off Baja California, between 29° 30′ N. and 32° N., combining seismic profiling, dredging, and lithologic and biostratigraphic analyses. Bathymetry and seismic profiling show a ridge-basin structure and a distinct structural and topographic escarpment between the deep-sea floor and the Continental Margin throughout. Much of the southern borderland is shallower than 2,000 m and no true oceanic depths occur. Dredgings on ridges of the southern borderland brought up acidic and basic igneous rocks, metamorphosed sediments, phosphorite, and strata like those of the Cenozoic marine sections of southern California and the northern part of the Continental Borderland. Biofacies of the sedimentary rocks include upper Paleocene, middle Miocene, upper Miocene, and Pleistocene assemblages, all comparable to those of the biofacies of similar ages in southern California. The Miocene biofacies are in distinct contrast to oceanic benthic biofacies of the middle and upper Miocene in the experimental Mohole, just to the west. Upper Miocene biofacies atop Soledad Ridge, and on many other ridges of the Continental Borderland, are deep water in character as are those of the Miocene Monterey Formation of California. They show major uplift of the ridges since their deposition. Planktonic biofacies of late Miocene strata on Soledad Ridge indicate that this deep-water sediment correlates with Neogene Zones 17 and 18. The age of Neogene Zone 18, correlated with the Gilbert and lowermost Gauss Magnetic Epochs, indicates that uplift of the ridge commenced later than about 4 m.y. ago. It is suggested that formation of the Continental Borderland structural patter n correlates in time with the opening of the Gulf of California, which developed in the last 4 m.y. Uplift, averaging from 6 cm to 9 cm per year for the past 11,000 yrs, characterizesbasins just to the north of the southern borderland. This has contributed in large part to the southern borderland having depths some 450 m greater than those of the Continental Borderland off southern California. Bathymetry, structure, varied continental lithology, sedimentary relations, and paleoecology of marine strata exposed on ridges of the southern borderland are consistent with similar characteristics of the northern part of the Continental Borderland. They indicate that the southern borderland is not a rhombochasm but that the entire Continental Borderland is one major structural ridge-basin unit.