Geology of the southern Ridge basin, Los Angeles County, California: with a section on economic geology

Abstract

The marine Modelo formation and the overlying non-marine Ridge Route formation, as exposed in the southern part of the Ridge Basin, are similar in general appearance and consist of alternating beds of sandstone and siltstone. The contact between the formations is not a sharp break. Interfingering of the two sedimentary units was not observed, but such interbedding may exist. The two formations differ in strike by as much as 35 degrees in the area studied, although the strike changes uniformly from one formation to the other. This difference in attitude is believed to be a depositional feature, rather than one resulting from structural disturbance. The Modelo formation thins markedly in a northerly direction, and the sea in which it was formed appears to have transgressed the area from south to north during a part of upper Miocene time. Subsequently the sea retreated from the area, and non-marine Ridge Route beds were probably deposited concomitantly from a northerly direction. There is no evidence of a considerable time apse between the periods of marine and non-marine deposition. The lowermost beds of the Ridge Route formation may be fluviatile, but the remainder of the formation appears to be of lacustrine origin, in post-Pliocene time the region was folded into the asymmetrically synclinal Ridge Basin. The folding probably was related to displacements along the adjacent San Gabriel fault.

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