Abstract

A provenance and paleocurrent study of the middle and upper Pliocene continental Paso Robles Formation in the Salinas Valley area, California, adds to the knowledge of the late Cenozoic geologic history of the California Coast Ranges. The geographical distribution of heavy minerals and pebbles in the Paso Robles Formation and in streams presently draining proposed source areas leads to the same conclusions as do measurements of foreset beds, pebble imbrication, and channels in the formation. Uplift in the Santa Lucia and La Panza ranges initiated Paso Robles deposition in early Pliocene time. The paleodrainage was southeastward from the Santa Lucia Range and northward from the La Panza Range, continuing across the present site of the Temblor Range and into a marine basin at the present site of the southern San Joaquin Valley. The Salinas River was a relatively unimportant stream during Paso Robles deposition.

Near the end of the Pliocene, uplift in the Temblor Range and southwestward tilting of the Gabilan Mesa brought about the end of Paso Robles deposition by defeating the southeast-flowing drainage and creating the conditions for its capture by the modern Salinas River. Lithologic differences between juxtaposed beds of the Paso Robles Formation on either side of the San Andreas fault suggest that about 25 miles of right-lateral movement has occurred since deposition of the formation.

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