INTRODUCTION TO THE FACIES OF THE RIDGE ROUTE FORMATION, RIDGE BASIN, SOUTHERN CALIFORNIA

The upper Miocene nonmarine Ridge Route Formation is over 9,000 m thick and consists of alluvial fan, fluvial, and shoreline facies. It records marginal sedimentation on the northeast side of the basin. The alluvial fan facies consists of white to red conglomerate and breccia and sandy griiss which is transitional into finer-grained strata to the southwest. The alluvial fan facies formed on basementrock highlands or along faults. Two fluvial styles of sedimentation are interpreted: (1) Deposition in small braided channels and upon fluvial outwash plains resulting from sheet-flood processes, and (2) Deposition in larger meandering-channel complexes with levee and interchannel deposits formed by pointbar accretion and overbank flooding. These strata are composed mainly of cross-bedded and laminated sandstone, interbedded with mudstone. The shoreline facies includes beach, nearshore, and fluvial-deltaic deposits which interfinger with Peace Valley Formation shales to the southwest. These latter facies consist of interbedded carbonate and clastic strata containing mollusks, stromatolites, ostracodes, oolites, as well as interbeds of coarse terrigenous sediment. Paleocurrents indicate that the alluvial fan, fluvial, and shoreline facies of the Ridge Route Formation were derived from the northeast and are laterally transitional into one another.