Molluscan paleontology of the Pliocene Peace Valley beds and Ridge Route formation (Ridge Basin group), Ridge Basin, southern California

The Ridge Basin, 90 km northwest of Los Angeles, California, lies within the San Gabriel, San Andreas, Liebre, and Clearwater fault zones. The basin is an elongated, wedge-shaped, intermontane, Miocene-Pliocene basin filled with about 12 000 m of marine, lacustrine, fluvial, and alluvial sediment. More than half of the thickness of the Ridge Route formation and about one fifth of the Peace Valley beds was examined for molluscs along the northeastern side of the basin and more than 600 m of section were measured. The nonmarine molluscs of the Ridge Route formation and Peace Valley beds consist of ten species: one unionid, Anodonta oregonensis (Lea); three sphaeriids, Musculium lacustre (Muller), M. transversum (Say), and Pisidium (Cyclocalyx) nitidum Jenyns; four hydrobiids, Fontelicella aff. F. (F.) truckeensis Yen, Tryonia aff. T. protea Gould, and Hydrobiidae species 1 and 2; one lymnaeid, ?Stagnicola sp.; and one planorbid. ?Planorbula aff. P. armigera (Say). All of the hydrobiids occur stratigraphically in the lower portion of the Ridge Route formation and Peace Valley beds. The other nonmarine molluscs occur in the Ridge Route formation and Peace Valley beds above the hydrobiids except in measured section 3, where Fontelicella aff. F. (F.) truckeensis and Hydrobiidae species 2 are associated with Musculium lacustre, M. transversum, and ?Planorbula aff. P. armigera. Freshwater environments are indicated by the presence of Anodonta Musculium, Pisidium, Fontelicella, ?Stagnicola, and ?Planorbula, whereas brackish water environments may be indicated by Tryonia or hydrobiids. Tryonia aff. T. protea is interpreted to tolerate a wide salinity range. Stagnant water is indicated by Stagnicola and Planorbula. Shallow freshwater is suggested by ?Planorbula aff. P. armigera or Pisidium (Cyclocalyx) nitidum. ?Stagnicola, together with ?Planorbula aff. P. armigera, is assumed to represent shallow ponds or marshes on a floodplain. Ridge Basin molluscs occur in sediment ranging from claystone to conglomerate although the majority of the molluscan fauna is found in siltstone, very fine-grained wacke, and claystone. The lack of molluscs in the mudstone of the lower Peace Valley beds (lacustrine facies), and the presence of the fauna in the nearly stratigraphic equivalent sandstone and siltstone units of the Ridge Route formation (fluvial-deltaic facies), may suggest fluctuating salinities in the lacustrine environment with penecontemporaneous stable salinity conditions in the fluvial-deltaic environment. Deep water, high turbidity with sediment influx, or reducing conditions are alternative explanations for the paucity of molluscs. Ridge Basin nonmarine molluscs do not suggest an age for the basin. Chlamys (Chlamys) cf. C. (C.) hodgei suggests a Miocene or later age for the lowermost Ridge Route formation (Wilson 1978). The ostracodes probably indicate a late Miocene age for the lower Ridge Basin Group; however, recent magnetostratigraphy suggests an early Pliocene age for the lowermost Ridge Basin Group (Ensley and Verosub 1979)