Late Jurassic ammonites from the western Sierra Nevada, California

Abstract

The ammonites from the Jurassic strata in the western part of the Sierra Nevada represent the Callovian, Oxfordian, and the lower part of the Kimmeridgian stages of the Upper Jurassic. There is no faunal evidence for the presence of any older Jurassic rocks, although the lower and middle parts of the Cosumnes formation have not furnished any fossils and could be in part older than the Callovian. The upper part of the Cosumnes formation has furnished one fragmentary ammonite indicative of a Callovian age. The lower part of the overlying Logtown Ridge formation has furnished many ammonites of early to middle Callovian age not older than the European zone of Sigaloceras calloviense. Because the formations grade into each other, the part of the Cosumnes formation that contains the Callovian ammonite must be of early Callovian age older than that zone. The early Callovian is definitely represented by the ammonites Kepplerites (Seymourites) and K. (Gowericeras) obtained near Colfax, Calif. These indicate an age slightly older than the ammonites in the lower part of the Logtown Ridge formation on the Cosumnes River. The late Callovian is represented by a large specimen of Peltoceras obtained near Indian Creek in Amador County, presumably from the Logtown Ridge formation. There is no faunal record of the early Oxfordian in California. The late Oxfordian to early Kimmeridgian is represented by the ammonite Idoceras obtained near the top of the Logtown Ridge formation on the Cosumnes River. The late Oxfordian is represented in the Mariposa formation by species of Perisphinctes (Dichotomosphinctes) and P. (Discosphinctes) that are virtually identical with species in Mexico and Cuba. These ammonites show that the lower part of the Mariposa formation and similar slatey units are locally of the same approximate age as the upper part of the Logtown Ridge formation on the Cosumnes River. The early Kimmeridgian is represented in the Mariposa formation by Amoeboceras (Amoebites) and probably by Subdichotomoceras in association with the pelecypod Buchia concentrica (Sowerby). The late Oxfordian to early Kimmeridgian is represented in the Monte de Oro formation by an ammonite, Perisphinctes (Dichotomosphinctes) cf. P. elisabethaeformis Burckhardt, and by two crushed specimens of Buchia cf. B. concentrica (Sowerby). The Mariposa and Monte de Oro formations have not furnished any fossils that are younger than the early Kimmeridgian. The same is true of the Galice formation of southwestern Oregon and northwestern California which is lithologically similar to the Mariposa and likewise is characterized by the presence of Perisphinctes (Dichotomosphinctes} and Buchia concentrica (Sowerby). As the Galice formation is overlain with pronounced angular unconformity by beds of middle to late Portlandian age (Riddle formation), the middle Kimmeridgian to early Portlandian time appears to be represented, at least in part, by the unconformity. The affinities of the Callovian ammonites in the Sierra Nevada are mainly boreal as indicated by the presence of such ammonites as Cadoceras, Paracadoceras, Pseudocadoceras, Kepplerites, and Gowericeras. The affinities of the late Oxfordian ammonites are mostly with ammonites in Mexico and Cuba but partly with those in Alaska. The early Kimmeridgian ammonite Amoeboceras indicates boreal affinities as the genus is rare in the Tethyan region. These affinities indicate that the seas in California were connected freely northward with Alaska and southward with southern Mexico.