Abstract: Composition and Provenance of the Puente Formation (Miocene), Los Angeles Basin

Salvatore Critelli, Peter E. Rumelhart, Raymond V. Ingersoll

The Puente Formation (PFm) is a middle to upper Miocene clastic unit lying unconformably on the middle to lower Miocene El Modeno Volcanics and Topanga Group, within the Los Angeles basin (LAB). The PFm, about 3900m thick, is composed of sandstone, conglomerate, and mudrock deposited on a submarine fan at bathyal depths. Several intrabasinal discordances suggest active tectonics during deposition. The succession consists of two main upward thickening and coarsening megacycles reflecting submarine fan progradation. The PFm is characterized up-section by: (1) thin-bedded fine sandstone and shale (La Vida M.) grading to thick-bedded coarse sandstone and conglomerate (Soquel M.); (2) thin-bedded siltstone, mudrock and sandstone (Yorba M.) grading to thick- to ve y thick-bedded coarse-grained sandstone and conglomerate (Sycamore Canyon M.).

Sandstones of the PFm are quartzofeldspathic and suggest a probable local provenance from the plutonic, volcanic, and metamorphic rocks of the San Gabriel Mountains. Petrological parameters, however, suggest variable contribution of these source rock units through time. Coarse-grained plutonic rock fragments are abundant for the entire succession and consist of plagioclase-rich plutonic rocks perhaps sourced from the Lowe granodiorite. Microlitic, lathwork to felsitic volcanic lithic grains are also present in the lower and middle part. In the Yorba M. there is a local increase of volcanic detritus (Lv/L = 0.80), represented by larger volcanic lithics and abundant volcaniclastic matrix. Metamorphic detritus is not very abundant; it is concentrated in the La Vida M.

In this part of the sequence, there is also an intrabasinal contribution

(intraclasts and bioclasts) from shelfal areas. The sandstones of the Sycamore Canyon M. are dominantly plutoniclastic, as shown by abundant hornblende. The up-section increase of plutonic detritus is also suggested by the composition of the upper Miocene-lower Pliocene Capistrano Formation that has an identical composition as the Sycamore Canyon petrofacies.

A comparison with other sandstones deposited within LAB suggests quartzofeldspathic detritus related to the progressive unroofing of arc-related plutonic rocks. However, Neogene transrotational tectonics, responsible for opening of the LAB, has resulted in exhumation of other source rocks that locally contributed to the LAB. The PFm, therefore, represents sedimentation during tectonically active time in the evolution of southern California.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994