Abstract

The Puente Formation is a Middle-Upper Miocene clastic unit lying unconformably on the Lower-Middle Miocene El Modeno Volcanics and Topanga Group, in the Los Angeles basin. The Puente Formation, about 3900 m thick, is composed of conglomerate, sandstone, and mudrock deposited as a submarine fan at bathyal depths. Several intrabasinal discordances suggest tectonic activity during deposition. The succession consists of two main upward-thickening and - coarsening megacycles, reflecting submarine-fan progradation. The Puente Formation is characterized up-section by: (1) thin-bedded sandstone and shale ( La Vida Member ) grading to thick-bedded sandstone and conglomerate ( Soquel Member ); (2) thin-bedded mudrock and sandstone ( Yorba Member ) grading to thick- to very thick-bedded sandstone and conglomerate ( Sycamore Canyon Member ). Sandstones of the Puente Formation are quartzofeldspathic (Qm 35 F 54 Lt 11 ); their compositions suggest local provenance from the plutonic, volcanic, and metamorphic rocks of the San Gabriel Mountains and surrounding areas. Petrologic parameters, however, suggest variable contribution of these source rocks through time. Four petrofacies, with distinctive parameters, coincide with the lithostratigraphic subdivisions. Coarse-grained plutonic rock fragments are abundant throughout the succession and consist of plagioclase-rich plutonic rocks, probably sourced, in part, from the Lowe Granodiorite. Lathwork, microlitic to felsitic volcanic lithic grains are also present in the lower and middle part. In the La Vida petrofacies, there is also an intrabasinal contribution (intraclasts and bioclasts) from shelfal areas. In the Yorba petrofacies there is a local increase of volcanic detritus (Lv/L = 0.81), represented by coarser volcanic lithics and abundant volcaniclastic matrix. Metamorphic detritus is not very abundant. The Sycamore Canyon petrofacies is dominantly plutoniclastic (Rg/R = 0.90; hornblende-bearing plutonic rock fragments), including very abundant hornblende grains. The plutonic detritus is dominantly plagioclase-orthoclase-biotite-bearing in the lower part, and hornblende-bearing in the upper part, suggesting unroofing of the Lowe Granodiorite Complex as a key element of uplift of the San Gabriel Mountains. Other Neogene sandstones deposited in the Los Angeles Basin also consist dominantly of plutoniclastic detritus related to the unroofing of arc-related plutonic rocks (dissected magmatic arc). For example, the up-section increase in plutonic detritus is consistent with the composition of the Upper Miocene-Lower Pliocene Capistrano Formation (Qm 50 F 47 Lt 3 ), which has a composition identical to the Sycamore Canyon petrofacies. There is consistent provenance signal in spite of complex transrotational tectonics, responsible for opening of the Los Angeles Basin, and later transpressional processes, which are still active. Detailed provenance study of the Puente Formation and related units provides important constraints on paleogeographic and paleotectonic reconstructions of southern California basins and uplifts.

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