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Description
Supermature, ultrastable puddingstones collected from conglomerate outcrops in the Cretaceous Valle Group and the Eocene Las Palmas Gravels may not be derived from the unroofing of the Peninsular Ranges batholith (PRB). A detailed petrographic analysis along with framework grain provenance diagrams suggest a tectonic provenance other than the PRB for the Valle Group and possibly the Las Pal mas Gravels as well. Although the Valle Group and Las Palmas Gravels puddingstone cobbles are similar there are significant differences. The QFL plots for the Valle Group display high percentages of quartz, zero feldspar, and/or high percentages oflithics (chert), along with silica cement and no matrix. The QFL plots for the Las Palmas Gravels samples, except for three, contain moderate percentages of quartz, low percentages of feldspar, and high percentages of lithics (i.e. chert and volcanics), along with abundant hematite cement and no matrix. For the Valle Group samples this implies a hot and extremely wet climate in order to remove all but the most stable minerals. Furthermore, Valle Group cobbles contained well rounded quartzite clasts within the cobbles. This implies the weathering and erosion of a metasedimentary sequence that had severe climatic conditions in order to destroy all unstable metasedimentary units within the sequence. The abundant hematite cement in the Las Palmas Gravels samples was probably the result of a hot climate with intermittent periods of moisture allowing for the alteration of iron-bearing detrital grains to form hematite cement. Rounded detrital grains of zircon, tourmaline, and some rutile are present in almost every cobble from both locations indicating a recycling event through an older sandstone originally derived from a plutonic source. However, abundant chert clasts that are angular to rounded are found in almost every sample; this implies an original depositional event. The provenance models used for this investigation do not account for every type of sediment origin and dispersal, therefore intuitive reasoning from previous studies are used along with these models to determine tectonic provenance for both samples.
