Depositional environments of an upper portion of the Modelo Formation, Los Angeles County, California

Abstract

A portion of the upper Miocene Modelo Formation exposed in the vicinity of Canton and Devil Canyons, Los Angeles County, California, represents lobes of a fan-delta and intervening finer grained interlobe deposits. Eleven lithosomes and three different, recurring combinations or associations of these lithosomes are recognized in the Modelo in this study area. A fossiliferous, yellowish-gray breccia, conglomerate, and sandstone association occurs at the base of the section as a series of obliquely stacked clastic wedges that pinch out progressively to the southeast upsection. The association is composed of six lithosomes which were deposited on the locally steep, distal, marine portion of the fan-delta mostly within the "transition zone" and upper slope environments of the fan-delta. Each wedge of the association represents a cycle of establishment, then abandonment, of an individual lobe of the marine portion of the fan-delta. An unfossiliferous, orange conglomerate and sandstone association occurs near the top of the Modelo Formation as a single clastic wedge that pinches out to the southeast. The association is composed of three lithosomes which were deposited on the proximal, subaerial portion of the fan-delta as multiple, superposed fan lobes in an arid climate. An interbedded mudstone and sandstone association, which interfingers with the two conglomerate and sandstone associations, is composed of three lithosomes which represent mostly deposition on the prodelta and some deposition on the shelf. The conglomerate clasts composed of Mendenhall Gneiss, quartz monzonite, anorthosite, and other anorthosite-related rocks come from local northeastern sources and from the San Gabriel Mountains to the southeast. Sandstone petrography indicates a source terrain of rugged relief and overall semi-arid climate. Measurement of imbricated conglomerate clasts indicates dispersal from a northeastern direction. The disparity between direction of dispersal and probable source terrain for clasts of anorthosite and anorthositerelated rocks substantiates 60 km (37 mi) of right-lateral movement along the San Gabriel fault during the Neogene. Influx of these clasts of anorthosite to form the fan-delta in the study area marks the inception of movement along the San Gabriel fault. The shoreline, which was to the northeast of the study area prior to deposition of the Modelo Formation, shifted to the southwest as sediment input temporarily filled the basin and the fan-delta became emergent, then transgressed rapidly back to northeast during deposition of the uppermost parts of the Modelo.