Depositional environments and paleogeography of the Lower Saugus Formation, Northern San Fernando Valley, Los Angeles County, California

The lower Saugus Formation along the southern foothills of the Santa Susana Mountains and the San Gabriel Mountains, northern San Fernando Valley, Los Angeles County, California is a sequence of Plio-Pleistocene deposits that records the progradation into the ocean of a large delta and some associated small streams. Study of these rocks reveals 12 lithosomes. The lithosomes include marine deposits of lithosomes A (slightly fossiliferous pebble conglomerate), B (fossiliferous sandstsone), C (slightly fossiliferous sandstone), D (fossiliferous muddy sandstone), and E (algal mudstone), and fluvial deposits of lithosomes F (unfossiliferous cobble conglomerate), G (laminated conglomeratic sandstone), H (structureless sandstone), I (muddy fine sandstone), J (red sandy mudstone), K (varicolored non-sandy mudstone), and L (white sandy limestone). Rocks in the middle part of the study area were deposited in a river-dominated, braided delta system at first and later in a meandering fluvial system. The marine portion of the delta system included deposits of the lower delta plain, small distributary channels, interdistributary bay, and delta front. Storms sometimes influenced the bay. The meandering river carried mainly sand- and mud-sized particles and formed mostly braided bar, point bar, flood basin, and backswamp deposits. One of the major channels of this river, the ancient Santa Clara River, flowed in the area of the San Fernando Reservoir during depostion of the lower part of the lower Saugus Formation and then possibly migrated to the east as the delta prograded to the south and west. Rocks in the eastern and western parts of the study area were deposited in two separate braided fluvial systems. A wide range of grain sizes in these braided streams was available. Gravelly braided bars are the main deposits in the two areas. Complete fining upward sedimentation cycles are common. Marine deposits are absent.. The San Gabriel Mountains were the major source area for the clasts in the rocks. The Simi Hills and other hills to the north provided a part of the clasts. The ancient Santa Claria River and other braided streams carried the clasts into the basin. Near the end of the Pliocene at the beginning of deposition of the lower Saugus Formation in this area, the San Gabriel Mountains stood as a highland to the east and the Simi Hills existed as an offshore island to the west. The area between Limekiln Canyon and the San Fernando Reservoir was a shallow, deltaic bay environment. As time passed into the early Pleistocene, the delta prograded southward and westward and the bay was filled with meandering river deposits. At the same time to the east, small braided rivers deposited detritus at the foot of the San Gabriel Mountains.