Fluvial-dominated deltaic deposits of the marine Plio-Pleistocene lower Saugus Formation, northern Simi Valley, California

Abstract

The Plio-Pleistocene marine lower Saugus Formation is well exposed in the Santa Susana Mountains north of Simi Valley, California. The lower Saugus is composed mostly of interfingering sandstone, coquinite, and conglomerate deposits, with rare occurrences of intrasparrudite limestone and laminated mudstone. The beds unconformably overlie the Miocene Modelo Formation throughout most of the study area. The lower Saugus, in the study area, is interpreted to represent a fluvial-dominated delta that is herein named the "Simi Delta". Lithosomes interpreted to be prodelta deposits include clast-supported phosphorite conglomerate and intensely bioturbated, fine to medium sandstone. Delta-front deposits include bedded medium sandstone, serpulid-bearing sandstone, and oyster-pecten coquinites. Distributary-mouth bar deposits are composed of well sorted, parallel-laminated, medium sandstone. Interdistributary bay deposits are composed of poorly sorted, silty, fine sandstone and commonly contain abundant macrofossils. A laminated, lensoidal mudstone, exposed in the Gillibrand Quarry area, is interpreted to represent an abandoned channel which was filled with clay. In Browns Canyon, an angular discordance which is interpreted to represent the transition between topset and foreset beds on a Gilbert-type delta is exposed stable or slightly emergent. This Gilbert-type delta requires a tectonic regime to form. In the Gillibrand Quarry area, where distributary-mouth bar deposits are exposed, a subsiding receiving basin is required. This difference in tectonic regimes suggests that the "Simi Delta" consisted of at least two different delta lobes, which were formed at two different times. Provenance includes an igneous, a high-rank metamorphic, and a reworked sedimentary source. The presence of anorthosite and gneissic clasts indicates that during the Plio-Pleistocene, the San Gabriel Mountain complex was a highland and was shedding these distinctive clasts into the westward-flowing Plio-Pleistocene Santa Clara River which deposited its sediment on the "Simi Delta". Between 1,000,000 and 70,000 years before present, the Santa Susana Mountains were uplifted causing the Pleistocene Santa Clara River to shift to its present course and abandon the "Simi Delta" depositional site.