Tectonic Implications of Tertiary Conglomerates from Santa Monica Mountains, California

Charles L. Lane

Approximately 1,300 conglomerate clasts were collected from 23 different localities within the Santa Monica Mountains (SMM), representing upper Oligocene to middle Miocene units. An additional 250 clasts were collected from the upper Oligocene to middle Miocene units from the Santa Ana Mountains (SAM) and San Joaquin Hills (SJH). Clasts were slabbed, etched, and stained for K-feldspar; they were then analyzed using Dickinsonian ternary diagrams. The Sespe, Topanga Canyon, and Calabasas conglomerates from the SMM appear to reflect the unroofing of a magmatic arc. Conglomerates from the Sespe-Vaqueros Formations (undifferentiated) and Topanga Formation (undifferentiated) from the SAM and SJH reflect a source area that is more volcanic in nature than that indicated by conglo erates from correlative units in the SMM. Additionally, SMM volcanic clasts are more commonly dacitic to andesitic in contrast to the more rhyolitic metatuffs found in the SAM. Conglomerates from the Vaqueros Formation from the western SMM reflect a different source area than either the SAM clasts or clasts from other formations within the SMM. This western SMM clast population consists primarily of quartzites and gneisses, and probably represents a polycyclic deposit; paleocurrent data indicate a source nearly opposite to that for other clasts found in the SMM.

Paleocurrent data, when considered together with the clast information, suggest that Vaqueros Formation clasts in the western SMM were derived from a trench-slope area, whereas all other clasts in the SMM, SAM, and SJH were derived from a magmatic arc, probably located in the San Gabriel Mountains area. These data indicate the possibility of an arc-trench boundary configuration for this region as late as the early Miocene.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.