Abstract: Petrology and Provenance of the Modelo Formation, Santa Monica Mountains, Southern California: Constraints on Displacement Along the San Gabriel Fault

Peter E. Rumelhart, Raymond V. Ingersoll

The Modelo Formation of the central and eastern Santa Monica Mountains is an upper Miocene siliceous-diatomaceous shale and sandstone submarine-fan sequence, which was deposited in the northern Los Angeles basin. The graded, poorly sorted, and angular sandstones comprise channelized mid-fan facies deposited at middle-to-upper-bathyal depths; they are arkosic (58% feldspar) with variable amounts of matrix. Published paleocurrent data yield a south-southwest flow direction. The direction of the paleocurrents constrains the timing of deposition to be after rotation of the Transverse Ranges (after 15 Ma.). Biostratigraphic data indicate that deposition commenced during the late Mohnian (after 10.8 Ma.).

Gazzi-Dickinson sandstone modes from the basal section (QFL=49-36-15) differ from the remaining section (QFL=32-58-10). Plagioclase/ total feldspar values are 0.32 and 0.68, respectively. Dominant lithic types in the basal section are quartz-mica-feldspar aggregates, with minor amounts of volcanic, meta-igneous, and sedimentary lithics. Dominant lithic types in the rest of the section are andesitic volcanic, meta-igneous, and quartz-mica-feldspar aggregates, with minor sedimentary lithics. Typical LmLvLs percentages above the basal section are 46-48-6. Distinctive meta-igneous fragments found above the basal section are characterized by aggregates of recrystallized feldspar. Lithic modes are rich in volcanic and meta-igneous types with progressive enrichment in metasedimentary lithic ragments upsection.

QFL plots indicate that the provenance was basement uplift to transitional arc (e.g., Dickinson, 1985). The meta-igneous lithic fragments and the high feldspar content of the rocks suggest an anorthositic, gneissic, tonalitic, or mylonitic source terrane. Sandstone modes and conglomerate clasts indicate that the basal section was derived locally from the Santa Monica Slate. The upper section represents a volcanic unroofing sequence, as evidenced by decreasing volcanic lithic fragments upsection.

Crowell (1982) suggested 60 km of right slip on the San Gabriel fault between 12 and 5 Ma. Palinspastic restoration places

the Santa Monica Mountains immediately south of the San Gabriel Mountains, which were the likely source for Modelo sandstones. Based on plagioclase composition (95% oligoclase and albite) of Modelo sandstone, the anorthosite body in the western San Gabriel Mountains is an unlikely source. However, a source in the mylonitic, gneissic, and tonalitic bodies of the eastern San Gabriel Mountains could explain the observed petrography. This source for the Modelo is consistent with 60 km of movement on the San Gabriel fault.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994