The Los Angeles Basin contains what is known as the "Great Unconformity" which has been interpreted as a large scale erosional event in the basement rock unit. This unconformity is used to correlate strata throughout the basin. The record of the Cenozoic activity begins above this unconformity. The stratigraphic record for this basin indicates that it began as a non-marine environment and then transgressed to a deep ocean system. The oldest basement units of this basin are of both sedimentary and igneous origin. The sedimentary unit was metamorphosed as a result of slippage of the Newport–Inglewood fault and is known as the Catalina Schist. The Catalina Schist can be found on the southwestern edge of the basin and is predominantly a chlorite-quartz schist. Closer to the Newport–Inglewood fault zone, garnet-bearing schists and metagabbros occur. The Santa Monica Slate can be observed in the northwestern block of the basin. The eastern complex is characterized by Santiago Peak Volcanics. This rock unit contains andesitic breccias, flow, agglomerates and tuffs.
