Simi Valley (valley)

[5] The Sespe Formation consists of alluvial fan and floodplain deposits of interbedded pebble-cobble conglomerate, massive to thick-bedded sandstone, and thin-bedded siltstone and clay-stone.

[6] In the Las Posas Hills, Sespe Formation is unconformably overlain by marine sandstones of the middle Miocene Topanga Group that are interlayered with and intruded by basalt flows, breccia, and diabase dikes of the Conejo Volcanics.

Locally, Modelo Formation consists of interbedded diatomaceous shale, claystone, mudstone, and siltstone with minor sandstone, limestone, chert, and tuff beds.

The most widely exposed rock units in the area are the Plio-Pleistocene marine and non-marine Pico and Saugus Formations that crop out on the southern flank of South Mountain-Oak Ridge.

Pleistocene- to Holocene-age landslide deposits are widespread throughout the Simi quadrangle, especially in the finer grained Tertiary sedimentary units where bedding planes are dip slopes.

Seismic and well data from the San Fernando Valley document evolution of that region from mid-Miocene rifting to north-south contraction.

base of the Conejo Volcanics, identified in oil well logs, is inferred to have a dip-slip separation of about 425 to 550 m, suggesting a low long-term slip rate of about 0.03 mm/yr.

However, substantial late Quaternary offset is suggested by the presence of more than 150 m of Pleistocene and younger alluvium that fills the east-west trending, down-dropped bedrock trough beneath western Simi Valley.