Beneath L.A.!

Beneath L.A.!


The Geology of Los Angeles Basin

The Los Angeles Basin encompasses the coastal lowland area bordered by the Santa Monica mountains, Puente, Elysian, and Repetto hills to the north, and the Santa Ana mountains and San Joaquin hills to the southeast. In geological terms this basin can be classified as a pull-apart basin, forming where two overlapping faults create an area of extension, and causing the basin to sink down. This subsidence allows the accommodation space for the deposition of sediments. More than 9000 m of sediments accumulated in the basin, and includes the Vaqueros or Sespe formations (late Oligocene-early Miocene), Topanga Formation (early-middle Miocene), Puente Formation (middle-late Miocene), Monterey or Modelo formations (late Miocene), and the Fernando or Pico Formation (Pliocene). Overlying these are a number of Pleistocene units, including the Lomita Marl, Palos Verdes Sand, San Pedro Sand, Timms Point Silt, and numerous unnamed Quaternary marine terrace deposits.


Tuna Canyon Formation (Late Cretaceous)

Thumb_Inoceramus-subundatusInoceramus subundatus
Thumb_Pinna_Tuna-CanyonPinna sp. Thumb_MetaplacenticerasMetaplacenticeras sp.

Coal Canyon Formation (Paleocene)

Thumb_FusinusFusinus sp. Thumb_Turritella-infragranulataTurritella infragranulata Thumb_TeredoTeredolites isp.

Topanga Formation (Miocene)

The Topanga Formation comprises mixed sedimentary and volcanic rocks with an erosional base (unconformity) on underlying sediments. The formation consists of basal marine conglomeratic sandstone, a more volcanic middle layer with submarine lava flows and tuffs, and an upper sedimentary breccia, conglomerate, sandstone, and a siltstone.

Thumb_Dosinia pondersoaDosinia ponderosa
Thumb_Mytilus-mathewsoniiMytilus topangensis Thumb_Pinna_TopangaPinna sp. Thumb_Saxidomus-nuttalliSaxidomus nuttalli
Thumb_Calyptraea-mamillarisCalyptraea mamillaris Thumb_Calyptraea-spCrucibulum sp. Thumb_Ocenebra-topangensisOcenebra topangensis Thumb_Trophon-bartoniTrophon bartoni

Pico Formation (Pliocene)

Thumb_ARCO-tower-slab1Sectioned slab of Pico Formation
Thumb_ARCO-tower-polished-slab2Polished slab of Pico Formation Thumb_Metro-excavation-snailsNeverita reclusiana Thumb_Metro-excavation-shellsDentalium hexagonum

Palos Verdes Sand (Pleistocene)

Thumb_Dallocardia-quadragenariumDallocardia quadragenarium
Thumb_Macoma-sectaMacoma secta Thumb_Mactromeris-hemphilliMactromeris hemphillii Thumb_Crossata-californicaCrossata californiensis

Relevant literature

Yerkes, R.; McCulloch, T.; Schoellhamer, J.; Vedder, J. (1965). “Geology of the Los Angeles Basin, California- An Introduction”. Geological Professional Survey Paper: A1-A55.