Name: Rancho La Brea.
Location: Los Angeles, California.
Age: 38,000 years old to recent.

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Geological setting and age of Rancho La Brea


Geological setting:

The asphaltic deposits at Rancho La Brea consist of sands, clays, gravel and angular rubble whose thickness is between 12 and 58m in the area of the old Salt Lake Oil Field, located immediately north of the fossiliferous beds. Beneath are Tertiary marine shales and sandstones that are interbedded with oil sands, from which the asphalt is derived. The Tertiary sediments are folded and show a disconformity with the overlying Pleistocene sediment. Although the tar-free Pleistocene strata are nearly horizontal in orientation, they also appear to be deformed by the same event that has affected the Tertiary sediment. The Tertiary beds appear to be highly eroded and generally show large angles of elevation. The underlying alluvium appears to be part of a giant outwash fan, similar to those of neighbouring areas of the same age. The La Brea sediments have thus been deposited in the outwash plain between the Santa Monica Mountains and the Pacific Ocean; despite the volumes of work however, no formal names have yet been proposed for the rock units. The main bone accumulations are restricted to the upper 9m of the sediment and are underlain by marine rocks, which record the last known incursion into the area. Clasts in the outwash fan indicate that the alluvium had a northwest source area during the late Pleistocene and during the early Holocene was tectonically uplifted. As mentioned before, the asphalt itself is derived from the Salt Lake Oil Fields and wells up through faults and fissures to create the mires.

Age:

Radiometric dating has given ages from preserved wood and bones of 38,000 years for the oldest known material from the La Brea seeps and they are still ensnaring organisms today.

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