The fossil deposits of Rancho La Brea and the McKittrick
Tar Pits occur in the upper 8m of the late Pleistocene Palos Verdes
Sand Sequence. The deposits are made up of layers of fine to coarse grained
gravel, sand and clay. These sediments were laid down by fluvial and flood
plain systems from local sources in the Santa Monica Mountains.
The lower 37m of the Palos Verdes Sand are marine deposits, which overlies the early Pleistocene marine San Pedro formation, which in turn rests unconformably on the Miocene Montery Formation of marine mudstones and diatomaceous shales.
Within the Miocene Montery Formation particles of organic matter accumulated as the deposits were laid down. The organic matter represents preserved microscopic organisms, in particular diatoms, which form reservoirs of kerogen within the rock units. These petroleum reservoirs are structurally controlled and occur at about 350m depth.
Movements along the San Andreas Fault folded and cracked the rocks creating fissures and faults through which the petroleum was forced by heat and pressure to the surface, aided by the porous nature of the overlying sand.
At the surface the more volatile components evaporate and along with bacterial action (biodegradation) and oxidation, turn the oil into natural asphalt, commonly called tar. This sticky asphalt seeped to the surface, collecting as shallow pools in low lying areas such as stream beds and topographic lows, creating traps for the animals.
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