Abstract: Geologic Setting, Stratigraphy, and Mode of Fossil Accumulation, Hancock Park, Los Angeles, California
James P. Quinn
Based upon available geotechnical and oil industry subsurface data, the immediate area of Hancock Park is situated on a 65 to 135 ft (19-41 m) thick Pleistocene section that blankets steeply dipping
Puente Formation strata on the southwest limb of the northwest plunging Salt Lake anticline. Up-dip leakage from Delmontian "oil sands" appears to be the source of the hydrocarbons that saturate the Pleistocene section and seep in the vicinity of the park.
Three Pleistocene units are recognized in this area. The lower unit consists of coarse-grained sediments that may represent early Pleistocene fill of a southwest-trending channel cut into Tertiary strata. A west-thickening wedge of neritic and paralic sediments characterize the middle unit that appear to record middle Pleistocene glacio-eustatic sea level fluctuations. Above a basal revinement surface, the upper unit is composed of a lower sequence of Sangamon age neritic sand and an upper sequence of Wisconsin age fluviatile sediments. A detailed analysis of the stratigraphy of the fluviatile sequence has found that sand and clast composition changes up-section, suggesting an eastward shift in source area in response to west down-tilting of La Brea plain.
Concentrated masses of fossil bone in the fluviatile deposits at Rancho La Brea are primarily associated with asphalt "pipes." Many of these fossil deposits extend vertically across facies implying that these loci of bone accumulation were not substantially affected by depositional events and that these deposits are intrusive. Animal entrapment and subsequent trampling are proposed as significant mechanisms in the formation and taphonomy of the rich accumulations of bone at Hancock Park.
AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994