NOVEL RANCHO LA BREA FOSSIL DEPOSIT PRESERVES EVIDENCE OF FAULTING

Rancho La Brea (RLB, California, USA) is one of the most important fossil sites in the world, having yielded millions of specimens comprising more than 600 species of plants and animals preserved by asphalt over the past 50,000 years. Isolated asphaltic deposits, consisting of fossil material in a matrix comprising 80-90% sand and gravel and 10-20% asphalt, are presumed to have developed over hundreds to thousands of years from the interaction of fluvial sedimentation, seasonal ponding, and intermittent surficial asphalt seeps. RLB fossil deposits have previously been described as conical, chimney-like, pipes, columns, lenses, lensoid, tabular, pockets, fissures, small circular deposits, and irregular masses. A new deposit unearthed during the construction of a nearby parking structure has revealed an unusual new “tar pit” morphotype. Excavation of fossil deposit “Project 23 - Box 9” exposed an asphaltic deposit with a distinct Y-shape throughout multiple 25-cm levels. A central section of the fossil deposit appears as a thick E-W trending stem portion with two narrower asphaltic arms branching to the SW and NW. A third arm branching NE from the central section was observed only in several lower levels. Sharp contacts between asphaltic sands and surrounding non-asphaltic soil preserve features associated with faulting—centimeter-scale mullions and slickenlines, millimeter-scale planar asphaltic sand-filled sheets and fractures, and oriented angular peds of a non-asphaltic clayey silt B soil horizon with sharp contacts incorporated into the eastern stem portion of the asphaltic sand deposit. We interpret this unusual shape and the other fault-like features as preservation of faulting of an existing RLB fossil deposit that formed through asphaltic entrapment and alluvial deposition on a periodically flooded plain. Geochemical analyses are currently underway to determine the age of the sediments and fossils therein. This in turn will provide information on the timing of the faulting event.