Late Quaternary Asphalt Seeps and Paleontological site of La Brea Tar Pits

Los Angeles, California, sits atop a sedimentary basin bounded by the Transverse and Peninsular ranges. On the northern part of the basin lies an alluvial plain shed from the Santa Monica Mountains during the Pleistocene. In the vicinity of La Brea Tar Pits (Rancho La Brea), 45 meters of this alluvium, referred to as the Palos Verdes Sand, rests upon Middle Pleistocene shallow marine strata and thick, oil-rich Middle Miocene deep-marine deposits. Through multiple intervals of uplift and faulting, crude petroleum has seeped to the surface over the last ~60,000 years, forming shallow pools that have entrapped and preserved millions of fossils representing hundreds of species of plants, invertebrates, and vertebrates. These fossil remains, ranging from Mammuthus, Smilodon, and entire Juniperus trunks to seeds, insects and songbirds, form a continuous record of ecosystem structure and change across major climatic and geological boundaries. The asphaltic fossil preservation is so exquisite that original biotic materials including collagen, cellulose, and chitin are preserved, allowing geochemical analyses including radiocarbon dating and studies of stable isotopic signatures. The Rancho La Brea Lagerstätte therefore represents a unique opportunity to study the impacts of Earth processes like climate change on ecosystem dynamics, species adaptations, and extinction.