DOCUMENTING SEDIMENTARY AND TAPHONOMIC HETEROGENEITY AT RANCHO LA BREA: NEW INSIGHTS FROM QUANTITATIVE APPROACHES

The locality Rancho La Brea (the La Brea Tar Pits, Los Angeles, California) is famous for its rich Late Pleistocene fossil record. Excavations began in the early 1900s through exploration of over 100 deposits, which took the form of man-made “pits”. Though asphalt is an effective preservative agent for numerous biological tissues (bone, cellulose, chitin), these early efforts focused nearly exclusively on megafauna remains and did not document or sample sedimentary context, hindering our understanding of deposit formation and thus the utility of the fossils for paleoecological reconstructions.

This paucity of sedimentary data and focus on megafauna, coupled with the misconception of “pits” as natural features, has resulted in assumptions that the site’s fossil deposits resulted primarily from the entrapment of animals in sticky asphalt seeps, and the lack of stratigraphy as resulting from “churning”. Yet these hypotheses have not been thoroughly addressed from a geosciences perspective. Here we begin to quantitatively address taphonomic hypotheses using sediments themselves through the study of Rancho La Brea’s Project 23, a collection of 16 fossil deposits salvaged during construction on the grounds of the adjacent art museum in 2006. Fossil deposits of Project 23 have provided a remarkable opportunity to describe, sample, and map asphaltic deposits at fine scales. Excavations of these deposits to date have revealed distinct variations in asphalt content and grain size distributions among deposits; a range of bone abrasion, weathering, and diagenesis, dense sediment infillings of bone cavities, pedogenic clay coatings on bones, and root casts and carbonate nodules.

Knowledge of sediments is critical for interpreting the ecological significance of Rancho La Brea’s fossil record, distinguishing patterns between taphonomic biases and true ecological change, and provides context for comparing Rancho La Brea to other less well-studied asphaltic localities worldwide.