EXAMINING PRESERVATION BIAS AND DEPOSITIONAL SETTING WITH GRAIN SIZE AT LA BREA TARPITS FOSSIL BED, CALIFORNIA

Rancho La Brea of Los Angeles, California, is an iconic North American fossil locality representing a diverse record of plant, insect, and vertebrates spanning the last 50,000 years. Despite the exceptional fossil yield of these asphaltic sands and gravels resulting from over 100 years of excavation, there has been no systematic description of variation in grain size and asphalt concentration across the numerous deposits, and nearly all taphonomic interpretations have been drawn from studies of megafaunal mammal remains, rather than the sediments preserving them. A lack of radiocarbon stratigraphy on bones coupled with disarticulated skeletons and apparent abrasion or “pit wear”, have been taken to imply movement or churning of liquid asphalt or the activity of mammalian carnivores

We conducted a preliminary quantitative study of grain-size analysis to test three questions: 1) are visual descriptions of the matrix sufficient for understanding the depositional environment and related taphonomy of the site, 2) does grain size relate to variations in preservational quality of fossil material, 3) can paleoenvironments be reconstructed, or are depositional settings “churned” by asphalt and natural gas movement? Twenty-nine samples from five distinct deposits (“boxes” of Project 23) collected during a construction project adjacent to Hancock Park were selected for this project. Preliminary results suggest that mean grain size is overestimated in purely visual descriptions as fine silts and clays are mixed in with the asphalt. The amount of asphalt increases in a linear fashion with a greater percentage of fine material (< 63 microns) indicating that fines are underestimated in visual descriptions. There is no correlation between mean grain size and asphalt concentration. In contrast, some of the more delicate fossils, including birds and reptiles, are found in samples with a high percentage of coarse material. Plant fossils are found in samples with minimal coarse material. Insufficient samples have been processed to determine if there is sorting/stratigraphy in the deposits however, the initial data suggest that there is valuable information in a more quantitative matrix analysis.