NIFTY NUANCES: GEOLOGICAL ANALYSES REVEAL DIFFERENT DEPOSITIONAL ENVIRONMENTS IN A PRE-LGM FOSSIL DEPOSIT FROM RANCHO LA BREA

For decades, asphaltic deposits (“breas”) have been viewed through the lens of the famous and extensively-studied Rancho La Brea locality (RLB) in Los Angeles, California, USA. This locality has long been interpreted as forming through entrapment and preservation of biota in sticky surficial asphalt pools. However, increasing research on RLB and other breas deposits has revealed a diversity of depositional histories, including entrapment, fluvial activity, and faulting, that contribute to the formation of these fossil records. Understanding the complex interplay between these processes is important for interpreting their paleoecology and for conducting chronological, paleoecological, and biogeographical comparisons between sites.

One fossil deposit under current excavation at Rancho La Brea (Project 23, Deposit 2 [P23-2]) contains three distinct depositional facies preserving fossil material: dense accumulations (sticks, megafaunal bones) in asphaltic medium-grained sands; sparse/scattered accumulations (articulated insects, intact bird crania, rodent coprolites), in asphaltic silty fine sand and fine sandy silt; and thin interbeds of non-asphaltic fine sand and silt containing scattered small to medium-size mammal and bird bones, including an associated semi-articulated bird lower leg. A sharp, abrupt contact separates the two asphaltic facies. An erosional contact separates both asphaltic facies from the non-asphaltic sand beds, and exposes fossils eroding out of the coarser facies.

P23-2 represents the first Rancho La Brea deposit to be measured entirely with a surveying total station, greatly easing the geolocation of specific fossils and taphonomic and geological features. Excavation progress is also visualized via 3D scanning with a Lidar-enabled tablet, and photodocumented. Careful excavation and documentation of the geological features of this deposit presents a great opportunity to directly compare taphonomic indicators with depositional settings. These methods are advancing our ability to interpret the complex depositional history of one of Earth’s most important, but still incompletely understood, paleontological sites.