A paleodepositional reconstruction of middle Miocene cetacean bonebeds from the Topanga Formation, Northern San Joaquin Hills, Orange County, CA

Cetacean bonebeds containing multiple articulated individuals are extremely rare, with only a handful of occurrences documented worldwide. In 1997, three bonebeds containing mostly articulated and well preserved cetacean remains were uncovered during grading in sedimentary rocks assigned to the Paularino Member of the Topanga Formation near Bonita Canyon, Newport Beach, California. Multiple bonebeds within a single stratigraphic unit, such as those discovered at Bonita Canyon Planning Area 26, represent a rare class of deep marine fossil accumulations that have not been previously studied in detail. Taphonomic, stratigraphic, and geochemical data was collected from fourteen jacketed specimens removed from Bonita Canyon Planning Area 26 in order to determine the paleoenvironmental conditions that led to the accumulation of the bonebeds and to shed more light on these types of fossil accumulations. Results of this study indicate that the Bonita Canyon bonebeds do not represent a condensed facies, as previously proposed (The Keith Companies, 1998); rather they represent a composite concentration deposited in a continental shelf environment during a period of high sedimentation rates, where episodes of high energy gravity flows alternated with periods of low energy accumulation along a NE-SW trending submarine fan sequence. High net rates of sedimentation correspond with the creation of high accommodation space as the result of the opening of the Los Angeles Basin, a progressive marine transgression, and subsidence of the Topanga Basin as the result of the shift in the tectonic regime along western North America from a subduction zone to transform margin during the mid to late Miocene. Overall, this study provides valuable insight into the sedimentologic and taphonomic processes that lead to the formation of cetacean bonebeds, and demonstrate that high sedimentation rates may lead to well-preserved cetacean remains.