A TAPHONOMIC, STRATIGRAPHIC, AND GEOCHEMICAL ANALYSIS OF MIDDLE MIOCENE CETACEAN BONEBEDS FROM THE TOPANGA FORMATION, WESTERN SAN JOAQUIN HILLS, ORANGE COUNTY, CALIFORNIA

Cetacean bonebeds containing articulated remains are extremely rare with only a handful of occurrences documented worldwide. The Paularino Member of the Topanga Formation near Bonita Canyon, Newport Beach, California contains mostly articulated, middle Miocene cetacean bonebeds within three stratigraphic horizons. This study examines those bonebeds using a combination of taphonomic, stratigraphic, and geochemical analyses, along with previously collected data (Barnes, 1998) to determine the paleoenvironmental conditions that led to deposition of the cetacean bonebeds, and test the hypothesis of Barnes (1998) that the bonebeds represent a deep marine condensed facies. Two depositional facies exist within the rock units at the Bonita Canyon outcrop. The upper facies consists of heavily oxidized, coarser grained, conglomeritic sandstone interbedded with clayey siltstone and contains scrappy bone fragments with evidence of abrasion. In contrast, the lower facies consists of dark gray tuffaceous siltstone with some sandy lenses typical of the Paularino member and contains three bonebed horizons with the degree of preservation increasing with depth. Preliminary results indicate that the main bone-bearing facies of the Bonita Canyon outcrop signify a composite concentration with a complex depositional history. The lower two bone-bearing horizons were deposited in a low energy environment with relatively high rates of sedimentation where partially decomposed cetacean carcasses settled on the seafloor and were buried before significant scavenging could occur. In contrast, the bones in the uppermost horizon may have been exposed on the seafloor slightly longer, possibly due to changes in sedimentation rates, subjecting them to more scavenging and disarticulation.