Factors in palaeoclimate analysis based on the marine biota of the Conejo Volcanics (Miocene), Santa Monica Mountains, California

The fossils in the middle Miocene Conejo Volcanics lived on a middle to outer shelf at approximately 100 m depth. They were redeposited with their substrate sediment into a basin where submarine volcanic flows and hyaloclastic breccia were accumulating at bathyal depth. The sediments were deposited as lenses of primarily limestone but also sandstone. The complex depositional history of the biota within a tectonically active region provides the opportunity to investigate the diverse important factors essential in determining palaeoclimate. Palaeoclimate along the Pacific Coast during the Cenozoic has been largely determined by comparing the fossils with the biota diagnostic of present-day provinces. This is not possible for the Conejo Volcanics fauna because most of the shallow-water taxa on which the provinces are based are rare or absent in it, and because the regional palaeogeography that determined the Miocene provinces is not known. Geochemical data are essential to compensate for a lack of information about water depth and geography. A δ¹⁸O isotopic palaeotemperature of 15°C for a specimen of Amussiopecten in the Conejo Volcanics suggests a shallow-water setting off north-western Baja California. However at the depth at which the fossils lived, the equivalent present-day location would be farther south, off south-western Baja California. Incorporating the climatic effect of local Miocene geography, the modern equivalent latitudinal position of the Conejo Volcanics would be somewhat north of there, off central Baja California at 25 to 26°N. This location is about 9° south of the present location of the Conejo Volcanics and suggests that the marine climate was warmer in the mid-Miocene by about 5°C. However, because approximately one-half of the latitudinal difference is explained by tectonic translation through micro-plate rotation and San Andreas fault movement, the estimate of climatic change is correspondingly reduced to only a few degrees.