The Franciscan Complex: Problems of Instantaneous Generation and Missing Petroleum: ABSTRACT

Abstract

Organic matter in shales, limestones, and melange matrix of the Franciscan and Cedros subduction complexes, was studied, using Rock-Eval pyrolysis, to aid in understanding the nature and abundance of organic material in accretionary complexes. The largest quantities of total organic carbon (TOC) were found in the Calera Limestone from the San Francisco area (1-6% TOC). Previous workers interpreted the Calera Limestone as anoxic facies deposited on an oceanic rise of plateau and later accreted to the continental margin. Terrigenous sedimentary rocks of probable trench or trenchslope origin contain from 1-2% TOC. These values are slightly higher than the world average, due at least in part, to concentrated sampling on organic mudstones. These values may indicate deposition beneath a zone of upwelling, as has been previously hypothesized for the western North American continental margin, or deposition of terrestrial organic material from fluvial-deltaic sources. Because the organic material in the argillaceous rocks seems to be predominantly of terrestrial origin, a fluvial source is probably indicated, although locally, marine planktonic organic matter is preserved. Owing to rapid burial rates that accompany underthrusting in accretionary complexes (5-25 km/m.y.) and for geothermal gradients such as those measured in accretionary prisms (20-50{degree}C/km), large quantities of hydrocarbonsmore » may be rapidly generated in geologically instantaneous (less than 1 m.y.) time periods from rocks containing on the order of 1-3% TOC. The fate of this petroleum, whether vented or trapped in this environment, is unknown. The general absence of petroleum in accretionary complexes is puzzling, and may indicate large-scale venting. Such venting could conceivably be approximately 10% of the annual input of organic carbon into the sea by streams, and would be strongly periodic over geologic time.« less