Hydrocarbon seeps and their associated chemosymbiosis-based communities have been documented throughout modern oceans, as well as in Neoproterozoic through Pleistocene aged rocks in the stratigraphic record. A particularly good record of ancient hydrocarbon seeps occurs in Mesozoic strata (Tithonian-Maastrichtian) along the western North American continental margin, cropping out throughout the Great Valley Group forearc strata and the Franciscan Complex. This extensive record allows for the comparison of the paleoecological signatures of stratigraphically and spatially related hydrocarbon seeps, which are geotectonically, petrographically, and isotopically consistent with each other.
Ten hydrocarbon seeps were located throughout this region and their physical, morphological, and paleoecological signatures were documented. Analyses indicate that their chemical signatures are petrographically and paragenetically similar and their geochemical values are consistently anomalous, however, their physical and biological signatures are surprisingly variable. The localities are strikingly different from each other in regards to faunal composition and morphological characteristics. The deposits include variably sized lenses, pods, mounds, and continuous outcrops. Eighteen taxa were identified, the most common being vestimentiferan worm tubes. Gastropods are the most abundant clade, while bivalves are the most diverse. No taxon is found at all ten localities and each locality is dominated by a different taxon that is commonly only a minor constituent of another localities faunal assemblage. This variability indicates that, as with modern seeps, heterogeneity is driven largely by the ephemeral nature of the seep environment, including ebullition volume, rate, and area, rather than by the duration of fluid advection.
These ancient seep deposits cropping out in the GVG forearc strata and the Franciscan Complex preserve the physical, chemical, and biological signatures of a group of tectonically related ancient hydrocarbon seeps and are a record of 80 m.y. of intermittent fluid seepage, occurring over geologically short intervals. along the western North American continental margin. As the abundance of recognized hydrocarbon seeps in this area continues to increase, they can be viewed collectively as a quasi-continuous, dynamic fluid-sediment-biotic system, with the potential to reveal larger evolutionary, biogeographic, geotectonic, and geochemical patterns of hydrocarbon seep processes and associated faunas through geologic time.