File:MARUM-HTQ-01-HiRes.jpgA Hydrothermal Vent ecosystem is recorded in Figueroa Sulfide (Modern example from Mid-Atlantic Ridge)
The Figueroa Sulfide is located in the San Rafael Mountains and is part of the Franciscan Complex, which formed during subduction of the Farallon Plate. This geological formation includes altered basalts, serpentinites, cherts, and tectonically sheared mudstones.[3][6] The Figueroa Sulfide consists of a 1.5-meter-thick sulfide lens with a silicified core, flanked by more friable sulfides, and is structurally overlain by altered volcanics and cherts. The deposit shows evidence of tectonic brecciation and oxidation before silica deposition. Though small and not viable for large-scale mining, its mineralization and vent structures suggest similarities to modern hydrothermal vent systems.[3][6]
Biostratigraphic analysis of the Figueroa Sulfide, based on radiolarian fossils extracted from chert samples, indicates a late Pliensbachian to early Toarcian age. The presence of key radiolarian taxa, such as Parahsuumovale and Bagotumsp., supports this dating.[1][2] Taphonomically, fossils found in the deposit, including brachiopods, gastropods, and vestimentiferan tubes, are preserved as pyrite molds, replicating fine details of the original shells and tubes. These fossils suggest an ancient hydrothermal vent community, where organisms like tube worms and brachiopods likely relied on chemosynthetic bacteria for nutrition, mirroring modern vent ecosystems.[1][2]
The Figueroa sulfide fossil assemblage differs from modern vent communities in species diversity, composition, and trophic structure.[1][2] Unlike modern vents, the Figueroa Sulfide has only three species, lacks common taxa such as Vesicomyidae and Bathymodiolus bivalves, arthropods, and gastropods, and has no predators. Possible explanations for these differences include taphonomic bias, where only high-temperature vent species were fossilized, and a genuinely low-diversity Jurassic vent ecosystem. Some "missing" taxa, such as Vesicomyidae, likely evolved later, supported by their fossil record starting in the early Cretaceous and molecular divergence estimates.[1][2]
↑ 6.06.1Wahl, A.D. (1995). "The geology of the Franciscan Complex, San Rafael Mountains me´lange, California". Master's Thesis, University of California, Santa Barbara: 1–121.
