Ancient hydrocarbon seeps from the Mesozoic convergent margin of California: carbonate geochemistry, fluids and palaeoenvironments

Abstract

More than a dozen hydrocarbon seep-carbonate occurrences in late Jurassic to late Cretaceous forearc and accretionary prism strata, western California, accumulated in turbidite/fault-hosted or serpentine diapir-related settings. Three sites, Paskenta, Cold Fork of Cottonwood Creek and Wilbur Springs, were analyzed for their petrographic, geochemical and palaeoecological attributes, and each showed a three-stage development that recorded the evolution of fluids through reducing-oxidizing-reducing conditions. The first stage constituted diffusive, reduced fluid seepage (CH 4 , H 2 S) through seafloor sediments, as indicated by Fe-rich detrital micrite, corroded surfaces encrusted with framboidal pyrite, anhedral yellow calcite and negative cement stable isotopic signatures (d 13 C as low as À35.5% PDB; d 18 O as low as À10.8% PDB). Mega-invertebrates, adapted to reduced conditions and/or bacterial chemosymbiosis, colonized the sites during this earliest period of fluid seepage. A second, early stage of centralized venting at the seafloor followed, which was coincident with hydrocarbon migration, as evidenced by nonluminescent fibrous cements with d 13 C values as low as À43.7% PDB, elevated d 18 O (up to þ2.3% PDB), petroleum inclusions, marine borings and lack of pyrite. Throughout these early phases of hydrocarbon seepage, microbial sediments were preserved as layered and clotted, nondetrital micrites. A final late-stage of development marked a return to reducing conditions during burial diagenesis, as implied by pore-associated Mn-rich cement phases with bright cathodoluminescent patterns, and negative d 18 O signatures (as low as À14% PDB). These recurring patterns among sites highlight similarities in the hydrogeological evolution of the Mesozoic convergent margin of California, which influenced local geochemical conditions and organism responses. A comparison of stable carbon and oxygen isotopic data for 33 globally distributed seep-carbonates, ranging in age from Devonian to Recent, delineated three groupings that reflect variable fluid input, different tectono-sedimentary regimes and time-temperature-dependent burial diagenesis.

Paleocene methane seep and wood-fall marine environments from Spitsbergen, Svalbard

“…Campbell et al, 2002), and the Fossildalen seep carbonate micrites may have formed prior to the botryoidal carbonates. On average, the micrites of Fossildalen seep carbonates are more 13 C-depleted than the botryoidal cements, which may indicate that both phases formed in a partially closed system.…”

“…Campbell et al 2002Campbell et al , 2008Kiel and Peckmann, 2008) and its prevalence indicates that this mode of seepage contributed to the formation of carbonate in the Paleocene Fossildalen seep. Consequently, rare cavities within micrite and geopetal structures within thyasirid shells are indicative of localized focused flow and faster carbonate cementation (e.g.…”

Late Jurassic–Early Cretaceous hydrocarbon seep boulders from Novaya Zemlya and their faunas

“…Irrespective of the bathymetrical setting and their faunal composition, the seep carbonates display a set of characters allowing their straightforward interpretation in the fossil record (Campbell, 2006), such as significant contribution of isotopically light carbon (e.g. Kelly et al, 1995;Campbell et al, 2002;Peckmann et al, 2003;Campbell, 2006), textures suggesting formation under the influence of gaseous or liquid hydrocarbons (e.g. Peckmann et al, 2002;Campbell et al, 2008;Krauze et al, 2009) and lipid biomarkers suggestive of microbial processes accompanying methane oxidation (e.g.…”

“…7) and this indicates precipitation during active seepage of methane (e.g. Campbell et al, 2002;Campbell, 2006). The sediment was probably charged with gas, and some cavities within fossils were likely also partially filled with gas, as shown by asymmetrical cement fills (Fig.…”

“…These are the Oxfordian seeps in Beauvoisin, southern France (Gaillard et al, 1992;Kiel et al, 2010;Gaillard et al, 2011), the Tithonian seep in Alexander Island, Antarctica (Kelly et al, 1995;Kaim and Kelly, 2009), the Tithonian seeps in Paskenta and NW Berryessa, California, USA (e.g. Campbell et al, 2002;Kiel et al, 2008b;Jenkins et al, 2013), and the Late Tithonian seeps of Svalbard. The Cretaceous seeps are more common than those of the Jurassic and a fair number of both Early and Late Cretaceous aged seeps have been found (e.g.…”