Challenges to the ancient methane seep search strategy: The Bedford Canyon Formation (Middle Jurassic, Santa Ana Mountains, California)

Carbonates formed by anaerobic oxidation of methane at localized seeps have been recognized in the rock record based on a list of criteria, including anomalous deposition within detrital facies, complex petrofabrics, seep-endemic macrofauna and ¹³C-depleted isotopic signatures. While much emphasis has been placed on the latter, it is only one of several tools used for identification. This study reports on two different and rare limestone units in the forearc deposits of the Jurassic Bedford Canyon Formation in the Santa Ana Mountains, California (USA). Isolated, metre-scale limestone pods were found to host abundant endemic Anarhynchia dimerelloid brachiopods and display diverse petrofabrics, including multiple generations of cements. However, the δ¹³C values of the petrofabrics ranged from −14.5 to 4.52‰ Vienna PeeDee Belemnite. These values are only slightly ¹³C-depleted relative to contemporaneous seawater, thus likely recording a stronger component of oceanic dissolved inorganic carbon than hydrocarbons. Additionally, although the petrofabrics are common for seeps (primary micrite, microbialite, pelsparite, intraclastic micrite; early isopachous and botryoidal cements and yellow calcite; late-stage void-filling spar), the isotopic values did not follow a predicted trend of increasing δ¹³C with paragenetic sequence. Pelsparite δ¹³C values ranged from −10.65 to −7.09‰ Vienna PeeDee Belemnite while coarse spar and vein cements ranged from −11.42 to −2.44‰ Vienna PeeDee Belemnite. Olistostromes composed of brecciated pelmicrite sourced from shallow fringing-arc environments are typical of forearc settings. Olistostromes in the Bedford Canyon Formation contain much lower δ¹³C values than the more classically characterized seep deposits, ranging from −21.77 to −10.78‰ Vienna PeeDee Belemnite. While the original shallow-water source may not have a been a confined seep, the low δ¹³C values require the incorporation of carbon sourced from the oxidation of localized hydrocarbons. Together, these findings recommend a reconsideration of the weight placed on δ¹³C values as the key criterion for distinguishing seeps in the rock record.