NEW RECORDS OF MESOZOIC BRACHIOPODS (DIMERELLOID RHYNCHONELLIDS) FROM HYDROCARBON-SEEP DEPOSITS; ANARHYNCHIA FROM THE JURASSIC OF OREGON AND CALIFORNIA, USA; PEREGRINELLA FROM THE CRETACEOUS OF TIBET

We report new records of rhynchonellid brachiopod genera (Superfamily Dimerelloidea) from hydrocarbon-seep deposits. This is the first confirmed occurrence for Anarhynchia in such an association (although previously suspected, e.g., Campbell & Bottjer, 1995; Sandy, 1995) while for Peregrinella it is the 4^th (previous records CA, Campbell et al., 1993; Crimea, Kiel & Peckmann, 2008; Romania, Sandy et al., 2012). The association of Anarhynchiawith both hydrocarbon-seeps (herein) and hydrothermal-vent deposits (Jurassic of CA, Little et al., 2004) make this taxon unique.

Anarhynchia occurs in isolated limestone pods in clastic-dominated sequences near Seneca, Oregon, and Bedford Canyon, California. The monospecific mass-occurrences of Anarhynchia from Oregon are reminiscent of other localities interpreted as hydrocarbon-seep deposits. The dark micritic limestones yielded negative δ¹³C values of -20.1‰ to -15.0 (OR) and -23.7‰ to -3.1‰ (CA) reflecting hydrocarbon oxidation.

Peregrinella is one of the largest of Mesozoic brachiopods and with its highly disjunct distribution its mass occurrences in carbonate lenses have attracted much attention (e.g., Ager, 1965; Thieuloy, 1972). Peregrinella has been recorded from remote localities in Tibet; the material we investigated is from the Chebo section, northern Tibet (Sun, 1986). The rock contains petrographic seep indicators: clotted micrite and banded botryoidal cement. These phases yielded negative δ¹³C values of -24.5‰ to -21.9‰.

These Jurassic and Cretaceous brachiopod-rich deposits formed at hydrocarbon seeps, but it is not possible to determine further the composition of the fluids.

Acknowledgments: MRS, American Chemical Society - Petroleum Research Fund, Prof. Sun Dong Li, Nanjing Institute of Palaeontology.