Methane Seep Carbonates of the Bedford Canyon Formation (Early Jurassic): Insight into the Pre-Great Valley Record of Hydrocarbon Seeps

Abstract

Methane seeps of the Late Jurassic-Eocene Great Valley Group (GVG), California, have served as a model for studying this unique ecosystem for several decades. However, the pre-GVG record is poorly known. Starting in the Late Devonian through Late Jurassic, a series of arc collisions led to accreted terranes from Mexico north to Alaska. A handful of seeps are known from these terranes but their distribution is patchy and rare and often overprinted by metamorphic events. Recognition of seeps in the Bedford Canyon Formation (Early Jurassic, Santa Ana Mountains, California) extend the record of the pre-GVG seeps. The Bedford Canyon is interpreted to have formed in a slope setting adjacent to an island arc. The unit is comprised of silty and sandy turbidites with beds of conglomerate. Rare limestone units 1 to 10s of meters thick are olistrostromes sourced from shallow water carbonates. Fossils in these units are extremely rare. These units are primarily composed of peloidal limestone intraclasts, neomorphosed to fine and medium twinned spar. In contrast, several limestone bodies were mapped on the western slope near Bedford Canyon that are interpreted as methane seeps. The meter-scale outcrops are predominantly coquina of the brachiopod Anarhynchia, a member of the dimerelloids, an extinct group known only from methane seeps and one hydrothermal vent setting. In many samples, the rock is up to 90% brachiopod. Preservation of the shell ranges from well-preserved finely laminated fabric to coarsely crystalline spar. Fecal pellets, averaging 0.4 mm in long axis, are associated with the shells. A finer population of peloids, avg. 0.08 mm in diameter, are interpreted to be remnants of microbial mats. Throughout the unit are botryoidal cements, but the needles are replaced by coarse spar with curved twin planes. Other cements including fringing cements on grains and shells and coarse spar within articulated brachiopods. Other fossils are very rare and include poorly preserved radiolarians and a second unidentified smooth brachiopod. Very rare late-stage barite was also recognized. In addition to extending the record of island arc seeps, the Bedford Canyon locality offers new insight into the nature of brachiopod-dominated ecosystems. The dense, monospecific population, associated with microbialite, complex cements, and barite, argue for a chemosynthetic lifestyle.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)