An Early Jurassic brachiopod-dominated seep deposit enclosed by serpentinite, eastern Oregon, USA

Abstract

Many Paleozoic and Mesozoic hydrocarbon-seep deposits are typified by superabundant dimerelloid brachiopods. By contrast, Cenozoic and modern seeps have been and still are dominated by mollusks and tube worms. To date, five genera of the rhynchonellide superfamily Dimerelloidea have been confirmed to have lived at hydrocarbon seeps. Here, we look into the paleoecology of another dimerelloid genus, studying an unusual limestone body in the Blue Mountains Province of eastern Oregon, USA. This limestone block, found in the vicinity of the town of Seneca, is characterized by a mass occurrence of a dimerelloid brachiopod assigned to the species Sulcirostra paronai. The above block, along with two other sizeable exotic blocks of sedimentary rock scattered in its immediate periphery, and small pods of Sulcirostra limestone scattered in its wider periphery are enclosed in serpentinite. The brachiopod limestone predominantly consists of early diagenetic phases including clotted micrite and fibrous, banded and botryoidal cement. Negative δ13C values of these carbonate phases as low as −23.5‰ agree with precipitation resulting from the oxidation of hydrocarbons. Petrographic and isotopic evidence reveals that the dimerelloids lived at a seep on the Early Jurassic seafloor. The age assignment is based on Sinemurian ammonoids associated with Sulcirostra in one of the exotic blocks from the direct periphery of the main deposit. The exotic blocks are attributed to the Lower Jurassic Keller Creek Formation. The occurrence of the Sulcirostra limestone within serpentinite calls for an evaluation of the possibility that the Jurassic chemosynthesis-based ecosystem was driven by serpentinization, given the presence of chemosymbiotic communities on the modern seafloor where hydrogen and methane anomalies are generated by serpentinization. The nature of the contacts between the blocks of sedimentary rock including the Sulcirostra limestone and the surrounding serpentinite reveals that these lithologies were juxtaposed by tectonic processes, probably thrusting of the Keller Creek Formation strata, as well as other Aldrich Mountains Group rocks, over the serpentinites. No unambiguous evidence has been found that supports a derivation of seepage fluids from serpentinization, yet it is obvious that Sulcirostra lived at a hydrocarbon seep, confirming the association of the superfamily Dimerelloidea with seep environments for one more genus.