Miocene methane-derived carbonates from southwestern Washington, USA and a model for silicification at seeps

Exotic limestone masses with silicified fossils, enclosed within deep- water marine siliciclastic sediments of the Early to Middle Miocene Astoria Formation, are exposed along the north shore of the Columbia River in southwestern Washington, USA. Samples from four localities were studied to clarify the origin and diagenesis of these limestone deposits. The bioturbated and reworked limestones contain a faunal assemblage resembling that of modern and Cenozoic deep- water methane- seeps. Five phases make up the paragenetic sequence: (1) micrite and microspar; (2) fibrous, banded and botryoidal aragonite cement, partially replaced by silica or recrystallized to calcite; (3) yellow calcite; (4) quartz replacing carbonate phases and quartz cement; and (5) equant calcite spar and pseudospar. Layers of pyrite frequently separate different carbonate phases and generations, indicating periods of corrosion. Negative d 13 Ccarbonate values as low as) 37.6& V- PDB reveal an uptake of methane- derived carbon. In other cases, d 13 Ccarbonate values as high as 7.1& point to a residual, 13 C- enriched carbon pool affected by methanogenesis. Lipid biomarkers include 13 C- depleted, archaeal 2,6,10,15,19- pentamethylicosane (PMI; d 13 C:) 128&), crocetane and phytane, as well as various iso- and anteiso- carbon chains, most likely derived from sulphate- reducing bacteria. The biomarker inventory proves that the majority of the carbonates formed as a consequence of sulphate- dependent anaerobic oxidation of methane. Silicification of fossils and early diagenetic carbonate cements as well as the precipitation of quartz cement - also observed in other methane- seep limestones enclosed in sediments with abundant diatoms or radiolarians is a consequence of a preceding increase of alkalinity due to anaerobic oxidation of methane, inducing the dissolution of silica skeletons. Once anaerobic oxidation of methane has ceased, the pH drops again and silica phases can precipitate.