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

摘要: Kuechler, R.R., Birgel, D, Kiel, S, Freiwald, A, Goedert, J.L., Thiel, V & Peckmann, J. 2011: Miocene methane-derived carbonates from southwestern Washington, USA and a model for silicification at seeps. Lethaia, Vol. 45, pp. 259–273. Exotic limestone masses with silicified fossils, enclosed within deep-water marine siliciclastic sediments of the Early to Middle Astoria Formation, are exposed along north shore Columbia River in USA. Samples four localities were studied clarify origin diagenesis these deposits. The bioturbated reworked limestones contain faunal assemblage resembling that modern Cenozoic methane-seeps. Five phases make up paragenetic sequence: (1) micrite microspar; (2) fibrous, banded botryoidal aragonite cement, partially replaced by silica or recrystallized calcite; (3) yellow (4) quartz replacing carbonate cement; (5) equant calcite spar pseudospar. Layers pyrite frequently separate different generations, indicating periods corrosion. Negative δ13Ccarbonate values as low −37.6‰ V-PDB reveal an uptake carbon. In other cases, high 7.1‰ point residual, 13C-enriched carbon pool affected methanogenesis. Lipid biomarkers include 13C-depleted, archaeal 2,6,10,15,19-pentamethylicosane (PMI; δ13C: −128‰), crocetane phytane, well various iso- anteiso-carbon chains, most likely derived sulphate-reducing bacteria. biomarker inventory proves majority formed consequence sulphate-dependent anaerobic oxidation methane. Silicification fossils early diagenetic cements precipitation cement – also observed methane-seep abundant diatoms radiolarians is preceding increase alkalinity due methane, inducing dissolution skeletons. Once methane has ceased, pH drops again can precipitate. □Biomarkers, carbonates, isotopes, Miocene, silicification, Washington.