A thyasirid-dominated methane-seep deposit from Montañita, southwestern Ecuador, from the Oligocene-Miocene boundary

A mass occurrence of the thyasirid bivalve Thyasira montanita in a limestone bed, exposed at Punta Montañita on northern side Santa Elena peninsula southeastern Ecuador, is here identified as an ancient methane-seep deposit. The massive to nodular shows carbonate phases and microfabrics typical seep limestones, such banded botryoidal cements. Its δ13Ccarbonate values, low −52.6‰, suggest biogenic methane source. Furthermore, biomarkers ANME-2 archaea indicate high seepage rates. Tubular fossils previously interpreted worm tubes show cone-shaped projections internal structure consistent with that crustacean cuticle are fragments. Seep deposits occurrences bivalves have so far only been reported from Cretaceous, making deposit first example Cenozoic. faunal biomarker inventories this deposition open shelf; oysters upper surface likely secondary encrusters arrived after emergence shallower water.