The Late Eocene 'Whiskey Creek' methahe-seep deposit (Western Washington State) - Part II: Petrology, stable isotopes, and biogeochemistry

Abstract

The Late Eocene 'Whiskey Creek' deposit (Pysht Formation, Olympic Peninsula, Washington State) formed at a methane-seep. Early diagenetic micrites and aragonite cement have delta(13)C values as low as -36parts per thousand indicating that the seepage fluids contained methane. With respect to micrite samples, low delta(13)C values correlate with relatively high delta(13)O values and vice versa. Ongoing micrite formation after the cessation of the seepage during increased burial might have altered the isotopic composition of the microcrystalline carbonates toward lower delta(13)O values and higher delta(13)C values. Alternatively, the trend in isotope values may reflect a change in the composition of seepage fluids. The principal difference between these scenarios is the duration of seepage with respect to micrite formation. Two petrographically similar varieties of blocky calcite spar are related to different carbonate sources. The delta(13)C values range from -32 to -29parts per thousand for one type of blocky spar and are either the result of methane oxidation or indicate thermal decarboxylation of organic matter. Low delta(13)O values are in favour of the latter. For the other type of spar, delta(13)C values as high as +6parts per thousand indicate carbonate formation within the zone of methanogensis. The 'Whiskey Creek' limestone exhibits a chaotic fabric produced by a variety of processes, including bioturbation, concretionary carbonate formation, early in situ brecciation, carbonate corrosion, and late fracturing of the rock. Two varieties of micrite aggregates are responsible for the nodular fabric of the limestone. Smoothly-shaped pyritiferous micrite nodules are of diagenetic origin and formed in a manner similar to that which produces carbonate concretions. Apart from being induced by anaerobic oxidation of methane, their formation is proposed to be linked to iron reduction and sulphide formation. The second, dominant variety is represented by irregularly-shaped, nodular to angular micrite aggregates surrounded by massive rims of pyrite, resulting from carbonate corrosion. A pure, fluorescent seam-micrite, constructive in origin, lines cavities or surrounds micritic aggregates. Among authigenic non-carbonate minerals, large quartz crystals and quartz replacing aragonite cement are common constituents of the limestone. Gypsum and an unidentified prismatic calcium sulphate mineral formed in the limestone matrix. The prismatic mineral is tentatively identified as bassanite, the Ca-sulphate hemihydrate. Gypsum is the potential precursor of bassanite, the formation of which could have been induced by elevated temperatures, highly saline solutions, or both. The wide range of delta(34)S values obtained from 'Whiskey Creek' pyrite and the strong depletion in (34)S (S values as low as -15parts per thousand) reveal that bacterial sulphate reduction was the sulphide generating process. Comparative analyses of pyrite from other seep deposits yielded even lower values (as low as -28parts per thousand) possibly indicating the involvement of microbial disproportionation of sulphur.