MODERN & ANCIENT METHANE CLATHRATE HYDRATE ACCUMULATIONS: A COMPARATIVE SEDIMENTOLOGY STUDY OF MESOZOIC DEPOSITS OF THE WESTERN INTERIOR SEAWAY OF NORTH AMERICA

In modern seafloors methane clathrate hydrates are common deposits accumulating well below storm wave-base and photic zones, as their evolution is pressure and temperature dependent. Chemosymbiotic organisms capable of exploiting methane and hydrogen sulfide from bacterial and inorganic anaerobic methane oxidation (AMO) thrive in these depositional environments. Typically, hydrates fill pore spaces of coarser sediments, or are displacive and form nodules and sheet-cracked layers in mudstones. Diagenetic processes resulting from AMO lead to the accumulation of carbonates and induration with high and low magnesium calcite, ferroan calcite, aragonite, siderite and/or dolomite. Stable carbon isotopes of these deposits typically are depleted in ¹³C, bestowing a telltale methanogenic signature to the rocks.

Limestones with abundant coquinoid nymphalucinid bivalves, pellet grainstones, packstones and wackestones, and mm to cm large irregular sparry masses occur in the Teepee Buttes (TP.B.), Pierre Shale Formation (Upper Cretaceous), South Central Colorado. Sparry masses are nodular and comprise three phases of primary calcite cements that in order are high magnesium botryoidal fibrous, ferroan “dendrolublinite” and ferroan blocky. Botryoidal cements line the periphery of sparry masses. Fibers are directed towards the interior of the mass and have d¹³C_pdb ratios that range from -44.8 to -46.3 ‰, reflecting a methanogenic source. Significantly, modern lucinids are mixotrophic opportunists as they are both pellet producing filter feeders, and chemosymbiotic with sulfate reducing bacteria that enable them to exploit zones of AMO. TP.B. nymphalucinids behaved likewise. Lastly, sparry masses are nodular and displacive, not unlike modern “clathrite” carbonate nodules. TP.B. limestones thus resemble “clathrites” and would have formed during sedimentation of the Pierre Shale Formation in response to methane clathrate hydrate accumulations in the Mesozoic Western Interior Seaway.