Methane Clathrate Hydrates, Subseafloor Clam Communities & Carbonate Cement Diagenesis in The Pierre Shale: A Paradigm Shift in The Origin of Mixed Carbonate And Siliciclastic Sediments

Krause, Federico F.¹, Selim G. Sayegh¹, Renee Perez¹, Jesse Clark¹ (1) University of Calgary, Calgary, AB

Below modern seafloors, at water depths exceeding both storm wave-base and photic basement, methane clathrate hydrates are widespread. However, because these hydrates are inherently unstable, the prevailing historical consensus maintains that these materials lack a stratigraphic record.

In the Mesozoic Western Interior Seaway, calcite that is strongly depleted in ¹³C has been identified in limestone nodules of the Teepee Buttes Member of the Pierre Shale Formation. Nodules are encased in shale and consist of an immense number of coquinoid, infaunal, nymphalucinid bivalves; pellet grainstones, packstones, wackestones and mudstones; and centimeter-sized, irregularly nodular spar-filled vugs.

The bivalves belong to a well-known group of chemosymbiotic, infaunal pelecypods, the Lucinidae. In modern oceans this bivalve group occupies almost every possible marine seafloor environment known, as they are capable of living under anaerobic conditions. In contrast to other chemosymbiotic organisms, lucinids are mixotrophic opportunists. Modern lucinids combine siphonate filter feeding and chemosymbiosis with sulfide oxidizing bacteria, a behaviour that allows them to exploit sediment zones of anaerobic methane oxidation and sulfate reduction.

The sparry nodular vugs consist of three primary calcite cement phases that in paragenetic order are high-magnesium, botryoidal fibrous; ferroan "dendrolublinite;" and ferroan blocky. The botryoidal fibrous cements have ?¹³C_PDB ratios that range from -41.5‰ to -46.3‰, a signal that is indicative of a methanogenic carbon source. ?¹³C_PDB ratios of dendrolublinite and blocky calcites have greater separation, ranging from -12.6‰ to -40.8‰ and -15.1‰ to -44.5‰, respectively. These cements also originate from the diagenesis of methane, but followed a geochemical path that incorporated the reduction of iron. All three sparry cements grew from "free surfaces" into open space. In modern marine shales the enclathration of methane gas is often nodular and displacive, and is also accompanied by carbonate mineral precipitation.