Microenvironmental constraint on δ13C depletion: Garudamangalam Sandstone, Cauvery Basin, India. (March 2018)

• Record Type: Journal Article
• Authors: Chakraborty, Nivedita; Sarkar, Subir; Mandal, Anudeb; Mandal, Sunipa; Bumby, Adam
• Is Part Of: Marine and petroleum geology. Volume 91(2018)
• Page Start: 776
• Page End: 784
• Publication Date: 2018-03
• Subjects: δ13C depletion -- Methane -- Methanotrophs transportation -- Tidal slackening -- Garudamangalam sandstone; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2018.01.025
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Within the hydrocarbon-producing Cretaceous marine Uttatur Group, Cauvery Basin, India, the Garudamangalam Sandstone Formation is at top of a TST-HST transit. The δ 13 C value is expectedly depleted within the calcareous Garudamangalam Sandstone, which is the top most unit of the Uttatur Group, overlying the Karai Shale. The calcareous sandstone was deposited in a coastal setting around a shore-parallel river mouth bar. Instances of excessive depletion of δ 13 C up to −44.5‰ in the carbonate cement is suggestive of methane generation and its subsequent sequestration. The common occurrence of early diagenetic pyrite in these rocks testifies to the proliferation of sulfate-reducing bacteria and is suggestive of methane generation beneath the sulfate reduction zone. Upward-moving diffusive methane was possibly consumed by methanotrophs at the base of the sulfate-reduction zone. Abundant fabric-selective carbonate cement corroborates microbially-controlled anaerobic oxidation of methane. The presumed high rate of nutrient supply, abundance of vegetative material and moderately high organic carbon content in sediments (av. 1.6%), support this contention. All the samples which have the greatest δ 13 C depletion are characterized by enriched organic carbon and are derived from a tidal inlet-mouth facies, and selectively from mud drapes on cross-bedding in tidal strata. Calcarenite at the base of the same cross-strata are invariably much less depleted in δC 13 . This range of relationships indicates the transport of methanotrophs that settled on foreset beds mostly as tides slackened under the broader control of neap-spring cycles. Highlights: Methanotrophs settling on tidal slackening. Methanogenesis. Marked laterally variable degree of δ 13 C depletion. Tidal inlet mouths across shore-parallel delta favored.