Formation of methane-derived dolomite concretions in Okinawa-Jima, Southwest Japan during the Early Pleistocene “Chinen Disturbance”

• Published in: Chishitsugaku zasshi Vol. 129; no. 1; pp. 415 - 434
• Main Authors: Matsumoto, Ryo, Takeuchi, Rika, Nakagawa, Hiroshi, Sato, Tokiyuki, Iryu, Yasufumi, Matsuda, Hiroki, Komatsubara, Junko, Sato, Michio
• Format: Journal Article
• Language: Japanese; English
• Published: Tokyo The Geological Society of Japan 09.08.2023; Japan Science and Technology Agency
• Subjects: Benthos; Chinen disturbance; Concretions; Dissociation; Disturbance; Dolomite; dolomite concretion; Dolostone; Environmental changes; Foraminifera; Hydrates; Mass transfer; MDAC; Methane; methane hydrate; methane-derived carbonates; Miocene; MTD; Offshore; Okinawa-Jima; Outcrops; Oxidation; Pleistocene; Pliocene; Sedimentary facies; Sedimentary structures; Transition zone; Unconformity; Uplift; Water depth; Zonation; Japan; Okinawa
• ISSN: 0016-7630; 1349-9963
• DOI: 10.5575/geosoc.2023.0008

A thick deposit of the muddy Shimajiri Group (Upper Miocene-Early Pleistocene) is overlain by the calcareous sandy Chinen Formation (Pleistocene) in southern Okinawa-Jima, Southwest Japan. The abrupt facies change has been explained by a significant unconformity resulted from subaerial erosion (the Shimajiri disturbance) or the consequence of rapid and regional uplift leading to an abrupt environmental change (the Chinen disturbance). We identified sandy mass transfer deposits (MTD) directly overlying the Shinzato Formation-the uppermost part of the Shimajiri Group-in the large Urizun outcrop. The Shinzato-Chinen transition zone yields numerous dolomite concretions. The dolomite is strongly depleted in 13C, with δ13C values of −23.77‰ to −31.50‰, suggesting that the concretions were derived from anaerobic oxidation of methane (AOM). Zonation in δ18O values across the concretions, from 3.89‰ in the centers to 2.25‰ at the rims, suggest that the precipitation temperature increased from 12.3±2.0 to 19.5±2.0°C, corresponding to a shallowing of the water depth from 440±100 to 190±60 m. The estimate is consistent with the analysis of benthic foraminiferal assemblage. Given the wide distribution of the Bottom Simulating Reflector (BSR) offshore of Okinawa-Jima, methane hydrates are assumed to have persisted in the Shimajiri Group throughout the Pliocene-Pleistocene. Abrupt uplift and relative sea-level fall during the Chinen disturbance at 2 Ma must have triggered abrupt changes in facies and a massive methane hydrate dissociation. This disturbance resulted in a high methane flux, increased AOM, and the formation of the dolomite concretions in the Shinzato-Chinen transition zone. Methane-derived dolomite concretions are thus a geochemical record of an abrupt tectonic disturbance and hydrate dissociation offshore of Okinawa-Jima.