Abstract

Carbonate concretions are zones of preferential cementation in sedimentary rocks. Concretion formation is driven by various biotic or abiotic diagenetic reactions that lead to localized carbonate-mineral saturation. However, many reactions can promote carbonate saturation, lending ambiguity to mechanisms of concretion formation in any given deposit. Large (up to ∼ 2 m across), ellipsoidal carbonate concretions occur in late Miocene (∼ 10 Ma) sandstone turbidites of the Castaic Formation and Ridge Basin Group exposed in Ridge Basin, California, USA. The concretions are absent from shale interbeds, suggesting that the host lithology plays a role in concretion authigenesis. Concretion carbonate carbon-isotope (δ¹³C_carb) compositions range from –12.6 to –1.5‰ VPDB, indicating variable carbon contribution from organic sources. Petrographic data and contents of concretion total inorganic carbon (TIC) indicate that cementation occurred after significant burial compaction. Concretion clumped-isotope temperatures and oxygen isotope compositions (δ¹⁸O_carb) range from 45 to 85°C and –13.5 to –11.0‰ VPDB, respectively, and further support relatively hot and deep formation. The average concretion carbonate strontium-isotope composition (⁸⁷Sr/⁸⁶Sr_carb) of ∼ 0.7117 falls between the ⁸⁷Sr/⁸⁶Sr composition of contemporaneous (∼ 10 Ma) seawater and the detrital (noncarbonate cement) fraction contained in the concretions. The ⁸⁷Sr/⁸⁶Sr_carb values suggest partial Sr incorporation from reactive silicates in the host turbidite sandstones. These findings indicate that concretion formation was deep and likely driven by the reaction of organic-derived CO₂ with turbidite silicates, providing sources of calcium and alkalinity for carbonate authigenesis.