Geochemical characterization of the Salada Formation (Cenomanian–Turonian) in relation to Oceanic Anoxic Event 2, Colombia

Widespread organic-rich sediments of the Cenomanian–Turonian archived environmental conditions associated with Oceanic Anoxic Event (OAE2), a time of expansive deoxygenation that affected ocean chemistry and marine ecosystems. Previous studies of these deposits, especially in marginal semi-restricted basins such as the coeval “La Luna Sea” (Cenomanian–Santonian) in northwestern South America, revealed thick successions with anomalous accumulation of organic matter (OM). The mechanisms and sources of these OM and the geochemical variations during the Cenomanian–Turonian remain poorly understood. To address this issue, we conducted a multi-proxy characterization, including petrography, SEM, TOC, TIC, δ13Corg, Rock-Eval, n-alkanes, and major element geochemistry in 54 m section of the Salada Formation (Cenomanian–Turonian) at la Molinilla Creek, Lebrija, Colombia.

The Salada Formation comprises a succession of interbedded grayish black (N2) to dark gray (N3) calcareous mudstones, unbioturbated OM-rich shales, light gray (N7) marlstones and limestones with shell fragments and inoceramid impressions. Petrographic analyses revealed abundant planktonic foraminifera and fewer coccoliths in the carbonate components. TOC ranges 0.34–8.3 wt%; TIC 1.26–84.24 wt%, and δ13Corg -29.30‰– -26.66‰. Rock-Eval pyrolysis yielded S1 values ranging 0.59–2.52 mg HC/g, S2 5.21–17.17 mg HC/g, S3 0.16–0.88 mg CO2/g, Tmax 446–454°C, HI 189–265 mg HC/g TOC, OI 3–17 mg CO2/g TOC, and PI 0.06–0.15. N-alkanes oscillate between nC8 and nC28. Major elements Al (1.24–15.73 wt %), Si (1.93–54.64 wt %), Ti (0.01–0.49 wt%), Ca (0.86–34.27 wt%), P (0.12–4.11 wt%) and Fe (0.19–18.85 wt %) vary throughout.

The lithologic, fossil, and geochemical characteristics suggest a pelagic environment under oxygen-deficient conditions. Major element fluctuations indicate episodic enhanced terrigenous influxes. Rock-Eval values reveal mixed type II/III kerogen, while the abundance of short-chain n-alkanes indicates predominant marine OM. The carbon isotope curve shows a δ13Corg positive shift of 1.81‰ at 6.45 m which is interpreted to represent the first carbon isotope excursion of OAE2.