Characterization of the Weissert Oceanic Anoxic Event in Lower Cretaceous limestones of the Guaniguanico Terrain, Sierra de los Órganos, Western Cuba

Johanset Orihuela ^1* Mihaela Melinte-Dobrinescu ² Florentin J-M. R. Maurrasse ¹

• ¹ Florida International University, Miami, United States
• ² National Institute of Marine Geology and Geoecology (GeoEcoMar), Bucharest, Romania

We present a high-resolution chemostratigraphic assessment of Lower Cretaceous organic-rich hemipelagic limestones from Sierra de los Órganos, Pinar del Río, Western Cuba. The studied sedimentary sequence consists of alternating thin grayish black (N2) chert-bearing carbonaceous marlstones and medium dark gray (N5) limestones of a 30-meter quarry outcrop located near the city of Pons. Detailed lithostratigraphy, carbon geochemistry, trace element analysis, and biostratigraphy were performed on a ~3-meter interval at the base of the succession to characterize presumed deoxygenation conditions associated with the deposit. The carbonaceous marlstones yield a total inorganic carbon content (TIC) ranging from 44.7% to 77.3% and total organic carbon content (TOC) fluctuates between 3.5% and 10.82%. By contrast, the medium dark gray limestones have TIC values exceeding 90% and an average TOC of 2.3%. Petrographic analyses, radiography, and scanning electron microscopy imaging (SEM-EDS) revealed an isotropic fabric throughout the lighter micritic limestone, with a bioturbation index (BI) greater than 3, whereas the carbonaceous layers show BI consistently less than 2 and include single framboids and bundles of cubic pyrite. The carbon isotopic (δ 13 Corg) analysis gives an average value of approximately -27.3 ‰, with a main peak reaching around -25.56 ‰, corresponding to a ~1.7 ‰ excursion. This carbon isotopic excursion (CIE) correlates with the late Valanginian nannofossil subzone NK3B, as indicated by the presence of Calcicalathina oblongata and the disappearance of Rucinolithus wisei. The occurrence of calpionellids Tintinnopsella carpathica and Calpionellites cf. darderi supports the correlation up to the earliest Hauterivian. Enhanced values of redox-sensitive trace elements (V, Ni, Cr, Mo, TI, U, and S) associated with highest TOC suggest severe deoxygenation conditions during the δ 13 Corg CIEs archived in this section. These excursions also coincide with increased enrichment of major elements (Al, Si, Li, and Ti), thus indicative of enhanced terrigenous fluxes at these intervals. The results provide robust evidence supporting the occurrence of the Valanginian Weissert Oceanic Anoxic Event in the Yucatan-Florida passive margin. These findings significantly enhance our comprehension of oxygendeficient conditions linked to the global Weissert Event within the regional Proto-Caribbean Seaway-Basin.