Long-term burial of terrestrial organic matter in mid-Cretaceous syn-rift marine deposits: Environmental and tectonic controls (Basque-Cantabrian Basin, western Pyrenees)

In the Basque–Cantabrian rift basin, thick mid-Cretaceous organic-rich marine successions were deposited and later exposed during basin inversion, providing an outcrop analogue for North Atlantic black shales. Rock-Eval pyrolysis, total organic carbon (TOC), and organic carbon isotope analyses were carried out on marine sedimentary deposits ranging from deltaic to deep-water environments across the basin. The results indicate a strong control of environmental conditions and tectonic evolution on the type, distribution, and long-term burial of organic matter. Organic matter burial increased markedly during a ∼15 My interval (early Albian-early Cenomanian), with TOC values reaching up to 14.8%, coinciding with the syn-rift climax of basin development. During this phase, basin compartmentalization, restricted water-mass circulation, and the establishment of hypoxic to anoxic bottom-water conditions enhanced organic matter preservation. Preserved organic matter across all depositional environments is of terrestrial origin. High terrigenous sediment supply and the development of river plumes likely inhibited marine primary productivity. Maximum TOC values occur in prodeltaic deposits associated with northern fan-delta and southern deltaic systems, decreasing basinward due to prolonged oxygen exposure and partial remineralization during transport. In contrast, fault-controlled submarine canyons along the narrow northern margin enabled rapid transfer of plant debris to submarine fans, making turbidite systems—particularly thick-bedded lobes and Bouma Tb divisions—efficient organic carbon sinks. Differences between the organic-rich northern margin and the poorer southern margin reflect basin asymmetry, influencing climate, vegetation density, transport distances, and burial efficiency.