Vitrinite reflectance patterns from the Riosa-Olloniego Sector (Carboniferous, Cantabrian Zone, NW Spain): Overburden implications

Vitrinite reflectance is a parameter of organic matter maturity considered of paramount importance to reconstruct the history of sedimentary basins (palaeotemperature, depth of burial, unconformities, tectonic overprinting, local thermal anomalies). Although sensitive to external factors (vitrinite alteration, maceral composition, lithology) that may harm its reliability, it is still the most robust tool widely applied in sedimentary basins. In this work, vitrinite reflectance is used to study the post-sedimentary evolution of a Carboniferous (Moscovian, Asturian) succession (Riosa-Olloniego Sector, Central Asturian Coalfield) of the Cantabrian Zone foreland basin (NW Spain). Fourteen samples (coals, carbonaceous shales and channel sandstones with rip-up coaly clasts and plant fragments) from an 800-m-thick, mixed continental–marine interval and two localities, underground (coal mine) and on surface, respectively, were subjected to chemical and petrographic analyses. Attention was paid to the influence of the above-mentioned external factors. The organic matter plots in the Kerogen type III field and has a Bituminous C-B coal rank, with generally low S content. Reflectance values range from 0.79 to 1.07 Ro% with a gradient of ∼0.3 Ro%/km for both mine and surface samples, which plot along similar trends but with the latter yielding comparatively lower values. Palaeotemperatures calculated from vitrinite reflectance using the most general models yielded palaeogeothermal gradients of 23–36 °C/km and maximum burial depths of ∼3.9–5.2 km, depending on the model. It was concluded that the vitrinite reflectance was mainly governed by sedimentary burial, with differences between mine and surface values reflecting variable tectonic overburden under a neighbouring nappe (Aramo Nappe). • Vitrinite reflectance in the study area is mainly determined by sedimentary burial. • Nappe overburden leads to lateral vitrinite reflectance variation at kilometre scale. • No evidence found of vitrinite alteration or lithology affecting reflectance. • Occurrence directly below marine shale results in vitrinite reflectance suppression.