Assessing the factors controlling high sedimentation rates from the latest Barremian–earliest Aptian in the hemipelagic setting of the restricted Organyà Basin, NE Spain

The Organyà Basin, south-central Spanish Pyrenees, developed as a marginal depocenter during a rapid extensional phase of anticlockwise rotation of the Iberian plate. As a result of increased subsidence, an important change in sedimentation occurred from the late Barremian to the Aptian leading to unusually high sediment accumulation rates. Approximately 1000m of hemipelagic marls and limestones accumulated during this time interval. Here we studied the basal 85m of the hemipelagic facies of the El Pui section, Organyà Basin, that are characterized by alternating 15cm - ~3m thick beds of limestone and marls. Geochemical analyses indicate high total inorganic carbon (TIC) values (average 70%) suggesting enhanced CaCO ₃ production and deposition. SEM analyses of the samples indicate high abundance of calcareous nannofossils, which together with the absence of shallow water taxa characteristic of the Urgonian Carbonate platform of Organyà, and the lack of sedimentary facies attributable to carbonate platform components point to nannofossils as the main source for the elevated TIC. Organic-rich levels (total organic carbon (TOC) up to 1.74%) concurrent with positive excursions up to 2‰ in δ ¹³C _org, imply enhanced preservation of organic matter (OM) in the basin. In addition, pronounced peaks of δ ¹³C _org higher than the global average suggest superimposed local factors related to intensified ¹²C removal due to primary productivity. Biomarker analyses and the δ ¹³C _org profile suggest an autochthonous origin of the OM from phytoplankton and possible additional contributions from microbial communities. X-ray diffraction (XRD) results attest for sustained terrestrial fluxes as the source of nutrients to the basin because of a 30% average non-carbonate bulk mineral content in the sediment. The non-carbonate fraction is dominated by quartz (average, 14%) whereas the clay mineral assemblages are characterized by high illite content (>73 relative%) with minor concentrations of kaolinite (<5%), illite /smectite mixed layers (<17%) and chlorite (<15%), consistent with a provenance from the Paleozoic metamorphic terranes adjacent to the Organyà Basin. The integrated results suggest a high sediment accumulation rate (5cm/ky-7.5cm/ky) and enhanced carbon burial during the latest Barremian-earliest Aptian in the hemipelagic setting of the El Pui section.