Dolomite-rich condensed sections in overbank deposits of turbidite channels: the Eocene Hecho Group, south-central Pyrenees, Spain

High-resolution correlation of monotonous thin-bedded turbidites is desirable but usually difficult. Within the Eocene turbidites of the Hecho Group, South-Central Pyrenees, correlation has been facilitated by the occurrence of decimetre-thick, yellowish beds of micritic limestones and marlstones. These yellow beds (YB) are developed in the overbank deposits of major turbidite channel complexes (lower muddy stage), and occur at top of sandy deposits within the lowstand wedge of a third-order depositional sequence, as single or multiple beds related to high-frequency, fourth-order depositional cycles. The upper muddy stage, which comprises the rest of the lowstand systems tract, has no YB. The YB are interpreted as condensed sections where zones of burrowed and abundant planktonic and benthic microfossil assemblages are temporarily sustained by oxygen and nutriments delivered by turbidite discharges. The YB are, thus, reliable markers of the early lowstand wedge prograding complex, and coeval with initiation of delta restoration. Whole-rock and XRD data indicate that most YB contain detrital clay fraction and consistent excess of Fe, Mn, and P contents, which are ascribed to condensation driving redox relocations (hardground). YB are richer in calcite than the host claystones, indicating colonization by biogenic algal, and richer in dolomite. Dolomite varies in shape, zonation and chemistry ranging from early nearly pure to later more ferroan and formed under increasingly reducing, pre-compactional conditions. The 18OVPDB low values (–10.4 to –6.2 ‰) of Fe-dolomite that broadly correlates with calcite 18OVPDB (-8.1 to-5.6 ‰) suggest formation at higher temperature and/or isotopic re-equilibration with modified pore-water. Likewise, dolomite has more radiogenic 87Sr/86Sr (0.707926 and 0.707876) than ambient seawater, suggesting partial derivation from detrital aluminosilicates. Dolomitic YB have an outstanding potential as seals in the overbank deposits of major channel complexes, and their recognition demonstrates that the sequence stratigraphic framework relates to early diagenesis in this deep marine environment, thus providing a valuable tool for predicting deep reservoir properties.