A massive input of coarse-grained siliciclastics in the Pyrenean Basin during the PETM: the missing ingredient of a coeval abrupt change in hydrological regime

The Paleocene–Eocene thermal maximum (PETM) is represented in numerous shallow and deep marine sections of the south-central and western Pyrenees by a 2–4 m thick unit (locally ca. 20 m) of clays or marly clays intercalated within a carbonate-dominated succession. The massive input of fine-grained terrestrial siliciclastics into the Pyrenean Gulf recorded by that unit has been attributed to an abrupt hydrological change during the PETM. However, the nature of such change remains controversial. Here we show that, in addition to fine-grained deposits, large volumes of coarse-grained siliciclastics were brought into the basin that were mostly accumulated in incised valleys and a long-lived deep-sea channel, both spatially restricted settings. The occurrence of these coarse-grained deposits had been known for some time, but their correlation with the PETM is reported here for the first time. The bulk of incised valley PETM deposits are cross-bedded sands and pebbly sands, almost exclusively made of quartz, currently being actively quarried. Proof of their belonging to the PETM include: (1) their stratigraphic position, sandwiched between upper Thanetian and lower Ilerdian marine carbonates, (2) organic carbon isotope data, and (3) the fact that clay minerals from the sand matrix are more than 80 % kaolinite. The axially-flowing deep-sea channel existed throughout Paleocene times in the Pyrenean Basin, within which coarse-grained calciclastic turbidites, and lesser volumes of siliciclastic turbidites, were accumulated. This Paleocene succession is capped by thick-bedded turbiditic quartz sandstones and pebbly sandstones, here assigned to the PETM based on calcareous nannoplankton, clay mineral and organic carbon isotopic data. The large and simultaneous increase in coarse- and fine-grained terrestrial siliciclastic material delivered to the Pyrenean Gulf is related to an increased intra-annual humidity gradient. During the PETM longer and drier summer seasons facilitated the erosion of landscapes, whereas a dramatic enhancement of precipitation extremes during the wet seasons led to intensified flood events with rivers carrying greater volumes of bed and suspended loads. This scenario argues against the possibility that PETM kaolinites indicate a coeval warm and humid climate in northern Spain. Instead, the erosion of thick Cretaceous lateritic profiles developed in the Hercinian basement is proposed here as the most likely alternative.