High-resolution Sequence Stratigraphy and Facies Assemblage in Flood-Dominated and Mixed Deltaic Systems: Insights from Early Eocene Figols Group, South-central Pyrenean Basin

The upper part of the Early Eocene Figols Group cropping out near the town of Tremp (province of Lerida, Spain), is a wonderfully exposed sedimentary succession mainly constituted by mouth bar deposits and delta-front sandstone lobes, forming a fluvio-deltaic system dominated by catastrophic fluvial floods. These deltaic sandstones are vertically associated with carbonate and hybrid (mixed carbonate and siliciclastic) sediments that are physically related to a carbonate ramp platform which crops out in the southernmost part of the study area. The vertical and lateral juxtaposition of these depositional systems allows investigating how the stratigraphic controls (as climate and relative sea level) affect the resulting facies assemblages. The studied succession has been subdivided following a pragmatic physical stratigraphic approach into three 3rd order large-scale depositional sequences (FG2, FG3, and FG4), six 4th order small-scale depositional sequences and 21 EDSs (elementary depositional sequences). The latter are considered the stratigraphic building block of the succession, made of 10s of meter scale forestepping sandstones followed by, backstepping sandstones, bioclastic and mudstone sediments. During the lowstand system tract (LST), the depositional sequences FG2 and FG3 are characterized by the occurrence of flood-dominated deltaic systems (mouth bars and lobes). These deltaic deposits mainly developed in the northern sector characterized by higher subsidence. The overlain transgressive system tract (TST) is dominated by hybrid sediments and carbonates that initially accumulated in the southern zone and then progressively spread out over the entire area. The highstand system tract (HST) is mainly constituted by shelfal fine-grained sediments reflecting the reactivation of the fluvial input and deltaic clastic deposition in a more landward position. This sequential model resulted to be applicable at all hierarchical scales and in particular to the EDS scale. In FG4, the fluvio-deltaic systems became coarser-grained and channelized, the carbonate production on the shelf decreased and lagoonal fines were accumulated so as to separate vertically the fluvio-deltaic bodies. Syn-sedimentary tectonic activity capable to produce local variations of the available accommodation space is inferred by the geometry and facies distribution shown in the high-resolution correlation framework, particularly if considered at the physical scale of EDSs.