BASIN-WIDE OUTCROP CORRELATION OF FLUVIAL STRATA AND IMPLICATIONS FOR SUBSURFACE FLUVIAL SYSTEMS: THE PERMO-TRIASSIC CENTRAL IBERIAN BASIN, SPAIN

The relationships between large-scale depositional processes and the stratigraphic record are relatively well understood for marine deposits, but less so for fluvial systems. For example, the origin and distribution of channel stacking patterns in alluvial-fluvial strata and their direct relationship to base level changes is, at best, controversial. Here, we present a study of the Permo-Triassic Central Iberian Basin (CIB), NE Spain, and discuss the implications for correlation and sequence stratigraphy of fluvial strata.

The basin developed as a fault-bound, intracratonic rift basin during the Early-Middle Permian. A long-lived, axially flowing fluvial system developed in the Early Triassic (Buntsandstein facies) and was overlain by marine carbonates of the Tethyan transgression in the Late Anisian (Muschelkalk facies). The sedimentary fluvial architecture is characterised by basal, alluvial fan conglomerates, followed by sandy braided rivers. Isolated single- or multi-storey amalgamated complexes mark the top part of the sequence. The net-to-gross ratio decreases up section from 90% to 40-50% for the whole of the basin and reflects increasing channel stability and greater occurrence of mature paleosols.

Correlation of the Buntsandstein fluvial facies along a c. 100 km, faulted basin margin was achieved by a detailed sedimentological field study in combination with a chemostratigraphic analysis. Similar trends and sequence configurations in all chemostratigraphic profiles provide evidence for a major fluvial system along the western basin margin during the Early to Middle Triassic. This research identifies the important non-marine to marine transition characterised by significant changes in the Buntsandstein fluvial succession. A reduction in accommodation space and lowering of basin floor gradients are associated with the marine transgression and opening of the Tethyan seaway. The research correlates syn-rift Triassic fluvial sandstones over a distance of c. 100 km along the SE margin of the CIB and attributes changing fluvial architecture to a transition from a predominantly tectonic to a climatically driven fluvial system. The results have important implications for the temporal and spatial prediction of fluvial architecture in the subsurface and its transitions during a marine transgression.