Volumetric budget and grain-size fractionation of a geological sediment routing system: Eocene Escanilla Formation, south-central Pyrenees

Abstract

The supply of sediment and its characteristic grain-size mix are key controls on depositional facies and stratigraphic architectures in sedimentary basins. Consequently, constraints on sediment caliber, budgets, and fl uxes are a prerequisite for effective stratigraphic prediction. Here, we investigate a mid- to late Eocene (41.6–33.9 Ma) sediment routing system in the Spanish Pyrenees. We derive a full volumetric sediment budget, weighted for grain-size fractions, partitioned between terrestrial and marine depositional sectors, and we quantify sediment fl uxes between depocenters. The paleo–sediment routing system was controlled by syndepositional thrust tectonics and consisted of two major feeder systems eroding the high Pyre nees that supplied a river system draining parallel to the regional tectonic strike and that ultimately exported sediment to coastal, shallow-marine and deep-marine depo centers. We show signifi cant changes in both the volume and grain-size distribution of sediment eroded from the Pyrenean mountain belt during three different time intervals in the mid- to late Eocene, which controlled the characteristics of stratigraphy preserved in a series of wedge-top basins. The time-averaged sediment discharge from source areas increased from ~250 km 3 /m.y. to 700 km 3 /m.y. over the 7.7 m.y. interval investigated. This temporal increase in sediment supply caused major westward progradation of facies belts and led to substantial sediment bypass through the terrestrial routing system to the (initially) marine Jaca Basin. The grain-size mix, measured as size fractions of gravel, sand, and fi ner than sand, also changed over the three time intervals. Integration of volumetric and grain-size information from source to sink provides an estimate of the long-term grain-size distribution of the sediment supply, comprising 9% gravel, 24% sand, and 67% fi ner than sand. The techniques and concepts used in the Escanilla study can profi tably be applied to paleo–sediment routing systems in other tectonic and climatic settings and to catchments with a range of bedrock lithology and vegetation. This will promote a better generic understanding of the dynamics of source-tosink systems and provide a powerful tool for forward stratigraphic modeling. The sediment routing system approach has the potential to contribute strongly to new models of sequence stratigraphy.