Although the stratigraphy of sedimentary basins depends on the balance between the magnitude and grain‐size characteristics of the sediment supply (Qs) and the spatial distribution of tectonic subsidence generating accommodation σ(x), Qs is problematical to measure in present‐day sediment routing systems and formidably difficult to predict in their ancient counterparts. This challenge was tackled by treating the sediment discharge from the outlet of mountain catchments as the result of incision by a drainage network with a bulk diffusivity based on the length over which the mean annual rainfall is concentrated. The size, relief and slope of palaeo‐catchments acting as feeders for sediment routing systems are used to run simulations of sediment discharge and bulk diffusivity for a range of annual precipitation values. A wide range of observable geological phenomena can be used to converge on the most likely solutions for Qs, including depositional volumes in the basin, and bedrock thermochronology and detrital cosmogenic nuclide dating to constrain catchment erosion rates. Modelled sediment discharges can be checked with estimates derived from global regressions. The sediment efflux of mountain catchments serves as a boundary condition for down‐system sediment transport and deposition. Variations in the volumetric ratio of sediment supply to available accommodation, Qs/σ(x), determines patterns of transverse versus longitudinal (axial) sediment dispersal. The volumetric ratio may change as a result of variations in climatic parameters, tectonic uplift rate and catchment expansion. An abrupt climate change to higher precipitation values promotes higher Qs/σ(x), but transient landscape response causes a return to values close to the baseline, generating a distinctive down‐system extension of a gravel ‘spike’. Catchment expansion has a similar, but more prolonged, effect on gravel progradation. In contrast, a change in tectonic forcing, such as an increase in slip rate on a border fault, causes little change in Qs/σ(x), because increased subsidence compensates for the increased sediment supply. Studies of mid Eocene–Oligocene sediment routing systems in the south‐central Pyrenees allow the discrimination of different types of proximal wedge‐top sedimentary systems on the basis of the volumetric ratio of Qs to accommodation σ(x): (i) small, steep, local fan systems in tectonically ponded, underfilled basins, supplied by low sediment discharges; (ii) tectonically guided, long‐range, axial systems fed by large sediment discharges from widely spaced palaeovalleys; and (iii) large, shallow‐sloping transverse megafans burying underlying defunct or active tectonic structures, supplied by high to very high sediment discharges. Understanding the role of variations in Qs helps to explain the syntectonic evolution of proximal foreland basin systems. The Oligocene–Miocene North Alpine Foreland Basin, Switzerland, is qualitatively identified as a high‐Qs example, the Miocene–Recent northern Apennines of Italy as a low‐Qs...
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.
Stratigraphic grain-size trends record tectonic and climatic signals. Here, we show how measurements of sediment calibre and clast lithology can be used to identify changes in accommodation space and sediment budget, using examples from Palaeogene syntectonic clastic deposits in the southern Pyrenees. We identify a mid Eocene interval of rapid grain-size fining, driven by local tectonic subsidence; a late Eocene interval of diminished local accommodation generation; and an Oligocene interval showing order-of-magnitude lower grain-size fining rates, driven by increased sediment supply. Our results demonstrate that grain-size trends provide a powerful means to explore the tectonic and climatic boundary conditions governing sediment routing systems.
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