[1] Regional grain size trends in fluvial successions can reveal important information regarding the dynamics of sediment routing systems. Self-similar solutions for downsystem grain size fining have recently been proposed to explore how key variables, such as the spatial distribution of deposition, sediment discharge, and sediment supply characteristics, control spatial distribution of grain size in fluvial successions over time scales of 10 4 -10 6 years. We explore the sensitivity of these solutions to changes in key variables and assess their applicability to ancient fluvial successions. Several sensitivity analyses are presented to investigate the relative control of the key model variables on the spatial pattern of down-system grain size fining in fluvial successions. Sensitivity analyses demonstrate that (1) an increase in the initial value of sediment discharge to a basin causes a decrease in the rate of grain size fining in fluvial successions, an effect that becomes nonlinear for large values of initial sediment discharge; (2) a short-wavelength/ high-amplitude subsidence regime generates a greater rate of down-system grain size fining and a long-wavelength/lower-amplitude subsidence regime generates a lesser rate of down-system grain size fining in fluvial successions; and (3) an increase in the spread of grain sizes in the sediment supply generates a greater rate of down-system grain size fining. We apply this modeling technique to grain size data sets collected from two time surfaces within conglomerates of the Upper Eocene Montsor Fan Succession of the Pobla Basin, Spanish Pyrenees. These data sets exhibit approximately self-similar grain size distributions; further, the observed increase in down-system grain size fining associated with smaller depositional system lengths provides support for the application of self-similar solutions to fluvial successions. By applying these solutions to carefully collected grain size data from fluvial successions, we are able to relate explicitly the initial grain size supplied to the system, the spatial distribution of subsidence and the sediment discharge into the basin to the rate of grain size fining in fluvial successions. This method thus offers a powerful means of elucidating sediment routing system dynamics over time.

The Pyrenees represents an orogen that developed diachronously, from east to west, between the Late Cretaceous and Miocene. Here, we use detrital zircon fission-track thermochronological analyses and U-Pb geochronology, interpreted within the context of the thermal and tectono-sedimentary development of the orogen, to construct a 3-stage model for south-central Pyrenean sediment routing system evolution as follows: (1) Late Cretaceous to Paleocene: Oblique convergence and topographic growth initiates in the eastern Pyrenees. After erosion and removal of the "cover layer", south-central Pyrenean basins are supplied with zircons cooled during the Late Cretaceous (ϳ78 Ma), with a fission-track lag time of ca. 15 Myr, that record early Pyrenean exhumation. The zircons are sourced from the eastern, not central, Pyrenees. Orogen-parallel sediment routing systems dominate; (2) Early to Middle Eocene: After a period of quiescence, plate convergence rates increase. Uplift of the central Pyrenees supplies the south-central Pyrenean basins with zircons sourced from the central Pyrenean cover layer. Out-of-sequence thrusting recycles the early foredeep deposits and their associated thermochronological signals. The sediment routing systems begin to transition from orogen-parallel to orogen-transverse states; (3) Late Eocene to Miocene: Uplift and exhumation of the western Pyrenees begins. Zircons exhumed and cooled during the Oligocene (ϳ30 Ma) in response to duplex stacking in the central Axial Zone, reach the south-central Pyrenean wedge-top and foreland basins with a lag time of ca. 3 Myr. Orogen-transverse sediment routing systems become fully established. Our results extend the exhumational history of the Pyrenees beyond that shown from bedrock studies and reveal that significant topography existed in the Pyrenees in the Paleocene. Furthermore, our data demonstrate the successive change from orogen-parallel to orogen-transverse sediment dispersal along strike, coeval with diachronous mountain growth. This study has implications for understanding the evolution of synorogenic sediment routing systems, migrating depocenters and the redistribution of mass by surface processes that may drive any coupling with tectonics during oblique orogenic development.

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...

Downstream grain-size fi ning in stratigraphy is driven primarily by selective deposition of sediment, and the long-term effi ciency of this process is determined by:(1) the magnitude and characteristics of the input sediment supply; (2) the spatial distribution of subsidence rate, which creates accommodation for sediment preservation; and (3) the dynamics of sediment transport and deposition. A key challenge is to determine how these fi rst two factors control the caliber and spatial distribution of deposits over time scales of 10 4 -10 6 yr without incorporating sediment transport details that are largely unknowable for time-averaged stratigraphy in the geological past. We address this using grain-size data collected from fl uvial conglomerates in the Eocene Pobla Basin, Spanish Pyrenees, a synorogenic basin where the timing of sediment deposition is well-constrained; the sediment budget is closed; and good exposure enables time lines within stratigraphy to be picked out unambiguously. For successive stratigraphic horizons, downstream trends in grain size and composition are derived for basin-fi lling sediment-routing systems with length scales of 6 and 40 km, respectively. Our data show that the rate of grain-size fi ning varies over time and with system length and can be linked to changes in source area. These results are contrasted with grain-size data from the Antist Group, a 60-km-long Oligocene system that mantles the Southern Pyrenees, where very slow rates of grainsize fi ning on the wedge top of this foldand-thrust belt are observed. We apply a self-similarity-based selective deposition model to quantify the competing controls of tectonic subsidence and sediment supply on derived grain-size trends, and model results are compared with independent constraints on the Eocene-Oligocene evolution of the Pyrenees. Our results suggest that it is now possible to invert time-averaged grain-size trends in stratigraphy to gain quantitative information on the geological boundary conditions governing the evolution of sedimentary basins.

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