A B S T R A C TA key factor in the downstream dispersal and fractionation of sediment is the grain size distribution of sediment supplied by upstream catchments. Modeling of the grain size distribution of modern bedload in the main trunk channels of tectonically uplifting catchments, including the sediment at their outlets, and the weathering products of a range of bedrock lithologies in southern Italy and Sicily reveals fractal dimensions of 2.3-2.7, similar to the fractal dimension of many natural materials undergoing fragmentation. We examine the impact of changing statistical properties of the grain size distribution of the sediment supply in simulating grain size trends in sedimentary basins. Model simulations show a marked movement of the gravel front and patterns of progradation and retrogradation in basin stratigraphy. These grain size trends and sedimentary architectures are generated simply by variations in the grain size mix of the sediment supply, without variations in base level or sediment discharge. Variation in the grain size distribution of the sediment supply may therefore act as a first-order control on sequence stratigraphic architectures in sedimentary basins.

Calculation of the total depositional volume of an ancient source-to-sink system, combined with estimates of the area of catchments acting as source regions using provenance methods, is used to evaluate average catchment erosion rates on a million year time scale. These rates are compared with values derived from thermochronological methods. Using the mid-to late Eocene (33.9-41.6 Ma) Escanilla palaeo-sediment routing system from the south-central Pyrenean orogenic wedge-top zone as an example, c. 3500 ± 300 km 3 of solid particulate sediment was derived from two catchments in the south-central Pyrenees over a 7.7 myr period, equivalent to a mean erosion rate of c. 0.15-0.18 mm a −1 . Average exhumation rates in contributing catchments over the same time interval are estimated at 0.2-0.3 mm a −1 based on apatite fission-track analysis of pebbles in proximal conglomerates, and 0.23-0.34 mm a −1 from fission-track analysis of detrital apatites sampling a wider range of grain size. Sediment supply progressively increased during the mid-to late Eocene time period, at least in part driven by catchment expansion deep into the Pyrenean Axial Zone at c. 39 Ma. The consistency of the rates of catchment-averaged erosion calculated from different methods builds confidence that source areas have been connected to depositional sinks correctly.

Sediment is fractionated by size during its cascade from source to sink in sediment routing systems. It is anticipated, therefore, that the grain size distribution of sediment will undergo downsystem changes as a result of fluvial sorting processes and selective deposition. We assess this hypothesis by comparing grain size statistical properties of samples from within the erosional source region with those southcentral Pyrenees, the lognormal and truncated Pareto models provide excellent fits for distances of up to 80 km from the depositional apex, whereas the Weibull fit progressively worsens with increasing transport distance. A similar trend is found in the Miocene-Pliocene gravels of the Nebraskan Great Plains over a distance of >300 km. Despite the large fractionation in mean grain size and gravel percentage from source region to depositional sink, particle size distributions therefore appear to maintain lognormality over a wide range of transport distance. Use of statistical models enables downsystem fractionation of sediment released from source regions to be better understood and predicted and is a potentially valuable tool in sourcetosink approaches to basin analysis.

The Sarah Formation is a glaciogenic sedimentary unit deposited along the Gondwana margin during the latest Ordovician ice age and represents a major hydrocarbon reservoir in northern Saudi Arabia. Large‐scale glacial palaeo‐valleys cut into the Qasim Formation and were infilled by the Sarah Formation. Post‐glacial transgression in the earliest Silurian resulted in the deposition of the Qusaiba Shale Member and associated organic‐rich basal source rocks, which cap the Sarah Formation infilled palaeo‐valleys. This unique setting makes the Sarah Formation an important emerging exploration target in Saudi Arabia. This study focuses on the facies and depositional architecture in seismic‐scale outcrops of the Sarah Formation in north‐western Saudi Arabia. The Rahal Dhab palaeo‐valley provides a 100 km long dip‐oriented cross‐section which has been covered by 24 vertical sections, sedimentary architectural analyses at metre to kilometre scale and by three cored shallow boreholes. In the Rahal Dhab palaeo‐valley, the Sarah Formation was deposited in a proglacial setting that ranged from marginal marine to offshore prodelta and is made up of three units: (i) the Sarah Sandstone; (ii) the Sarah Shale; and (iii) the Uqlah Member. This study shows the relationships between these three units and architectural controls on reservoir quality in this system. This paper contributes to the regional understanding of the Sarah Formation, and the new depositional model of the Rahal Dhab palaeo‐valley provides an outcrop‐reservoir analogue for hydrocarbon exploration in adjacent areas.