ALBERT

Notes: The Grès de Champsaur turbidite system, deposited in a distal setting in the Alpine Foreland Basin of south-eastern France, exhibits a repeated upsection alternation in sand body geometry between incised channels and sheet sands. The channels form symmetric lenticular erosional features, of width 900–1000 m (measured between the lateral limits of incision) and depth 65–115 m, and can be traced axially for up to 5 km. In each case, the channel fill is capped by a laterally persistent sandy sheet-form interval, which lies upon a fine-grained substrate beyond the channel margins. No intrachannel elements have been traced into the substrate sequence, suggesting that, before infill, the channels acted as open sea-floor conduits of essentially the same dimensions as the preserved channel deposits. The channels are vertically stacked, although axial erosion juxtaposes younger channel axis deposits against the fill of older channels and their channel-capping sheet sandstones to produce an apparently well-connected composite sandstone body geometry. The predominant channel-fill facies comprises coarse-grained, amalgamated sandstones, which are commonly parallel- or cross-stratified. Subsidiary facies of finer grained sandstone–mudstone couplets and clast-bearing muddy debrites are commonly preserved as erosional remnants, suggesting a complex channel history of aggradation and erosion. The repeated cycles of channel incision, infill and transition to sheet sandstone development indicate repetitive incision and healing of the palaeo-sea floor. A model is proposed that links incision to the development of relatively steep axial gradients (parallel to the mean dispersal direction) and the return to sheet-form deposition to the re-establishment of lower axial gradients, with the repetitive switch between incisional channels and sheet sandstones driven by changes in sediment input rate against a background of ongoing sea-floor tilting.

Notes: The influence of palaeodrainage characteristics, palaeogeography and tectonic setting are rarely considered as controls on stratigraphic organization in palaeovalley or incised valley systems. This study is an examination of the influence of source region vs. downstream base level controls on the sedimentary architecture of a set of bedrock-confined palaeovalleys developed along the distal margin of the Alpine foreland basin in south-eastern France. Three distinct facies associations are observed within the palaeovalley fills. Fluvial facies association A is mainly dominated by poorly sorted, highly disorganized, clast-to-matrix-supported cobble-to-boulder conglomerates that are interpreted as streamflood deposits. Facies association B comprises mainly yellow siltstones and is interpreted as recording deposition in an estuarine basin environment. Estuarine marine facies association C comprises interstratified estuarine siltstones and clean, well-sorted washover sandstones. The sedimentary characteristics of the valley fill successions are related to the proximity of depositional sites to sediment source areas. Palaeovalleys located proximal to structurally controlled basement palaeohighs are entirely dominated by coarse fluvial streamflood deposits. In contrast, distal palaeovalley segments, which are located several kilometres downstream, contain successions showing upward transition from coarse fluvial facies into estuarine central basin fines, and finally into estuarine-marginal marine facies. Facies distributions suggest that the fluvial deposits form wedge-shaped, downstream-thinning sediment bodies, whereas the estuarine deposits form an upstream-thinning wedge. The vertical stacking of fluvial to estuarine to marginal marine depositional environments records the fluvial aggradation and subsequent transgression of relatively small bedrock-confined river valleys, which drained a rugged, upland terrain. Facies geometries suggest that a fluvial sediment wedge initially prograded downvalley, in response to high bed load sediment yields. Subsequently, palaeovalleys became drowned during the passage of a marine transgression, with the establishment of estuarine conditions. Initial fluvial aggradation and subsequent marine flooding of the palaeovalleys is a consequence of the interaction of high local rates of sediment supply and relative sea-level rise driven by flexural subsidence of the basin.