Coastal plain sedimentation in the late Devonian of Southern Ireland; hummocky cross-stratification in fluvial deposits?

Upper flow regime sheets, lenses and scour fills: Extending the range of architectural elements for fluvial sediment bodies

“…Upstream-dipping cross-bedding (or "backset" bedding) seems to be a characteristic of upper flow regime conditions, specifically within the antidune and chute and pool stability fields (Jopling and Richardson, 1966;Harms et al, 1975;Alexander et al, 2001; Breakspear, unpublished data: Figure 3A, B). Examples have been described from the ancient by Cotter and Graham (1991) and Browne and Plint (1994), among others, and interpreted in ways that are compatible with this hypothesis.…”

“…Some variations to plane bedding are, however, worthy of note. Various authors have described minor, low-angle discordant bedding (Figure 2A, B) within predominantly planar-bedded sands and sandstones, both from the modern (e.g., McKee et al, 1967;Stear, 1985) and ancient (e.g., Tunbridge, 1981;Cotter and Graham, 1991;Browne and Plint, 1994). This is at a considerably larger scale than the low amplitude bed wave structures noted above.…”

“…Fluvial sandstone bodies that contain convex-upward bedding structures have proved historically difficult to interpret, invoking confusion with hummocky cross-stratification (e.g., Rust and Gibling, 1990a;Cotter and Graham, 1991). Nonetheless, these and other authors have used the occurrences of symmetrical or near-sym- , showing a form set of slightly sinuous-crested, ripple-scale bedforms that in cross-section show very low-angle cross-lamination or no internal structure.…”

“…Few examples have been described from either the modern or ancient where such upper flow regime structures dominate large-scale geomorphic/architectural elements (but see McKee et al, 1967;Picard and High, 1973;Tunbridge, 1981, for examples). Furthermore, several authors have pointed out ambiguities between the distinguishing criteria for upper flow regime structures and those for combined wave-current flow structures such as hummocky crossstratification (e.g., Rust and Gibling, 1990a;Cotter and Graham, 1991;Alexander et al, 2001). The reader is referred to Bridge (2003) for a more detailed review of bedforms and resultant sedimentary structures in fluvial environments.…”

Architecture and development of the alluvial sediments of the Upper Jurassic Tordillo Formation in the Cañada Ancha Valley, northern Neuquén Basin, Argentina

“…During high discharge events, flow also expanded over these contemporaneous floodplain depositing sheet sandstones within splay complexes. Under these conditions, sandstone beds under near-critical flow conditions may form hummocky-like structures (Cotter and Graham, 1991 ).…”