Sedimentology of continental erg-margin interactions
Modern ergs are characterized by bedform types and distributions that reflect the directional variability and result and drift potential of the wind regime. Sand accumulates in the erg-centre, an area of low resultant drift potential where large, slow-moving linear and star draas act as sediment sinks. The thickest ancient aeolian sequences contain stacked transverse and oblique draa and dry interdune deposits. Only these forms are able to transport sand rapidly enough to aggrade by autocyclic bedform climb. Subsidence probably played a crucial role in their accumulation. Todays ergs are poor analogues for ancient erg sequences due to the rapidity of recent climatic change and because they do not appear to be aggrading by bedform climb. The sand saturation concept introduced by Wilson (1971) provides a valuable tool for understanding the nature of ancient ergs and regional bounding surfaces. Undersaturated conditions cannot form dunes, but under metasaturated conditions, dunes with deflationary interdunes may be sustained, but these are incapable of preservation by bedform climb. Only under fully sand-saturated conditions can ergs aggrade by bedform climb. The presence of deflationary interdunes in ancient erg-margin sequences indicates metasaturated conditions and implies that preservation only follows subsidence or water-table rise. Two extremes or erg-margin interaction are identified here. One, the Kayenta-Navajo transition, occurred at the margin of an established erg. Fluvial-aeolian interactions form small-scale cycles that reflect the continual interplay between the active erg and adjacent fluvial system. Large-scale cycles appear to represent a climatically-controlled alternation between active and denudated erg phases, the latter allowing advance of fluvial systems over the former erg during wetter phases. The opposite extreme occurred during deposition of the Ormskirk sandstone in the Irish Sea. Here, conditions were predominantly unsaturated because, though aqueous and aeolian processes provided an abundant supply of sand, this was almost immediately removed from the aeolian system by the continually rising water-table, thus restricting erg and dunefield development to elevated areas.