Coastal alluvial fans (fan deltas) of the Gulf of Aqaba (Gulf of Eilat), Red Sea

Abstract

Coastal sediments of the Gulf of Aqaba are dominated by alluvial fans that prograde directly into the sea. The fans can be subdivided into four types: (1) largely inactive alluvial fans that merge into a braided fluvial system and pass seaward into sabkha flats, lagoons, mangroves and fringing reefs; (2) large alluvial fans that pass directly into the sea with one major entrenched channel and a fringing reef with a large incised canyon; both of these were formed during the Pleistocene, present fluvial activity is confined to the entrenched channels; (3) medium-sized (1–2 km long, 3–4 km wide) moderate to highly active alluvial fans with fringing reefs and backreef lagoons; and (4) small short-headed wadis that empty directly into the sea. The scale, overall sediment body geometry and facies associations of type (3) coastal alluvial fans (fan deltas) provide a close and useful modern analogue for many ancient fan-delta sedimentary sequences. On subaerial parts of the fan, disorganised cobbles and boulders, at the apex, deposited by debris flows pass downslope into longitudinal bars deposited during the high flood stage of periodic flash-flood events. The bars extend over the entire fan surface becoming progressively smaller and finer grained down fan. In general, the fans are characterised by a low proportion of floodplain deposits and extensive modification by aeolian processes, producing widespread gravel pavements and small dune fields over inactive areas of the lower fan. In the marine environment the fans are modified by a combination of wave action and longshore drift. Sand beaches are characterised by low-angle seaward-dipping lamination. On shingle beaches all gravel clasts have a strong preferred seaward dipping orientation. In areas where the fringing reefs are situated offshore from the fan, mixed quartz-bioclastic sand-filled lagoons develop. The nearshore lagoon areas are characterised by large sand bars orientated parallel to the shore. These pass seawards into the reef flat. Where the fringing reef lies adjacent to the foreshore, fluvial activity erodes and emphasises irregularities in the reef and redeposits coarse gravel onto and through the reef. Much of the coarse material is incorporated into the framework. Initial stabilisation is by a variety of encrusting organisms, mainly coralline algae. This organic cementation is followed by later recolonisation by corals.