Mixed siliciclastic-carbonate-evaporite sedimentation in an arid eolian landscape: The Khor Al Adaid tide-dominated coastal embayment, Qatar

Abstract

The Khor Al Adaid embayment of southern Qatar represents a unique shallow-water mixed siliciclastic‑carbonate coastal depositional system that developed in a hyper-arid climatic setting over the past 6000 years. The embayment, which was formed during the Flandrian transgression as a result of flooding across a partially fault-controlled incised fluvial drainage, is supplied by quartz-rich sands delivered by wind-blown dunes migrating southward across the surface of Qatar. These offshore-migrating eolian-derived sediments are being redistributed by tidal currents in an otherwise low-energy coastal zone, where in situ formation of carbonate mud and a low-diversity skeletal assemblage is ongoing within salinity-restricted environments. Three depositional sectors are delineated: 1) an energetic, linear, fault-controlled Entrance Channel into which the eolian dunes spill directly; 2) a relatively deep (up to 20 m) Outer Lagoon, interpreted to represent a flooded karst-collapse structure; and 3) a sprawling, low-energy, shallow (<2 m) Inner Lagoon occupying low-lying areas between deflated eolian dunes.Physical oceanographic modeling, integrating multi-seasonal current meter and tidal gage measurements, demonstrates tidal current flow velocities are relatively high in both the Entrance Channel and at the constricted entrance to the Inner Lagoon. Associated flow expansion into less confined areas results in deposition of the eolian-derived sands as flood-tidal deltas, one in the Outer, and two in the Inner Lagoon. A weakly-developed ebb-tidal delta occurs where the Entrance Channel debouches into the Arabian Gulf. Flood-tide dominance is also apparent in Entrance Channel deposits, where sand accumulates in seaward-terminating “ebb barbs” along the margins of the flood-dominant thalweg. Such flood-tide dominance of the thalweg is unusual and likely reflects the absence of river discharge. Evidence of significant inverse estuarine circulation (seaward flow of a brine along the embayment floor) is mostly absent in spite of the landward increase of salinity, where concentrations reach more than double normal seawater salinity in the Inner Lagoon. Modeling results show that seaward-flowing brines formed in the Inner Lagoon are trapped in the relatively deep Outer Lagoon, and that mixing by tidal currents in the energetic Entrance Channel precludes the formation of vertical density gradients there. Because siliciclastic mud is essentially absent, and most of the sediment forming the deltas consists of sand, upper intertidal deltaic deposits that would normally consist of mud are absent, resulting in the flood-tide deltas having a pronounced lobate geometry similar to that of fluvial deltas formed predominantly of sand.Away from tidal deltas, low-energy lagoons are floored by carbonate mud of local production, with minor gypsum precipitating in the Inner Lagoon. Wind-generated waves only locally influence sedimentation, forming complex nearshore bars along some lagoonal shorelines. The overall landward increase in salinity is accompanied by a decrease in the diversity of benthic fauna and their skeletal remains. A fairly diverse faunal assemblage is observed in the Entrance Channel near the Arabian Gulf, including colonial corals, whereas the inner lagoon assemblage is dominated gastropods belonging to Pirenella cingulata. The remnants of large eolian dunes are preserved in the low-energy setting of the Inner Lagoon, segmenting the waterbody, and locally increasing restriction. The sedimentology of the arid-zone coastal Khor Al Adaid embayment may serve as an analogue for environmental settings that were perhaps more commonplace in arid zones of flooded continents during greenhouse times.