Are carbonate barrier islands mobile? Insights from a mid to late‐Holocene system, Al Ruwais, northern Qatar

Abstract

Barrier islands are important landforms in many coastal systems around the globe. Studies of modern barrier island systems are mostly limited to those of siliciclastic realms, where the islands are recognized as mobile features that form on transgressive coastlines and migrate landward as sea‐level rises. Barrier islands of the ‘Great Pearl Bank’ along the United Arab Emirates coast are the best‐known carbonate examples. These Holocene islands, however, are interpreted to be anchored by older deposits and immobile. The mid‐Holocene to late‐Holocene depositional system at Al Ruwais, northern Qatar, provides an example of a mobile carbonate barrier island system, perhaps more similar to siliciclastic equivalents. Sedimentological and petrographic analyses, as well as 14C‐dating of shells and biogenic remains from vibracored sediments and surface deposits, show that after 7000 years ago a barrier system with a narrow back‐barrier lagoon formed along what is now an exposed coastal zone, while, contemporaneously, a laterally‐extensive coral reef was forming immediately offshore. After 1400 years ago the barrier system was forced to step ca 3 km seaward in response to a sea‐level fall of less than 2 m, where it re‐established itself directly on the mid‐Holocene reef. Since that time, the barrier has retreated landward as much as 1000 m to its current position, exposing previously‐deposited back‐barrier lagoonal sediment at the open‐coast shoreline. In modern neritic warm‐water carbonate settings mobile barrier island systems are rare. Their construction and migration may be inhibited by reef formation, early cementation, and the relative inefficiency of sourcing beach sediments from open carbonate shelves. Carbonate barrier island systems likely formed more commonly during geological periods when ramps and unrimmed shelves predominated and in calcite seas, when meteoric cementation was minimized as a result of initial calcitic allochem mineralogy. As with their siliciclastic analogues, however, recognition of the influence of these transient landforms in the rock record is challenging.

Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems

“…Most recently, Rivers et al . (2019b) showed that mid‐Holocene deposits are mostly formed of aragonite and calcite (Fig. 3A), and what little dolomite is observed is derived from underlying Eocene rocks.…”

“…In Qatar the vast majority of nearshore modern and Holocene sediments are uncemented. Minor exceptions include thin (centimetre‐thick) intervals of cemented/lithified sediments at the base of tidal channels (Shinn, 1973b), or intermittently associated with back barrier mid‐Holocene deposits such as the stromatolites referenced above at Al Ruwais (Rivers et al ., 2019b). The only major exception is beachrock cementation (Fig.…”

“…5), are interpreted here to represent transgressive barrier islands or land‐attached spits, with landward‐dipping cross‐beds formed by barrier washover processes. They appear exactly analogous to barrier island deposits found to have formed over the past 6000 years at Al Ruwais, along the windward margin of Qatar, which are lithified by beachrock‐type cementation (Rivers et al ., 2019b). Stained thin‐sections from several of the Pleistocene marine outcrops confirm that these rocks are not dolomitized to any observable degree, and demonstrate that they bear micritic meniscus‐type calcite cements (Fig.…”

Sediment-filled karst depressions and <i>riyad</i> – key archaeological environments of south Qatar

“…The rate of aeolian sand sedimentation of 1-2 mm yr −1 in the upper 3 m of QAT 63 averaged over the time between ca. 7500 and 5800 years ago as inferred from OSL data is driven by two factors: (i) greater sand availability further north (upwind), as large parts of the Qatar peninsula were then still covered by dune fields , and, (ii) at that time, relative sea level, which is coupled with groundwater levels in low-lying areas near the Qatari coast (Macumber, 2011), rose by several metres (Lambeck, 1996), reaching a highstand around 6000 years ago (Perthuisot, 1977;Engel and Brückner, 2014;Parker et al, 2018;Strohmenger and Jameson, 2018;Rivers et al, 2020). A rising groundwater table and capillary fringe stabilize newly deposited sand in dry climates and lead to net accumulation.…”

“…It seems that after the sea-level highstand phase (+2-3 m, ca. 6000-4500 years ago; Vita-Finzi, 1978;Engel and Brückner, 2014;Parker et al, 2018;Strohmenger and Jameson, 2018;Rivers et al, 2020), deflation began due to the lowering of the groundwater and the capillary fringe along with sea level, thereby affecting the lowering Stokes surface. The characteristic mounds covering specific areas of the Asaila basin (Figs.…”