Carbonates and evaporites of Paleogene age form the shallow-aquifer rocks mantling most of Qatar, including the Paleocene and Lower Eocene Umm er Radhuma Formation and the Lower to Middle Eocene Rus and Dammam Formations. A core-based study was carried out to improve general understanding of the stratigraphic controls on aquifer matrix properties in Qatar. A cumulative total of 377 m of 10-cm-diameter core was recovered from three boreholes in central and northern Qatar, drilled to depths of greater than 120 m. Sedimentological attributes of these rocks were investigated through core and thin-section description, X-ray diffraction-based mineralogical assessment, as well as whole-rock stable isotopic analysis and integrated with interpretation of gamma-ray logs. Stratigraphic correlation of the penetrated intervals was then undertaken using sequence stratigraphic concepts and isotope stratigraphy (δ 13 C trends) in the context of recently published regional paleomaps and structural studies. In the area of Qatar, the Umm er Radhuma Formation and the overlying Traina Member of the Rus Formation were deposited in marine settings of two different basins. These basins, which extended to the south and north of Qatar, respectively, are interpreted to have been separated by a topographic high, the location of which was controlled by the presence of high-angle normal faults. The southern basin was more restricted and was the site of extensive evaporite and clay-rich siliciclastic deposition during early stages of Rus Formation. Similar evaporites and fine siliciclastic deposits are not observed in time-equivalent strata of the northern basin. During subsequent deposition of the Al Khor Member of the Rus Formation, as well as the Dammam Formation, the basins appear to have been interconnected, and fine-grained siliciclastic deposits are interbedded with, but subordinate to, carbonate strata across Qatar. Most rocks recovered for this study are dolomitic, and dolomitic rocks free of other mineral phases tend to have significant porosity (20-50%) and permeability (10-1000 mD). Decreased connectivity, flow, and storage capacity are caused by (1) the presence of gypsum beds and nodules (only southern Qatar, upper Umm er Radhuma Formation, and Rus Formation), (2) the presence of pore-occluding clays (typically palygorskite) to varying degrees in all formations, and (3) the occurrence of diagenetic calcites, most commonly in the Dammam Formation. Aquifer quality of the near-surface rocks of Qatar is in large part a function of their depositional history and is to a degree predictable using reconstruction of basin architecture, as well as sequence stratigraphic concepts.

The southern margin of the Arabian Gulf is a ''classic'' shallow-water, evaporative, carbonate-producing setting. The sediments and early diagenetic products creating the ''Great Pearl Bank'' of the United Arab Emirates to the east and accumulating in the coastal regions of Qatar to the west have long been studied as modern analogs for ancient evaporitic carbonate deposits of the rock record. An integrated study measuring the chemistry of Qatar subtidal coastal waters, evaporating tidal-pond waters (to halite saturation), and meteoric pond waters was undertaken encompassing both the dry (fall) and wet (winter/spring) seasons of 2016-2017. Measured parameters included temperature, pH, dissolved oxygen, and alkalinity, as well as major-ion (Na þ , Ca 2þ , Mg 2þ , K þ , Sr 2þ , Cl -, and SO 4 2-) and stable-isotope (d 18 O and dD) composition. Initial concentration by evaporation (to~90 practical salinity units (psu)) is interpreted to drive minor diagenetic aragonite precipitation. Further evaporation initially causes minor aragonite dissolution followed by gypsum and halite precipitation. One pond showed evidence of ongoing replacive dolomitization interpreted to be driven by H 2 S formation and oxidation in association with microbial breakdown of organic matter. The stable-isotope composition of water in restricted ponds is a function of the degree of evaporation and dilution by meteoric waters during the wet season. Unexpectedly, beyond 350 psu, d 18 O and dD continue to rise reaching values greater than 12 and 60%, respectively. The slope of the d 18 O-dD regression line exhibits no differences between dry and wet seasons.During collection of coastal waters (up to~90 psu), live Pirenella cingulata (previously Cerithidea cingulata) gastropods were collected and their shells analyzed for d 18 O and d 13 C, as well as Sr 2þ and Ca 2þ concentration. The d 18 O of water and Pirenella from the same sample site exhibits a strong correlation (R 2 ! 0.85) with a slope of~1, suggesting that the shells may be a useful chemical archive for the isotopic composition of past oceans. The d 18 O and d 13 C of the shells correlate positively, likely reflecting greater sequestration of 12 C into organic matter in more restrictive evaporative settings. The intercept of the d 18 O and d 13 C correlation shifts between dry and wet seasons, and is interpreted to reflect average seawater temperature differences during recent growth. There is also a strong correlation (R 2 ! 0.85) between shell d 18 O and measured water salinity, reflecting their mutual control by degree of evaporation. The Sr 2þ content of the gastropods does not correlate well with any measured oceanographic parameter, or show evidence of systematic seasonal change.

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.