Geology of the Gheweifat and As Sila 1:100 000 map sheet, United Arab Emirates

This Sheet Description describes the Quaternary and solid geology of the Gheweifat and As Sila 1:100 000 scale geological map. The oldest rocks in the area are the Miocene rocks of the Ras Khumeis, Dam, Shuwaihat and Baynunah formations. The Ras Khumeis and Dam formations (Gheweifat and Sila members) are spectacularly exposed across the headlands of Ras Khumeis, Ras Hayusa and Ras Mushairib and comprise variable proportions of grey and pinkish grey carbonate mudstones, shelly limestones, stromatolitic limestones and nodular gypsum beds. The carbonates are predominantly shallow marine but, particularly toward the top in the Sila Member, include rooted palaeosols, collapse breccias and freshwater faunas indicating periodic shifts to non-marine conditions. The overlying Shuwaihat Formation completes the transition to fully terrestrial environments and is characterised by grey and red sandstones and mudstones deposited in a range of aeolian, fluvial and sabkha environments. Quaternary and Holocene sediments are present throughout much of the area. Pale brown sands and gravels of Hofuf Formation rest unconformably on the Dam and Shuwaihat formations forming a 1-5 m thick cap on the As Sila-Gheweifat plateau. The gravels include quartzite, limestone and igneous clasts brought into the area by major river systems draining large parts of the Arabian Shield. Gravels of a comparable composition and grain-size occur at much lower altitudes to the east of Sabkha Mati where they form a thin scattered veneer called the Sabkha Mati Gravels. To the north and south of Ba’ya well-cemented pale brown shelly limestones up to 2 m thick form broad elevated platforms just above high water mark. These limestones are included with the Marawah Formation. The Abu Dhabi Formation is dominated by shelly beach ridges, intertidal deposits and algal mats which have accreted during a cycle of relative sea level rise and fall during the Holocene. These deposits underlie much of Sabkha Mati adjacent to the As Sila escarpment and occur in coastal embayments between the headlands in the north of the district. A small proportion of the district has a cover of unconsolidated aeolian sand either as a thin veneer or as Zibar dune fields to the east of Sabkha Mati. Sabkha Mati is one of the largest coastal sabkhas in the UAE and formed by the wind deflation of Holocene marine deposits close to the level of the saline water table