The geology of Saudi Arabia includes Precambrian igneous and metamorphic basement rocks, exposed across much of the country. Thick sedimentary sequences from the Phanerozoic [ citation needed ](including sandstone, anhydrite, dolomite, limestone, chert and marl) dominate much of the country's surface and host oil.
Saudi Arabia is underlain by the Precambrian igneous and metamorphic rocks of the Proterozoic Arabian Craton. Up to 5.5 km (3.4 mi) of sedimentary rocks from the Cambrian through the Pliocene accumulated on the shield, including many oil-bearing units.
Within these younger rocks, a large western facing escarpment formed in central Saudi Arabia, capped with limestone. Basement rock dips away from the escarpment with thicker sediments in the Rub' al Khali and the Persian Gulf region. [1]
2 km (1.2 mi) of Paleozoic rocks are exposed in the northwest, with the lowest 600 m (2,000 ft) correlated with the Cambrian in Jordan. The top 300 m (980 ft) is limestone, although most units from the Ordovician, Silurian and Devonian are terrestrial sandstone or shale.
The Triassic begins with the Khuff Formation shallow water limestone and ascends through 500 m (1,600 ft) of Jurassic shelf limestone and marine shale, overlain by fossil-rich rocks from the late Jurassic and early Cretaceous. Jurassic rocks form much of the escarpment in central Saudi Arabia, although discontinuous sedimentation led to the deposition of calcarenite and anhydrite. The alternating layers of the Arab Formation took shape during the time and hold extensive oil and gas resources. [2] [3] [4] The Kimmeridgian age Jubaila Limestone contains the high productivity Arab-D member in the Ghawar oil field and a 1.1 km (0.68 mi)-long sequence of aphanitic and calcarenite limestone. It often weathers to prominent scarps without talus. [5]
For the most part, Middle Cretaceous rocks include thick sandstone, although some marine rocks are present to the north and Jordan. A combined sequence of 500 m (1,600 ft) of limestone and dolomite spans the Cretaceous into the Paleogene. [6]
The Umm er Rhaduma Formation, named by S.B. Henry and C.W. Brown in 1935 is the oldest Cenozoic sedimentary unit in Saudi Arabia, from the Paleocene to the early Eocene. It is visible in the walls of Wadi al Batin and overlies the dolomite and limestone of the Aruma Formation. The formation spans 1200 kilometers from Wadi Jabaliyah to the Iraq-Saudi Arabia border. Karst commonly forms in the uplands along with river channels. Northwest of Al Batin, the rock has very poor internal drainage. Drilling revealed two monocline structures were related to the dissolution and collapse of anhydrite layers. Dolomite, limestone and chert with some fossiliferous units are particularly common. Dissolved hydrogen gas renders water from the unit poor quality near Ras Tenura, Abqaiq and Nariya. Saudi Aramco developed the unit to supply water to its outlying operations. [7] [8]
The unit formation is overlain by the early Eocene Rus Formation, named for Umm ar Ru'us. Originally known as the Chalky Zone, it was renamed in 1946. It occurs only a few places, extending 180 kilometers north of Wadi as Sahba and occupying the breached center of the Dammam Dome. Chalk and limestone ascend to gypsum-bearing and calcareous shale. Coastal Saudi Arabia was only submerged briefly in the Cenozoic, limiting marine deposition, although the few Miocene and Pliocene marine rock are divided into the Dam, Hofuf and Hadrukh formations. [9]
The early and middle Eocene Damman Formation is named for the Dammam Dome and includes basal marl, limestone and dolomite as well as the Saila Shale Member. It is exposed above the Umm er Rhaduma but largely overlapped by Miocene and Pliocene sediments and occurs in larger areas near the border with Qatar and in a five kilometer wide, 180 kilometer long belt from Wadi as Sahba. The marl, limestone and shale units taken together do store groundwater and communities in the Eastern Province often draw on the Alat aquifer in the Khobar Member.
Groundwater conditions vary widely within Cenozoic units. Al Hasa and Al Qatif are examples of oases, fed by artesian wells. Al Fufuf has a well-developed Miocene-Pliocene aquifer, likely fed in part by an underlying Eocene aquifer. Neogene groundwater supplies other scattered locations in the Eastern Province. Paleogene gravels form patches in the Wadi Nayyal and Wadi as Sahba. The round quartz pebbles can reach 10 centimeters in diameter, often accompanied by limestone pebbles. In the past, a river may have crossed the Al Aramah escarpment via the Wadi as Sahba structural trench. At the northwest edge of Harrat Hutaym are marl, sandstone and basal conglomerate deposits with ostracode fossils from a freshwater environment, inferred to be from the Pliocene. Small, similar deposits are scattered across the Rub al Khali.
The Al Harrah volcanic complex is the only eruptive feature in the sediment-covered areas of Saudi Arabia. Its olivine basalt and aplite dikes formed during the Miocene, Pliocene and early Quaternary, spanning 250 kilometers to the northwest from Ath Thayat into Jordan. In the northwest, duricrust carapace is particularly common on Paleozoic and Mesozoic units due to greater moisture. [10]
In the last 2.5 million years of the Quaternary, limestone-quartz terrace gravel deposited in Wadi al Batin, occurring 60 to 90 kilometers north of the Trans-Arabian Pipeline. Sheet gravel blankets the Ad Dibdibah Plain, with quartz, carbonate and metamorphic rock grains spanning into Iraq and Kuwait. Al Harrah lava field basalt pebbles make up most of the gravel in the As Sahn Plain. Deltaic sheet gravels are also common in Wadi as Sahba-Wadi and Dawasir-Wadi Najran. In the Al Aramah and Hit escarpment are dissected, older limestone gravels. An Nafud and Ar Rub al Khali have silt, gravel, sabkha, unconsolidated sands and coral limestone from the geologically recent past. Half of Saudi Arabia with sedimentary cover is blanketed in eolian sands, covering an area of 600,000 square kilometers. Lake beds outcrop in the Rub al Khali while marine terraces are common along the Persian Gulf. [11]
The Gulf of Suez Rift is a continental rift zone that was active between the Late Oligocene and the end of the Miocene. It represented a continuation of the Red Sea Rift until break-up occurred in the middle Miocene, with most of the displacement on the newly developed Red Sea spreading centre being accommodated by the Dead Sea Transform. During its brief post-rift history, the deepest part of the remnant rift topography has been filled by the sea, creating the Gulf of Suez.
The Pyrenees are a 430-kilometre-long, roughly east–west striking, intracontinental mountain chain that divide France, Spain, and Andorra. The belt has an extended, polycyclic geological evolution dating back to the Precambrian. The chain's present configuration is due to the collision between the microcontinent Iberia and the southwestern promontory of the European Plate. The two continents were approaching each other since the onset of the Upper Cretaceous (Albian/Cenomanian) about 100 million years ago and were consequently colliding during the Paleogene (Eocene/Oligocene) 55 to 25 million years ago. After its uplift, the chain experienced intense erosion and isostatic readjustments. A cross-section through the chain shows an asymmetric flower-like structure with steeper dips on the French side. The Pyrenees are not solely the result of compressional forces, but also show an important sinistral shearing.
The geology of Lebanon remains poorly studied prior to the Jurassic. The country is heavily dominated by limestone, sandstone, other sedimentary rocks, and basalt, defined by its tectonic history. In Lebanon, 70% of exposed rocks are limestone karst.
The geology of Somalia is built on more than 700 million year old igneous and metamorphic crystalline basement rock, which outcrops at some places in northern Somalia. These ancient units are covered in thick layers of sedimentary rock formed in the last 200 million years and influenced by the rifting apart of the Somali Plate and the Arabian Plate. The geology of Somaliland, the de facto independent country recognized as part of Somalia, is to some degree better studied than that of Somalia as a whole. Instability related to the Somali Civil War and previous political upheaval has limited geologic research in places while heightening the importance of groundwater resources for vulnerable populations.
The geology of Somaliland is very closely related to the geology of Somalia. Somaliland is a de facto independent country within the boundaries that the international community recognizes as Somalia. Because it encompasses the former territory of British Somaliland, the region is historically better researched than former Italian Somaliland. Somaliland is built on more than 700 million year old igneous and metamorphic crystalline basement rock.. These ancient units are covered in thick layers of sedimentary rock formed in the last 200 million years and influenced by the rifting apart of the Somali Plate and the Arabian Plate.
The geology of Morocco formed beginning up to two billion years ago, in the Paleoproterozoic and potentially even earlier. It was affected by the Pan-African orogeny, although the later Hercynian orogeny produced fewer changes and left the Maseta Domain, a large area of remnant Paleozoic massifs. During the Paleozoic, extensive sedimentary deposits preserved marine fossils. Throughout the Mesozoic, the rifting apart of Pangaea to form the Atlantic Ocean created basins and fault blocks, which were blanketed in terrestrial and marine sediments—particularly as a major marine transgression flooded much of the region. In the Cenozoic, a microcontinent covered in sedimentary rocks from the Triassic and Cretaceous collided with northern Morocco, forming the Rif region. Morocco has extensive phosphate and salt reserves, as well as resources such as lead, zinc, copper and silver.
The geology of Georgia is the study of rocks, minerals, water, landforms and geologic history in Georgia. The country is dominated by the Caucasus Mountains at the junction of the Eurasian Plate and the Afro-Arabian Plate, and rock units from the Mesozoic and Cenozoic are particularly prevalent. For much of its geologic history, until the uplift of the Caucasus, Georgia was submerged by marine transgression events. Geologic research for 150 years by Georgian and Russian geologists has shed significant light on the region and since the 1970s has been augmented with the understanding of plate tectonics.
The geology of Bosnia & Herzegovina is the study of rocks, minerals, water, landforms and geologic history in the country. The oldest rocks exposed at or near the surface date to the Paleozoic and the Precambrian geologic history of the region remains poorly understood. Complex assemblages of flysch, ophiolite, mélange and igneous plutons together with thick sedimentary units are a defining characteristic of the Dinaric Alps, also known as the Dinaride Mountains, which dominate much of the country's landscape.
The geology of Moldova encompasses basement rocks from the Precambrian dating back more than 2.5 billion years, overlain by thick sequences of Proterozoic, Paleozoic, Mesozoic and Cenozoic sedimentary rocks.
The geology of Bahrain is poorly studied before the Cenozoic. Extensive sedimentary formations from the Eocene through recent times cover much of the island.
The geology of Mississippi includes some deep igneous and metamorphic crystalline basement rocks from the Precambrian known only from boreholes in the north, as well as sedimentary sequences from the Paleozoic. The region long experienced shallow marine conditions during the tectonic evolutions of the Mesozoic and Cenozoic, as coastal plain sediments accumulated up to 45,000 feet thick, including limestone, dolomite, marl, anhydrite and sandstone layers, with some oil and gas occurrences and the remnants of Cretaceous volcanic activity in some locations.
The geology of Utah, in the western United States, includes rocks formed at the edge of the proto-North American continent during the Precambrian. A shallow marine sedimentary environment covered the region for much of the Paleozoic and Mesozoic, followed by dryland conditions, volcanism, and the formation of the basin and range terrain in the Cenozoic.
The geology of North Dakota includes thick sequences oil and coal bearing sedimentary rocks formed in shallow seas in the Paleozoic and Mesozoic, as well as terrestrial deposits from the Cenozoic on top of ancient Precambrian crystalline basement rocks. The state has extensive oil and gas, sand and gravel, coal, groundwater and other natural resources.
The geology of Kuwait includes extremely thick, oil and gas-bearing sedimentary sequences from the Mesozoic and Cenozoic. Kuwait is a country in Western Asia, situated in the northern edge of Eastern Arabia at the tip of the Persian Gulf.
The geology of Afghanistan includes nearly one billion year old rocks from the Precambrian. The region experienced widespread marine transgressions and deposition during the Paleozoic and Mesozoic, that continued into the Cenozoic with the uplift of the Hindu Kush mountains.
The geology of Lithuania consists of ancient Proterozoic basement rock overlain by thick sequences of Paleozoic, Mesozoic and Cenozoic marine sedimentary rocks, with some oil reserves, abundant limestone, dolomite, phosphorite and glauconite. Lithuania is a country in the Baltic region of northern-eastern Europe.
The geology of Israel includes igneous and metamorphic crystalline basement rocks from the Precambrian overlain by a lengthy sequence of sedimentary rocks extending up to the Pleistocene and overlain with alluvium, sand dunes and playa deposits.
The geology of Syria includes ancient metamorphic rocks from the Precambrian belonging to the Arabian Craton, as well as numerous marine sedimentary rocks and some erupted basalt up to recent times.
The geology of the United Arab Emirates includes very thick Paleozoic, Mesozoic and Cenozoic marine and continental sedimentary rocks overlying deeply buried Precambrian. The region has extensive oil and gas resources and was deformed during the last several million years by more distant tectonic events.
The geology of Italy includes mountain ranges such as the Alps and the Apennines formed from the uplift of igneous and primarily marine sedimentary rocks all formed since the Paleozoic. Some active volcanoes are located in Insular Italy.
