Geology of Saudi Arab

Saudi Arabia is situated in the southern part of the Arabian plate, one of the youngest of the 10 or more plates that make up the present-day surface of the Earth. In this area, the plate comprises a crystalline basement of Precambrian continental crust about 40-45 km thick and mostly 870-550 million years old, an overlying sequence of younger Phanerozoic sedimentary rocks that range in age from Cambrian (540 Ma) to the Pleistocene and in thickness from zero to 10 km, surficial Cenozoic flood basalt, and Paleogene-Holocene intracontinental and, now, oceanic basins along the Red Sea and Gulf of Aden

Prior to the opening and uplift of the rifted margins of the Red Sea and Gulf of Aden, Phanerozoic rocks covered and concealed the basement rocks, but erosion and unroofing since then exposes them as the Arabian Shield, in the west, and in minor outcrops elsewhere. The Precambrian includes terranes of volcanic, sedimentary, and calc-alkaline intrusive rocks that form a collage of strongly deformed magmatic arcs, other types of less deformed volcanic and sedimentary rocks that constitute overlap assemblages deposited in sag and rift basins, bodies of gneiss that reflect local uplift, and large amounts of late- to posttectonic granitoids that result from the development of voluminous, post-orogeny anatextic magma. The Precambrian terranes converged and amalgamated between 780-650 Ma during orogenic events that involved deformation, metamorphism, and uplift, culminating in the Nabitah orogeny, and were partly covered by overlap assemblages and intruded by plutonic rocks during a subsequent 100 million-year period of orogenic collapse, extension, exhumation, and strike-slip faulting.

The Phanerozoic rocks are unconformable on the Precambrian, and mostly little deformed, affected by open folds and block faulting in the Arabian Platform (east and north of the exposed shield) and in the Red Sea and Gulf of Aden basins. The rocks in the Arabian Platform accumulated on a stable marine-to-fluviatile shelf. Uplift and collapse of arches and basins, movements on fault blocks, and the migrating of shorelines back and forward across this shelf resulted in the intercalations and facies migrations of sandstones, siltstones, carbonates, and evaporites (salt basins) that characterize the Phanerozoic of this region. The Cenozoic sedimentary, evaporitic, and minor volcanic rocks that fill the Red Sea basin were deposited in an initial intracontinental rift that evolved, with ongoing spreading, into the present-day narrow marine basin.

The separation of Arabian and Africa, which began about 25 million years ago, entailed rifting and sea-floor spreading along the axes of the Red Sea and Gulf of Aden and the northward drift of the Arabian Plate and eventual collision with Eurasia. During this period, in addition to the formation of new oceanic crust and sedimentation in the Red Sea and Gulf of Aden basins, the western and southern margins of the Arabian Plate were uplifted and partly covered by subaerial flood basalt, resulting in the creation of the Red Sea Escarpment and fields of lava (harrat), and the northern and northeastern margins were sutured to rocks in Iran and Turkey, causing crustal shortening and the formation of the Zagros fold-and-thrust belt.

The rocks of Saudi Arabia range in age from the Precambrian to the present day, forming part of a larger unit that includes the Arabian Peninsula and is known as the Arabian Plate. Some Precambrian rocks in this region date back to the Archean (nearly 3 million years ago) but most are Neoproterozoic (1000-540 Ma*). They originated as volcanic islands or as chains of volcanoes along spreading centers and subduction zones in a Neoproterozoic ocean and against ancient continental margins, and were folded and uplifted toward the end of the Precambrian as a large belt of mountains. The mountains existed between about 680-540 Ma and were part of one of the largest mountain belts ever known to have existed on Earth. By the end of the Precambrian, the mountains had been eroded and only their roots are preserved, exposed in western Saudi Arabia in the Arabian shield.

The younger rocks in Saudi Arabia belong to the Paleozoic (540-250 Ma), Mesozoic (250-65 Ma), and Cenozoic (65 Ma to Recent) (collectively referred to as Phanerozoic cover), and crop out as relatively flat lying beds of sedimentary rocks such as sandstone, siltstone, limestone, and evaporites (salt deposits), and volcanic rocks. The rocks were deposited unconformably on the underlying Precambrian basement, in riverbeds, in glacial valleys, and in shallow seas, or were extruded from subaerial volcanoes. The rocks north and east of the Arabian shield are referred to as the Arabian Platform; those on the shield are mainly harrat (fields of Cenozoic flood basalt); and those west of the shield are Cenozoic rocks that occupy the Red Sea basin. The youngest deposits in the region include coral limestone and unconsolidated sand, silt, gravel, and sabkhah, which accumulated in the sand seas of Ar Rub al Khali and An Nafud, filled dried-up lake beds and wadis, and fringed the coastlines.

The Precambrian contain most of Saudi Arabia’s known metal deposits of gold, silver, copper, zinc, iron, and magnesium. The Phanerozoic cover contains the oil resources and deposits of bauxite (the source of aluminum), phosphate, clay, limestone, silica sand, and lightweight aggregate that are of increasing importance to the industrial development of the Kingdom.