The Geology of Yemen.
The Afro-Arabian Plate drifted to the north-east, closing the Tethyan sea-way. It collided with Eurasia in the mid-Late Tertiary, forming the Zagros mountains and Iranian fold-belt, when the Arabian Plate was detached from Africa by the Aden-Red Sea rift which split the shield.
Being high on the Arabian Shield, Yemen had a stable geology with erratic distribution of continental sediments through the Paleozoic. The first marine transgression occurred in the mid-Jurassic from the south and south-east. That was associated with a broad sag which preceded the opening of the Indian Ocean and resulted in deposition of the trans-gressive Kohlan sandstone and wide-spread shallow shelfal deposits of the lower Aniran (Massive Amran facies), grading into the Marker Shale facies.
The opening of the Indian Ocean in the Upper Jurassic coincided with a rift across the shield, resulting in the Ma'reb-Jawf graben, where oil production at Alef was located. Rapid subsidence of the graben floor accommodated thick syn-rift deposits of Turbidite facies. Away from that, shallow marine conditions continued. The rift lasted less than 10m years. The Tithonian was followed by a sag. That, combined with a lowering of eustatic sea level, resulted in exposure and erosion of broad shelfal areas, and deposition of the detritus in the sag basin. The Alef sandstone was the result, followed by the regional Arabian evaporitic cycle of the late Tithonian. Massive salt deposits filled the sag basin. Away from the sag basin, erosion probably continued.
In the Cretaceous through the Paleocene, the Tawilah-Medj Zir sandstone was deposited in low-lying areas in a marginal marine to continental environment, during a time of stable tectonics. Stable conditions persisted until the development of the Red Sea-Gulf of Aden rifts in the Oligo-Miocene, which resulted in a massive uplift along the rift, crustal thinning, extension, and massive out-pourings of volcanics, intrusion of dikes along fractures, and local intrusion of basic and acidic plutons, stocks and volcanic necks. High heat flow was associated with this crustal thinning and igneous intrusion. This volcanic activity continued in the Quaternary.Uplift and eastward tilt-ing continues up to now, with historical volcanic flows north-west of San'a', active fumaroles and volcanic vents east of Dhamar and sporadic earthquakes.
The stratigraphic column of Yemen comprises of sediments ranging from Paleozoic to Recent and can be sub-divided to the following units:
A Pre-Cambrian-Infra-Cambrian Basement Complex, composed of eroded igneous and metamorphic rocks including gneiss, schist, meta-volcanics and meta-sediments.
Wajid (Cambrian to Carboniferous) consists of sandstone and minor carbonates. It is un-conformably over-lying the Permian Akbra formation (Fm) or the Jurassic Kohan Fm.
Akbra (Upper Carboniferous to Lower Permian) Fm is considered roughly contemporaneous with and consisting mostly of mudstone of fluvio-glacial sediments - equivalent to the Unayzah Fm in southern Saudi Arabia.
Kohlan (Bathonian to Kimmeridgean) Fm is the basal Jurassic sandstone. It is a trans-gressive clastic sequence found throughout the basin as localised erosional remnants. This unit is composed of sandstone with minor amounts of conglomerate, shale and limestone. The age was poorly known, however, in the Bathonian. Yemen was subjected to a marine trans-gression. The equivalent unit is distributed in Somalia and Ethiopia as the Adigrat sandstone.
Shuqra (Oxfordian to Kimmeridgian) Fm was deposited in a broad shallow shelf. The unit is mainly a platform carbonate with reefal build-ups. The basal part is often sandy due to a re-working of the Kohlan. The carbonate is of wacke-stone and packstone and is locally silty and argillaceous. Inter-bedded mudstone is common. The lower-most part could be the Callovian.
Arwa (Kimmeridgian to Tithonian) Fm is composed of argillaceous limestone, deep-water lime mudstone, and locally minor packstone, wackestone, grainstone, and black shale. It represents the initiation of rifting in the Ma'reb-Jawf- Shabwa rift system. It conformably overlies the Shuqra Fm.
Meem (Kimmeridgian to Tithonian) Fm consists of various facies including shallow to relatively deep marine carbonate, shallow marine sandstone, turbidite and organically rich mudstone. It is conformably over-laying the Arwa Fm.
Lam (Tithonian) Fm is similar in the lithology to the Meem. But Lam is dominant in sandstone. The sandstone is interpreted to be turbidite and occurs in thin beds within the mudstone.
Tawillah (Hauterivian to Maastrichian) group consists mainly of fluvial to continental clastics mainly sandstones inter-bedded with siltstone, marl and shale. The Tawillah group un-conformably over-lies the Amran group.
The Arabian Peninsula is part of the Pre-Cambrian Arabian-Nubian Shield and can be sub-divided into two adjacent structural regions: The internal stable area (Arabian Platform), which includes the Pre-Cambrian basement rocks and the over-lying sedimentary cover. The mobile belt, which includes Zagros and the Oman mountain series.
During the Late Paleozoic to the Early Mesozoic the trends of the basement rocks were oriented N-S, NW-SE, NE-SW, and subordinately E-W. These influenced inter-basins rifting direction or elevated uplift/high as differing stress fields built up during various phases of the Gondwana fragmentation.
During the Late Jurassic to the Early Cretaceous the ancient NW-SE Wajid Faults were rejuvenated, with final separation of the Arabian Plate from Africa's in the Neogene along the ENE-WSW (Gulf of Aden) and NNW-SSE (Red Sea) trend. This had profound impacts on the geology of the area.
In the northern and eastern parts of the Arabian Plate, the basement is depressed and thickly covered with sediments (Arabian Platform Zone), whereas in the southern and western parts, the Pre-Cambrian crystalline basement and its sedimentary cover are up-lifted and partly exposed. The major parts of Yemen are in the Arabian platform and its tectonics and geological architectures are directly related to the tectonic evolution of the Arabian Plate. The tectonic evolution of Yemen resulted in the following up-lifts and sedimentary basins:
Paleozoic: including the southern flank of Rub' al-Khali (empty quarter) and the eastern Socotra Basin. They are probably related to the Karoo rifting episode of Africa (Permian-Triassic phases).
Mesozoic: including ad-Dhali, Sab'atayn, Say'un-Masila, Jiza-Qamar, Balhaf and the south Socotra Basin, which trends NW-SE (Wajid Fault System).
Tertiary: including Mukalla-Sayhut, Aden-Abyan which extends parallel to the ENE-WSW Gulf of Aden trend and Tihama Basin along the NNW-SSE Red Sea trends.
The major basement up-lift (Mahafid High) separates ad-Dhali and Sab'atayn Basins, while Jabal al-Aswad separates the Hajar sector of Sab'atayn from the Balhaf Basin. The Jahi-Mukalla high separates the Sab'atayn-Balhaf Basins from the Say'un-Masila Basin and Fartaq high separates the Say'un-Masila Basin from the Jiza-Qamar Basin. The Hadramaut Arch separates the Mesozoic Basins from the Rub' al-Khali Basin.