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Geologie Ománu (Oman geology)

04.03.2012  |  56× přečteno      vytisknout článek 

Geologie Ománu (Oman geology)

The oldest rocks in Oman outcrop most extensively in Dhofar, east of Marbat and on the Hallaniyat islands.  They are crystalline basement rocks of the granite clan, gneisses and mica schists.  Granodiorite also outcrops in a small area near Jabal Ja'alan at the southern edge of the Wahibah Sands.  

Late Proterozoic to Cambrian rocks outcrop in Al Huqf on the eastern side of the region of Al Wusta just south of the turning to Masirah Island, and on Saih Hatat south of Muscat.  They have a largely terrigeneous and volcanic origin typical of continental shelf conditions with a transgression to a marine environment in later times.

Largely sediments

 

Diamictite, Mistal Formation, 
Ghubrah Bowl

Volcano-sedimentary rocks outcrop extensively in the centre of the Ghubrah Bowl on Al Jabal al Akhdar.  Small pebbles of both igneous and sedimentary rocks are embedded in a tuffitic or sandstone matrix.  The volcanics are succeeded by pillow lavas and greywackes.  The sequence is thought to represent a series of submarine debris flows dating from the end of the Precambrian era.
 

Glacial tillite, 
Wadi bani Kharus

Recent work suggests that this rock found at the end of Wadi bani Kharus is a glacial tillite dating from approximately 750Ma at the end of the Precambrian. 
 

Salt dome in the 
Fuhud oil-field

The complex geology exposed on the mountains continues southwards beneath the desert sands.  Salt domes dating from the Cambrian era punctuate the desert sands south of the northern Oman mountains. 

As a result, Oman has relatively small reserves of oil sited in structural traps, some of which are adjacent to the salt domes.  The oil can be difficult to extract and PDO has pioneered methods of horizontal drilling to reach it.

 

Polygonal structures in ancient evaporites

The relatively low density of the salt dome rocks has resulted in their diapiric rise to the surface.  Gypsum, anhydrite and some rarer evaporite minerals outcrop as well as halite.

Polygonal structures on the surface of these ancient rocks are similar to those seen on present day evaporite deposits in hot dry deserts.

 

Unconformity between Permian and pre-Permian rocks, 
Wadi bani Kharus

Stromatolite-bearing limestones were deposited in a shallow sea that existed at the end of the Precambrian era.  These limestones belonging to the Kharus Formation are a source of hydrocarbons found in rocks in the south of Oman.  Here in Wadi bani Kharus in the northern Oman mountains, they dip steeply northwards beneath the overlying Saiq Formation, largely dolomites, of Permian age.

The age gap of 300 My represents a period of uplift before marine sedimentation began again in the Permian, continuing for 190 My well into the Cretaceous.

 

Palaeozoic-Mesozoic boundary, Wadi bani Kharus

The Open University Geological Society toured northern Oman in early 1998 following Hanna's guidebook extensively.  We weren't altogether sure we'd found the Palaeozoic-Mesozoic boundary in Wadi bani Kharus, but it does mark a major geological extinction.  You should find brachiopods and crinoids in the underlying Permian dolomite in contrast to the overlying fossil-poor Triassic mudstones and limestones.
  Here's the OUGS field trip party from 1998.  Wadi bani Kharus.
 

The Ghubrah Bowl, Wadi Mistal

The sedimentary rocks on Al Jabal al Akhdar have been weathered and exposed to reveal the succession from the Precambrian to the Cretaceous.  The Ghubrah Bowl and the many wadis flowing both north and south from the watershed offer invaluable opportunities to examine the sequences.
 

View northwards over 
Al Jabal al Akhdar 
from Sharfat al Alamayn

The region of Al Jabal al Akhdar is an anticline plunging gently westwards.  Its sedimentary and volcanic rocks formed an autochthonous platform over which contemporary and younger deep ocean sediments were thrust along with the overlying mantle ophiolites during the Cretaceous era.
 

Rocks of Al Hajar supergroup outcrop behind the American-British Academy in Muscat

Palaeozoic sediments of the Al Hajar supergroup form the northern flank of both the Al Jabal al Akhdar and the Seih Hatat anticlines.  They dip steeply to the north.

This picture also shows a small wedge of ophiolite on the extreme left hand side.  The contact is marked by a wide crush zone in which the ophiolite has been serpentinized.  Fragments of ophiolite found on top of Al Jabal al Akhdar show that the mantle sequence once covered the sedimentary platform.

 

Shelf carbonates of the Natih Formation on Al Hajar dipping south from Sharfat al Alamayn

These are some of the upper sedimentary units found on Al Jabal al Akhdar.  Thinly bedded limestones with varying clay content and black shale and oyster layers form the upper unit of the Natih Formation, dating from the Middle Cretaceous. 

These deposits were laid down in a partly euxinic (reducing) environment.  The Natih Formation is a widespread oil-bearing horizon in Oman

 

Shelly fossils in Natih Formation

Massive bioclastic limestones form a lower unit.  The presence of these rudist fossils in rocks over 2,000 m high testify to the degree of uplift.  The fossils are the remains of animals that lived in shallow marine and reef conditions on the continental shelf.

Mostly ophiolites

 The northern Oman mountains form an arc extending for 700 km from Musandam in the north to the east coast at Ras al Hadd.  The mountains define an obduction zone  where the mid-oceanic rocks and deep ocean sediments of the ancient Tethys ocean were thrust upwards and over the continental shelf and slope rocks of the Arabian platform.  

More nad photos on:http://www.geocities.com/suonnoch/Oman/geology_of_oman.htm

ZDROJ: www.geocities.com

Although not clearly evident in this picture, Tertiary limestones overlie ophiolite on the left hand side of the road.  Al Hajar supergroup sediments dip almost vertically on the right hand side of the road. 


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