prospecting in the sedimentary formations of the red

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prospecting in the sedimentary formations of the red sea coast between yanbu al bahr and maqna 1968-1979 : 

prospecting in the sedimentary formations of the red sea coast between yanbu al bahr and maqna 1968-1979 Muhammed Hussain Tajuddin kau

INTRODUCTION : 

INTRODUCTION Study of the sedimentary formation of the continental margin on the Saudi Arabian side of the Red Sea rift

LOCATION AND GEOLOGIC SETTING : 

LOCATION AND GEOLOGIC SETTING The geology and morphology of the west coast of Saudi Arabia are closely associated with and have been conditioned by their position in relation to the Red sea rift. Two geomor-phologic assemblages may be distinguished : 1.Hijaz range, which forms a very deeply dissected plateau of Precambrian rocks partially overlain by the Paleozoic cover 2. the coastal plain which is less than 200m in altitude and generally 5 to 10 km wide although reaching

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These features are controlled by the fracture tectonics of the Red Sea rift which broke up the Precambrian basement into fault-bounded blocks, mainly aligned as follows 1.The Yanbu al Bahar coastal plain fracture bounded to the north by east-west faults, to the north of which N. 45° W. 2. The Ra’s Mahâr coastal margin and the Umm Lajj block marked by north and N. 50° W.fault Here the plain becomes progressively narrower to the north. 3. The Samnah graben and the Wadi al hamd plain, which widens northwards for a distance of 100 km, basement rocks moved some 30 km westward by left.

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4. The Al Wajh block and narrow coastal fringe trending N. 20° W., where the basement lies close to the coastline due to the faulting mentioned above. 5. The Alzam embayment and trough the north half of the Al Wajh block is affected by large N. 50° W.-striking faults to the east and these bound the Alam subsidence trough which lies oblique to the coastline. 6. The Duba block with a narrow coastal plain oriented N. 20° W. The plain widens in the Ash Sharmah area . 7.Wadi ifal plain and the Maqna massif. To the north of Aynnah fractures striking E.-W. ‘to N. 7G° W.

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Methods used correspond to the various phases of mineral exploration: (1) reconnaissance and prospecting (2) surface work and detailed mapping (3) geochemistry and geophysics (4) drilling.

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GEOLOGY the results of local geologic studies have been used to reconstitute the lithostratigraphy tectonics and paleogeographic evolution of the sedimentary of the littoral zone data used are mostly from surface work, but information provided by drill holes is also referred to. LITHOSTRATIGRAPHY The areas which offer the best outcrop conditions are those where the coastal plain is wideYanbu al Bahr Jabal Dhaylan Azlam and the Maqna Aynuah area

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Silicified limestone formation The calcareous layers display whole fossil impressions and fragments of bivalve, gastropod and ostracod Dhaylan formation The Dhaylan formation is characterized by a predominance of reddish-gray to purple conglomerate and sandstone, and unconformably overlies the Cretaceous It is devoid of fossils and has been ascribed to the Oligocene due to the presence of detrital fragments and its location between dated Cretaceous and Miocene formations

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Basal conglomerate Lithology -The lower part of the basal conglomerate is found m various positions in relation to underlying rocks; either directly overlying the Precambrian basement or channeling all phases of the Cretaceous-Bocene formations It comprises sand and purple clayey sandstone incorporating angular detrital fragments of varying nature and size, with the latter ranging from pebbles to boulders. Red sandstone member Although the lower part of the Qligocene formation is only represented by sporadic deposits, the red sea sandstone member forms a continuous cover which overlies the conglomerate or the underlying Cretaceous or, in places, the Precambrian basement.

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Basalt flows and dikes Basalt flows or dikes only occur in the northeastern extremity of the Alam trough several small outcrops of basalt are dotted along a fault trending north-northwest. The of these are located at the boundary between the basal conglomerate and the red san members, where they appear to be intercalated as peneconcordant flows. Raghama formation The Raghama formation of Miocene age is much better represented than the d of the pre-Oligocene cover, and is closely linked with the littoral zone of the Red Sea as r both facies and position; sedimentation was controlled by the palogeographic blocks c by rift faulting.

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Lower member The lower member consists o conglomerate and sandstone, gypsum, and deposits, totaling about 600 m thicknees. It unconformably overlies the Oligocene Dhaylan formation or the basement. They are two type of contact: 1- directly against the basement with very sharp dips 2- in apparent continuity with the underlying Oligocene sandstone. Middle member The middle member is composed of various calcareous and manly deposits and two units have been identified.

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1-Lower assemblage: Rhythmic alternation of marl and of yellow to red calcarenite in decimeter-sized beds; echinoids, gastropods, bivalves and tubes of burrowing organisms are present. 2-Upper assemblage—The sequences are thinner than in the lower assemblage, and gypsum is also present The thickness are about 500 m . Upper member 1-influences present. The formation is composed of beds of gypsum massive at the base, The most common section, measured on the south edge of the Maqna massif (the Raghama gypsum) is as follows:

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6- 2 m: limestone and beige dotomite with black patina. 5- 50 m: two or three beds of gypsum with clayey intercalations, in places passing to more detrita facies. 4- 30 m reference horizon: dark and composed’ of brown dolomite, clay, marl, greenish brown silt and sandstone. 3- 10 m: the first band of gypsum with a calcitic or dolomitic matrix. 2- 20 m: sandstone and conglomerate 1- 10m green clay stone overlain by a bed of dolomite .

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Pliocene-Quaternary these units are referred to as ‘terraces’ flat surfaces with scattered pebbles Two Pliocene-Quaternary assemblages have been identified 1- These deposits are the most coarse grained and the richest in reworked siliceous pebbles. 2- These deposits alternate with rocks assigned with certainty to the Quaternary.

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TECTONICS The general structure of the coastal strip is that of rigid platform affected by fracture tectonics. The sedimentary cover has undergone deformation in response to movements in the basement Local geology The northwest of yanbu al bahr the coastal plain is 7 to 30 km wide and is composed mainly of Miocene and Pliocene quaternary sedimentary units

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Conclusions on the Yanbu’ al Bahr area In the Yanbu’ al Bahr area, three types of rnineralization can be identified 1- Barite and galena stockwork in the basement. 2- Fracture filling in Miocene sandstone. 3- lead-zinc mineralization in detrital and Miocene carbonate rocks. Description of mineral occurrences The first mineralized occurrence to be found in coastal sedimentary strata was discovered in the Jabal Dhaylãn it comprised Oligocene copper-bearing. Subsequently numerous occurrences have been discovered, not only in Oligocene sandstone(copper and lead-zinc) but particularly in Miocene facies (lead-zinc and barium).

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There is two types of mineralization have been identified: 1-copper mineralization with small amounts of associated zinc in places 2-lead-zinc mineralization with lead clearly predominant. The barium, lead and zinc mineralization discovered in this area is mainly located in the Miocene carbonate layers at the base of the gypsum. Barite, copper, lead, and fluorite occurrences have been inventoried in the basement Maqna massif 1-very small outcrops. 2- composed of conglomerate, gypsum, and reef limestone at the base. The mineralization present occurs in two types 1- impregnating dolomite.2-filling fractures and breccia.

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