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From the point
of view of a geologist, Jordan is a paradise. The climate and the
geographical conditions of the country permit the geologist to conduct filed
work all year round and examine every outcrop and any rock sequence without
difficulties.
Pre-Cambrian igneous metamorphic rocks cover an area of approximately 1800
sq. km in south Jordan. These rocks are exposed from the area south of Aqaba
for approximately 70 km towards north along the east side of Wadi Araba
Rift. They are also exposed east of the Rift and south of the Ras en Neqb
escarpment, dipping east of Wadi Rum Underneath thick Paleo-zoic sandstones
series.
The basement complex is composed mostly of biotiteaplite-granite, two
mica-aplitegranite, granodiorite, quartzdiorite, alaskite, and diorite.
These rocks are intruded by numerous acidic, intermediate and basic dike
rocks of 14 different types.
Based on correlation with similar igneous in the Sinai desert and the Red
Sea area, the Jordanian basement complex is found to be around 590 million
years old.
The highly metamorphosed rocks occurring in the Wadi Araba are thought to
represent the remnants of the eldest early Algoncian geosynclinal
sedimentation.
Uplift and erosion preceded the deposition of the Upper Algoncian sediments
which overlie the older rocks with an angular unconformity. These sediments,
the Saramuj Conglo-merates and the Slate Gerywacke Series, indicate
epimentamor-phic processes. The entire pre-Cambrian basement complex is
considered to belong to the northern rim of the African Craton, which was
consolidated relatively late.
The paleogeographic evolution of the country since the beginning of the
Cambrian is believed to be influenced by the following factors:
1. The Tethys-Ocean,which occupied the area in the W and NW , and
transgressed several times over parts of, or over the whole country.
2. The Wadi Araba-Jordan geosutrue between the Palestine and East Jordan
blocks.
3. The Nubo-Arabian Shield in SE and S, from where me-chanical and chemical
weathering products were trans-ported into the shelf of the Tethys.
During Lower Cambrian to Middle Cambrian, the first transgression of the
Tethys reached the E-rim of Central Wadi Araba. To the E & SE of the limit
of the transgression the continental deposition of sandstone continued in
epeirogenic basin during a dominantly dry climate period. In middle Cambrian
a regression occurred since continental sandstones were deposited in
northern Wadi Araba, on top of the marine sandstones.
In the Lower Ordovician the ocean invaded the flat hinter-land and covered
the entire SE of Jordan. Marine shallow water type sediments dominant by
fine sandstones and sandy shales were deposited during Middle and Upper
Ordovician until the Llandoverian.
During Upper Silurian continental deposition of the clastics resumed, as
observed in the SE corner of the country.
Lower Devonian sediments equivalent to the Tabuk for-mation of Saudi Arabia
may exist beneath Mesozoic sediments in the Jafr, Wadi Sirhan and Azraq
Troughs.
Palaeontological studies carried out by Olexcon 1967 on material collected
from Safra Well (44 kms east of Amman) indicate that sedimentation persisted
at least locally in the area into the carboniferous period. Anyhow it is
believed again that remnants of the Carboniferous and Permian sediments
might be preserved in the Jafr, Azraq, Wadi sirhan Troughs and in the
northwestern part of the country.
During the Permain the Paleozoic rocks were tilted east-wards and eroded.
Sedimentation resumed during the Lower Triassic, sedi-ments of this age
represent the oldest Mesozoic sediments in the country. Outcrops of these
sediments were seen to occur in the lower reaches of wadis which breach the
rift escarpment between the Wadi Hisban and north of El Lisan Area.
Sedimentation during this age was restricted only in the northern and
western parts of the country (north of El lisan Area) while further east and
southeast erosion prevailed on the continent.
At the end of the Triassic the sea regressed and the whole area was
subjected to erosion. During the Triassic phase of sedimentation a sequence
of arenaceous, argillaceous and car-bonate deposits accumulated.
In the Middle Jurassic the sea regressed again. Outcrops of Jurassic
sediments were restricted to the Jordan Valley side wadis in a region
extending southwards for about 20 kilometers from Wadi Zerqa. During this
period, sandstones, clays and dolomitic limestones accumulated. Erosion on
the continent continued in the South and East.
It has been noticed that Jurassic and Triassic shore lines run more or less
in parallel. Jurassic-Triassic sediments were seen to occur beneath the
lower Cretaceous sandstones (Kur-nub Sandstone) in many drilled wells in the
northern and north-eastern parts of the country.
The Lower Cretaceous sandstones are neritic to littoral north and northwest
of the Jurassic shoreline. South and Southeast of this shoreline, the lower
part of the Lower Cretaceous succession, both marine and brackish influences
have been encountered in Southeast Jordan.
The Lower Cretaceous sandstones of the continental origin extend to the east
and to the extreme southeast where they become diachronous and their top
becomes younger from west to east.
The middle Cretaceous (Cenomanian-Turonian) sediments are fully marine and
thick in the west, northwest and north Jordan . On the contrary in the
south, southeast and east, they wedge out, calcareous contents decrease and
the arenaceous facies prevail.
The Upper Cretaceous (Santonian-Maestrichtian) marine sediments have been
developed into two different character-ristic sedimentary successions:
1. The chalky limestones, sandy limestones, silicified limestones,
sandstones, chert, phosphate and Tripoli deposits were originated in what is
called "Gentle Swells System".
2. The thick bituminized marls. Marly limestone, compacted dolomites and
limestones, interbedded with black brown chert layers were deposited in
basins and troughs with local euxinic environments.
The Tertiary marine sediments of the Paleocene and the major part of the
Eocene were deposited in a similar "gentle swells and basins system".
Along Wadi Araba – Jordan rift, coarse clastic weathering products from the
bordering, uplifted areas were deposited at the same time together with
taphrogenic tectonism in the Oligocene. Locally, in the central part of the
graben, thick rock salt was deposited in the Oligo-Miocene to Pliocene.
During Cenozoic the Jordan rift was either occupied by fresh water lakes
draining west to the Tethys, or it formed an inland depression with
terrestrial or lacustrine sediments. Also marine deposits during Miocene
extended for 100 km east of the rift into El Azraq – Wadi Sirhan depressions
and even east of the Jebel Ed-Drouz area.
A major phase of taphrogenic movements took place in the plio-pleistocene.
The major graben movements ended at the time of the Middle Pleistocene
basalt vulcanism. More likely a fresh water lake (Samra Lake) covered the
northern part of the rift. This lake slowly changed to the brackish Lisan
Lake, Lisan Maris extend from approximately 50 km south of the Dead Sea to
the north almost as far as Lake Tiberias. Not before the Upper Pleistocene
did the Lisan Lake change into the present Dead Sea.
Throughout the Quaternary detritus was transported into the Rift Valley, and
the extension Azraq-Wadi Sirhan and El-Jafr depressions of East Jordan.
These depressions were partly covered by fresh and brackish water lakes
during fluvial pe-riods of the Pleistocene, while fluviatile conglomerates
of the same age spread over wide areas along the eastern slopes of the
mountain ridges bordering the east side of the Wadi Araba – Jordan Rift.
From Petroleum Geology point of view one can state that parts of Jordan are
considered of interest for oil exploration. In such areas the requirements
of oil generation and accumulation listed below are existing:
1) Thick marine sediments.
2) The occurrence of deep source rocks.
3) Adequate proportion of reservoir rocks to cap rocks.
4) Suitable trapping conditions either structural or Stra-tigraphical.
In addition the existence of seepages and asphaltic rocks in some area are
considered as "another indication".
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