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Cambro-Ordovician ? strata
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In the northeastern part of Jabal Arkenu
the methamorphic basement overlain unconformably 200 m of sandstone and quartz
sandstone which more or less conglomeratic in the upper part and is referred
to the Cambrian Hassaouna Formation. A sharp erosional surface separates this
formation from approximately 150 m of upper sandstones referred to the Upper
Ordovician Memouniat Formation.. In the latter formation, apart from fine and
medium sandstones, numerous levels of conglomerate are present. Beds with
Tigillites have been observed in the Hassaouna Formation. Metamorphic sandstones
have been observed at Jabal Qardah (30 km west of Jabal Arkenu).
To the west of the last relief at Jabal Qardah, some scattered outcrops
of the basement (!)
are overlain by well-layered sandstones which
are generally medium and fine grained and reddish on the surface. A level was
discovered with frequent Tigillites. In the western direction, amongst the sandstone,
siltstone levels appear in which it is sometimes possible to observe frequent
and notably developed Arthrophycus (Harlania).
The same Situation is to be found 20 km WNW of Jabal Qardah. At the base, in
the southern zone of the relief, sandstones are present both well-layered or
very thick-bedded and pale in colour. On this eroded surface, there are deposits
of darker, more cemented, medium- and coarse-grained and often cross-bedded,
sandstones. More silty levels, soft at the base, sometimes contain abundant,
long Arthrophycus. These upper sandstones are more than 40 m thick. In
the northern part of this relief, volcanic rocks of a basaltic type have intruded
along a NW-SE fracture. In this sequence, as at Jabal Arkenu, a lower formation
(Hassaouna of Cambrian age) and an upper formation (Memouniat of Upper Ordovician
age) could be recognized.
In the Jabal Asba area the Cambro-Ordovician is present in various outcrops.
In the extreme SW, facing Jabal al Bahari, two distinct formations outcrop. The
lower one (Hassaouna Formation) is more to the east and is represented by thick-bedded
sandstones, medium to very coarse grained and sometimes with foresets or cross-bedding.
Layered sandstones (Memouniat Formation) follow, often with parallel laminations,
fine- and medium-grained, from buff to violet, with some harder levels rich in
ferruginous cement. In the central western zone of Jabal Asba (section 4), the
lower 57 m are represented by thick beds of sandstones, fine- to coarse-grained,
generally with parallel lamination. Unconformably, and with a fine conglomerate
at the base, approximately 7 m of buff-brown to reddish sandstone follow, medium-
to coarse-grained, with clear cross-bedding, which fill all the eroded parts
of the underlying formation and are overlain by the shales of the Tanezzuft Formation.
These sandstones must be considered as belonging to the Upper Ordovician Memouniat
Formation. Similar sandstones of the Hassaouna and Memouniat Formations were
found in the NE area of Jabal Asba (section
13). Sometimes there are beds with abundant
and large fossil (bioturbations) traces.
Remark: The series
of this sedimentary units were named by KLITZSCH (Geology of Egypt) as
Umm Ras Formation and classified as Silurian. This contrast must be clarified.
In any case fossil traces are no useful marks for a dating. Here a suggestion
for the correlation of both geological
sequences.
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Sandstone wall; Jabal Arkenu
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Massive Ordovician (?) sandstone; Jabal
Arkenu
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Jabal Arkenu; uplifted Ordovician (?)
sandstones
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Shared massive Ordovician (?) sandstone;
Jabal Babein
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A cross-bedded sandstone; Jabal Babein
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Plug of uplifted Ordovician (?) sandstone;
Jabal Babein
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In the foreground a dyke of erected
layers; Jabal Babein
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Two formations were recognized, Tanezzuft and
Akakus, which are typical in the Silurian of the surrounding Palaeozoic basins.
Tanezzuft Formation (Lower Silurian).
Approximately 30 km NE, in Jabal Asba, the Tanezzuft Formation outcrops
over large extensions. In the SW area of this relief, the
section 2 has shown 40-50 m of greenish and grey shales, sometimes, on the surface,
whitish or purple, very thinly layered, soft, often with coating and dendrites
of manganese, and with some interbeddings of greyish, hard micaceous siltstone.
Some graptolites of the species Climacograptus medius (TORNQUIST) were
found. The Chitinozoa assemblage from 27-33 m consists Ancyrochitina.
According to the graptolites the interval from 18-31 m belongs to Lower Llandovery.
In core 36-37 m most of the Chitinozoa are absent and the assemblage is dominated
by the species Conochitina chydaea JENKINS in association with Rhabdochitina
cf. magna EISENACK and Lagenochitina sp.; these species suggest an
Upper Ordovician age (Caradocian-Ashgillian). In the same cores, traces of small
Pelecypoda and carbonaceous matter were found. The depositional environment of
this formation is open marine and its thickness in the central and northern areas
of Jabal Asba is 24 and 18.5 m respectively. At the base, in contact with the
sandstones of the Memouniat Formation, a thin level was observed with abundant
traces due, in all probability, to worms. This characteristic level was commonly
observed at the Tanezzuft-Memouniat boundary.
Akakus Formation (Lower-Middle Silurian).
In the area this formation is well characterized from the lithologic and palaeontologic
point of view, but it shows very limited thicknesses everywhere. Region of
Jabal Qardah (section 10): 25 m of dark grey and grey-green, flaggy siltstone,
with thin interbeddings of fine-grained, cemented sandstone and micaceous shales.
Rare indeterminable fossil traces were observed on some surfaces. It is thought
that the highest part of this formation is eroded, due to the absence of Arthrophycus
and Cruziana beds, generally widespread in the whole region, and of the Tadrart
sandstones; in this section the Akakus Formation is unconformably overlain by
the Middle-Upper Devonian Binem Formation. Southern part of Jabal Asba
(section 2): this formation is represented by only 2-5 m of siltstone
and fine-grained sandstone, greenish-grey to reddish, with some interbeddings
of silty-micaceous shales. In the highest part there is a siltstone bed, hard,
yellow and very heavy with characteristic stromatolitic structure. This
is a very widespread level in the highest part of the Akakus Formation in this
region. There are extremely abundant sedimentary structures (ripple marks, flute
casts, etc.) and also fossil traces (Cruziana furcifera, Arthrophycus sp.,
etc.). The boundary with the underlying Tanezzuft Formation is rather gradual,
whilst it is very sharp, marked even with a base conglomerate, with the overlying
Tadrart Formation. Central part of Jabal Asba
(section 4): 9 m of siltstones and silty-micaceous, multicoloured shales, giving
way to fine-grained, cemented sandstones. Abundant sedimentary structures (flute
casts, ripple marks, etc.), bioturbations and fossil remains including numerous
Arthrophycus sp. and Cruziana sp. were observed. A similar
sequence, 5 m thick, was found in the section
13 both from the lithological-sedimentological
and the paleontological point of view.
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Silurian Cruziana -
trilobite traces - Akakus Formation, northern Jabal Asba
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Arthrophycus
trace fossils in Ordovician strata near Jabal Asba
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The dominantly continental sandstones of the
Lower Devonian, locally with shale interbeddings, have been referred to an undifferentiated
complex of the Tadrart Formation; the overlying, dominantly marine, sandstones
and shales have been referred to the Binem Formation.
Tadrart Formation (Lower Devonian)
In the southern region, in the Jabal Qardah, this formation may be completely
absent, as has been observed (section 10) where the Akakus Formation is directly overlain by
the Zoophycus sanstones of the Binem Formation. Differently the northern part.
In the section 2
of Jabal Asba: 62 m of light nut-brown
to grey-violet, fine to extremely coarse grained sandstones with thin and irregular
beds of fine conglomerate; a conglomerate bed is present at the base, at the
contact with the Akakus Formation. In general this is a very porous sandstone,
slightly cemented and medium to very thick bedded; some thinner levels are of
dark, hard ironstone. Sedimentary structures are represented by cross-beddings,
foresets, laminations and slumping phenomena. In some cases there are well developed
phenomena of overturned cross-bedding or very dipping beds of sandstones and
gravels, probably due to fluvial or dune deposits. Central part of Jabal Asba
(section 4): 53 m of fine to very coarse grained sandstones, sometimes
with irregular beds or lenses of fine conglomerate, with some thin (l 5-20 cm)
very irregular interbeddings of siltstones and greenish-greysilty shales. The
sandstones are generally very thick bedded and with cross-beddings, overturned
cross-beddings, foresets, parallel and cross-laminations, slumpings, etc. In
this sequence there are also dark, hard beds with abundant ferruginous cement;
one of these beds, 30-40 cm thick, marks the boundary with the overlying Binem
Formation. In the section 13 the thickness of the Tadrart is 41 m. The beds of siltstones
and silty shales are more numerous and thicker compared with the above-mentioned
sections. There are thick beds of red and violet-red hard sandstone, with ferruginous
cement, such as usually marks the boundary of this formation with the Binem.
Binem Formation (Middle-Upper Devonian).
The Binem Formation has never been formally described and published. The type
section is located in the Tibesti area, probably in the area of Wadi Binem-Wadi
Ounga. Another suggested reference section in Libya is represented by coordinates
lat 21°52'08"N and long 19°44'10"E.
In the Jabal Qardah (section 10), the greatest thickness of Binem Formation was measured
(160 m). As already stated for this section the Akakus is unconformably overlain
by multicoloured (whitish, pale grey, purple, etc.), fine-grained sandstones,
generally cemented, with some very thin beds of reddish hard siltstone, with
ferruginous cement and sometimes with abundant Zoophycus.
At the top, the section is formed of sandstones as above, sometimes only slightly
cemented, with some interbeddings of siltstone and silty shales and thin beds,
or crusts, of hard, red-purple siltstones. Beds with abundant Zoophycus
(their size varying from a few cm up to 20 cm) and a few other fossil traces
are common. A wide sandy plain separates the top of this formation from the first
outcrops of Continental Mesozoic.
Another outcrop of Binem sandstones lies 20 km N of Jabal Qardah (section 9); 31
m are exposed but it was not possible to see the lower and upper boundaries of
the formation. At Jabal Asba this formation is well exposed and
more complete. In the southern part (section
3) a thickness of 35 m was measured, represented
by pale grey and purple sandstones, often brownish-red or brick red on the surface,
from fine- to medium-grained, well layered, often with cross-beddings, sometimes
passing to very slightly cemented siltstone. Rare indeterminable fossil traces
were observed. Section 5: 22 m of whitish, pale grey, fine to very fine grained,
more or less cemented sandstones with some thin interbeddings of whitish siltstone
or very hard reddish clay. Zoophycus of varying size are distributed at
various levels throughout the section.
Section 4: 45 m of brown-red, reddish,
purple and greyish sandstones with grains ranging from medium-fine to coarse,
generally well layered, with some harder levels cemented by iron minerals.
Section 6: 52 m were measured, 22 m of which are fine, pale grey and purple,
well-layered sandstone, with frequent Zoophycus and 30 m are red-brown
and buff-brown sandstones, fine- to coarse-grained, often cross-bedded, rarely
with fossil traces. In the section 13 the thickness of the Binem is 56 m. The upper part
of several sections show thick-bedded sandstones with common vertical, parallel
burrows (cf. Tigillites).
The sandstone outcropping in the W and NW areas of Jabal Asba is to be
added to the Binem Formation. These sandstones are fine-grained, buff, whitish,
pale grey, sometimes purple, porous, only slightly cemented. They are often thinly
bedded, with common interbeddings of whitish flaggy siltstone. Morphologically
they are similar to the sandstones of the upper member and may contain rare fossil
remains, including Zoophycus. In the section 7 these sandstones have
a thickness of 47 m; of remarkable interest is the presence, at the base of the
section, of fine, whitish, crumbly sandstones with very common traces and
remains of plants (even longer than l m) at longitude 24°23'38" and
latitude 23°15' 25". At the top of the section there are, again, rare Zoophycus
sp. and other organic traces.
The Binem Formation outcrops NE of southern Jabal Asba show numerous sandstone-siltstone
beds with Bifungites fezzanensis DESIO common. Extremely extended outcrops
of the Binem, particularly the Zoophycus sandstones, are located in the most
northern area of Jabal Asba and on the SW side of Jabal Rukn.
Around
Jabal Asba the sediments of the Middle-Upper Devonian are unconformably
overlain, towards the W, by Continental Mesozoic, while, immediately to the N
of lat 23°30' N, they are overlain by sequences of Carboniferous sandstones.
On account of their lithological, sedimentological, palaeontological and environmental
characteristics they were referred to the Dalma Formation (from the high hill
of Jabal Zalmah (Jabal Dalma), NNE of Kufra with coordinates: lat 25°34'10" N
and long 23°55'15"E).
Two sections were measured and sampled in this formation. The first (section 11), 30
km N of the eastern side of Jabal Cudi, shows 95 m of buff-brown, greenish-yellow
to grey-purple sandstones, mainly medium and fine grained, sometimes even coarse-grained
in the lower third, well-bedded (with common cross-beddings), generally hard
and cemented in the higher part, more crumbly in the lower part. In the higher
part the sandstones are sometimes very fine grained, thinly bedded and with numerous
traces of bioturbations or other indeterminable fossil traces. In the central
part of the section, plant remains, including probable roots, have been observed.
Further below there are traces and small trunks of plants
belonging to the Licopodophyta and again some beds with common traces
of "roots". These have been completely replaced by sandstone. About 10 m from
the base of the section, large concretions of fine sandstone are common; these
are well cemented (sometimes quartzites), long and nearly cylindrical in shape
(up to more than 2 m); the size and shape of most of these concretions recall
trees completely replaced by sandstone. This characteristic level is to be found
for several kilometres in the area W of this place. The
section 12 (approximately 45 km to ENE) has shown 205 m of Carboniferous
sediments; it was made in a group of hills, a synclinal fold, located immediately
N of outcrops in the Binem. At the base the outcrops are represented by more
than 45 m of fine-grained sandstones, with some thin levels or lenses of conglomerate,
generally only slightly cemented on the surface, which form steep and almost
inaccessible walls. These are followed by 5 m of whitish-grey, very thinly bedded,
quite soft siltstone with numerous plant moulds (Licopodophyta). Subsequently
there are fine- and medium-grained sandstones, varying in colour from pale grey
to purple, generally only very slightly cemented, thick-bedded in the lower part
and subsequently more thinly bedded and with some siltstone beds. There are small
irregular beds of conglomerate, cross-beddings, foresets, parallel and cross-laminations.
Remains and moulds of plants (Licopodophyta) were observed in the lower
part. A further 21 m of fine sandstones and siltstones follow, more or less clayey,
thinly bedded and multicoloured with common bioturbations and rare specimens
of large Bifungites sp. (5-6 cm in size).
The upper part of the section is represented by sandstones, coarse- to fine-grained,
sometimes with irregular beds of fine conglomerate, generally only slightly cemented,
harder in the higher part, with some levels of clayey and flaggy siltstone. Numerous
beds have provided various moulds and remains of plants (Licopodophyta)
and of roots; bioturbations occur locally. This sequence is unconformably overlain
by the Continental Mesozoic.
Carboniferous layers on top
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