The Jabal Asba and surrounding in Libya

- southern extension of the Abu Ras and Gilf Kebir Plateaus in Egypt -

Norbert Brügge, Germany
Dipl.-Geol.

Lithostratigraphic sequences
(in sense BELLINI & MASSA, "The Geology of Libya", 1980)
 

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.

Two formations were recognized, Tanezzuft and Akakus, which are typical in the Silurian of the surrounding Palaeozoic basins.
Tanezzuft Formation (Lower Silurian).
The Tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins, but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by AGIP between 1978 and 1981.
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.

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.

Northern Jabal Asba: Carboniferous layers on top

Geological map with location of  sections
by BELLINI & MASSA

Generalized geological profile
Source: BELLINI & MASSA, 1980

Palaeozoic of northern Jabal Asba. Bottom sandstones: Ordovician. Middle soft unit: Tanezzuft and Akakus formations. Cliff on top: Devonian sandstones

      
Broken Ordovician Tigillites layer in the foreground.   (Source: Maria Emilia Peroschi, Italy)