Geological Society, London, Special Publications

doi: 10.1144/SP326.9 2009; v. 326; p. 181-191Geological Society, London, Special Publications

 H. Benshati, A. Khoja and M. Sola Infracambrian sediments in Libyan sedimentary basins  

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Infracambrian sediments in Libyan sedimentary basins

H. BENSHATI1,2*, A. KHOJA3 & M. SOLA3

1National Oil Corporation, Tripoli, Libya2Present address: BG Group, Thames Valley Park, Reading RG6 1PT, UK

3British Gas, Tripoli, Libya

*Corresponding author (e-mail: Hesham.Benshati@bg-group.com)

Abstract: Infracambrian sediments are widely distributed in Libya, outcropping on the easternand western margins of Al Kufrah Basin and the eastern margin of Murzuq Basin. The sedimentshave been penetrated in the Central Cyrenaica Platform, Concession 10, NW Sirte Basin, and BlockNC115, NW Murzuq Basin.

There are two main subdivisions. The first is metamorphosed due to local volcanism in theMurzuq and Al Kufrah basins. The second is unaltered and has been penetrated in the NW SirteBasin. It occurs as outcropping limestone on the eastern margin of the Murzuq Basin.

These sediments generally show lateral thickness variability, with the thickest section, approxi-mately 991–1067 m, in the Cyrenaica Platform. Individual units show thinning towards the Pre-cambrian basement highs, and both fining-up and coarsening-up successions. Two-dimensionalseismic data acquired by the operating companies AGIP and AGOCO in the southern Al KufrahBasin image strata presumed to be Infracambrian.

The Infracambrian sediments were probably deposited as lens-shaped bodies in palaeo-lows(graben, half-graben and troughs) alongside Precambrian basement highs, trending NW–SE inthe Cyrenaica platform and NE–SW in the Al Kufrah and Murzuq basins. These sedimentsshow lateral facies changes and their nomenclature differs across the basins. Palynological andpalaeontological studies suggest a Late Riphean age.

Libya, on the southern Mediterranean coast, coversan area of nearly 1.8 million km2 (Fig. 1). A series ofmajor tectonic events have led to the developmentof five sedimentary basins. A single Palaeozoicintracratonic basin was fragmented into the Gha-dames, Murzuq and Al Kufrah basins in the Hercy-nian Orogeny. The Botnan Basin is also Palaeozoic,while the Sirte Basin is Mesozoic, as is the Cyre-naica Platform (Fig. 1).

Drilling activities in the Libyan basins and theCyrenaica Platform – Concession 10 (NW SirteBasin) and Block NC115 (NW Murzuq Basin) –have penetrated the widely distributed Infracam-brian sediments. The sediments also crop out onthe eastern and western margins of Al KufrahBasin in the Jabal Arkenu and Jabal Nuqay areas,respectively, and on the eastern margin of MurzuqBasin in the Mourizidie area (Sola & Worsley2000; Fig. 1).

The nomenclature of these sediments variesacross the basins. They are assigned to the Infracam-brian Sediments in the Cyrenaica Platform and theSirte Basin, the Arkenu Formation in the easternAl Kufrah Basin, the Bir Bayia Formation near BirBayia in the western Al Kufrah Basin and the Mour-izidie Formation on the eastern margin of theMurzuq Basin.

Palynological and palaeontological studies onditch cutting and core samples of these sediments

collected from wells drilled in the CyrenaicaPlatform and Concession 10, NW Sirte Basin indi-cate a Precambrian, Late Riphean age (El-Arnautiet al. 1988; Underwood 1991. Geophysical andgeological assessment of Al Kufra Basin, SoutheastLibya. Unpublished Forum Exploration PetroleumReport, NIOC Libya).

The sediments are unconformably overlain byCambrian or younger sediments, and they uncon-formably overlie Precambrian basement (Mamgain1980; El-Mehdi et al. 2004).

Cyrenaica Platform

The Cyrenaica Platform lies in NE Libya and extendseast into Egypt. It is bounded to the north by JabalAkhdar and to the south by Jabal Dolma, and separ-ated from the Sirte Basin (Ajedabia Trough) to thewest by a major NW–SE-trending fault.

Wells drilled in the Cyrenaica Platform haveencountered Infracambrian sediments (Fig. 2).C1-82 well, central Cyrenaica Platform, reachedthe Precambrian basement rock beneath approxi-mately 670 m of Infracambrian sediments; while,to the west in D1-31 well, the thickness of the Infra-cambrian is more than about 990 m and, to the northin C1-125 well, it is more than approximately 230 mthick, but neither of these reached the Precambrianbasement (El-Arnauti et al. 1988).

From: CRAIG, J., THUROW, J., THUSU, B., WHITHAM, A. & ABUTARRUMA, Y. (eds) Global Neoproterozoic PetroleumSystems: The Emerging Potential in North Africa. Geological Society, London, Special Publications, 326, 181–191.DOI: 10.1144/SP326.9 0305-8719/09/$15.00 # The Geological Society of London 2009.

An isopach map of the Infracambrian sedi-ments reveals the development of two possiblyfault-bounded lows between the Precambrian base-ment highs. One low, in the Central CyrenaicaPlatform, has a NW–SE trend with a maximumthickness of around 1070 m. The second low, inthe south, has a NE–SW trend and is more than1060 m thick (El-Arnauti et al. 1988) (Fig. 3).

An east–west section in the central CyrenaicaPlatform shows that the Infracambrian sedimentsare unconformably overlain by Upper Ordovicianformations – the Cambrian and Lower Ordovicianformations are absent due to erosion or non-deposition. The Infracambrian sediments restunconformably on Precambrian basement rock andthin toward the basement high, suggesting thesynrift deposition of likely fluvial and alluvial sedi-ments with an erosional base. These have been sub-divided into three main units (El-Arnauti et al.1988) (Fig. 4).

The lower unit is comprised of red–brown,coarse- to very-coarse-grained, angular–subangular,subrounded, conglomeratic sandstones. The middleunit consists of of red, medium- to coarse-grained,subangular, subrounded, micaceous sandstones.

The upper unit comprises light grey–white, fine- tovery-fine-grained, micaceous, glauconitic sand-stones. The overall package is a fining-up successionsuggesting an alluvial–fluvial type deposition.

The localities where Infracambrian sedimentsare deposited match the lows between the Precam-brian basement highs, with the thickest sectionbeing present in the deepest part of these lows(Mamgain 1980) (Fig. 3).

Sirte Basin – Concession 10

Sirte Basin is a rift basin that evolved during the LateCretaceous, in Cenomanian time. Concession 10 liesin the NW of the basin (Klitzsch 1971, 2000; Fig. 1).

A1-10 is a dry well drilled in Central Concession10 by Veba Oil Operation in 1958. Drillingencountered approximately 106 m of Infracambriansediments. These are unconformably overlain byLower Ordovician (Tremadocian) sediments, andare composed of shales with thin quartzites. Theshales are dark grey–black, hard, non-calcareous,micaceous and arenitic. The quartzite is light grey,fine–medium grained, argillaceous and fractured,with abundant feldspar grains up to 5 cm.

Fig. 1. The major sedimentary basins of Libya, illustrating the distribution of Infracambrian sediments, both subcropand outcrop.

H. BENSHATI ET AL.182

Correlation between A1-10 Well in the SirteBasin and B1-31 Well in Central Cyrenaica (c.650 km apart) showed a SP (spontaneous potential)log response match (Fig. 5).

Palynological data (Underwood 1991. Geophysi-cal and geological assessment of Al Kufra Basin,Southeast Libya. Unpublished Forum ExplorationPetroleum Report, NIOC Libya) on Core 26, from3195.2 to 3200.4 m, suggest a Precambrian age forthese sediments. No Tremadocian microfossilshave been found below 3094.6 m, and so this depthis considered to be the top of the Infracambrian.

Murzuq Basin – Block NC115

Murzuq Basin is a Palaeozoic intracratonic basinthat extends south into Niger and west intoAlgeria. It is separated from the Hamada Basin(Ghadames) to the north by the Gargaf Arc and

Atshan Saddle and from the Al Kufrah Basin tothe east by the Tripoli–Tibisti Massif, andbounded by the Hoggar Massif to the SW. BlockNC115 is a Repsol-operated licence located in theNW Murzuq Basin (Jacque 1962; Sola & Worsley2000; Fig. 1) and information related to this con-cession is derived from an unpublished NOC data-base that includes well reports.

Drilling activities have provided almost com-plete cover of Block NC115. Some of these wells,such as B31-NC115, S1-NC115 and H27-NC115,have penetrated the Infracambrian MourizidieFormation (Aziz 2000; Burollet 1963; Fig. 6).

The Mourizidie Formation is unconformablyoverlain by the lower member of the CambrianHassaouna Formation. In proximal areas, the sedi-ments are light grey conglomeratic sandstones,bedded sandstones, siltstone and claystone. Thesandstones are grey, fine–medium grained, poorly

Fig. 2. Cyrenaica Platform: the locations of wells penetrating Infracambrian sediments.

LIBYAN SEDIMENTARY BASINS 183

Fig. 3. Cyrenaica Platform: Infracambrian sediments isopach map.

Fig. 4. East–West Stratigraphic section across the Central Cyrenaica Platform.

Fig. 5. SP log correlation between A1-10 and B1-31.

Fig. 6. NE-SW stratigraphic cross-section across NC115.

to moderately sorted, well cemented, kaolinitic andmicaceous, with quartz overgrowth and no porosity.The siltstones are dark grey, blocky, very fine andmicaceous, with siderite concretions, and are wellcemented (Davidson et al. 2000; Sola & Worsley2000; Fig. 6). The geological map of the area andthe line of the cross-section are shown in Figure 7.

East Murzuq Basin – the

Mourizidie area

Infracambrian sediments, known here as the Mour-izidie Formation (Burollet 1963), crop out in theMourizidie area, in the east Murzuq Basin (Jacque1962; Aziz 2000; Fig. 7).

Fig. 7. Location and geological map of the Mourizidie area and the line of the cross-section in Figure 6. There is a smalloutcrop a few metres thick of sandstones and limestone (see Fig. 8).

H. BENSHATI ET AL.186

These Infracambrian sediments are composed oftwo different sedimentary units. There is a very smallexposure, 2–5 m thick, consisting of brown–white,fine–medium grained, subangular, subrounded,quartzitic sandstones, and a light grey–whitish,fractured, cryptocrystalline limestone (Aziz 2000;Fig. 8).

East Al Kufrah Basin – Jabal Arkenu area

Al Kufrah Basin, SE Libya, is a Palaeozoic intracra-tonic basin. It extends north to Jabal Dalma, NE intoEgypt, east to Jabal Awainat, south and SE intoSudan and Chad, and west to the Sirte–TibistiMassif (Klitzsch 1981, 2000; El-Mehdi et al.2004; Fig. 1).

Jabal Arkenu is an igneous ring complex situatedin the east Al Kufrah Basin, where Infracambriansediments crop out as a NE-trending belt depositedalongside the Precambrian basement highs. TheInfracambrian lies unconformably on the Precam-brian basement and is unconformably overlain bythe lower member of the Cambrian HassaounaFormation (Saıd et al. 2000; Fig. 9).

South of Jabal Arkenu, the sediments arealluvial–fluvial sandstones. To the NE of Jabal

Arkenu they change to shallow-marine sandstonesand thermally metamorphosed limestones (marbles),and further NE to metamorphosed limestones under-lain by marginal-marine iron-bearing quartzite. Thislateral change is evidence for an interfingering rela-tionship between these sedimentary facies (Fig. 9). Adepositional model for Infracambrian sediments hasbeen constructed to illustrate their distribution andlateral facies changes (Saıd et al. 2000; Fig. 10).

Two intersecting seismic lines, SL KU91-03and SL KU89-06 in the Al Kufrah Basin demon-strate a NE-trending half-graben filled with Infra-cambrian sediments that can be subdivided intothree main units based on seismic attributes and seq-uence stratigraphy. These sediments have been fol-ded and intensively faulted by the still-active Trans-African Sahara lineament, which extends from WestAfrica to the Middle East. They also show thinningtowards the Precambrian basement highs that mightprovide stratigraphic traps (Fig. 11a, b).

West Al Kufrah Basin – Jabal Nuqay area

Jabal Nugay, west Al Kufrah Basin, is a Cenozoic(post-Eocene) extrusive volcanic composed ofbasalt and phonolite (Fig. 12).

Fig. 8. Limestone of the Mourizidie Formation, Murzuq Basin.

LIBYAN SEDIMENTARY BASINS 187

Fig. 9. NE-trending belt of Infracambrian sediments, showing lateral facies variability, alongside the PrecambrianJabal Arkenu igneous ring complex in the eastern Al Kufrah Basin (Saıd et al. 2000).

Fig. 10. Depositional model of the Infracambrian sediments in Figure 9: proximal and distal alluvial fan depositsinterfinger with iron-bearing quartzites, which in turn interfinger with slightly metamorphosed limestone.

Fig. 11. (a) NE–SW seismic line KU91-03 across an Infracambrian graben in the north Al Kufrah Basin. Thinningtowards the Precambrian highs may provide stratigraphic traps. (b) NW–SE seismic line KU89-06 across anInfracambrian graben in the north Al Kufrah Basin. Thinning towards the Precambrian highs may providestratigraphic traps.

LIBYAN SEDIMENTARY BASINS 189

Infracambrian sediments, known here as the BirBayia Formation (Selley 1971), crop out near BirBayia in the Jabal Nuqay area as a small exposurea few metres thick unconformably overlain by theCambrian Hassaouna Formation.

The sediments are red–violet feldspathic sand-stones, interbedded with thinly bedded siltstone andclaystone, representing a marine transgression on abraided alluvial plain (Selley 1971).

Conclusions

Infracambrian sediments are widely distributed inonshore Libyan basins and are interpreted as

lens-shaped bodies deposited in palaeo-lows along-side the Precambrian basement highs, having aNW–SE trend in the Cyrenaica Platform, and aNE–SW trend in the Al Kufrah and Murzuq basins.

They are mainly clastic in the CyrenaicaPlatform, and both clastic and carbonate inMurzuq and Al Kufrah basins. Limestones nearthe Jabal Arkenu intrusion are now considered tobe marbles.

Palaeontological and palynological studies onditch cuttings and core samples indicate a Precam-brian, Late Riphean age for these sediments.They show lateral facies changes from continentalalluvial sands to marginal marine sandstones,

Fig. 12. Location and geological map of the Jabal Nuqay area. A few metres of Infracambrian sediments – sandstone,siltone and claystone – represent a marine transgression over an alluvial braided plain.

H. BENSHATI ET AL.190

iron-bearing quartzite and limestone to marinesandstones.

Seismic data show a graben structure with Infra-cambrian sediment being deposited, demonstratingthickening and thinning of this sedimentary unit.

We would like to express our thanks and appreciationto the Exploration Department of the National OilCorporation, Libya, the Earth Sciences Society of Libyaand the Geological Society of London for their supportand encouragement to in producing this paper.

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