Доклади на Българската академия на наукитеComptes rendus de l’Academie bulgare des Sciences

Tome 71, No 4, 2018

GEOLOGY

Stratigraphy

NEW DATA FROM GRAPTOLITES AND QUARTZARENITES FOR THE ORDOVICIAN–SILURIANBOUNDARY IN THE MURZUQ BASIN, LIBYA

Valeri Sachanski, David Loydell∗, Athanas Chatalov∗∗

(Submitted by Academician T. Nikolov on March 13, 2018)

Abstract

The graptolite-bearing interval in well A1-NC101, Libya indicates a Hir-nantian age (persculptus graptolite Zone) for the lowermost Tanezzuft Fm basedon the presence and stratigraphical distribution of N.? pseudovenustus. In theC1-NC101 core, the presence of N. inazaouae (recognised for the first timeoutside Algeria) and M. parvulus indicates either the uppermost Ordovician orthe lowermost Silurian, but the revised total stratigraphical range of N. tar-guii (ascensus-acuminatus graptolite Zone) suggests an earliest Rhuddanianage. Several specimens of N. targuii are longer than those previously recorded.Pure quartz arenites of the uppermost Mamuniyat Fm (well A1-NC101) weredeposited during the end-Hirnantian deglaciation probably in a nearshore ma-rine environment. Their provenance was associated with sedimentary recyclingof mature sands that were formed across North Gondwana in Cambrian andpre-glacial Ordovician times.

Key words: graptolites, shales, sandstones, Ordovician–Silurian bound-ary, Murzuq Basin

Introduction. The Murzuq Basin is a large intracratonic sag basin whichdeveloped on the passive margin of Gondwana during the Palaeozoic [1]. Atpresent most of the basin area is situated in SW Libya (Fig. 1A), where its sed-imentary fill (up to 3500 m thick) consists of Cambrian to Cretaceous deposits.The Bulgarian state oil company BOCO, which is no longer operational, was

DOI:10.7546/CRABS.2018.04.

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Fig. 1. A – Location of the Murzuq Basin [3] and the two studied wells; B – Lithologic log ofthe cored intervals and vertical distribution of the identified graptolite taxa

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awarded an exploration permit (Concession NC101) in the north central flankof the basin in 1980. BOCO drilled a total of 17 exploration wells and madeseveral oil discoveries. In most wells lower Silurian shales of the Tanezzuft Fmoverlie Upper Ordovician sandstones of the Mamuniyat Fm [2]. The latter is amajor hydrocarbon reservoir in the Murzuq Basin and “hot shales” in the lowerpart of the Tanezzuft Fm are the principal source rock. The two units combinedcomprise the Mamuniyat-Tanezzuft Petroleum System [1,2]. Great efforts havebeen made to identify the Ordovician–Silurian boundary within the TanezzuftFm using graptolites, acritarchs and chitinozoans [3–5]. Available data from thesubsurface of the basin are scarce, and recent studies of cored wells have shownthat the oldest drilled strata of that unit have a Rhuddanian age [3,5]. The basalpart of the Tanfezzuft Fm in outcrops has been also referred to the lower Llan-dovery [4]. This paper presents biostratigraphical and sedimentological resultsfrom a study of the lower Tanezzuft Fm and uppermost Mamuniyat Fm in wellsA1-NC101 and C1-NC101.

Materials. The core samples were collected by Hristo Spassov in March1983 and are stored now at the Geological Institute of the Bulgarian Academyof Sciences (BAS) The cored interval in A1-NC101 is from 2255.0 to 2311.0 mand in C1-NC101 is from 1831.9 to 2193.0 m. The core has a diameter of 10 cm.Lithologically, there is little variation throughout the core: it is dominated byhard, dark grey shales with rare, thin-bedded, current-laminated, siltstone to finesandstone intercalations, all belonging to the Tanezzuft Fm (Fig. 1B). The onlyexception is in the core from A1-NC101, at a depth of 2311.0 m, where there islight grey, massive, very coarse grained, quartzose sandstone which is typical ofthe upper parts of the Mamuniyat Fm [2]. Two thin sections were prepared fromthese sandstones for sedimentological studies.

Thirteen samples (four from A1-NC101 and nine from C1-NC101) yieldedgraptolites. The number of examined graptolites is 30. Of these, three samplesfrom A1-NC101 and seven samples from C1-NC101 yielded a total of 16 grapto-lites identifiable to species level. Undeterminable specimens are distal fragmentsor complete specimens that are poorly preserved or preserved in scalariform view.Almost all specimens are flattened with periderm or a residue from it. A speci-men from C1-NC101 (depth of 2193.0 m) is preserved in low relief, and anotherspecimen from the same well (depth of 2189.15 m) is preserved in part-relief asa steinkern of iron oxides pseudomorphing pyrite. There is no tectonic deforma-tion of the rhabdosomes. All figured specimens are housed in the Laboratory ofGeocollections at the Geological Institute of BAS.

Palaeontological and biostratigraphical results (lower TanezzuftFm). The vertical occurrence of graptolites in the cores from the two wellsis depicted in Fig. 1B. The graptolite assemblages are entirely of biserial taxaand generally of low diversity. There is a mixture of cosmopolitan taxa and thoseconsidered to be endemic to Gondwana.

508 V. Sachanski, D. Loydell, A. Chatalov

Fig. 2. Graptolites from the A1-NC101 core: a – Normalograptus aff. ajjeri sensuLegrand [6], Inv.-Nr. F.015316, 2307.6 m; b – Normalograptus aff. ajjeri sensu Legrand [6],Inv.-Nr. F.015317, 2307.9 m; c, d – Normalograptus? pseudovenustus (Legrand), Inv.-Nr.F.015318, 2309.2 m; e – Normalograptus ajjeri (Legrand), Inv.-Nr. F.015318, 2309.20 m.

Scale bar is 1 mm across

The graptolite-bearing interval in the A1-NC101 core is 1.9 m thick (depthfrom 2307.6 to 2309.5 m). The graptolites identifiable to species level includeNormalograptus aff. ajjeri sensu Legrand, Normalograptus ajjeri (Legrand), andNormalograptus? pseudovenustus (Legrand). They are illustrated in Fig. 2.

Legrand [6] recognised several “formes” of aff. ajjeri, some of which arecharacterized by a broader proximal end that is typical for N. ajjeri sensu stricto.Most previous records of N. normalis are N. ajjeri, which has a long stratigraph-ical range from the Hirnantian through to the lower Aeronian [7].

Normalograptus? pseudovenustus has a distinctive, somewhat protracted pro-ximal end (see [8], Fig. 3A). Legrand [8] recorded a maximum rhabdosome widthof 1.6–2.0 mm. N.? pseudovenustus is a reliable index species for an interval justbelow the Ordovician–Silurian boundary (e.g., [9], and references therein). Basedon the range of this species, the graptolite-bearing shales overlying the sandstonesof the Mamuniyat Fm are inferred to be of latest Ordovician age, and thereforethe studied core interval includes the Ordovician–Silurian boundary.

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510 V. Sachanski, D. Loydell, A. Chatalov

The graptolite-bearing interval in the C1-NC101 core is nearly 15 m thick(depth from 2178.05 to 2193.00 m). A diverse shelly fauna (bivalves and bra-chiopods) occurs from 1831.9 to 2117.65 m. One orthoconic nautiloid cephalopodwas found at a depth of 2193.0 m. The graptolites identifiable to species level in-clude Normalograptus normalis (Lapworth), Normalograptus inazaouae Legrand,Metabolograptus parvulus (Lapworth), and Normalograptus targuii Legrand.They are illustrated in Fig. 3.

N. normalis is a long-ranging taxon, from the upper Katian to probably thelower Aeronian [10].

The maximum width (2 mm) and glyptograptid appearance of some thecaeof the specimen depicted in Fig. 3a match N. inazaouae [11]. This taxon isrecognized here for the first time outside Algeria. According to Legrand [11],its stratigraphical range is from the uppermost Ordovician up to probably thelowermost Llandovery.

M. parvulus (Fig. 3h) is similar in appearance to some specimens illustratedfrom the USA [12]. This well-known and widely recorded species has a range inthe “standard” biozonation from the persculptus graptolite Zone up to probablythe lower vesiculosus graptolite Zone [3,7].

Both Legrand ([11], pl. 12, Fig. 12) and Loydell ([7], Fig. 18K) figuredspecimens of N. targuii with only a few thecal pairs that narrow distally (sim-ilar to the one from well C1-N101, depth 2178.85 m, Fig. 3g). This species ischaracterized by a slight inclination of the supragenicular walls throughout. Theslightly greater rhabdosome width of the specimens in Fig. 3c, d, compared topreviously recorded specimens, is a result of their considerably greater length.The material studied by Legrand [11] was collected from a geographically iso-lated exposure lacking stratigraphically diagnostic associated graptolites. Theage was considered to be probably Late Ordovician. The Jordanian specimensare from the upper ascensus-acuminatus graptolite Zone [7]. The stratigraphi-cally lowest Silurian graptolite assemblages (lubricus graptolite Zone and lowerascensus-acuminatus graptolite Zone) from Saudi Arabia contain M. parvulusand N. targuii [13]. The stratigraphical range of N. targuii in the E1-NC174 core,Murzuq Basin, Libya [3] is within the lower tilokensis graptolite Zone, which iscorrelated with the peri-Gondwanan lower ascensus graptolite Zone [14]. All thisimplies that the total stratigraphical range of N. targuii, and thus the age ofLegrand’s [11] material, is younger than he tentatively suggested [7].

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Fig. 3. Graptolites from the C1-NC101 core: a – Normalograptus inazaouae Legrand,Inv.-Nr. F.015324, 2189.15 m; b, d – Normalograptus targuii Legrand, Inv.-Nr. F.015328,2193.0 m; c – Normalograptus targuii Legrand, Inv.-Nr. F.015323, 2188.9 m; e – Nor-malograptus targuii Legrand, Inv.-Nr. F.015326, 2190.45 m; f – Normalograptus nor-malis (Lapworth), Inv.-Nr. F.015327, 2192.2 m; g – Normalograptus targuii Legrand, Inv.-Nr. F.015321, 2178.85 m; h – Metabolograptus parvulus (Lapworth), Inv.-Nr. F.015322,2188.00 m. Scale bar is 1 mm across

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Fig. 4. Supermature quartz arenites of the uppermost Mamuniyat Fm: a – Bimodal textureshowing evidence of strong compaction (see text). Some detrital grains have thin quartz over-growths (arrows); b – Well rounded quartz grain with concavity (arrows) suggesting eolianabrasion; c, d – Fine-grained quartz “matrix” and microquartz cement (arrows) produced by

cataclasis and related dissolution/precipitation processes. Zircon grains are encircled

Sedimentological notes (uppermost Mamuniyat Fm). In thin sec-tions, the sandstones are defined as quartz arenites consisting of > 99% detri-tal quartz. Bimodal grain size distribution (Fig. 4a) is a characteristic featurewith the coarser fraction (1.2–2 mm) including well rounded, locally concavegrains (Fig. 4b), and the finer fraction (< 0.6 mm) including subangular to sub-rounded grains. The detrital quartz comprises monocrystalline and polycrys-talline (< 15%) grains, some of which have thin quartz overgrowths. Minortextural components (< 1%) are abraded heavy mineral grains (zircon, rutile,epidote), chert lithics, and intergranular quartz cement. Strong compaction is ev-idenced by the close grain packing, triple junctions, and pressure solution effects.Brittle deformation and related dissolution/precipitation processes produced alocally cataclastic quartz “matrix” and microquartz cement (Fig. 4c, d).

Deposition of the Mamuniyat Fm in the Murzuq Basin was closely relatedto the Hirnantian glaciation over a large part of Gondwana [15]. On the basisof outcrop and subsurface studies, the sand-dominated upper part of the unit

512 V. Sachanski, D. Loydell, A. Chatalov

has been interpreted as deposited during glacial retreat and post-glacial isostaticrebound in various environments, from (glacio-)fluvial braidplain to glaciomarineshelf [15–17]. Quartzitic sandstone is the main lithofacies in the central part ofthe Murzuq Basin [2], and the sampled quartz arenites from well A1-NC101 rep-resent a typical example. The high compositional maturity of these sandstonesmust have resulted from intense chemical weathering in the source area becausephysical processes (abrasion, hydraulic sorting, recycling) have limited efficiencyin progressive enrichment in relatively durable grains [18]. Such extreme chem-ical weathering and erosion on North Gondwana occurred during the Cambrianand pre-glacial Ordovician and produced large volumes of first-cycle quartz-richsands [19]. The sandy material was later incorporated in the expanding ice-sheetand then released from the melting ice during the end-Hirnantian deglaciation insubaerial and subaqueous environments. Therefore, the provenance of the quartzarenites in the upper Mamuniyat Fm was associated with sedimentary recyclingof mature siliciclastics. Some textural features of the sandstones, such as therounded and locally concave morphology of the quartz grains, suggest prolongedaeolian abrasion in the source area [18], while the bimodality of grain size androundness may reflect mixing of aeolian dune sands with finer interdune mate-rial [20]. The lack of syndepositional muddy matrix and abundant cement, aswell as the close boundary with the shales of the Tanezzuft Fm, imply that thequartz arenites were deposited in a nearshore marine environment where win-nowing and additional reworking occurred and the high sedimentation rate led torapid mechanical compaction. Later pressure solution, cataclasis and cementationsignificantly reduced the sandstone porosity.

Conclusions. The new palaeontological and biostratigraphical results al-low identification of the Ordovician–Silurian boundary in the subsurface of theMurzuq Basin. The presence of N.? pseudovenustus in the lowermost Tanez-zuft Fm indicates a Hirnantian age (persculptus graptolite Zone), while the othergraptolite taxa bracket a stratigraphical interval from the uppermost Ordovicianto the lowermost Silurian. In addition, the species N. inazaouae is recognised forthe first time outside Algeria and some specimens of N. targuii appear to be thelongest figured. Pure quartz arenites of the uppermost Mamuniyat Fm were de-posited during the end-Hirnantian deglaciation as recycled products of Cambrianto pre-glacial Ordovician mature sands from North Gondwana.

Acknowledgements. The paper benefited from the constructive reviews ofIliana Boncheva and Lubomir Metodiev.

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University of Mining and Geology“St. Ivan Rilski”

Studentski Grad, Prof. Boyan Kamenov St1700 Sofia, Bulgaria

andGeological Institute

Bulgarian Academy of SciencesAcad. G. Bonchev St, Bl. 24

1113 Sofia, Bulgariae-mail: v sachanski@geology.bas.bg

∗School of Earth andEnvironmental Sciences

University of PortsmouthBurnaby Road

Portsmouth PO1 3QL, UKe-mail: david.loydell@port.ac.uk

∗∗Faculty of Geology and GeographySofia University “St. Kliment Ohridski”

15, Tsar Osvoboditel Blvd1504 Sofia, Bulgaria

e-mail: chatalov@gea.uni-sofia.bg

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