new data from graptolites and quartz arenites for the ordovician ...

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Mar 13, 2018 - deposited during the end-Hirnantian deglaciation probably in a nearshore ma- rine environment. Their provenance was associated with ...
Доклади на Българската академия на науките Comptes rendus de l’Acad´emie bulgare des Sciences Tome 71, No 4, 2018

GEOLOGY Stratigraphy

NEW DATA FROM GRAPTOLITES AND QUARTZ ARENITES FOR THE ORDOVICIAN–SILURIAN BOUNDARY 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 Hirnantian age (persculptus graptolite Zone) for the lowermost Tanezzuft Fm based on the presence and stratigraphical distribution of N.? pseudovenustus. In the C1-NC101 core, the presence of N. inazaouae (recognised for the first time outside Algeria) and M. parvulus indicates either the uppermost Ordovician or the lowermost Silurian, but the revised total stratigraphical range of N. targuii (ascensus-acuminatus graptolite Zone) suggests an earliest Rhuddanian age. Several specimens of N. targuii are longer than those previously recorded. Pure quartz arenites of the uppermost Mamuniyat Fm (well A1-NC101) were deposited during the end-Hirnantian deglaciation probably in a nearshore marine environment. Their provenance was associated with sedimentary recycling of mature sands that were formed across North Gondwana in Cambrian and pre-glacial Ordovician times. Key words: graptolites, shales, sandstones, Ordovician–Silurian boundary, Murzuq Basin

Introduction. The Murzuq Basin is a large intracratonic sag basin which developed on the passive margin of Gondwana during the Palaeozoic [1 ]. At present most of the basin area is situated in SW Libya (Fig. 1A), where its sedimentary 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 of the cored intervals and vertical distribution of the identified graptolite taxa

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awarded an exploration permit (Concession NC101) in the north central flank of the basin in 1980. BOCO drilled a total of 17 exploration wells and made several oil discoveries. In most wells lower Silurian shales of the Tanezzuft Fm overlie Upper Ordovician sandstones of the Mamuniyat Fm [2 ]. The latter is a major hydrocarbon reservoir in the Murzuq Basin and “hot shales” in the lower part of the Tanezzuft Fm are the principal source rock. The two units combined comprise the Mamuniyat-Tanezzuft Petroleum System [1, 2 ]. Great efforts have been made to identify the Ordovician–Silurian boundary within the Tanezzuft Fm using graptolites, acritarchs and chitinozoans [3–5 ]. Available data from the subsurface of the basin are scarce, and recent studies of cored wells have shown that the oldest drilled strata of that unit have a Rhuddanian age [3, 5 ]. The basal part of the Tanfezzuft Fm in outcrops has been also referred to the lower Llandovery [4 ]. This paper presents biostratigraphical and sedimentological results from a study of the lower Tanezzuft Fm and uppermost Mamuniyat Fm in wells A1-NC101 and C1-NC101. Materials. The core samples were collected by Hristo Spassov in March 1983 and are stored now at the Geological Institute of the Bulgarian Academy of Sciences (BAS) The cored interval in A1-NC101 is from 2255.0 to 2311.0 m and 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 by hard, dark grey shales with rare, thin-bedded, current-laminated, siltstone to fine sandstone intercalations, all belonging to the Tanezzuft Fm (Fig. 1B). The only exception is in the core from A1-NC101, at a depth of 2311.0 m, where there is light grey, massive, very coarse grained, quartzose sandstone which is typical of the upper parts of the Mamuniyat Fm [2 ]. Two thin sections were prepared from these sandstones for sedimentological studies. Thirteen samples (four from A1-NC101 and nine from C1-NC101) yielded graptolites. The number of examined graptolites is 30. Of these, three samples from A1-NC101 and seven samples from C1-NC101 yielded a total of 16 graptolites identifiable to species level. Undeterminable specimens are distal fragments or complete specimens that are poorly preserved or preserved in scalariform view. Almost all specimens are flattened with periderm or a residue from it. A specimen from C1-NC101 (depth of 2193.0 m) is preserved in low relief, and another specimen from the same well (depth of 2189.15 m) is preserved in part-relief as a steinkern of iron oxides pseudomorphing pyrite. There is no tectonic deformation of the rhabdosomes. All figured specimens are housed in the Laboratory of Geocollections at the Geological Institute of BAS. Palaeontological and biostratigraphical results (lower Tanezzuft Fm). The vertical occurrence of graptolites in the cores from the two wells is depicted in Fig. 1B. The graptolite assemblages are entirely of biserial taxa and generally of low diversity. There is a mixture of cosmopolitan taxa and those considered to be endemic to Gondwana. 508

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Fig. 2. Graptolites from the A1-NC101 core: a – Normalograptus aff. ajjeri sensu Legrand [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 (depth from 2307.6 to 2309.5 m). The graptolites identifiable to species level include Normalograptus aff. ajjeri sensu Legrand, Normalograptus ajjeri (Legrand), and Normalograptus? pseudovenustus (Legrand). They are illustrated in Fig. 2. Legrand [6 ] recognised several “formes” of aff. ajjeri, some of which are characterized 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 stratigraphical range from the Hirnantian through to the lower Aeronian [7 ]. Normalograptus? pseudovenustus has a distinctive, somewhat protracted proximal end (see [8 ], Fig. 3A). Legrand [8 ] recorded a maximum rhabdosome width of 1.6–2.0 mm. N.? pseudovenustus is a reliable index species for an interval just below the Ordovician–Silurian boundary (e.g., [9 ], and references therein). Based on the range of this species, the graptolite-bearing shales overlying the sandstones of the Mamuniyat Fm are inferred to be of latest Ordovician age, and therefore the studied core interval includes the Ordovician–Silurian boundary. 5

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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 brachiopods) occurs from 1831.9 to 2117.65 m. One orthoconic nautiloid cephalopod was found at a depth of 2193.0 m. The graptolites identifiable to species level include 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 the lower Aeronian [10 ]. The maximum width (2 mm) and glyptograptid appearance of some thecae of the specimen depicted in Fig. 3a match N. inazaouae [11 ]. This taxon is recognized here for the first time outside Algeria. According to Legrand [11 ], its stratigraphical range is from the uppermost Ordovician up to probably the lowermost Llandovery. M. parvulus (Fig. 3h) is similar in appearance to some specimens illustrated from the USA [12 ]. This well-known and widely recorded species has a range in the “standard” biozonation from the persculptus graptolite Zone up to probably the lower vesiculosus graptolite Zone [3, 7 ]. Both Legrand ([11 ], pl. 12, Fig. 12) and Loydell ([7 ], Fig. 18K) figured specimens of N. targuii with only a few thecal pairs that narrow distally (similar to the one from well C1-N101, depth 2178.85 m, Fig. 3g). This species is characterized by a slight inclination of the supragenicular walls throughout. The slightly greater rhabdosome width of the specimens in Fig. 3c, d, compared to previously recorded specimens, is a result of their considerably greater length. The material studied by Legrand [11 ] was collected from a geographically isolated exposure lacking stratigraphically diagnostic associated graptolites. The age was considered to be probably Late Ordovician. The Jordanian specimens are from the upper ascensus-acuminatus graptolite Zone [7 ]. The stratigraphically lowest Silurian graptolite assemblages (lubricus graptolite Zone and lower ascensus-acuminatus graptolite Zone) from Saudi Arabia contain M. parvulus and 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 is correlated with the peri-Gondwanan lower ascensus graptolite Zone [14 ]. All this implies that the total stratigraphical range of N. targuii, and thus the age of Legrand’s [11 ] material, is younger than he tentatively suggested [7 ].



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 – Normalograptus targuii Legrand, Inv.-Nr. F.015326, 2190.45 m; f – Normalograptus normalis (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 texture showing evidence of strong compaction (see text). Some detrital grains have thin quartz overgrowths (arrows); b – Well rounded quartz grain with concavity (arrows) suggesting eolian abrasion; 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 sections, the sandstones are defined as quartz arenites consisting of > 99% detrital quartz. Bimodal grain size distribution (Fig. 4a) is a characteristic feature with the coarser fraction (1.2–2 mm) including well rounded, locally concave grains (Fig. 4b), and the finer fraction (< 0.6 mm) including subangular to subrounded grains. The detrital quartz comprises monocrystalline and polycrystalline (< 15%) grains, some of which have thin quartz overgrowths. Minor textural components (< 1%) are abraded heavy mineral grains (zircon, rutile, epidote), chert lithics, and intergranular quartz cement. Strong compaction is evidenced by the close grain packing, triple junctions, and pressure solution effects. Brittle deformation and related dissolution/precipitation processes produced a locally cataclastic quartz “matrix” and microquartz cement (Fig. 4c, d). Deposition of the Mamuniyat Fm in the Murzuq Basin was closely related to the Hirnantian glaciation over a large part of Gondwana [15 ]. On the basis of outcrop and subsurface studies, the sand-dominated upper part of the unit 512

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has been interpreted as deposited during glacial retreat and post-glacial isostatic rebound in various environments, from (glacio-)fluvial braidplain to glaciomarine shelf [15–17 ]. Quartzitic sandstone is the main lithofacies in the central part of the Murzuq Basin [2 ], and the sampled quartz arenites from well A1-NC101 represent a typical example. The high compositional maturity of these sandstones must have resulted from intense chemical weathering in the source area because physical processes (abrasion, hydraulic sorting, recycling) have limited efficiency in progressive enrichment in relatively durable grains [18 ]. Such extreme chemical weathering and erosion on North Gondwana occurred during the Cambrian and pre-glacial Ordovician and produced large volumes of first-cycle quartz-rich sands [19 ]. The sandy material was later incorporated in the expanding ice-sheet and then released from the melting ice during the end-Hirnantian deglaciation in subaerial and subaqueous environments. Therefore, the provenance of the quartz arenites in the upper Mamuniyat Fm was associated with sedimentary recycling of mature siliciclastics. Some textural features of the sandstones, such as the rounded and locally concave morphology of the quartz grains, suggest prolonged aeolian abrasion in the source area [18 ], while the bimodality of grain size and roundness may reflect mixing of aeolian dune sands with finer interdune material [20 ]. The lack of syndepositional muddy matrix and abundant cement, as well as the close boundary with the shales of the Tanezzuft Fm, imply that the quartz arenites were deposited in a nearshore marine environment where winnowing and additional reworking occurred and the high sedimentation rate led to rapid mechanical compaction. Later pressure solution, cataclasis and cementation significantly reduced the sandstone porosity. Conclusions. The new palaeontological and biostratigraphical results allow identification of the Ordovician–Silurian boundary in the subsurface of the Murzuq Basin. The presence of N.? pseudovenustus in the lowermost Tanezzuft Fm indicates a Hirnantian age (persculptus graptolite Zone), while the other graptolite taxa bracket a stratigraphical interval from the uppermost Ordovician to the lowermost Silurian. In addition, the species N. inazaouae is recognised for the first time outside Algeria and some specimens of N. targuii appear to be the longest figured. Pure quartz arenites of the uppermost Mamuniyat Fm were deposited during the end-Hirnantian deglaciation as recycled products of Cambrian to pre-glacial Ordovician mature sands from North Gondwana. Acknowledgements. The paper benefited from the constructive reviews of Iliana Boncheva and Lubomir Metodiev.

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[18 ] Garzanti E. (2017) The maturity myth in sedimentology and provenance analysis, J. Sediment. Res., 87(4), 353–365. [19 ] Avigad D., A. Sandler, K. Kolodner, R. J. Stern, M. McWilliams et al. (2005) Mass production of Cambro–Ordovician quartz-rich sandstone as a consequence of chemical weathering of Pan-African terranes: environmental implications, Earth Planet. Sci. Lett., 240(3/4), 818–826. [20 ] Folk R. L. (1968) Bimodal supermature sandstones: product of the desert floor. Proc. 23rd Int. Geol. Congr., 8, 9–32. University of Mining and Geology “St. Ivan Rilski” Studentski Grad, Prof. Boyan Kamenov St 1700 Sofia, Bulgaria and Geological Institute Bulgarian Academy of Sciences Acad. G. Bonchev St, Bl. 24 1113 Sofia, Bulgaria e-mail: v [email protected]



School of Earth and Environmental Sciences University of Portsmouth Burnaby Road Portsmouth PO1 3QL, UK e-mail: [email protected]

∗∗

Faculty of Geology and Geography Sofia University “St. Kliment Ohridski” 15, Tsar Osvoboditel Blvd 1504 Sofia, Bulgaria e-mail: [email protected]

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