Graptolite biostratigraphy of the E1-NC174 core, Rhuddanian (lower Llandovery, Silurian), Murzuq Basin (Libya) DAVID K. LOYDELL The graptolite biostratigraphy of the Rhuddanian (lower Llandovery, Silurian) of the E1-NC174 core, Murzuq Basin, Libya is described. The generally low diversity graptolite assemblages include both North African endemic taxa and cosmopolitan species. The lower part of the cored interval, below the ‘hot’ shale, is dominated by Normalograptus tilokensis, a species previously considered to be confined to the Upper Ordovician, but which almost certainly ranges into (or perhaps occurs only in) the lower Silurian. As in the BG-14 core, Jordan, the base of the ‘hot’ shale lies just above the first appearances of Neodiplograptus africanus and Normalograptus rectangularis. The latter species domi- nates throughout the mid Rhuddanian ‘hot’ shale interval, with the highest strata in the core, of late Rhuddanian age, yielding abundant Neodiplograptus fezzanensis. • Key words: Silurian, Libya, graptolite, Llandovery, Rhuddanian, hot shale, North Africa, Tanezzuft. LOYDELL, D. 2011. Graptolite biostratigraphy of the E1-NC174 core, Rhuddanian (lower Llandovery, Silurian), Murzuq Basin (Libya). Bulletin of Geosciences 87(4), 651–660 (3 figures, 7 tables). Czech Geological Survey, Prague. ISSN 1214-1119. Manuscript received August 19, 2011; accepted in revised form January 3, 2012; published online April 17, 2012; issued October 17, 2012. David K. Loydell, School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK; [email protected] This paper represents the first published detailed graptolite gamma ray curves demonstrating the presence of a ‘hot’ biostratigraphical study of a ‘hot’ shale core from North shale in the well. More detailed TOC and gamma ray Africa. curves were produced by Lüning et al. (2003) and sup- The E1-NC174 well (Fig. 1) was drilled in the plemented with Rock-Eval pyrolysis data, organic geo- Murzuq Basin of SW Libya. This large, intracratonic chemical biomarker analyses, pyrite framboid data and structural basin extends east and south from Libya into some very limited graptolite biostratigraphy (from four western Algeria and northern Niger. Important hydro- core samples, based on an unpublished 1998 report for carbon source rocks (organic-, and uranium-rich – hence LASMO by Loydell). ‘hot’ – shales) occur within the basin where they overlie In November 1999 the opportunity arose to sample the glacial deposits (often acting as a reservoir, with the E1-NC174 core more thoroughly, for graptolite bio- ‘hot’ shales acting as a seal) of Late Ordovician age. stratigraphical work and palynological studies. Funding The cored interval is from 7237´(2205.84 m) to from Eni, from 2005–2007, enabled examination of this 7294´(2223.21 m); the core has a diameter of 4˝ material. The present paper is based upon study of 82 (10 cm). Note that feet and inches are used herein as samples from depths ranging from 7237´2˝ (2205.89 m) these are the units used on the core storage boxes, in the to 7293´10˝ (2223.16 m). Of these, 67 samples yielded original core description and in the catalogue of graptolites identifiable to species level, with the total graptolites from the core. Lithologically, there is very number of graptolites examined in excess of 500. In little variation throughout the core: it is dominated terms of preservation, with the exception of three-di- throughout by hard, dark grey shales, all belonging to the mensional pyrite internal moulds from 7266´9˝ and Tanezzuft Formation. 7267´½˝, virtually all specimens are preserved flattened Well logs and the core derived from the E1-NC-174 (the few exceptions are preserved in very low relief). well have been the subject of considerable previous There is no tectonic deformation of the rhabdosomes. study. Published work includes that of Lüning et al. All figured specimens are housed at the British Geologi- (2000) who presented maps together with TOC and cal Survey, Keyworth. DOI 10.3140/bull.geosci.1311 651 Bulletin of Geosciences Vol. 87, 4, 2012 Graptolite biostratigraphy of the E1-NC174 core and age of the hot shale Figure 2 illustrates the stratigraphical ranges of graptolites within the E1-NC174 core, and shows the three biozones recognised. The graptolite assemblages are generally of low diversity and are dominated by biserials: uniserial graptolites were recorded from only four samples. There is a mixture of cosmopolitan taxa and those considered to be endemic to Gondwana and some peri-Gondwanan terranes, the latter exhibiting the high level of intraspecific variation that characterizes graptolites from these regions. There is thus a rather surprising difference in the nature of the as- Figure 1. Location of the E1-NC174 well, Libya. semblages from those from neighbouring Algeria; Philippe Legrand’s extensive investigations of the Upper Ordovi- cian and lower Silurian graptolites of Algeria (comprehen- Previous work on Libyan graptolites sively reviewed in Legrand 1999) emphasized the endemi- city of the taxa found there. Whilst several of these There have been several published works which include endemics have also been found in the E1-NC174 core, they graptolite data from Libya: the summary of these here is occur together with cosmopolitan taxa, notably several limited to papers with illustrations and/or descriptions; species of Normalograptus, Huttagraptus acinaces papers with faunal lists only are excluded. (Törnquist, 1899) (Fig. 3O) and Metaclimacograptus hug- Desio (1940) described 12 graptolite taxa from western hesi (Nicholson, 1869) (Fig. 3R). The presence of these Libya. He erected two new species, both of which have cosmopolitan taxa, together with knowledge gained from subsequently been shown to be common and strati- work in Jordan (Loydell 2007) where again Gondwanan graphically important: Climacograptus (now Paraclima- endemics and cosmopolitan species co-occur, enables cograptus) libycus and Diplograptus (now Neodiplo- more confident integration of North African graptolite bio- graptus) fezzanensis. Some of Desio’s material was stratigraphy with the ‘standard’ graptolite biozonation. re-examined by Štorch & Massa (2006) and in some cases Graptolites from the core are illustrated in Fig. 3. Virtually assigned to different taxa. all of the species are well-known and have been described Jaeger (1976) illustrated two specimens from SW thoroughly by Legrand (1970, 1977, 1986a, 1999, 2001) Libya as Climacograptus innotatus brasiliensis Ruede- and/or Loydell (2007); thus only a few comments regarding mann in Maury, 1929. Loydell (2007) referred this material distinguishing features are made below. A large number of to Paraclimacograptus libycus. measurements of rhabdosome dorso-ventral width and thecal Parizek et al. (1984) illustrated some lower and mid- spacing was made during the identification work. These mea- dle Llandovery graptolites from the Al Qarqaf Arch re- surements are included here (Tables 1–7) as they may be of gion. use to workers in the future when identifying, or studying El-Chair et al. (1985) illustrated C. innotatus intraspecific variation in, the taxa concerned. brasiliensis from Tahale, SW Libya: this material also ap- Many, including the lowest, of the samples from the pears to be P. libycus (based upon the distinguishing fea- lower part of the core bear monospecific assemblages of tures of this taxon noted by Štorch & Massa 2006). Lüning Normalograptus tilokensis (Legrand, 1986a) (Fig. 3D–H). et al. (1999) illustrated Neodiplograptus fezzanensis from As noted in the original description (Legrand 1986a), the Kufra Basin, SE Libya. N. tilokensis differs from the rather similar N. rectan- Štorch & Massa (2004) provided a very useful over- gularis in being more robust (most noticeable proximally, view of the Llandovery and Wenlock graptolites of Libya. although there is some overlap: compare DVW figures in Štorch & Massa (2006) described 23 species from the Tables 1 and 3), in having a long, robust virgella and by its Aeronian of the Al Qarqaf Arch region; and Štorch & distal diminution in width. Legrand (e.g. 1986a, 1999, Massa (2007) described three species from the middle 2003, 2009) used N. tilokensis as a biozonal index and as- Telychian from cores in NW Libya. signed the biozone tentatively to the uppermost Hirnantian, Graptolites from neighbouring Algeria have been stud- having stated (Legrand 1986a) that the North African ied over a long period by Legrand (1969, 1970, 1977, 1978, tilokensis and overlying kiliani biozones are ‘the very ap- 1979, 1986a, 1986b, 1995, 1998, 1999, 2000, 2001, 2002, proximate equivalent’ of the persculptus Biozone. Subse- 2003, 2009) whose papers include descriptions of many quently, Legrand (2009) considered the tilokensis Biozone important species found also in Libya. to be equivalent to a Hirnantian post-persculptus Biozone 652 David K. Loydell Graptolite biostratigraphy of the lower Llandovery in Murzuq Basin (Libya) Figure 2. Stratigraphical ranges of graptolites in the E1-NC174 core, Libya. TOC and gamma-ray curves are from Lüning et al. (2003); the highest val- ues (the ‘hot’ shale) are shaded. 653 Bulletin of Geosciences Vol. 87, 4, 2012 ‘interregnum’ such has been recognised in Sweden Rhuddanian Coronograptus cyphus Biozone and the low- (Koren’ et al. 2003) and is present also in China (Chen et ermost Aeronian Demirastrites triangulatus Biozone. The al. 2005; see Loydell 2007, p. 8). Lüning et al. (2000, top of the core,