Latest Ordovician-earliest Silurian acritarchs and chitinozoans from subsurface samples in Jebel Asba, Kufra Basin, SE Libya

Bindra Thusu (1) , Sayed Rasul (1) , Florentin Paris (2) , Guido Meinhold (3, 4) , James P. Howard (3) , Youssef Abutarruma (5) , Andrew G. Whitham (3)

1 Maghreb Petroleum Research Group
2 GR - Géosciences Rennes
3 CASP
4 GZG - Geowissenschaftliches Zentrum Göttingen
5 Mellitah Oil & Gas B.V.

Latest Ordovician-earliest Silurian Tanezzuft Formation shales recovered from core material of the shallow borehole JA-2 drilled in Jebel Asba at the eastern margin of the Kufra Basin, southeastern Libya, yielded well-diversified palynomorph assemblages with transparent and brownish to yellowish vesicles and organic matter (visual kerogen Types 1 and 2) from depth interval 46.20 to 67.82 m. In addition, miospores including cryptospores, and Tasmanites sp. ("Tasmanites with nodules"), scolecodonts, and a stratigraphically significant palaeo-marker, the enigmatic, tubular organic structure Tortotubus protuberans, were also recorded frequently in most samples. Kerogen colour based on miospores (TAI < 3) and chitinozoan reflectance indicate an immature facies for oil generation. The two uppermost samples (from 33.33 m and 46.20 m depths) and the lowermost ones (from 67.92 to 73.21 m depth) contain rare palynomorphs and other organic remains and have been partially affected by oxidation. Furthermore, palynological and palynofacies analyses were carried out on cuttings from an old well (UN-REMSA well), ca. 530 m towards the NNE from well JA-2. The composition of the organic residue is similar in both wells. However, the UN-REMSA well yields fairly numerous chitinozoans, scolecodonts and biofilms but lacks the "thread-like structures" and "Tasmanites with nodules" observed in well JA-2. All the investigated samples in well JA-2 are dominated by a single chitinozoan species, Euconochitina moussegoudaensis Paris (in Le Hérissé et al., 2013). Based on correlation with chitinozoan-bearing strata around the Ordovician-Silurian boundary, the analysed samples from well JA-2 and from the UN-REMSA well are regarded as post-glacial, but still of either latest Hirnantian age, or at least no younger than earliest Rhuddanian. A well-diversified acritarch, miospore and cryptospore assemblage recorded in well JA-2 supports a marginal marine (nearshore) depositional environment. This assemblage is no older than earliest Rhuddanian yet the latest Hirnantian age of the assemblage cannot be completely ruled out as our current knowledge on the post-glacial, latest Hirnantian acritarch and miospore assemblages is poorly documented in North Africa.

Mots clés: biostratigraphy; acritarch; chitinozoan; Hirnantian; Silurian; Gondwana; Libya

Domaines: Sciences de la Terre

Bindra Thusu, Sayed Rasul, Florentin Paris, Guido Meinhold, James P. Howard, et al.. Latest Ordovician-earliest Silurian acritarchs and chitinozoans from subsurface samples in Jebel Asba, Kufra Basin, SE Libya. Review of Palaeobotany and Palynology, 2013, 197, pp.90-118. ⟨10.1016/j.revpalbo.2013.05.006⟩. ⟨insu-00843501⟩