Early Palaeozoic evolution of Libya: perspectives from Jabal Eghei with implications for hydrocarbon exploration in Al Kufrah Basin

Abstract

This paper presents new stratigraphic and sedimentological data of the Ordovician, Silurian, and Mesozoic succession exposed on the western flank of Al Kufrah Basin. Field data (logged sections, photographs, palaeocurrent analyses) are presented from the Jabal Eghei region. This region lies ca. 200 km E of the closest stratigraphic tie point at Mourizidie on the eastern flank of the Murzuq Basin. The succession starts with the Hawaz Formation (Middle Ordovician) comprising >100 m of cross-bedded and bioturbated sandstones that are interpreted as deposits of tidal currents in an open shelf setting. The contact between the Hawaz and Mamuniyat formations is an erosional unconformity, incised during advance of Late Ordovician ice sheets towards the NE. The Mamuniyat Formation comprises >150 m of massive and graded sandstones tentatively assigned to the Hirnantian, and contains an intraformational, soft-sediment striated surface that is interpreted to record re-advance of ice sheets over Jabal Eghei. The outcrop section suggests the sandstone would form an excellent reservoir in the subsurface. The Mamuniyat Formation is overlain by the Tanezzuft Formation (uppermost Ordovician-lowermost Silurian). This includes sandy limestone/calcareous sandstone, a Planolites horizon, and then 50 m of interbedded shale, silt and fine-grained, graded and hummocky cross-stratified sandstone recording deposition from both shallow marine turbidity currents and storm flows. A striated pavement in the lower part of this sequence is overlain by calcareous lonestone-bearing intervals (interpreted as ice-rafted debris). These features testify to late phases of glacial advance probably post-dating the regional Hirnantian glacial maximum. The basal Silurian 'hot shale' facies is not developed in this area, probably because late glacial advance suppressed the preservation of organic matter. The upper part of the Tanezzuft Formation is truncated by an unconformity above which palaeosol-bearing fluvial deposits (undifferentiated Mesozoic) occur.

An exhumed Paleozoic glacial landscape in Chad

“…No Paleozoic glacially striated surfaces of any type have hitherto been described from this area. In the Libyan portion of the basin, striated surfaces and glacial deposits are recognized in Jabal Azbah at the eastern flank of the basin (Late Ordovician; Le Heron et al, 2010) and in correlative strata in Jabal Eghei to the west (Le Heron et al, 2015). The character of striated surfaces is identical to those reported from more accessible areas (e.g., Deynoux and Ghienne, 2004;Le Heron et al, 2005;Denis et al, 2010;Girard et al, 2015).…”

The search for ‘hot shales’ in the western Kufra Basin, Libya: geochemical and mineralogical characterisation of outcrops, and insights into latest Ordovician climate

“…2). As this thickness was observed where the Mamuniyat Formation is unconformably overlain by Mesozoic sandstone (Le Heron et al 2015), it should be regarded as a minimum value.…”

“…1). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 A detailed description of the stratigraphy and facies interpretation of the sedimentary strata of Jabal Eghei is given by Le Heron et al (2013, 2015 and is summarised briefly below. The oldest studied succession, the Hawaz Formation, composed of a cross-bedded and a Skolithos-burrowed sandstone facies association, occupies the westernmost part of the study area, although isolated outcrops have also been found near the eastern margin of the area.…”

Tracking the isostatic and eustatic signals in an end Ordovician–early Silurian deglacial sedimentary record (Algeria – Libya border)

“…The present contribution focuses on the stratigraphic architecture of the uppermost Ordovician (‘upper Hirnantian’) to Early Silurian sand‐prone, shallow‐marine, postglacial strata that develop over relict palaeorelief forms linked to glacial erosion (depression) and/or positive relief inherited from glaciogenic depocentres (Lüning et al ., ; Ghienne et al ., ; Moreau, ; Girard et al ., , ). Related sediment bodies together form a deglacial transgressive package in place (TA2 of Moreau, ) bounded at its base by a late glacial erosion surface of regional extent (GES6 of Girard et al ., ) and sealed by diachronous, uppermost Hirnantian to Early Silurian offshore shales, including the ‘hot shales’ interval (the Tanzuft Formation in Libya; Lüning et al ., ; Moreau, ; Le Heron et al ., ). A minimum time range of 5 Myr is documented between the older shales deposited in palaeodepressions and the younger shales deposited over palaeohighs (Lüning et al ., ; Fello et al ., ; Storch & Massa, ; Moreau, ; Le Heron et al ., ).…”

“…During the end‐Ordovician glaciation (Ghienne et al ., , ; Le Heron & Craig, ; Le Heron, ) the North Gondwana platform (present‐day West and North Africa and Arabia), located at high southern latitudes, was repeatedly covered by large‐scale ice sheets (Sutcliffe et al ., ; Brenchley et al ., ; Ghienne, ; Le Heron et al ., ; Loi et al ., ; Nutz et al ., ). In south‐west Libya, between two and five glacial sequences are identified (Ghienne et al ., , , ; Le Heron et al ., , , ; El‐Ghali, ; Moreau et al ., ; Moreau, ; Moreau & Ghienne, ; Girard et al ., ,b, ). In Algeria two main glacial sequences are identified (Hirst et al ., ; Eschard et al ., ; Roussé et al ., ; Deschamps et al ., ).…”