Glaciation and deglaciation of the Libyan Desert: The Late Ordovician record

Abstract

Detailed outcrop studies at the flanks of Al Kufrah Basin, Libya, reveal the nature of glacially-related sedimentation and post-depositional deformation styles produced in association with the Late Ordovician glaciation, during which ice sheets expanded northward over North Africa to deposit the Mamuniyat Formation. At the SE basin flank (Jabal Azbah), the Mamuniyat Formation is sand-dominated, and incises interfingering braidplain and shallow marine deposits of the Hawaz Formation. The glacially-related sediments include intercalations of mud-chip bearing tabular sandstones and intraformational conglomerates, which are interpreted as turbidite and debrite facies respectively. These record aggradation of an extensive sediment wedge in front of a stable former ice margin. An increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites. A widespread surface, bearing streamlined NW–SE striking ridges and grooves, punctuates this succession. The structures on the surface are interpreted to have formed during a regional north-westward ice advance. Above, siltstones bearing Arthrophycus burrows, and Orthocone-bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath. By contrast, the northern basin margin (Jabal az-Zalmah) is appreciably different in recording shallower water/paralic sedimentation styles and major glaciotectonic deformation features, although facies analysis also reveals northward deepening. Here, a siltstone wedging from 8 to 50 m toward the north was deposited (lower delta plain), succeeded by climbing ripple cross-laminated sandstones up to 60 m in thickness (distal through proximal delta mouth bar deposits) with occasional diamictite interbeds. These rocks are deformed by thrusts and > 50 m amplitude fault-propagation folds, the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation. Integrating observations from both basin margins, a model of fluvial-dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed.

Introduction

The sedimentary record of the Late Ordovician is extensive in North Africa. In recent years, intensive efforts to unravel the sedimentology and stratigraphy of Hirnantian glacial deposits have been made, most particularly in western Libya, subsurface Algeria, and at outcrop in Morocco (e.g. Ghienne et al., 2007, Le Heron et al., 2007, Le Heron and Craig, 2008). As a sand-dominated package, Late Ordovician glaciogenic deposits are of prime interest in eastern Algeria and western Libya as a hydrocarbon reservoir within a proven petroleum system that is sourced and sealed by lowermost Silurian shale (Le Heron et al., 2009).

In North Africa, the easternmost Late Ordovician glaciogenic deposits yet described occur on the Gargaf Arch, in western Libya (Ghienne et al., 2003, Le Heron et al., 2004). On the Arabian Peninsula, the westernmost occurrence of glaciogenic rocks is on the eastern flank of the Arabian shield in the Al Qasim district (Clark-Lowes, 2005). Consequently, there exists a 2500 km wide data gap between W Libya and NW Saudi Arabia where there is sparse evidence for Late Ordovician glacial deposits. Limited data to date have been published documenting the presence of streamlined subglacial bedforms and striae on Jabal az-Zalmah at the northern Al Kufrah Basin margin (Le Heron and Craig, 2008), but no further data have been forthcoming. The objective of this paper, therefore, is to present a high quality sedimentological dataset for Al Kufrah Basin, with data collected from outcrops at the northern and eastern basin margins (Fig. 1). Our goal is to evaluate the nature of Late Ordovician glaciation and deglaciation in the eastern Sahara.

Al Kufrah Basin occupies an area of approximately 400,000 km2 of the eastern Sahara, mostly in SE Libya but extending also into NE Chad, NW Sudan and SW Egypt. The basin is currently a frontier basin for hydrocarbon exploration. It forms a broadly NNE-trending depression that is bounded by four uplifted basement highs, the Tibesti Massif in the west, the Jabal Aweinat Massif (including Jabal Azbah) in the east, the Ennedi and Borkou mountains in the south, and Jabal az-Zalmah in the north (Fig. 1). Outcrops of Lower Palaeozoic strata at these locations reveal the nature of the sedimentary fill of the basin. As part of a potential petroleum system sourced and sealed by Silurian shales, Late Ordovician glacially-related deposits have already proven to have been a major hydrocarbon reservoir across western Libya in the northern Murzuq Basin (e.g. El Feel/Elephant Field: Craig et al., 2008) and in eastern Algeria (e.g. the Tiguentourine/In Amenas fields: Hirst et al., 2002). Therefore, using the central Saharan basins as an analogue, these “glaciogenic reservoirs” are also expected to be of major importance in Al Kufrah Basin. Consequently, a detailed evaluation of the stratigraphic architecture of Late Ordovician strata in this region is of great commercial as well as academic interest.

The academic significance of these strata hinges on the fact that reconstructing the behaviour and dynamics of former ice masses such as the Late Ordovician Saharan ice sheet (Le Heron and Craig, 2008, Le Heron and Dowdeswell, 2009) provides critical boundary data that can be fed into palaeo-climate models. Palaeogeographic models, or gross depositional environment maps, are underpinned by fundamental sedimentological data collected at outcrop. Studies of ancient glacial deposits from elsewhere have proposed that the aerial extent of the glacial sedimentary record may even be inversely proportional to the dimensions of the ice sheets that created them (González-Bonorino and Eyles, 1995). Therefore, the collection and interpretation of new sedimentological data from ancient glacial successions remain pertinent and integral in Palaeozoic palaeoclimatic research.

The impetus for geological research in Al Kufrah Basin has been provided, to a large extent, by the Libyan National Oil Company (NOC) leasing acreage in the basin, aggressively from 2005 onwards. Aside from two AGIP wells drilled in the late 1970s/early 1980s, a hiatus ensued before drilling recommenced recently first with Remsa shallow boreholes in Jabal al-Hawaish, an exploration well of RWE in 2007, and finally with the StatoilHydro well spudded in late November 2008. Geological research in Al Kufrah Basin has advanced rapidly in recent years by a joint mapping project between Egyptian and Libyan authorities. This collaboration produced 1:200,000 geological maps for the south-eastern part of the basin in the Jabal Arkenu, Jabal Rūkn and Jabal Azbah areas (Saïd et al., 2000). In these areas, the names of most stratigraphic units in the Lower Palaeozoic derive from type sections at the flanks of the Murzuq Basin in western Libya, located either in Wadi Tannezuft (e.g. Tannezuft Formation; Radulovic, 1984) or the Gargaf Arch (e.g. Hasawnah Formation, Hawaz Formation, Mamuniyat Formation; Parizek et al., 1984, Gundobin, 1985).