A giant Late Triassic–Early Jurassic evaporitic basin on the Saharan Platform, North Africa

Abstract

This paper describes the Late Triassic and Early Jurassic evaporites of the Berkine/Ghadames Basin in eastern Algeria and western Libya, which is important because it forms the regional seal to a number of hydrocarbon reservoirs in the Triassic Argilo-Gréseux Inférieur. This evaporitic succession spans the time interval Norian to Late Liassic and is divided into three contrasting units (S1, S2 and S3). S1 consists of five evaporitic cycles dominated by mudstone and halite, which represent the main salt deposits of the Berkine Basin. At this time the Berkine/Ghadames basin was a restricted evaporitic basin with a barrier to the north (Medenine High) separating the basin from the developing peri-Tethys and North Atlantic. The succession thickens to the west, thins around the Hass Messouad Ridge and continues across the Maghreb into the western High Atlas of Morocco, a distance of some 1500 km. S2 is represented by the basin-wide development of a carbonate platform in Pliensbachian times. This is the product of a relative sealevel rise concomitant with the developing extension of the region and may reflect the opening of an Atlantic gateway. S3 comprises five cycles that are predominantly mudstones, fine-grained carbonates and anhydrites. Mounded structures, evident on seismic sections, may represent the development of patch reefs and the whole succession is interpreted as a shallow-marine carbonate–evaporite ramp. Similar thickness variations to those observed in Sequence 1 continue through Sequence 3 and reflect the on-going influence of differential subsidence along basement lineaments. A number of factors controlled the development of the evaporite basin. These include Late Triassic sealevel rise and flooding of a sub-sealevel basin, combined with globally warm climatic conditions and on-going extensional tectonics in the peri-Tethyan and Atlantic areas. Restricted circulation was caused by an east–west oriented sill to the north (Medenine High in central Tunisia) and the basin was episodically refilled as global sealevel rose during the Early Mesozoic and extensional tectonics continued.

<sup>17</sup>O excess traces atmospheric nitrate in paleo-groundwater of the Saharan desert

“…Dietzel and Kirchhoff, 2003). Thus, from the solution chemistry of the groundwater as well as from the known occurrence of evaporites on the Saharan platform (Turner and Sherif, 2007), and occasionally documented gypsum in core samples from Hasouna well fields (Sahli, 2006), we reasonably assume salt dissolution to be the dominant source of the dissolved SO 2− 4 . This conclusion is also supported by the above-mentioned narrow range of stable isotope data with respect to SO 2− 4 , with no correlation of isotope data and SO 2− 4 concentrations (Fig.…”

From Neoproterozoic to Early Cenozoic: exploring the potential of older and deeper hydrocarbon plays across North Africa and the Middle East

“…These include, at the base, the important Lower Triassic fluvio-deltaic reservoir sandstones of the TAGI/TAGS in the Algerian portion and the broadly equivalent Ouled Cheb and Ras Hamia formations in the Libyan and Tunisian portions of the Ghadames Basin. The early postHercynian sedimentation in the Oued Maya, Berkine and Ghadames basins of eastern Algeria and western Libya was characterized by the deposition of a complex system of Triassic fluvial, aeolian and sabkha sediments across the subtle erosional topography of the Hercynian unconformity surface with regional drainage towards the NNE into the developing Neo-Tethys rift (Turner et al 2001;Turner & Sherif 2007). Sedimentation and associated volcanism during the Late Triassic and Early Jurassic was increasingly influenced by NE -SW trending extensional faults.…”

Integrated petroleum systems and play fairway analysis in a complex Palaeozoic basin: Ghadames-Illizi Basin, North Africa

“…The Djebel Tebaga -Medenine High continued to be an important structure throughout the Jurassic, although it is unlikely ever to have been elevated above sea-level. The Jurassic sequences thin over it (Benton et al 2000;Turner & Sherif 2007) and there are some important facies changes from south to north across it, but it seems likely that the Jurassic was only removed by later (Cretaceous -Aptian) erosion.…”

“…Post-rift thermal subsidence after Early Triassic rifting created a broad area of Late Triassic and Jurassic evaporitic deposition in a basin that also extended northwestwards into the Moroccan Atlas (Turner & Sherif 2007). Up to 1250 m of evaporitic facies accumulated in the basin, typically with halite in the centre of the basin grading outward through anhydrites to interbedded dolomites and shales toward the edges of the basin (Turner & Sherif 2007).…”

“…Up to 1250 m of evaporitic facies accumulated in the basin, typically with halite in the centre of the basin grading outward through anhydrites to interbedded dolomites and shales toward the edges of the basin (Turner & Sherif 2007). To the north of the Medenine High Mid Triassic evaporites are also present (Acheche et al 2001) and are widespread across the Mediterranean, but the Jurassic is dominated by platform carbonates (Soussi & Ben Ismail 2000).…”