The Late Ordovician deglaciation sequence of the SW Murzuq Basin (Libya)

Abstract

Rocks of Late Ordovician to Silurian age are well exposed on the western rim of the Murzuq Basin (Ghat-Tikiumit area, Libya) where seismic-scale exposures allow spectacular insights into the growth and decay of the Late Ordovician (Hirnantian) ice sheet.The ¢nal deglaciation left a complex topography with a combination of subglacial morphologies and proglacial depositional systems.This paper documents the glacial and proglacial palaeo -topography that controls the accumulation of a postglacial transgressive depositional system and the Rhuddanian (Early Silurian) shales.The glacial relief directly contributed to an important hiatus, with the Rhuddanian deposits at the base of the remnant glacial troughs being 3 Ma older than at the top of the topographic highs.The source-rock in the Murzuq Basin is of Early Rhuddanian age, so it is present only in the deepest part whereas geomorphic traps are formed within the highs of the relict postglacial topography.The transgressive system, recognised for its good reservoir potential, is considered to play a key-role in the petroleum system, linking the source rock deposited in the ancient topographic lows with the reservoir rocks in the topographic highs.This study aims to demonstrate the importance of palaeo -glaciological reconstructions for petroleum exploration of the Ordovician^Silurian in North Africa.

Did lingering ice sheets moderate anoxia in the Early Palaeozoic of Libya?

“…Gindre et al 2012), and the term Dohone Formation remains obscure, we use the term 'Tanezzuft Formation' in preference. The Tanezzuft Formation shows a diachronous base, and whereas it is no older than latest Hirnantian in SW Libya (Moreau 2011, and references therein) it is considerably younger on the Gargaf Arch in central Libya . This diachroneity reflects onlap onto a late Hirnantian glacial topography at the onset of post-glacial transgression (Moreau 2011).…”

“…Directly overlying Hirnantian glacial deposits, they were deposited following the recession of Late Ordovician ice sheets, which carved a series of palaeovalleys as they advanced northward (e.g. Le Heron Moreau 2011). The ice sheets grew to their maximum extent in the Hirnantian, which left a distinct yet only partially understood stable isotopic excursion in low-latitude carbonates known as the Hirnantian Isotopic Curve Excursion, or HICE (Delabroye & Vecoli 2010).…”

“…These shales include the widespread Tanezzuft Formation, which laps onto an inherited glacial topography (Moreau 2011, and references therein) and has a disconformable base that corresponds to a well-documented c. 3 Ma diachroneity. The Tanezzuft Formation is preserved in outcrop belts of the Saharan intracratonic depressions including the petroliferous Murzuq and Al Kufrah basins (Fig.…”

Glaciogenic reservoirs and hydrocarbon systems: an introduction

“…This region straddles southeastern Algeria and southwestern Libya. The Late Ordovician glaciogenic succession in the Libyan part of the Tassili N'Ajjer region has been studied in detail Le Heron et al 2006;Moreau 2011), but comparatively little has been published to date on the Algerian side since the landmark memoir of Beuf et al (1971), with the exception of Eschard et al (2005). Spectacular successions crop out in the Djanet -Illizi region, where the Late Ordovician succession has been subject to some limited study (Hirst et al 2002;Le Heron 2010b;Hirst 2012).…”

“…For example, we recognize glaciogenic reservoirs, deglacial source rocks, glaciogenic seals, glaciogenic deformation (glaciotectonics and glacial loading/unloading cycles), glacial sculpting of reservoir rocks or cold climate conditions leading to gas hydrate formation. Source rock deposition linked with glaciation tends to be favoured by remnant topographic relief linked with glacial sculpting and transgressive and high sea-level stands causing condensed organic-rich sediment accumulation (Lüning et al 2000;Craig et al 2009;Moreau 2011;. A common glaciogenic reservoir and source rock system is deposited following a glacial advance (lowstand), with subsequent deposition of thick clastic reservoir systems during the ice retreat phase.…”