Channelized sandstone bodies (‘cordons’) in the Tassili N'Ajjer (Algeria & Libya): snapshots of a Late Ordovician proglacial outwash plain

Abstract

The architecture, distribution and development of channelized sandstone bodies are described from Late Ordovician paraglacial successions of the Tassili N'Ajjer (SE Algeria and SW Libya) based on satellite images and field data (sedimentary logs, photomosaics). Sandstone bodies have a ribbon-like form at outcrop (often referred to as ‘cordons’ in the literature). They typify a fluvioglacial outwash plain deposited between a continental ice front and a marine delta-front zone. Channelized sandstone bodies are straight to sinuous, with widths (W) in the 60–600 m range, thicknesses (T) in the 5–30 m range and they have a mean W/T ratio of 16.5. They develop within an aggradational–progradational sand-dominated deltaic topset succession including at its distal end a terminal distributary channel and mouth-bar environments. The architecture of channel bodies and the related depositional facies, which includes climbing-dune cross-stratification, indicates that channelized sandstone bodies represent plugs of isolated channels related to high-magnitude flood events (glacier outbursts). These plugs form fossilized networks of both braided channels and interlaced anastomosed channels, offering snapshots of an outburst-related unconfined proglacial outwash braidplain constituted by the amalgamation of adjacent, elongated outwash fans.

Glaciogenic reservoirs and hydrocarbon systems: an introduction

“…New outcrop work and new core results from In Amenas that strengthen this interpretation are presented by Hirst (2012). Girard et al (2012) interpret the 'cordons' as fluvial channel plugs deposited on a subaerial outwash plain, supporting the original interpretation of Beuf et al (1971). We recommend that readers consider both authors' arguments and make up their own mind concerning the environmental setting of these fascinating sandbodies.…”

“…Analogues for the reservoirs filling deeply buried Ordovician tunnel valleys in North Africa and the Middle East can be found in the coeval deposits that crop out in the same regions Douillet et al 2012;Girard et al 2012;Hirst 2012). Studies of these outcrops aim to understand the facies associations and stratigraphy, providing insight on the genetic and infill processes associated with these glacial incisions (e.g.…”

“…The Hirnantian glaciation is clearly recorded in the stable isotope signature of low-latitude carbonates (HICE: Delabroye & Vecoli 2010). The glacial record of the Hirnantian maximum is well expressed in parts of the Gondwana supercontinent, particularly in North Africa, Arabia and South Africa, where extensive, laterally continuous outcrops in Saudi Arabia (Keller et al 2011), Jordan (Douillet et al 2012), Algeria (Girard et al 2012;Hirst 2012) and Libya Girard et al 2012) allow detailed investigation of the sedimentology and facies of the glacial succession. Boreholes drilled for petroleum exploration also provide high-quality subsurface data sets in these areas (e.g.…”

“…Analysis of core samples from the subsurface glaciogenic sequences of the In Amenas area reveals some striking similarities with facies observed at outcrop in the Tassili N'Ajjer (Hirst 2012). The papers by Hirst (2012) and Girard et al (2012) both provide detailed photographic and descriptive records of the Algerian Tassili N'Ajjer outcrops, which are remote, mostly inaccessible by road, and subject to local security restrictions.…”

“…First described by Beuf et al (1971), this unit comprises several cross-cutting generations of sinuous channels in a finer-grained background lithology. The finer matrix is typically deflated, leaving the more resistant sandbodies standing out in bold relief (see satellite images in Hirst 2012; Girard et al 2012). Two alternative interpretations have been proposed for the 'cordons'.…”