J. P. P. Hirst¹ and A. Benbakir²

¹BP Amoco Exploration, Sunbury, U.K.
²SONATRACH, PED, 8 Chemin du Reservoir, Hydra, Algiers, Algeria

ABSTRACT: Turbidite Channels and Dune Structures, Late Ordovician, Algeria

The Late Ordovician (Ashgillian) glacial event in SE Algeria was a period when lithologically varied mass flow sediments were deposited in a proglacial, marine setting. Excellent exposures in the south Illizi Basin (Tassili N’Ajjer region) allow the study of the external geometry and internal facies of both amalgamated, sheet-like and isolated, channelised turbidite sandstones. Sedimentary structures within these geometrically distinct deposits are dominated by dune forms which often have rounded tops and a symmetrical form.

The Late Ordovician glaciation was a short lived event which affected large areas of the African craton; during this time, the palaeo-south pole was situated close to the study area. Erosion at the onset of the glacial advance resulted in a complex topography of palaeovalleys being scoured into the underlying succession. Considerable variation in the infill of these depressions is noted both at outcrop and in the subsurface. Locally derived blocks from the valley margin sequences are found in some depressions as are poorly sorted intervals of coarse sands and gravels which were deposited in proximity to the ice front. However, the dominant depositional process was one of mass flow resulting in a range of debris flows (diamictites) and turbidite sandstones. These were interspersed with pelagic sediments which include dropstones; rare graptolites suggest a marine setting.

As this topography was infilled, depositional events became more widespread. Towards the top of the syn-glacial succession, an amalgamated complex of sheet-like turbidite units is widely recognised (Grès à Rides Unit); in detail, the sheet-like components tend to vary in thickness laterally and are locally channelised. This amalgamated complex was abruptly succeeded by a low net to gross interval of channelised turbidites. Preferential erosion of the sediments encasing the channel fill sandstones has resulted in excellent plan-form outcrops enabling their orientations and sinuosity to be studied; in rare instances, cross-cutting relationships are noted. Although the channels were sinuous, lateral migration does not appear to have been a feature. The orientation of the many channel fills exposed indicates a consistent palaeoslope towards the north north-west.

Sedimentary structures within both the sheet complexes and the channels are mostly dunes; many of these dune forms are near symmetrical with typical wavelengths of 2 to 2.5m and amplitudes of several 10’s centimetres. Examples of asymmetrical forms with slip faces are uncommon but tend to be of similar dimensions. Crests of these structures are straight to gently sinuous and nonbifurcating. Typically vertical accretion was steep (climbing dunes); harmonic stacks of symmetrical dunes without erosional cross-cuts are observed over thicknesses of >3m. The presence of well formed channels suggests depositional events were of relatively long duration, possibly driven by subglacial outflows of sediment charged melt water. The thick, harmonic stacks of dune forms point to steady, depletive flows and the symmetry and crest rounding of many of the structures may be associated with high sediment transport rates.

The observations made at outcrop have been extrapolated into the subsurface some 250kms to the north where many of the features can be recognised; reservoir quality and geometry is controlled by glacial topography and the varied nature of the syn-glacial infill.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado