The Cambrian-Ordovician succession in the Ougarta Range (western Algeria, North Africa) and interference of the Late Ordovician glaciation on the development of the Lower Palaeozoic transgression on northern Gondwana

 

Authors: Ghienne J-F, Boumendjel K, Paris F, Videt B, Racheboeuf P, Salem HA

Published in: Bulletin of Geosciences, volume 82, issue 3; pages: 183 - 214; Received 25 June 2007; Accepted in revised form 13 September 2007;

Keywords: Cambrian, Ordovician, North Gondwana, transgression, glaciation, sequence stratigraphy,

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Supplementary material

Appendix 1 (3070 kB)

Fig. 8. Facies and sequence stratigraphic interpretations in the Cambrian-Ordovician succession of the Ougarta Range


Appendix 2 (5917 kB)

Dating of the Cambrian-Ordovician succession in the Ougarta Range


 

Abstract

Depositional environments in the 1000-2000 m thick Cambrian-Ordovician succession range from inner-shelf siltstones to fluvial sandstones. Six transgressive-regressive (T-R), low-frequency (15-25 Ma) depositional sequences have been differentiated and correlated with both northern distal (Anti-Atlas, Morocco) and southern proximal (Ahnet, southern Algeria) areas. Sequence 1 (“Lower” Cambrian?) comprises fluvial deposits truncating volcanics and deformed Panafrican basement rocks. Marine strata appeared in Sequence 2 (lower and middle part of the “Middle” Cambrian) but fluvial conditions maintained to the South. Sequence 3 (upper “Middle” Cambrian) is erosionally truncated but further develops to the south, possibly comprising “Upper” Cambrian deposits. In Sequence 4 (Lower Ordovician), fluvial deposits are no longer present and inner-shelf siltstones reached southern Ougarta. Sequences 5 and 6, upper Lower-Middle Ordovician and Upper Ordovician, develop inner-shelf conditions throughout the study area at maximum flooding. Hirnantian glacial strata form a lowstand wedge in the upper part of the Sequence 6 regressive system tract. This succession designates a first-order transgression initiating in the Cambrian. It was temporarily perturbed just before its end by the Hirnantian glaciation. The related erosion led to a discontinuity, angular at basin scale, falsely suggesting a pre-glacial tectonic event. The post-glacial transgression, very fast relative to the Cambrian-Ordovician tempo, resulted in a drastic backstepping of the shorelines that shifted drastically to the south in the early Silurian. After deglaciation, the first-order inversion trend occurred between retrograding strata of the Cambrian-Ordovician succession and the prograding Siluro-Devonian wedge. This event marks in North Africa the maximum flooding of the Lower Palaeozoic megasequence.

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