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

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.