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
Аннотация: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.
