The palynological study of the Silurian and Lower Devonian strata in twenty-five petroleum boreholes in the western part of the Great Erg has led to a biostratigraphic zonation into four zones and nine sub-zones. The fossils collected in these boreholes have been used to attempt the correlation of these zones to chronostratigraphic units (from Middle Llandovery to the Siegenian).
The geographic extension of each zone shows the approximate palaeogeographic history of these deposits and in particular demonstrates an important regressive phase from the centre of the basin towards the north.