Sedimentary Structure and Sequence Stratigraphy of the Upper Paleocene-lower Eocene (Ayaycha Formation) from the Eastern Gafsa Basin, Southern Tunisia

Abstract

Detailed sedimentological and sequence stratigraphic analysis of the upper Paleocene–lower Eocene shallow-marine limestones exposed in the Eastern Gafsa basin in southern Tunisia provides a new insight into the sedimentary response to climate and sea-level changes revealed in the southern Tethysian margin. The 81 m thick Ayaycha Formation is composed of three units. The lower unit is built up of channelized fossiliferous limestones superposed by the sequence of alternated thin–to medium-bedded limestone and marls. The second unit is dominated by marls, and the third unit is formed by channelized limestones lying within bioclastic limestones. Based on their lithological features and strata geometries, the Ayaycha Formation exhibits seven facies evolving from offshore to intertidal environments. Clay paragenesis shows that the lower unit was accumulated under the warm and seasonally contrasting climate, or probably, repeated change of dry and humid seasons. The lower unit corresponds to the upper Paleocene deposits. The upper Paleocene-lower Eocene, middle and upper units were accumulated under warm climate and cover the time interval of global paleoecological crisis known as the Paleocene–Eocene Thermal Maximum (PETM). The consequence of this global warming was the sea level rise, which correspond to the transgressive pulse occurred prior to the Paleocene–Eocene boundary. Integrated sequence stratigraphic analysis showed that the Ayaycha Fm is formed by the stacking of thirteen third-order depositional sequences. Each depositional sequence results from a transgressive-regressive cycle in shallow marine environments.