Phosphate genesis and concentration a response to sea level fluctuation in shallow marine environments of the lower Eocene deposits in the southern Tethyan margin: case study of the Gafsa Basin, southern Tunisia

摘要

Detailed analysis of the Chouabine Formation exposed in the western part of the Gafsa basin, southern Tunisia shows the presence of six principal facies. Vertical and lateral evolution of facies showed a gradual transition from intertidal to circatidal environments summarized in a carbonate ramp model: homoclinal ramp under the control of synsedimentary faults and, episodically, swept by upwelling currents. Phosphogenesis indicates that early diagenetic organic matter degradation can be a source of authigenic phosphate during the first stage of transgression. The concentration of phosphate was under the control of hydrodynamic processes as testified by angular unconformities, erosive surfaces, current structures and reworked clasts. Phosphate genesis seems under the control of sea level changes. It begins with the upper LST where diagenetic processes allow the dissolution of fauna remains (bones and fish teeth) and a prelude precipitation of authigenic phosphates. During the TST stage, transgression was associated with upwelling currents, which enable faunal blooming. Phosphatization, which requires suboxic conditions, occurs in shells or in troughs, preferentially in infratidal to circatidal environments. During the HST, the shift of the sea level leads to a sea level fall and, thus, to a basinward migration of depositional environments. Accordingly, infratidal environment becomes intertidal. A detailed analysis of facies in the field shows the fossilization of current structures (cross-bedding structures capped by planar laminations and sheet deposits). Tidal currents sweep the deposits and allow phosphate concentration and deposition in troughs. The model proposed does not necessitate major rises and falls of the relative sea level to produce economic phosphorite, but emphasizes the interplay of both autocyclic and allocyclic controls to form phosphate strata during the highstand system tract.