Gafsa Basin is one of the most geologically investigated areas in southern Tunisia. During the Late Paleocene-Early Eocene interval, the platform exhibits a complex topography, organized into horsts and grabens and showing clear variations in terms of facies and thickness. This distribution is under the control of halokinesis and the reactivation of Gafsa Fault. Sedimentological approaches, diagenetic processes, depositional environments, and sequential evolution have rarely been discussed, especially, within the Western part of the Gafsa Basin. Six sections were surveyed in the Tamerza area (Western part of Gafsa Basin). Detailed analysis of facies, based on the pick of washed marls and clays, microscopic investigation, geochemistry of trace elements and clay mineralogy was carried on these deposits. These varied approaches allow us to characterize 20 depositional sequences within Late Paleocene –Early Eocene interval. In fact, we’ve recognized 7 sequences within the Thelja Formation, 6 within the Chouabine Formation and 7 within the Kef Eddour Formation. These sequences are correlative through the studied area. The characterization of the sequential sequences was at the base of accurate datations of the Danian-Thanetian and the Thanetian- Ypresian transitions, from previous works. The comparison of the sequences with the global charte of Snedden and Liu (2010) shows correspondences across third order sequences.
Sea level fluctuation, recorded in the studied area, allow us to define 20 depositional sequences. The sequences SI, SIV, SV, SXVI and SXVII correspond to major eustatic fluctuations, respectively, Th1, Th4, Th5, Yp10 and Lu1. However, SII, SIII, SVI, SVII, SVIII, SIX, SX, XI, SXII, SXIII, SXIV, SXV and SXVIII, correspond to minor eustatic fluctuations represented, respectively, by Th2, Th3, Th6, Yp1, Yp2, Yp3, Yp4, Yp5, Yp6, Yp7, Yp8, Yp9 and Lu2. The two sequences SXIX and SXX correspond to medium eustatic fluctuations, respectively, Lu3 and Lu4.
A particular attention is paid to both SII and SIII which are corresponding to sebkha deposits. On another hand, SVI, SVII, SVIII, SIX, SX, XI, SXII, SXIII, SXIV, SXV and SXVIII are corresponding to repetitive sequences made of the succession of three main terms represented by marls, phosphates and carbonates along a shallow marine platform.
Vertical evolution of facies and the organization of deposits along the studied section permit us to characterize a long term trend of sea level fluctuations. Indeed, the sharp transition from Late Paleocene evaporite sequences to Early Eocene carbonates and phosphates, mainly, rich in foraminifera deal with a major transgressive event.
