Docum. Lab. Géol. Lyon, 2002, n° 156

243 SEDIMENTARY SEQUENCES, SEISMIC STRATIGRAPHY AND VELOCITY STRATIGRAPHY : AN INTEGRATED APPROACH IN STUDY OF THE UPPER CRETACEOUS BASINS, CENTRAL ATLAS OF TUNISIA

A several possible controls were used to follow the evolution of Tunisian Atlas basins. The first step in constructing and comparing histories of the northern and southern basins of Gafsa and the accommodation zone that lies between them has been the establishment of an interbasinal stratigraphy that reveals the evolution of central and southern Atlas of Tunisia. This stratigraphy is based on the seismic data, which are calibrated to wells and surface geologic studies collected in the area. A seismic stratigraphic scheme was developed within each basin by comparison of reflection geometries and lapout patterns. The construction of this interbasinal stratigraphic development is checked for consistency using the approximate synchrony of fault movement and basement subsidence within the study area. The stratigraphies of the northern and southern basins are critically dependent on the sequence development within and near the accommodation zone that separates the two basins where sediment supplies offset the subsidence rate were nearly identical. There are strong differences in facies and relative rates of basin infilling through the basins of Gafsa. The northern was initially sediment starved. In the Souinia-Majoura area, the development of the sequence boundaries separating the Zebbag and Aleg Formations appears to have been controlled by uplift during deposition of sequences 3 and 4. Also during the same period, the sediment flux decreased significantly in the area of Kasserine Island. Toward the strike-slip of Gafsa, a thicker sediment of several sets of Top-lapping and prograding clinforms indicative of relatively shallow-water deposits. However, the southern basin had a much greater initial subsidence as well as a greater thickness of basin fill composing the sequence sets. The structures define six distinct sequences sets were defined by thickening up toward the western border fault of Gafsa. The eastern edge of the basin is highly faulted and has a relatively thin cover of onlapping sediments. The results of the subsidence analysis suggest that the geometrical evolution as well as the formation of the six sedimentary sequences were primarily controlled by the interaction between tectonic subsidence and sediment supply. The development of the Upper Cretaceous Formations was caused by an enhanced subsidence rates in the distal reaches. Furthermore, because the sediment flux increased simultaneously, shallow-marine condition prevailed during this time interval. However, the flooding surfaces separating S2-S3, S3-S4, S4-S5 and S5-S6, could be the result either of a short-term decrease of the sediment flux at a constant sea level or of an enhanced increase of the eustatic sea level at a constant sediment supply rate. Transgressions of the Aleg and Abiod Formations appear to have been caused by an increase of the basin width by renewed subsidence in the distal part of the southern basin of Gafsa. During the same time interval, however, no subsidence is recorded at the Souinia-Majoura area. The main inference from the sediment distribution and sequence stratigraphy is that the Upper Cretaceous sedimentation volumes and rates must have increased significantly in the depressions. Moreover, well data and surface geologic controls suggest coeval changes in depositional environment throughout the Gafsa basin. Because the control is poor and sediments are buried, we turn to seismic velocity as an indicator of basin-wide changes in lithology and physical sediment properties. Expanded spread and seismic profiles in the Gafsa basins have recorded a continuous increase in Upper Cretaceous sediment velocity with depth. To further evaluate regional trends in the lateral and vertical velocity distribution, we have analyzed interval velocity through the seismic lines. The analysis, illustrated by a margin transect in a relatively simple depositional and structural setting, reveals : (1) a division in to three velocity zones ; (2) a continuous increase in velocity with depth in units of both the northern and southern basins of Gafsa area and (3) a velocity inversion below the base of the Late Cretaceous horizon. We ascribe the vertical velocity-depth increases within the main sediment units to overburden as compaction reduces the porosity. The lateral changes probably reflect lithology of sediment lapout. The interpretation of seismic-velocity data within a seismic-stratigraphic context has the objectives to accurate estimation of interval velocities for individual seismic sequences and application of these estimates to prediction of subsurface parameters by comparison of velocity maps with seismic facies and structure maps and with other seismic parameters such as amplitude and continuity. All of these concepts are embodied in a systematic procedure using sequence stratigraphy, seismic stratigraphy, seismic velocity and well data to map