Factors controlling sedimentation and sequence stratigraphy evolution in shallow marine (carbonates) platform: example of Middle Eocene deposits from Gafsa Basin

摘要

Based on their facies characteristics and stratal geometries, the carbonate deposits of the Middle Eocene outcropping in the Tamerza area (Western Gafsa Basin) led to the recognition of nine facies that were deposited in shallow marine environment summarized in a carbonate ramp model: homoclinal ramp under an arid climate. The depiction of facies features and the sequential analysis help to determine the depositional environments and establish the linkage between facies evolution, eustasy, climate and tectonics. The vertical evolution of facies shows a regressive trend that records a transition from infratidal to supratidal environments. The stacking pattern revealed several kinds of meter-scale, shallowing-upward cycles. Each cycle starts with relatively open-marine deposits, which are overlain by shallower and more restricted facies (tidal flat progradation). The varied facies, which are described within the Kef Eddour Formation, compose seven depositional sequences, mainly made of carbonates, marls and phosphates. Fauna associations and sedimentary structures imply a shallow subtidal to intertidal, moderately to high-energy platform. Seven sequences are interpreted as depositional sequences showing retrogradational (lowstand systems tract), aggradational (transgressive systems tract) and progradational (highstand systems tract) packages of facies associations. The seven depositional sequences which show a hierarchical organization of many cycles, as described above, suggested that eustatic sea-level oscillations caused by cyclic perturbations of the Earth's orbit played a fundamental role in determining the formation of hierarchical cyclic rhythmicity.