Abstract

Rhythmic bedding forms one of the most characteristic features of the Late Permian in southern Tunisia. For almost a century, these bedding cycles have been interpreted to be only a part of reefal complex structure. These bedding cycles have been also identified in several neighboring wells, as well as in Libyan Jeffara, with different depositional environmental settings, interpreted as a climate variations controlled by cyclic variations in the Earth's orbit. A great variety of bedding cycles, from centimeter to meter-scale, have been described in outcrops and cores. However, exploration geophysical well logs are rarely used for this purpose, although these allow the study of thick successions of Middle-Late (either to early) Permian at decimeter-scale resolution and are widely available in the Jeffara Basin (Southern Tunisia). Therefore, a frequency analysis has been carried out of the Permian interval in well logs of four (04) available boreholes from the Jeffara basin sector. The interval contains cyclic intervals in most of the areas that underwent gradual subsidence during deposition of the Permian. In contrast, predominantly chaotic log responses are encountered in the Southwestern of the basin (northern limit of the Bou Nemcha-Telemzane complex high, BTC). Based on the calculated frequency spectra, a selection of logs showing pronounced cyclicity was made possible and used in order to identify the astronomical periodicity type represented in the sediments (i.e. eccentricity, obliquity or precession cycles). Alternatively, the Permian interval contains cycles which could be interpreted to represent 100 ky eccentricity and 40 ky obliquity Milankovitch cycles. A gradual decrease of cycle wavelength can be traced along the Roadian–Dzhulfian (and either Changhsingian) interval of several wells that probably reflects a regional decrease in primary carbonate production during this period. In this study the Permian system, in Tunisia, is revised for the first time as it is assigned to the international system nomenclature (standard chronostratigraphy). Using time series analysis, sedimentation rate and duration intervals have been also calculated.

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