oa Climate Sequence Stratygraphy Unraveled by the Analysis of the Clay Content Evolution

Case Study - The Triassic Formation of the Hassi R’mel Field (Algeria). In the sedimentary recordings, clays, if inherited can give a fundamental a reliable and fundamental indication on the continental contributions. The examples studied come from central sahara wells of various basins. The triassic shale sandstone formation of the central sahara is about 200 meters thick, made of two series of the shale sandstone separated by volcanic rocks. The clay fraction was studied by using diffractometer and the following results were obtained: - from the point of view of the clay fraction constitution a) in oued mya basin the principal minerals are illite, chlorite and mixed layers; b) on the other hand in the berkine basin, there is an association of illite, chlorite mixed layers and kaolinite -from the point of view of the characteristic of clay: during the interpretation of the diffractogramms and by superimposing them we remark that: the intensity of illite peak (I001) and chlorite peak (I002) vary considerably with the depth. The peak variation does not occur at the random. In fact, as observed the intensity of the illite peak (I001) decrease from the bottom to the top in a metric level or profile. Thus, it was observed that at the bottom of the profile, illite shows a high intensity (I001) peak, with a good cristallinity, and as one goes up in the profile(i.e.a decrease in the depth) the peak intensity becomes weaker and sometimes, nothing can be noticed on the diffractogramm. The explanation of the loss of the intensity is not due to the reduction in the partiules but to the worse cristalline organisation of the crystalline system. Indeed the climate which characterized the triassic period is arid or semi arid, he is marked by alternation of wet and dry period. The state of the clay fraction, which depends of the source of the sedimentary material that is homogenous in the sedimentary environment system. In period of dryness, the evaporation is intense, concentrates the solutions which become more agressive and start to initially deteriorate the sedimentary unit with the contatc water sediment then penetrates in the sedimentary unit according to its permeability. This proves that the substance analysed is amorphous by XRD, revealing thus a high weathering of the clay fraction. This decrease in the intensity of the I001 peak of illite from the bottom to the top of the level is spectacular. This phenomenon is observed both in the clay fraction content in the sand and shale samples. The repetition of this phenomenon in the space et time and at the regional scale shows the regularity of the climate.This evolution of the clay fraction is similar to the present day pedogenetic one. Thus, degradations or tranformations by degradations seem to be an overall mechanism in the meteoric alteration and pedogenetic process. This permits us to define sections in the sequence called genetic sequences.