Variation of the Descriptive Grain Size Distribution Along Cores From the Sebkha of Sidi El Hani, Eastern Tunisia

Introduction

Since the saline environment of Sidi El Hani is sedimentologically a clay pan (Essefi, 2009), sediments grain size in its cores is generally fine; it ranges mainly between Ф≥9 (less than 2 µm) and Ф=-1 (2mm). Even though traditional methods of classification (e.g., Cailleux & Tricart, 1962) give an idea about the origin (e.g., the cumulative curves of Cailleux & Tricart (1962) are divided into aeolian, deltaic, etc..), they are still far from being efficient to classify sediments within cores where slight modifications in the grain size distribution are still undetectable and the interplay between different depositional environments risks to be interpreted as one same context. Thus, these classification schemes could not distinguish such a fine grain size distribution. The task, then, becomes more complicated when trying to correlate between cores. Consequently, to do an efficient study of the grain size distribution and a coherent correlation between cores, it is a must first to calculate weights of the fine grains by a statistical extrapolation and then to find the appropriate method of classification; which may be a genetic or a descriptive classification.

As for the descriptive classification, a good few of new or modified approaches has been proposed or investigated, e.g., a simplified classification of intertidal Wadden Sea sediments on the basis of sand/mud ratios (Reineck & Siefert, 1980), a hydrodynamically based classification of fine-grained estuarine sediments using sand/silt/clay ratios (Pejrup, 1988) and a classification for coarse-grained sediments based on gravel/sand/mud ratios (Blair and McPherson, 1999); but differences between the adopted descriptive systems of classification such as the difference between Schepard (1954) and Folk (1966, 1971, 1974), impose the question about their usefulness. Genetically speaking, the genetic approach of Cailleux and Tricart (1962) has always been trusted to infer the depositional environment of any sample; but none can deny that the comparison has always been based on a visual similarity between cumulative curves of the studied sample and these reference curves. Actually, an as yet open debate is still between geologists whether to use descriptive or genetic classification. On the one hand, Rodgers (1950) made it clear that a descriptive, objective and precise classifications and nomenclatures in sedimentology are ideally adequate rather than a genetic distinction. On the other hand, since the interpretation of sedimentary deposits looks for reconstructing depositional processes, environments and facies, it may be actually desirable for classifications to have genetic basics (Folk et al., 1970). In this study, the descriptive method was tested. In order to solve the problem of having fine grain size, this method followed two lines of attack: Inclusive Graphic Statistics (IG) and Moments Statistics (MM). Eventhough it has given special care to methods of classification, this study has not deviated from its primary purpose of investigating the sedimentary dynamics in Sidi El Hani clay pan. Accordingly, the use of these methods permitted correlations between Sidi El Hani cores, which in turn permitted the identification of the sedimentary dynamics in different areas from Sidi El Hani clay pan.