Comment on ‘Geodynamic evolution of Jbel Cheid (Northern Tunisian Atlas) from geophysical and geological data’ by R. Benassi (2011)

Abstract

Benassi (2011) provides a valuable contribution and much needed data (gravity measurements, edge enhancement of Bouger anomalies and 2.5D forward modelling of geologically realistic profiles) regarding the problem of the geodynamic evolution of Jebel Ech Cheid salt body. However, when we viewed the previously published and the new gathered data, we believe the evidence better supports folded sheets of allochthonous salt rather than diapiric walls. For elucidating the geodynamic evolution model of Jebel Ech Cheid salt body (Fig. 1), R. Benassi (2011) supposed an unfractured substratum (Fig.7 of R. Benassi (2011)) which is completely different of the previous interpretations (Ben Slama et al. 2008; Ben Slama et al. 2010; Ben Slama 2011). This structural problem has been a subject of discussion and heated debate between different research groups in the neighbouring salt structures of Jebel Ech Cheid (Jallouli et al. 2005; Talbot 2005). This discussion focuses on the structural and the gravimetric responses of the Jebel Ech Cheid salt body published by R. Benassi (2011). Taking into account many facts (rock densities, geological data, etc.), we believe that in his interpretation R. Benassi (2011) has completely ignored the average rocks densities published by Japan and Metal mining (2001) which can greatly influence the gravimetric responses and consequently the interpretation (Table 1). The laboratory test published by Japan and Metal mining (2001, p.43) and resulting from several samples within the Jebel Ech Cheid salt structure for the Quaternary, Tertiary, Cretaceous and Triassic deposits. The problem is that R. Benassi (2011) has used an average density of the Cretaceous series (2.31 g/cm), which is very close to the average density of the Triassic material (2.32 g/cm) deduced from the values presented in Japan and Metal mining (2001) report. While the gravity map shows (Fig. 2) the variation of the gradient density between the Triassic complex and enclosing series is net (Fig. 2). In addition, the values of average densities of Jurassic, Cretaceous and Tertiary have almost the same values of mean density (2.57 g/cm) which implies a net contrast density between Jurassic, Cretaceous and Tertiary series and Triassic density which is illustrated in the gravimetric map (Fig. 2). Nevertheless, in our analysis we discovered an important dissolved Halite mass coming from the principal salt structure (Ben Slama 2011) (Fig. 3). The salt material of Jebel Ech Cheid has a low-density, probably originated from a shallow level (≃100 m) such as in other salt bodies of Northern Tunisia, also we pointed out the existence of many salt springs around the salt structure which indicate the low density of Triassic complex (Vila et al. 1999, 2002; Ghanmi et al. 2001, M.-M. Ben Slama (*) :M. Ghanmi : F. Zargouni U.R. 11 E.S. 13, Geomatique, Geologie structurale et Appliquee, Faculte des Sciences de Tunis, 2092 Tunis, Tunisia e-mail: mohamedmontassar.benslama@fst.rnu.tn