Glauconitic-rich strata from Oligo-Miocene shallow-marine siliciclastic deposits of the northern margin of Africa (Tunisia): geochemical approach for basin analysis

Glauconitic minerals have long been appreciated as a reliable tool for sequence stratigraphic interpretation. A degree of maturity and occurrence of glauconite are closely related within a system tract context and substrate paleoenvironmental conditions. Within Oligo-Miocene shallow marine deposits of northern Tunisian outcrops, the glauconite occurs as thin (few centimeters) to moderate thick (1 m) clay and fine grain size sandstone horizons which are superbly exposed at many localities. Sedimentological investigations data indicate that these glaucony-bearing strata are deposited under shallow water shelfal and lagoonal siliciclastic depositional systems. Geochemical analysis from nine glaucony samples shows that Oligocene glaucony is invariably more evolved (K2O = 6–8 %) than Miocene glaucony, which is typically slightly evolved (K2O = 4–6 %). Vertical changes in glaucony maturity are consistent with sequence-stratigraphic interpretation, showing in general an upward increase in the transgressive systems tract. Maximum glaucony concentration is recorded corresponding to the maximum flooding surface/condensed section. Comparison of glaucony characteristics across different depositional systems at the same stratigraphic level shows a slight decrease in maturity and abundance from distal to proximal locations. This lateral tendency reflects more suitable conditions for glauconitization in open-marine environments than in shallow waters.