Glaucony-bearing horizons from shallow-marine siliciclastic deposits of the northern margin of Africa: a case study from the Oligo-Miocene of northern Tunisia

In spite of our rather comprehensive knowledge on the glauconitic minerals as a reliable tool for basin analysis, we have almost no data from the northern margin of the African continent. Oligo-Miocene shallow-marine deposits are widely exposed in northern, northeastern and central Tunisia. Particularly, northern Tunisian outcrops host numerous glaucony-rich strata, which are superbly exposed at many localities and provide good sites for the study of the green grains. Sedimentological data indicate that the Oligocene glaucony-bearing strata cropping out at Jebel Harb and Jebel Hajra Touila were deposited in shallow-water and shelf environments, respectively, whereas Miocene deposits at Jebel Ben Amara and Gorge de Kasseb are of shelfal and shoreface origin, respectively. Geochemical analyses of nine glaucony samples show that Oligocene glaucony is invariably more evolved (K2O = 6-8%) than its Miocene counterpart, which is typically slightly evolved (K2O = 4-6%). Vertical changes in glaucony maturity are consistent with the sequence-stratigraphic interpretation, showing in general an upward increase in the transgressive systems tract. Maximum glaucony concentration is recorded in correspondence of 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.