First biostratigraphic (palynological) dating of Middle Cambrian strata in the subsurface of northwestern Algeria, North Africa (borehole AMG-1): implications for regional stratigraphy
Résumé
The first biostratigraphic (palynological) evidence of Middle to Late Cambrian sedimentation in the Algerian subsurface is presented and discussed in this paper, resulting from the palynological analysis of pre-Ordovician clastic sequences penetrated by borehole AMG-1 in northwestern Algeria. Well preserved and moderately diversified acritarchs occur in a transgressive sequence comprising the upper third of the Sebkhet el Mellah Formation and most part of the Aïn en Nechea Formation. This acritarch suite corresponds to the Cristallinium cambriense ƒ Eliasum/Timofeevia acritarch Superzone established in Morocco as well as to the Adara alea acritarch Zone of eastern Newfondland, both directly correlated to the Paradoxides paradoxissimus Trilobite Zone of Middle Cambrian age. This stratigraphic interval correlates clearly with the mixed carbonatesiliciclastic successions outcropping in the Tacheddirt Valley of the Anti Atlas, Morocco. The uppermost part of the Aïn en Nechea Formation is possibly attributed to the Late Cambrian, based on the absence of the typical Middle Cambrian species, and the presence of taxa of Timofeevia and Cristallinium, associated to an increase in abundance and diversity of diacromorphs acritarchs, suggesting that the Late Cambrian stratal sequences must be considerably reduced and/or partly eroded in the study area. These results provide the basis for improved correlation with the acritarch-bearing Middle Cambrian sediments of the Anti Atlas in Morocco, and with the outcrops of the Ougarta Ranges further south in Algeria, facilitating future sequence stratigraphic studies of the pre-Ordovician clastic successions of the North Saharan Platform, with a higher resolution than previuosly available.