New insights in late Miocene lower Chelif basin biostratigraphy based on planktonic foraminifera (Algeria)

Abstract

Quantitative and qualitative analyses based on planktonic foraminifera of two sections (Oued Derdoussa and Djebel Meni) from lower Chelif basin (northern Algeria) enable us to identify for the first time a sequence of bioevents calibrated with the geomagnetic polarity time scale. The identified bioevents of late Miocene formations are useful for a high-resolution correlation in the whole western Mediterranean at local and regional scales. In particular, this work reveals that the sequence covers an interval of time that extends from the upper Tortonian up to the pre-evaporite Messinian period. Indeed, the base of the succession starts with a change in the coiling direction (from dextral to sinistral) of Neogloboquadrina acostaensis as the first bioevent. This later corresponds to the bioevent (1–8) assigned to the Tortonian sediments. This period is marked by the presence of Globorotalia menardii in sinistral coiled form that was substituted gradually with dextral coiled. The Tortonian/Messinian (T/M) boundary coincides to some extent with a sharp replacement of G. menardii group (I and II) with the first common occurrence (FCO) of Globorotalia miotumida plexus few meters below the development of the Tripoli diatomite formation (bioevent 9). This later is dominated mainly by highly convex species (G. miotumida plexus) marked by the presence of Globorotalia mediterranea in contrast with the marly formation at the base of the section. In this part, the recorded bioevents (10 to 17) are assigned to the Messinian sediments. Thus, the Tripoli formation shows the influx of Globorotalia nicolae and a small incursion of the dextrally coiled of Globorotalia scitula at the top of Oued Derdoussa section as a local bioevent that may be related to the local palaeoenvironment. We notice the absence of the change in the coiling from sinistral to dextral of N. acostaensis during the upper Messinian, this may be due to the coincidence of this bioevent with the barren levels.