Viviana Reale

Distribution of the calcareous nannofossil Reticulofenestra asanoi within the Early-Middle Pleistocene transition in the Mediterranean Sea and Atlantic Ocean: correlation with magneto- and oxygen isotope stratigraphy

Abstract Quantitative analyses of the calcareous nannofossil Reticulofenestra asanoi and related species have been performed on the Early-Middle Pleistocene transition in the Mediterranean Sea (ODP Sites 976 and 963) and Atlantic Ocean (DSDP Hole 610A) in order to improve the understanding of their stratigraphic distributions. Abundance patterns have allowed the identification of the lowest common occurrence (LCO) and highest common occurrence (HCO) of R. asanoi in a short interval below and above the lowest occurrence of Gephyrocapsa sp. 3. Correlation with oxygen isotope stratigraphy at Site 976 places the LCO of R. asanoi at the Marine Isotope Stage (MIS) 34-33 transition and its HCO at the MIS 23-22 transition. At Site 963, the HCO of R. asanoi (estimated age of 0.96 Ma correlated to MIS 25) is regarded as ‘artificially’ low, and its highest occurrence (estimated age 0.90 Ma correlated to MIS 23) has therefore been used for bio-chronostratigraphic correlation. The LCO of R. asanoi is estimated at 1.05 Ma at Site 963 and 1.17 Ma at Hole 610A, which suggests correlation to MIS 30 and MIS 35, respectively. These data suggest a possible diachrony for the LCO of R. asanoi.

THE MID- JURASSIC MARINE TRANSGRESSION IN EAST AFRICA: NEW DATA ON THE DEPOSITIONAL ENVIRONMENT AND AGE OF THE LOWER KAMBE FORMATION (AALENIAN TO BAJOCIAN) IN THE MOMBASA AREA (S.E. KENYA)

The Lower Kambe Formation crops out from Mombasa towards the northeast, along the coastal area of Kenya; it was deposited during the first phases of the Middle Jurassic marine transgression. The lithology of the Lower Kambe Fm. varies passing from the southern areas towards the north: near Mombasa, this formation consists of an alternance of calcareous and marly intervals; the marly levels become thinner and tend to disappear towards the north, where the succession becomes entirely calcareous and shows features of a carbonate platform. According to field observations, in the Mwachi River area near Mombasa, four lithological units crop out, that we consider as informal members, namely from bottom : (a) Calcarenitic member (Cam), well bedded calcarenites and some conglomerates; (b) Lower Shaly member (LSm), marly shales with marly limestone beds; (c) Conglomeratic member (Cgm), alternance of calcarenitic beds and levels of conglomerates; (d) Upper Shaly member (USm), marly shales and siltstones with thin beds of calcilutites and quartz sandstones. The sedimentological features of the Cam point to a shoreline environment, possibly shoreface; those of the LSm to a mid/outer ramp; the onset of tectonically active slopes is evidenced by the occurrence of gravity flow structures in the fine-grained sediments. At the top of LSm a marked shallowing of the deposition environment occurred. The Cgm was deposited on a inner ramp, where polymictic coarse deposits (arenaceous, shaly and carbonate clasts) indicating a strong erosion of the coastal areas, were intercalated to the wave-winnowed calcarenites; also the calcarenitic upper portion of the Cgm was deposited on an high energy inner ramp depositional environment. Upwards, a deepening occurred, as the fine sediments of the USm point to a mid/outer ramp. The age of the Lower Kambe Formation has been determined by several Authors on the basis of ammonites; in this work calcareous nannofossils stratigraphy is provided. The age given by ammonites is Bajocian; the calcareous nannofossil assemblage, found in various localities along the Mwachi River, indicates an Aalenian - Bajocian p.p. age. The discrepancy is possibly due to a complex geological situation (faults ?) in the localities with ammonites.