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)

Abstract

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.