ABSTRACT: Facies and sequential analysis applied to the lower Eocene deposits cropping out in the Oued Thelja area provided important information on sediment cyclicity and paleoclimatic conditions. Facies stacking patterns allowed for the reconstruction of depositional environments with relation to sea level changes. The Chouabine Formation is formed by the succession of four sequences including marls, phosphorites, cherts and carbonates. Facies analysis of limestones and washed clays yielded six main facies. The gradual transition of facies from intertidal to circatidal environments suggests the sedimentation occurred on a homoclinal ramp, episodically, swept by upwelling currents. Clay paragenesis shows the presence of sepiolite, palygorskite clinoptilolite and dolomites indicating containment conditions. The richness of the lower part in smectites advocates the transgressive trend of the Chouabine Formation. Sequential analysis shows the stacking of seven third order sequences. Each depositional sequence resulted from a transgressive-regressive cycle within shallow marine environments. Most depositional sequences exhibit a gullied and burrowed base, which, corresponds to a transgressive surface. A lowstand system tract (LST) corresponding to marl/limestone alternations contains a badly-preserved fauna and was recorded within the fourth sequence. The transgressive system tract (TST) is delineated by enrichment in planktonic foraminifera and coincides with the early stage genesis of phosphorites. The highstand system tract (HST) composed of lumachellic beds and exhibits a regressive trend, which is marked within its uppermost part by the drop in planktonic microfauna associated with the decrease in benthic foraminifera.
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