Microfacies of the Jurassic limestone in some sections of east and
central Ethiopia
Gilamichael K. Domenico
Microfacies studies of the Jurassic Limestone, (Upper Hamanlei & Urandab
formations) or the Antalo Limestone have been carried out in Eastern
Ethiopia (Bedessa area) and in the Blue Nile Basin of central Ethiopia.The
purpose of the studies is to reconstruct the Upper Jurassic paleogeography of
the country and east Africa in general.
The study shows that in all sections of the Bedessa area, carbonate
sedimentation took place in prevailingly low energy shallow water
environment occasionally interrupted by episodes indicating a relatively
high energy environment. The deposition of the Upper Hamanlei formation
(Bathonian- Early Kimmeridgian) took place on shelf areas, in a lagoon type
environment. A later deepening episode with sporadic influx from the open
sea is evident by the overlying Urandab Formation (Kimmeridgian).The
biostratigraphic subdivision was carried out mainly on the basis of
foraminifera and dasycladacean algae.
The study in the Blue nile also indicates that normal marine conditions
prevailed during the Late Callovian to Oxfordian. The marine transgression
reached on the platform reached its peak in the Oxfordian and is represented
by a thick stratigraphic interval made up of marl- limestone alternations
deposited under shallow open marine conditions. In the Kimmeridgian, a
general regressive trend is documented by abundant high energy shallow
water limestones.
Both regional tectonic and eustatic phenomena are the causes for the major
transgression.