Microfacies Analysis of Mesozoic Carbonate Units of Dire Dawa Area, South Eastern Ethiopia
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Date
2014-05-05
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Addis Ababa University
Abstract
The lithostratigraphy and microfacies analysis of three selected stratigraphic sections were
studied using field data and about thirty four thin sections analysis. These were used to
understand the depositional environments, diagenetic settings and age of the carbonate
succession (the Antalo Limestone Formation) of the Dire Dawa area, in South Eastern
Ethiopia.
The field observations, microfacies analysis, and biostratigraphic examinations on the
carbonate successions of the area, which sandwiched between lower and Upper Sandstone
units, reveal that: these carbonate units have about (~306 m) thickness , have been
deposited during Bathonian-Kimmeridgian time; and can be sub-divided into three sub-units,
according to their facies contents and stratigraphic positions. These, from older to younger
are: (1) The lower sub-unit (Bathonian-Callovian) which is (~119m) thick and consists of
mixed siliciclastic - carbonates units of tidalflat-lagoon deposits; and peloidal and ooilitic
grainstone facies of carbonate shoal/barrier deposits of the shallow marine setting, (2)The
middle sub-unit(Oxfordian-lower Kimmeridgian) is about (~84m ) thick , conformably
overlies the lower sub-unit , consists of the foreshoal facies of thick, allochemical rich
wackestone-grainstone limestones, mainly dominated by reworked intraclastic grains and
rare amounts of patches of colonial organisms, and (3) The upper sub-unit (Kimmeridgian)
is about (~103m) thick, overlies the middle sub-unit conformably and topped unconformably
by the Upper Sandstone. It consist of low energy deposits of fine grained dark micritic
limestones with some cherts and clastic shale interbedded at its lower; and some colonial and
carbonate buildup organism bearing layers at its upper part.
The overall microfacies and facies successions throughout the unit shows a carbonate ramp
depositional setting , under which various submarine depositional environments ranging
from shoreline carbonate deposits( including low energy tidalflat and lagoon) and high
energy platform margin carbonate sand bodies) of inner ramp , foreshoal deposits of midramp
to the offshore deposits(basin margins and open sea) of outer ramp, successively from
bottom to top.
The diagenetic sequence of these carbonate units, when ordered form early to late stage,
comprises the following: micritization, dissolution, early marine cementation, meteoric
calcite cementation, mechanical and chemical compaction, fractures; stylolitization,
dolomitization, silicification and burial cementation. These processes have taken place with
varying intensities and occurrence throughout the area.
Lastly deposition and facies patterns throughout this unit are related to already established
paleogeoraphic changes particularly involving the transgression of the sea on the horn of
Africa. These deposits are also correlated with some of equivalent deposits in major
Ethiopian sedimentary basins and with the Sana a basin of Yemen.
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Keywords
Dire Dawa, Microfacies, Diagenesis, Depositional Environments, Ramp