The diagenetic development of the Callovian to Kimmeridgian

The diagenetic development of the Callovian to Kimmeridgian carbonate
sequence in the Blue Nile Canyon
Gilamichael K. Domenico
This study focuses on the major diagenetic features of the Jurassic carbonate succession
in the Blue Nile canyon of central Ethiopia. These features include cementation,
micritization, neomorphism, compaction, dolomitization and silicification.
Cementation is characterized by isopachous calcite, granular cementation, syntaxial
overgrowths and dog tooth types. It is extensive and took place at least in three phases;
marine, meteoric phreatic and burial. Micritization is evident by micritic envelopes
developed mostly on mollusk grains. Neo morphism is commonly observed in
aragonitic grains mostly bivalves.
Compaction is characterized by mechanical and chemical types. The former includes
grain to grain contacts (concavo- convex contacts), and fracturing of micritic envelopes
while the latter is evident by pressure solution (stylolites).
Mechanical compaction prevails among the Kimmeridgian oolitic grainstones and
packestones of the Gohatsion sections, while pressure solution is mainly a peculiar
feature of the Oxfordian wackestones and mudstones of Dejen sections.
Dolomitization was mostly post compactional. Most of the dolomite is dedolomitized
and is found predominantly as calcite rhombohedra.Some of the rhombohedra consist of
dolomite and calcite hence indicating incomplete replacement.
Late diagenetic replacement by silica led to the formation of chert nodules.However
the timing of the silicification event is not easily understood .Porosity is generally low
(< 5%) as a result of intense primary cementation.