Karstic dissolution, porosity evolution in Albian Mfamosing limestone, Calabar Flank, South-eastern Nigeria. – Aderogba
The Mfamosing Limestone is a shallow marine carbonate platform that was karstified as a result of subaerial exposure.
Karstification of the limestone first occurred during late-Albian sealevel lowstand. During the deposition of the limestone,
the porosity was originally dominated by primary pores (intergranular and intragranular). During early karstification, the
original depositional fabric and related primary porosity was overprinted by episodes of dissolution and cementation
processes. The primary pores were filled with calcite cement and internal sediments while fine secondary (matrix) pores
and conduit pores (vugs, caverns, fractures) were formed during this period. The high depositional porosity which started
the paragenesis was later concluded by meteoric calcite cement during karstification. Other dissolution processes of great
importance occur during the mesogenetic (burial) period when metalliferous basinal brine enlarged previously formed
karst-solution cavities, and created additional channelways and openings for the precipitation of base metal deposits and
its associated gaugue minerals. At present, the limestone body is undergoing a second period of karst formation. The result
of this late sub burial stage is similar to that of earlier diagenetic sub burial stage.