Burial dissolution porosity in Albian Mfamosing limestone, Calabar flank, Southeastern Nigeria. – Aderogba
ABSTRACT
The distribution of porosity in the Albian Mfamosing Limestone was affected by, meteoric and burial diagenesis. During early subaerial exposure (meteoric diagenesis) primary pores were filled with calcite cement and internal sediments while fine secondary (matrix) pores and conduit pores (vugs, caverns, fractures) were formed. During burial, unlithified sediments (including grainstones) lost porosity by mechanical and chemical compaction. Calcium carbonate that was dissolved during chemical compaction was reprecipitated as burial calcite cement. Also, metalliferous basinal hot fluids derived during the compaction of overlying shale formations, (Ekenkpon and Nkporo) was precipitated as cement and replacement. Evidence for burial dissolution in this limestone include: 1) secondary porosity cutting across pressure solution seams, 2) dissolution of grains already sutured by pressure solution, 3) secondary porosity preserved along stylolites and burial fractures, or termination of these fabrics into such pores; and 4) leaching of calcitic grains. Unlike karstic meteoric dissolution, burial dissolution is controlled on a larger scale by joint systems. Most solution-enlarged joints often show evidence of corrosion of walls. These joint systems help in enhancing permeability and acting as a pathway linking other types of pores.