Fabrics of meteoric phreatic diagenesis: inferences from the petrographic analysis of the Turonian Wadatta Limestone, Nigeria

Abstract

The Wadatta Limestone, composed of biosparite and biomicrite, is a marine member of the otherwise fluvial Makurdi Formation (Turonian). The carbonate unit exhibits calcite cement precipitation in both primary and secondary pore spaces. Replacement fabrics were created when the original aragonite of shells was dissolved and simultaneously precipitated as calcite crystals of various sizes, in some cases with oriented inclusions left as relics of the former texture. The original microcrystalline aragonite matrix was neomorphosed, mostly beyond the micrite barrier, into microspar and pseudospar. These diagenetic effects—precipitation of sparry cement and calcitization of both molluscan shells and matrix—are diagnostic of meteoric phreatic environment. The change from marine to freshwater phreatic condition is suggested to have transpired when extensive regimes of meteoric groundwater flushed away marine waters from the carbonate deposit after the Turonian regression led to subaerial exposure in thet part of the Benue Trough.