On the origin of feldspatic mudrocks associated with quartz arenites

Abstract

Two lithostratigraphic units in southeastern Nigeria, the Maastrichtian Ajali and the Eocene Nanka Formations, described as quartz arenites, have been found to contain significant amounts of feldspars in their intercalated mudrocks. If intense chemical weathering prior to transport could eliminate feldspars from the source rocks of these formations, it ought to have been reflected equally in both the sandstones and the mudrocks. Diagenetic elimination (intrastratal solution) may be discounted in these cases on the grounds of lack of petrographic evidence as well as its demonstrable ineffectiveness in the older (Turonian) Makurdi Formation of the same area whose sandstones and mudstones both are dominantly feldspathic or arkosic. It is therefore suggested that feldspars, which are characteristically mechanically unstable, become progressively comminuted along cleavage and twin composition planes, and bypass the coarser bedload during transport. The result is the impoverishment of bedload in feldspar and simultaneous enrichment of suspension load. The impervious nature of mudrocks protects the feldspars from intrastratal solution. The result is a feldspathic mudrock in a feldspar-poor sandstone. Comminution and sorting together are therefore considered effective processes that are capable of producing quartz arenites in a sedimentary system.