ABSTRACT

SEM observations of lightly etched thin sections of Bahamian sediments reveal an unusual process of micritization that involves carbonate precipitation in microborings concurrent with endolithic activity. A coccoid cyanobacterium, tentatively identified as Solentia sp., bores tunnels, which initially penetrate just beneath grain surfaces and eventually extend throughout the entire grain. These tunnels are filled by radial fibrous aragonite, which is precipitated as the microorganism advances. Extensive multicyclic repetitions of this process result in obliteration of original grain textures with almost complete preservation of grain margins and rare empty bore holes. The rapidly filled tunnels cannot be detected by resin cast embedding techniques that are commonly used to study microboring. This type of multicyclic boring and concurrent filling of bore holes forms micritized grains that can be difficult or impossible to distinguish from micritized grains formed by recrystallization.