ABSTRACT

The sandstones of the Bogota stratigraphic section are largely orthoquartzites. Quartz types include undulose quartz, semi-composite quartz, composite quartz and stretched quartz. Sandstones have undergone a variety of diagenetic alterations which include: 1) formation of quartz grain overgrowths among clean sandstones; 2) precipitation of carbonate cement; 3) recrystallization of carbonate cement or matrix: 4) ion exchange and changes in lattice structures among clay minerals yielding vermicular kaolinite, illite, sericite, muscovite and chlorite; 5) grain replacement by clay minerals, sericite, or carbonate following several specific replacement patterns, including preferential alteration of polycrystalline quartz types; 6) replacement by iron oxides of all matrices, cements and alt red zones; 7) formation of complex microstylolite grain contacts, inclusion trains, comb-like groups of inclusion trains, and grain fractures in response to grain stress.

Increase in the amount of detrital matrix inhibits the formation of overgrowths, inclusion trains, comb-like groups of trains and development of microstylolites along grain contacts. Appearance of microstylolites is enhanced by the presence of polycrystalline quartz and/or the presence of intergranular illite-sericite. Among poorly sorted sandstones larger grains take up most of the deformational stress.