Introduction

Carbonates are an important group of rocks that make up nearly one-quarter of all rocks in the sedimentary record. They differ fundamentally from siliciclastic sedimentary rocks both in composition and in depositional origin. Siliciclastic sedimentary rocks are composed of constituents that originated outside the depositional basin and were transported as solids into the basin. By contrast, carbonate rocks are composed of intrabasinal sediments, which were deposited in situ by precipitation of calcium carbonate through inorganic and biochemical processes. They have little or no provenance significance, unlike siliciclastic sedimentary rocks; however, the textures and structures that characterize carbonate rocks have considerable significance with respect to interpreting depositional conditions (e.g., water depth, water energy, biological activity). Thus, petrographic study of carbonate rocks has customarily focused on observation of carbonate grains (including fossils), textures, and structures that are important to interpretation of depositional conditions.

Carbonate rocks also have considerable economic significance, particularly as reservoir rocks for petroleum. Their importance as reservoir rocks depends upon the degree to which original (depositional) porosity (40–80 percent) is preserved during diagenesis. Porosity is lost owing to physical compaction, chemical compaction (pressure solution), and cementation. Diagenesis can also bring about pervasive changes in mineral composition (e.g., alteration of aragonite to calcite or dolomite and alteration of calcite to dolomite). The role of cathodoluminescence imaging in the study of carbonate rocks has been confined mainly to study of carbonate diagenesis, although it plays some role in identification of carbonate grains, textures, and structures.