COMPLEX ROLES FOR CALCITE IN ORGANIC-RICH MUDROCKS: USING MICROPETROGRAPHY TO UNDERSTAND GRAINS, FOSSILS, AND CEMENTS

X-ray diffraction (XRD) and X-ray fluorescence (XRF) can provide estimates of calcite content in mudrocks. However, calcite can take many forms such as biological grains (fossils), nonbiological grains, and cement, which have differing impacts on rock properties such as pore systems and rock brittleness and also implications for depositional models. XRD and XRF provide no information on calcite distribution, which is important to understand rock brittleness. Micropetrographic examination of samples in the scanning electron microscope (SEM) can elucidate the different forms.

Calcite cements can be pore-filling or replacement. Pore-filling cement in intraparticle pores (e. g. foraminifera chambers) plays a different role than pore-filling cement in interparticle pores. Both fill pore space, but cement in interparticle pores lithifies the rock increasing brittleness and reducing permeability. Replacement calcite has variable influence on rock properties depending on what is replaced and how it is distributed. For example, calcitization of opaline fossils leads to dissolved silica which can recrystallize as clay-size quartz in the mudrock matrix thus increasing brittleness. Partial calcite replacement of feldspars is also common, but this late process does not add to a rigid rock framework.

Calcareous fossils are common hosts for intraparticle pores. Biota types and whether they are transported provide data on depositional setting and bottom conditions. For example, benthic foraminifers suggest relatively oxic conditions at the seafloor if not transported, whereas transported fossils provide no environmental clues but do indicate depositional processes.

Calcareous nonbiological grains are common in some mudrocks. The two most common sources are transported grains from the shelf and, in post-Triassic rocks, coccolith-rich fecal pellets. Both of these components can have significant intraparticle pores but provide different data on depositional setting.

To understand the roles of calcite in mudrocks, micropetrographic examination, preferably in the SEM, is necessary.