Chapter 8: Cementation in Mudrocks: Brief Review with Examples from Cratonic Basin Mudrocks

Abstract This chapter reviews petrographic evidence concerning the mechanisms by which mudrocks lithify. Mudrocks clearly undergo processes analogous to compaction and cementation in sandstone and limestone, although the relative importance of these two processes in the diagenesis of mudrocks remains uncertain. Cement in mudrocks can be demonstrated to fill both primary and secondary pores. Inter- and intragranular cements are observed in mudrocks as well as cement within fracture fills. Thus, cements in mudrocks take the full range of form and distribution as observed for cements in sandstone and limestone. Displacive precipitation is a chemical-mechanical process observed with particular frequency in mudrocks, which contrasts with the common cementation processes in sandstones and limestones. Overall, the same authigenic minerals common in sandstone and limestone dominate the authigenic assemblages in mudrocks. Sediment accumulation rate is a significant factor in mudrock cementation. In situations of slow sediment accumulation, cement emplacement, typically in the form of highly localized carbonate and phosphate minerals, takes place near the sediment-water interface. In contrast, rapidly deposited mudrocks tend to lithify at greater depths in response to thermally controlled diagenetic reaction of the detrital assemblage. Authigenic quartz in mudrocks, a topic of particular interest for an understanding of mudrock mechanical properties, requires further study. Grain replacements and fracture fills of quartz are readily documented in mudrocks, but convincing demonstration of intergranular quartz cement remains elusive. High-resolution imaging by transmission electron microscopy may be required to fully resolve issues surrounding the emplacement of authigenic quartz into the minute pores of mudrocks.