Abstract

Kinetic barriers inhibit quartz nucleation and growth at lower temperatures (<50°C [<122°F]). Thus, under ordinary geothermal gradients, the formation of authigenic quartz in fine-grained systems is preceded inevitably by the early stages of compaction. Nucleation sites for quartz precipitation and the abundance and sizes of pores into which quartz cement can be emplaced are limited by the compactional state at the time of precipitation. The two main types of grain alteration that are proposed to yield authigenic quartz, dissolution of biogenic opal and illitization of smectite, occur in different temperature ranges and contrasting compactional regimes. This chapter summarizes petrographic observations on quartz components (grains and cement) by high-resolution cathodoluminescence (CL) and X-ray elemental mapping in 11 mudrock units ranging in age from Ordovician to Oligocene. The amounts of quartz cement observed provide constraints on the sources of silica for the formation of authigenic quartz and mass and volume balances of silica generation and precipitation in mudrock diagenesis.

The size (1–3 μm), spatial distribution, and abundance (typically 30–40% of rock volume) of authigenic microquartz that arise from the biogenic opal pathway are consistent with the compactional state of mud in the temperature range of the opal-A to opal-CT transition. Mudrocks that are clearly cemented by authigenic microquartz contain a volume of quartz in excess of amounts potentially generated by illitization (up to about 13% of rock volume). In the absence of abundant biogenic silica and consequent early cementation that inhibits compaction, the most common mudrock in the temperature range of illitization (>~80°C [>~176°F]) have few available nucleation surfaces for quartz precipitation and little available pore space (mostly nanometer scale) to accommodate pore-filling crystal growth. Sites for higher temperature quartz precipitation, synchronous with illitization, are mostly restricted to localized packing flaws at contacts between silt-size particles and constitute a trivial volume of the rock. Thus, tarls tend to feature diagenesis dominated by compaction that dramatically reduces pore space prior to the onset of significant reactions of the grain component such as albitization and illitization. The absence of discernible cementation in most deep basinal mudrocks raises the possibility that mechanical compaction persists as a mechanism of porosity decline to greater depths in mud than in sand.