MINERAL DIAGENESIS IN SILT- AND CLAY-RICH MUDSTONES: MACROSCOPIC TO MICROSCOPIC CHARACTERISTICS

Significant research has been undertaken upon siliceous and carbonate-rich organic-rich mudstones, due to their role as economic shale gas and shale oil reservoirs. However, silt- and clay-rich mudstones are the most abundant in the rock record, commonly have low organic matter contents and in many cases also act as economic shale reservoirs. However, the detailed nature of diagenesis within these rocks is poorly constrained. We document details of diagenetic alteration within some Mesozoic mudstones from the USA and the UK (Lower Jurassic mudstones). We highlight the reactive nature of these rocks, both with respect to carbonate and silicate minerals.

In all the mudstones, macroscopic carbonate cements concretionary horizons are present (up to 0.3 m thick), capping upward-coarsening sediment packages 1- 3 m thick, interpreted to be parasequences. Cementation is interpreted to result from breaks in sediment accumulation below marine flooding surfaces and highlights the importance of macroscopic scale diagenetic carbonate mobility in these mudstones.

Precipitation of quartz and alumino-silicate minerals are also important in these mudstones. Kaolinite is present both in uncompacted test of organisms and as vein fills in septarian concretions, is interpreted to have occurred prior to significant compaction and indicates that both Si and Al were mobile during early diagenesis, possibly a result of Al-mobilization by organic acids generated during organic matter oxidation reactions, with the Al sourced from poorly crystalline detrital aluminium oxides and clay minerals. Quartz cement takes the form of quartz overgrowths and microcrystalline quartz crystals. Recrystallization of biogenic silica (opal-A) and the smectite-to-illite transformation are likely important sources for quartz cements. Partial to nearly complete albite replacement of large pyrite- and kaolinite-filled bioclasts is also abundant in deeper buried mudstones, along with iron-rich chlorite, sometimes as kaolinite pseudomorphs. These mineral phases highlight that microscopic-scale diagenetic mobility of silica and aluminium is important, even within mudstones lacking obvious sources of biogenic silica and is likely to be an important processes in a wide range of mudstones.