DIAGENESIS IN MUDSTONES: FRIEND OR FOE FOR STRATIGRAPHERS? (Invited Presentation)

Mudstones, depending on their compositional, grain size, fabric and component origins, play important roles in hydrocarbon systems as sources, seals and unconventional reservoirs, and in stratigraphy as repositories of ancient climate / environmental information. Most investigations of mudstones have assumed their constituents are derived from terrestrial inputs to, and biological production within, a basin, and that these components are modified by diagenesis during burial. With the main overprinting effects of diagenesis being compaction, nodule formation, and the transformation of poorly crystalline siliciclastic and oxide detrital assemblages to a more stable clay mineral and silica-rich mineral assemblage. Recent optical and electron optical studies demonstrate that in some units diagenesis, involving clay mineral authigenesis and silica precipitation, has also contributed significant volumes of materials to their inventories. Crucially, these changes are occurring after deposition, but prior to significant compaction. The presence of these materials complicates interpretations of mineral proxies in fine-grained successions and their origins and significance are investigated.

High-resolution microscopy of ancient mudstone samples (Upper Jurassic through to Upper-Cretaceous aged samples from outcrops of the Greenhorn and Kimmeridge Clay Formations) reveals that they comprise a range of lithofacies from calcareous, and siliceous-calcareous mudstones through to argillaceous-siliceous mudstones and that they all contain a significant diagenetic component. These include early pyrite and non-ferroan calcite / dolomite cements as well as later pore filling materials such as silica and kaolinite. Importantly, these materials occlude water filled pore space, and because their precipitation pre-dates compaction, contribute a significant component to the overall rock volume.

Here the origin of the solutes and the timing of cement precipitation are discussed in the context of pre-compaction pore water evolution. In particular, the likely mechanisms that underpin Al-mobility are reviewed, and the implications of these data for provenance studies that use fine grained sediments as records of climate change are considered.