DIAGENETIC PATHWAYS IN FINE GRAINED SEDIMENTS ARE DRIVEN BY PRIMARY SEDIMENTOLOGICAL COMPOSITION: PREDICTING MUDSTONE PROPERTIES

The diagenetic pathways taken by marine unconsolidated muds are a key factor in controlling the mineralogical, chemical and physical properties of resulting mudstones. Here, we show that these pathways, particularly during early diagenesis, are largely pre-determined by the initial mineralogical composition of the sediment. The solid inputs that are important drivers of porewater composition and diagenetic mineral precipitation are organic carbon, iron oxides, biogenic silica and carbonate material. We consider here three “end-member” cases: moderate (”typical”) concentrations of iron oxides, enhanced iron oxide inputs, and negligible iron oxide inputs in both biogenic-silica dominated sediments and carbonate-dominated sediment. In these different circumstances these solids interact with porewaters (commonly via redox reactions) producing predictable strata-bound diagenetic assemblages.

Within muds containing iron oxide and organic matter contents that are within the ranges of the concentrations found on most modern shelf settings, bacterial sulfate and iron reduction dominate and the resulting mineral assemblage is one of pyrite and iron-poor carbonate cements. In contrast, in the case of sediments enriched in reducible iron oxides bacterial iron reduction can dominate and the resulting iron-enriched sulfide-poor porewaters lead to the precipitation of iron carbonate and iron silicate minerals. Finally, in sediments deposited in systems that are starved of detrital minerals and enriched in production-derived materials, iron oxides are generally negligible and biogenic silica or carbonate material can dominate. The lack of iron oxides, coupled with high bacterial sulfate-reduction rates typical of such productivity-enriched sediments, leads to the dominance of sulfide oxidation and the resulting dissolution of carbonate minerals and possible co-precipitation of phosphate minerals. Additionally, biogenic silica undergoes diagenetic transformations to opal-CT and quartz, leading to quartz-cemented mudstones.

These varying pathways, driven by initial mineral inputs, result therefore in a wide range of mudstone types with contrasting, but predictable, mineralogy, organic matter preservation, mechanical properties and geophysical well-log responses.