Abstract

Compositional variability in fine-grained sedimentary successions is commonly used as a proxy to inform interpretations of environments of deposition and paleoclimate. In fine-grained organic carbon-rich sediments such variability is widely used to chart the impacts of varying primary production, clastic inputs to the basin, and water-column stratification. Recent high-resolution imaging and compositional data suggests that many of the components used as proxies to chart these differences are complex mixtures of material derived from a range of sources, that have been significantly altered by diagenesis. The aims of this study, using Cenomanian to Turonian mudstones collected from the Portland Core as a natural laboratory, are to investigate the origins of the main rock-forming components in these organic carbon-rich units, to determine the impact of diagenesis as a modifier of original composition, and test the reliability of compositional proxies used commonly to appraise their origins.

High-resolution image analysis of these mudstones reveals that they are composed of multi-component mixtures of calcareous materials composed of planktonic fossils including coccoliths and foraminifera, as well as a nektonic and benthic fauna in addition to siliciclastic detritus composed of silt and clay minerals and aggregate grains of mixed biogenic and clastic detritus. Added to this mix are large volumes of organic carbon. In the studied succession these materials are organized into thin (< 10 mm), sharp-based beds. These beds are commonly normally graded, exhibiting homogeneous-looking bases, ripple-laminated medial parts, and burrow-mottled tops. The primary intergranular and intragranular porosity has been occluded partially by a complex mixture of pre-compaction cements, including zoned calcite, pyrite, microcrystalline quartz, and kaolinite.

The similar compositions of rock-forming materials derived from continental weathering, water-column productivity, and early diagenesis indicate that provenance interpretations of bulk sample composition and other mineralogical proxies are very difficult to interpret consistently. The overprinting effects of diagenesis are commonly ignored. For instance, much of the quartz, calcite, and clay minerals, rather than being just detritus, are cementing components. In addition, the preserved fabric data indicate that these particularly organic-carbon-rich fine-grained sediments are not simply the products of suspension settling from buoyant plumes through a stratified water column. Instead, they are a combination of suspension settling and transport via bedload-delivered materials to the sediment–water interface. This material was then eroded, mixed, and sorted as it was being transported to ultimate sites of deposition before being colonized by an infauna. Finally, it was cemented by a range of pre-compaction diagenetic processes. These include calcite and pyrite cements that precipitated in the dysoxic and early sulfidic zones and whose solutes were supplied by either in-situ breakdown of organic carbon or diffusion from overlying seawater. These were followed by microcrystalline quartz and kaolinite cements, which precipitated in the methanogenic zone, whose solutes were derived mainly from the breakdown of chemically reactive materials buried with the sediment, in a pore-water system that was not connected directly to the overlying seawater.

The apparent similarity of these materials in hand specimen and bulk mineral compositions to one another belies their very complex origins. Proxy interpretations of provenance should be performed with great care, particularly if they are being used to inform subtle climate changes and changing environments of deposition.