Sandstone deposits held together with clay matrixes have been reported from almost all geological time periods. This is somewhat surprising, as the physical processes which form sand deposits are usually very good at separating particles by grain sizes, hence we have sandy beaches and sand dune fields in deserts, rather than sand-and-clay deposits in these places. A number of possible explanations for the formation of clay matrixes in sandstone deposits have been put forward, including subsequent infilling, simultaneous deposition of clay particles, and bioturbation, but none of these has been generally accepted.
In a paper published in the journal Geology on 17 August 2017, Luke Woodridge, Richard Worden, Josh Griffiths, and Anu Thompson of the School of Environmental Sciences at the University of Liverpool, and Peter Chung of the School of Geographical and Earth Sciences at the University of Glasgow, describe a possible method by which clay particles may become adhered to sand grains by the formation of biofilms in intertidal environments, thereby giving a method for the formation of sandstones held together by clay matrixes.
A variety of organisms form biofilms in intertidal sandy sediments, including Diatoms (single-celled Algae with silica shells), Euglenids (single-celled flagelate Algae), Crysophyceans (Golden Algae), Dinoflagellates (shelled flagellate Algae), and a variety of Bacteria. Woodridge et al. concentrated on the role of motile epipelic Diatoms, the dominant group of biofilm-forming organisms in sediments in northeastern Europe.
Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells.
Motile epipelic Diatoms excrete strands of extracellular polymeric substances, which they used to attach themselves to grains of sand. This both serves to anchor them within the sediments, and facilitates their movement within the sediment, enabling them to move up and down in response to environmental stimuli, such as daylight. These strands eventually become detached from the Diatoms, but remain attached to the sand grains, forming a web of mucus strands (biofilm) that bind the sand together. Clay particles are known to adhere to biofilms, particularly in the presence of divalent cations such as Magnesium (Mg²⁺) and Calcium (Ca²⁺), both of which are present in seawater, thereby providing a potential method for the formation of clay coatings on sand grains.
Woodridge et al. collected sand samples from the Ravenglass Estuary in Cumbria, northwest England, which is considered to be an analogue for the environments in which about 54% of the sandstones with clay matrixes found in the rock record formed. These grains were found to be partially covered by a network of fibrous filaments, to which were adhered a variety of silt and clay particles, as well as organic material, including Diatoms.
Backscattered electron and environmental scanning electron microscope (SEM) images of clay-coated grains. Arrows indicate clay coatings. Dashed lines outline the extent of the biofilm coats on the grain surface. (A) SEM image of loose sediment. (B) Thin section of clay-coated sand grains from an intertidal estuarine setting. (C) Environmental SEM image of hydrated sediment. Triangle in top right points to a diatom with excreted extracellular polymeric substance grain attachment outlined. Woodridge et al. (2017).
Woodridge et al. found that the grains were covered by the biofilm to a variable extent, with grains having as little as 0.5% to as much as 87% of their surface covered. They then mapped the distribution of particles with different coverages within the estuary, which showed that coverage was most extensive in the upper and middle estuary, on tidal bars and tidal flats that correspond to tidally exposed or shallow waters. They also mapped the distribution of Chlorophyll-a concentration in sediments (a proxy for the presence of Diatoms) across the estuary, finding a strong correlation with the distribution of biofilms, strongly supporting the idea that the Diatoms are responsible for the biofilms covering the sand grains.
Maps of clay-coated sand grain distribution and biomarker proxy for tidal flat biofilm abundance on sand grains. (A) Clay-coat coverage of sand grains established from quantitative petrography. Rivers Irt, Mite, and Esk are indicated. (B) Chlorophyll-a concentrations. Darker shades in (A) and (B) represent greater extent of clay-coat coverage and greater abundance of sediment biofilm (chl-a—chlorophyll-a). Woodridge et al. (2017).
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