This is an expanded version of an article originally written for the National Trust’s White Cliffs of Dover volunteers’ intra-web site. Those photographs included which were taken on National Trust property are reproduced here under licence granted by National Trust (Enterprises) Limited. Parts of this note assume some familiarity with the National Trust’s Fan Bay Deep Shelter. This is a system of Second World War tunnels situated between Dover and South Foreland and can be visited by taking a “hard hat” tour lead by enthusiastic and knowledgeable volunteers. Opening times can be found on the National Trust’s White Cliffs of Dover web site.
Short answer:
Flint was formed in chalk after it was deposited on the seabed as a chalky ooze, but before it was compressed into chalk rock. Silica, which was dissolved in sea water, precipitated within permeable pathways in this sediment. These pathways may have been animal burrows, fracture planes caused by stresses in the seabed, or changes in composition of the original sediments. After time, the precipitated silica hardened to become flint, a micro-crystalline form of silica, which took the form of the original cavities, so they can be regarded as internal moulds of cavities.
Long answer:
Flint is a form of silica, other types include many well-known gemstones such as rock crystal (quartz), amethyst and citrine. Some are made of microscopic crystals such as Chalcedony and Chert. Some are non-crystalline such as Opal. Chert when found in chalk is usually called flint and those found from Norfolk southwards, are characteristically black or very dark grey.
Flint is a hard mineral so, how is it that it is found embedded in chalk, a soft white limestone? And why do flints occur in layers? Photo 1 shows these layers in Langdon Cliff, near Dover. Firstly, before looking at their possible origins, it may help an understanding by considering some key features of the environment in which flint was formed – the chalk seabed.
Chalk was formed from microscopic shells (coccoliths) originating from a type of calcareous algae called coccolithophores. These were sufficiently numerous to form algal blooms in the warm seas of the “hot house” climate prevalent in the Upper Cretaceous, about 101 to 66 million years ago. As they died, a steady shower of coccoliths accumulated as a chalky ooze on the seabed which over millions of years compacted to form chalk.
The chalk seabed was not a uniform medium, its permeability was affected by biological and tectonic processes after its initial deposition.
The soft seabed made an ideal environment for burrowing animals. When the burrows became disused, they were in-filled with debris, some organic and some chalky silt. But this burrow infill would remain less compacted than the host sediment and so provided a more permeable path for fluids within the seabed sediment.
The chalk beneath seabed was also subjected to episodes of deep seated (tectonic) uplift and tilting which caused the chalk to fracture which introduced permeable planes within the chalk. The tectonic movements also caused brief but vigorous currents flowing over the seabed. Where these currents were sufficient to scour the surface of the seabed, they introduced a discontinuity in the sediment between it and succeeding sediments resulting in more permeable planes within the chalk.
The mechanism of formation of the chalk flints outlined below is based on those described by Bromley et al., (1975) and Clayton, (1986) although a description of the chemical processes is not included here. The Bromley model describes a situation where silica is precipitated directly as small nodules. In the Clayton model the silica passes through several intermediate phases before precipitating as micro-crystalline quartz (flint).
In the Clayton model, flint formation was initiated at shallow depths in the chalk sediment after it had been deposited but before it had been become fully compacted into solid chalk. Silica is soluble in water, given enough time, and was present in the sea water and the pore water within the sediment close to the seabed. It probably originated from silica spicules from organic sources, particularly sponges, and perhaps from inorganic sources such as sub-sea volcanic eruptions.
The first requirement for the formation of flint was for the dissolved silica to be precipitated. Both the Bromley and Clayton models assume that this was initiated by the effects of anaerobic decomposition of organic matter in the sediment (decomposition by bacteria in the absence of oxygen) but in the Clayton model acidic conditions produced by the bacteria promoted silica precipitation in permeable pathways present in the chalk.
The processes which initiated anaerobic decomposition varied and these influenced the shape of the resulting flints. Clearly, all required that oxygen is depleted so that anaerobic decomposition can start. The simplest initiator is burial below the seabed to a depth at which oxygen cannot be replenished by the downward diffusion of oxygenated water. This boundary between deoxygenating (reducing) and oxygenating conditions is the Redox Boundary. If the organic matter and bacteria were evenly distributed in the sediment and the permeability of the sediment was uniform, this boundary would be flat zone parallel to the seabed.
In practice the depth and shape of the Redox Boundary depends on the permeability of the sediment, which affects the penetration of oxygenated water, and the concentration of decaying organic matter, which affects the rate of oxygen depletion by anaerobic bacteria. A large mass of decaying matter would attract a large concentration of bacteria with a high oxygen demand resulting in local reducing conditions at the core of the mass, even if it was above the main Redox Boundary. In this situation the Redox Boundary could be a three-dimensional surface surrounding the decaying mass where the centre of the mass is anaerobic, and the surrounding volume is aerobic.
The types of flint in chalk are distinguished by their shape. The commonest are the “burrow form” flints which are the knobbly shaped flints which you may see used as building stones or come across in fields in the chalk lands of southern England. A less common form are the sheet flints, which as the name suggests, form extensive sheets, usually less than 50mm thick. A third type, the Paramoudra flints, sometimes called Pot Stones, are roughly cylindrical in form and up to 0.5m in diameter, and Giant Paramoudras which are aggregates of flints which can be several metres diameter.
In the case of the burrow-form flints, as their names suggests, silica was precipitated in the burrows excavated by animals living in the seabed. The silica took on a shape approximating to enclosing burrow, hence the irregular knobbly shapes.
Sheet flints were formed on fault planes or bedding planes. A good example of sheet flint can be seen in the National Trust’s Fan Bay Deep Shelter shown in Photo 2. In this example, the sheet flint is parallel with the bedding so according to the model, the silica rich water preferentially followed a more permeable discontinuity in the bedding, formed by either a lithological variation or a bedding plane fracture.
But it is difficult account for the lens of chalk in this example by simple sedimentation. The lens is enveloped by a surface which combines on each side of the lens; deposition of successive layers could not form such a surface. Some post-depositional process must have occurred to produce this feature. It may be that this sheet flint is following a scoured surface produced by a short-lived current flowing across the seabed, of the type caused by tectonic movements affecting the sea bed. You can see similar lens structures in current bedded sandstones.
A diversion.
If you are visiting the Fan Bay tunnels, a rare example of fossil is displayed in the roof above your head at the location of Photo 2. Follow this link to see Cladoceramus undulatoplicatus
Paramoudras can be divided into normal Paramoudras and Giant Paramoudras. They are entirely different things, and it would be appropriate to use the term Pot Stone for the normal Paramudras.
The normal Paramoudras are roughly cylindrical in shape, usually less than 0.5m in diameter and slightly taller. They frequently have a hole through the vertical axis or, if not, a hard chalky core. Some can be pebble sized with a small central hole penetrating them. Photo 3 shows a slightly wave battered example of a Paramoudra (without a chalky core) on the beach at Ness Point southwest of St Margaret’s Bay. Photo 4 shows one in-situ in the cliffs at Pegwell Bay in north Kent showing how it protrudes from a flint band (tentatively identified as Bedwell’s Columnar Flint).
The chalky core frequently has a trace fossil (a tube or track of a creature) in its core. According to the Redox Boundary model, this creature would have been the origin of the organic matter decomposing in anaerobic conditions. If this is the case, the Redox Boundary would have been a vertical cylinder surrounding a decomposing core. So, for this variety of Paramoudra flint, it is the shape of the decaying matter which controlled the form of the flints rather than the shape of the permeable pathway.
Giant Paramoudras can reach 6 metres high and up to two metres in diameter and rather than being a single flint they are an aggregate of flints. These were described as fossilised sponges in the 19th century and Yeomans, (2018) has recently spent much time refining this idea. Bromley et al., (1975) drew attention to a narrow tube 5cm diameter running the full length of the structure. It is difficult to believe in this instance that this housed a creature which was the source of the required organic matter. The idea of a worm, or similar, up to 6m long living the thousands of years required to grow within the accumulating chalk sediment is a bit far-fetched. So, the sponge origin is the most likely for the Giant Paramoudras.
Photo 5 shows the top of a partly submerged Giant Paramoudra on the shore at West Runton Beach, Norfolk. This example is about a metre and a half in diameter. It also shows a gap in the circumference which Yeomans, (2018) refers to as the “exhalent dip” found in most Paramoudras. When seen in section, in cliff faces, they show an irregular shape which when they pass through a flint band may extend into it.
Yeomans (2018) has identified a large Cretaceous seabed reef on the foreshore at West Runton, comprised of Giant Paramoudra flints. They show a north easterly alignment which he attributes to the effect of the prevailing sea-bed currents.
The final question which remains to be answered, when considering the Redox Boundary model, is why doesn’t flint occur as a continuous mass within the chalk? This would be the expected outcome if a constant accumulation of sediment was continually raising the base of the oxygenated layer in the sediment.
The flints in Photo 1 are arranged in bands. It has been suggested that variations in the type of sedimentation produced permeable layers allowing an increase in the concentration of silica on these bedding surfaces. This may explain sheet flints and flint bands which are of limited horizontal extent, but a key feature of most flint bands is that they can be traced over hundreds of kilometres. Such widespread features are most unlikely to be the result of sedimentary controls but point to a global influence.
A convincing explanation for such an influence is that the Cretaceous hot house climate periodically cooled, killing off the algal blooms and the supply of coccoliths which virtually stopped sediment accumulation. This pause in sedimentation allowed the Redox Boundary to remain at the same depth in the sediment long enough for the silica precipitation process to complete. This temporary climate cooling would have been a global effect, hence the extensive distribution of the flint bands at the same level throughout the chalk seabed.
If this idea is correct, what caused the periodic climate cooling? Two sets of time intervals have been identified between the flint bands, one of approximately 21,000 years and the other of 41,000 years. These are similar to the periodicity of two Milankovitch Cycles which are regular variations in the orbit of the Earth around the Sun. These affect the amount of heat from the Sun reaching the Earth and it is suggested that flint formation coincided with the cooler parts of the Milankovitch cycles.
Andrew Coleman
Rev. 22/03/2024
References:
Bromley, G. R., Schulz, M.-G., & Peake, N. B. (1975). Paramoudras: Giant Flints, Long Burrows and the Early Diagenesis of Chalks. The Royal Danish Academy of Sciences and Letters., 20(10), 1–31.
Clayton, C. J. (1986). The chemical behaviour of flint formation in Upper Cretaceous chalks. In G. D. G. Sieveking & M. B. Hart (Eds.), The scientific behaviour of flint and chert (pp. 43–54).
Yeomans, R. (2018). Paramoudra: Observations on large flint structures from the Chalk (Upper Cretaceous) and flint formation. Proceedings of the Yorkshire Geological Society, 62(3), 210–216. https://doi.org/10.1144/pygs2017-005
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