Rory Mortimore (UK)
Flint in the Late Cretaceous Chalk: links across the European platform
In a recent issue of this journal Paul Taylor wrote “We are very fortunate in Britain to host one of the most remarkable deposits in the entire geological record, the Chalk” (Deposits Issue 55, 2018, p.35, see Bryozoans in the English Chalk). Perhaps equally remarkable are the bands of flint associated with the pure white chalks (Figs. 1 to 3). Flints have attracted human attention since pre-historic times with some flint bands providing the preferred source rock for manufacturing stone-age tools (for example, the Late Turonian Floorstone Flint at Grimes Graves near Brandon in Norfolk, England (Figs. 4 to 6a and b; Mortimore and Wood, 1986), or the geologically much younger Early Campanian flints in the Harrow Hill Flint Mines in Sussex, England Fig. 7).
Subsequently, Brandon flints were used as the vital spark for guns (that is, gun-flints, Skertchley, 1879; Shepherd, 1972) and these have been found as far afield as eastern North America (used in weapons of the American revolution) to the Fijian Islands in the Pacific (from Royal Navy guns).
In the modern era flint remains a material of concern in engineering causing damage to core-drilling operations, tunnelling machines (Fig. 8) and cable trenching machines onshore and offshore northwest Europe. Flint also impacts the ease with which piles can be driven into chalk. To fully appreciate flint as a material and to assess the impact of flints on engineering operations has required establishing a detailed flint stratigraphy and a flint band correlation framework for each project (for example, Mortimore and Wood, 1986; Warren and Mortimore, 2003; Mortimore et al., 2011; Mortimore, 2012). The Seven Sisters Flint Band is an example of a flint seam that can be traced widely from Yorkshire (the Eppleworth Flint, Fig. 4) to southeast Devon, and along the south coast of England (Fig. 1) and across the English Channel to France (Fig. 2) as far as the Yonne in the eastern Paris Basin (Fig. 3).
As a result of recognising that individual flint bands and groups of flint bands could be widely recognised and correlated, a flint stratigraphy (Fig. 5) has been developed for each region of the UK including Northern Ireland (Fletcher, 1978), the Northern Province, England (Wood and Smith, 1978) and the Southern or Anglo-Paris Basin Province (Mortimore, 1986; Mortimore and Pomerol, 1987).
The Late Turonian Brandon flint succession forming part of the Grimes Graves flints in Norfolk were first described by Skertchley (1893), further developed during the CERN investigations (Ward, Burland and Gallois, 1968) and correlated with the Turonian flint successions of the Southern and Northern provinces of England (Mortimore and Wood, 1986).
Other flint stratigraphies included the Campanian to Early Maastrichtian flints of the north Norfolk coast (Peake and Hancock, 1960, 1972) and those of Maastricht in the Netherlands (Felder, 1975a,b; 1981; Felder et al. 1979). These studies illustrated the remarkable continuity of many marker flint bands and subsequently provided the framework for assessing the risk flints posed to engineering operations. In some areas, borehole geophysics, especially various forms of electrical resistivity logs, has also been used to distinguish the size as well as continuity of flint bands (Mortimore, 2014).
In addition to developing a flint stratigraphy, the general interest in flint has continued to raise many questions including:
- What is flint?
- Why do flints have funny shapes?
- How has flint formed?
- Where has all the silica come from to make flint?
What is flint?
Flint is a dense, cryptocrystalline silica rock (SiO2), often referred to as Chalcedony. Flints are generally a mixture of two minerals – quartz and moganite (a polymorph of quartz). Flint also contains impurities derived from its parent chalk sediment including carbonate and has a very fine-grained structure (size of grains 0.5-20 μm) reflecting the background chalk mud which flint has replaced. This structure gives flint its dull and almost opaque colour.
Colour variations are largely related to impurities, especially the presence of carbonate which gives Northern Province flints their grey colour. Pink-purple cortices to flints appear to be common at specific stratigraphical levels (see below). The interior of flint can also have up to 1% to 5% porosity, whereas the cortex of flints can range from 3% to 12% porosity (Lautridou et al., 1986). Chert is chemically the same as flint but is generally a coarser and greyer material. Chert is used as a more general term to cover all types of siliceous rock including flint (for example, greensand cherts, radiolarian cherts and so on). For further discussion on the origin and usage of the terms flint and chert, see Shepherd, 1972 pp. 35-36.
Why do flints have funny shapes?
Bromley (1967, 1975) established that the common nodular, horn-flints forming Thalassinoides branching networks (Figs. 11 to 13) were related to animal burrows into the Chalk seabed especially crustacean burrowers (Callianassid shrimps and lobsters).
Subsequently, Bromley (1975, 1996) and co-workers (Bromley and Ekdale, 1984, 1986; Bromley, Schulz and Peake 1975) illustrated the wide variety of trace fossils and other fabrics in the Chalk preserved in flint and imparting a range of shapes and sizes to flints. As shown by these authors, flints generally ‘over-grow’ and are therefore larger than the original burrow (for example, Figs. 12 and 13).
Paramoudra flints (Bromley, Schulz and Peake, 1975) are the most extreme example of such overgrowth surrounding a trace fossil, producing enormous columnar flints (Figs. 15 to 18). Other trace fossils commonly found preserved in flint are the spectacular spiralling marginal tubes of the late stage trace maker Zoophycos and flints peppered with small (millimetre size) branching networks of another late stage trace maker Chondrites (see illustrations in Mortimore, 2014). Many of the small ‘finger’ flints and extreme spikes on nodular flints are related to Zoophycos.
Other flints have formed in and around macro-fossils, such as the commonly found flint fossil sea urchins Micraster and Echinocorys,and the many fossil sponges preserved in flint (for example, Shepherd, 1972). All fossils in the pure white chalks have been found preserved in flint, including very small meso-fossils such as crinoids, bryozoans, bivalves and brachiopods, and very large ammonites (Parapusozia sp.) found in the Early Campanian Newhaven Chalk on the Sussex coast.
A feature of Paramoudra flint development is the central millimetre thin organic trace fossil Bathichnus paramoudrae Bromley, Sculz and Peake, which is often pyritic (Fig. 18 a and b) and surrounded by a column of altered, hardened chalk and then the column of flint (Bromley, Schulz and Peake, 1975; Clayton, 1986). In some cases, a hardened column of chalk may have formed without the outer flint also forming. Such hardened chalk must have formed early as animal burrowers, such as a shrimp or lobster, have encountered this difficult substrate and been forced to go around it, producing the wonderful ring flint (Fig. 14), a Thalassinoides network with a difference!
At the same horizon as this ‘ring-flint’ shown in Fig. 14 are Paramoudra flints (Figs. 15 to 18), a band that can be traced from the French coastline of Upper Normandy at St Pierre-en-Port (Fig. 16) and Criel Plage (Fig. 17) to the Autoroute 16 road cuttings near Dannes and Camiers (Nord Pas De Calais and Somme).
This same interval of Paramoudra flints yields columnar flints in chalk pits at Lewes (Bridgewick Pit below the Lewes Marl and the Lewes Tubular Flints) and the Paramoudra flints on the Yorkshire coast at North Sea Landings (Flamborough Head, Fig. 16a to c) below the Ulceby Marl (the equivalent of the Lewes Marl in the Northern Province). This is also the horizon of flint maxima (Fig. 4, Mortimore and Wood, 1986). Cliff-fall examples from these same Paramoudra flint intervals occur further north on the Yorkshire coast at Speeton and Bempton Cliffs (Figs. 16 and 19a, b).
There are many other horizons of Paramoudra in the Chalk, for example, Northern Ireland (Fletcher 1976) and Norfolk (Bromley Schulz and Peake, 1975; Wood, 1988). Some horizons may have the trace fossil and surrounding altered chalk without the flint formation. An example may be the millimetre thin dark grey traces with pale chalk surrounds found in the Chalk below the sub-Plenus Marls erosion surface at Beachy Head and Eastbourne Sussex (Fig. 19).
How has flint formed? Where has all the silica come from? And what controls the conditions that allow individual flint bands to be correlated over very long distances?
The typical Thalassinoides burrow-form flints illustrated in Figs. 12-14 and the columnar Paramoudra flints (Figs. 15 to 18a and b) suggest that flints form within the seabed sediment in pure white chalks and the shapes of flint are controlled by the burrow-networks and trace makers. This requires dissolving chalk carbonate and precipitating silica to form flint by replacing the carbonate, a process that must occur post-sedimentation (Fig. 20). The burrow surrounding the vertical column (Figs. 15 to 17) and the preservation of trace fossils uncompacted in flints suggests that the flint forming process developed early pre-compaction.
This observation is supported by the sinking of denser flint into soft unconsolidated chalk (Fig. 16c). The regular seams of flint (Figs. 1 and 2) imply a regular layering of the chemical environment in which flint can form, at a constant depth in the sediment, a chemical mixing zone parallel to but below the seabed (Figs. 21 to 23).
As flint is made of silica, a source of silica is required in a ‘pure’ carbonate rock such as chalk. Many sources of silica are recognised as contributing to flint formation, including the siliceous skeletons (sponge spicules) of the hexactinellid sponges which colonised chalk sea floors in regular beds (Fig. 21a). Other sources of silica include microfossils such as radiolaria and silica microspherules (for example, Clayton, 1986). These forms of ‘biogenic’ silica (Opal-CT) have been converted to more stable α-quartz chalcedonic flints seen today.
Two main theories have evolved to explain the process of conversion biogenic Opal-CT to α-quartz leading to two models for the formation of flint. In Model 1 (Fig. 22 and 23), silica deposited within the sediment as biogenic silica is converted over time to α-quartz through a process that involves bacterial decay of organic matter (Clayton, 1986) and temperature change with depth of burial (Madsen and Stemmerik, 2010). The process of flint formation requires the chalk carbonate to be replaced by silica, either by dissolution-precipitation or by solid state cation exchange.
The main pathways for the chemical changes are the Thalassinoides burrow networks, where better permeability is preserved in contrast to the more cemented and consolidated surrounding chalk. As Clayton (1986) illustrated, the trace fossils such as Bathichnus paramoudrae are also strong controlling influences on flint formation. Such a model involving bacterial decay of organic matter was also developed by Jeans (1980) to explain various stages of cementation leading to nodular chalk and red chalk.
In contrast to Model 1 where alteration of biogenic Opal-CT to α-quartz occurs within the chalk below the seabed, Model 2 (Fig. 23) suggests that biogenic Opal-CT is converted to α-quartz in the water column above the seabed before being sedimented onto the chalk sea floor (Lindgreen and Jakobsen, 2012). In this second model, a layer of flint is, therefore, directly formed on the sea floor. Variation in coccolith carbonate production in the oceans and shelf seas and volcanic activity are also considered to be important processes in this model (Fig. 23) where a change in the pH in the water column above the seabed is required.
Model 1 more easily explains the silicification of trace fossils and development of the typical Thalassinoides burrow-form flints, which requires a ‘replacement-process’ within the sediment (Fig. 23). Similarly, the formation of Paramoudra flints requires a range of mineralisation around a column within the sediment. Model 2 might help to explain the formation of some more continuous thick Northern Province tabular flints, which appear to form a layer.
The nano-particles of α-quartz identified by Lindgreen and Jakobsen (2012) may be a feature of deeper water North Sea chalks. Perhaps both models (Fig. 23) are involved at different times in different conditions in flint formation although Model 2 requires more evidence to prove it works in the chalk successions discussed here.
Whichever model for flint formation is accepted, flints provide vital clues to sedimentary processes on the Chalk sea floor picking out erosional channels and channel-fills (Fig. 24) as well as slump folds and tectonic structures (Mortimore, 2011, 2014, 2019).
Flint bands provide a detailed stratigraphy for investigating sedimentary events in the Chalk and assessing risk to engineering operations. Each flint band has uniquely shaped flints generally related to animal burrows or feeding traces within the Chalk seabed and a characteristic concentration of flint within the band. These unique features are retained over vast distances across the UK and France providing the evidence for a correlation framework for flint bearing Chalk. Fossils associated with the flints provide supporting evidence for the correlations.
One such flint horizon is the St-Pierre-en-Port Paramoudra Flints in the Late Turonian between the Bridgewick and Lewes Marls of southern England (Figs. 4 and 20) and northern France and the North Ormsby and Ulceby marls of the Northern Province Chalk. These flints, in the interval of flint maxima (Fig. 4), illustrate the processes involved in flint formation. Two models for flint formation are considered. Model 1 (bacterial decay cycles within the seabed sediment, Figs. 22 and 23), is more generally applicable to formation of burrow-form flints. Model 2 (direct deposition of α-quartz nanoparticles, Fig. 23) may be more applicable to solid tabular flints and perhaps flints formed in deeper water chalks although this is very uncertain.
Flint colour appears to have both stratigraphical and palaeogeographical controls. Pink-purple flints are especially characteristic of the Late Turonian lower Lewes Nodular Chalk Formation (Figs. 4 and 20). It may be no coincidence that this same Late Turonian interval contains several volcanogenic marl seams and, perhaps, the pink colour reflects volcanic material input to the Chalk-sea. Grey flints are a feature of the English Northern Province Chalk. The grey colour appears to reflect the increased carbonate contained in these grey flints (Aliyu et al., 2017).
Correlation of the flint bands across the Anglo-Paris Basin was undertaken over 25 years with Bernard Pomerol (Paris) and through the Transitional and Northern Chalk Provinces of England with the late Chris Wood. Chris Jeans is thanked for cutting and polishing the Yorkshire Paramoudra Flint (Fig. 18b) and for the many discussions on flint formation and helpful comments on an early manuscript of this paper. Chris Clayton is also thanked for his discussions on flint formation on joint field work in the Northern Province Chalk in the 1980s.
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