Chalk Stratigraphy in the UK

Stratigraphy is the study of rock strata in terms of time and space, and deals with the correlation of rocks between different localities. Correlation may involve the use of:

• Fossils ('biostratigraphy'),
• Rock Units ('lithostratigraphy'), and
• Geological time-units or intervals - 'chronostratigraphy'.

Biostratigraphy, and the consideration of fossils is covered in ......
Lithostratigraphy is discussed below and in ..... and the rock units as Chalk formations are listed in Chalk Formations.
Chronostratigraphy is also described, in tabular form, as the geologic Periods, Epochs, and Stages.

The following discussion covers:

• History
• Naming Stages
• The Boreal and Tethyan Realms
• The Northern and Southern Provinces
• Fauna Migration
• Chalk
• Tectonics
• Rhythms in the Chalk - Milankovich Cycles

History

Nicolaus Steno, a Dane working in Florence in the mid-17th century, studied hills around Tuscany and became convinced that many rock formations originated as layers of sediment, laid down in orderly succession. Thus, the Earth's crust contained a chronological history of geological events, that could be unravelled by careful study of each succeeding layer. Publishing in 1669, he was careful to explain these findings in accordance with the strict Church dogma of the story of Creation - The Deluge and collapse of oceans into pre-exisiting caverns.

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Naming Stages

Period and stage names have traditionally been based upon observation: the appearance of the strata (type of rock, or lithology), and most importantly, the fossils that appear within it. There is thus an obvious geographical problem, as different fauna thrive in different geographical areas and climates. Only free-swimming (pelagic) marine fauna tend to spread world-wide, and even these fauna are limited and differ under conditions of varying oceanic temperatures.
A single fossil (or a few, known as a 'fauna suite') [is this a recognised term?] will be selected as characteristic 'zonal markers' and be used to define stages or substages. However, observation in different areas will define different stages based on their local fauna, and correlation between these different zones is often unclear.

This is a particular problem in the British Isles, when defining the Cretaceous, from its base throughout the Period.

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The Boreal and Tethyan Realms

At the end of the Jurassic and during the Cretaceous, the fauna of 'Europe' was divided into two major "biogeographical provinces" - i.e. 2 separate areas with different ecology, so different fossil assemblages were preserved and are now observed.
In the South was the 'Tethyan Realm', based on the Tethys Ocean, whereas further north was the cooler 'Boreal Realm' - which was further subdivided into two subprovinces, those of the 'Central Russia' and 'Central Europe' Realms.

The Boreal Realm

In the North, the Boreal Realm was characterised by belemnites, which arrived in 'Britain' in either short-term migrations of pulses of cold water, or during intermittent phases of shallowing (falls in sea level).
A [scheme] using this Boreal zonation is useful in the Chalk of Northern England; but is not apparent in the Chalk of the south.

[Do I want this para??] This high-latitude boreal zone, defined in offshore Norway (and equating to the boundary between Volgian and Ryazanian in Siberia) is (confusingly!) an ammonite zone. As it may coincide with the top of the British Portlandian (in Dorset), it is generally more useful for local zonation within the UK.
[If i use it i need more detail?]

The Tethyan Realm

In the South, the Tethyan Realm deposits are characterised by ammonites, and by planktonic foraminifera.
These provide an international standard (as plankton is more able to spread), so the Tethyan Realm definition is that that has gained international status.
However; the majority of these micro-fossils were absent in the colder north seas, so the two distinct biostratigraphies are only very tenuously correlated at many levels.

Britain therefore favours a national, rather than the international, zonal system, which is based on benthic species within the UK area.
[Does it? rephrase!]

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The Northern and Southern Provinces

Within the UK, the Boreal Realm resulted in what we now recognise as the Northern Province of Chalk, which outcrops in Yorkshire, while the Tethyan Realm results in the Southern Province Chalk, which outcrops from Dorset across to Kent.
The Chalks of these Provinces are also known as 'Grey Chalk' (northern) and 'White Chalk' (southern).
[Possibly, or I may have got geographical boundaries muddled up with lithological. Perhaps a definition of Grey and White, then "Grey Chalk characterises the Northern Provine and White the SOuthern"? ]

However, there is a distinct regions between where the lithologies and fossils are [inter-bedded], which is thus known as the Transitional Province - for example, alternations of both Boreal and Tethyan fauna suites of fossils can be seen at Speeton in Yorkshire.

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Fauna Migration

A further confusion arises as climates change. As climates and temperature change, different fauna migrate geographically with time. Thus, the changeovers in fauna that are used to characterise a stage boundary may occur at different times in the different provinces, and involve different fauna suites.

This leads to further difficulties in correlation when attempting to set dates to specific stages that are defined by fauna that may migrate with time. [This is a fairly useless sentence - but i need a little more padding in this para.]

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Tectonics

As discussed in the palaeogeography pages, the Cretaceous was a time of great tectonic activity, with volcanism due to both oceans rifting apart, and Africa "scraping along and under" Europe (and the various smaller continental plates in the Tethyan Ocean). Thus, the area of Europe was subject to many stresses of all varieties (compressional, extensional and strike-slip), which affected the ancient bedrock and Variscan structures.
Tectonic movements, combined with sea-level changes (that were only partially due to that tectonic activity) thus led to the complex stratigraphy of the Chalk, and the changes on the structural geometry and sedimentation patterns.

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Chalk

The nature of chalk has already been discussed in detail in the Chalk page. The following is a brief summary, with further comment on its importance in correlation.
Specifically to the UK regions, it is now generally agreed that the Chalk formed at depths between 100m and 500m across the UK, with the Northern Chalk at the greater depths. (recall, however, that the coccolithophores themselves, lived in the top 40m, in the photic zone.) Southern Chalk is a more shallow-water deposit.

The chalk is formed of coccoliths, and is a fine-grained, low-magnesium calcite limestone (micrite).
[However;] silicates have also been found as post-depositional coatings on coccoliths, which may have a profound effect on the properties of chalk.
like what? hardening it? more detail

Chalk also contains calcispheres and foraminifera.
Chalk, being biogenic, had to wait for the Mesozoic and the evolution of calcareous plankton to evolve - there is no (coccolith) chalk from the Palaeozoic, any more than there is coal from before the [Devonian], when trees evolved.

The 'Grey Chalk' of the Northern Province contains much clay, which (mixed with chalk) produced marl.
The 'White Chalk' of the Southern Province, in contrast, is on average more then >98% pure CaCO₃, a purity which reflects the greater distance that the land was from the sea at the time - due to the late Cretaceous marine transgression over the continents.

The marl-seams are useful in long-range correlation as they form conspicuous marker-beds, that can be traced over wide areas. That many of the marl seams were volcanic in origin has long been suspected, and has now been proved.

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Rhythms in the Chalk

As the Chalk is studied in greater detail and at more sites, more is continually being discovered.
Since the 1930's, Milankovich cycles have been considered the cause and catalyst of Ice Ages. However, more recently, evidence of this orbital forcing in older, Cenozoic, strata has been found.

(A longer description of Milankovich cycles can be read at Orbital Forcing, but a brief summary follows). Skip to Evidence in the Chalk if you don't need this.

Milankovich Cycles

The Milankovich cycles are three orbital cycles which are thought to influence the climate of Earth, by changing the incidence and distribution of solar radiation that reaches it. These are:

• Eccentricity of the Earth's orbit. This cycle has a periodicity of 110,000 years, and affects the total amount of solar radiation reaching the Earth. An elliptical orbit exaggerates the seasons in the hemisphere in which winter occurs at aphelion and summer at perihelion.
• Change in the angle of tilt of the Earth's axis (21.8° - 24.4° ). This cycle has a periodicity of 40,000 years. It changes the effective latitude of the Tropics, with a higher tilt giving warmer summers and colder winters.
• Precession of the equinoxes. This cycle has a periodicity of 22,000 years. Solstices and equinoxes move clockwise around the orbit. It changes the position in the Earth's orbit at which the seasons occur.

The three cycles operate independently of each other, and combine to produce variations in the intensity of the seasons and the amount of solar radiation reaching the Earth.

Evidence in the Chalk

Within the Southern Chalk, the sedimentation has been shown to be affected by the precession cycle of the Milankovich Band.
A background alteration of more, and less, calcareous layers has established a 'cyclostratigraphy' for parts of the Cenomanian and Turonian successions. These cycles have been analysed (using oxygen isotope data of the carbonates) to reveal an inferred seawater palaeo-temperature difference of 4^oC between the marl - limestone alternations at Folkestone in the South East, where the marl indicates cooler conditions and the limestone warmer.
Using absolute dating, it has been shown that the marl-limestone couplets of the Cenomanian succession reflects orbital forcing of sedimentation, related to the precession cycle of the Milankovich Band.

This cyclicity is particularly useful in the search for oil, and a more technical description of the appearance of Milankovich cycles in Cretaceous Chalk can be seen in a discussion of Chalk in the Danish Sea.

Additionally to these Milankovich cycles, the Chalk bears evidence of geochemical signals relating to oceanographic pulses, climate change and volcanic events. Isotopic variations for carbon and oxygen, and peaks of manganese, iridium and strontium, have all been used as stratigraphical marker beds.

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For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk