Cretaceous: the upper chalks
The cretaceous experienced a number of significant events which changed life and earth itself. One event which is clearly visible in Britain is the deposition of the upper chalks in the late cretaceous.
The late cretaceous chalks can be traced throughout southern England with successions from the Cenomanian to the early Maastrichtian.
The upper chalks were deposited through-out the five stages in the Late Cretaceous; Cenomanian, Turonian, Coniacian, Santonian, Campanian and the Maastrichtian.
Much of the geological foundation of southern England is shaped and sculpted by a thick bed of white chalk. The chalk is made of limestone with nodules of flint in the upper beds. The chalks have gained much interest as they hold key scientific information which is fundamental in future construction projects, development and the protection of existing groundwater resources and utilizing materials for industrial processes. The fossil content of these chalks have also made major key geological contributions. Also in addition with analysis of structures and sedimentlogy of chalks, modeling has allowed us a glimpse into the climate and biodiversity of this time period.
The late cretaceous lasted from 99.6 - 65.5 Ma. During this period sea levels were extremely high which lead to the deposition of chalk on the British Isles and also forming on many other continental regions of the world. During the Late Cretaceous the Atlantic Ocean began to lengthen and widen. The ocean was now circulating vast amounts of water north and south. Along with the rise in sea levels the breakup of Gondwanaland and Laurasia was taking place. The cause of these events may have been as a result of the following factors:
1. The expulsion of sea water onto continents by bulging ocean basins.
2. High levels of carbon dioxide keeping atmosphere much warmer during winter (DeConto et al.,1998)
3. High sea levels due to very little water being trapped as ice (Hays and Pitman, 1973; Jenkyns, 1980).
The oceanic anoxic event which deposited the black shale formation is an indicator of the above factors taking place.
Closer to the end of the cretaceous period dramatic changes had occurred on earth with the disappearance of many fossil groups. The K-T extinction marked the end of the cretaceous.
Life was abundant during the cretaceous, with dinosaurs dominating the herbivore guild in the late cretaceous and the oceans saw the final radiation of pliosaurs. The diversity and biomass of marine organisms flourished in this period. Examples of this are the domination of angiosperm plants in the late cretaceous and the abundance of calcareous nannoplankton. For marine life the driver for the abundance in life may have been because of the gradual rise of sea levels and change in ocean chemistry due to the active mid ocean ridges in this time period. Water temperatures were rising along with the rise in methane and carbon dioxide.
Palaeogeography, Climate and Sea Level curves:
The Boreal Realm and Tethyan Realm influenced the distribution of fossils and sediments in the upper cretaceous stratigraphy in Britain. The North Sea was covered by the northern Boreal Realm which was assumed to be colder water. Modern researchers have imagined the Boreal Realm to be an area where belemnites thrived and later migrating southwards with surges of colder water or during stages of shallowing sea levels. This stage is better applied to more northern and shallower shelf parts of Britain rather than southern England.
Common fossil groups of the Tethyan Realm deposits include; Ammonites and the unicellular planktonic foraminifera. Many of these planktonic fossils are absent in the Boreal sea thus the two distinct biostratigraphies. These two realms are reflected in the deposits of the upper cretaceous deposits in Britain. The northern province of Britain mirrors the northern Boreal Realm and the southern province relates more to the Tethyan Realm. Between the two provinces the lithologies and fossils indicate a transitional province.
During the late cretaceous there was high sea levels and significant tectonic change globally. Gondwanaland and Laurasia, the super continents were breaking up. Africa began to under-ride Europe and shear along the southern edge. This activity converted tensional stress to compressional stress tectonics. Such tectonics can be used to explain geometry of sedimentary bodies in the chalk and the anomalous stratigraphies found in all provinces and also be related to erosional channels, slumping and major hiatuses. Parasequences and episodic sedimentation can also be explained using such tectonics.
Upper cretaceous series: Chalk
Chalk is entirely planktonic and biogenic. It is formed of millions of calcareous nannoplankton, known as coccolith algae which flourished on the surfaces of oceans and seas and as time progressed settled on the sea bed. Where combined with the remains of living bivalves, foraminifera and ostracods to form the main components of the chalk. Chalk is usually very soft. Fine grained and composed of very pure white limestone. The chalk formed in varying depths, from shallower shelf areas to deeper parts of the basin. The chalk in the north of England is considered to be deeper water deposits in relation to the same chalk deposited in southern England.
As you move upwards through the rock column the chalk changes composition. The chalk contains a considerable amount of clay which produces marl. This marl is part of the grey chalk subgroup which is significantly different from the white chalk subgroup which on average contains 98% pure calcium carbonate. This is due to the late cretaceous transgression of the sea onto the continents. Seams of marl can also be identified in the chalk. Some of the marl seams are of volcanic origin, with distinctive brown colour and some with green and grey colours. REE analysis of the clay minerals suggest the marl seams analyzed in the Northern Province may have been airborne volcanic ash. Newhaven Chalk and Flamborough Chalk formations show similar marl seam characteristics.
Flint is another component of chalk and they appear in nodular seams, flat beds and linings to fractures. Flint is also a key characteristic of the upper part of the chalk sequence. The flint is a siliceous material, which is hard and brittle. Flint formed by replacing chalk and is also excellent in preserving trace fossils.
Syndepositional hardening of seabed sediments can be identified by the occurrence of nodular chalks and hardgrounds. Early nodules to fully developed hardgrounds can be used to recognize a lithification series. Sediment can also be lost through dissolution. A significant amount of chalk has been lost to pressure solution. The chalks have stratigraphical differences in strength, hardness, porosity and density. In areas of hard chalk, pressure solution has caused stylolites to form.
Chert, greensands and limestones were deposited in the upper cretaceous due to the transgression of the upper cretaceous sea onto the shelf areas of the United Kingdom. The transgression created shallow water deposits of glauconite rich greensands and marls, chert beds and quartz sands.
There are two sub groups; the grey chalk subgroup and the white chalk subgroup.
The grey chalk subgroup starts from the base Glauconitic Marl and ends at the base of the Plenus Marls Member. The white chalk begins at the Plenus Marls and follows all the way up the chalk.
Cyclostratigraphy, episodic events and chemostratigraphy
Cyclostratigraphy can be recognized using background alternation of more or less calcareous layers. These cycles are analyzed using oxygen (18O/16O) isotopes of the carbonates. The dating had given an inferred sea water palaeotemperature difference of 40C between the marls which was cooler and limestone which was warmer in the Cenomanian succession. The volcanic ash beds interbedded with marine sediments rich in fossil are indicators of episodic events. Along with these volcanic episodic events, fossil assemblages can be used to correlate more authentic time framework of these deposits. In the chalk geochemical signatures are also left behind which can tell us about oceanographic pulses, climate change and volcanic events. This can be done by looking at the variations in isotope of carbon (13C) and oxygen (18O), combined with peaks of manganese (Pomerol, 1976, 1983) and iridium (Pratt et al., 1991) and strontium are used as stratigraphical marker beds.
The changes in fauna within the chalk vary significantly from one horizon to the other. This reflects how evolutionary or environmental changes affect the fauna in any one region varied with time. The way a fossil is preserved also effects the Biostratigraphy; ammonites are commonly found within the grey chalk but rarely within the white chalk. Some fossils are present through distinct periods and it is these which can be used as zone fossils. However a zone fossil does not have to be present in all sedimentary rock types worldwide.
Deposits of the upper chalks in Britain: Cenomanian
The lower slopes of Eggardon Hill, Dorset contain exposures of the Eggardon Grit Member and are overlain by the fine-grained pale grey coccolith rich limestone from the chalk group. The Cenomanian strata can be identified from any major surface erosion and facies change.
The only strong piece of evidence for the Cenomanian age strata can be found in Babcombe Copse Quarry, Bovey Basin. This is based on records of ammonite in the Cullum Sands with Cherts. The Albian Cenomanian boundary is poorly defined in the Haldon Hills succession. There is no distinct major facies change or unconformity at this boundary.
Turonian & Coniacian
The chalks exposed on the Dover cliffs were deposited during the Turonian and Coniacian stages. At this time this region lay beneath relatively shallow sea. The purity of the chalk indicates its formation took place far from land. Within the chalks there are visible horizontal layers of flint formed from the silica of sea sponges. The chalks saw a series of ice ages. The glaciers carved and eroded the chalk. This produced valleys between hills. As the ice melted a lake was created which today is known as the North Sea which eventually over flowed and joined the Atlantic. Due to major tectonic uplift as a result of the European ad African plates colliding, the chalk today appears be above sea level. After the last age an increase in sea levels have caused coastlines to move inland, which has exposed chalk to a high degree of erosion which has created the faces of the white cliffs.
Santonian & Campanian
Phosphatic chalks have been deposited in southern England during these two stages. These deposits are abundant in brown phosphate. The phosphate can be found filled and coated on echinoid test, macrofossil fragments, faescal pellets, intraclasts and vertebrate remains. Phosphatic chalks accumulated in varying depths of 70m to a few hundred meters in oxic, high current, normal saline conditions in a continental shelf environment. Sedimentation rates were reduced by high surface productivity and moderate organic carbon fluxes. However the high rate of sediment mixing led to hardground surfaces and carbonate grains to be precipitated with phosphate. Phosphate precipitation thrived due to bacterial decomposition of organic matter which created the necessary geochemical environment for the phosphate to precipitate. It also raised phosphate content in pore waters. Phosphatization was a rapid process, the precipitated phosphate eventually transformed to carbonate fluorapatite.
The Lower Maastrichtian chalks can be found on Norfolk. The exposure are scattered in various places, largely in abandoned quarries, pits, coastal cliffs or glacially transported erratics. Some erratics studied show complex overfolding with shallow dipping northern limbs and vertical to overturned southern limbs. Others show one erratic on top of another erratic. It is now thought the method of erosion of the chalk was initiated by increased load due to the accumulation of the ice sheets which than raised the surface of the chalk causing the salt doming to occur. Due to the chalk being raised the chalk now could be eroded by surges within the ice sheets. The chalk is than deposited on the flanks.
The Overstrand Hotel Lower Mass in the overstrand area is one of the largest mass chalk outcrops, it show frequent overthrusting. The outcrop is a tabular body with some 100m in length and 13m high. Looking closely at the outcrop, three marly layers containing quarts pebbles can be identified, these may have been thrust planes. The outcrop had flint bands developed throughout the section.
There are no particular diagnostic planktonic species recorded in this succession. The succession generally conveys a Boreal feel with none of the Tethyan signatures. The Norfolk Campanian and Maastrichtian succession are dominated by smaller benthonic taxa. Indicating deposition in an shallow to deep marine environment.
The chalk is ideal for correlating information as many individual beds can be correlated over great distances. We can trace the upper Turonian marl seam known as the tephro-event stratigraphy, from Sussex all the way north to Yorkshire, moving on eastward to Germany and then southward to the Paris basin. However although correlation may seem very simple, complex tectonic structures underlie these correlations.
The upper cretaceous deposits are incomplete onshore up to the Campanian stage and follow off shore. However the deposits in Norfolk end in the lower Maastrichtian. More complete successions can be found offshore in the North Sea basin, central English Channel and Western Approaches Basins.
The cretaceous period has a very diverse range of geology. This geology has been imprinted in the chalk deposits of the late cretaceous. By studying the upper chalks we gain a glimpse of how life thrived in the cretaceous environment. The chalks also tell us that the global climate during the cretaceous was significantly warmer and sea levels were higher. In addition there were high concentrations of carbon dioxide within the atmosphere. This is an ideal analogue to use in understanding the modern greenhouse environment.
The structures within the chalks tell us the cretaceous was a period of major tectonic reorganization. By using this data we may be able to understand modern seismic hazards and gain a greater understanding of the resources; oil and gas that we have become so reliant on had formed at this time period.