The volcanic events
Thin marly beds (5 in 20 cms) fit at various levels of the chalky series. Initially, in Germany of the North, the study pétrographique of these levels rich in clayey minerals showed that they contained volcanic splinters of bone and crystals of sanidine. This indicates that they derive from the fallout of volcanic ashes. The change in situ volcanic products leads to the training of qualified clayey minerals of bentonites (consult this bibliography)). The granular frequent glauconie in the coats of the beginning of the upper Cretaceous, in England, in The Scotland and in Ireland, is considered by Jean (2006) as resulting from the glauconitisation of péné-contemporary volcanic ashes bound to the continental break and to the opening of Atlantic Ocean
You should not confuse these bentonites in discreet beds with bentonites (or smectites) spread in very small concentration in the normal chalk or in the impure chalks. It is also necessary to distinguish them from detrital marly levels having for origin the continental erosion.
The geochemical data show the composition of ashes is of type rhyolitique. The volcanism which produced them is of explosive type intra-stick. Thick clouds or clouds cinéritiques must be thrown and pulled in the high troposphere and scattered on considerable areas (in the style of volcanoes current as Pinnatubo, Bulusan, the Holy Mount Helens, Redoubt or Eyjafjöll). The sedimentation and thus the burying in the chalky mud protects the track of an episode which is almost immediate in the geologic scale. In factual stratigraphy, we qualify these marks as téphro-events. A téphro-event is a perfect isochronal which connects various cups and his interest is essential to make a high-resolution stratigraphy.
In the outcrop, a bentonite appears sometimes as a homogeneous coat, but more generally as other marly levels with a structure flaser. In this case, the darker clayey nets wrap clearer chalky lenses. This structure recovers at the same time from a mechanical compaction and from a dissolution by pressure during the diagenèse, and also during the more premature bioturbation.
To identify a specific bentonite, represented by a particular name, should contain a double analysis:
- Criteria giving evidence of his volcanic origin,
- Intrinsic or relational peculiarities giving evidence of his uniqueness.
But it is difficult to respect such a rigor on the ground and the identification (the given name) remains more or less hypothetical.
Geochemical arguments for a volcanic origin
1) The presence of minerals or typically volcanic rocky elements (splinters of bone of glass or crystals automorphs of sanidine, of quartz or of zircon, association heulandite-clinoptilolite). Their preservation is sometimes possible, as in the German "tuffs".
2) The profile of rare earths normalized with regard to Cody Shales (shale-normalised rare - earth element or SNREE) is a tool used to differentiate the detrital or volcanic origin of bentonites (in particular, the negative anomaly europium cinérites).
3) Zr, Nb and Th is elements in track in stronger concentration, whereas Ba, Rb and K is less plentiful.
Arguments of identification
Geochemical reports can serve as "fingerprint" to characterize a marl, for example a scandium, a titanium, a vanadium and supposed yttrium to be more plentiful in the detrital marls.
On the ground, the association and the position with regard to other levels marks are widely used.
In the Anglo-Parisian Pond, chalks, particularly those of Turonien, contain marly insertions of some centimeters in some decimeters of thickness. These exceptional levels contrast with him taking chalky and were the object of specific carrying studies on:
- The mineralogical nature of the clayey fraction,
- The mineralogical nature of insoluble minerals in the acid,
- The geochemistry of elements in track and rare earths.
The dominant clayey mineral is the montmorillonite (or bentonite) of the group of smectites rich in Mg. It is the same mineral which is present in minor quantity in the ordinary chalk and which finds itself here distributed in great quantities (until 50 % of the rock) in a very short time. Bentonites can result from the change either of continental silicates (bentonites detrital), or of volcanic ashes thrown in the atmosphere during violent eruptions (bentonites cinéritiques).
Bentonites cinéritiques can present glass splinters of bone or crystals automorphs of sanidine, of quartz or of zircon. The profile of rare earths normalized with regard to Cody Shales is a tool used to differentiate the detrital or volcanic origin of bentonites (in particular, the negative anomaly europium cinérites). Zr, Nb and Th is elements in track in stronger concentration, whereas Ba, Rb and K is less plentiful.
The horizontal continuity of the levels bentonitiques is remarkable. They follow each other in England, in France and in Germany. Here is a picture recapitulating their usual names and their presumed nature.
|England the South||England is||Germany||Bentonite|
|Shoreham 2||Little Weighton 2||?||***|
|Shoreham 1||Little Weighton 1||?|
|Bridgewick 2||Thornton Curtis||ME|
|Bridgewick 1||North Ormsby||YOU||***|
|Southerham 1||Melton Ross||TC2||***|
|Glynde 1||Barton 1||TC||***|
|New Pit 2|
|New Pit 1||Croxton|
Picture of the téphro-events in "British Upper Cretaceous Stratigraphy"
In East Anglia (Bristow, on 1990), marl Mount Ephraim is correlée with her m. Southerham 1 and Twin Marls with the marl Caburn.
The position of the broadcasting volcanoes is very hypothetical. He could involve volcanoes rhyolitiques situated in the Atlantic Ocean the rising North, between Groënland and Norway, in the North of the Ditch of Rockall. Volcanic tracks offshore are described in the ditch of the Western Approaches, off current Manche (Evans, on 1990) and in the pond of the North Sea (Jean and al ., on 2000).