BioStratigraphy

Working notes from the chapter of the BUCS book

Biostratigraphy is the study of place, date, and correlation, using fossils.

The following discussion covers:

• Historical Framework
• Zone markers
• Correlation

Historical Framework

By the middle of the 19^th century, there was already enough known about the distribution of fossils (a study initiated by William Smith in ....) to produce a list of typical fossils for the subdivisions of the Upper Cretaceous Series into stages. The first [data] was drawn from the Paris and Aquitaine Basins in France and Maastrichtian in the Netherlands. This was later extended to Kent, then the whole of England and Northern Ireland.
Until the 1970's, the zones thus marked were used as the framework for Chalk studies in the UK.

More recently, work on two internationally important groups, the ammonites and the inoceramid bivalves, has revolutionised global correlation in the Upper Cretaceous succession.
However; apart from a few isolated horizons of preservation such as hard-grounds, the originally-aragonite-shelled ammonites are generally rare in white chalk facies. Consequently, zonal schemes worked out in Central Europe using the calcite shells of inoceramid bivalves have been increasingly applied to the zonation and long-range correlation of English Chalk.

In 1984, the Upper Cretaceous stages were finally recommended, and the base of the Upper Cretaceous Series is (currently!) defined in Europe and is taken as the base of the Cenomanian Stage.

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Zone markers

The basal marker of the Cenomanian Stage (and the Upper Cretaceous Series) is the first appearance of the planktonic foraminifera Rotalipora globotruncanoides. This is virtually coincident, but slightly lower than the entry of the ammonite Mantelliceras mantelli, the zonal index fossil of the basal Cenomanian ammonite Zone.
In southern England (as in much of northern Europe), sedimentation is missing for perhaps 1 - 2 million years, between the Albian and Cenomanian Stages.

The Upper Cretaceous Series is now divided into six stages, each of which is further divided into substages and zones, based on a wide range of fossil taxa.

Formerly, the basal boundary of a stage was generally marked by the base of a fossil zone. However, alternative criteria are now permitted, so basal boundaries may be marked, for example, by any of:

• The first appearance of a particular fossil
• The last appearance of a particular fossil
• palaeomagnetic reversal
• a geochemical event (such as ....? does this include volcanic?.
• a cyclostratigraphical couplet (such as one representing a Milankovich cycle - any others?

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Correlation

The Chalk in unusual, in that many individual beds can be correlated over great distances. For example, the Upper Turonian marker marl seams are traceable from Sussex to Yorkshire in the north, to Germany in the east, and through the Paris Basin in the south.
Get OU book eg
These marls form part of a tephro-event stratigraphy, and both geographically diverse marls, and tuffs in northern Germany, can be linked to a single event.

Another long-distance feature that can be used in correlation is the palaeomagnetic reversal, from the long Cretaceous Quiet Zone, to the Lower Campanian strata; although the stratigraphical positioning of this reversal is not universally agreed. Such lateral correlations do break-down over major tectonic structures.
Whatever this all means

The preserved onshore Upper Cretaceous deposits of the British Isles are incomplete, generally ending in the Campanian Stage (as they do in Portsdown).
However, in Norfolk they end in the Lower Maastrichtian, and offshore in the North Sea Basin and central English Channel the successions are more complete and can locally extend up to the Cretaceous-Tertiary boundary.

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