Magnetostratigraphy of lower cretaceous and oligocene formations in the UK and northwest Belgium

Ting, Fung (1991) Magnetostratigraphy of lower cretaceous and oligocene formations in the UK and northwest Belgium. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The magnetostratigraphy of certain Lower Cretaceous sedimentary sequences (the Wessex and Weald Clay Formations) of southern England and Oligocene sequences of southwest England (the Bovey Formation) and northwest Belgium (the Boom Clay Formation) has been investigated. Five sections through the Wessex Formation along the coast of southern England are well-correlated using combined geological, sedimentological and magnetic evidence (including NRM intensity variations and magnetic polarity reversals sequences). The results indicate that below the magnetic reversal found in the Vectis Formation by Kerth & Hailwood (1988), four other significant reversed polarity events occur within the Wessex Formation. When combined with available biostratigraphic evidence (pollen palynomorphs, Hughes & McDougall 1989) this magnetic polarity sequence can be correlated with the geomagnetic polarity time scale (GPTS) between Chron CM1 and Chron CM5 (or possibly Chron CM8?). This confirms that the Wessex Formation spans the Barremian and upper Hauterivian stages. The age of the Coarse Quartz Grit (CQG) at Worbarrow Bay can be estimated as 123Ma, using the GPTS. However, the CQG layers at other sections occur at different positions in the polarity sequence, and clearly have different (younger) ages than the CQG at Worbarrow. Preliminary results from two short sections through the Weald Clay Formation of southeast England indicate the presence of a magnetic reversal in Bed 2 of Thurrell et al (1978) in this formation. A reliable magnetic polarity sequence has been obtained for the Oligocene Bovey Formation of SW England, even though this formation is very weakly magnetised. When combined with age constraints based on tectonic controls of the fault basin and limited biostratigraphic evidence, this magnetic polarity sequence can be correlated with Chron C10 to C12 of the Middle to Early Oligocene GPTS, spanning the time interval 30.25 to im34.0 Ma. Although serious magnetic overprints, believed to be due to secondary diagenesis, have affected the upper part of the Boom Clay Formation of Belgium, a reasonably reliable magnetic polarity sequence has been defined for its lower part. The reliability is supported by the good repeatability of magnetic polarities within small sampling intervals. When combined with available detailed biostratigraphic evidence, the Boom Clay magnetostratigraphy can be correlated with Chron C11 to C12 on the GPTS of Berggren et al (1985), indicating an age range of 30.5 to 33.5Ma.

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