The magnetostratigraphy of Coniacian-Late Campanian chalk sequences of Southern England

Abstract

Results from a detailed palaeomagnetic study of the Late Cretaceous sequences of Culver Cliff and Scratchell's Bay (Isle of Wight) and Seaford Head, East Sussex are presented. The sections range in age from Latest Turonian to early Late Campanian and consist of white chalk with flints. Hand samples and large volume rock cores have been collected. Both thermal and AF demagnetisation have been used to remove magnetic overprints. Measurements have been carried out using a CCL 'discrete sample' and a 2-G 'wholecore' cryogenic magnetometer. Average NRM intensities range between 0.0015 and 1.6008 mA/m. By carrying out repeat measurements on large volume samples, reliable determination of the remanence of such weakly magnetic rock has been possible. A reliability classification scheme is proposed to provide an objective means of assessing the quality of the palaeomagnetic results obtained from thermal and AF demagnetisation.

IRM acquisition experiments suggest the presence of single domain and multi-domain titanomagnetite. Mixtures of hematite and titanomagnetite also occur. Magnetic mineral extractions carried out by Dr. M. Hounslow have revealed the presence of detrital titanomagnetite and hematite preserved as inclusions within silicate grains. Moreover, the finest portion of the magnetic extract ("10%) have revealed bacterial magnetite preserved as individual grains and chains. Sample horizons which contain the highest proportion of bacterial magnetite appear to have higher NRM intensities.

Geomagnetic polarity zones representing Chrons C33N, C33R and C34N have been located and reliably tied to the macrofossil (Dr. A S. Gale, pers. comm.) and nannofossil (Dr. J.A. Burnett, pers. comm.) biostratigraphic zones in the sections studied. A standard magnetic polarity stratigraphy for the Late Cretaceous is proposed.

These studies have thus provided magnetostratigraphic age calibration points of 83.000 and 78.781 Ma (the C34N/C33R and C33R/C33N boundaries of Cande & Kent, 1992) for the English Chalk successions. By constructing a composite magnetostratigraphic section, and by assuming that deposition of the Chalk was relatively constant, a magnetostratigraphic time scale for southern England is proposed and used to calibrate the ^^Sr/^Sr curve of McArthur et al. (1992) and to date Santonian-Campanian chalk-flint cycles.

The characteristic rhythmic bedding of the Upper Cretaceous pelagic carbonate sequences of the UK have been interpreted as the result of orbital variations. Though measurement of chalk-flint cycles within the English Chalk have been previously attempted, difficulties in locating and dating Late Cretaceous stage boundaries has proved a major hindrance in such studies. By determining the mean frequency of the chalk-flint cycles, inaccuracies in section and cycle measurement can be minimised, and an 'idealised' number of cycles for the chalk representing Chron C33R calculated. A histogram of the chalk-flint cycle thickness for strata enclosed within a 170 metre thick, reverse polarity magnetozone representing Chron C33R at Scratchell's Bay reveals a mean cycle thickness of 0.7 metres. A duration of 17, 362 years is inferred for chalk-flint cycles which probably represent the quasi-periodic orbital precession cycle (18kyr).

By employing the best quality palaeomagnetic data, a palaeopole for southern England, during Coniacian-Late Canipanian times (78-85 Ma), is proposed (Long.=184°E, Lat.=73.0"N). However, though the Q value (van der Voo, 1988) of this palaeopole is low (Q=5) this pole position helps to confirm the palaeomagnetic pole of Heller & Channell (1979) derived from Late Cretaceous limestone (83 Ma) of the Miinster Basin, Germany (Long.=181°E, Lat.=76°N). Thus, during Coniacian-Late Canipanian times southern England lay at a palaeolatitude of 34±7°N.

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