The petrology of clay-rich beds in the Turonian (Upper Cretaceous) of the Anglo-Paris Basin

Wray, David Stanley (1990) The petrology of clay-rich beds in the Turonian (Upper Cretaceous) of the Anglo-Paris Basin. Doctoral thesis, City of London Polytechnic.


Clay-rich beds ('marl bands') in the Turonian of the Anglo-Paris Basin are primary features reflecting a sudden, sharp increases in non-carbonate deposition on the sea-floor. They differ from omission surfaces because they are not rhythmic, and usually only occur during eustatic falls in sea level. Subsequent to their deposition they have been modified by bioturbation and diagenesis. Marl bands containing a marl seem originated from a single, large influx of non-carbonate material. Flaser marls may have been formed from either a localised decrease in the amount of noncarbonate materiel deposited, or the deposition of several smaller pulses of non-carbonate material interspersed with white chalk sedimentation.

Mineralogical investigation shows that they contain the same non-carbonate constituents as adjacent white chalks although the relative proportions of the two main clay minerals (smectite and illite) differ markedly in higher Turonian marls. The variation in clay mineral composition seems to reflect fluctuations in sea-level.

Dinoflagellate cysts and the calcareous nannofauna do not appear to have been affected by marl band formation. In contrast, benthie foraminifera reflect the change in suface conditions with a marked decrease in the epifauna, and little change in the infauna.

Stratigraphically, geochemical patterns reflect changes in the overall composition of the clay fraction, making it posslble to suggest in which of the two principal clay minerals each clay associated element occurs. Rare-earth element patterns show that the non-carbonate fraction of both white and marly chalk is of detrital origin. Oxygen stable isotope data implies that the formation of a marl is accompanied by a small (2 -4[degree]C drop in ocean temperature.

Analysis of a series of samples from widely spaced localities within the Basin shows that individual marls can be geochemically fingerprinted, and confirms the established lithostatigraphic correlations. Using this method, samples of prevlously uncorrelated marls from within and below the Chalk Rock and Spurious Chalk Rock can be allocated 'Basin' names. An attempt to correlate Northern Province marls with those in the Anglo-Paris Basin was unsuccessful.

It is proposed that marls are formed as a result of erosion of clay-rich sediments from the borders of proximal emergent massifs at times of increased current activity induced by eustatic sea-level falls.

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