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1 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
2 BP Research Centre, Sunbury-on-Thames, UK
Natural gamma-ray spectrometry on coastal sections from Redcar and Robin Hood's Bay permits the radioactive characterization of five different mudstone units and the construction of an outcrop gamma-ray log for the Lower Lias. Correlation of these five units with their equivalents in the gamma-ray log of the Felixkirk Borehole, located 40 km to the west, has been established. Statistical comparison of tie points in the composite coastal section shows there to be a stratigraphic gap of 10 m between the two localities relative to the continuous section in the Felixkirk Borehole.
The distribution of Th (ppm), K (%) and U (ppm) are generally highly correlated with the total radioactivity. However, anomalies do occur: a peak in the Th/K ratio can be related to an iron-oolitic horizon marking a condensed sequence in the early/mid-Sinemurian; a sharp fall in the Th/K ratio followed by high U values coincides with the early-Pliensbachian transgression. Comparison with mineralogical (clays) and chemical (Si/Al and Al/K ratio, V, Total Organic Carbon) profiles of the same section allows for an evaluation of the significance of radioactive element distribution in terms of grainsize, run-off and oxygenation indicators.
A spline curve fitted through the total radioactivity data emphasizes low frequency cyclicity (wavelengths of between 10 and 30 m) in the outcrop section. These cycles appear to be correlatable with cycles of comparable wavelengths in the low-pass filtered gamma-ray log of the Felixkirk Borehole. Sedimentological evidence confirmed high radioactivity peaks correspond to transgressive phases. The gamma-ray signal of the Felixkirk borehole has also been analysed for high frequency cyclicity. With an overall rate of accumulation of about 13m/Ma variation in the Milankovitch band is marked by units 5 m or less in thickness. Evidence for eccentricity, obliquity and precession cycles has been found both in borehole and in outcrop. A typical shift from lower to higher frequencies occurs consistently in transgressive phases, and is related to increased sedimentation rates.
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