Fossil-wood carbon-isotope stratigraphy of the non-marine Wealden Group (Lower Cretaceous, southern England)

Journal of the Geological Society, Jan 2004 by Robinson, Stuart A, Hesselbo, Stephen P

A composite fossil-wood carbon-isotope curve for the Early Cretaceous

A problem with the data from Worbarrow Bay, Dorset, is the uncertainty of the age range of the section (i.e. Hughes & Croxton 1973; Oldham 1976; Hughes & McDougall 1990), which makes it difficult to assign an independent stratigraphic tie-point from which to start chemostratigraphic correlation with the marine record. In the following analysis we have combined the Worbarrow Bay data with the Isle of Wight data to produce a composite fossil-wood carbon-isotope curve for the Early Cretaceous.

To produce this composite curve some assumptions have to be made. Of critical importance is the relative stratigraphic position of the Wessex Formation-Vectis Formation boundary on the Isle of Wight and at Worbarrow Bay. Ruffell & Batten (1994) suggested that 7 m of shale at the very top of the Wessex Formation in Worbarrow Bay are a lateral equivalent of the Vectis Formation at Swanage. Strahan (1898, p. 127) noted that at Corfe railway station (between Swanage and Worbarrow) the Vectis loses 'its distinctive character as a shale, and . . . [passes] . . . westwards into sand and sandy clays'. However, further to the west (at Mupe Bay; Fig. 2) there may have been significant erosion of the Wessex Formation, before deposition of the Lower Greensand (Arkell 1947). Given the westward attenuation, possible erosion and lack of diagnostic fossils, it is impossible to say with any certainty that lateral equivalents of the Vectis Formation are definitely present at Worbarrow Bay. Our own observations at Worbarrow Bay show that there is a 4.5 m thick package of grey and green mudstone and fine sandstone below sediments that are clearly of the Lower Greensand. Whether these mudstones are equivalent to the Vectis Formation is not entirely clear. For the purposes of this composite curve it has been assumed that the Wealden Group-Lower Greensand boundary at Worbarrow Bay is age-equivalent to the top of the Wessex Formation on the Isle of Wight, and that the Vectis Formation, or lateral equivalents of it, are not present at Worbarrow Bay. If the following exercise is repeated with a thin (

The thickness of the Wessex Formation in Worbarrow Bay is c. 376 m (including 41 m at the base which are not exposed), whereas in the Arreton Borehole on the Isle of Wight there is c. 557 m (Fig. 1; McLean 1953; Falcon & Kent 1960). On the basis of seismic interpretations, the Wessex Formation on the west coast of the Isle of Wight has a total thickness similar to that in the Arreton Borehole (see Butler 1998). This suggests that the Wessex Formation in the Isle of Wight area is c. 1.5 times thicker than it is at Worbarrow Bay. To be able to plot all the data from Worbarrow Bay, Hanover Point and Sudmoor Point on the same composite depth scale, the Worbarrow Bay depth values have thus been scaled by a factor of c. 1.5. Figure 8 shows all the [delta]^sup 13^C^sub wood^ data from Worbarrow Bay, Hanover Point and Sudmoor Point, and those of Grocke et al. (1999), at their composite stratigraphic heights relative to the top of the Wessex Formation (0 m is equivalent to the top of the Wessex Formation). In Figure 9 the complete composite fossil-wood dataset is shown along with a three-point running average. The results of the biostratigraphic work at Worbarrow Bay by Hughes & Croxton (1973) and Hughes & McDougall (1990) are shown, scaled to the composite depth scale. Figure 9 also shows a smoothed composite carbonate carbon-isotope curve for the Berriasian-Aptian from Tethyan sections (references in figure caption), which has been tentatively correlated with the fossil-wood data, on the basis of similarities in the shape of the curves. The Barremian and Aptian have been correlated as they were in Figure 7. The tentative correlation between the Wealden composite isotope curve and the Tethyan carbon-isotope curve suggests that the Valanginian-Hauterivian boundary occurs below the Coarse Quartz Grit and that the Valanginian is relatively condensed in the English sections compared with the Hauterivian and Barremian.