Palaeomagnetic Correlation of the Upper Triassic of Somerset, England, with Continental Europe and Eastern North America
Briden, J. C., Daniels, B. A., Journal of the Geological Society
Abstract: Three Normal and three Reversed palaeomagnetic polarity zones were identified in the exposed 67 m section of Mercia Mudstone at St Audrie's Bay, Somerset, which has been regarded as being of Norian age on limited biostratigraphic evidence. The polarity sequence is correlated with magnetozones E 15 to E 17 in the Newark Basin, New Jersey, further supporting the inferred Norian age and tentatively placing it in the mid-Norian. Correlations with the northern North Sea and the Alps are also suggested. Results from Doniford Bay, where tectonic dips are steeper but which cannot be correlated precisely with the St Audrie's section, are incorporated for the calculation of the mean direction of Characteristic Remanent Magnetization which has declination 30.8 deg and inclination + 34.3 deg (N= 27 horizons, alpha^sub 95^ = 5. 1 deg ). The corresponding palaeomagnetic pole is 49.6 deg N, 128.4 deg E (dp = 3.3 deg, dm = 5.8 deg), which also correlates with the Norian of the Newark Basin, while possibly indicating that small adjustments should be made to the orientation of all or part of Britain within the 1965 Bullard et aL configuration as a representation of Late Triassic geography.
Keywords: Triassic, Norian, Somerset, magnetostratigraphy, palaeomagnetism.
Forty years after the pioneering palaeomagnetic studies of the Triassic in Britain (Clegg et al. 1954; Creer 1957), the palaeomagnetism of these rocks is again of interest for both palaeogeographic and stratigraphic reasons. For example, as uncertainties in knowledge of the past configurations of continents shrink to the scale of hundreds of kilometres, such studies can begin to contribute to the history of individual Mesozoic sedimentary basins that have subsequently been fragmented by continental break-up. Resolution of controversy over the precise form of apparent polar wander (apw) paths, as is currently an issue with respect to North America, becomes crucial in this context. For palaeoenvironmental studies, the discovery of chronometers in the geological record (e.g. facies significance of Milankovitch cyclicity), and the worldwide identification of particular geomagnetic reversals that are potentially precise correlation tools on the order of a thousand years, lends a new importance to complete determination of the Geomagnetic Polarity Time Scale back through the Mesozoic and Palaeozoic.
The palacomagnetic database that is vital to progress on such problems in the Triassic is sparse. No pre-Callovian ocean-floor magnetic anomalies exist, and exposures of Triassic sedimentary rocks are commonly tectonized or diagenetically altered (as in the Alps) or have poor biostratigraphic control (as in the non-marine basins of western Europe and North America). Nevertheless significant progress has recently been made both in the Alpine and Mediterranean sequences (e.g. Gallet et al. 1993, 1994) and notably in the Newark Basin, New Jersey (e.g. Kent et al. 1995). The study described here is a further contribution to the database.
The sequence at St Audrie's Bay and Doniford Bay, Somerset
Late Triassic and Early Jurassic rocks, slightly flexured and somewhat disrupted by normal faulting, are well exposed over some 18 krn between Hinckley Point and Blue Anchor on the Bristol Channel coast of Somerset. The location of this and previously published studies of late Triassic rocks from Britain is shown in Fig. 1. The present study concentrates on the upper part of the Mercia Mudstone Group (formerly 'Keuper Red Marls' and 'Tea Green Marls') in St Audrie's Bay from UK National Grid Ref. [ST 1085 4305] to [1047 43101@ latitude 5 deg 11'N, longitude 3 deg 17'W. Here the exposed sequence consists of 67m of red calcareous mudstones and siltstones with interbedded greenish beds of similar lithology, overlain by c. 5 rn of 'Tea Green Marl'. It has been logged by the Geological Survey (Whittaker & Green 1983, fig. 9); their section is reproduced as Fig. …