Magnetostratigraphy and biostratigraphy of the Upper Triassic and lowermost Jurassic succession, St. Audrie's Bay, UK
Abstract The St. Audrie's Bay section in west Somerset comprises the uppermost Mercia Mudstone Group, the Penarth Group and the basal Lias Group and includes a candidate Global Stratotype Section and Point for the base of the Jurassic. The magnetostratigraphy has been evaluated through 122 m of this section at 147 stratigraphic levels, which range in age from mid-Norian to earliest Hettangian. In red dolomitic mudstones, the remanence is carried predominantly by haematite, whereas in non-red lithologies, it is mostly carried by magnetite. The mean virtual geomagnetic poles fall near the mean Upper Triassic and Lower Jurassic apparent polar wander track and display the start of the northeast-directed track typical of the Jurassic. The magnetostratigraphy comprises nine major magnetozones, five of normal polarity and four reversed, together with several minor magnetozones. In the Mercia Mudstone Group, the 68 m of the Twyning Mudstone Formation examined includes three major normal magnetozones (SA2n, SA3n and SA4n) and the Blue Anchor Formation has predominantly reversed polarity (SA4r) except at its top, in the Williton Member. The Penarth Group and basal Lias Group have predominantly normal polarity (SA5 to SA6n) but short reversals occur within the Westbury Formation and at the base of the Lilstock Formation and the Lias Group (SA5r). This magnetostratigraphy is a good match with that found in the upper part of the Newark Supergroup succession in the eastern USA. The distinctive long reversal (SA4r) in the Blue Anchor Formation is equivalent to Newark Supergroup magnetozone interval E18r to E20r. Magnetozone SA5n, located mainly in the Penarth Group, probably equates with part of E22n and all of E23n in the Newark Supergroup. The reversed magnetozone, SA5r, at the base of the Lias Group, may correspond either with E23r in the Exeter Member (Passaic Formation) in the Newark Supergroup or with an undetected reverse polarity interval within the Newark Basin flood basalts. A change in the composition and diversity of terrestrial microfloras that occurs in the upper part of the Penarth Group at St. Audrie's Bay and elsewhere in the UK, is similar to that interpreted as marking the Triassic–Jurassic boundary in the Newark Supergroup. At St. Audrie's Bay, this change occurs c. 0.6 m below SA5r, within the Rhaetian, whereas in the Newark Supergroup, it occurs c. 20 m above the potentially equivalent E23r. Reconciliation of these disparities requires that either the microfloral changes are not synchronous between these locations or the change in the Newark Supergroup is time-equivalent with late Rhaetian conodont bearing strata. The correlation of marine and nonmarine Upper Triassic magnetostratigraphies is revaluated with the new data from St. Audrie's Bay, indicating the Twyning Mudstone and Blue Anchor formations are mid to late Alaunian (mid-Norian) in age. The Sevatian (late Norian) is represented by the Williton Member and the lower part of the Westbury Formation, but is incomplete because of disconformities at the base of the Williton Member and Penarth Group.