An Anisian, Ladinian and Carnian non-marine magnetostratigraphic reference section: the coastal exposures between Budleigh Salterton and Branscombe, South Devon, U.K.

 M.W. Hounslow ⁽¹⁾, Gregg McIntosh⁽³⁾, Ramues Gallois ⁽²⁾, and Guy Jenkins⁽¹⁾

⁽¹⁾ CEMP, Environment Lancaster, Geography Department, Lancaster University,Lancaster, LA1 4YW, m.hounslow@lancaster.ac.uk;

 ⁽²⁾ 292 Stoke Valley Rd., Exeter EX4 5ER

  ⁽³⁾ Departamento de Geofisica y Meteorologia, Facultad de CC Fisicas,

Universidad Complutense de Madrid, Avda Complutense s/n, 28040 Madrid, Spain

The most readily accessible and most complete outcrop exposures of the non-marine middle and Upper Triassic in Western Europe occur on the South Devon coast between Budleigh Salterton and Pinhay Bay. The magnetostratigraphy through 550m of these sediments has been evaluated, through the Sherwood Sandstone Group (SSG) and the lower 308m of the Mercia Mudstone Group (MMG). The MMG is a largely monotonous sequence of 435m of mostly red, gypsum and dolomite bearing, lacustrine/loessic mudstones, without sedimentological evidence of significant hiatus. The 242m of the SSG is mostly composed of stacked fluvial channel sandstones. The biostratigraphic age of the succession is constrained by a Rhaetian age for the top-most MMG, an upper Carnian age (Tuvalian) for the Arden Sandstone Formation within the middle of the MMG, and an Anisian age for the Otter Sandstone Formation of the SSG.

The palaeomagnetic directions are compatible with other Middle Triassic palaeomagnetic data from cratonic Europe. The data defines 42 magnetozones with the thickest at ~40m and the shortest at ~2m. The magnetostratigraphy from the lower part of the SSG indicates the interval across the Olenekian/Anisian boundary is preserved above a major palaeosol. The magnetostratigraphy of the Otter Sandstone Formation (upper part of SSG) provides a good match to other published marine Anisian magnetostratigraphies. This, and the magnetostratigraphy from the overlying MMG, suggests the Otter Sandstone Formation represents the entire Anisian. The magnetozones from the lower part of the MMG show a close match to the marine-Ladinian composite magnetostratigraphy of Muttoni et al. (2000), indicating the SSG/MMG boundary is at the level of the Trammeri conodont zone. A dominantly reverse polarity interval above the Arden Sandstone Formation is not matched in any published marine Carnian record, but is similar in style to the E8 to E12 chrons from the Newark Supergroup. The Devon MMG magnetostratigraphy appears to stitch together the Ladinian marine composite and the base of the Newark Supergroup magnetostratigraphy. However, these correlations produce contradictions between the palynology-based ages of the Arden Sandstone Formation and the Lockatong and Stockton formations of the Newark Supergroup. One resolution to this problem requires that the Carnian/Norian boundary is some 1000m lower than commonly accepted, within the Newark Supergroup.

Reference

Muttoni , G, Gaetani, M., Budurov, K., Zagorchev, I., Trifonova, E., Ivanova, D., Petrunova, L., & Lowrie,W. (2000). Middle Triassic paleomagnetic data from northern Bulgaria: constraints on Tethyan magnetostratigraphy and paleogeography. Palaeogeog. Palaeoclimt. Palaeoecol., 160, 223-237.