Evolving stratigraphy of a Middle Triassic fluvial-dominated sheet sandstone: The Otter Sandstone Formation of the Wessex Basin (UK)

The Middle Triassic (Anisian) Otter Sandstone Formation of the Wessex Basin (UK) has long been considered a single undivided geological formation, but building on previous work, it is shown here to have a remarkably well-defined internal stratigraphy that is evident not only in the fluvial sedimentary architecture but also in the colour, grain size, sorting, and whole-rock geochemistry of the sandstones. The Otter Sandstone Formation is here divided into four named members of very different character (West Down Member, Otterton Ledge Member, Chiselbury Bay Member, and Pennington Point Member). The study highlights the importance of outcrop-to-subsurface correlation in stratigraphic studies, which suggests several major unconformities within the Otter Sandstone Formation. The Otter Sandstone Formation provides evidence for an increase in humidity throughout the early Middle Triassic, with an upward decrease in the proportion of calcrete and an apparent increase in channel size. Whole-rock major and trace geochemistry (chemostratigraphy) is widely used in the subdivision and correlation of red-bed sheet sandstones, and it is shown here that data, even when used in a very raw and unprocessed way, can be a useful stratigraphic discriminant. Fluvial sheet sandstones are often regarded as relatively homogeneous aquifer units, but this study shows that changes in fluvial style and background climate can introduce major changes to the geometry and physical properties of the aquifer or reservoir. The recognition of thin marker beds such as palaeosols can be particularly critical for the correlation of thick fluvial sheet sandstones.