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

Abstract

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.

Sedimentary flow heterogeneities in the Triassic U.K. Sherwood Sandstone Group: Insights for hydrocarbon exploration

“…; Hounslow & McIntosh, ; Worden et al, ). This arises from its tectonic history; rifting in the later parts of the Olenekian supported thermal subsidence in creating accommodation space (Newell, , ; Ruffell & Shelton, ). Thus, the coupling of low subsidence and proximity to the fluvial sediment source does not favour aeolian preservation in the Sherwood Sandstone Group, here (Kocurek & Havholm, ; McKie & Williams, ; Rodríguez‐López et al, ).…”

“…Evidence of wetter climatic conditions is present in the Sherwood Sandstone Group of the Wessex Basin in Southern England (see Figures and ). Here, the appearance of rhizocretions in the uppermost part of the fluvial succession indicates a subhumid climate during late Anisian time (Newell, ). However, this is likely local to this basin and cannot be used as a palaeoclimatic indicator at the scale of the entire Triassic U.K. Sherwood Sandstone Group.…”

“…A paucity of diagenetic studies, which might have been useful for the reconstruction of burial depths, means that tectonic subsidence rates of other Triassic basins in England are poorly constrained. However, the thickness of the Sherwood Sandstone succession in each Triassic sedimentary basin is well constrained by the availability of numerous seismic lines and boreholes (Allen et al, ; Ambrose et al, ; Edmunds & Smedley, ; Kattenhorn & Pollard, ; Newell, , ; Nirex, ). Furthermore, the age (Induan–Ladinian) of the Triassic Sherwood Sandstone Group succession is known from radiometric and palaeomagnetic dating, as well as stratigraphic relationships with adjacent units (Ambrose et al, ; British Geological Survey, ; Hounslow & McIntosh, ; Warrington et al, ).…”

“…Palaeocurrent data (Figure a) from fluvial facies are chiefly derived from cross‐stratification of sand‐prone channel fills (Edwards, ; Gaunt, ; Gaunt, Fletcher, & Wood, ; N. S. Jones & Ambrose, ; Medici et al, ; Powell, Cooper, & Benfield, ; Smith, ; Smith & Francis, ; Steel & Thompson, ; Wakefield et al, ). Palaeocurrents from fluvial channel lithofacies indicate a northward palaeoflow from the southern Wessex Basin via a series of linked basins to the eastern Irish Sea, Vale of Eden, and Carlisle basins in northern regions (Brookfield, ; N. S. Jones & Ambrose, ; McKie & Williams, ; Newell, , ; Smith, ).…”

“…(a) Pre‐Hardegsen unconformity (Ambrose et al, ; Bouch et al, ; Evans et al, ; N. S. Jones & Ambrose, ; Smith, ; Warrington et al, ). (b) Post‐Hardegsen unconformity (Ambrose et al, ; Brookfield, , ; Evans et al, ; N. S. Jones & Ambrose, ; Newell, ; Purvis & Wright, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”