Rifts, rivers and climate recovery: A new model for the Triassic of England

Abstract

Triassic basins of England developed under a regime of largely WE extension and progressed from non-marine fluvial and aeolian sedimentation (Sherwood Sandstone Group), through semi-marine playa lacustrine deposits (Mercia Mudstone Group) to fully marine environments (Blue Anchor Formation and Penarth Group). A new tectono-stratigraphic model for the Sherwood Sandstone Group is proposed in which two major long-distance river systems developed under conditions of relative fault inactivity in the Early Triassic (Budleigh Salterton Pebble Beds and equivalent) and Middle Triassic (Otter Sandstone and equivalent). These are separated by a late Early Triassic syn-rift succession of fluvio-aeolian sandstones (Wildmoor and Wilmslow sandstone formations) and playa lacustrine muds (Nettlecombe Formation) which show major thickness variation and localisation with hanging wall basins. The partitioning of syn-rift deposits into mudstones in upstream basins (close to the source of water and sediment) and clean aeolian or fluvio-aeolian sandstones in downstream basins is similar to the pattern observed in the underlying Late Permian. Under conditions of rapid tectonic subsidence chains of extensional basins may become disconnected with upstream basins (Wessex Basin) acting as traps for fines and water permitting more aeolian activity in temporarily unlinked downstream basins (Worcester and Cheshire basins). In addition to tectonic controls, fluctuating climate, relief related to limestone resilience in arid settings, the smoothing effect of fill and spill sedimentation and Tethyan sea-level change all contributed toward the observed Triassic stratigraphy in England.

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

“…This arid zone unit may in fact have greater affinity with the Nettlecombe Formation (Figure b), an anhydritic playa lacustrine deposit that overlies the Budleigh Salterton Pebble Beds and accumulated in the hanging wall basin of the Wynford‐Hooke Fault during a time of relatively active extension. Using tectono‐stratigraphic principles, Newell () suggested that the Nettlecombe Formation is a possible correlative of the Wildmoor Sandstone and Wilmslow Sandstone formations of the English Midlands. Both these units include extensive aeolian deposits and may have been deposited during a relatively arid climatic phase in the late Early Triassic that occurred between the two major long‐distance river systems of the Budleigh Salterton Pebble Beds and the Otter Sandstone Formation.…”

“…The coastal outcrop of the Otter Sandstone Formation near Sidmouth in south Devon is the most southerly exposure of the parent Sherwood Sandstone Group in onshore England (Figure ). Traced northward from the south Devon coast, the Sherwood Sandstone Group crops around the periphery of a series of linked extensional basins that include the Wessex Basin, the Worcester Graben, and Cheshire Basin (Figure ; Newell, ). At subcrop in southern Britain, the Sherwood Sandstone Group forms a N–S‐trending belt that cuts between the Welsh Massif and London‐Brabant Massif before bifurcating around the Pennines to extend into the East Irish Sea Basin and onto the East England Shelf.…”

“…Thinning across the Cranborne‐Fordingbridge High and the Mendip High probably reflects a combination of structural control and the underlying Variscan bedrock, which here comprises a W–E‐trending belt of Carboniferous limestone. Under the relatively arid Triassic climate, the limestones would have been resistant to erosion and probably formed an elevated topographic block (Newell, ; Simms, ).…”

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

“…Lithostratigraphic scheme and nomenclature of the Permo‐Triassic succession in sedimentary basins of Great Britain (based on Ambrose, Hough, Smith, & Warrington, ; N. S. Jones & Ambrose, ; Holliday, Jones, & McMillan, ; Hounslow & McIntosh, ; Hounslow & Morton, ; Hounslow & Ruffell, ; Hounslow et al, ; Newell, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…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, ).…”