WGCG Bridgenorth Field Trip – Led by Stuart Burley, Mike Allen & Ray Pratt

Bridgnorth, straddling the Severn Valley in a spectacular setting, is well known for its striking topography, defined by the Bridgnorth Sandstone Formation, a bright orange-red, soft sandstone of Permian age, some ≈ 256-260 million years ago. The succession is composed of well-sorted, quartz arenite sandstones organised into large scale sets of cross-strata and has long been interpreted as an aeolian dune deposit (e.g. Shotton, 1937). It is one of several lower Permian aeolian dune-field (‘erg’) successions present within the United Kingdom and adjacent offshore regions and is thought to be broadly time-equivalent to the desert sandstones of the Kinnerton Sandstone Formation in Cheshire and the Rotliegend Group in the Southern North Sea Basin, including the Leman Sandstone, which formed the main gas-bearing interval. The Bridgenorth Sandstone Formation is considered to be the time equivalent of the Clent Breccias, and possibly the Kenilworth Sandstone Formation.

The Permo-Triassic successions of the West Midlands mainly overlie Carboniferous basement rocks that were deformed during the Variscan Orogeny and partly eroded during early Permian sub-aerial exposure. The Bridgenorth Sandstone Formation occurs in the Stafford Basin, a major half-graben structure filled principally by Permian-Triassic deposits of continental origin. It forms part of a chain of interconnected rift basins that extend from Lancashire in the north to Warwickshire and beyond in the south, forming a large rift basin system in which a variety of non-marine environments developed including large ergs, small isolated dune fields, sandsheets, basin margin alluvial fans and, in the case of the Irish Sea Basin, a large interior-draining playa lake. The Variscan highlands occupied an equatorial latitude and the Stafford Basin lay approximately 400 km further north under the influence of a semi-arid to arid climatic regime. Aridity was maintained because the region occupied an interior continental position within the Pangaea supercontinent, away from any maritime influence, and because it lay in the rain shadow of the Variscan mountain chain that stretched east-west through Europe.

Within the Stafford Basin, the Bridgnorth Sandstone formed as the product of the migration of straight-crested and crescentic dunes that coalesced laterally to form elongate ridges (‘draa’). Many bedform slip faces were orientated transverse to the palaeo-wind with bedform migration occurring to the west. Along slope winds modified the dunes on a seasonal basis. The Stafford Basin contains only 200-300 m of Permian strata. By analysing the shape slip face orientations of these fossilised dunes the predominant wind direction when the dunes were formed can be determined. Analysis indicates winds blew from the east and south-east which is consistent with sub-tropical trade winds found at 20 degrees north of the equator today.

Overlying the Bridgenorth Sandstone Formation is the basal conglomerate of the Kidderminster Formation (formerly known as the Bunter Pebble Beds) within the Sherwood Sandstone Group. These conglomerates were laid down approximately 250 million years ago during the early Triassic. They represent massive high-energy river systems that swept across the region during periods of flash flooding, eroding deeply into the landscape.

On this field trip we will learn how to recognise aeolian dune sandstones and compare the Bridgnorth Sandstone Formation with the depositional characteristics of the Kenilworth Sandstone Formation.

To register for this trip please email [email protected]

For more details please view the event flyer by clicking here