Three new early Namurian (E 1c -E 2a ) marine bands in central and northern England, UK, and their bearing on correlations with the Askrigg Block

Abstract

SUMMARY Three new marine bands are recognized in the exceptionally thick Lower Namurian succession of the northern part of the Central Pennine Basin, namely the Blacko (late Pendleian), Cravenoceras gressinghamense (early Arnsbergian) and Saleswheel (early Arnsbergian) marine bands. The stratotype of the Blacko Marine Band (E 1c 2) is within delta-slope siltstone and sandstone of the Surgill Shale Member at the top of the turbiditic Pendle Grit Formation of the Blacko Borehole (near Colne). It comprises marginal marine siltstone and contains the bivalve Sanguinolites. The band is correlated with a bivalve-spat phase in the Widmerpool Gulf, and a Lingula band lying within the delta-top Grassington Grit Formation on the south-eastern part of the Askrigg Block. The Cravenoceras gressinghamense Marine Band (E 2a 2α) succeeds the Eumorphoceras ferrimontanum Marine Band (E 2a 2) in the Roeburndale and Sabden Shale formations. At its type locality, along Gressingham Beck, Hornby, near Lancaster, it has yielded the new name-bearing ammonoid. It also contains the lowest appearance of Selenimyalina variabilis (Hind) and is characterized by the presence of Posidonia lamellosa (de Koninck). In the Widmerpool Gulf, the band is represented by the Posidonia phase at two stratigraphical levels. The Saleswheel Marine Band (E 2a 2β) lies between the C. gressinghamense and Eumorphoceras yatesae (E2 a 3) marine bands. Its type locality is in the lower part of the Sabden Shale Formation at Ribchester, near Blackburn, where it contains Anthracoceras. The marine band thickens dramatically towards the Lancaster district into siltstone beds of the Close Hill Siltstone Member containing a Sanguinolites -dominated faunal phase. In the Widmerpool Gulf it is represented by a Lingula band. The dramatic thickness and facies changes of early Namurian marine bands across the Central Pennine Basin and on to the Askrigg Block appear to be controlled primarily by proximity to sediment supply, availability of accommodation space, and the duration, rate and amplitude of glacio-eustatic marine flooding events. A regional non-sequence is developed at the base of the Ward’s Stone Sandstone (and equivalents), associated with marked tectonic discordance. This boundary, together with the newly resolved marine band stratigraphy, necessitates lithostratigraphical revision and re-correlation of early Namurian sandstones in the northern part of the Central Pennine Basin and the Askrigg Block. One new species, Cravenoceras gressinghamense sp. nov., is described.