Revisiting the black shale depositional enigma: Transport processes and contrasting sediment sources in a heterolithic basin fill – Bowland Basin, England

Factors that control the accumulation of organic-rich shales are keenly debated and include basin redox variations, sediment provenance and diverse depositional processes. The relative importance of hemipelagic settling versus sediment gravity flows has been especially contentious in recent years. This study examines the Bowland Shale, a thick succession of organic-rich mudrock with subsidiary facies, from the late Mississippian Bowland Basin of northern England that records a broad range of depositional processes. Interbedded amongst the mudrocks are several elongate, calciturbidite fans ca 10 km in length, sourced from a small carbonate platform to the south-east of the basin, whilst a turbidite body of siliciclastic sand entered the basin from the east. An intrabasinal high in the north-west of the basin deflected the progradation of the turbidite sandstones and was likely also responsible for the reflection of the carbonate-carrying sediment gravity flows generating combined flow structures in the calciturbidite fans. Abundant, fine calcareous detritus was also sourced from the south-east, forming an apron of calcareous mudstone delivered by low-strength debris flows. Interbedded amongst these diverse facies types, the Bowland Shale primarily consists of hemipelagic, organic-rich shale with a fabric consisting of compacted clay lenses (0.05–0.4 mm in width), hemipelagic components (including larval shells of bivalves, goniatites and syngenetic framboids) and organic filaments (marine snow). The lenses are interpreted to be faecal pellets formed above the redoxcline before settling to the seabed. An alternative idea, that has gained much traction in mudrock studies, is that the clay lenses are transported intraclasts and that black shales are substantially the product of deposition from traction currents sourced from adjacent basin margins. This idea is problematic because it fails to address why basinal shales have a euxinic geochemical signature rather than recording the well-oxygenated conditions of the purported source area.