Palynological and palaeobotanical investigation of the Carboniferous deposits of the Bristol Coalfield, U.K.; biostratigraphy, systematics and palaeoecology

This multi-disciplinary study of the Pennsylvanian-aged Warwickshire Group of the Bristol Coalfield presents the first secure biostratigraphical dating of this historically contentious sequence. Stratigraphical revisions and reinterpretation also enable accurate infrabasin correlation between the Coalpit Heath Syncline and the southern limb of the Kingswood Anticline. Using newly produced palynological (miospores and megaspores) and palaeobotanical biozonations the Winterbourne Formation and Downend Member are mid- to late Bolsovian in age, featuring assemblages corresponding to the SL palynological Biozone and the Laveineopteris rarinervis palaeobotanical Sub-Zone. The base of the Asturian, roughly approximating the base of the OT Biozone, occurs within the lowermost 120m of the Mangotsfield Member; the uppermost division of the Pennant Sandstone Formation. A stratigraphical gap is highlighted, for the first time, between the Pennant Sandstone and Grovesend formations, encompassing the early to mid-Asturian (absent Linopteris obliqua palaeobotanical Biozone and Lobatopteris micromiltonii palaeobotanical Sub-Biozone assemblages). This correlates to similar hiatuses in both nearby coalfields and several European basins, related to the Leonian Phase of uplift (part of the Variscan Orogeny). Both palynological and palaeobotanical data sets are interpreted within a facies context, to highlight a diverse patchwork of plant communities. Bolsovian peat mires were dominated by lepidodendrids and ferns, and were replaced by tree fern and fern mires in the late Asturian, likely due to uplift-induced alterations in drainage. High diversity pteridosperm, sphenophyte and fern clastic swamps fringed and infiltrated these mires. Bolsovian to early Asturian braided fluvial systems created disturbed riparian niches that were colonised by low diversity pteridosperm-sphenophyte communities and Sigillaria-sphenophyte-fern-cordaitalean communities. High levels of cordaite pollen adds to the growing body of evidence that these plants were not merely restricted to ‘upland’ areas. Evidence suggests that marattialean tree ferns may have originated within clastic environments within the Bolsovian, before becoming dominant members of Asturian peat mires.