Stratigraphy and palynology of the Pennsylvanian continental Buçaco Basin (NW Iberia)

Abstract

The Buçaco Basin is a Pennsylvanian continental basin located along an important NNW–SSE strike structure (Porto-Tomar-Ferreira do Alentejo shear zone) that separates the Ossa-Morena and Central Iberian Zones of the Iberian Variscan Fold Belt in central western Portugal. The shear zone controlled the sedimentation in the basin and probably its post-sedimentary evolution. Sedimentation is initially alluvial with characteristic red sandstones, breccias and conglomerates. A gradual change to a fluvial (and probably lacustrine) type of sedimentation is observed with finning-upward cycles of gravel conglomerates, sandstones and organic-rich mudstones with occasional coal seams. Three representative sections were sampled for palynology and seventeen samples yielded sporomorphs with moderate to poor preservation. The palynological content from the alluvial sediments shows low diversity and poorly preserved assemblages dominated by Triquitrites spp., Densosporites spp., Laevigatosporites spp., and other taxa associated with siliciclastic environments or rheophytic mires. The fluvial and lacustrine sediments show a dramatic increase in diversity with an abundant, typical peatland microflora including sporomorphs such as Endosporites spp., Lycospora spp. and Monoletes spp., but also marginal peat and siliciclastic substrate taxa such as Densosporites spp., Latensina/Cordaitina spp., and Florinites spp. Other common taxa are Cheiledonites spp., Crassispora spp., Dictyotriletes-like miospores (mostly fragments), Potonieisporites spp., and Wilsonites spp. The presence and considerable abundance of Potonieisporites novicus and Cheiledonites cf. major is indicative of the middle to upper Potonieisporites novicus-bhardwajii–Cheiledonites major (NBM) miospore biozone of Western Europe, corresponding to the late Stephanian (early Gzhelian).