Miospore distribution and sedimentological facies distribution as an insight to changing terrestrial palaeoequatorial floral communities during a Pennsylvanian glacio-eustatic sea level cycle

Abstract

Systematic trends in miospore assemblages are known to characterise systems tracts in Carboniferous successions and could relate either to changing delivery mechanisms or to the influence of fluctuating sea levels on coastal plain vegetation. A succession of deltaic Pennsylvanian mudstones exposed in the UK Pennine Basin provides a record of continuous sedimentation at a location that remained submerged through a complete glacio-eustatic sea level cycle. Changing sedimentological facies, identified from thin sections enable the succession to be linked to an existing sequence stratigraphic framework and provide a context for the miospore trends through the succession. An abundant and diverse microflora is present in all samples examined. Detailed palynological and palaeoecological analyses have identified some significant trends and key taxa at important stratigraphic surfaces. This succession has provided a rare opportunity to study miospore assemblages in the late highstand and the lowstand systems tracts that are normally either removed by erosion or represented by sandstones. In the mid-late highstand, forest mire abundances (dominated by Lycospora) peak at 81.75% falling to 39.5% in the early lowstand, suggesting that lycopsid-dominated forests characterised coastal plains in the highstand and that there was a major floral turnover during significant sea level falls. The early lowstand sand- and silt-bearing mudstones are also associated with an abrupt increase in Florinites, Calamospora, Punctatisporites and Spelaeotriletes arenaceus. Maximum flooding surfaces, characterised by clay-rich lenticular mudstone facies, have some similar miospore characteristics to the sequence boundaries, including low abundances of the forest mire group and peaks in Punctatisporites, but are distinguished by high abundances of upland miospores. Changes in the diversity and abundance of miospores through the studied section appear to relate to their location within the stratigraphic framework, rather than directly to the facies and sedimentary processes. Therefore the greatest control on miospore trends and distributions appears to be sea level changes influencing vegetation in the coastal areas rather than the depositional mechanisms transporting sediment on to the shelf.