Recognition and documentation of depositional sequences from outcrop: an example from the Aptian and Albian on the eastern margin of the Wessex Basin

Abstract

Stratigraphic data relating to the Aptian and Albian sediments around Folkestone in southeast England, on the eastern margin of the Wessex Basin, are plotted graphically to illustrate biostratigraphic gaps and condensed intervals. These are then used to suggest candidate sequence boundaries, notated LG1-4 and Gl-3, which are named after the Lower Greensand and Gault. The biostratigraphic subzones apparently unrepresented in the area are: LG1 obsoletus and bodei; LG2 gracite and debile; LG3 schrammeni; Gl bulliensis. Candidate sequence boundaries G2 (cristatum Subzone) and G3 (auritus Subzone) are erosion surfaces correlating with probable biostratigraphic gaps elsewhere in the basin. The age of LG4 is uncertain but must lie within the mammillatum Superzone. All candidate sequence boundaries correspond closely to those predicted by recent global sea-level cycle charts, and additionally there is a close match between predicted maximum flooding surfaces and the occurrence of condensed intervals. Most of the candidate sequence boundaries could have been formed as a result of rapid, and at least regionally extensive, relative sea-level falls, but the sedimentological contexts of LG3 and Gl suggest that these may have been generated by transgression-related sediment condensation. The minor sequence boundary in the loricatus Zone (intermedius Subzone) illustrated in the cycle charts cannot be unambiguously recognized at outcrop in the Folkestone area. Additional sequence boundaries not shown in the cycle charts may also be present in the Wessex Basin.