|
Sedimentological Advances |
School of Geosciences, Queen’s University, Belfast BT7 1NN, Northern Ireland, UK
Three sequence boundaries (SB1, SB2 and SB3) are documented in the Upper Aptian martinioides to jacobi zones of the Lower Greensand Group of southern England. Correlation shows how folding affects only the strata of the lowest depositional sequence, indicating a discrete phase of tectonism in the nutfieldiensis zone/subzone (SB2). No similar tectonic deformation occurs at the sequence boundaries above or below. The regional features of this unconformity surface demonstrate that tectonically enhanced sequence boundaries may be discerned from those of eustatic/global tectonic origin. The tectonically enhanced sequence boundary (SB2) was initially interpreted as a transgressive surface at individual outcrops, but the regional pattern of truncation indicates that the transgressive surface is amalgamated onto a sequence boundary. Comparison of the geometry of each sequence on a regional (basin-wide) scale shows clearly the structural origin to truncation. Correlation of the three depositional sequences is made using their bounding surfaces, and the variable deposits of fuller’s earth bentonite, estuarine sand and lagoonal muds between SB1 and SB2 are resolved as time-equivalent facies deposited at the turn-around from sealevel lowstand to transgression. This correlation can be compared to that achieved by two conflicting ammonite zonal/subzonal schemes: the older biostratigraphy suggests diachroneity of the deformed lower sequence, whilst a new zonation shows this same sequence to be approximately synchronous.
