Lithofacies and sedimentary cycles within the Late Dinantian (late Brigantian) of Fife and East Lothian: is a sequence stratigraphical approach valid?

Abstract

ABSTRACT The late Brigantian topmost parts of the Pathhead Formation (Aberlady Formation in East Lothian) and the succeeding Lower Limestone Formation crop out widely in Fife and East Lothian. The successions include nine deltaic, coastal floodplain and marine shelf cycles (cyclothems), of which the lowest examined terminates the Pathhead and Aberlady Formations and the remaining eight constitute the Lower Limestone Formation. The cyclothems conform broadly to the ‘Yoredale” transgressive/regressive pattern in which a transgressive marine shelf phase is succeeded by delta progradation and terminates with a fluvial delta plain phase. Cycles may combine to form compound cyclothems up to more than 50 m thick, in which a basal, typically complete initial cycle of Yoredale pattern is succeeded by up to five base-absent minor cycles. These are thinner, more variable and less laterally persistent units in which the marine phase is weakly represented or absent. Cyclothems reflect successive marine flooding events, possibly under eustatic control, succeeded by delta progradation and, ultimately, leading to extensive palaeosol formation, including coal seams. Sedimentation and palaeosol formation were partly controlled by fault-induced differential subsidence and are likely to have been related to autocyclic processes. Local uplift and subsidence associated with vulcanicity, as at Kinghorn and Elie, have led to thickening or thinning of sediments accumulated in a given time period. Initial cycles initiate longer-period allocycles, corresponding broadly to third-order Exxon Production & Research (EPR) Type 1 sequences having a periodicity of around 1 Ma, within the Milankovitch orbital band. Two parasequences constitute each initial cycle: a lower, initiated on a marine flooding surface, and an upper, bounded by the base of the lowest thick sandstone in the cycle; cyclothem bases and sequence bases thus alternate. Parasequences and sequences are less well defined in minor cycles due to the problem of tracing the combined disconformity and soil profile of the underclay beyond the edge of channel sandstones. Minor cycles were controlled primarily by short-period autocyclic sedimentary and, or, tectonic processes, including delta-lobe switching and differential subsidence. Although we have attempted to interpret the deposits of Fife and the Lothians in terms of sequence stratigraphy, we are not fully convinced that the patterns of associated changes widely recognised within the framework of sequence stratigraphy can be confidently applied in succesions in which autocyclic changes feature strongly in an area undergoing active basin subsidence associated with strike-slip faulting. There is no doubt that some of the cyclicity discerned in the late Brigantian successions of eastern Scotland was related to eustatic sea level changes, which gave rise to the widespread limestone platforms or marine bands. The formation of eight cyclothems within the 2·5–3·5 Ma of late Brigantian suggests a cyclicity of about 400 ka, which corresponds to the long period eccentricity cycles of Milankovitch rather than the 0·5–5·0 Ma of third-order EPR cycles.