Print this pageThe Sedimentological and Geochemical Variability of Maximum Flooding Surfaces and Their Application for Improved Sequence Stratigraphical Correlation
DOCHERTY, MATTHEW
The maximum flooding surface is a key sequence stratigraphical surface that is fundamentally important in dividing the depositional sequence and developing regional correlations. The surface commonly contains condensed pelagic and hemipelagic material, is enriched in bioclastic debris, organic matter, authigenic minerals and uranium. They are known to have distinct biostratigraphic, isotopic and geochemical signatures which facilitate their identification in the subsurface. This presentation reports on a high resolution study of the superbly exposed San Andres formation of Last Chance Canyon, USA and a subsurface study from the Inner Moray Firth, UK. Both studies indicate marked variability of maximum flooding surfaces in terms of sedimentology and geochemistry. This variability has major implications for the construction of regional correlations based on the identification of maximum flooding surfaces as condensed shale horizons and peaks on gamma-ray curves.
The San Andres formation contains two well exposed maximum flooding surfaces. Gamma-ray data from these surfaces indicates there is significant variability from outer shelf to basin centre environments. In distal locations the surfaces are recognised as gamma-ray spikes while in more proximal locations the surfaces are characterised by a decrease in the gamma-ray response.
Within the Lower Jurassic of the Inner Moray Firth several regionally correlatable distal marine shale horizons are identified. The differing petrographic and geochemical signatures of these maximum flooding surfaces can be used to aid age determination and improve correlations in areas of poor biostratigraphical control. Their variability, however, emphasises the need for caution when identifying maximum flooding surfaces from gamma-ray spikes alone.