Forward modelling of the sequence stratigraphic architecture of shelf cyclothems: application to Late Pliocene sequences, Wanganui Basin (New Zealand)

We demonstrate a procedure for forward modelling of the sequence stratigraphic architecture (i.e. positions of systems tract boundaries) of shelf cyclothems as observed at outcrop sites or in well logs, The I-D model is based on the fact that the deposition of facies within shelf sequences and those defining the key stratigraphic surfaces between systems tracts, respond directly to changes in depth between the sea surface and seafloor. This accommodation-bathymetry model incorporates three reference surfaces, sea surface, seafloor and lower sequence boundary, each of which varies independently as a result of changes in the rates of eustatic sea level, sedimentation and subsidence. The model input and boundary conditions are an eustatic sea-level curve (i.e. late Cenozoic deep-sea delta(18)O record), an independent chronology for the outcrop or well section and a subsidence curve; the model outputs are changes in accommodation and palaeobathymetry throughout the sequence, and the time 'missing' at erosional sequence boundaries, The main variable in the model, i.e. sedimentation rate, is established by comparison of a model output (bathymetry) with the model target (palaeobathymetry) established from the faunal and facies content of the sequence being modelled, When the model output and target an compatible, the model sedimentation rate curve is adopted and the theoretical positions of the systems tract boundaries can be read off the model in relation to the lower sequence boundary, and compared with independent field-based interpretations of their stratigraphic position. The model is designed to predict the architecture of sequences as they appear at particular outcrop sites or in well sections, Its usefulness lies in establishing the timing of accumulation of systems tracts in relation to stages in a eustatic sea-level cycle; in particular, distinguishing between units deposited during highstand versus regressive systems tracts, The model is applied to 6th-order (41 h-a) Plio-Pleistocene cyclothems in Wanganui Basin (New Zealand), where it distinguishes the boundary between highstand systems tracts (HST) and regressive systems tracts (RST). The modelling has implications for the definition of systems tracts, especially in late Cenozoic successions for which the record of eustatic sea-level changes is known independently. (C) 1998 Elsevier Science B.V. All rights reserved.