Sequence concepts at seismic and outcrop scale: the distinction between physical and conceptual stratigraphic surfaces

Abstract

The sequence stratigraphic terms maximum flooding surface and downlap surface, as currently applied, are ambiguous. Examples of these intra-sequence surfaces are summarised from high frequency mid-Pleistocene sequences and from a Cretaceous–Recent seismic megasequence, both from New Zealand. At any one locality, a mid-Pleistocene sequence contains up to four stratal discontinuities, in ascending order: the sequence boundary, ravinement surface, local flooding surface and downlap surface. These physical surfaces, which occur in outcrop, are regionally diachronous and should be differentiated from theoretical isochronous horizons such as the time of maximum flooding (horizon corresponding to maximum shoreline transgression) and the time of peak eustatic or local relative sea-level (horizons corresponding to the highpoint of the eustatic and relative sea-level cycles, respectively). In seismic studies, the boundary between the transgressive and highstand systems tracts is usually located at the downlap surface. On the basis of a major thermo-tectonic sea-level cycle in the Canterbury Basin, it is shown that the downlap surface is not a single regional surface, and that the change in slope associated with toes of successive prograding clinoforms rises in stratigraphic height basinwards. The downlap surface therefore does not usually coincide with the maximum flooding horizon. In Plio–Pleistocene cyclothems, a discrete unit — the mid-cycle shellbed — straddles the contact between the transgressive and highstand systems tracts. This unit might be classified within its own systems tract (the condensed section systems tract; CSST). Alternatively, the position of the boundary between the transgressive and highstand systems tracts can remain unspecified or unknowable.