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Characterising Sequence Components Using Fossils
We need to establish characteristic paleontological signatures or fingerprints for surfaces and components of a sequence. In order to do that we need to know how the fossils – as living organisms – respond to changes in sea level and, to an extent, sediment supply.
Sequence surfaces provide the broad features that separate individual sequences and sequence components and also the mechanism by which sequences are biostratigraphically calibrated and therefore correlated on a well-to-well and eventually regional basis. The three main surfaces which occur within an individual sequence are:
- the sequence boundary (SB)
- the maximum regressive surface (MRS) (sometimes called the “transgressive surface”)
- the maximum flooding surface (MFS).
The SB is a negative surface in that, in its proximal expression as an unconformity, it represents a period of missing time. The same may also be said of the MRS. The MFS is a positive surface expressed within an actual rock unit representing (more-or-less) continuous deposition. The importance of the surfaces is that, because they are eustatically controlled, they are laterally extensive. Furthermore if they can be biostratigraphically calibrated with consistent marker species, they can be correlated at basin, region and even global scales. The rock units between the surfaces are called Systems Tracts. They effectively build the rock record. The principle systems tracts are a) the lowstand systems tract comprising the lowstand wedge and fan b) the transgressive systems tract c) the highstand systems tract including the shelf-margin wedge