Sequence stratigraphy and eustatic sea-level change: the role of micropalaeontology

Following the May 1992 meeting in Dijon, which initiated an international project on the “Sequence Stratigraphy of European Basins”, it seems an appropriate time to consider the contribution micropalaeontology can make to the science of sequence stratigraphy. In this short note, we assume that readers are familiar with sequence stratigraphic terminology; if not, see Van Wagoner et al. (1988). WHAT ARE THE CHALLENGES FACING SEQUENCE STRATIGRAPHY? Demonstrating global eustatic sea-level change. We accept that the basic sequence stratigraphy model put forward by Peter Vail and his colleagues (see Van Wagoner et al., 1988 for a summary) is a powerful tool for describing many sedimentary successions, and that the associated eustatic sea-level curve (Haq et al., 1987) has some validity. Our own observations on numerous sedimentary sequences around the world suggest that local and global eustatic events exist, and that relative sea-level curves can be constructed, but it should be remembered that the timing and magnitude of many global eustatic events are still to be established. As most workers in the field will be aware, much of the evidence to support the Haq et al. curve has not been published. The Sequence Stratigraphy of European Basins Project will go some way to rectify this, but it should be borne in mind that there can be an unfortunate tendency to use the Haq et al. curve for dating in its own right - i.e. fitting relative sea-level changes seen in a succession to the curve. If this is done, then the global . . .

Following the May 1992 meeting in Dijon, which initiated an international project on the "Sequence Stratigraphy of European Basins", it seems a n appropriate time to consider the contribution micropalaeontology can make to the science of sequence stratigraphy. In this short note, we assume that readers are familiar with sequence stratigraphic terminology; if not, see Van Wagoner et al. (1988).

WHAT ARE THE CHALLENGES FACING SEQUENCE STRATIGRAPHY?
Demonstrating global eustatic sea-level chan e. We accept that the basic sequence stratigraphy model put forwarfby Peter Vail and his colleagues (see Van Wagoner et al., 1988 for a summary) is a powerful tool for describing many sedimentary successions, and that the associated eustatic sea-level curve (Haqet a/., 1987) has some validity. Our own observations on numerous sedimentary sequences around the world suggest that local and global eustatic events exist, and that relative sea-level curves can be constructed, but it should be remembered that the timing and magnitude of many global eustatic events are still to be established. As moqt workers in the field will be aware, much of the evidence to support the Haq et al. curve has not been published. The Sequence Stratigraphy of European Basins Project will go some way to rectify this, but it should be borne in mind that there can be an unfortunate tendency to use the Haq et a/. curve for dating in its own righti.e. fitting relative sea-level changes seen in a succession to the curve. If this is done, then the global eustatic curve will become no more than a self-fulfilling prophecy. Recognising sequence boundaries, maximum flooding surfaces and systems tracts in individual successions. The basic sequence stratigraphic model is established. Geologists are now attempting to identify sequence boundaries, maximum flooding surfaces and systems tractsinnumeroussuccessions around the world. Systems tracts and sequence boundaries were originally defined by seismic geometries. Later they were recognised by stratal patterns and facies variations in outcro , and in the subsurface by wireline logresponses. However, microparaeontology can also play an important role in recognising systems tracts and key surfaces.

HOW CAN MICROPALAEONTOLOGY HELP?
Before answering this question it is important to stress that the application of micro alaeontology to sequence stratigraphv has not yet been establishecfb the publication of controlled case studies. Much work needs to ge undertaken to pro1.e the applications of rnicropalaeontology. However, these are some of the possible applications that need to be explored: Dating: For building a local sea-level curi'e that can contribute to a globalcurve,itisimportanttoestablish theageofa succession and this isan obvious role for micropalaeontolog!, rvith its applicabili ty to both outcrop and well sections. Micropalaeontologists i\Al be aivare that microfossilsareoften a more suitable tool than macrofossils for dating a succession because of their relative ease of extraction, abundance and often their known relationship to established zonal schemes and hence geologic time. Correlation: Sequencestratigraphic modelsare based on a relative (or preferably absolute) chronostratigraphic framework which requires a detailed but robust biozonation scheme of the type often provided by micropalaeontology. Many micropalaeontologists have a good perception of age control and correlation across facies because they work on microfossil groups that inhabited a range of palaeoenvironments. Maximum flooding surface recognition: Because of its widespread nature and thus correlation potential, one of the most critical surfaces to recognise in the sequence stratigraphic analysis of a succession is the maximum flooding surface (MFS). A number of workers ( e g Loutit et al., 1988;Vail & Wornardt, 1990) have emphasised the point that the MFS can often be recognised by an palaeontological abundance peak, especially of lanktonic microfossils, together with the potential condensation orbiozones. We agree, but urge readers to consider the point that plankton abundance peaks can occur for other, more localised, reasons (e.g. climatic controls on u welling). Taphonomic controls should also be considered. It shouldge remembered that microfossil abundancecan also occur on other downlapped Tethynn Consultants, Branshaw House,Downgate, Callington, Cornwall, PL17 SJX, U.K.
surfaces and/or condensed intervals (e.g. the top of a lowstand fan or flooding surfaces within a transgressive systems tract). Similarly, a number oi workers (e.g. Ellison, 1989) have suggested changes in the abundance of mangrove pollen type can be used to determine relative sea-level changes in suitable successions. Again this needs to be applied critically, considering other possible controls on abundance. Sequence boundary recognition: The relative sea-level fall leading to the formation of a sequence boundary causes a basinwards shift in facies and the development of an unconformity on the shelf. This should be reflected in a dramatic change in microfossil assemblages, together with missing biozones (if the zonation is of sufficient resolution). Techniques such as graphic correlation can be used to demonstrate the presence of a hiatus. In the sediments directly overlying the unconformity reworking of microfossils from the underlying sequence may be observed, together with the contemporaneous transortation of biota into lowstand fans (although in carbonate settings Righstand shedding can result in similar patterns of redeposition). Systems Tract Characterisation: Systems tracts prograde, retrograde or aggrade, depending on their position during sea-level change. From the detailed study of biofacies, shallowing-up or deepening-up trendscanbe established, which indicate likely systems tract. Furthermore, it seems possible that for given environmental settings at given geological times, systems tracts could be characterised by particular assemblages (i.e. the composition, diversi and density of assemblages and the morphology and size of inzviduals) that are themselves the response to relative sea-level change. Opportunistic taxa may characterise a transgressive system tract, whilst species diversification takes lace during the highstand systems tract. This is a particular appEcation of micropalaeontology that requires further research.

CONCLUSIONS
Our mainmessa e isoneof caution. Most activemicropalaeontologists will be aware &at our subject area (not withstanding the classic limitations of sampling, greservation, resolution, endemism, etc.) has a critical role to p ay in uilding sequence stratigraphic models and developing relative sea-level curves. One only has to examine the brochures of biostratigraphic service companies to see that many micropalaeontologists have seized the o portunity to use micropalaeontolo y as more than just a simpg "dating" tool. We agree, but woufd remind readers that the applications of micro alaeontology to sequence stratigraphy are model driven and yet toge proven. We advocate that the empirical evidence behind the models be sought out and documented.