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1 Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK
2 Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
The value of quantitative surveys of ancient reef slopes for palaeobathymetric analysis is reviewed. Reefs are selected for palaeobathymetric analysis because they are characterized by in situ preservation of benthic communities which are often depth related. In addition, if the reef crest and slope are preserved then ancient water depths can be measured.
The zonation of living reefs has been measured using semi-quantitative phytosociological methods as well as a range of plot and plotless techniques, but there is little agreement as to a single best method. For this study a modified line intercept transect method is selected as it may be used on both living reefs and on various types of outcrops of fossil reefs. This method minimizes problems arising from outcrop conditions, is faster than quadrat methods, can be used to assess different frame-building taxa, matrix, cement and porosity along ancient reef surfaces, and is also frequently used in studies of living reefs.
The zonation of corals and coralline algae on living and fossil reefs can be characterized by three parameters: framework density, diversity and species abundance. The framework density of reef corals is normally greatest on external reef slopes between 1 and 30 m and decreases into shallow reef crest environments, and towards shorelines. Coralline algal frameworks are highest in reef crest environments and may also replace coral frameworks in deeper reef-slope environments. Variations in diversity are complex and relate to biogeographical constraints as well as local variations of physical factors and biological factors. The distribution of reef-building assemblages is often expressed as a zonation which may relate to depth but also to hydraulic energy. Examples of coral and coralline algal zonation are presented and discussed from Indo-Pacific and Caribbean reefs.
Three case studies are presented based on the authors’ work on reef zonation. The Seychelles reefs (Indian Ocean) are used to illustrate depth and water energy related zonation of present-day living corals. Coralline algal zonation is illustrated from present-day St Croix reefs (Caribbean), together with preserved zones in sections through these reefs. Coral and coralline algal zonation on fossil reefs are characterized using our modified line intercept transect technique on exposures from late Miocene reefs of Mallorca (Spain).
This review of reef zonation and introduction of a new technique for surveying ancient reef communities to establish depth related zones has important advantages over previous qualitative approaches. Direct measurements of palaeodepths in the field make subjective palaeobathymetric interpretations unnecessary and avoids the problems of assuming uniformitarianism. In situ organism assemblages are sampled which minimizes the effects of taphonomic changes on community composition. The technique allows the quantification of framework density, diversity, species abundance and zonation to be compared between different fossil reefs and between living and fossil reefs for the first time.
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