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114                                      WOLFGANG SCHLAGER


                                                                   determine from regional information the location of the section
                                                                         with respect to the long-term platform edge
                          select reference profile

                                                                                   section location
       determine elevation of the platform top in question with respect to reference profile:
                                                                  seaward of platform edge  landward of platform edge
            flat top below reference level  flat top above reference level
                                                               all shoalwater intercalations and   deepening  shoaling
                                                               wave-cut terraces in slope deposits
                                  retrograding     prograding
                                                                         LST                TST              HST
                   LST              TST              HST     recognition of TST and HST seaward   LST is represented only by
               mostly prograding  (dA/dt > dG/dt)  (dA/dt < dG/dt)  of platform edge is speculative:  exposure surfaces and
                (dA/dt < dG/dt)                              TST: platform debris in slope deposits   terrestrial deposits
                                                             low and decreasing upward
                                                             HST: platform debris in slope deposits
                                                             high and increasing upward

        Fig. 7.11.— Flow chart for identification of systems tracts when  Fig. 7.12.— Identification of systems tracts in a single section
       a cross section of the carbonate platform is available, in particular  or borehole. This technique is more speculative than the cross-
       the position of the platform margin.                  section approach, relying on proxy indicators such as deepening
                                                             and shoaling trends. The position of the section with respect to the
                                                             long-term margin is required as input.


       or by a change of sediment supply (or by a combination           The testimony of ancient systems tracts
       of both, of course).  This conclusion is inevitable if one
       defines relative sea-level change not simply as a change in  Handford and Loucks (1993) offer an extensive compi-
       water depth but as a change in the distance between the  lation on the facies patterns of systems tracts, relying on
       sea surface and some deep stratigraphic reference level,  ancient examples and principles of carbonate sedimentol-
       such as the top of basement or a deep stratigraphic marker  ogy. Their results agree well with the observations on the
       (p.91).  With good reason, sequence stratigraphers have  Holocene summarized above. Handford and Loucks’ (1993)
       insisted on this definition in several strategic papers (e.g.  lowstand tracts are narrow, favorable for reefs and carbon-
       Vail et al., 1977; Posamentier et al., 1988; Jervey, 1988). I  ate sands; transgressive tracts are well flushed, reefs again
       recommend to accept this definition with its implications.  thrive and spread over the platform top, tidal flats are nar-
       Shoaling/deepening trends should be mapped and corre-  row or absent; in highstand tracts, the platform interiors
       lated where possible, but their relation to sea-level change  tend to become restricted and more muddy, tidal flats ex-
       should be left open until independent evidence removes  pand. On the slopes, slumps and sediment gravity flows
       the ambiguity. Such evidence may consist of correlating  occur throughout a sequence cycle as long as the slope is
       the shoaling or deepening trends with distinct exposure  suffiently steep. For mixed carbonate-siliciclastic systems,
       rhythms or with orbital cycles that have the additional  Handford and Loucks (1993) predict that in the lowstand
       advantage that their sea-level component is by definition  tract, incised valleys breach the narrow carbonate belt and
       eustatic.                                             funnel siliciclastics to the basin. In the highstand tract, the
                                                             landward part of the platform is filled by siliciclastics form-
       Flow charts for identification of carbonate systems tracts are  ing a coastal plain under humid conditions or a salina under
       shown in Figs 7.11 and 7.12. Fig. 7.11 is based on the geo-  arid conditions.
       metric definitions of Fig. 7.3. This procedure is applicable  Homewood (1996) approached systems-tract anatomy
       if one has information on the shore-to-basin cross section of  and facies from a theoretical point of view. He argued for
       the platform, in particular the position of the platform edge.  strong coupling among the rate of accommodation creation,
       Seismic profiles, large outcrops or well-correlated series of  ecologic conditions and carbonate production. Such feed-
       boreholes or outcrops may provide this information. How-  backs are very likely but I doubt that they are simple enough
       ever, there is great demand to recognize sequences and sys-  for reliable predictions. For instance, Homewood (1996,
       tems tracts also in single boreholes or outcrop sections. Un-  p. 711-714) predicts that transgressive tracts will be domi-
       der these circumstances, the diagnostic criteria for systems  nated by r-strategists among the biota, highstand tracts by
       tracts must be inferred from circumstantial evidence. Fig.  K-strategists such as reef builders (p. 9). The analysis of
       7.12 offers a procedure for these situations. However, iden-  Holocene systems tracts given above indicates very nearly
       tification of systems tracts in single sections remains more  the opposite pattern: reefs thrive during the late part of
       speculative and less reliable than the cross-section method.  the transgressive tract and their domain shrinks during the
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