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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