Page 95 - Carbonate Sedimentology and Sequence Stratigraphy

86 WOLFGANG SCHLAGER

has been dated as late Albian to Cenomanian and was ten- lowstand position to an elevation above the old shelf mar-
tatively correlated with the 94-My lowstand on the curve gin and depositional environments shift landward. Finally,
of Haq et al. (1987) (Winker and Buffler, 1988). How- the highstand systems tract consists of the depositional sys-
ever, at least two other lowstands, at 96 and 98 My are tems developed then sea level stands above the old shelf
equally possible given the stratigraphic data. Furthermore, margin and depositional environments and facies belts pro-
there is a mismatch between the prominence of the se- grade seaward. The standard model postulates further that
quence boundary and the amplitude of the invoked sea- systems tracts follow each other in regular fashion. The
level events. The MCSB is arguably the most prominent lowstand systems tract immediately overlies the sequence
sequence boundary in the Gulf whereas the amplitudes of boundary, the transgressive sytems tract occupies the mid-
the mid-Cretaceous sea-level falls are very modest. On the dle, the highstand tract the top of a sequence. Systems tracts
curve by Haq et al. (1987) they are predated by two pro- in sequence stratigraphy were originally defined by lap-out
nounced Valanginian lowstands and postdated by a Turo- patterns at the base and top, internal bedding, stacking pat-
nian one. All of them have about twice the amplitude of terns and position within a sequence (Posamentier et al.,
the mid-Cretaceous events, yet none of them has a compa- 1988, 110; Van Wagoner et al., 1988, 42; Emery et al., 1996,
rable seismic expression in the Gulf. Thus, the postulated p. 26). All these criteria are based on geometry. The charac-
correlation to eustatic events fails to explain the prominence terization of sequence systems tracts in terms of facies are a
of the MCSB. What sets the MCSB apart from all other se- later addition.
quence boundaries in the Gulf is the associated change in The standard model assumes that the fall of sea level from
depositional regime: the mid-Cretaceous sequence bound- highstand to lowstand position does not leave a significant
ary marks the termination of the rim of carbonate platforms sediment record. Subsequent work on outcrops and cores
around the Gulf and the spread of pelagic deposits and ma- has shown that this generalization is not justified. There ex-
rine hardgrounds, later to be covered by Tertiary siliciclas- ists a growing number of examples where the retreating sea
tics (Schlager et al., 1984; Buffler, 1991). The change in depo- has left a significant sediment accumulation that records the
sition goes hand in hand with a drastic change in the input downward shift of the shoreline and shelf surface (Hunt and
and dispersal of sediment in the basin (Fig. 6.2). The un- Tucker, 1992; Nummedal et al., 1995; Naish and Kamp, 1997;
conformable nature of this boundary is accentuated by the Belopolsky and Droxler, 2004). These observations agree
fact that the platforms had high and steep flanks when they with theoretical considerations (Nummedal et al., 1993) and
were drowned and this pronounced relief tends to amplify insights from numerical modeling. Numerical models al-
ocean currents (see chapter 5). low one to explore under what circumstances the falling sea
mayproduce a sedimentaccumulation. Itturns outthatthe
parameter space for generating a falling-stage systems tract
Systems tracts
is quite large while the space for creating the geometry of
the standard model is small. To produce the erosional un-
The term “depositional system” was introduced by Fisher conformity of the standard model one has to assume either
and McCowan (1967) for a three- dimensional assemblage intensive terrestrial erosion or a highly asymmetric sea-level
of lithofacies genetically linked by a common set of depo- cycle with a rapid fall (Figs 6.4, 6.5). Extreme erosion rates,
sitional processes. Rivers, deltas and slopes are examples on the other hand, are incompatible with the very minor ero-
of depositional systems. Coeval systems are often linked by sion shown in the classic diagram of the standard model. In
lateral transitions, for instance along a topographic gradient, summary, field observations and numerical models indicate
to form systems tracts. The most common example of a sys- that sea-level falls are likely to produce a sediment record.
tems tract is the succession of systems encountered in a tra- The situation shown in the standard model, i.e. only modest
verse from basin margin to deep water. Such a transect may erosion of the highstand tract and no sedimentation during
cross the systems river, delta, shelf, slope and basin floor. sea-level fall, requires a highly asymmetric sea-level cycle
Sequence stratigraphy has adopted and somewhat mod- with a very rapid fall. Diagrams relating the standard model
ified the concept of systems tracts. The standard model to a symmetric sine wave of sea level are misleading.
of sequence stratigraphy stipulates that the systems tract Several different names have been suggested for the sed-
from basin margin to deep water varies in a systematic iment body formed during sea-level fall: Forced-regressive
fashion during a sea-level cycle such that lowstand, trans- wedge systems tract (Hunt and Tucker, 1992), falling-stage
gressive and highstand systems tracts can be distinguished systems tract (Nummedal et al., 1995) and regressive sys-
(Posamentier and Vail, 1988). Fig. 6.3 shows the standard tems tract (Naish and Kamp, 1997) are commonly used. I
model applicable to siliciclastics, tropical carbonate ramps prefer “falling-stage systems tract (FST)”because it refers to
and cool-water carbonates. For systems-tract definitions of the critical process - the relative fall of sea level that can be
rimmed platforms see chapter 7. The lowstand systems directly deduced from the geometry or the facies pattern of
tract consists of the suite of depositional systems developed the systems tract. The phrase “forced-regressive wedge sys-
when relative sea level has fallen below an earlier shelf mar- tems tract” is a bit awkward and the term “regressive sys-
gin. The transgressive systems tract consists of the depo- tems tract” does not distinguish between forced regression
sitional systems developed when sea level rises from its and depositional regression, for instance during highstand