Page 128 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 119

ognized. However, lithoclasts do not prove lowstand condi- A) steep slope, short cycles (modern Bahamas)
tions at the platform top because erosion may cut into older
material on the flanks at any time during platform history 100m
(Grammer et al., 1993). Collapse of the Bahama margin in
Exuma Sound produced high admixture of lithoclasts in a
debris flow from the Sangamonian highstand (Crevello and
Schlager, 1980). 30-90°
Limitations of the compositional tool are the same as for upper slope
highstand shedding. A platform that builds an extensive
lowstand wedge during a long sea-level cycle may well ex-
port what looks compositionally like highstand material if
the platform is large enough to develop a protected interior
zone with its own characteristic sediment.
B) gentle slope, long cycles (Neogene Bahamas)
The model of systems tracts assumes that sea level con-
trols the presence or absence of submarine fans as well as
their position higher or lower on the toe-of-slope. A view 100m
at modern oceans as well as hydrodynamic theory indicate
that fan development is largely a function of the transport
capacity and competence of turbidity currents. These pa-
rameters, in turn, depend on sediment supply and topogra- 1.5-5°
phy (slope angle, size of valleys to channel the flows etc.). slope low-stand wedge
Sea level can influence sediment supply and, to a lesser ex-
tent, topography but in doing so it must compete with other Fig. 7.17.— A) Highstand shedding is expected to be most pro-
nounced around steep-sided platforms and during short sea-level
processes.
cycles. B) Gentle flanks and long cycles lead to growth of a broad
For carbonate platforms, Schlager and Camber (1986)
lowstand wedge that may produce bank-top sediment and dampen
have proposed a model of slope evolution as a function of
the effect of highstand shedding. After Schlager (1992), modified.
the height of the platform (Fig. 3.12). This model predicts
that the depocenter will shift basinward as the slope angle
increases. Steep slopes can be bypassed and even eroded during lowstands but are “not specific to times of falling
by turbidity currents and fans may onlap these slopes irre- or lowstands of relative sea level” (1993, p. 314). Spence
spective of sea-level position. The position and shape of the and Tucker (1997) also emphasize that margin failure and
gravity-displaced sediment bodies is largely a function of to- concomitant deposition of megabreccias may take place
pography. Carbonate sediments, especially reefs, are known during highstands as well as lowstands. However, they
for their ability to rapidly build steep relief. Depocenters of also conclude that lowstands may favor slope failure and
gravity-displaced sediments will shift equally rapidly in re- megabreccia formation by excess pore-water pressure in
sponse to the changes in sea-floor relief. Thus, fans and sub- confined aquifers and by stress increase in the rock fabric.
marine canyons in carbonate terrains are rather unreliable Below follows a closer look at these mechanisms.
indicators of sea-level movements.
Failure by overpressured pore fluids. Slope sediments in which
Megabreccias and sea level overpressures are generated during sea-level lowering will
have less shear strength and thus be prone to failure. Con-
In chapter 4, megabreccias were introduced as common fined aquifers seem favorable for such overpressure genera-
and conspicuous features on the slopes and basins of the T tion and failure because pore pressures in these aquifers are
and M factories. They merit further discussion here because controlled by high water tables in the platform while local
their specific position in sea-level controlled sequences is hydrostatic pressure is markedly reduced by reduction of
controversial. the overlying column of sea water. (Fig. 7.18 The question
The standard model as summarized by Vail (1987) and is: under what circumstances will significant overpressures
Van Wagoner et al. (1987) makes no specific statements develop in young carbonate rocks, considering their high
regarding carbonate rocks. Sarg (1988), who contributed intrinsic permeability and the likely addition of cavernous
the pioneer paper on carbonate sequence stratigraphy, karst systems during lowstands?
closely followed the standard model with regard to gravity- The Pleistocene/Neogene of Great Bahama Banks may
displaced sediments. He concluded that most carbonate serve as a case in point because the depositional anatomy
material, including megabreccias, is shed during lowstands and physical properties are fairly fairly well known (see p.
of sea level from the collapsing, oversteepened platform 133ff). The Bahamian shoal-water facies have a mean ma-
margins. Similar conclusions were reached by Vail et al. trix permeability of about 10 millidarcy (Melim et al. 2001)
(1991, p. 656) and Jacquin et al. (1991). Hunt and Tucker plus a well-connected network of karst conduits that sup-
(1993) note that megabreccias may be particularly abundant ports long-distance circulation (Whitaker and Smart, 1990).