6.3 SEDIMENTARY MODELS
247
6.3.2.7.4 Carbonate shelves
A general theory of carbonate shelf sea sedimentation was put forward, based largely
on the study of Paleozoic deposits of the Williston basin, North America (Shaw, 1964;
Irwin, 1965; Heckel, 1972). The thesis on which this model is based states that in quiescent tectonic epochs of the past there were broad stable subhorizontal shelves with gradients of less than one in a thousand. These gently sloping surfaces were intersected
by two horizontal surfaces of great significance: sea level and effective wave base (what
might now be called the "mudline"). The intersections of these surfaces with the sea
bed define three sedimentary environments. In the deepest part of the shelf, below effective wave base, fine-grained mud settles out of suspension. Resultant sedimentary
facies are laminated shales and calcilutites, sometimes with chert bands, and a biota
of sparse well-preserved macrofossils and pelagic foraminifera. Upslope of the point
at which effective wave base impinges on the seabed is a high-energy environment. Because of the gentle gradient of the shelf, this belt may be tens of kilometers wide. This is
a zone of shoals and bars. The resultant sedimentary facies include biogenic reefs, crossbedded oolites, and skeletal and mature quartz sands. To the lee of this high-energy
belt is a sheltered zone which may stretch for hundreds of kilometers to the shoreline.
This low-energy environment generates pelmicrites, micrites, dolomicrites, and evaporites in the lagoons, tidal flats, and sabkhas of arid carbonate realms (Warren, 1989; Kendall and Harwood, 1996). Clays, sands, and peats form in the analogous environments
of humid terrigenous realms. Regressions and transgressions cause the three facies belts
to migrate to and fro over each other in a cyclic manner.
The X-Y-Z zone model was based on the study of ancient limestones. Subsequent
research into recent carbonate environments has allowed much more detailed carbonate facies models to be devised, with up to eight separate facies (Wilson, 1975; Enos,
1983; Scholle et al., 1983b; Wilson and Jordan, 1983; Tucker and Wright, 1990; Wright
and Burchette, 1996, 1999). A major distinction is made between carbonate ramps and
rimmed carbonate platforms (Ahr, 1973; Read, 1985). Ramps are gently sloping surfaces, broadly comparable to the X-Y-Z zone model. Ramps tend to accrete across a
shelf depositing a sequence of pelagic muds, overlain by shallow water high-energy
skeletal and/or oolitic sand, succeeded in turn by lagoonal and intertidal muds. Rimmed
carbonate platforms, in contrast, drop sharply off from shallow to deep water, and are
thus also referred to as the carbonate drop-off model. There is a close correlation between the grain type and texture of carbonates and their depositional environment, for
reasons explained in some detail in Chapter 9. Figure 6.52 illustrates this correlation for
carbonate accretionary ramps and rimmed platforms. Modern examples of these two
models are briefly described and illustrated next.
The modern northeastern coast of Arabia is an example of a modern carbonate ramp
(Fig. 6.53). In the deeper water of the Gulf, below about 30 m, lime mud is being deposited. As water depth gradually shallows toward the Arabian Shield, skeletal wackestones pass shoreward, via skeletal packstones, into shallow water oolite grainstones and
reefs that accrete around Pleistocene limestone islands (Purser, 1973). Carbonate muds,
algal stromatolites, and evaporites form in sabkhas (Arabic - salt marsh) in sheltered
coastal lagoons and embayments (Evans et al., 1969). These form upward-fining sequences analogous to those of temperate terrigenous intertidal flats (Fig. 6.51).
247
6.3.2.7.4 Carbonate shelves
A general theory of carbonate shelf sea sedimentation was put forward, based largely
on the study of Paleozoic deposits of the Williston basin, North America (Shaw, 1964;
Irwin, 1965; Heckel, 1972). The thesis on which this model is based states that in quiescent tectonic epochs of the past there were broad stable subhorizontal shelves with gradients of less than one in a thousand. These gently sloping surfaces were intersected
by two horizontal surfaces of great significance: sea level and effective wave base (what
might now be called the "mudline"). The intersections of these surfaces with the sea
bed define three sedimentary environments. In the deepest part of the shelf, below effective wave base, fine-grained mud settles out of suspension. Resultant sedimentary
facies are laminated shales and calcilutites, sometimes with chert bands, and a biota
of sparse well-preserved macrofossils and pelagic foraminifera. Upslope of the point
at which effective wave base impinges on the seabed is a high-energy environment. Because of the gentle gradient of the shelf, this belt may be tens of kilometers wide. This is
a zone of shoals and bars. The resultant sedimentary facies include biogenic reefs, crossbedded oolites, and skeletal and mature quartz sands. To the lee of this high-energy
belt is a sheltered zone which may stretch for hundreds of kilometers to the shoreline.
This low-energy environment generates pelmicrites, micrites, dolomicrites, and evaporites in the lagoons, tidal flats, and sabkhas of arid carbonate realms (Warren, 1989; Kendall and Harwood, 1996). Clays, sands, and peats form in the analogous environments
of humid terrigenous realms. Regressions and transgressions cause the three facies belts
to migrate to and fro over each other in a cyclic manner.
The X-Y-Z zone model was based on the study of ancient limestones. Subsequent
research into recent carbonate environments has allowed much more detailed carbonate facies models to be devised, with up to eight separate facies (Wilson, 1975; Enos,
1983; Scholle et al., 1983b; Wilson and Jordan, 1983; Tucker and Wright, 1990; Wright
and Burchette, 1996, 1999). A major distinction is made between carbonate ramps and
rimmed carbonate platforms (Ahr, 1973; Read, 1985). Ramps are gently sloping surfaces, broadly comparable to the X-Y-Z zone model. Ramps tend to accrete across a
shelf depositing a sequence of pelagic muds, overlain by shallow water high-energy
skeletal and/or oolitic sand, succeeded in turn by lagoonal and intertidal muds. Rimmed
carbonate platforms, in contrast, drop sharply off from shallow to deep water, and are
thus also referred to as the carbonate drop-off model. There is a close correlation between the grain type and texture of carbonates and their depositional environment, for
reasons explained in some detail in Chapter 9. Figure 6.52 illustrates this correlation for
carbonate accretionary ramps and rimmed platforms. Modern examples of these two
models are briefly described and illustrated next.
The modern northeastern coast of Arabia is an example of a modern carbonate ramp
(Fig. 6.53). In the deeper water of the Gulf, below about 30 m, lime mud is being deposited. As water depth gradually shallows toward the Arabian Shield, skeletal wackestones pass shoreward, via skeletal packstones, into shallow water oolite grainstones and
reefs that accrete around Pleistocene limestone islands (Purser, 1973). Carbonate muds,
algal stromatolites, and evaporites form in sabkhas (Arabic - salt marsh) in sheltered
coastal lagoons and embayments (Evans et al., 1969). These form upward-fining sequences analogous to those of temperate terrigenous intertidal flats (Fig. 6.51).