286
6 DEPOSITIONAL SYSTEMS
sea level change (e.g., Stille, 1924, 1936; Sloss, 1963; Dott, 1994). Not everyone is prepared to accept, however, that sea level changes are global and eustatic. There is plenty
of evidence for global climatic change, and evidence for global changes in sea level.
Every geologist knows, however, that sea level varies at a point in response to:
1. Global changes in ocean water volume -- probably driven by climatic change
2. Local tectonic uplift and subsidence of the crust- probably driven by plate movements
3. Local changes in the availability of sediment supply- probably driven by climatic
change, changing rates of sediment runoff, and the initiation and extinction of drainage systems.
Sadly it is normally impossible to determine which of these mechanisms, singly or together, may be responsible for a particular sea level change in a given area. There is
obviously no universal benchmark or Plimsoll line against which global changes in sea
level may be calibrated, and which may be used therefore to differentiate the various
processes of sea level change. This point is demonstrated by many modern deltas. The
modern Mississippi (Fig. 6.33) and the Arctic fan deltas of Spitzbergen (Martinsen,
1995) show that regressions (high stand systems tract) occur at the active river mouth.
Simultaneously, a few kilometers away, the sea advances across a subsiding compacting
abandoned delta lobe depositing a transgressive systems tract.
6.4.4 Cyclostratigraphy
Cyclicity, the repetition of genetic increments of strata, is a common feature in many
stratigraphic sections. This is a topic that has been widely studied and written about for
many years (e.g., Merriam, 1965; Duff et al., 1967; Schwarzacher, 1975). Twenhofel remarked in 1926 "The finding of rhythms by geologists seems at the present time to be
a pleasant diversion" (p. 612).
This chapter has decribed many sedimentary models that produce predictable facies
sequences. The formation of genetically related sequences is an obvious and integral
part of sedimentation. The origin of cyclicity, the phenomenon of repeated sequences,
has aroused lively debate. Beerbower (1964) made an important distinction between
autocyclic mechanisms, which are an integral part of the sedimentary model, and allocyclie mechanisms, which originate outside the model. The to-and-fro migration of a
river channel and the switching of a delta distributary can generate cyclic sedimentation in a steadily subsiding sedimentary basin. These are examples of autocyclicity. Not
all sedimentary models have this ability, however, and external mechanisms must be
sought to explain cyclicity where it is present (Frostick and Steel, 1994).
Mechanisms capable of causing allocyclic sedimentation can be seen operating in the
recent past. The worldwide distribution of raised beach deposits, and relict terrestrial
sediments on continental shelves, provide evidence of sea level changes correlatable
with the waxing and waning of the ice caps. Individual raised beaches are not always of
constant elevation when traced along the coast. These testify to tectonic tilting of the
earth's crust, sometimes in isostatic response to the vanishing load of ice caps. Cyclic
climatic variations are known both in historic times and, from pollen analyses, from prehistoric time. These facts show that transgressions and regressions capable of generating sedimentary cycles may occur due to climatic, eustatic, and tectonic changes. It is
not always possible, however, to detect which of these mechanisms caused any specific
6 DEPOSITIONAL SYSTEMS
sea level change (e.g., Stille, 1924, 1936; Sloss, 1963; Dott, 1994). Not everyone is prepared to accept, however, that sea level changes are global and eustatic. There is plenty
of evidence for global climatic change, and evidence for global changes in sea level.
Every geologist knows, however, that sea level varies at a point in response to:
1. Global changes in ocean water volume -- probably driven by climatic change
2. Local tectonic uplift and subsidence of the crust- probably driven by plate movements
3. Local changes in the availability of sediment supply- probably driven by climatic
change, changing rates of sediment runoff, and the initiation and extinction of drainage systems.
Sadly it is normally impossible to determine which of these mechanisms, singly or together, may be responsible for a particular sea level change in a given area. There is
obviously no universal benchmark or Plimsoll line against which global changes in sea
level may be calibrated, and which may be used therefore to differentiate the various
processes of sea level change. This point is demonstrated by many modern deltas. The
modern Mississippi (Fig. 6.33) and the Arctic fan deltas of Spitzbergen (Martinsen,
1995) show that regressions (high stand systems tract) occur at the active river mouth.
Simultaneously, a few kilometers away, the sea advances across a subsiding compacting
abandoned delta lobe depositing a transgressive systems tract.
6.4.4 Cyclostratigraphy
Cyclicity, the repetition of genetic increments of strata, is a common feature in many
stratigraphic sections. This is a topic that has been widely studied and written about for
many years (e.g., Merriam, 1965; Duff et al., 1967; Schwarzacher, 1975). Twenhofel remarked in 1926 "The finding of rhythms by geologists seems at the present time to be
a pleasant diversion" (p. 612).
This chapter has decribed many sedimentary models that produce predictable facies
sequences. The formation of genetically related sequences is an obvious and integral
part of sedimentation. The origin of cyclicity, the phenomenon of repeated sequences,
has aroused lively debate. Beerbower (1964) made an important distinction between
autocyclic mechanisms, which are an integral part of the sedimentary model, and allocyclie mechanisms, which originate outside the model. The to-and-fro migration of a
river channel and the switching of a delta distributary can generate cyclic sedimentation in a steadily subsiding sedimentary basin. These are examples of autocyclicity. Not
all sedimentary models have this ability, however, and external mechanisms must be
sought to explain cyclicity where it is present (Frostick and Steel, 1994).
Mechanisms capable of causing allocyclic sedimentation can be seen operating in the
recent past. The worldwide distribution of raised beach deposits, and relict terrestrial
sediments on continental shelves, provide evidence of sea level changes correlatable
with the waxing and waning of the ice caps. Individual raised beaches are not always of
constant elevation when traced along the coast. These testify to tectonic tilting of the
earth's crust, sometimes in isostatic response to the vanishing load of ice caps. Cyclic
climatic variations are known both in historic times and, from pollen analyses, from prehistoric time. These facts show that transgressions and regressions capable of generating sedimentary cycles may occur due to climatic, eustatic, and tectonic changes. It is
not always possible, however, to detect which of these mechanisms caused any specific