High-Resolution Forward Stratigraphic Modeling of Ca2-Carbonate Platforms ...
315
3.3
Sea Level
Although the intensive Zechstein research of the past 10 years has shed new light
on topics like relative shifts of coastal onlap and Zechstein sequence stratigraphy
in general (Tucker 1991j Strohmenger et a1.1993a, bj 1996a, b) the mechanisms
responsible for sea-level fluctuations are not clear and a quantification of sealevel change amplitudes and frequencies was not possible so far.
However, the high-resolution forward stratigraphic modeling of the Ca2 provides the possibility to fix amplitudes and frequencies of sea-level change, which
affect Ca2 sedimentation at least throughout the southern margin of the Southern Permian Basin, since the recently established sequence stratigraphic framework (Strohmenger et al. 1996a) and the resulting relative sea level interpretation can first be tested by comparing model results and "reality" at different locations. If the model results resemble the actual data, the values used for sea-level frequencies and amplitudes provide one reasonable set of input values.
The simulation program PHIL 5.1 either uses digitized eustatic sea-level
curves as input or constructs a curve by using input sea-level cycle periods, their
amplitudes, phases and character (sawtooth or sinusoidal).
The Permian period is generally assumed to be a time of sea-level fall, since
the formation of the supercontinent Pangea was nearly finished (first-order lowering). This class of sea-level order, which seems to be caused by accretion and
splitting of supercontinents (Vail et al. 1991), is far beyond the time scale of high
resolution stratigraphic modeling and therefore a quantification of duration and
amplitudes was not possible in this study.
A sinusoidal cycle with a period of 20 Ma and a magnitude of 100 m (e.g. Revelle 1990) was chosen for the second-order eustatic sea-level curve (Table 1). In
accordance with the general sea-level fall in Permian times, this cycle had to be
shifted in its phase about 160°, which results in a slight sea-level fall of secondorder magnitude during the simulated Ca2 time (Strohmenger et aI.1996a).
Third-order sequence cycles are fundamental units in sequence stratigraphy
and are generally characterized by a duration between 1 and 10 Ma (Plint et al.
1992). Vail et al. (1991) suggest a duration of 0.5-5.0 Ma. In the reconstruction
of the Ca2 sea level this cycle plays a major role, since it governs the main sequence stratigraphic units. The cycle duration of 1.1 my (Table 2) was chosen
according to the observed Ca2 facies and sequence stratigraphic patterns which
Table 1 Duration of sea-level orders
Order
PLINT et al. (1992)
Reconstr. Ca2 sea level
Duration
Duration
2
10-100 Ma
20Ma
3
1-10 Ma
1.1 Ma
4
200-500 ka
335 ka
5
10-200 ka
167 ka
315
3.3
Sea Level
Although the intensive Zechstein research of the past 10 years has shed new light
on topics like relative shifts of coastal onlap and Zechstein sequence stratigraphy
in general (Tucker 1991j Strohmenger et a1.1993a, bj 1996a, b) the mechanisms
responsible for sea-level fluctuations are not clear and a quantification of sealevel change amplitudes and frequencies was not possible so far.
However, the high-resolution forward stratigraphic modeling of the Ca2 provides the possibility to fix amplitudes and frequencies of sea-level change, which
affect Ca2 sedimentation at least throughout the southern margin of the Southern Permian Basin, since the recently established sequence stratigraphic framework (Strohmenger et al. 1996a) and the resulting relative sea level interpretation can first be tested by comparing model results and "reality" at different locations. If the model results resemble the actual data, the values used for sea-level frequencies and amplitudes provide one reasonable set of input values.
The simulation program PHIL 5.1 either uses digitized eustatic sea-level
curves as input or constructs a curve by using input sea-level cycle periods, their
amplitudes, phases and character (sawtooth or sinusoidal).
The Permian period is generally assumed to be a time of sea-level fall, since
the formation of the supercontinent Pangea was nearly finished (first-order lowering). This class of sea-level order, which seems to be caused by accretion and
splitting of supercontinents (Vail et al. 1991), is far beyond the time scale of high
resolution stratigraphic modeling and therefore a quantification of duration and
amplitudes was not possible in this study.
A sinusoidal cycle with a period of 20 Ma and a magnitude of 100 m (e.g. Revelle 1990) was chosen for the second-order eustatic sea-level curve (Table 1). In
accordance with the general sea-level fall in Permian times, this cycle had to be
shifted in its phase about 160°, which results in a slight sea-level fall of secondorder magnitude during the simulated Ca2 time (Strohmenger et aI.1996a).
Third-order sequence cycles are fundamental units in sequence stratigraphy
and are generally characterized by a duration between 1 and 10 Ma (Plint et al.
1992). Vail et al. (1991) suggest a duration of 0.5-5.0 Ma. In the reconstruction
of the Ca2 sea level this cycle plays a major role, since it governs the main sequence stratigraphic units. The cycle duration of 1.1 my (Table 2) was chosen
according to the observed Ca2 facies and sequence stratigraphic patterns which
Table 1 Duration of sea-level orders
Order
PLINT et al. (1992)
Reconstr. Ca2 sea level
Duration
Duration
2
10-100 Ma
20Ma
3
1-10 Ma
1.1 Ma
4
200-500 ka
335 ka
5
10-200 ka
167 ka