High-Resolution Forward Stratigraphic Modeling of Ca2-Carbonate Platforms ...
317
phase shift of -200° (Table 2). These features and their superimposition on the
third-order sawtooth cycle lead to a subdivision of the third-order cycle. As simulations show, the time between the transgressive systems tract (TST) of Zechstein sequence ZS3 and the early highstand systems tract (HST) of Zechstein sequence ZS4 is characterized by more than four fourth-order cycles (Figs. 10, 11).
Three of the Ca2 cycles occur during the slow, but steady sea-level rise caused by
the third-order cycle during Zechstein sequence ZS3 (late lowstand?), transgressive and highstand systems tract (Ca2 cycle I, II and III according to Strohmenger et al. 1993a, b, c, 1996a, b). One cycle is situated in the lowstand systems
tract (LST) of Zechstein sequence ZS4 (Ca21A2 cycle I) and an additional one
(Ca21 A2 cycle II) , which was not modeled completely, occurs during the late lowstand and the following early transgressive systems tract during A2 time of
Zechstein sequence ZS4. Although the fourth-order cycles, in general, follow the
sea-level trend given by the third-order cycle, they have their own considerable
impact on Ca2 sedimentation, as they are responsible for the buildup of major
cycles (e.g. the two major shallowing-upward cycles (Ca2 cycle II and III) preserved on the Ca2 platform (Fig.s). Furthermore, each of the fourth-order cycles
in Ca2 platform position reflects a rapid initial transgression and a long stage of
slow sea level fall. That is why, in contrast to the reconstruction of the third-order sequence, the fourth-order cycles are characterized by an "80% sawtooth",
which means a short and rapid transgression followed by a long stage of slow
sea-level fall.
Fifth- and higher-order cycles are thought to last between 10 and 200 ka (Plint
et al. 1992) and belong, like fourth -order cycles, to the Milankovitch frequencies.
Apart from the already described fourth-order cycles, fifth-order cycles playa
major role in Ca2 sedimentation. They form small-scale shallowing-upward cycles, called parasequences (Strohmenger et al. 1996a; Strohmenger and Strauss
1996), and are an important parameter, especially in platform sedimentation
during Ca2 time. Although the reconstruction of the sea level suggests the occurrence of at least nine fifth-order cycles during the time span between the early
TST of Zechstein sequence ZS3 and the early TST of Zechstein sequence ZS4,
only those four which shape the Ca2-highstand systems tract are obviously preserved (PS 4, 5, 6, and 7). Those three fifth-order parasequences (PS 1,2, and 3)
which occur during the Ca2 transgressive systems tract of Zechstein sequence
ZS3 mainly affect Ca2 sedimentation on the preexisting Al slope. However, as
simulations show, the shallow-water components of these sediments were reworked immediately after their deposition and probably resedimented as turbidites along the Ca2 slope. The last 500 ka of Ca2 time (LST and early TST of
Zechstein sequence ZS4) are characterized by the occurrence of another two
complete fifth-order parasequences (PSI and PS2 within Ca21A2 cycle I) and
parts of a third one (PS3 within Ca21A2 cycle II). Parasequences PSI and PS2 of
Ca21 A2 cycle I developed during Ca2 low-stand systems tract of Zechstein sequence ZS4 (Ca2lowstand wedge, LSW). With the onset of Ca21A2 cycle IIa the
Ca21 A2 platform of Zechstein sequence ZS4 was finally flooded (A2 transgressive systems tract). All fifth-order sequences are reconstructed by having a fre-
317
phase shift of -200° (Table 2). These features and their superimposition on the
third-order sawtooth cycle lead to a subdivision of the third-order cycle. As simulations show, the time between the transgressive systems tract (TST) of Zechstein sequence ZS3 and the early highstand systems tract (HST) of Zechstein sequence ZS4 is characterized by more than four fourth-order cycles (Figs. 10, 11).
Three of the Ca2 cycles occur during the slow, but steady sea-level rise caused by
the third-order cycle during Zechstein sequence ZS3 (late lowstand?), transgressive and highstand systems tract (Ca2 cycle I, II and III according to Strohmenger et al. 1993a, b, c, 1996a, b). One cycle is situated in the lowstand systems
tract (LST) of Zechstein sequence ZS4 (Ca21A2 cycle I) and an additional one
(Ca21 A2 cycle II) , which was not modeled completely, occurs during the late lowstand and the following early transgressive systems tract during A2 time of
Zechstein sequence ZS4. Although the fourth-order cycles, in general, follow the
sea-level trend given by the third-order cycle, they have their own considerable
impact on Ca2 sedimentation, as they are responsible for the buildup of major
cycles (e.g. the two major shallowing-upward cycles (Ca2 cycle II and III) preserved on the Ca2 platform (Fig.s). Furthermore, each of the fourth-order cycles
in Ca2 platform position reflects a rapid initial transgression and a long stage of
slow sea level fall. That is why, in contrast to the reconstruction of the third-order sequence, the fourth-order cycles are characterized by an "80% sawtooth",
which means a short and rapid transgression followed by a long stage of slow
sea-level fall.
Fifth- and higher-order cycles are thought to last between 10 and 200 ka (Plint
et al. 1992) and belong, like fourth -order cycles, to the Milankovitch frequencies.
Apart from the already described fourth-order cycles, fifth-order cycles playa
major role in Ca2 sedimentation. They form small-scale shallowing-upward cycles, called parasequences (Strohmenger et al. 1996a; Strohmenger and Strauss
1996), and are an important parameter, especially in platform sedimentation
during Ca2 time. Although the reconstruction of the sea level suggests the occurrence of at least nine fifth-order cycles during the time span between the early
TST of Zechstein sequence ZS3 and the early TST of Zechstein sequence ZS4,
only those four which shape the Ca2-highstand systems tract are obviously preserved (PS 4, 5, 6, and 7). Those three fifth-order parasequences (PS 1,2, and 3)
which occur during the Ca2 transgressive systems tract of Zechstein sequence
ZS3 mainly affect Ca2 sedimentation on the preexisting Al slope. However, as
simulations show, the shallow-water components of these sediments were reworked immediately after their deposition and probably resedimented as turbidites along the Ca2 slope. The last 500 ka of Ca2 time (LST and early TST of
Zechstein sequence ZS4) are characterized by the occurrence of another two
complete fifth-order parasequences (PSI and PS2 within Ca21A2 cycle I) and
parts of a third one (PS3 within Ca21A2 cycle II). Parasequences PSI and PS2 of
Ca21 A2 cycle I developed during Ca2 low-stand systems tract of Zechstein sequence ZS4 (Ca2lowstand wedge, LSW). With the onset of Ca21A2 cycle IIa the
Ca21 A2 platform of Zechstein sequence ZS4 was finally flooded (A2 transgressive systems tract). All fifth-order sequences are reconstructed by having a fre-