High-Resolution Forward Stratigraphic Modeling of Ca2-Carbonate Platforms ...
313
Missing data mostly affect important sedimentation-governing parameters like
sea-level amplitudes and/or their frequencies, spatial and temporal subsidence
rates, and sediment production rates/sediment supply. For modeling purposes,
most of these data require a detailed quantification. However, even the extensive
Ca2 data set provides only a few reliable data and a lot of important parameters
had to be estimated during numerous modeling iterations. In the present paper
only the most important input parameters, which are time, subsidence rates, sealevel fluctuations and sediment production rates, will be discussed.
3.1
Time Setting
To assign the amount of time, that geological formations need for their build-up is
generally very difficult, because sedimentation is governed by many different variables (Schlager 1993). Estimation of these processes and especially their impact
on the resulting sediments is further complicated by frequent changes in sediment
mineralogy as is the case in the Upper Permian of northern Central Europe. The
Upper Permian in this area is characterized by an alternation between evaporite
and carbonate formations, which means a change in water chemistry and production rates. Customary methods of time estimation, like biostratigraphy, do not apply in most of the Upper Permian strata because time periods are too short and
fossils are extremely rare. A petrographic method of estimating time in (deep-water) evaporites is counting varves (Richter-Bernburg 1985). Although varves seem
to be induced by yearly processes and therefore are able to record very short -term
phenomena, they are dependent on steady-state conditions and consequently restricted to both deeper-water environments ("Linienanhydrit") and time periods
without changes in the sedimentary system. However, a typical feature of evaporites is the differential precipitation rate, so thickness patterns give no hints on sedimentation periods (Fiichtbauer 1968). As a result of his recently performed integrated time analysis Menning (1995) suggests a duration of about 7 Ma. for the
seven common Zechstein cycles (Zl-Z7). According to stratigraphic simulations,
this assumption seems to be reasonable. The Stassfurt cycle Z2 (Stassfurt-Carbonate, or Ca2 + Basal Anhydrite, or A2 + Stassfurt Salt, or Na2) was estimated by
Menning (1995) to last about 1.5 Ma. Following our simulation results, this seems
to be somewhat too short. Taking into account the duration of the observed sealevel fluctuations and estimating carbonate production rates within a reasonable
range, the Ca2 itself needs about 1.5 Ma. Assuming shorter time periods would result in higher frequencies of sea-level fluctuations and higher carbonate production rates, both of which are unrealistic in such a restricted sedimentary basin.
3.2
Subsidence Rates
The North European Zechstein Basins are genetically classified as resulting from
rifting. They formed along intracontinental convergence belts (Sengor 1995).
313
Missing data mostly affect important sedimentation-governing parameters like
sea-level amplitudes and/or their frequencies, spatial and temporal subsidence
rates, and sediment production rates/sediment supply. For modeling purposes,
most of these data require a detailed quantification. However, even the extensive
Ca2 data set provides only a few reliable data and a lot of important parameters
had to be estimated during numerous modeling iterations. In the present paper
only the most important input parameters, which are time, subsidence rates, sealevel fluctuations and sediment production rates, will be discussed.
3.1
Time Setting
To assign the amount of time, that geological formations need for their build-up is
generally very difficult, because sedimentation is governed by many different variables (Schlager 1993). Estimation of these processes and especially their impact
on the resulting sediments is further complicated by frequent changes in sediment
mineralogy as is the case in the Upper Permian of northern Central Europe. The
Upper Permian in this area is characterized by an alternation between evaporite
and carbonate formations, which means a change in water chemistry and production rates. Customary methods of time estimation, like biostratigraphy, do not apply in most of the Upper Permian strata because time periods are too short and
fossils are extremely rare. A petrographic method of estimating time in (deep-water) evaporites is counting varves (Richter-Bernburg 1985). Although varves seem
to be induced by yearly processes and therefore are able to record very short -term
phenomena, they are dependent on steady-state conditions and consequently restricted to both deeper-water environments ("Linienanhydrit") and time periods
without changes in the sedimentary system. However, a typical feature of evaporites is the differential precipitation rate, so thickness patterns give no hints on sedimentation periods (Fiichtbauer 1968). As a result of his recently performed integrated time analysis Menning (1995) suggests a duration of about 7 Ma. for the
seven common Zechstein cycles (Zl-Z7). According to stratigraphic simulations,
this assumption seems to be reasonable. The Stassfurt cycle Z2 (Stassfurt-Carbonate, or Ca2 + Basal Anhydrite, or A2 + Stassfurt Salt, or Na2) was estimated by
Menning (1995) to last about 1.5 Ma. Following our simulation results, this seems
to be somewhat too short. Taking into account the duration of the observed sealevel fluctuations and estimating carbonate production rates within a reasonable
range, the Ca2 itself needs about 1.5 Ma. Assuming shorter time periods would result in higher frequencies of sea-level fluctuations and higher carbonate production rates, both of which are unrealistic in such a restricted sedimentary basin.
3.2
Subsidence Rates
The North European Zechstein Basins are genetically classified as resulting from
rifting. They formed along intracontinental convergence belts (Sengor 1995).