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Earth & Life (http://www.geofinds.com), 2006-10-1, Vol.1. , No.1: 31-33.
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A method to calculate bottom subsidence for restoring eustasy sea-level changes*
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Abstract To exclude the basin subsidence caused by sediment loading is a critical step in restoring the magnitude of ancient eustatic changes. Since the basin subsidence is caused by the weight of the deposits, the magnitude of the subsidence should be in proportion to the weight of the deposits, i.e., the original thickness of the deposits. To several stratigraphic sections under the same tectonic settings, the ratio of the difference in the original thickness of any two sections to the difference in their subsidence magnitudes should be a constant, which can be called sediment-loading subsidence coefficient and can be easily calculated. Then, the basin subsidence of a section can be obtained via multiplying the original thickness of the section with the sediment-loading subsidence coefficient. Keywords: eustatic sea-level change; sediment loading subsidence; sea-level change
1. IntroductionTo restore ancient eustatic change amplitudes and find the pattern of Phanerozoic eustatic changes is the basis for predicting future eustatic changes. A key step in restoring ancient eustatic change is to recognize and exclude the magnitude of basin subsidence caused by sediment loading and the vertical tectonic motion distances of the basin basement. Generally, restoring ancient eustatic change amplitudes include two steps. Firstly, relative sea-level change amplitudes are to be calculated. After that, as the second steps, the basin subsidence and vertical tectonic motion distances are to be recognized and excluded, so that the relative sea-level change amplitude are converted to eustatic change amplitude. Many methods have been developed to determine relative sea-level change amplitude (Fischer, 1964; Harris et al., 1984; Seiglie and Moussa, 1984; Weimer, 1984; Cisne et al., 1984; Cisne and Gildner, 1988; Hallam, 1978; Goldstein and Franseen, 1995), such as those of Hallam, Cisne et al. But no effective methods have been proposed to determine basin subsidence. A formula to calculate basin subsidence is S = Hw rw/rm + Ts rs/rm (1) Here S, rw, rm, Hw, rs, Ts means bottom subsidence amplitude, density of water, density of mantle, sea-level change height, density of deposits, original thickness of deposits, respectively (Burton et al., 1987; Rowan et al., 1996). This formula means that the weight of the deposits make the mantle under the earth crust to move aside and the weight of the mantle moved aside is equal to the weight of the deposits on the basinal bottom. The basis of the formula is isostasy, which can be easily understood from Archimedes’ formula to calculate buoyancy. According to the formula (1), 10 m thick deposits will cause 6.7 m high subsidence of the basinal bottom. Such a magnitude of subsidence seems to be too large to be credible and isostasy is not always true. So this formula has not been generally accepted for determining basin subsidence. Up to now, there are no effective methods to determine ancient basin subsidence and determining of ancient eustatic change amplitudes is generally impossible (Soreghan and Giles,1999). So, it is significant to develop some effective methods to determine ancient basin subsidence amplitudes. |
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*Supported by National Natural Scientific Foundation of China (No. 40472015) and the State Key Laboratory of Modern Paleontology and Stratigraphy (No. 053102), as well as the Key Laboratory for Minerals and Resources, Chinese Academy of Sciences. |
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