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|  | Stratigraphic CorrelationsSubsidence and fault displacement around producing fields also can be measured directly by using basic coring techniques. Correlation of shallow stratigraphic markers in sediment cores provides a basis for determining the magnitude of displacement relative to some datum such as mean higher high water. Figure 6 shows that the correlated beds have been displaced progressively downward 35-63 centimeters (cm) comparedto the adjacent marsh surface. Preservationof the marsh sediments beneath ameter of water is clear evidence that themarsh surface subsided first to form theopen water area. Subsequent erosion ofthe marsh sediments was minor comparedto subsidence.Subsidence RatesOne way of detecting artificially induced subsidence around a producing field is by comparing the measured rate of subsidence there with natural subsidence rates in the same region. The wetland surface at Port Neches subsided about 63 cm in 22 years (figs. 2 and 6). These values yield a historical rate of wetland subsidence of about 3 cm/yr. Some of the highest natural rates of subsidence in the Mississippi Delta of southern Louisiana are on the order of 1 cm/yr (Roberts and others, 1994). Thus, the short-term historical subsidence rate at Port Neches (3 cm/yr) is three times higher than the highest subsidence rates in a region that is known for rapid subsidence. It is 75 times higher than the subsidence rate of 0.04 cm/yr in southwestern Louisiana, which is the closest area having both a similar geologic setting and estimated subsidence rates. « Faults & Releveling Surveys | Environmental Implications » |
