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Research Articles: Diagenesis
Diagenetic Salinity Cycles and Sea Level Along a Major Unconformity, Monte Composauro, Italy
In shallow-water carbonates, position of the water table and the chemistry of early diagenetic fluids are controlled in large part by sea-level and climate fluctuations. These fluctuations may result in cyclic progression from marine diagenesis or sedimentation, to mixing zone, meteoric, mixing zone, and then marine diagenesis or sedimentation. Herein we term such a cyclic progression as a diagenetic salinity cycle. This diagenetic study along a single unconformity in Cretaceous limestones of Monte Camposauro, southern Apennines, Italy, demonstrates the presence of four diagenetic salinity cycles and their link to relative changes in sea level. Data consist of field observations, transmitted light and cathodoluminescence microscopy, fluid inclusions, and carbon and oxygen isotopes. They show that twenty-one unconformity-related diagenetic features predated deposition of marine limestones above the unconformity. Fossiliferous internal sediments and stable-isotope data from radiaxial fibrous calcite and recrystallized aragonite are used to indicate marine diagenesis. Fluid-inclusion and stable-isotope data are used to indicate precipitation of calcite cement in all mixing ratios of the mixing-zone environment and in the freshwater phreatic zone. Paleocave morphology indicates vadose and water-table dissolution, and bauxite sediments support extensive vadose diagenesis. In each diagenetic salinity cycle, there is evidence for relative fall or rise of sea level, indicated by transitions from marine sedimentation to mixing zone and freshwater phreatic diagenesis, marine phreatic to freshwater vadose diagenesis, vadose diagenesis to marine phreatic diagenesis, and freshwater phreatic diagenesis to normal marine sedimentation.
During the time interval of the development of the unconformity (beginning some time in the Aptian and ending in the early middle Cenomanian) at least seven third-order eustatic fluctuations have been hypothesized during an overall second-order eustatic rise. At a given elevation, the third-order eustatic fluctuations would not be able to produce four diagenetic salinity cycles without tectonic uplift of 3.5 to 8 mm/ky. Our study illustrates that the formation and the timing of diagenetic salinity cycles are highly sensitive to paleoelevation.
Diagenetic salinity cycles, preserved along unconformities, can be used to interpret relative changes of sea level and to predict the presence of depositional sequences in downdip positions. Conversely, downdip depositional sequences, tectonic history, and sea-level history can be used to predict some aspects of diagenetic alteration updip.
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