DIAGENETIC SALINITY CYCLES RECORD KASRT-WATER GEOCHEMISTRY

Paleokarst surfaces normally represent hiatus in the sedimentary record. However, in coastal environments, the position of the water table and chemistry of fluids below such surfaces is closely controlled by climate changes and relative sea-level fluctuations. Therefore, a diagenetic succession below a paleokarst surface has great potential for providing a detailed paleoenvironmental record. This paleoenvironmental record can be used to predict the presence of depositional sequences in downdip positions, and conversely, downdip depositional sequences and sea-level history can be used to predict some aspects of diagenetic alteration updip. Here, we develop the concept of "diagenetic salinity cycles" as the geochemical record of paleoenvironmental change. A complete diagenetic salinity cycle starts with marine diagenesis followed by a progression from mixed, fresh, mixed, and marine. Five diagenetic salninity cycles are identified along a single paleokarst surface (Albian to upper Turonian) in Cretaceous limestones of Monte Camposauro, Southern Appenines, Italy. Data consist of field observations, transmitted light and cathodoluminescence microscopy, fluid inclusions and stable isotopes. For cycles #1, 3, 4 and 5, there is strong evidence for relative falls or rises in sea level indicated by transitions from marine phreatic to vadose, vadose zone to marine phreatic, and freshwater phreatic to normal marine sedimentation recorded by the filling of the cavities. During this time interval, there were at least five higher amplitude eustatic fluctuations combined with tectonism, which could have been expressed as diagenetic salinity cycles. As relative sea-level change cannot be demonstrated for diagenetic salinity cycle #2, climatic changes must also be considered. This study has shown that along a single, long-lived unconformity, there is a diagenetic record consisting of diagenetic salinity cycles. These diagenetic salinity cycles are the record of changing fluid chemistry induced by relative sea-level fluctuations and climate change. They link diagenetic history of coastal paleokarst to sequence stratigraphy and paleoclimate.