A 136 m section of Pleistocene-Holocene carbonate sediments in the Ashdod Yam 15/0 water well, which penetrates the coastal plain aquifer of Israel, was chosen for a study of carbonate diagenesis. The following diagenetic parameters were used to diagnose vadose, phreatic and marine diagenetic environments: original textures of aragonitic molluscan shells, kind and distribution of carbonate cements, trace-element (Sr and Mg) concentration and carbonate mineralogy. Comparison of the diagenetic parameters with the present hydrologic regime in the borehole indicates the following: 1) the preservation of vadose features well within the present fresh-water phreatic zone; 2) abundant intergranular and moldic porosity in a leached interval coinciding with a brackish-water mixing zone; 3) clear signs of fresh-water alteration, including leaching and recementation of molluscs and complete occlusion of pore space by drusy cement, in a permeable layer in a highly saline zone of sea-water intrusion; 4) the preservation of original metastable carbonate mineralogy, molluscan microstructure, and marine cements in the thin permeable layers shielded by impermeable shales; 5) extensive fresh-water alteration (coalescive neomorphic recrystallization, leaching, and occlusion of pore space by drusy cement) in thick permeable intervals.

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