Subaerial exposure and porosity generation in the Aymamon Limestone

Three regional hydrogeologic units compose the Puerto Rico North Coast Aquifer System: an upper aquifer, an intervening confining unit, and a lower aquifer. The upper aquifer consists mainly of the Aymamon Limestone. The patterns of porosity within the Aymamon Limestone are variable and irregular. These porosity variations are part the result of caliche formation and karstification induced by Tertiary sea level fluctuations. Cyclic subaerial exposures are recorded at different depths giving rise to a variable sequence of karst and caliche zones. In some case karst develops immediately above a soil zone creating a very abrupt change in porosity. In general, porosity of 30 to 40% abruptly decreases to 10 to 15% below an exposure surface and only gradually returns to higher values with depth.;Immediately above the caliche surfaces, a leached zone is common. This indicates two possible situations either: (1) case-hardened caliche surfaces control the vertical and horizontal water movement in the vadose zone or (2) they are simply more resistant to dissolution. Caliche surfaces may serve as partial aquitards above which groundwater would locally pond and move laterally.;Those successive karst-soil episodes suggest that fourth or higher-order eustatic cycles in combination with a tilting northward movement of the carbonate ramp, during the middle Tertiary, are probably responsible for the transgressive-regressive cycles in the Aymamon.