Climate, duration, and mineralogy controls on meteoric diagenesis, La Molata, southeast Spain

摘要

We isolated the impact of climate, duration, and mineralogy on porosity enhancement and cementation associated with seven surfaces of subaerial exposure in Miocene carbonate strata of southeast Spain. We integrated new and published petrographic and geochemical studies on the same strata to quantify the approximate impact of each event of subaerial exposure. The results quantified that only minor dissolution and/or cementation occurred when subaerial exposure was short-lived (<533 thousand years) and in an arid climate, or when carbonate sediments were composed primarily of calcite. For such exposure surfaces, alteration was confined to the uppermost 0.5-2 m with 2%-5% porosity from dissolution and 3% cementation. After deposition of the last Miocene carbonate sequence, dolomitization and dissolution occurred during the initial stages of sea-level fall, associated with mixing during influx of meteoric water. This resulted in dissolution to create 10%-20% porosity over 83% of the platform system, and this indicated that even incipient subaerial exposure can lead to major porosity enhancement during times of high freshwater recharge and hydrogeology that promotes mixing. During a long-lived period of subaerial exposure (greater than 5.3 million years) associated with wet climate, major amounts of cementation reduced porosity in the phreatic zone and some porosity was enhanced in the vadose zone. This included 25% freshwater calcite cement affecting 53% of the platform. Dissolution during and after calcite cementation was approximately 8% throughout the platform. These observations predict that duration and climate combine to have the most significant impacts on porosity associated with subaerial exposure.