Climate, Duration and Mineralogy Controls on Meteoric Diagenesis

Abstract

For subsurface prediction of porosity distribution in carbonates that have undergone subaerial exposure, it is critical to start to quantify the diagenetic effect of the following factors: rainfall/recharge, duration of subaerial exposure and original mineralogy. This project examines the diagenetic alteration associated with seven subaerial exposure surfaces in Miocene La Molata carbonates in SE Spain. For short-lived and/or arid events of subaerial exposure, and subaerial exposure of sediment with calcitic mineralogy, diagenetic alteration occur in the uppermost 0.5 to 2 m but not noticeably below it. Over this thin interval there was minor dissolution (2-5%) and cementation (typically less than 3%). During the inception of end-Miocene subaerial exposure, mixing of freshwater with evaporated seawater led to dolomitization (greater than 90%) and dissolution (approximately 10-20%) over 83% of the carbonate system. Although short lived, this had the greatest impact on formation of secondary porosity. This surprising result suggests that a climate that allows for significant freshwater recharge and a hydroGeology that promotes mixing can have a profound impact on the creation of porosity, despite only incipient, short-lived, or updip subaerial exposure. During long-lived subaerial exposure after the Miocene, that included times of wet climate, calcite cement (25%) precipitated from fresh water in two different zones. This affected 53% of the overall carbonate system. Later dissolution in the vadose zone enhanced porosity as well. The amount of dissolution during and after calcite cementation is estimated to be 8% throughout the entire carbonate system. Although subaerial exposure has been thought to result in extensive alteration of carbonates, this study shows that some events of exposure have little affect and others have great affect. Short-lived subaerial exposure in an arid climatic setting is not likely to have great impact. Decreasing aridity and increased rainfall correlates to an increase in diagenetic alteration by meteoric waters. Carbonates composed of calcitic mineralogy are more resistant to alteration from meteoric diagenesis. During times of wet climate and high recharge, a hydroGeology promoting mixing between seawater and freshwater can promote great porosity enhancement, despite short duration. Given a long period of subaerial exposure during wet climate, volumetrically major amounts of cementation and dissolution are predicted.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014