Diagenetic controls on porosity and permeability in Miocene carbonates, La Molata, Spain

Abstract

This diagenetic study of Miocene strata from La Molata, southeastern Spain, evaluates controls on meteoric diagenesis and low-temperature dolomitization. There is a well-constrained sequence-stratigraphic framework and sea-level history that, combined with an absence of deep burial, allows for improved conceptual models of carbonate diagenesis, porosity and permeability. The methods combined field data, transmitted light and cathodoluminescence petrography, XRD, fluid inclusions, carbon and oxygen isotopes, Sr concentration and 87Sr/86Sr. The carbonate platform was extensively dolomitized at the end of the Miocene but before Pliocene deposition. Amount of dolomite increases basinward and down section; limestone is restricted to the most proximal area. Cathodoluminescence petrography shows dolomite is closely associated with dissolution and shows consistent zonation throughout the area. Fluid inclusions and carbon and oxygen isotopes indicate dolomitization was from mixtures of freshwater and evaporated seawater (43 ppt). The distribution of carbon and oxygen isotopes on the platform, and the Sr data indicate vertical flow of freshwater into the carbonate system. Thus, dolomitization is due to ascending freshwater-mesohaline mixing. This mechanism is broadly applicable to carbonate platforms with hydrogeology that permits recharge of freshwater below a platform saturated with evaporated seawater. Minor dissolution and cementation, associated with meteoric diagenesis along the first six sequence boundaries, occurred during short-lived and/or arid events of subaerial exposure. The seventh surface was exposed subaerially in a wetter climate for the last 5-million years, and this led to extensive cementation. These constraints show the diagenetic effects of subaerial exposure may be in part predictable with constraints on climate and duration along sequence boundaries. Calcite cements precipitated in two zones that cut across stratigraphy. Petrography, fluid inclusions, and carbon and oxygen isotopes indicate a freshwater phreatic origin. The two cemented zones were associated with two different water tables formed during relative sea-level fall and erosional downcutting of the landscape. In cemented zones, calcite cementation reduced porosity and permeability with relationships that are lithofacies specific. Petrography and field work indicate later dissolution, closely associated with vadose processes and fracture control. These observations serve to calibrate GPR observations suggesting fracture control on porosity.