Giant calcite concretions in aeolian dune sandstones; sedimentological and architectural controls on diagenetic heterogeneity, mid-Cretaceous Iberian Desert System, Spain

Abstract

Aeoliandunesandstones of the Iberian erg system (Cretaceous, Spain) host giantcalciteconcretions that constitute heterogeneities of diagenetic origin within a potential aeolian reservoir. The giantcalciteconcretions developed in large-scale aeoliandune foresets, at the transition between aeoliandune toeset and damp interdune elements, and in medium-scale superimposed aeoliandune sets. The chemical composition of the giantconcretions is very homogeneous. They formed during early burial by low Mg-calcite precipitation from meteoric pore waters. Carbonate components with yellow/orange luminescence form the nuclei of the poikilotopic calcite cement. These cements postdate earlier diagenetic features, characterized by early mechanical compaction, Fe-oxide cements and clay rims around windblown quartz grains resulting from the redistribution of aeolian dust over the grain surfaces. The intergranular volume (IGV) in friable aeoliansandstone ranges from 7.3 to 15.3%, whereas in cemented aeoliansandstone it is 18.6 to 25.3%. The giant-calciteconcretions developed during early diagenesis under the influence of meteoric waters associated with the groundwater flow of the desert basin, although local (e.g. activity of fluid flow through extensional faults) and/or other regional controls (e.g. variations of the phreatic level associated with a variable water influx to the erg system and varying sea level) could have favoured the local development of giant-calciteconcretions. The spatial distribution pattern of carbonate grains and the main bounding surfaces determined the spatial distribution of the concretions. In particular, the geometry of the giantcalciteconcretions is closely associated with main bounding aeolian surfaces. Thus, interdune, superimposition and reactivation surfaces exerted a control on the concretion geometries ranging from flat and tabular ones (e.g. bounded by interdunes) to wedge-shaped concretions at the dune foresets (e.g. bounded by superimposition and reactivation surfaces) determining the spatial distribution of the heterogeneities of diagenetic origin in the aeolian reservoir