Influence of Host-Rock Texture on the Morphology of Carbonate Concretions in a Meteoric Diagenetic Environment

The integrated stratigraphic-petrographic–geochemical study of Quaternary marine terraces that have undergone well-constrained sea-level and water-table oscillation(s) over geologically short time periods can provide important clues on diagenetic processes at the marine–continental interface. The Punta di i Cani marine terrace (Quaternary; Saint-Florent, Corsica) unconformably overlies the well-cemented, low-permeability arenites of the Miocene Monte S. Angelo Formation and is erosively overlain by unlithified, high-permeability Holocene continental sediments. The terrace is composed of a retrogradational, fining-upward succession, from fluvial pebbly sandstone to beach and shoreface deposits. The entire thickness of the terrace hosts concretions made of low-Mg calcite. Concretion geometry is linked to the sedimentological facies of the host sediment: (i) massive fluvial pebbly sandstone at the base of the terrace contains knobby, subhorizontal concretions somewhat resembling turbidite flute casts; (ii) very low-angle cross-bedded, well sorted beach deposits contain pipy, subhorizontal concretions; (iii) trough-crossbedded upper-shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae, locally obliterated by a patchy, isotropic cementation. Overall, the degree of cementation decreases from the bottom to the top of the terrace. Cement chemistry and morphology, as well as stable carbon and oxygen isotopic signatures, indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast. Elongate concretions are invariably parallel to the topographic gradient, consistent with the notion that the growth of concretions was controlled by advection. Smectitic vadose cement is locally present; its crosscutting relationships with calcite cement point to periodic oscillations of the water table. The results of this study (i) show that elongate concretions can be used as interstitial paleoflow indicators and (ii) suggest that concretion geometry in an advection-dominated diagenetic environment can depend on the small-scale permeability distribution, as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies.