Abstract

Highly elongate vertical and horizontal calcite-cemented concretions in Pleistocene shallow-marine and coastal eolian sandstones are mixtures of terrigenous sand and carbonate biogenic grains cemented to various degrees by low-Mg calcite. Petrographic and stable carbon and oxygen isotopic data support the earlier hypothesis of Mazzanti and Parea (1979) that vertically oriented concretions formed by the precipitation of vadose meteoric water percolating downward toward the water table and that horizontal concretions were precipitated from phreatic groundwater flowing toward the coast. Many vertical concretions started growth as rhizocretions composed of micrite but enlarged and coalesced when equant sparry calcite was precipitated from percolating vadose water. Horizontal concretions, cemented entirely by subequant calcite spar, have coalesced into nearly completely cemented beds. Decimeter-scale domains of curved horizontal concretion elements reflect the influence of adjacent phreatic flow domains, each with their own particular flow paths.

Aragonite and high-Mg calcite shells in the sands were the main source of carbon in calcite cement. Isotopically depleted δ13C values of bulk samples (-6 to -8.5‰, V-PDB), however, require the input of a light carbon source, probably soil CO2. δ18O values of bulk samples are compatible with cementation chiefly by meteoric water at temperatures between 7° and 22°C. The values are typical of air, and inferentially groundwater, temperatures today.

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