Calcretes from a palaeosinkhole in jurassic palaeokarst (subbetic, southern Spain)

Abstract

Calcretes have been recognized in the bottom of a palaeokarstic sinkhole etched into Early Jurassic carbonates from the Subbetic (Southern Spain). These are 3 to 10 cm thick and they show an internal structure in which three units can be distinguished. The lower unit is composed of alternate irregular bands of unfossiliferous micrite and sparry calcite. In the micrite, several features point out the activity of endolithic microorganisms as fungi and bacteria. The middle unit, which has a laminated structure with palisade calcite spar and some vadose silt is interpreted as a speleothem and, finally, the upper unit shows calcrete pisoids and rhizocretions. The isotope ratio analysis gives results in which δ 13C ranges from −10.1‰ to −10.9‰ and δ 18O ranges from −5.7‰ to −7.0‰. These values are very similar to those from palaeosols of other ages and localities, which is an important argument favouring emersion stages, with continental deposition in an island with an arid or semi-arid climate. Nevertheless, these calcretes outcrop in association with a palaeokarstic relief reflecting wet climate before the calcrete formation.