CALCRETES FROM A PALAEOSINKHOLE IN JURASSIC PALEOKARST (SUBBETIC, SOUTHERN SPAIN)

Calcretes have been recognized in the bottom of a palaeokarstic sinkho le etched into Early Jurassic carbonates from the Subbetic (Southern S pain). 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 cal cite. In the micrite, several features point out the activity of endol ithic 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 cal crete pisoids and rhizocretions. The isotope ratio analysis gives resu lts in which deltaC-13 ranges from -10.1 parts per thousand to -10.9 p arts per thousand and deltaO-18 ranges from -5.7 parts per thousand to -7.0 parts per thousand. These values are very similar to those from palaeosols of other ages and localities, which is an important argumen t favouring emersion stages; with continental deposition in an island with an arid or semi-arid climate. Nevertheless, these calcretes outcr op in association with a palaeokarstic relief reflecting wet climate b efore the calcrete formation.