A geochemical study of the materials associated with the JURASSIC paleokarst of the Sierra Gorda (Internal Subbetic, Southern Spain)

Abstract

A dense network of cavities formed by a Jurassic palaeokarst exists within the Mesozoic materials of the Sierra Gorda (Subbetic Zone, Southern Spain). These cavities are best developed in the southern part of the sierra, where they penetrate more than 100 metres down into the underlying rock (Gavilán Fm.). The walls of these cavities are often coated in speleothems, while the rest of the cavity is filled in with carbonate sediment (crinoidallimestones, pelagic limestones and/or laminated sediment) as well as calcitic cements. The limestone filling of many of the cavities contains pelagic marine fauna, thus indicating that they are neptunian dykes. We have studied the speleothems covering the walls of the karstic cavities and also the cement and carbonatesediment infillings, using both cathodoluminescence and trace-element microanalysis. These results were then compared with those for the host rocks. Various phases of infilling can be discerned (precipitation and sedimentation) under differing genetic conditions at different times. One of the main features of these conditions was changes in energy. During high-energy episodes, caused by wave and current action, non-luminescent, calcitic cements with a lesser Mn content were formed, whilst during low energy periods cements with a dull and/or zoned luminescence, in which the Fe/Mn ratio was at its highest, were generated. These alternations in energy conditions reflect the fluctuations in the relative sea level in the region, due to eustatic and/or tectonic events. A comparison of the results of the tests described above with those obtained from isotope analysis (δ13C and δ18O) leads us to affirm that the cements and speleothems were created under phreatic marine conditions but that fresh water, probably flowing from adjacent islands, had an influence on the formation of the sediments within the cavities. The palaeotemperatures and palaeosalinity have been calculated from the δ18O data and Sr/Na ratios respectively. The palaeosalinity results also point to fresh water having been involved in the generation of the internal sediments.