Geochemistry of speleothems from Jurassic palaeokarst (Subbetic, southern Spain)

Abstract

Speleothems growing on Jurassic palaeokarstic cavity walls from the Subbetic (Southern Spain) have been geochemically studied. These speleothems contain more than one generation of low-magnesian calcite (LMC) cements which grow into the cavities. The simplest case is composed of two generations of precipitate: the first cement is a non-luminescent (NL) radiaxial fibrous calcite (RFC) with a Fe Mn ratio between 2.8 and 6.1; the second cement is a zoned dull luminescent (ZDL) columnar, palisade calcite (CPC) with a lower Fe Mn ratio. The more complex case is composed of several generations of precipitate (NL-RFC and ZDL-CPC), with interbedded mechanically deposited internal sediments. Petrographic and isotopic data of the host rocks suggest that they have been formed in the marine phreatic environment. In contrast, the isotopic composition of the internal sediment and of non-skeletal grains of the host rocks suggest that they underwent diagenetic alteration in the marine-meteoric mixing zone. The isotopic composition of cements (in speleothems and complete infillings of small cavities) shows a typical meteoric “inverted J” trend. The alternation of the NL and ZDL cement stages in the speleothems is interpreted to be a result of hydrodynamic changes in the cementation environments. These changes can be related to sea-level fluctuations which affected the Subbetic continental margin during the Jurassic.