Meteoric water participation in the genesis of Jurassic cherts in the Subbetic of southern Spain — a significant indicator of penecontemporaneous emergence

Abstract

In the eastern sector of the Betic Cordillera outcrops of the Internal Subbetic display chert lenses in the upper parts of oolitic limestones (Camarena Formation, Mid-Jurassic). The stratigraphy indicates a sedimentary record related to synsedimentary tectonics (tilting). The sediments were deposited in different tectonic blocks, at varying depths. The Camarena Formation is mainly made up of shallow platform oolitic limestones and was deposited on a `pelagic' swell, a shallow isolated platform that developed far from continental areas. The chert lenses appear in those stratigraphic units deposited in the lowermost sunken fault blocks which display biosiliceous facies overlying the Camarena Formation. The chert mainly consists of megaquartz, formed by replacement of the oolitic host rocks without an opaline precursor. The part of the host that has been most affected by the silicification is the carbonate cement. The petrological features indicate a slow replacement from solutions which were poor in silica and cations. Comparing the textures and fabric of the cherts and their host rocks, it is deduced that the silicification took place after compaction, cementation, fracturing and local dissolution of the oolitic limestones. The stable isotopic data (δ 18O, δ 13C) obtained from the chert, oolitic host rock and overlain biosiliceous rocks display significant differences. The host rocks (Camarena Formation) and overlying biosiliceous rocks have isotopic signatures that are typical of marine environments or shallow-marine diagenesis. However, the chert isotopic values are interpreted to suggest meteoric water participation in the silicification process. The timing of the chertification is suggested to have been between the Bathonian (beginning of the deposition of the biosiliceous source facies) and the Aptian (first appearance of this chert as clasts in breccia beds). In palaeogeographic terms the optimum timing would have been during emergence (i.e. between the Bathonian and Tithonian). The exposed areas could have acted as recharge areas of meteoric water into the regional aquifer system. The involvement of meteoric water in the silicification processes gives new support to the hypothesis that pelagic swells were subjected to episodic exposure.