METEORIC WATER PARTICIPATION IN THE GENESIS OF JURASSIC CHERTS IN THESUBBETIC OF SOUTHERN SPAIN - A SIGNIFICANT INDICATOR OF PENECONTEMPORANEOUS EMERGENCE

In the eastern sector of the Betic Cordillera outcrops of the Internal Subbetic display chert lenses in the upper parts of oolitic limestone s (Camarena Formation, Mid-Jurassic). The stratigraphy indicates a sed imentary record related to synsedimentary tectonics (tilting). The sed iments 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 a ppear in those stratigraphic units deposited in the lowermost sunken f ault blocks which display biosiliceous facies overlying the Camarena F ormation. The chert mainly consists of megaquartz, formed by replaceme nt of the oolitic host rocks without an opaline precursor. The part of the host that has been most affected by the silicification is the car bonate cement. The petrological features indicate a slow replacement f rom solutions which were poor in silica and cations. Comparing the tex tures and fabric of the charts and their host rocks, it is deduced tha t the silicification took place after compaction, cementation, fractur ing and local dissolution of the oolitic limestones. The stable isotop ic data (delta(18)O, delta(13)C) 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 sh allow-marine diagenesis. However, the chert isotopic values are interp reted to suggest meteoric water participation in the silicification pr ocess. The timing of the chertification is suggested to have been betw een the Bathonian (beginning of the deposition of the biosiliceous sou rce facies) and the Aptian (first appearance of this chert as clasts i n breccia beds). In palaeogeographic terms the optimum timing would ha ve been during emergence (i.e. between the Bathonian and Tithonian). T he 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, (C) 1998 Elsevier Science B.V, All rights reserved.