Los sedimentos con diatomeas del Mioceno en la Cuenca del Guadalquivir: edad y composición litológica

Abstract

The lithological composition and biostratigraphy of diatomaceous marls have been defined in 93 outcrops of the Guadalquivir Basin. The diatomaceous marls display a wide variety of lithological types, mainly comprised of opal A (10-85 %) (mainly diatoms and to a lesser extent silicoflagellates, spicules and radiolarians), calcite (5-80 %) (principally foraminifers, nannoplankton, sparite, micrite and microspar), c1ay minerals (5-40 %) (smectite in greater quantities than illite; and kaolinite in small quantities) and terrigenous quartz (< 15 %). The diatom assemblages in the marls reflect the occurrence of temperate to warm water conditions in this area during middle and late Miocene. The diatoms are dominated by the fol1owing upwelling species: Thalassiosira nitzshoides, Thalassiotrix longuissima and Thalassionema baci//aris. Benthic diatoms are scarce but they have great species diversity. However, in sorne outcrops the genus Delphineis is more frequent thus indicating environments c10ser to shore areas. Three episodes based on Barron's (1985) low latitude zonation have been defined. The Lower Episode includes Cestodiscus peplum and Coscinodiscus lewisianus Zone and is assigned to the upper Langhian- lower Serravalian. After a period of no diatom occurrence (e. gigas var. diorama Zone is missing), the Middle Episode is defined by Craspedodiscus coscinodiscus and Actinocyclus moronensis Zones and covers the latest Serravalian-early Tortonian. The Upper Episode corresponds to Thalassiosira yabei Zone and it is assigned to middle Tortonian. The fundamental types of dilutants in diatomic sedimentation (biogenic calcareous, terrigenous and c1ay minerals), do not vary throughout the three episodes but there are great variations in the proportions. The coastal upwelling regime produces tongue-like patterns of sedimentation and the sedimentary record displays different changes of facies. The Upper Episode, where the c1ay-terrigenous dilutants predominate over calcareous dilutants and Delphineis appear in greater quantities, is interpreted as having been formed in an environment closer to the shore than other episodes.