Olistoliths from the Middle Jurassic in Cretaceous materials of the Fardes Formation biostratigraphy (Subbetic Zone, Betic Cordillera)

Abstract

The Middle Jurassic olistoliths studied are found within Cretaceous materials of the south-eastern part of the Montes Orientales region (province of Granada, Betic Cordillera), specifically in the Río Fardes sector. The cretaceous materials are here characterized by pelagic and hemipelagic facies with abundant turbidite and olistostrome insertions in a deep basin marine environment (Middle Subbetic), bounded to the SE by a pelagic ridge (Internal Subbetic). The transition between the two domains was an area with active paleoslopes which facilitated both the exposure and denudation of Jurassic and Cretaceous materials and the development of a clear synsedimentary tectonics (slumps and olistostromes). Paleocurrents show that the source area of materials forming the thickest beds was located towards the S-SW, though for the thinner beds they are more widely dispersed. The sections under study have been dated by means of calcareous nannoplankton, planktonic foraminifera and radiolaria. The calcareous nannofossil assemblages enable us to distinguish Sissingh's zones CC-9, CC-10 and CC-13 for the Lower Cenomanian-Coniacian, which in turn determines the position of the Jurassic olistoliths within the sections. The precise age of the materials where the large-sized olistoliths are embedded is Coniacian (Marthasterites furcatus zone of calcareous nannoplankton). Olistoliths from the Middle Jurassic are often sufficiently exposed and stratified to allow sampling level by level and the establishing of a detailed biostratigraphy. After a study of the ammonite fauna collected in two olistoliths we were able to distinguish the Murchisonae and Concavum zones in the Aalenian, and the Discites, Laeviuscula, Sauzei and Humphriesianum zones in the Bajocian. In those areas with S-SW paleocurrents Aalenian materials are not found, but the Bajocian materials of some nearby sectors belonging to the Internal Subbetic have a similar lithology and faunal content.