The lower/upper Bajocian boundary (Middle Jurassic) in the Casa Chimeneas section: Sierra de Ricote (Murcia Region), southern Spain

Abstract

The Lower Bajocian/Upper Bajocian transition was analysed in the Casa Chimeneas section (JRi3), Sierra de Ricote (Murcia), which is located in the Median Subbetic paleogeographic domain. This stratigraphic section, consisting of an alternation of marls and marly limestones with radiolarians, finely shelled bivalves, Zoophycos and frequent ammonites, represents the best stratigraphic sequence of the Subbetic (Betic Range) and, also possibly, of the Mediterranean province (westernmost Tethyan Realm) to carry out this analysis. Ammonite have been sampled since the 90s increasing the collection available, which, together with the analysis of calcareous nannofossil assemblages and the carbon-isotope δ13C curve, allows accurately modify the position of the Lower Bajocian/Upper Bajocian boundary in this stratigraphic reference section. Ammonite assemblages are dominated by Oppeliinae, Phylloceratoidea and Sphaeroceratidae, but other taxa with more biostratigraphical value (Stephanoceratinae, Cadomitinae, Frebolditinae, Leptosphinctinae and Parkinsoniidae) are also common. The absence of the typical Teloceras and the scarcity of Caumonstisphinctes make difficult to recognize the base of the Upper Bajocian. The appearance of the species Leptosphinctes constrictus, the last records of the genus Paviceras, and the species Masckeites? aff. exilis and Subcollina ochoterenai are the most significant bioevents marking this boundary. The first specimens, but not first occurrences, which would be recorded below the oldest studied sample, of the calcareous nannofossils Cyclagelosphaera margerelii and Crepidolithus crassus are identified in the base of the Blagdeni Subzone, and those of Discorhabdus ignotus and Ethmorhabdus gallicus within the upper part of the same subzone. The last occurrences (LOs) of Hexalithus magharensis and Carinolithus superbus are located within the Niortense Zone. Significant fluctuations are recorded in the δ13C curve in the Lower Bajocian/Upper Bajocian transition coinciding with the turnover of ammonites and the increase in diversity of calcareous nannofossils. Some ammonite taxa with significant biostratigraphic value are herein described and illustrated for the first time in Sierra de Ricote.