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RESEARCH PRODUCT

A foraminiferal assemblage as a bioevent marker of the main Ladinian transgressive stage in the Betic Cordillera, southern Spain

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Abstract The Betic Cordillera comprises the mountain range in the southern Iberian Peninsula that extends from Cadiz to Alicante, which displays typical features of alpine cordilleras. The Betic Cordillera includes two large geological tectonic domains, namely an External Zone, and an Internal Zone. The External Zone is essentially composed of epicontinental Triassic rocks that consist of the Buntsandstein, Muschelkalk and Keuper facies. The Buntsandstein facies rarely occur in outcrops, whereas the Keuper stratigraphic successions are truncated. The Muschelkalk facies frequently displays complete successions that are very useful for interpreting the Ladinian stage palaeogeography. The Muschelkalk facies consists of a lower carbonate member formed by three thick, massive limestone beds with intervals of thin-bedded marly limestone, and an upper marly member with intercalation of distinct beds of limestones and marls, where tempestites predominate. The first member has been interpreted as a transgressive systems track in which subtidal and ramp deposits can be identified. The second member is related to a highstand systems track, and consists of marine-platform and coastal deposits, which correspond to a shallowing-upward sequence. We describe and interpret a distinct bioclastic limestone bed with high concentrations of Involutinidae benthic foraminifers, that has been found in the Muschelkalk of several distant sections. This unusual bed is just above the third bed of massive Muschelkalk limestone, and consists of a bioclastic wackestone–packstone, up to 8 cm thick, with many Involutinidae of Ladinian age. The most abundant species are: Lamelliconus gr. biconvexus – ventroplanus, Lamelliconus cordevolicus and Triadodiscus eomesozoicus . These foraminiferal concentrations form a well-defined marker bed that may be related to a bioevent. The appearance of these foraminiferal taxa, which are also very common in the Triassic Alpine realm, coincides with the maximum flooding interval of a transgressive sequence. We surmise that these Tethyan foraminifers moved into newly developed coastal zones during maximum transgression and, at times, produced prolific shallow-water communities, which were reworked by storm currents that caused their unusual concentrations in deeper water. These concentrations thus generated the marker beds of foraminiferal packstones that can be found over distant outcrops in the region. Similar involutinid foraminiferal assemblages occur in the upper Muschelkalk of the Iberian Ranges and Catalan Coastal Ranges, as well as in the upper part of the transgressive systems track of the UAA-2.2 third-order cycle identified in the Muschelkalk of the Pyrenees Basin. Thus, this foraminiferal assemblage can be considered as an extensive bioevent marker related to an important transgressive stage of Ladinian age.