A refugium for charophytes during the maximum post-Palaeozoic sea-level highstand in the Turonian of Tarragona (Catalonia, Spain)

Abstract

During the Cenomanian–Turonian interval, Europe was largely submerged under a shallow tropical sea within the Cretaceous Tethyan Archipelago, limiting non-marine lacustrine habitats to a few coastal lakes on the islands. This study reports an island refugium for charophytes in the Upper Cretaceous of Tarragona located at the palaeo-shores of the former Ebro Massif. The Upper Cretaceous of Tarragona comprises three carbonate formations that record a Cenomanian–Turonian transgressive-regressive sequence. This sequence is represented by a shallow marine platform to pelagic facies at the base, overlain by lacustrine and palustrine facies at the top. These non-marine deposits are newly attributed to the Turonian, based on the stratigraphic context and the presence of the species Atopochara trivolvis var. multivolvis. In addition to this dominant species, the charophyte assemblage contains a clavatoroidean species, represented by the thallus Munieria grambastii forma sarda, and is associated with freshwater gastropods. A. trivolvis var. multivolvis had a wide distribution in the Northern Hemisphere at a palaeolatitude of around 40ºN, occurring in the United States, Spain, France, and Armenia. The European localities suggest that the island charophyte flora in the Cretaceous Tethyan Archipelago was an impoverished version of the prehighstand Early Cretaceous flora, which was dominated by clavatoraceans. This contrasts with coeval floras from the mainland (Chinese and Argentinian basins), where Turonian charophyte communities were diverse and already dominated by modern characean genera. The island lakes acted as refugia for the last clavatoracean-dominated charophyte communities before their complete replacement by modern characean communities during the latest Cretaceous.