Abstract

Carbonate lithofacies have been commonly used as paleoclimatic indicators although classifications of carbonate deposits based on temperature and climate ignore a wide spectrum of other controlling factors that should be taken into account in order to discriminate between warm and temperate marine paleoenvironments. Therefore, the analysis of other evidences that support these results is strongly suggested. The tortonian Basal Trangressive Complex (BTC), locally named Niebla Fm. in the eastern sector of the province of Huelva, record the marine transgression of the southern Iberian margin and the beginning of the Neogene deposition in the Guadalquivir Basin. Along the northern margin of this basin the BTC includes heterogeneous siliciclastic/carbonate facies and a very complete paleontological record. Although previous studies have focused on the climate characteristics of the BTC, mainly based on the paleoecology of red algae, bryozoans, macroforaminifers and bivalves, a review of this issue is still necessary. Several of these evidences point out non-tropical conditions for shallow marine environments but warmer conditions are inferred by the occurrence of the aragonite bivalve Isognomon in an accumulation bed in the middle part of the Niebla Fm. The recent founding of the scleractinian tropical coral Siderastrea radians in the Arroyo Tariquejos outcrop (Lepe-Cartaya, western sector of Huelva province) allows us to make some clarifications about the climate setting of this Atlantic domain sector in the Late Tortonian. The corals occur covering the basal transgressive surface of the unit, developed in a bioeroded substrate constituted by Carboniferous shales. Hence, the appearance of both subtropical and tropical taxons as Heterostegina, the red algae Lithoporella and the selaceans genus Carcharocles and Isurus, as well as other evidences such as the heterozoan carbonate sediment associations described in the Niebla Fm., denote a general subtropical setting and temperatures of the waters very close to the reef-threshold value. On the other hand, this general trend was punctuated by a warning episode recorded within the sedimentary succession by the Isognomon bed and the, probably coetaneous, bioherms of S. radians. This climatic event was accompanied by significative paleogeographic changes in the Guadalquivir Basin, such as a transgressive process that drive the rapid marine flooding of the Iberian foreland in the surrounding of Lepe-Cartaya area, and the migration of the shoreline some tens of kilometers inland, a quick deepening in the marine basin, and an abrupt change in the sedimentation from siliciclastic to carbonate, all of them recorded in the Niebla Fm.