THE CHALLENGE OF CLASSIFYING ANCIENT FRESHWATER WETLANDS: AN EXAMPLE FROM THE PLIOCENE-PLEISTOCENE GUADIX BASIN (S. SPAIN) (Invited Presentation)

Freshwater wetlands cover nowadays about 11% of the Earth’s surface. Although it is difficult to assess their abundance in the past, it is logical to assume that they should be present in the geologic record. However, there are few examples of ancient wetlands in the literature, mostly showing no established parameters for recognition of wetland deposits in the sedimentary record. The aim of this study is to provide objective tools in recognizing and classifying ancient freshwater wetlands using as an example the palustrine sediments of the Guadix Basin of southern Spain.

The continental fill (Tortonian to Late Pleistocene in age) of the Guadix Basin (Betic Cordillera, Spain) is divided into three genetic units, of which only the two youngest (Pliocene-Pleistocene units V and VI) crop out in the study area. Both of these units are characterized by a fluvial system flowing along the axis of the basin, draining towards a shallow lake located to the northeast in the neighboring Baza Basin and being fed by two alluvial fan transverse systems. Through detailed stratigraphic, sedimentologic, and petrographic studies of Units V and VI, carried out as part of Projects AHOB-3 (funded by the Leverhulme Trust Foundation) and CGL2009-07830/BTE (awarded by the Spanish Ministry of Science and Innovation), a series of both siliciclastic and carbonate architectural elements have been identified, resulting in the interpretation of fluvial-palustrine facies in the so-called “Axial System”. In the central sector of the Guadix Basin, palustrine siliciclastics are identified in association with palustrine carbonates, coal seams, and other floodplain sediments with some of them showing incipient paleosol development. The paleoenvironment represented by these facies has been interpreted as an ancient analogue for modern wetlands.

The categorization of such ancient wetlands is based on the existing classification systems for modern wetlands and a selection of the key features that can be deduced from the geologic record. Parameters such as type and composition of the substrate, hydrology, pH, and stable isotopic geochemistry are applied to characterize ancient wetland paleoenvironments. As a result, a relatively detailed classification of the Guadix Basin ancient wetlands is proposed.