Pliocene-Pleistocene palustrine sediments of the Guadix Basin (Betic Cordillera, S. Spain): geologic record of ancient wetlands

The continental fill of the Guadix Basin (Betic Cordillera, S. Spain) is represented by its three youngest units (from IV to VI) and is Upper Tortonian (Pliocene) to Late Pleistocene in age. Detailed stratigraphic, sedimentologic, and petrographic studies of Units V and VI identifies both siliciclastic and carbonate facies as well as architectural elements resulting in the interpretation of fluvial-palustrine facies in the so-called Axial system, which flowed longitudinally through the basin. It was transversally fed by marginal alluvial fans, draining towards a shallow lake located to the northeast in the neighboring Baza Basin. 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 is an ancient analog for modern wetlands.
Unit V comprises mostly fine sediments associated with isolated low-sinuosity channels and rare palustrine carbonate beds with immature paleosol development. It is interpreted as a high accommodation systems tract (HAS), in which the available accommodation space is high in relation to the sediment input. On the other hand, Unit VI, contains in the western and northern zones of the basin vertically-stacked carbonate palustrine sequences. These successions are interpreted as occurring within a relatively reduced accommodation area receiving sediment from a source area with more carbonates (low accommodation systems tract or LAS).
Two types of laterally-associated architectural elements are interpreted as palustrine in origin in both units: (1) palustrine carbonates (CPm) and (2) palustrine-lacustrine siliciclastic beds (FPb). These palustrine facies appear laterally and vertically related to the floodplain siliciclastic sediments in Unit V. Facies relationships in Unit VI are difficult to assess because of less extensive exposure.
The interpretation of the palustrine deposits in Units V and VI is based on an extensive review of classic examples of palustrine facies from the geologic record together with modern studies. The features described for some sub-environments in the Florida Everglades (USA) and the Recent sedimentation in Las Tablas de Daimiel wetlands (Spain) are strikingly similar to those of the Guadix Basin palustrine facies. The sedimentologic and petrologic features point to a fluctuating base level with seasonal subaerial exposure. Together with the laterally extensive palustrine beds and the lack of deep lacustrine facies in the area, these deposits can be interpreted as marsh wetlands developed during Pliocene and Pleistocene times in the central sector of the Guadix Basin. To confirm this interpretation and refine paleoclimatic reconstructions, stable isotopic analyses (C,O) will be performed on the palustrine carbonates. The research is funded by projects CGL2009-07830/BTE, AHOB-3 and the Working Group RNM-369JA.