DISTRIBUTION OF EARLY MIOCENE ICHNOFACIES AND PALEOSOLS OF DISTAL FLUVIAL SYSTEMS, EBRO BASIN, SPAIN

Conventional facies analysis of floodplain and fluvial channel deposits only provide a limited amount of information that can be used in paleoenvironmental interpretation. The application of ichnofacies and paleosol studies provides a more sensitive set of indicators enabling sedimentological models based on limited information to now be refined.

Early Miocene deposits exposed in the Ebro Basin, Spain, have been identified as proximal alluvial fan, fluvial and lacustrine continental facies. The fluvial facies form the Luna and Huesca Distributary Systems. In the distal fluvial system ephemeral lacustrine deposits, channel, and overbank deposits interfinger with pedogenically altered mudstones.

Four ichnofacies have been identified within these deposits and are associated with three distinct paleosol types. The Coprinisphaera isp. ichnofacies is found in sandy Entisol-like paleosols (early successional soils forming on crevasse splays), and the Scolicia isp. ichnofacies is found in gleyed Entisol-like paleosols forming on limestone (subaerially exposed ephemeral lacustrine deposits) and the Celliforma isp. ichnofacies in Inceptisol-like paleosols (poorly developed soils). Alfisol-like paleosols (open woodland soils of well-drained alluvial plains) contain the drab Stirpitus ichnofacies and are the most widely distributed ichnofacies in the distal fluvial deposits of the Luna and Huesca Systems. The distribution of ichnofacies is representative of changes in base level with the rapidity of water table fluctuations controlling the degree of overprinting of ichnofacies.

Preliminary paleoenvironmental reconstructions suggests a mosaic of ecotypes with areas of dry, open woodland composed of trees, some grassy vegetation and scrublands, with a strongly seasonal, sub-humid climate (mean annual temperature 10-12^oC +/- 4^oC, mean annual precipitation 458-883mmyr^-1 +/-196mmyr^-1), somewhat different to the present-day Ebro Basin (MAT 14^oC, MAP 320mmyr^-1) and previous interpretations of an arid to semi-arid environment based on sedimentological criteria alone (Nichols, 1987). Each ecotype and associated ichnofauna were controlled primarily by fluctuating paleohydrological conditions.