The Gediz River fluvial archive: A benchmark for Quaternary research in Western Anatolia

Abstract

The Gediz River, one of the principal rivers of Western Anatolia, has an extensive Pleistocene fluvial archive that potentially offers a unique window into fluvial system behaviour on the western margins of Asia during the Quaternary. In this paper we review our work on the Quaternary Gediz River Project (2001–2010) and present new data which leads to a revised stratigraphical model for the Early Pleistocene development of this fluvial system. In previous work we confirmed the preservation of eleven buried Early Pleistocene fluvial terraces of the Gediz River (designated GT11, the oldest and highest, to GT1, the youngest and lowest) which lie beneath the basalt-covered plateaux of the Kula Volcanic Province. Deciphering the information locked in this fluvial archive requires the construction of a robust geochronology. Fortunately, the Gediz archive provides ample opportunity for age-constraint based upon age estimates derived from basaltic lava flows that repeatedly entered the palaeo-Gediz valley floors. In this paper we present, for the first time, our complete dataset of 40Ar/39Ar age estimates and associated palaeomagnetic measurements. These data, which can be directly related to the underlying fluvial deposits, provide age constraints critical to our understanding of this sequence. The new chronology establishes the onset of Quaternary volcanism at ∼1320ka (MIS42). This volcanism, which is associated with GT6, confirms a pre-MIS42 age for terraces GT11-GT7. Evidence from the colluvial sequences directly overlying these early terraces suggests that they formed in response to hydrological and sediment budget changes forced by climate-driven vegetation change. The cyclic formation of terraces and their timing suggests they represent the obliquity-driven climate changes of the Early Pleistocene. By way of contrast the GT5-GT1 terrace sequence, constrained by a lava flow with an age estimate of ∼1247ka, span the time-interval MIS42 – MIS38 and therefore do not match the frequency of climate change as previously suggested. The onset of volcanism breaks the simple linkage of terracing to climate-driven change. These younger terraces more likely reflect a localized terracing process triggered by base level changes forced by volcanic eruptions and associated reactivation of pre-existing faults, lava dam construction, landsliding and subsequent lava-dammed lake drainage. Establishing a firm stratigraphy and geochronology for the Early Pleistocene archive provides a secure framework for future exploitation of this part of the archive and sets the standard as we begin our work on the Middle-Late Pleistocene sequence. We believe this work forms a benchmark study for detailed Quaternary research in Turkey.