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

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

The Early Pleistocene development of the Gediz River, Western Turkey: An uplift-driven, climate-controlled system?

This paper reports the latest details from an on-going investigation of the Early Pleistocene buried river terrace sequence of the Gediz River ∼40 km upstream of the Alaşehir graben in the Kula volcanic province, Western Turkey. Using clast lithology to characterise sediment provenance, we demonstrate that the buried Early Pleistocene terrace sequence of the palaeo-Gediz is overlain by the deposits of two carbonate-rich, northerly-derived, tributary systems. The surface form of these carbonate-rich deposits suggests deposition on alluvial fans, an interpretation supported by limited palaeocurrent data and upwards-coarsening sequences, suggesting fan progradation. It is argued that the formation and preservation of the palaeo-Gediz terrace sequence is intimately related to fan deposition, fan head entrenchment and fan progradation. Terrace formation is the result of incision in the main Gediz valley, a response to long-term uplift, during periods of lower sediment supply. Under low sediment supply conditions inferred fan head entrenchment is associated with fan toe progradation. This progradation would have led to the burial of the newly formed terrace of the Gediz. Subsequent higher sediment supply conditions would have led to renewed fan deposition. The direct coupling of the fan system with the main Gediz River during these periods would also result in deposition within the trunk river. The primary control of sediment supply is considered here to reflect changing vegetation cover, a function of changing Quaternary climates. These inferred controls suggest that the Gediz Early Pleistocene terrace sequence is an uplift-driven, climate-controlled system

The Pliocene initiation and Early Pleistocene volcanic disruption of the palaeo-Gediz fluvial system, Western Turkey.

In this paper, we report our latest observations concerning a Pliocene and Early Pleistocene record from Western Turkey. The sedimentary sequence described comprises the fluvial deposits of an Early Pleistocene palaeo-Gediz river system and its tributaries prior to the onset of volcanism around Kula and the subsequent lacustrine, volcaniclastic and fluvial deposits associated with the first phase of volcanism (similar to 1.2 Ma) in this area. Early development of an east-west drainage system in this area resulted from tectonic adjustments to north-south extension and the formation of east-west-oriented grabens. Headward erosion of drainage entering the main Alasehir graben led to the progressive capture of pre-existing drainage systems as eastward (headward) erosion upstream tapped drainage networks previously formed in internally draining NNE-SSW-oriented basins. Within one of these, the Selendi Basin, part of this evolutionary sequence is preserved as a buried river terrace sequence. Eleven terraces are preserved beneath alluvial fan sediments that are, in turn, capped by basaltic lava flows. Using the available geochronology these terraces are considered to represent sedimentation-incision cycles which span the period similar to 1.67-1.2 Ma. Although progressive valley incision is a fluvial system response to regional uplift, the frequency of terrace formation within this time period suggests that the terrace formation resulted from sediment/water supply changes, a consequence of obliquity-driven climate changes. The production of sub-parallel terraces suggests that during this period the river was able to attain a quasi-equilibrium longitudinal profile adjusted to the regional uplift rate. Thus, the incision rate of 0.16 mm a(-1) during this period is believed to closely mirror the regional uplift rate. After the onset of volcanism at similar to 1.2 Ma, there is a destruction of the dynamic link between fluvial system behaviour and climate change. The repeated damming of the trunk river and its tributaries led to the construction of complex stratigraphic relationships. During the first phase of volcanism the palaeo-Gediz river was dammed on numerous occasions leading to the formation of a series of lakes upstream of the dams in the palaeo-Gediz valley. Variations in lake level forced localised base-level changes that resulted in complex fluvial system response and considerable periods of disequilibrium in profile adjustment. Furthermore, response to these base-level changes most likely disrupted the timing of the incisional adjustment to the on-going regional uplift, thus making the use of this part of the archive for inferring regional uplift rates untenable. (c) 2007 Elsevier Ltd. All rights reserved

The obliquity-controlled early Pleistocene terrace sequence of the Gediz river, western Turkey

The buried Early Pleistocene river terrace record of the Gediz River, around Kula, western Turkey has previously been considered to span the time interval equivalent to Marine Isotope Stages (MIS) 58-37 (c. 1.6-1.2 Ma), with the frequency of terrace formation mirroring obliquity-driven climate change. Whereas progressive Pleistocene incision of the Gediz River is seen as a response to regional uplift, the timing of fluvial incision, leading to terrace formation and subsequent new floodplain development, is believed to be climate-controlled with incision-deposition cycles resulting from varying sediment-discharge conditions, a direct consequence of changing climate and related vegetation change. New outcrop observations downstream of the original field area, alongside recently published geochronological data and improved understanding of the volcanic sequence, all now suggest that the previously published interpretation is incorrect. Here we present a revised stratigraphy based upon terrace gradients of c. 0.004-0.005 (previously 0.001), in which 11 terraces are identified but only terraces GT11 (the oldest) to GT6 (pre-lava incursion) predate volcanism. The available geochronology suggests that terraces GT6 (post-first lava incursion) to GT1 relate to the time interval MIS38-28 (c. 1.26-1 Ma). However, despite penecontemporaneous volcanism terrace formation continues to reflect sediment-discharge changes predominantly controlled by regional climate change