Early Pleistocene river terraces of the Gediz River, Turkey: the role of faulting, fracturing, volcanism and travertines in their genesis

Reconstructing and interpreting past hydrological system changes from the Quaternary sediment-landform record presents many challenges especially when trying to establish causality. In such circumstances attempting to isolate individual drivers is perhaps unrealistic and thus inferences are made with respect to the combined influence of multiple controls and constraints. In this paper we seek to interpret the landscape responses recorded in the sediment-landform record of the Early Pleistocene Gediz valley, western Turkey, within the context of interacting geodynamic and hydrodynamic system changes. Our earlier work on the Gediz sequence established the presence of a rich fluvial archive constrained by a comprehensive geochronology from overlying lava flows. Repeated damming of the Gediz valley floor and consequent re-routing of water flow, testifies to a direct connection between fluvial activity and volcanism. This volcanism has been linked to regional tectonic extension with regional uplift promoted as a major driver of the progressive river incision of the Gediz River over the Quaternary. This incision has been punctuated by periods of deposition dictated by sediment and water budgets controlled principally by fluctuating climate and consequent vegetation change. Recently however, the significance of more local fault movements and fracturing has been recognised and here we examine and attempt to quantify these geodynamics in greater detail. In addition, we examine and describe, for the first time, extensive associated travertine deposits. Isotopic analysis of these travertines confirms their thermogenic origin and directly links their creation to the faulting/fracturing, volcanism and the recycling of calcium-rich groundwater. The formation of mounds and sheets extend onto their contemporary valley floors, thus directly connecting the fluvial and travertine archives and allowing linkage between surface and subsurface hydrological change. By utilising both the fluvial and travertine archives a more complex picture of landscape evolution emerges. The youngest terraces (<GT6) appear to be driven by local fault movement associated with the onset of volcanism and thus the link with fluctuating climates, as demonstrated in the higher terraces (GT7 +), is unclear. Detailed observations suggest formation of these lower terraces is also conditional on the local substrates. Here it appears that GT5-GT1 form in response to the creation of a knickpoint where the river intersects the faults in areas of hard basement rock. Hence, this response is highly localized, dependent on the immediate geometry and structure of the underlying strata. The presence of complex basement topography also plays a key role in generating ground water pressure and thus the creation of travertine mounds. The Gediz sequence demonstrates increasing complexity. There is no reason to suggest this is unusual. Although climate and tectonics remain as key drivers the local landscape response is also conditioned by many additional factors. While it is tempting to search for global patterns (“the big picture”), such reduced complexity may be over simplistic. Recognising complexity requires detailed observation and therefore more emphasis on “the smaller picture” which can only be obtained through extensive field observation.