Morphometric and Sedimentological Features of Terrace Systems in the Middle Sakarya Valley (İnhisar-Gemiciköy, Bilecik)
Ebubekir Karakoca, Levent UncuSakarya River, which is the largest river in northwest Anatolia, drains waters from a large area known as the Middle Sakarya Plateau. The most striking geomorphological features of the Sakarya River, and its tributaries in this area, are the joining gorges which connect the basins and terrace systems together. This study aims to reveal the morphometric and sedimentological features of the terrace systems in the area between the Sakarya River, the Inhisar, and the Gemiciköy settlements. The Sakarya River, which has settled in the region from the Upper Pliocene, has formed terrace systems that can be monitored in several steps throughout the valley. They are monitored under the control of the climatic changes during the Quaternary, the sea level changes in the Black Sea, and the cyclical regional/local tectonic movements. Multiple terraces determined in this study area including the following. Starting from the valley floor of the Sakarya River, two terraces (+19 m and +11 m height) are determined around Gemiciköy, three terraces (+36 m, +24 m and +12 m height) are determined around Hamitabat village, and four terraces (+52 m, +34 m, +18 m and +10 m height) are determined in Yakacık village. These terrace formations, which are distributed irregularly along the valley of the Sakarya River, are mostly semi-rounded from large gravel at the high elevations, and at the lower elevations, terrace formations are made of small gravel and floodplain sediments.
Orta Sakarya Vadisi Akarsu Seki Sistemlerinin Morfometrik ve Sedimantolojik Özellikleri (İnhisar-Gemiciköy Arası, Bilecik)
Ebubekir Karakoca, Levent UncuKuzeybatı Anadolu’nun en büyük akarsuyu olan Sakarya Nehri, orta çığırında “Orta Sakarya Platoları” olarak bilinen geniş bir sahanın sularını akaçlamaktadır. Bu saha da Sakarya Nehri vadisi boyunca, bölgenin tektonik ve jeomorfolojik gelişiminin ortaya konulabilmesi açısından son derece önemli olan boğazlar ve seki sistemlerinin varlığı dikkati çekmektedir. Bu çalışmanın amacı, Sakarya Nehri vadisinin Bilecik iline bağlı İnhisar ve Gemiciköy yerleşmeleri arasında kalan kesiminde gözlenen seki sistemlerinin morfometrik ve sedimantolojik özelliklerini ortaya koymaktır. Üst Pliyosen’den itibaren bölgeye yerleşen Sakarya Nehri bir yandan Orta Sakarya Platolarında açmış olduğu boğazları derinleştirirken diğer yandan Kuvaterner sırasındaki iklim değişmeleri, Karadeniz’deki kaide seviyesi oynamaları ve süregelen bölgesel/lokal tektonik hareketlerin kontrolü altında vadisi boyunca birkaç basamak halinde izlenebilen seki sistemlerini geliştirmiştir. Çalışma kapsamında, Sakarya Nehri’nin vadi tabanından itibaren, Gemiciköy çevresinde iki (+19 m ve +11 m yükseklikte), Hamitabat köyü çevresinde üç (+36 m, +24 m ve +12 m yükseklikte), Yakacık köyü çevresinde ise dört (+52 m, +34 m, +18 m ve +10 m yükseklikte) seviye halinde seki basamakları tespit edilmiştir. Sakarya Nehri’nin vadisi boyunca düzensiz bir dağılış gösteren bu seki basamaklarının yüksekte bulunanları yarı yuvarlanmış-yuvarlanmış iri çakıllardan, alçakta olanları ise az yuvarlanmış küçük çakıllar ve ince taneli taşkın ovası sedimanlarından oluşmaktadır.
River systems are areas where changes in both the erosion and accumulation processes can be observed over short distances. In particular, global climatic changes, sea level changes and local and/or regional tectonic movements during Quaternary have been recorded in river valleys (Schumm et al., 2000; Vandenberghe, 2002; Westaway et al., 2003). The most important geomorphological units that these records have kept are the terrace systems responsible for the deposition and splitting processes. For this reason, quantitative studies in river valleys, (sedimentological analyzes, morphometric calculations and dating) based on field observations of the terrace systems, have great importance for understanding the Quaternary period.
By its geographical location, Turkey is located in the temporal-subtropical zone, which is the most determinant force on geomorphological forming in rivers terraces. Therefore, large river valleys were greatly affected by hydrological changes, induced by climate changes in Quaternary and local/regional tectonic movements, that developed in parallel with the rising tendency of Anatolia. As a result, terrace systems (which are widely observed) have been developed throughout large river valleys. The significantly differ in origin and characteristics.
Studies on river terraces in our country started in the 1940s and increased rapidly in recent years. These studies (İlgüz, 1940; Pfannenstiel, 1941; İnandık, 1955; Akkan, 1970; Erol, 1973), which were based on the geomorphological and sedimentological features of landforms, have gained a quantitative dimension, including geochronological data such as deposition/cleavage dates. In Turkey, said studies are still carried out in various sections of large rivers such as the Kızılırmak River (Dogan, 2011; Çiner et al., 2015; Berndt et al., 2018), the Yesilırmak River (Erturaç and Kıyak, 2017; McClain et al., 2017), the Tigris River (Bridgland et al., 2007; Karadoğan and Kuzucuoğlu, 2017), the Gediz River (Westaway et al., 2003; Maddy et al ., 2016; Maddy et al., 2020), Göksu (Avşin et al., 2019), the Melendiz River (Doğan et al., 2019), the Asi River (Bridgland et al., 2012), the Filyos River (McClain et al., 2019) and the Sakarya River (Erturaç et al., 2019a; Erturaç et al., 2019b).
The lower and middle parts of the Sakarya River valley, the largest river of Northwest Anatolia, are also among the areas where terrace systems can be widely observed. Erturaç et al. (2019) detailed the origins and geochronological developments of the terrace systems. They were observed in three steps in the lower part of the Sakarya River, between the Geyve Gorge and Karasu. However, there are not any publications on the terrace systems observed in the central part of the Sakarya River valley. This study aims to fill this gap in literature. It reveals the morphometric and sedimentological features of the terrace systems observed in the 38 km length part of the middle of the Sakarya River, between the İnhisar and Gemiciköy settlements in Bilecik province.
The terrace steps identified between İnhisar and Gemiciköy are concentrated on three main areas: Yakacık Village, Hamitabat and Gemiciköy. Terrace levels descend in the flow direction of the stream which includes four levels in Yakacık village (+10 m, +18 m, +34 m and +52 m), three levels around Hamitabad (+12 m, +24 m and +36 m), and two levels in Gemiciköy (+10 m and +19 m). The common feature of these terraces is that they are preserved on a single slope of the river valley, and they do not show regular continuity.
Around Yakacik, gravel scattered on the terrace (at +52 m height) was discovered. All other terraces feature alluvial filling terraces with varying thickness. The fillings of the high terraces (S3 and S2) that we detected in the study area begin with channel deposits consisting of coarse-rounded-coarse gravel at the bottom, and continue with small gravel and finely grained flood plain materials. This indicates that when terrace fillings began to accumulate, there was a change from an irregular, but high energy flow, regime to a more regular and weak energy flow regime over time. The fillings belonging to the youngest benches (S4), developed in the immediate vicinity of the valley floor of Sakarya, consist of finely grained flood plain sediments and sands that contain very little gravel, and and they are the work of this low flow regime.
As revealed by Erturaç et al. (2019), in the terrace systems of the lower ground of the Sakarya River, the global climate changes and the sea level changes during the Quaternary period played an important role in the formation of terraces detected in the study area. The first findings of this study revealed that local and/or regional tectonic movements were developed due to the K-G-squeezing regime, which prevailed in the region during the Quaternary. This study also revealed the effects from the formation of the terraces in the middle ground of the Sakarya River.
