The Aksu Basin is a sedimentary basin in southwestern Turkey, around the present-day Aksu River. Located at the intersection of several major tectonic systems, in the Isparta Angle, the Aksu Basin covers an area of some 2000 square kilometers. Together with the Köprü Çay Basin and the Manavgat Basin, the Aksu Basin forms part of the broader Antalya Basin. It forms a graben relative to the surrounding Anatolian plateau.
The Aksu Basin has been gradually filling up with sediment since Neogene times.
General description The Aksu Basin is located at an important intersection of several tectonic systems. To the north, the Anatolian continental plateau is undergoing uplift, while to the south the African and Eurasian plates are colliding, producing features like the Mediterranean Ridge and the Hellenic subduction zone. On the west, the Aksu Basin is bounded by the Bey Dağları platform carbonates, where the crust is extending to produce a series of grabens and horsts that are mostly east-west aligned. To the east, the Aksu Thrust separates the Aksu Basin from the Köprü Çay Basin.
Part of the broader Antalya Basin, which is itself located within the Isparta Angle, the Aksu Basin can be divided into two sub-basins: in the north is the older Aksu-Karpuzçay sub-basin, and in the south is the younger Yenimahalle-Çalkaya sub-basin. The southern part of the basin is "now very heavily karstified and vegetated."
The Aksu Basin overlies basement rocks of several types: Bey Dağları platform carbonates, Alanya Metamorphics, Antalya Nappes (made of ophiolite) and Lycian Nappes (platform carbonate). An unconformity separates the basin from these basement rocks.
In front of the mouth of the Aksu River, there is a broad, shallow marine shelf offshore where the present-day sediments of the Aksu River are deposited. In the past, this area was deeper; according to Poisson et al., there was likely a canyon, as there still is over by the Düden and Karaman mouths near Antalya, but this canyon has since been filled in by Quaternary deposition from the Aksu River.
The Aksu Basin is considered a sedimentary basin and a foreland basin. It has been infilling since Neogene times.
There are five fan deltas in the Aksu Basin: Kapıkaya, Kozan, Karadağ, Kargı, and Bucak. There is also the Eskiköy alluvial fan.
Along with the neighboring Köpru and Manavgat Basins, the Aksu Basin has been of interest to geologists since the 1910s thanks to its "rich tectonic complexities and well-exposed structural and sedimentary features".
The Aksu Basin covers some 2000 square kilometers.
Evolution The Aksu Basin is formed by a combination of several geological processes, including uplift of the Tauride Mountains to the north, rifting within the Isparta Angle, and relative subsidence of the region to the south that now forms Antalya Bay.
According to Glover and Robertson, the upper Aksu Basin was formed by "erosion, then subsidence" in the Miocene: a foreland basin, after and related to the SE-ward emplacement of the Lycian Nappes. Then in the Pliocene and Pleistocene there was crustal extension or transtension to form the lower Aksu Basin.
The pre-Miocene basement primarily consisted of the Bey Dağlari platform, surrounded by the Antalya Nappes, and covered by a thin layer of Paleogene sedimentary deposition. This formed "an eroded flat surface [that] covered a large area in SW Turkey". Beginning in the Late Oligocene, and continuing through the Burdigalian, rising sea levels caused a marine transgression in the area. The shallow marine Karabayir Limestones were deposited in the north and west, forming a large platform around what is now the Aksu Basin. Meanwhile, to the south, a molassic basin was formed; this was the beginning of the Antalya Basin as a whole.
Miocene
By the Early Miocene, a "right-lateral transform fault plate boundary" had formed between the Cyprus Trench and the Isparta Angle. This led to the creation of "a hybrid, terrestrial-shallow marine accommodation space" on the eastern side of the Isparta Angle, where sediments from the Antalya Complex further east were deposited.
In the Middle Miocene ( 16-11 MYA), the Aksu Basin was formed as a graben cutting through the Karabayir platform. Meanwhile, the surrounding continental areas were being pushed upward; erosion from these areas provided a lot of sediment that was poured into the emerging Aksu Basin. According to Kaya, this was especially the case with the rapidly uplifting Antalya Complex to the east; it "provided the topographic gradient and gravitational potential energy for the necessary fluvial erosion and transport for sediment supply". Poisson et al. instead highlight the basin's northern and western margins, where a series of fan deltas at prehistoric river mouths left thick sediment deposits – for example, the present-day Kapıkaya and Kargı fan deltas.
The Aksu and Karpuzçay Formations were deposited during this period in what Kaya describes as "fluvial, deltaic to beach environments". Coral reefs fringed the coastline. These reefs lay on a "warm, well-aerated shallow marine shelf" in the photic zone. The low species diversity – primarily colonies of Porites and Tarbaellastraea – may indicate a "stressed" environment. The shore was "medium-high wave energy-dominated", and the climate was temperate to subtropical. The presence of coral fossils among the fan delta deposits is likely from periods when sudden sea level increase left the deltas underwater. Traces of coral reefs at Kargı and Sütçüler have been dated to Late Tortonian times.
Around 5.6 MYA, the Mediterranean region was hit by the Messinian salinity crisis. Sea levels dropped tremendously, and there was rapid erosion and desiccation. Growth of the fan deltas stopped as the receding coastline left the entire Aksu Basin on land. Deep gorges were carved in the areas that were now exposed as dry land. One of them, now a deep undersea canyon in the Gulf of Antalya, is still traceable in front of the present day Düden and Karaman river mouths. A similar canyon likely existed on the Aksu, but it has since been buried by Quaternary sedimentation on the marine shelf.
Poisson et al. reconstructed the course of the Aksu River around the end of the Messinian ( 5.3 MYA) this way: the river flowed through the Eskiköy Canyon, then "crossed the Antalya alluvial plain before joining the Antalya abyssal plain, through the actively cutting Antalya canyon".
Poisson et al. also argued that the Gebiz Limestone was formed during the Messinian, implying that there was still at least a pocket of sea in the Aksu Basin even while the sea retreated. They characterize this environment as "very shallow marine", with coral reefs, which then gradually became more restricted as it gave way to land and eventually dried up completely. This characterization is disputed – Üner et al. instead describe the Gebiz Limestone as being deposited after sea levels rose again; Kaya attributes the Gebiz Limestone to the "latest Miocene-early Pliocene", describing it as being deposited by a NW-SE-oriented lake that formed during this period.
According to Kaya's interpretation, the Aksu Basin was a "fluvial, fluvial-delta, beach setting" during the Middle Miocene ( 16-11 MYA), then "a lacustrine and lagoon environment" by the end of the Miocene (i.e. 5.3 MYA). Then, during the early Pliocene ( 5.3-3.6 MYA), there was a relatively brief (at least geologically speaking) period of tidal flat and very shallow marine conditions. Later, there was a "return to entirely terrestrial conditions", and deposition by the ancient Aksu River formed the Belkis Conglomerate during the Pleistocene period.
Pliocene Sea levels rose again in the early Pliocene, during the Zanclean period ( 5.3-3.6 MYA). The southern part of the Aksu Basin was submerged, while the northern part remained dry land. The Messinian canyons became filled in by new deposition. The Eskiköy canyon was at first completely submerged, as fossils of nannoplankton and planktonic foraminifera found in marl deposits suggest a shallow, "open-marine" environment. It then became "completely filled" by these marl deposits, then deltaic conglomerate deposits, and finally lacustrine marl deposits.
Meanwhile, by the latest Miocene/early Pliocene, continued tectonic activity had caused the Aksu Basin (specifically the present-day northern part, the Aksu-Karpuzçay sub-basin) to become uplifted and tilted towards the south. Deposition was now happening further south, forming the Yenimahalle-Çalkaya sub-basin. Sediment was now coming from the Bey Dağları platform to the west, which had been uplifted in the meantime.
Rising sea levels had left the Yenimahalle-Çalkaya sub-basin underwater, leading to what Üner et al. describe as a shallow marine shelf environment. Üner et al. attribute the alluvial fan-delta of the Eskiköy Formation, and the "shallow marine siltstone-marl alternations" of the Yenimahalle Formation to this period. According to Kaya, the Yenimahalle and Çalkaya Formations were deposited during this period.
In the Late Pliocene ( 3.6-2.6 MYA), sea levels dropped and the southern Aksu Basin once again became dry land. On the western side of the basin, the Antalya Fault spawned cold springs, which produced the Antalya tufa and travertine deposits.
Quaternary During the Quaternary period, the Aksu River cut a new gorge – not in the same place as the Eskiköy canyon; the river's course had changed in the meantime.
Pleistocene Since in the early-mid Pleistocene, Earth has experienced an alternating series of glacial and interglacial periods, which "superimposed their effects" on the ongoing geological processes in the region. Anatolia has been experiencing relative uplift, while the Aksu Basin has been experiencing relative subsidence to form an extensional graben. A fluctuating climate affected the river systems, and aggradation in the central part of the basin created fluvial terraces. Terra rossa-type soils formed on top of Mesozoic limestone and were redeposited into channels during flash floods. At the beginning of the middle Pleistocene, global fluctuations in sea level increased, and a marine incursion may have eroded the lower part of the basin, although no marine sediments have been found in this area. Also around the mid-Pleistocene, a small fan was deposited at the mouth of the Karaman River. Also during this time, the most recent layer of the Aksu Basin was formed: the Belkis Conglomerate, in the middle Pleistocene, by fluvial terraces.
Now At the present day, tectonic activity in the Aksu Basin is minimal, and there is also relatively little sedimentary deposition. Instead, erosion is the primary activity, such as cave systems and sinkholes forming in the Antalya Tufa. The Aksu River also continues to form fluvial terraces along its course.
Component strata The stratigraphy of the Aksu Basin is well documented, although the specific delineation of formation names, ages, and boundaries are varied. This article follows Kaya's terminology.
Aksu Formation