Plio-quaternary sediments and neotectonics of the Isparta angle, S.W. Turkey

An integrated approach, combining sedimentology, structural geology and geomorphology, has been used to reconstruct the Pliocene and Quaternary evolution of the Aksu basin, located on the Mediterranean coast of Turkey. The present basin opened in the Lower Miocene (Flecker, PhD, 1995). The shallow marine, bioclastic Gebiz Limestone, originally thought to be Messinian in age is believed to date from the late Tortonian. Selenitic gypsum in the Aksu basin is related to the Messinian dessication of the Mediterranean Sea, which produced widespread evaponte deposits. Right-lateral shear is recognised during the late Miocene-early Pliocene. Pre-existing structural lineaments, that were orientated sub-parallel to the shear direction, were reactivated as right-lateral shear zones. Between these defined lineaments broader zones of distributed shear are thought to have deformed by clockwise rotation of small blocks bounded by sinistral fault planes. Flooding of the Mediterranean at the beginning of the Pliocene resulted in deposition of the Yenimahalle Formation. The onshore sediments of the Yenimahalle Formation are dated as Globorotalia puncticulata-Globorotalia margaritae zone, using planktic foraminifera. Benthic foraminifera assemblages show that maximum water depth at this time was of the order of 150m. Seismic profiles in Antalya Bay indicate that the Pliocene sediment package is approximately 500m thick and that it was deposited onto a block-faulted topography. Three transgressive phases are recognised, that can be related to sealevel changes during the Lower Pliocene highstand. The Yenimahalle Formation shallows and coarsens upwards in the upper 25m of section. The sediments change from blue-grey silts to yellow sands containing gravel lenses, shallow-water burrows and large macrofossils. The benthic foraminifera assemblage of the upper part of the section indicates a water depth of 50m or less. The Yenimahalle Formation grades into the calkaya Formation. This consists of non-marine fanglomerates in the far north; braided stream deposits in the northern Aksu basin; and deltaic marine and transitional marine deposits (marine sands and conglomerates, muds, soils and coals) in the south. A tuff deposit in the calkaya Formation is interpreted as the basal surge deposit of a small phreatomagmatic eruption. K-Ar dates from biotites extracted from pumice clasts in the tuff were inconclusive, due to excess atmospheric argon. Along the steep basin margin in the west, fanglomerates were deposited during the development of smooth mature slope surfaces, which are thought to have begun to evolve in the early Pliocene and to have continued to develop until the early Quaternary. A period of normal faulting occurred in the ? Late Pliocene/Early Quaternary, mostly affecting the western basin margin, although it is also recognised in the east. As a result the Pliocene sediments were tilted towards the west. This normal faulting event is thought to be associated with a change of regional stress field, which facilitated gravitational collapse of the Aksu basin, and which has been recognised elsewhere in Turkey. Uplift of the Bey Dag Ian, possibly related to underthrusting of the Anaximander mountains, is also thought to be a factor in the development of the Kerner normal faults. During the Early-mid Quaternary deposition of the Antalya tufa infilled the fault block topography. U-Th dating shows the majority of the tufa to be >400,000 years old. A complex depositional environment of tufa depositing lakes, swamps and streams existed. With time the flow of water supplied to the basin reduced and depocentres became less defined. The tufa became less lacustrine and more paludal. During the Quaternary, and probably also during the Pliocene, regional uplift affected the area. Thus relative sea-level fell, although relative subsidence of the basin with respect to the Taurus Mountains occurred. The interaction of this uplift with glacial climate fluctuations has produced the terracing of the tufa and the conglomerate terraces of the Aksu river. Karstification and stream erosion have subsequently modified the tufa terraces. Terra rossa soils are the most recent sediments documented in the basin.