Geology and early paleozoic depositional features of Beykoz (İstanbul) region

Within the continental fragment termed the Intra-Pontid Shelf, the Istanbul Zone is composed of mid- to high-grade crystalline basement rocks of Neoproterozoic origin. These rocks encompass volcanic arc remnants and fragments of continental crust, primarily observed to the east of Istanbul. This foundational complex is subsequently covered by a continuous and well-developed sedimentary sequence extending from the Lower Ordovician to the Lower-Middle Carboniferous. Notably, the foliation during the Carboniferous period signifies a crucial phase denoting a regressive shortening progression. The Calidonian Orogeny, a significant tectonic event during this epoch, led to the folding and faulting of the Paleozoic succession. Overlying this process, red sandstones and conglomerates formed between the Upper Permian and Lower Triassic, creating an unconformity that distinctly outlines the geological evolution of the region. The Istanbul Stack is an example of a sedimentary basin that evolved over a defined time span from the Lower Ordovician-Tramadocene to the Upper Silurian-Ludlovian. This evolutionary process is characterized by the interactions of several sedimentation environments and tectonic events. Initially, sedimentation started in an offshore deep shelf environment, represented by the deposition of the Kocatöngel Formation, during which clayey silty shales were particularly prominently deposited. Shoaling of the shelf and transformation to shelf is realized with the deposition of silty sand and shale successions of the Bakacak Formation. The regressive process is completed with the land formation of the region and the continuation of sedimentation in the river environment and the arkosic deposition of the Kurtköy Formation in the river environment. In the later stages, the lowering of the region and the deposition of the pelagic unit is determined by the quartz arenite deposits of the Aydos Formation. The deposition of the Gozdag Formation takes place in a reefal and shallow shelf near the open sea, while the formation of the Dolayoba Formation depends on the basin reaching the reefal level. Heavy mineral analyses and petrographic studies reveal the provenance process of Erkan Paeozoic units. It is determined that the source of the deposited material of the Kocatöngel Formation varies in four stages from lower to upper part. The lower part is fed by high-grade regional metamorphic and granitic felsic igneous rocks, while the second sedimentary band originated from low-grade metamorphic and ultramafic rocks. In the middle levels, the deposited material was generally fed by basin low-grade metamorphic sources. The upper parts of the formation were mostly fed by contact and regional metamorphic rocks. It is shown that the elements feeding the basin of the Bakacak Formation are derived from the material of acidic igneous and felsic rocks. It shows that the elements feeding the basin of the Kurtköy Formation are generally derived from the material of metamorphic, low-order metamorphic, metamorphic and felsic rocks of old sedimentary origin. Based on the high degree of abrasion and rounded shapes of the minerals during transportation, it is concluded that the source is distant and the transportation distance is long for the Bakacak Formation, while the source is distant and the transportation distance varies between medium and long for the Kurtköy Formation. In tectonic terms, the study area has been under the influence of two Northeast-Southwest trending tectonic compression, Northeast-Southwest trending compression and Northwest-Southeast trending tensional tectonic regimes after the Early Paleozoic.