Stratigraphical and morphological aspects of the northern Marmara shelf based on seismic data

Abstract

Recent sedimentation and Plio-Quaternary stratigraphy of northern Marmara shelf has been investigated by high-resolution seismic datasets. Seismic data indicate that a thin cover of Plio-Quaternary unit (termed Unit 1) overlies the Miocene/Oligocene age older sediments (termed Unit 2). The erosional surface between Unit 1 and Unit 2 is the regional unconformity for the shelf formed during the last sea level low stand. Unit 1 is subdivided into two sub-units as Unit 1a for Holocene deposits accumulated after the last glacial maximum (LGM) and Unit 1b for fluvial sediments deposited during the last low stand of the Marmara Sea. The thickness of the Holocene sediments is maximum at SW of Bosphorus outlet, in the Büyükçekmece Bay and along the coastal area between Silivri and Büyükçekmece reaching approximately 32 m. Mean sedimentation rate of the entire northern shelf is calculated as 0.4 m/1000 yr for the last 12,000 yr based on the Holocene sediment thickness. There are four depressions in the western part of the shelf, which correspond to palaeolakes during the LGM filled by Plio-Quaternary sediments. Transition from lacustrine to marine conditions in the palaeolakes occurred when the Marmara Sea level exceeded −62 m threshold depth during the sea level rise following the LGM at approximately 12,500-13,000 yr before present (BP).

Seismic stratigraphy of the north-western Sea of Marmara shelf along the North Anatolian Fault system

“…The internal character of Unit U1a also suggests that this sub-unit is of marine origin and was deposited during a transgressive phase. We did not observe stratigraphic unconformity below -85 m, which is consistent with other studies and suggets the presence of a paleo-shoreline and paleodeltas around the Sea of Marmara Çağatay et al, 2003;Polonia et al, 2004;Gökaşan et al, 2005;Eriş et al, 2007;Gasperini et al, 2011Gasperini et al, , 2018Köprülü et al, 2016, Nasıf et al, 2019.…”

“…The stratigraphic setting is characterized by young and relatively thin sediments compared to the accommodation spaces overlying a shelf-crossing acoustic basement, an observation in agreement with other works, e.g. Aksu et al [1999], Çağatay et al [2009], Tur et al [2014], Vardar et al [2018], Nasıf et al [2019]. On the other hand, our stratigraphic analysis suggests that the presence of two sub-units, U1b and U1c, were controlled by particular sedimentary and tectonic effects occurring between Tekirdağ and Kumbağ, and also between Gaziköy and Şarköy.…”

“…Conversely, unconformity above sub-unit U1c displays an erosional character, indicating that this unit was deposited during the "falling" and/or "lowstand" phase. Unit U1b can be correlated with such seismo-stratigraphic units identified for the northern shelf of the Sea of Marmara by , Çağatay et al [2003], Gökaşan et al [2005], Eriş et al [2007], Köprülü et al [2016], Vardar et al [2018] and Nasıf et al [2019]. However, lowstand deltas such as our Unit U1c are not very common in the Sea of Marmara, those reported by Smith [1995] and are an exception.…”

“…The sedimentary succession deposited along the northern shelf of the Sea of Marmara consists of a young and relatively thin layer, overlying an acoustic basement not penetrated by high-frequency seismic sources [Aksu et al, 1999;Çağatay et al, 2009;Tur et al, 2014;Vardar et al, 2018;Nasıf et al, 2019]. The coarser basal sediments, however, derive from the erosion of tectonic highs, terrestrial inputs, and new hydrodynamic conditions after the LGM.…”

“…Aksu et al, 1999;Algan et al, 2001;Gökaşan et al, 2005;Eriş et al, 2007;Köprülü et al, 2016] are thicker and more complex, due to variable hydrodynamic conditions and high sediment input, compared to the other parts of the Sea of Marmara [Alavi et al, 1989]. In addition, several recent seismic data sets have also proven complex stratal developments in the lagoons of Büyükçekmece and Küçükcekmece [Vardar et al, 2018;Nasıf et al, 2019]. However, the available shallow seismic profiles do not suffice to explain the possible formation and evolution conditions on the northern shelf of the Sea of Marmara.…”