Investigation of sea level changes in Gulf of Gemlik with geophysical methods

Located to the east of the Sea of Marmara (SoM), the Gemlik Gulf, is a semi-enclosed basin such as the Gulf of İzmit, have similar oceanographic features. Although it is an active basin, it is diversed from the İzmit Gulf because it is not effected by 1999 earthquake as much as İzmit Gulf . The Gemlik Gulf has an earthquake risk, while it is on the mid-part of North Anatloin Fault Zone, therefore, the Gemlik Gulf must be investigated in terms of paleoseismology. The middle arm in the Gemlik Gulf, late Pliocene-Pleistocene period in the growing east-west right-lateral strike-slip faults control width of 36 km, 11 km in length. NW-SE Extendded Gemlik basin must be formed as result of transtention during the late Pleiocene-Pleistocene. The North Anatolian Fault reached the eastern Sea of Marmara ≥3.5 Ma ago, where its middle strand intersected the Thrace-Eskişehir Fault. GPS slip vectors measured on the Armutlu and Mudanya blocks show a displacement of 7–8 km during the last 3.5 Ma. The middle strand of the North Anatolian Fault zone has lower tectonic activity than the northern strand. Because uplifting of the southeast Marmara Sea region has been continuous since the late Pliocene, the presence of fluviatile, lacustrine and deltaic environments in the Gemlik pull-apart basin over the same period supports a lower level of tectonic activity. The transgressive Marmara Formation was deposited on top of these fluvio-lacustrine deposits following the Mediterranean inundation at 600 ka via the Strait of Çanakkale. The Gemlik basin was affected both by two major sea-level falls at 160–132 and 24–11 ka, and minor short-lived sea-level variations, as a result of becoming a lacustrine setting five times since ≥600 ka. During these lowstands, stacked delta successions were deposited around the lake and on the transgressed shelves of the Sea of Marmara. In addition to the Gemlik Gulf morpho-tectonic structure as detailed stratigraphy of the sediments of the late Pleistocene-Holocene period and facial features of these sediments it is not demonstrated in detail. This is why the Gemlik Gulf have late goal to uncover the paleoceanographic and palaeoseismologic evolution in the Quaternary, R/V URANIA research vessel obtained from completing the sea trials in 2013. Shallow-seismic and core data were obtained and used in this thesis. High resolution seismic profiles, multibeam bathymetry of the Gemlik Gulf, and multi-proxy analyses of cores from the Gulf's depocentre has been used to study the sedimentary processes in the Gemlik Gulf. Some of the dense chirp profiles and multibeam bathymetry data belong to Gemlik Gulf, acquired onboard R/V Urania during "MARM13" cruise in 2013 and "MARM05" cruise in 2005, are studied. One piston-gravity 8.6 m core with undisturbed tops were recovered from -78 m water depth on board during cruise in respectively. Closely spaced Chirp sub-bottom profilling data was acquired by the 16 transducers, hull mounted Benthos (Datasonics) Chirp-II profiler, with operating frequencies ranging between 2 and 7 kHz. Seismic traces, which have been extracted from chirp profiles crossing the core locations, have been correlated with sediment cores by generating stynthetic traces with using the P-wave velocity and gamma density measurements from Multi-sensor Core Logger. The autocorrelation of the Chirp pulse(zero-phase Klauder wavelet) is used as a source wavelet for the synthetic seismic traces. The source wavelet, which has been convolved with the reflectivity series obtained through acoustic impedance calculations using gamma-density and P-wave velocity core logs, has been generated as having identical pramateres such as sampling rate, sweep start frequency, sweep end frequency of with the real traces. In order to eliminate the ringing noises which is a result of convolution process, a source signature deconvolution is applied to the synthetic traces, and subsequently the instantaneous amplitude of the traces is computed by Hilbert Transform. The instantaneous amplitude, that is proportional to the square root of the total energy of the seismic trace at an instant of time, measures the reflectivity strength, thereby the stratigraphy could be relatively well defined. The multibeam bathymetric map of the basin shows that the study areais characterized by NW-SE trending transtensional oblique faults, delta lobes of the Kocadere River to the east. Furthermore, the faults observed both in high resolution seismic sections and multibeam bathymetric map have NW-SE orientations and high-angle dip-slip geometries, are most probably associated with secondary elements of the Middle strand of the NAF in the Gemlik Gulf. The high resolution seismic profiles show an erosional surface in the deep basinal parts, that was formed during Late Glacial period when the water level was 100 m, or more lower than present. This surface is overlain by a highly reflective lacustrine unit deposited until about the first marine transgression at 12 14C ka BP. The topmost unit is represented by an up to 15 m-thick Holocene mud drape. The base of the Holocene above the delta in the eastern part of the Gemlik Gulf had been dated at 11,250±60 14C (calib.) BP by previous works on the study area.The oblique clinoform reflectors have been interpreted on seismic sections as prograding deltaic sediments, and foreset directions of the deltaic unit indicate that the delta was sourced from the Kocadere River. This delta lobe is offset by the secondary faults of the Middle strand of the NAF. The dimly trace of the main NAF, according to the multibeam bathymetry map, is interpreted in the northern part of lacustrine delta lobe. Gulf of Gemlik, from the beginning of the Holocene sea level changes were revealed simultaneously with the Marmara with various research studies carried out before that. Today, with the bay in the Marmara and transition bathymetry data is up to date with -55 m depth in the presence of an observable threshold indicates that the water may be cut at different times of transition with the Marmara. This thesis aims the late Pleistocene to Holocene sealevel changes and the water exchanges between the Gemlik Gulf and the Marmara under the transtensional tectonic regime. For this purpose, we used the shallow seismic reflection data in this study together with a piston core that was recovered from the northern shelf of the Gemlik Gulf. Studying of high-resolution seismic and bathymetric dataallow us to examine the current morphology of the Gulf of Gemlik and stratigraphy of the sediments that were deposited during the Late Pleistocene-Holocene period. Seismic stratigraphic studies revealed facies characteristics of the main depositional units. P-wave and density measurements of Core M13-08 by the multi-sensor core logging were used for producing synthetic seismogram. This was matched with the seismic reflection profile on the basis of which the core sediments can be compared on the basis of lenght of the core. However, five different sedimentary units have been differentiated in 280 seismic profiles and their boundaries have been defined as continuous seismic reflection surfaces . Using the same general character as the reflection of seismicreflection profiles and differentiated seismic unit in water depths through which they also limit the volume of the surface were obtained important information about water level changes in the gulf. Both revealed the relative stratigraphic position of taking into account. The oldest of these mirror images within Unit S5 shelf unit represented by deltaic sediments in the gulf, this unit precipitated between 23 to 21 ka BP. It also belongs to the unit the water level in the gulf below -65 m foreset/topset is indicate the transition. This unit unconformably overlying Unit-S4 seismic images, the formation of the more old-S3 exceeds the unit overlying mud mats should be as transgressive of this unit BP 21-18 thousand years shows that between sediments. Last Glacial Maximum (LGM), which terminate at 18 ka BP in the coming year as determined by the erosional channels with radiocarbon age obtained from seismic profiles is found to -110 m up to scratch. Seismic reflection images can be clearly observed that the vaccine should be understood that channels the transgressive deposits on the surface reflection structure exhibits the Unit-S3, radiocarbon dates obtained from core 18-15,6 ka BP between were deposited at a time when the water level started to rise again in the gulf. This period shows that post-glacial period, humid climate with the Gulf around the bay fed by drainage.and when the water level of the Marmara in the previous time period considered to be at about -110 m level rise of water in the gulf in around 18-15,6 ka BP. This thesis shows also water level collation with the Gemlik Gulf and the Sea of Marmara before Holocene transition. Paleo-bathymetric contour maps produced from the data in 18 ka BP that remains to water level connection with the gulf and in the Sea of Marmara. Before Holocene 15,6 to 12,1 ka BP between the sediments with radiocarbon ages determined that the unit-S2 consists of accumulated clinoform shelf edge and slope indicating that the overall experience of the gulf in this period, the water level is regressive feature. Unit-S2 in seismic reflection images overlying and also Unit-S1 is also on the main disharmony forming the base -65 m observed to occur at a water depth "Berm". The presence before the Holocene in the water level almost to be at these depths It indicates that it should. From the M13-08 12,1 ka BP thought to result from a contour map of this disharmony in the gulf has been reviewed, removable out this time of shoreline. In the way of providing water to pass through the sea sediments which uncovered marine Unit-S1 indicates that caused the seismic character of which also reached the Unit-S1 and Mediterranean waters and increase the water level in the gulf.