Yedikule-Büyükçekmece (İstanbul) sığ deniz sismiği verilerinin değerlendirilmesi

Abstract

Marmara Denizi'nde Yedikule-Büyükçekmece (İstanbul) açıklarında, yaklaşık -100 metre derinliğe kadar olan bir alanın sığ deniz jeofiziği ve jeolojisini incelemek amacıyla, K-G doğrultulu 43 adet ve D-B doğrultulu 10 adet 345 km toplam hat uzunluklu, çözüm gücü yüksek sığ sismik yansıma verileri toplanmış ve sismik stratigrafik ve yapısal unsur değerlendirmeleri yapılmıştır. Batimetrik inceleme sonucu, sahanın batı kısımlarının kıyılarda eğimli, açıklarda düzleşmekte olduğu, doğuda kıyılarda -25 metrelerde geniş bir sığ düzlüğün olduğu, ayrıca sahayı güneyde sınırlayan, D-B doğrultusunda uzanan dik bir yamacın varlığı belirlenmiştir. Sahada genç çökellerin (Kuvaterner-Holosen?), kalınlığı genelde kıyılardan açığa gittikçe 9 metrelerden 1 metrelere doğru azalmaktadır. Genç çökel tabakasının altında yer alan temel kaya topografyası (paleotopoğrafya) bir uyumsuzluk yüzeyi olarak belirlenmiştir. Kara jeolojisinden elde edilen bilgilerden temel kayanın, sahanın batısında Oligosen?, doğusunda da Üst Miyosen? yaşlı birimlerden oluştuğu ihtimali kuvvetlidir. Sahayı güneyde sınırlayan, D-B doğrultulu normal faylanma, ve buna paralel fayların varlığı önemli yapısal unsur değişimleri olarak belirlenmiştir

High resolution shallow seismic data were acquired at northern Marmara Sea, along offshore Yedikule-Büyükçekmece (Istanbul, Turkey) for shallow marine geophysics. Seismic profiling is achieved by using high resolution Uniboom seismic data collection system. The data reveals striking examples of submarine geology, such as angular unconformity surfaces, faults, deformed layers, ancient river beds, man made effects on the sea bottom, and layers with various internal reflection configurations. The basement surface is a truncational, angular unconformity surface above which marine sediments (Quaternary- Holocene?) are superposed. In general, the sediments are thicker along the coast and at the east of the area, and thinner at offshore. At the west of the area, the overall seismic stratigraphy shows a deformed basement located at the lowermost level, possibly Oligocene age at the west, and upper Miocene age at the east. The seismic stratigraphy above the unconformity surface can be divided into 4 levels. The trace of a possible major faulting, possibly one of the normal faults belonging to pull-apart basins of strike-slip North Anatolian Fault Zone (NAFZ), is observed as a strong bathymetry change on offshore, extending in east- west. Minor old and young faults parallel to this major event, and evidences of Küçükçekmece basin are observable from the seismic data. The coast of northern Marmara Sea has a great potential as a new urban area of İstanbul metropolitan. The metropolitan has been expanding toward west along the northern coast of Marmara Sea. Construction of large number of housings, industrial sites, harbor facilities, natural gas pipelines, and sewage disposal systems are among increasing activities. Although, the land geology has been extensively studied (Komut and Barka, 1994; Meriç et al., 1991; Sayar, 1954), the marine geophysics and geology are not well known. However, large industrial and urban investment in the area requires good knowledge of the shallow marine geophysics and geology in the near future. Moreover, the area is located at the north of North Anatolian Fault Zone (NAFZ) which is associated with high seismicity. As a result, the region possesses a great potential of earthquake hazard (Barka and Cadinsky- Cade, 1988). Investigation of marine geophysics and geology is VI therefore important as far as the earthquake safety is concerned. To investigate the marine geology and geophysics of the area, high resolution shallow seismic data are acquired along offshore Yedikule-Büyükçekmece at northern Marmara Sea, Turkey, by the cooperation of Turkish Naval Forces, Department of Navigation, Hydrography and Oceanography (SHOD) and İstanbul Technical University (İTÜ) in a joint Marine Geology and Geophysics project supported by TÜBİTAK. The seismic data has been acquired by using Uniboom high resolution seismic data collection system. The system consists of EG&G manufactured Model-234 energy unit, Model-230-1 transducer, Model- 265 hydrophone streamer, and Model-255 recorder. High frequency band (100 Hz-8 KHz) of the system allows the collection of high resolution seismic data. The resolution is good enough to discriminate layers with about 30 cm thickness when good Signal-to-Noise (S/N) data are available. During the data collection, following field and instrumentation parameters were selected: Seismic system Energy level Vessel speed Record length Trigger interval Gain Variabie gain Filter type Profile lengths Profile intervals Uniboom seismic system (ModeI-230) 300 Watts 4-4.5 mile/h Maximum 200 ms 3*100 ms Around 60 dB Adjusted according to record quality Low-cut 1000 Hz (adjusted) 5 to 10 km 0.5 and 1 km There are 43 profiles perpendicular and 10 profiles parallel to the coast with 345 km total profile length. The profile interval is 0.5 km between Yedikule and Küçükçekmece and 1 km between Küçükçekmece and Büyükçekmece. Location map of seismic profiles is shown in Figure 3.2. The numbers on the profiles which also appear on the seismic records denote the location of the seismic events. The lengths of 43 VII profiles perpendicular to the coast are limited by -100 m of bathymetry, due to limited penetration capacity of the marine seismic source, and by the -10 m of bathymetry for safety operation of the research vessel. There are 10 control profiles parallel to the coast, crossing the perpendicular ones. The labels on the map show the major urban and industrial sites of İstanbul metropolitan along the northern coast of Marmara Sea. Yedikule, Bakırköy, Yeşilköy and Küçükçekmece are mostly residence areas with intensive housing and population, Ambarlı and Büyükçekmece are mostly industrial areas. The data reveal striking examples of submarine geophysics and geology, such as unconformity surfaces, faults, deformed layers, landslides, ancient river beds, man made artifacts, and layers with various internal reflection configurations. The bathymetry map of the area obtained from the seismic sections is seen in Figure 6.4. Note that two major slopes develop in the area, one by the coast and the other one on offshore. The slope by the coast is East-West oriented between Büyükçekmece with an average dip of 2 to 3 degree. In front of the Küçükçekmece bay, the sea bottom gets gradually shallower. By the coast of Yeşilköy, the orientation of the slope near the coast becomes Northwest-Southeast, and extends offshore Yedikule where it makes a sharp turn towards Northeast- Southwest. On the other hand, the coast line between the Yeşilköy and Yedikule is Northeast-Southwest oriented; for this reason, a shallow and more or less flat platform develops at the western part of the investigated area. The second slope appears as a strong sea bottom morphology change at the further south. A sudden decrease in bathymetry from -100 meters to -140 meters is observed depending on the penetration capacity of the marine seismic source. This sudden drop in the bathymetry which may be an indication of normal faulting limits the entire area and extends in East-West. Between these two slopes, the sea bottom is relatively flat, and gradually gets deeper from -60 meters to -90 meters over 3 to 5 km distance. The interpretation of the seismic sections reveals a truncational, angular unconformity surface which was identified on the seismic sections due to its continuous high amplitude reflections and rough surface (Figure 6.7). This unconformity surface is followed almost all over the area and form the top of the deformed basement formation of possibly Oligocene age to the west of Küçükçekmece and possible VIH upper Miocene age to the east of Küçükçekmece. The unconformity surface continues underneath another one, under the shallow, flat platform, offshore Bakırköy. The reflections between the two unconformity surfaces show subparallel reflections of possible upper Miocene or younger formations. The topographic map of unconformity surface obtained from the seismic sections is seen in Figure 6.7. Note the striking similarity between the bathymetry and basement maps. In Figure 6.12, the general seismic stratigraphy of the area is shown. In general, the seismic stratigraphy of offshore Yedikule- Büyükçekmece shows deformed basement formation (A) of Oligocene age to the west of Küçükçekmece and upper Miocene age to the east of Küçükçekmece. At further east, underneath the shallow, flat platform offshore Bakırköy, upper Miocene or younger age formations (B) with subparallel reflection configuration form the basement. The top of basement is a truncational unconformity surface above which the marine sediments (Quatemary-Holocene?) are placed. The seismic stratigraphy within the marine sediments may be divided into four levels. The lowermost level is a unit with chaotic reflection configuration (C) which may be an indication of an old land slide or fan delta deposits due to faulting. This unit is lens shaped and interestingly is located down dip of the slope by the coast line. When the borders of unit (C) is marked (Figure 6.13), it is seen that unit (C) is well correlated with the slope by the coast. It is well developed offshore Küçükçekmece and appears in two pieces offshore Yeşilköy. This correlation indicates a possible land slide due to the slope. Another interpretation is that unit (C) is fan delta deposits of an old faulting which developed the slope by the coast and the shallow, flat platform offshore Bakırköy. However, direct evidences of this faulting on the seismic sections are not strongly observed. Above unit (C), a unit with frequent reflection configuration (D1) is observed. Unit (D1) is onlap to unit (C) and its borders is shown in Figure 6.13. Note unit (D1) is also develops in front of the slope by the coast and possibly becomes wider to the east. The frequent internal reflection configuration of unit (D1) indicates that this unit consists of thin layers with different acoustic impedances. Above unit (D1), a reflection free zone characterizes an homogenous unit (D2) which is seen almost all over the area (Figure 6.12). Unit (D3) with parallel reflection configuration is deposited over unit (D2) mostly to the west of Yeşilköy (Figure 6.12). The uppermost level is the mud and sandy mud (D4) overlapping the most part of the area which is well known from the sea bottom samples. The velocity of wave propagation usually increases with depth because the sediments becomes more and more consolidated with increasing depth. For this reason an average P-wave ix velocity of 1900 m/s is assumed to figure out the average thickness of the marine sediments. The sediment thickness map is shown in Figure 6.6. In general, the marine sediments are thicker by the coast (6-9 meters) and to the east of the area and thinner (1 meters) on offshore. Thickest sedimentation is by the coast between the Ambarlı and Küçükçekmece, within Küçükçekmece bay, and on offshore Yedikule in the form of small sediment packets. The structural interpretation of the seismic sections shows the mark of a possible major normal fault (F1) observed as a strong sea bed bathymetry change extendinding East-West on offshore (Figure 6.9). The basement formation (A) is strongly deformed by fault F1 as seen in Figure 6.9, and deformational fissures are well developed and observed in some seismic sections as seen in Figure 6.10. Parallel to this major fault, another one (F2) is also developed further north (Figure 6.9). Between faults (F1) and (F2), some minor faults which effect the bathymetry are also determined. It is seen that (F1), (F2) and the faults in between all together can be taken as a normal fault zone. This fault zone is placed possibly between the strike-slip faults forming the north branches of the NAFZ in the Sea of Marmara (Figure 6.1 1)