Marmara Denizi'nde, Büyükçekmece (istanbul)-marmara Ereğlisi (tekirdağ) Kıyı Kesiminin Deniz Jeolojisi Ve Kuvaterner Evrimi

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Tarih
1996
Yazarlar
Yılmaz, Birol
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
Marmara Denizi kuzey şelfinde, Büyükçekmece (İstanbul) - Marmara Ereğlisi (Tekirdağ) açıklarında, yaklaşık -100 metre derinliğe kadar olan bir alanın deniz jeolojisini incelemek ve Kuvaterner evrimini açıklamak amacıyla çözüm gücü yüksek sığ sismik yansıma verileri ile karot ve yüzey örnekleri toplanmış, birimlerin sismik stratigrafisi ortaya çıkarılmıştır. Ayrıca, kara jeolojisi çalışmasıyla stratigrafik birimlerin deneştirmesi yapılmıştır. Çalışma bölgesinin kıyı kesiminden toplanan sığ sismik veriler bu bölgede deniz tabanının batimetrik olarak kıyıdan -50 metreye kadar düşük eğimli, -50 ile -100 metre izobatları arasının düzlük, -100 metreden itibaren de yaklaşık D-B doğrultulu çok dik bir yamaç şeklinde olduğunu göstermiştir. Bu yamaç, Marmara Denizi kuzeyinde gelişmekte olan pull-apart havzayı kuzeyden sınırlayan normal fay zonunun oluşturduğu fay dikliğidir. -50 metreye kadar olan düşük eğimli kesim ise sadece temel birimleri (A) etkileyen bir normal faylanmanın sonucudur. -50 ile -100 metre izobatları arasındaki düzlükte yaklaşık D-B uzanımlı pull-apart havzacıklar görülür. Kuzeyden ve güneyden normal faylarla sınırlı bu havzacıklar yaklaşık NE- SW doğrultulu yanal atımlı fayların oluşturduğu sırtlarla birbirlerinden ayrılmışlardır. Stratigrafik olarak bölge temelini, inceleme alanının kıyı kesimlerinde yüzeyleyen Oligosen yaşlı Gürpınar Formasyonu oluşturur. Formasyonun alt düzeyleri deniz içinde kaldığından kalınlığı hakkında kesin bir şey söylenemez. Bu temel üzerinde açısal uyumsuz olarak, Pleyistosen yaşlı Marmara Formasyonu ve Holosen yaşlı Kuşdili Formasyonu ile alüvyon örtü izlenir. Temel üzerinde yer alan bu çökellerin kalınlığı aradaki uyumsuzluk yüzeyinin paleotopoğrafyasına bağlı olarak 2-20 m. arasında değişir. Geç Kuvaterner yaşlı tortulların çökelimi, kıyıya paralel uzanımlı normal fayın yamacında moloz akımlarıyla depolanmış kaotik (düzensiz) yansımalı ve merceksel geometrili birimler (B1) ya da iki pull-apart havzayı ayıran basınç sırtlarında belirgin downlap yansımalı (B2) birimlerle başlar. Pull-apart havzacıklar içinde olasılıkla tatlı su ortamında, sismik olarak seyrek ve sık ardalanmalı birimler (C1.C2) gelişmiştir. Bu çökeller (C1.C2) sismik kesitlerde kaotik (B1) ve downlap yansımalı (B2) birimlerle geçişli olarak izlenirler. İç yansımasız (reflection free) birimler (D1.D2), denizel transgresyon sonucu kaotik çökelleri (B1.B2) ve havza içi çökellerini (C1.C2) onlap şeklinde aşarak gelişmiştir. Bu çökellerin üstünde ise, inceleme alanının tümünde izlenen bir güncel çamur düzeyi (E) bulunur. Karotlarda görülen bu düzeyin varlığı olasılıkla Holosen içinde deniz su derinliğinin artmasıyla bölgenin çamur çökelim alanı şekline dönüştüğünü belirtmektedir.
High resolution shallow seismic data, gravity core and dredge samples were collected from the Northern shore of the Sea of Marmara in order to study the marine geology of the region and to explain the Quaternary evolution of the region up to the -100 m. isobath. The seismic stratigraphy of this region was constructed and the structural interpretations were carried out. Geology of the neighbouring land area was also investigated and the correlation of the units found by seismic stratigraphy and by the direct observations on land was discussed. Çubuklu research vessel of Turkish Naval Forces was used to collect the seismic data, gravity cores and orange-peel samples. The seismic data were recorded uniboom and sparker systems. During the marine studies, 62 seismic profiles were made. 55 of them are perpendicular to the coast line, 3 of them lying in the E-W, 3 of them NW-SE and one of them is lying in the NE-SW directions. The bathymetry, basement morphology, loose sediment thickness, structure and the distribution of the various seismic stratigraphical units were evaluated by means of arious maps prepared from the processing of the seismic data. The strongest reflections are observed coming from the sea-floor and the basement paleo surface. Due to the structural disturbances, it is not possible to see all seismic stratigraphical units in one section. The basement of the area (A) is folded and observed as a even-way reflection configurations. The upper surface of this unit is in the form of angular unconformity indicating intense truncations. First unit observed on the basement is a possible fault scree deposit with chaotic internal arrangement (B1). Equivalent of this unit is situated on the structurally elevated basements between the two pull-apart depressions with downlap reflection configurations (B2). C1 of rare and C2 of frequent VIII reflection configuration young deposits possibly of lacustrine origin are developed in the pull-aparts. All these units (B1, B2, C1, C2) are seen laterally and vertically passing into each others. During the time of deposition of the C2, it seems that the climatical conditions were changed and the areal extensions of the lakes were considerably enlarged. The later unit of D is formed as onlaps by the sea transgression upto the normal fault-zone in the north. This unit is divided lower (D1) and upper (D2) subunits by their differences of the seismic velocities and densities. The uppermost seismic stratigraphical unit is the muddy horizon which is seen in the cores. The orange-peel and core samples indicate that the mean grain-size decreases from the sand to clay-size towards the offshore area. The sea-floor is in the form of gently inclined surface (~ 1.5°) between the coast line and the -50 m. isobath, it forms of a flat area between -50 and -1 00 m. isobaths. It is in the form of a steeply inclined slope after the -1 00 m. isobath. 5 dperessions of pull-apart origin is located on the flattened part of the sea-floor. These basinlets are separated from each others by pressure- ridges and limited by E-W lying gravity faults from the north and the south. The steep slope limits the area-studied to the south is a normal fault zone limiting the pull-apart basin to the north. It is seen in the seismic sections and also on the bathymetrical and the basement paleotopographical maps that the basement morphology bears close similarities with the recent sea-floor morphology must have played a significant role in the evolution of the recent sea-floor morphology. The young sedimentary cycle (B,C,D,E) is unconforamble on the basement (A). Basically two different kinds of faults namely dip-slip normal and strike-slip faults are present in the area-studied. A normal fault zone is present at -30 m. isobath line parallel to the shore-line (F1). This fault zone, effects the basement. This zone lies in the WNW-ESE direction between Büyükçekmece and Silivri. It lies roughly in the E-W direction to the further west. A 15-20 m. dip-slip has occurred to the south creating a plane of 4-6 km. The seismic stratigraphic unit B1 was been deposited in front of the fault plane to the south. Strike-slip faults (F2) dissected this plane in the NE-SW direction creating small pull-apart depressions limited by nomal faults (F3). These pull-aparts are separated from each others by pressure-ridges are clearly seen as positive flower-structure created by strike-slip faulting (F2). IX The E-W lying normal faults (F4, F5) are seen at depths between -60 and -80 m. These faults created 15-7.5 m. fault-scarps observed in the bathymetry. The faulting effected D1, D2 and also E seismic stratigraphic unit indicating that the faulting occurred during the Late Holocene. The area investigated is limited by a very steep scarp after the -90 m. isobath to the south. This scarp is in fact a fault plane of F6 normal fault. It limits one of the largest pull-apart basin of the Sea of Marmara. It is an active fault at present because it created a considerable dip-slip offset to the south effecting the bathymetry. In conclusion this structural evolution of the area is the result of the shear zone which created the North Anatolian Fault Zone. Four different unconformity surfaces are observed between the seismic stratigraphic units. The first one (d1) is between the basement (A) and the young sediments (B,C,D,E) in the form of angular unconformity. Fault-scarp brecheas along the fault lines. The unit C has been deposited on this unconformity in the pull-apart depressions to the further south. At the southern most part of the area-studied D1, D2 are seen deposited onlaps above this surface. The second unconformity surface (d2) is seen between B1 and C1, C2 is of an erosional surface. The third unconformity surface (d3) is seen between the marine transgressional unit of D1 and the lower units (B2; C1.C2). The last one (d4) is observed between the basement (A) and the unit E. It is of Late Holocene time. Paleozoic-Mesozoic aged Istranca Massif is seen at the base of Thrace Basin (PAMİR ve BAYKAL, 1947). It is composed by various metamorphic and granitic rocks. Reefal carbonates (Soğucak Formation) and their lateral equivalent of basinal sequence of calciturbidite-mudstone alternation (Ceylan Formation) are unconformably observed on this basement. These sediments are of Middle-Upper Eocene (ÜNAL, 1967; KESKİN, 1974). Oligocene aged lacustrine-fluviatile deposits (Gürpınar Formation, SAYAR, 1977) are present lying unconformably on the basement and the Eocene deposits alike. The sedimentary column of the area and its near surroundings continues with a Upper Miocene sedimentary cycle composed of fluviatile coarse elastics (Çukurçeşme Formation) and brakish- water carbonate-marl and mudstone intercolations (Bakırköy Formation) (ARIÇ, 1955; SAYAR, 1976). A Pleistocene aged, shallow-marine sequence of unconsolidated elastics and sandy limestones with Ostrea, Cerastoderma are place to place unconformably observed on the older deposits (Marmara Formation; SAKINÇ ve YALTIRAK, 1995). Another loose basinal facies of sandy clays, clayey sand and dark grey mudstones with Murex and Cerastoderma of Holocene is seen above the older deposits (Kuşdili Formation; MERİÇ et.al., 1991a). An alluvium of coarse elastics form the upper most part of the stratigraphic column. The Gürpınar Formation constitutes the basement in the area-studied. Marmara Formation or its marinal equivalents are not observed in the seismic sections. In the marinal part of the investigated area, possible lacustrine fills (C1.C2) and their lateral equivalents of slope-debris and aeolian deposits are seen on the basement. These deposits and the basement are covered by marine units of D1, D2 as onlaps due to Flandrian transgression. The remnants of this transgression is also observed on land as a Holocene basin-fill to the north-west of Marmara Ereğlisi (Kumludere). The units of C1 and C2 are not present on land. Continental conditions have reigned in the investigated area, during the Oligocene-Pliocene period. In this period, various forms of continental deposition took-place such as mainly lacustrine in Oligocene, as braided river deposits in Late Miocene. Marine conditions returned to the area during the Tyrrehenian. Marmara Formation of clastic shore-line and shallow-marine deposits were deposited in due course. The sea-level was fallen below -90 m. during the Wurm Glaciation. Therefore the region must have become an erosional land area. In this period, Pleistocene shallow- marine deposits (Marmara Formation) had been completely eroded except a few occurrences such as the terrace-like remains around Marmara Ereğlisi. The Wurm Glaciation had reached its nadir at 25.000 B.P., from this time onwards interglacial conditions started to reign around the area-studied. In this period, a shear zone has been developed creating a series of pull- apart depressions and pressure-ridges under the influence of North Anatolian Fault Zone at 6 km. of the present shore-line of the area. In time, the pull-aparts had become lacustrine areas possibly accomodating clastic deposition (C1,C2). Fault brechias (B1) were formed to the south of the normal faults and some thin large-scale cross-bedded elastics of possible aeolien origin were formed on the pressure-ridges (B2). First marine deposits are seen covering these deposits and the basement are seen as onlaps (D1.D2) due to the Flandrian transgression. This transgression could come to the scarps of the normal fault zone which created the pull-apart depressions in the area-studied, between Büyükçekmece and Silivri. However, presence of the Holocene deposits around Marmara Ereğlisi imlies that this transgression might invaded considerable land area in this region. During the later Holocene, Büyükçekmece-Silivri part of the area was down-lifted tectonically and the sea transgressed to the present shore-line whereas the western part was uplifted exterminated the basinal conditions around Marmara Ereğlisi. Extensive deposition of muddy elastics in the eastern part of the area today supports this findings thus the tectonic origin of the drastic changes of environmental conditions. By this severe tectonics, the later Holocene deposits have been elevated upto 50 metres along the XI present shore of Büyükçekmece-Silivri-Marmara Ereğlisi. Presence of later Holocene deposits containing various red tile pieces within and above the archaeological remains at Marmara Ereğlisi also dates these movement occurring 5.000 years B.P. (ÖZDO?AN ve ÖZBAŞARAN-DEDE, 1990). This situation also explains that the area was tectonically elevated being quite independent of the sea-level changes in Marmara. By these latest movements, present morpholgy of the area-studied have been formed.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1996
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1996
Anahtar kelimeler
Deniz jeolojisi, Kuvaterner, Kıyı şeridi, Tekirdağ-Marmara Ereğlisi, İstanbul-Büyükçekmece Büyükçekmece, Marine geology, Quaternary, Coastal line, Tekirdağ-Marmara Ereğlisi, İstanbul-Büyükçekmece
Alıntı