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Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gürsökü Formation (Kavak–Samsun, N. Turkey)
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Journal of Asian Earth Sciences 31 (2008) 429–438 www.elsevier.com/locate/jaes
Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gu¨rso¨ku¨ Formation (Kavak–Samsun, N. Turkey) Kemal Akdag˘ *, M. Ziya Kırmacı Karadeniz Technical University, Department of Geological Engineering, 61080 Trabzon, Turkey Accepted 9 May 2007
Abstract The Late Cretaceous Gu¨rso¨ku¨ Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gu¨rso¨ku¨ Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous. 2007 Elsevier Ltd. All rights reserved. Keywords: Cretaceous; Gu¨rso¨ku¨ Formation; Forearc basin; Submarine slope; Submarine fans
1. Introduction Coarse-grained submarine fan systems and their depositional processes are well established in the literature (Mutti and Ricci Lucchi, 1978; Nelson, 1983; Normark, 1974; Walker, 1975, 1977; Surlyk, 1984; Lowe, 1982; Ineson, 1989; Lowe and Guy, 2000; Shanmugam, 1997, 2000, 2002). The distinction between submarine fans and slopes has been emphasized both in the generalized classification schemes of deep-marine depositional environments and in the descriptions of ancient and modern deepwater systems (e.g. Nelson and Nilsen, 1989). There are three major turbidite facies associations: slope, submarine fan, and basin-plain. The facies and their vertical organization into recognizable facies associations permit an analytical study of ancient turbidite basins. Prograding and retreating systems, or part of the same system, can be recognized and
*
Corresponding author. Tel.: +90 462 3772755; fax: +90 462 3247505. E-mail address: [email protected] (K. Akdag˘).
1367-9120/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2007.05.005
the tectonic history of the basins can be deduced from the cyclic nature of the deep-marine clastic deposits. The Sinop–Samsun Basin trending NW–SE is located in the central part of the Pontide mountain range and is bordered by large scale thrust faults to the north and south (Korkmaz, 1984; Pelin and Korkmaz, 1981). The study area, located in Kavak Town and surrounding areas, is situated in the eastern part of this basin (Fig. 1). The aim of this paper is to recognize and interpret the turbidites and associated facies, and to determine the depositional conditions and environment of the Gu¨rso¨ku¨ Formation in the area of study. 2. Geological setting The Sinop–Samsun Basin commenced its development as a rift associated with the volcanic arc in Aptian–Albian time by the northward subduction of the Neotethys oceanic plate beneath the southern margin of Eurasia (Yılmaz and Tu¨ysu¨z, 1988; Tu¨ysu¨z et al., 1990; Tu¨ysu¨z, 1999) and turned into an asymmetrical Forearc Basin in Campa-
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Fig. 1. Location of the study area and simplified geological map of the Sinop–Samsun Basin (from Korkmaz, 1984).
nian–Maastrichtian time as the rifted volcanic arc (Aydın et al.,1995; Tu¨ysu¨z, 1999; Leren, 2003). In this basin, Mesozoic sedimentation begins with the Early Jurassic Akgo¨l Formation consisting of siliciclastic sediments, which rests unconformably on pre-Jurassic basement rocks. The Akgo¨l Formation is unconformably overlain by the Bathonian– Kimmeridgian Akkaya Formation composed of reefal limestones (Gedik and Korkmaz, 1984; Aydın et al., 1986). Both formations are exposed outside of the study area and are unconformably overlain by the Barremian– Albian C ¸ ag˘layan Formation (Gedik and Korkmaz, 1984; Tu¨ysu¨z, 1990; Aydın et al., 1995). The C ¸ ag˘layan Formation, forming the oldest rocks in the study area, is composed of turbiditic sandstones, dark grey to black shales, conglomerates and large/over-sized blocks (Fig. 2). The Late Cretaceous sedimentary succession is up to 2500 m thick in the region. This sedimentary succession begins with the Turonian–Santonian Kapanbog˘azı Formation, which is composed of red pelagic micritic limestone resting unconformably on the Early Cretaceous siliciclastic rocks, and is overlain by turbiditic sandstone–shale intercalations
with volcanogenic horizons of the Late Cretaceous Yemisßlic¸ay Formation. The succession grades upward into the Campanian– Maastrichtian Gu¨rso¨ku¨ Formation (Aydın et al., 1986; Tu¨ysu¨z et al., 2004) consisting of turbiditic sandstone– shale–conglomerate and marl alternations, which is the subject of this study. Higher up in the section, the Early Campanian–Paleocene Akveren Formation, composed of carbonates, grades into Paleocene–Eocene carbonates and siliciclastic deposits that are unconformably covered by younger terrestrial to shallow marine rocks. 3. Methodology In this study, a geological map of Kavak area covering about 275 km2 has been compiled. In order to reveal the lithofacies development of the Gu¨rso¨ku¨ Formation, a detailed stratigraphic section was measured with a 1.5 m Jacob’s staff and 115 rock samples were systematically collected. Because of the heavy vegetation cover and the lack of continuous outcrops, lateral shifts were made in some
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Fig. 2. Geological map of the studied area.
places of the section by tracing key beds. The sectioning included macroscopical identification of primary sedimentary structures, determination of mean grain size using a magnifying glass (10·) and a grain size comparator. A total of 58 paleocurrent directions were measured during mapping and measuring of the section. All measurements were corrected in the field to compensate for folding. A total of 100 thin sections of sandstone samples were petrographically analyzed and 20 selected samples were point counted (n = 1000) with a standard polarizing microscope attached
with a swift point counter to determine relative percentages of particle types. The sandstones are classified according to Folk (1974) and the conglomerates according to Boggs (1995). 4. Facies analysis The Late Cretaceous Gu¨rso¨ku¨ Formation comprises a 1000 m thick succession of conglomerate, sandstone, mudstone and marl (Fig. 3). The succession crops out along
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Fig. 3. Detailed measured section of the Gu¨rso¨ku¨ Formation showing facies and facies associations. Facies 1. Thin-bedded sandstone and mudstone, Facies 2. Thick-bedded graded sandstone, Facies 3. Clast-supported conglomerate, Facies 4. Mud-supported conglomerate.
road cuts for a distance of about 10 km between Kavak and Samsun, nearly perpendicular to the axis of the basin (Fig. 2). Globotrucana sp. was found in marl beds but is not as abundant as in the formations below and above. Derivation of the sediments from a source area located in the SE is indicated by the measured palaeocurrents directions (Fig. 2). Well-rounded clasts of limestones and metamorphic rocks form over 80% of the conglomerate clasts. The sandstones are litharenitic (phyllarenitic) in composition. They are composed of 20–48% (average 35%) sedimentary rock fragments, 1–12% (average 4%) volcanic
rock fragments and 42–80% (average 61%) metamorphic rock fragments. Four lithofacies are recognized and the description and interpretation of these are given below and are summarized in Table 1. 4.1. Thin-bedded sandstone and mudstone (Facies 1) Thin bedded, typically 3–8 cm thick, laterally persistent beds of medium to very fine grained sandstones, which typically show either normal grading, parallel lamination or
Not applicable
Not applicable
Flat, sharp base
Boulder-pebble, mediumcoarse sand matrix
Block-boulder, sandy mud matrix
3. Clast-supported conglomerate (Fig. 7)
4. Mud-supported conglomerate (Fig. 8)
1.5–50 m
Sand: medium to very fine. Mud: clay to coarse silt Medium to coarse grained 1. Thin-bedded sandstone and mudstone (Figs. 4 and 5) 2. Thick-bedded graded sandstone (Fig. 6)
1.5–25 m
Flat, locally amalgamated and scoured Scoured, amalgamated 0.1–1 m
Mud-supported, rip-up clasts
Deposition from a traction carpet at the base of high energy high concentration turbulent flow Deposition from debris flow
Ta, Tab
Ta-e, Tb-e, Tcde
Sand deposited from low density turbidity currents, mud deposited from turbidity currents Sand deposited from sand-rich high density turbulent flows
Normal grading, parallel and cross lamination, linguoid ripples Normal grading, structureless or parallel laminated upper portions Clast-supported, normal grading, imbrication Flat, sharp base, transitional top Sand: 3–8 cm Mud: 1–30 cm
Interpretation Bed thickness Grain size Facies
Table 1 Sedimentary facies distinguished in the Gu¨rso¨ku¨ Formation
Boundaries
Structures/textures
Bouma (1962)
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cross lamination, are intercalated in grey to greenish-grey, 1–30 cm thick silty mudstone beds (Fig. 4). The mudstones are transitional with the underlying sandstones and their silt content decreases upward, passing upward into claystones. The cross-laminated sandstones show linguoid ripples on their upper bedding surfaces (Fig. 5). The sandstones are grey to greenish-grey in colour, with a brownish-grey weathering colour. They are composed of poorly to moderately sorted, angular to subangular grains of metamorphic, sedimentary and volcanic rocks. The graded sandstone beds (typically Ta of Bouma, 1962) are interpreted as the deposits of high density turbidity currents (Lowe, 1982) or concentrated density flows (Mulder and Alexander, 2001). The parallel and cross-laminated sandstones (typically Tb and Tc of Bouma, 1962) are interpreted as low density turbidity current deposits settled from traction (Lowe, 1982; Ineson, 1989) or concentrated density flows with lower particle concentration basal layers (Mulder and Alexander, 2001). The mudstones intercalated with sandstones are interpreted as settled from suspension but with some traction before deposition. 4.2. Thick-bedded graded sandstones (Facies 2) The medium to coarse-grained sandstone beds, typically 0.1–1 m thick, are laterally persistent at outcrop scale (several 10s of meters) (Fig. 6). They have flat, locally amalgamated and scoured bases, show normal grading (Ta of Bouma, 1962), and are structureless (massive) or the upper few centimeters may be parallel laminated. They are phyllarenitic in composition. Trace fossils, including (Nereites sp) are observed in places on the capping mudstone surfaces. The sharp, flat bases, normal grading and laminated tops (typically Ta, Tab of Bouma, 1962) suggest deposition from waning sand-rich high density turbulent flows (Lowe, 1982; Ineson, 1989; Hickson and Lowe, 2002) or concentrated density flows (Mulder and Alexander, 2001). Such massive, predominantly structureless, deepwater sandstones have been interpreted by Kneller and Branney (1995) as the deposits of relatively continuously fed, longduration turbidity currents, and by Shanmugam and Moiola (1995) as the deposits of cohesive, sandy debris flows. However, the presence of laminated tops argues against a sandy debris flow interpretation because this stratification forms solely beneath turbulent, tractional flows. The flat, non-erosional bases of sandstone beds indicate that turbulence near the base of the flows was damped, and sediment fallout rates were high, suppressing the erosional ability of the flow. 4.3. Clast-supported conglomerate (Facies 3) These pebble and boulder conglomerates, typically 1.5– 25 m (Fig. 7), form single, graded beds (except in the upper part of the section) and locally show scoured, amalgamated sequences (Fig. 3). In some outcrops, they show a-axis
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Fig. 4. Thin-bedded turbidite sandstones interbedded with mudstones (Facies 1). Ruler is 40 cm in length.
Fig. 5. A thin-bedded turbidite sandstone (Facies 1), showing linguoid ripples on the upper bedding surface. Current is from lower left to upper right. Hammer is 35 cm in length.
imbrication (Fig. 7). Beds are laterally persistent at outcrop scale (several hundreds of meters) and show locally scoured bases and complete grading from boulder size at the base to sand size at the top (coarse tail grading). The clasts are rounded to subangular and poorly sorted, and consist dominantly of sedimentary rocks (limestones 70–75%, sandstones 5–10%), with lesser amount of metamorphic rocks (schist and quartzite 20–25%) and volcanic rocks (5%). They are clast-supported, sand matrix conglomerates and in some outcrops include dispersed Late Cretaceous red pelagic micritic limestone clasts up to several meters in diameter. These beds are closely comparable to the graded-bed model of the clast-supported conglomerates of Walker
(1977, 1980) and to Lowe’s (1982) R3–S3 division, representing the deposit of gravelly high density turbidity currents, as they show complete grading. This suggests that they originated from a single depositional event with a-axis imbrication but not inverse grading and stratification. The preferred clast orientation and normal grading in deepwater conglomerates is interpreted by Lowe (1982) as a result of the traction and direct suspension sedimentation of gravel from high density turbidity currents. 4.4. Mud-supported conglomerate (Facies 4) Internally chaotic mud-supported conglomerate (petromict diamictites of Boggs, 1995) beds 1.5–50 m thick
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Fig. 6. Thick bedded, parallel sided turbidite sandstones (Ta, Tab; Facies 2). Note the amalgamation in the upper left of the photograph (arrow). Ruler in the middle of the photograph is 1 m in length.
Fig. 7. Clast-supported cobble conglomerate composed of rounded limestone clasts (Facies 3). Ruler is 1 m in length.
form isolated beds in the succession. The conglomerates are poorly sorted and non-graded, and have a silty mud matrix content ranging from 25% to 35% in volume. The pebbles are mainly composed of subangular to subrounded limestone with a lesser proportion of metamorphic and volcanic rock clasts. In addition, limestone blocks, boulders and plastically deformed rip-up clasts of sandstone and shale of intrabasinal strata (up to meter and decametersize) may occur randomly within the beds (Fig. 8). The beds have sharp scoured bases. The high matrix/clast ratio, internally chaotic nature, and contorted rip-up clasts of the underlying beds indicate deposition from viscous debris flows that may have
originated from slumps or slides on unstable slopes (Surlyk, 1978; Ineson, 1989; Mulder and Alexander, 2001). 5. Facies associations and depositional environments The four facies described above occur in two discrete facies associations recognized through analysis of 980 m of measured section. The thin-bedded sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets (mudstone association), are interpreted to represent a slope-apron environment and the conglomerates and sandstones (sandstone
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deposits (Facies 3). Erosional features are rare and bedding is typically parallel and locally persistent except where deformed by isolated slump folds (Fig. 9). Systematic vertical trends of grain size or bed thicknesses have not been observed. This facies association represents a low-energy, subwave base setting, characterized by sedimentation from suspension and low density turbidity currents which are intermittently interrupted by high density flow and debris flow deposits. Mass-flows and slumps are a characteristic component of the association and they indicate that the depositional setting is an unstable basin margin setting, probably flanked by a proximal fringe of slope debris. The intrabasinal shale and sandstone clasts and angular clasts of Jurassic carbonate rocks within Facies 4 were derived from slumping and sliding, probably reflecting episodic movements on the basin margin faults.
5.2. Sandstone–conglomerate association Fig. 8. Mud-supported conglomerate (a debrite, Facies 4) underlain by thin-bedded sandstones and mudstones (Facies 1). Ruler is 40 cm in length.
and conglomerate association) are interpreted as submarine fan deposits (Fig. 11). 5.1. Mudstone association This association forms a succession, up to 500 m thick, and is the typical host sediments of the Gu¨rso¨ku¨ Formation (Fig. 2). It consists largely of thinly interbedded sandstone and mudstone (Facies 1) together with muddy massflow deposits (Facies 4) and high density turbidity current
This facies association forms discrete green-brown sandrich packages (Facies 2) up to 10–50 m thick, alternating with conglomerates (Facies 3) in the upper part of the Gu¨rso¨ku¨ Formation. The sandstones and conglomerates assigned to the association show scoured and amalgamated beds (Fig. 10). A distinctive feature of the association is crude fining-upward sequences up to 4–5 m thick. The depositional processes dominating this association are high density turbidity currents (Lowe, 1982) and noncohesive debris flows (Mulder and Alexander, 2001). The deposits of cohesive debris flows (Mulder and Alexander, 2001) and slides form discrete beds within the section. Apart from crude fining-upward trends, the association shows no clear internal organization. It is proposed that this association represents sediment deposited at the
Fig. 9. Isolated slump folds in the mudstone association. Scale (circled) is 0.75 m in length.
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Fig. 10. Thick conglomerate bed with an erosional base (Facies 3, sandstone–conglomerate association). Ruler is 1 m in length.
Fig. 11. Schematic tectonic and paleogeographic setting of the Sinop–Samsun Basin and depositional model for facies associations of the Gu¨rso¨ku¨ Formation.
mouths of base of slope channels that funneled sediment into the basin or inner channeled zone of submarine fans. 6. Conclusions The Late Cretaceous Gu¨rso¨ku¨ Formation consists of four facies forming two facies associations. The thin-bedded sandstone and mudstone facies (Facies 1) was deposited by low density turbidity currents and concentrated density flows with lower particle concentration. The thick-bedded graded sandstone (Facies 2) was deposited by concentrated density flows. The clast-supported conglomerate (Facies 3) was deposited by gravelly, high density turbidity currents. The mud-supported conglomerate (Facies 4) was deposited by viscous debris flows. The mudstone facies association, consisting of Facies 1, 3 and 4 and comprising the lower part of the Gu¨rso¨ku¨ Formation, was deposited in a base-
of-slop setting. The sandstone–conglomerate association, consisting of Facies 2 and 3 and comprising the upper part of the Gu¨rso¨ku¨ Formation, consist of thick-bedded sandstones and conglomerates deposited by high density turbidity currents and non-cohesive debris flows at the mouths of base-of-slop channels that funneled sediment into the basin. The depositional setting of the Gu¨rso¨ku¨ Formation was probably a faulted basin margin setting, flanked by a proximal fringe of slope debris and submarine fans developed at the mouths of slope channels. References _ U ¨ ., Serdar, H.S., O ¨ zc¸elik, Y., Akarsu, I., ¨ ngo¨r, Aydın, M., S ß ahintu¨rk, O A., C ¸ okug˘rasß, R., Kasar, S., 1986. The geology of the area between Ballıdag˘ and C ¸ angaldag˘ (Kastamonu). Bulletin of the Geological Society of Turkey 29, 1–16 (in Turkish with English Abstract).
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¨ zaydın, S., Harput, B., 1995. Aydın, M., Demir, O., Serdar, H.S., O Tectono-sedimentary evolution and hydrocarbon potential of the Sinop–Boyabat Basin, North Turkey. In: A Erler, E. Tuncay, E. ¨ rc¸en (Eds.). Geology of the Black Sea Region. General Bingo¨l, S. O Directorate of Mineral Research and Exploration (MTA), Ankara, pp. 254–263. Boggs, S.J., 1995. Principles of Sedimentology and Stratigraphy. Prentice Hall, Upper Saddle River, NJ, 774 p. Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits. Elsevier, Amsterdam, 168 p. Folk, R.L., 1974. Petrology of Sedimentary Rocks. Hemphill, Austin, TX, 182 p. Gedik, A., Korkmaz, S., 1984. Geology of the Sinop basin and petroleum possibilities. Jeoloji Mu¨hendislig˘i 19, 53–80 (in Turkish with English Abstract). Hickson, T.A., Lowe, D.R., 2002. Facies architecture of a submarine fan channel–levee complex: the Juniper Ridge Conglomerate, Coalinga, California. Sedimentology 49, 335–352. Ineson, J.R., 1989. Coarse-grained submarine fan and slope apron deposits in a Cretaceous back-arc basin, Antarctica. Sedimentology 36, 793–819. Kneller, B.C., Branney, M.J., 1995. Sustained high-density turbidity currents and the deposition of thick massive sands. Sedimentology 42, 607–616. Korkmaz, S., 1984. Geological and geochemical investigation of the northeastern part of Boyabat (Sinop) from the point of petroleum ¨ niversitesi. Fen Bilimleri Enstitu¨su¨, possibilities. Karadeniz Teknik.U Trabzon, Ph.D. Thesis, 193 p. (in Turkish with English Abstract, unpublished). Leren, B.L.S., 2003. Late Cretaceous to Early Eocene sedimentation in the Sinop–Boyabat Basin: north-central Turkey: facies analysis of turbiditic to shallow–marine deposits. Candi. Scient. Thesis, Department of Earth Science, University of Bergen, 140 pp. Lowe, D.R., 1982. Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents. Journal of Sedimentary Petrology 52, 279–297. Lowe, D.R., Guy, M., 2000. Slurry-flow deposits in the Britannia Formation (Lower Cretaceous), North Sea: a new perspective on the turbidity current and debris flow problem. Sedimentology 47, 31–70. Mulder, T., Alexander, J., 2001. The physical character of subaqueous density flows and their deposits. Sedimentology 48, 269–299. Mutti, E., Ricci Lucchi, F., 1978. Turbidites of the northern Apennines: introduction to facies analysis. International Geological Review 20, 125–166. Nelson, C.H., 1983. Modern submarine fans and debris aprons: an update of the first half century. In: Revolution in the Earth Sciences: Advances in the Past Half Century Kendall/Hunt. Dubuque, Iowa, pp. 148–166. Nelson, C.H., Nilsen, T.H., 1989. Modern and Ancient Deep sea-fan Sedimentation. Society of Economic Paleontologist and Mineralogists, Short Course No. 14, 404 p. Normark, W.R., 1974. Submarine canyons and fan valleys Factors affecting growth patterns of deep-sea fans. In: Dott, R.H., Shaver,
R.H. (Eds.), Modern and Ancient Geosynclinal Sedimentation, vol. 19. Society of Economic Paleontologist and Mineralogists Special Publications, pp. 56–68. Pelin, S., Korkmaz, S., 1981. Petroleum potential of Black Sea. Karadeniz Technical University Earth Sciences Bulletin 1, 145–157 (in Turkish with English Abstract). Shanmugam, G., Moiola, R.J., 1995. Reinterpretation of depositional processes in a classic flysch sequence (Pennsylvanian Jackfork Group), Ouachita Mountains, Arkansas and Oklahoma. AAPG Bulletin 79, 672–698. Shanmugam, G., 1997. The Bouma sequence and the turbidite mind set. Earth-Science Reviews 42, 201–209. Shanmugam, G., 2000. 50 years of turbidite paradigm (1950s–1990s): deepwater processes and facies models – a critical perspective. Marine and Petroleum Geology 17, 285–342. Shanmugam, G., 2002. Ten turbidite myths. Earth-Science Reviews 58, 311–341. Surlyk, F., 1978. Submarine fan sedimentation along fault scarps on tilted fault blocks (Jurassic–Cretaceous boundary, East Greenland). Greenland Geological Survey Bulletin 128, 108. Surlyk, F., 1984. Fan–delta to submarine fan conglomerates of the Volgian–Valanginian Wallaston Forland Group, East Greenland. In: Koster, E.H., Steel, R.J. (Eds.), Sedimentology of Gravels and Conglomerates, vol. 10. Memoir of Canadian Society of Petroleum Geologists, pp. 359–382. Tu¨ysu¨z, O., 1990. Tectonic evolution of a part of the Thetyside orogenic collage: The Kargı Masif, Northen Turkey. Tectonics 9, 141–160. Tu¨ysu¨z, O., Yılmaz, Y., Yig˘itbasß, E., Serdar, H.S., 1990. Upper Jurassic– Lower Cretaceous stratigraphy of the central Pontides and its tectonic significance. Turkish Association of Petroleum Geologists – UCTEA Chamber of Petroleum Engineers. In: 8th Petroleum Congress of Turkey, Proceedings, pp. 340–350 (in Turkish with English Abstract). Tu¨ysu¨z, O., 1999. Geology of the Cretaceous sedimentary basins of the Western Pontides. Geological Journal 34, 75–93. Tu¨ysu¨z, O., Aksay, A., Yig˘ibasß, E., 2004. Batı Karadeniz Bo¨lgesi Litostratigrafi Birimleri. Stratigrafi Komitesi Litostratigrafi Birimleri Serisi–1. General Directorate of Mineral Research and Exploration, Ankara, 92 p. Walker, R.G., 1975. Generalized facies model for resedimented conglomerates of turbidite association. Geological Society of America Bulletin 86, 737–748. Walker, R.G., 1977. Deposition of upper Mesozoic resedimented conglomerates and associated turbidites in southwestern Oregon. Geological Society of America Bulletin 88, 273–285. Walker, R.G., 1980. Facies models. Turbidites and associated coarse clastic deposits. In: R.G. Walker (Ed.) Facies models. Geosciences Canada, Reprint Series 1, the Geological Association Canada, 211 p. Yılmaz, Y., Tu¨ysu¨z, O., 1988. An approach to the problem of reconstructing the Mesozoic tectonic units in the Kargı M assif and its surroundings. Turkish Association of Petroleum Geologists 1 (1), 73–86 (in Turkish with English Abstract).
Journal of Asian Earth Sciences 31 (2008) 429–438 www.elsevier.com/locate/jaes
Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gu¨rso¨ku¨ Formation (Kavak–Samsun, N. Turkey) Kemal Akdag˘ *, M. Ziya Kırmacı Karadeniz Technical University, Department of Geological Engineering, 61080 Trabzon, Turkey Accepted 9 May 2007
Abstract The Late Cretaceous Gu¨rso¨ku¨ Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gu¨rso¨ku¨ Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous. 2007 Elsevier Ltd. All rights reserved. Keywords: Cretaceous; Gu¨rso¨ku¨ Formation; Forearc basin; Submarine slope; Submarine fans
1. Introduction Coarse-grained submarine fan systems and their depositional processes are well established in the literature (Mutti and Ricci Lucchi, 1978; Nelson, 1983; Normark, 1974; Walker, 1975, 1977; Surlyk, 1984; Lowe, 1982; Ineson, 1989; Lowe and Guy, 2000; Shanmugam, 1997, 2000, 2002). The distinction between submarine fans and slopes has been emphasized both in the generalized classification schemes of deep-marine depositional environments and in the descriptions of ancient and modern deepwater systems (e.g. Nelson and Nilsen, 1989). There are three major turbidite facies associations: slope, submarine fan, and basin-plain. The facies and their vertical organization into recognizable facies associations permit an analytical study of ancient turbidite basins. Prograding and retreating systems, or part of the same system, can be recognized and
*
Corresponding author. Tel.: +90 462 3772755; fax: +90 462 3247505. E-mail address: [email protected] (K. Akdag˘).
1367-9120/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2007.05.005
the tectonic history of the basins can be deduced from the cyclic nature of the deep-marine clastic deposits. The Sinop–Samsun Basin trending NW–SE is located in the central part of the Pontide mountain range and is bordered by large scale thrust faults to the north and south (Korkmaz, 1984; Pelin and Korkmaz, 1981). The study area, located in Kavak Town and surrounding areas, is situated in the eastern part of this basin (Fig. 1). The aim of this paper is to recognize and interpret the turbidites and associated facies, and to determine the depositional conditions and environment of the Gu¨rso¨ku¨ Formation in the area of study. 2. Geological setting The Sinop–Samsun Basin commenced its development as a rift associated with the volcanic arc in Aptian–Albian time by the northward subduction of the Neotethys oceanic plate beneath the southern margin of Eurasia (Yılmaz and Tu¨ysu¨z, 1988; Tu¨ysu¨z et al., 1990; Tu¨ysu¨z, 1999) and turned into an asymmetrical Forearc Basin in Campa-
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Fig. 1. Location of the study area and simplified geological map of the Sinop–Samsun Basin (from Korkmaz, 1984).
nian–Maastrichtian time as the rifted volcanic arc (Aydın et al.,1995; Tu¨ysu¨z, 1999; Leren, 2003). In this basin, Mesozoic sedimentation begins with the Early Jurassic Akgo¨l Formation consisting of siliciclastic sediments, which rests unconformably on pre-Jurassic basement rocks. The Akgo¨l Formation is unconformably overlain by the Bathonian– Kimmeridgian Akkaya Formation composed of reefal limestones (Gedik and Korkmaz, 1984; Aydın et al., 1986). Both formations are exposed outside of the study area and are unconformably overlain by the Barremian– Albian C ¸ ag˘layan Formation (Gedik and Korkmaz, 1984; Tu¨ysu¨z, 1990; Aydın et al., 1995). The C ¸ ag˘layan Formation, forming the oldest rocks in the study area, is composed of turbiditic sandstones, dark grey to black shales, conglomerates and large/over-sized blocks (Fig. 2). The Late Cretaceous sedimentary succession is up to 2500 m thick in the region. This sedimentary succession begins with the Turonian–Santonian Kapanbog˘azı Formation, which is composed of red pelagic micritic limestone resting unconformably on the Early Cretaceous siliciclastic rocks, and is overlain by turbiditic sandstone–shale intercalations
with volcanogenic horizons of the Late Cretaceous Yemisßlic¸ay Formation. The succession grades upward into the Campanian– Maastrichtian Gu¨rso¨ku¨ Formation (Aydın et al., 1986; Tu¨ysu¨z et al., 2004) consisting of turbiditic sandstone– shale–conglomerate and marl alternations, which is the subject of this study. Higher up in the section, the Early Campanian–Paleocene Akveren Formation, composed of carbonates, grades into Paleocene–Eocene carbonates and siliciclastic deposits that are unconformably covered by younger terrestrial to shallow marine rocks. 3. Methodology In this study, a geological map of Kavak area covering about 275 km2 has been compiled. In order to reveal the lithofacies development of the Gu¨rso¨ku¨ Formation, a detailed stratigraphic section was measured with a 1.5 m Jacob’s staff and 115 rock samples were systematically collected. Because of the heavy vegetation cover and the lack of continuous outcrops, lateral shifts were made in some
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Fig. 2. Geological map of the studied area.
places of the section by tracing key beds. The sectioning included macroscopical identification of primary sedimentary structures, determination of mean grain size using a magnifying glass (10·) and a grain size comparator. A total of 58 paleocurrent directions were measured during mapping and measuring of the section. All measurements were corrected in the field to compensate for folding. A total of 100 thin sections of sandstone samples were petrographically analyzed and 20 selected samples were point counted (n = 1000) with a standard polarizing microscope attached
with a swift point counter to determine relative percentages of particle types. The sandstones are classified according to Folk (1974) and the conglomerates according to Boggs (1995). 4. Facies analysis The Late Cretaceous Gu¨rso¨ku¨ Formation comprises a 1000 m thick succession of conglomerate, sandstone, mudstone and marl (Fig. 3). The succession crops out along
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Fig. 3. Detailed measured section of the Gu¨rso¨ku¨ Formation showing facies and facies associations. Facies 1. Thin-bedded sandstone and mudstone, Facies 2. Thick-bedded graded sandstone, Facies 3. Clast-supported conglomerate, Facies 4. Mud-supported conglomerate.
road cuts for a distance of about 10 km between Kavak and Samsun, nearly perpendicular to the axis of the basin (Fig. 2). Globotrucana sp. was found in marl beds but is not as abundant as in the formations below and above. Derivation of the sediments from a source area located in the SE is indicated by the measured palaeocurrents directions (Fig. 2). Well-rounded clasts of limestones and metamorphic rocks form over 80% of the conglomerate clasts. The sandstones are litharenitic (phyllarenitic) in composition. They are composed of 20–48% (average 35%) sedimentary rock fragments, 1–12% (average 4%) volcanic
rock fragments and 42–80% (average 61%) metamorphic rock fragments. Four lithofacies are recognized and the description and interpretation of these are given below and are summarized in Table 1. 4.1. Thin-bedded sandstone and mudstone (Facies 1) Thin bedded, typically 3–8 cm thick, laterally persistent beds of medium to very fine grained sandstones, which typically show either normal grading, parallel lamination or
Not applicable
Not applicable
Flat, sharp base
Boulder-pebble, mediumcoarse sand matrix
Block-boulder, sandy mud matrix
3. Clast-supported conglomerate (Fig. 7)
4. Mud-supported conglomerate (Fig. 8)
1.5–50 m
Sand: medium to very fine. Mud: clay to coarse silt Medium to coarse grained 1. Thin-bedded sandstone and mudstone (Figs. 4 and 5) 2. Thick-bedded graded sandstone (Fig. 6)
1.5–25 m
Flat, locally amalgamated and scoured Scoured, amalgamated 0.1–1 m
Mud-supported, rip-up clasts
Deposition from a traction carpet at the base of high energy high concentration turbulent flow Deposition from debris flow
Ta, Tab
Ta-e, Tb-e, Tcde
Sand deposited from low density turbidity currents, mud deposited from turbidity currents Sand deposited from sand-rich high density turbulent flows
Normal grading, parallel and cross lamination, linguoid ripples Normal grading, structureless or parallel laminated upper portions Clast-supported, normal grading, imbrication Flat, sharp base, transitional top Sand: 3–8 cm Mud: 1–30 cm
Interpretation Bed thickness Grain size Facies
Table 1 Sedimentary facies distinguished in the Gu¨rso¨ku¨ Formation
Boundaries
Structures/textures
Bouma (1962)
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cross lamination, are intercalated in grey to greenish-grey, 1–30 cm thick silty mudstone beds (Fig. 4). The mudstones are transitional with the underlying sandstones and their silt content decreases upward, passing upward into claystones. The cross-laminated sandstones show linguoid ripples on their upper bedding surfaces (Fig. 5). The sandstones are grey to greenish-grey in colour, with a brownish-grey weathering colour. They are composed of poorly to moderately sorted, angular to subangular grains of metamorphic, sedimentary and volcanic rocks. The graded sandstone beds (typically Ta of Bouma, 1962) are interpreted as the deposits of high density turbidity currents (Lowe, 1982) or concentrated density flows (Mulder and Alexander, 2001). The parallel and cross-laminated sandstones (typically Tb and Tc of Bouma, 1962) are interpreted as low density turbidity current deposits settled from traction (Lowe, 1982; Ineson, 1989) or concentrated density flows with lower particle concentration basal layers (Mulder and Alexander, 2001). The mudstones intercalated with sandstones are interpreted as settled from suspension but with some traction before deposition. 4.2. Thick-bedded graded sandstones (Facies 2) The medium to coarse-grained sandstone beds, typically 0.1–1 m thick, are laterally persistent at outcrop scale (several 10s of meters) (Fig. 6). They have flat, locally amalgamated and scoured bases, show normal grading (Ta of Bouma, 1962), and are structureless (massive) or the upper few centimeters may be parallel laminated. They are phyllarenitic in composition. Trace fossils, including (Nereites sp) are observed in places on the capping mudstone surfaces. The sharp, flat bases, normal grading and laminated tops (typically Ta, Tab of Bouma, 1962) suggest deposition from waning sand-rich high density turbulent flows (Lowe, 1982; Ineson, 1989; Hickson and Lowe, 2002) or concentrated density flows (Mulder and Alexander, 2001). Such massive, predominantly structureless, deepwater sandstones have been interpreted by Kneller and Branney (1995) as the deposits of relatively continuously fed, longduration turbidity currents, and by Shanmugam and Moiola (1995) as the deposits of cohesive, sandy debris flows. However, the presence of laminated tops argues against a sandy debris flow interpretation because this stratification forms solely beneath turbulent, tractional flows. The flat, non-erosional bases of sandstone beds indicate that turbulence near the base of the flows was damped, and sediment fallout rates were high, suppressing the erosional ability of the flow. 4.3. Clast-supported conglomerate (Facies 3) These pebble and boulder conglomerates, typically 1.5– 25 m (Fig. 7), form single, graded beds (except in the upper part of the section) and locally show scoured, amalgamated sequences (Fig. 3). In some outcrops, they show a-axis
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Fig. 4. Thin-bedded turbidite sandstones interbedded with mudstones (Facies 1). Ruler is 40 cm in length.
Fig. 5. A thin-bedded turbidite sandstone (Facies 1), showing linguoid ripples on the upper bedding surface. Current is from lower left to upper right. Hammer is 35 cm in length.
imbrication (Fig. 7). Beds are laterally persistent at outcrop scale (several hundreds of meters) and show locally scoured bases and complete grading from boulder size at the base to sand size at the top (coarse tail grading). The clasts are rounded to subangular and poorly sorted, and consist dominantly of sedimentary rocks (limestones 70–75%, sandstones 5–10%), with lesser amount of metamorphic rocks (schist and quartzite 20–25%) and volcanic rocks (5%). They are clast-supported, sand matrix conglomerates and in some outcrops include dispersed Late Cretaceous red pelagic micritic limestone clasts up to several meters in diameter. These beds are closely comparable to the graded-bed model of the clast-supported conglomerates of Walker
(1977, 1980) and to Lowe’s (1982) R3–S3 division, representing the deposit of gravelly high density turbidity currents, as they show complete grading. This suggests that they originated from a single depositional event with a-axis imbrication but not inverse grading and stratification. The preferred clast orientation and normal grading in deepwater conglomerates is interpreted by Lowe (1982) as a result of the traction and direct suspension sedimentation of gravel from high density turbidity currents. 4.4. Mud-supported conglomerate (Facies 4) Internally chaotic mud-supported conglomerate (petromict diamictites of Boggs, 1995) beds 1.5–50 m thick
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Fig. 6. Thick bedded, parallel sided turbidite sandstones (Ta, Tab; Facies 2). Note the amalgamation in the upper left of the photograph (arrow). Ruler in the middle of the photograph is 1 m in length.
Fig. 7. Clast-supported cobble conglomerate composed of rounded limestone clasts (Facies 3). Ruler is 1 m in length.
form isolated beds in the succession. The conglomerates are poorly sorted and non-graded, and have a silty mud matrix content ranging from 25% to 35% in volume. The pebbles are mainly composed of subangular to subrounded limestone with a lesser proportion of metamorphic and volcanic rock clasts. In addition, limestone blocks, boulders and plastically deformed rip-up clasts of sandstone and shale of intrabasinal strata (up to meter and decametersize) may occur randomly within the beds (Fig. 8). The beds have sharp scoured bases. The high matrix/clast ratio, internally chaotic nature, and contorted rip-up clasts of the underlying beds indicate deposition from viscous debris flows that may have
originated from slumps or slides on unstable slopes (Surlyk, 1978; Ineson, 1989; Mulder and Alexander, 2001). 5. Facies associations and depositional environments The four facies described above occur in two discrete facies associations recognized through analysis of 980 m of measured section. The thin-bedded sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets (mudstone association), are interpreted to represent a slope-apron environment and the conglomerates and sandstones (sandstone
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deposits (Facies 3). Erosional features are rare and bedding is typically parallel and locally persistent except where deformed by isolated slump folds (Fig. 9). Systematic vertical trends of grain size or bed thicknesses have not been observed. This facies association represents a low-energy, subwave base setting, characterized by sedimentation from suspension and low density turbidity currents which are intermittently interrupted by high density flow and debris flow deposits. Mass-flows and slumps are a characteristic component of the association and they indicate that the depositional setting is an unstable basin margin setting, probably flanked by a proximal fringe of slope debris. The intrabasinal shale and sandstone clasts and angular clasts of Jurassic carbonate rocks within Facies 4 were derived from slumping and sliding, probably reflecting episodic movements on the basin margin faults.
5.2. Sandstone–conglomerate association Fig. 8. Mud-supported conglomerate (a debrite, Facies 4) underlain by thin-bedded sandstones and mudstones (Facies 1). Ruler is 40 cm in length.
and conglomerate association) are interpreted as submarine fan deposits (Fig. 11). 5.1. Mudstone association This association forms a succession, up to 500 m thick, and is the typical host sediments of the Gu¨rso¨ku¨ Formation (Fig. 2). It consists largely of thinly interbedded sandstone and mudstone (Facies 1) together with muddy massflow deposits (Facies 4) and high density turbidity current
This facies association forms discrete green-brown sandrich packages (Facies 2) up to 10–50 m thick, alternating with conglomerates (Facies 3) in the upper part of the Gu¨rso¨ku¨ Formation. The sandstones and conglomerates assigned to the association show scoured and amalgamated beds (Fig. 10). A distinctive feature of the association is crude fining-upward sequences up to 4–5 m thick. The depositional processes dominating this association are high density turbidity currents (Lowe, 1982) and noncohesive debris flows (Mulder and Alexander, 2001). The deposits of cohesive debris flows (Mulder and Alexander, 2001) and slides form discrete beds within the section. Apart from crude fining-upward trends, the association shows no clear internal organization. It is proposed that this association represents sediment deposited at the
Fig. 9. Isolated slump folds in the mudstone association. Scale (circled) is 0.75 m in length.
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Fig. 10. Thick conglomerate bed with an erosional base (Facies 3, sandstone–conglomerate association). Ruler is 1 m in length.
Fig. 11. Schematic tectonic and paleogeographic setting of the Sinop–Samsun Basin and depositional model for facies associations of the Gu¨rso¨ku¨ Formation.
mouths of base of slope channels that funneled sediment into the basin or inner channeled zone of submarine fans. 6. Conclusions The Late Cretaceous Gu¨rso¨ku¨ Formation consists of four facies forming two facies associations. The thin-bedded sandstone and mudstone facies (Facies 1) was deposited by low density turbidity currents and concentrated density flows with lower particle concentration. The thick-bedded graded sandstone (Facies 2) was deposited by concentrated density flows. The clast-supported conglomerate (Facies 3) was deposited by gravelly, high density turbidity currents. The mud-supported conglomerate (Facies 4) was deposited by viscous debris flows. The mudstone facies association, consisting of Facies 1, 3 and 4 and comprising the lower part of the Gu¨rso¨ku¨ Formation, was deposited in a base-
of-slop setting. The sandstone–conglomerate association, consisting of Facies 2 and 3 and comprising the upper part of the Gu¨rso¨ku¨ Formation, consist of thick-bedded sandstones and conglomerates deposited by high density turbidity currents and non-cohesive debris flows at the mouths of base-of-slop channels that funneled sediment into the basin. The depositional setting of the Gu¨rso¨ku¨ Formation was probably a faulted basin margin setting, flanked by a proximal fringe of slope debris and submarine fans developed at the mouths of slope channels. References _ U ¨ ., Serdar, H.S., O ¨ zc¸elik, Y., Akarsu, I., ¨ ngo¨r, Aydın, M., S ß ahintu¨rk, O A., C ¸ okug˘rasß, R., Kasar, S., 1986. The geology of the area between Ballıdag˘ and C ¸ angaldag˘ (Kastamonu). Bulletin of the Geological Society of Turkey 29, 1–16 (in Turkish with English Abstract).
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