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  3. Basin margin tectonics and morphology as controls of delta type and architecture: examples from the Mio-Pliocene Yalvaç Basin (SW Turkey)

Basin margin tectonics and morphology as controls of delta type and architecture: examples from the Mio-Pliocene Yalvaç Basin (SW Turkey)

This study describes the sedimentary facies and depositional architectures of Gilbert-type and shoal-water delta deposits developed on opposed margins of the extensional fluvio-lacustrine Yalvaç Basin during the late Cenozoic. The roles of syndepositional tectonism, basin dynamics, and hinterland morphology on the development of different delta types are assessed. This asymmetric trough initially opened as an intramontane molasse basin to the southwest of the Sultandağları massif. Its northern a

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  1. Turkish Journal of Earth Sciences Turkish J Earth Sci (2021) 30: 516-535 http://journals.tubitak.gov.tr/earth/ © TÜBİTAK Research Article doi:10.3906/yer-2009-8 Basin margin tectonics and morphology as controls of delta type and architecture: examples from the Mio-Pliocene Yalvaç Basin (SW Turkey) Ayhan ILGAR*, Ali ERGEN, Ercan TUNCAY, Alper BOZKURT Department of Geological Research, General Directorate of Mineral Research and Exploration, Ankara, Turkey Received: 10.09.2020 Accepted/Published Online: 28.03.2021 Final Version: 16.07.2021 Abstract: This study describes the sedimentary facies and depositional architectures of Gilbert-type and shoal-water delta deposits developed on opposed margins of the extensional fluvio-lacustrine Yalvaç Basin during the late Cenozoic. The roles of syndepositional tectonism, basin dynamics, and hinterland morphology on the development of different delta types are assessed. This asymmetric trough initially opened as an intramontane molasse basin to the southwest of the Sultandağları massif. Its northern and southern margins are bounded by normal faults, which controlled both tectono-sedimentary evolution of the basin and the surrounding palaeomorphology. The lacustrine deposits consist of thin-bedded limestones, marls, and medium to thick-bedded sandstones and conglomerates. The coarse-grained Gilbert-type delta, up to 150 m thick, was deposited just in the front of the steep northern basin margin. The clinoformal architecture of this delta comprises steeply inclined foreset beds, which are overlain by horizontal alluvial topset units and underlain by subhorizontal bottomset deposits. The Yarıkkaya normal fault controls the northern margin of the basin and plays an active role in both local uplift and subsidence, giving rise to new sediment sources and increasing the accommodation space. Abundant coarse-grained sediment supply into this relatively deep basin across the steep scarp of the Yarıkkaya Fault led to creation of a delta with Gilbert-type architecture. Contemporaneously, on the shallow southern margin of the Yalvaç Basin, several small shoal-water deltas up to 3.5 m thick, were deposited on a smooth basin floor. The gently mound-shaped depositional architecture of these shoal-water delta deposits comprise erosive-based mouth-bar and distributary channel deposits. These observations show that, in this case, the river deposited its bedload shortly after entering the basin, generating small shoal-water deltas that, through stacking over time and some lateral offset, built a shoal-water delta complex. Key words: Gilbert-type delta, shoal-water delta, asymmetric graben, fault-margin control 1. Introduction determines the delta thickness, and thus also the height Deltas are the basin margin systems that host a huge of its subaqueous slope (Nemec, 1990). These features volume of sediments carried by rivers from hinterland indicate that if an alluvial distributary system transports regions into basins. They create regressive shoreline sufficient bedload into a basin, which is relatively deep wedges and form typically coarsening-upward bed immediately adjacent to the mouth and the spreading packages as well as seaward-dipping clinoform bedding of the stream effluent as an axial turbulent, the resultant patterns (Barrell, 1912; Colella, 1988; Postma, 1990; deposits display Gilbert-type delta architecture (Nemec, Bhattacharya, 2010). Ignoring the water-level change, the 1990; Postma, 1990). However, if water depths seaward sedimentary characteristics of a delta system prograding of the mouth of a river are shallow or shoaling, turbulent within a low-energy basin vary according to the type of diffusion becomes restricted to the horizontal, and bottom alluvial feeder, the basin relief at the river mouth, and the friction then plays a major role (Wright, 1977), resulting in river mouth processes. These dynamic factors determine shoal-water delta architecture (Postma, 1990). the sedimentation rate on the subaqueous delta segment, Deltas are also sensitive recorders of tectonic, which controls its growth-style and thus its profile (Postma, climatic, and base-level conditions (Leeder et al., 1988; 1990). The terms “shoal-water profile” and “Gilbert- Colella and Prior, 1990; Gawthorpe and Colella, 1990; type profile” are used for gently inclined delta fronts and Ilgar and Nemec, 2005). The synsedimentary tectonism steeply inclined delta fronts, respectively (Nemec, 1990; controls the type and evolution of depositional facies at Postma, 1990). For a Gilbert-type delta, sediment supply the basin margin in an active extensional basin (Leeder determines the size of the delta; whereas, the water depth and Gawthorpe, 1987; Alexander and Leeder, 1987). The * Correspondence: ayhan.ilgar@mta.gov.tr 516 This work is licensed under a Creative Commons Attribution 4.0 International License.
  2. ILGAR et al. / Turkish J Earth Sci activity of a basin margin fault influences the sediment the Sultandağı sequence, the Beyşehir-Hoyran nappes, the supply derived from the uplifting footwall and creates Anamas-Akseki autochthon and the overlying Celeptaş depocentres on the subsiding hangingwall (Gawthorpe Formation (Özgül et al., 1991; Ergen et al., 2021) (Figure and Colella, 1990). Thus, the alluvial distributary system 1b). The Sultandağı sequence consists of early Cambrian that carries sediments to a tectonically controlled basin to Late Cretaceous autochthonous and allochthonous margin accumulates the sediments in relatively deep- metamorphic rocks (Özgül et al., 1991; Ergen et al., 2021), water conditions, causing the development of Gilbert- while the Anamas-Akseki autochthon is represented type deltas (Leeder and Gawthorpe, 1987; Nemec, 1990; by Jurassic to Cenomanian dolomites and limestones Postma, 1990). On the other hand, drainage systems (Şenel et al., 1992). These units are tectonically overlain developed on gently inclined topography and carrying by the Middle Triassic-Late Cretaceous Beyşehir-Hoyran sediments to the basin floor immediately basinward of the nappes, which are composed of peridotites, metamorphic- stream outlet tend to generate shoal-water deltas (Leeder sole rocks, and mélange (Gutnic et al., 1968; Monod, and Gawthorpe, 1987; Nemec, 1990; Postma, 1990; Ilgar 1977). The late Paleocene-Lutetian Celeptaş Formation, and Nemec, 2005). which consists of thin-bedded limestones, calcilutites, Deltas are common depositional systems in many and deep-marine turbidites, rests unconformably on the sedimentary basins and a variety of tectonic settings abovementioned units along a narrow, NW-SE trending (see Nemec and Steel, 1988; Colella and Prior, 1990; Oti belt between the Sultandağları and Anamas Mountains. and Postma, 1995). Although Gilbert-type deltas have The Neogene deposits of the Yalvaç Basin that overlie been reported from basins in many parts of the world, these bedrock units with an angular unconformity (Figure shoal-water delta deposits are less well defined, and the 1b and 2a) commence with the reddish conglomerates, coexistence of these different delta facies associations in sandstones, siltstones, and mudstones of the Bağkonak a single basin is even less well known (e.g. Dunne and Formation (Demirkol et al., 1977; Demirkol, 1982; Hempton, 1984; Sohn and Son, 2004; Ilgar and Nemec, Demirkol and Yetiş, 1983-1984; Yağmurlu, 1991; Koç et 2005; Ghinassi, 2007). The Yalvaç Basin is a typical example al, 2014). These fine to coarse clastic deposits, transported of this kind of basin that includes these delta types, mainly to the southwest from Sultandağları are interpreted with their distinctive morphotectonic features on either as stream-dominated alluvial fan deposits (Figure 2b). The margin. This account describes the sedimentary facies and Bağkonak Formation is thus the product of the coalescence depositional architectures of these delta systems, formed of several alluvial fans. The lacustrine carbonates and the on the margins of the extensional lacustrine Yalvaç Basin. clastic rocks in the Yalvaç Basin transgressively overlie Special attention is given here to the relationships between Bağkonak Formation. The lacustrine sediments consist syndepositional fault activity, basin floor, and hinterland morphology in order to assess the roles of tectonics and mainly of mudstones, marls, limestones (Figures 2c and morphology in the development of different delta facies 2d), algal limestones, sandstones, conglomerates, coal, and architectures. The present study also considers and tuffs. These units were deposited in Gilbert-type how the asymmetrical subsidence of a basin may cause delta, shoal-water delta, foreshore, shoreface, offshore- transgression on one margin and forced regression on the transition, offshore, and marsh subenvironments within other, through shoreline shifting which can result in the the basin. In previous studies, the facies assemblages development of different delta facies associations within that reflect deposition in these sub-environments were the same basin. described under the names of Göksöğüt, Madenli and Yarıkkaya formations (Demirkol et al., 1977; Demirkol, 2. Regional geological setting and stratigraphy 1982; Yağmurlu, 1991; Koç et al, 2014; Tuncer, 2020). The fluvio-lacustrine Yalvaç Basin opened in the NE part However, these facies assemblages are laterally and of the Isparta Angle as an intramontane molasse basin vertically transitional and alternate with each other several (Koç et al., 2014) formed in early Miocene times between times in the succession. Therefore, in this study, all the the rising Sultandağları and Anamas Mountains (Figures facies associations deposited in lacustrine environments 1a and 1b) and evolved as an asymmetric graben. Yalvaç have been assigned to the Yarıkkaya Formation, which has Basin is bounded on all sides by bordering normal faults, a widespread distribution in the region (Figure 1b). which controlled both tectono-sedimentary evolution In the central and the southern parts of the basin and of the basin and the surrounding palaeomorphology. in the surroundings of Yarıkkaya village in the north, this It is thought that the Yalvaç Basin formed as a result of formation is made up of alternating thin-bedded clayey orogenic collapse in early Miocene, following the nappe limestones and marls. Algal limestones and coarse-grained emplacement in the Central Taurides (Koçyiğit and Gilbert-type delta clastics overlie the clayey limestone Deveci, 2007; Koçyiğit et al., 2013). The basin bedrock and marl alternations seen on the northern margin of the units comprise (from north to south) rocks ascribed to basin. A shoal-water delta sequence, consisting mainly 517
  3. ILGAR et al. / Turkish J Earth Sci a BLACK SEA NAF A N AT O L I A N P LAT E Study Area EF EAF os Su agr tu -Z ta An Bitlis re Ispar A R A B I A N P LAT E gle NAF : North Anatolian Fault N SH EAF : East Anatolian Fault M PF EF : Ecemiş Fault W E BF BFPF : Burdur-Fethiye-Pliny Fault C DSF : Dead Sea Fault MEDITERRANEAN SEA ypru S MSH : Misis Structural High DSF s Ar 200 km c Cyprus b N Quaternary AGE FORMATION S Karamık 0 km 5 Yarıkkaya Fm. U marls, sandstones, Yarıkkaya Format on Bağkonak Format on 6 Yarıkkaya L conglomerates YALVAÇ BASIN 9 T De limestones, lta A algal limestones N 5 D A algal limestones Bağkonak Fm. Sağır 7 Ğ mudstones, 8 sandstones, L conglomerates A Körküler (alluv al fan depos ts) R I Late Paleocene Celeptaş Fm. Bedrock SULTANDAĞI BEYŞEHİR-HOYRAN ANAMAS-AKSEKİ Kumdanlı SEQUENCE NAPPES AUTOCHTHON Celeptaş Eyüpler YALVAÇ Normal fault Thrust Hüyüklü 2 Bağkonak 4 3 Dedeçam 11 e 11 1 Balcı GELENDOST 10 lt Y cı fau Bal LAKE EĞİRDİR ŞARKİKARAAĞAÇ ANAMAS MOUNTAINS Figure 1. (a) Location of the Yalvaç Basin and major tectonic lineaments of Turkey, depicted on the 90 m resolution SRTM image of Anatolia (after Jarvis et al., 2008)1. (b) Geological map of the Yalvaç Basin. The points 1-11 indicate outcrop localities to which the paper’s other figures refer. 1 Jarvis A, Reuter HI, Nelson A, Guevara E (2008). Hole-filled SRTM for the Globe, Version 4. CGIAR-CSI SRTM 90m Database. http://srtm.csi.cgiar. org. [accessed 15 05 2020] 518
  4. ILGAR et al. / Turkish J Earth Sci SW NE Bağkonak Fm. Bedrock Sultandağı sequence a SE NW Bağkonak Fm. b SE NW SE NW c 1m d Figure 2. Typical features of Neogene sedimentation in the Yalvaç Basin: (a) General view of the reddish coarse clastics of the Bağkonak Formation in unconformable contact with bedrock metamorphics of Sultandağı sequence. (b) The fining-upward packages of gravel and sand representing multistorey fluvial channel-fills of the stream-dominated alluvial fan deposits typical of the Bağkonak Formation. (c, d) The lacustrine deposits of the Yarıkkaya Formation transgressively overlie the Bağkonak Formation. The mudstones shown in (c) represents the offshore environment, whereas thin bedded of mudstone, marl, and limestone alternations (d) in indicate offshore- transition to shoreface environment. The hammer (scale) in (b) and (d) is 33 cm. Picture (a) is from locality 2, picture (b) from locality 3, pictures (c), and (d) from locality 4 in Figure 1b. of sandstone, pebbly sandstone, and a lesser amount closure of the basin. Thus, alluvial fan sediments were of conglomerate, was deposited to the east of Madenli deposited on top of the lacustrine sediments, which were village, in the south of the basin. Alluvial fan deposits of described using the term Kırkbaş Formation by previous the Bağkonak Formation, which has lateral and vertical researchers (Yağmurlu, 1991; Koç et al, 2014), over a very relationships with Yarıkkaya Formation on the basin large area between Körküler in the north and Hüyüklü margin, overlie the lacustrine deposits depending on the villages in the south. 519
  5. ILGAR et al. / Turkish J Earth Sci The age of the Yarıkkaya Formation, which overlies the is ca 12 km wide (Figure 1b) and is located just to the south Bağkonak Formation, was suggested as middle-late Miocene of Gelincikana Hill, the highest point of the Sultandağları by Yağmurlu (1991). Tuncer (2020) carried out a study in massif. The NW-SE trending Çakırçal and Sağır faults and the Yalvaç Basin in order to age the lacustrine deposits of the NE-SW trending Yarıkkaya Fault bound the basin the Yarıkkaya Formation based on ostracod, mammals, to the north (Figure 1b). Each of these main fractures is and palynomorphs. He assigned a late early Miocene- composed of several major and minor fault segments that early middle Miocene age to the lower levels (named as have similar strikes and dips. These faults controlled the Yarıkkaya Formation in his study) and a late Miocene- development of both basin structure and the morphology Pliocene age to the upper levels of the formation (named of surrounding regions in this northern sector. The faults, as Göksöğüt Formation in his study). Besides, Usta et al. generally form boundaries between the Jurassic bedrock (2019) determined a late Miocene age (MN 11 mammalian and the Neogene basin deposits, cut the basin sediments zone) based on their fossil findings in terrestrial deposits of as well as the bedrock. the uppermost part of the Bağkonak Formation. The Yarıkkaya Fault (Figures 3a and 3b), structurally In this study, nine samples collected from the marls and and morphologically one of the most important fractures mudstones of the uppermost part of the Yarıkkaya Formation in this region, extends across the northern sector of the (Figure 1b, location 1), a late Miocene-early Pliocene aged Basin with a strike of N 45°-53° E and a dip of 70°-90° ostracod assemblage including Heterocypris salina (Brady), SE. The Miocene drainage system feeding the Gilbert type Heterocypris salina salina (Brady), Heterocypris salina delta conveyed its load from the footwall of the Yarıkkaya barneri (Luttic), Candona parallela pannonica (Zalanyi), Fault, while the delta was built in front of this fault plane Candona aff. parallela pannonica (Zalanyi), Candona aff. (Figure 3a). The Çakırçal Fault, one of the largest fractures iliensis Mandelstam, Candona candida (Koch), Candona defining the northeastern margin of the basin, has a aff. candida (Koch), Cypridopsis aff. vidua (O. F. Müller), strike of N 34°-44° W and a dip of 62°-80° SW. Another Candona (Candona) cf. Churmensis Freels, Candona important fault in the northern part of the basin is the Sağır (Candona) aff. iliensis Mandelstam, Darwinula cylindrica Fault, which borders the northern sector of the basin to Straub, Candona neglecta Sars, Darwinula stevensoni (Brady the southwest and extends subparallel to the Çakılçal Fault ve Robertson), Candona (Candona) cf. marchica marchica (Figure 1b) with a N 15º W strike and dip of 65°-75° NE. Hartwaing, Candona (Pseudocandona) compressa (Koch), The Kumdanlı Fault cuts both the bedrock and Neogene Candona (Pseudocandona) cf. compressa (Koch), Ilyocypris fill-sediments in this northern sector. It strikes N 40° E gibba (Ramdohr), Cyclocypris ovum (Jurine), Ilyocypris and dips 72° NW. In aggregate, the plunge of slickenlines bradyi Sars, Zonocypris membranae Livantel, Candona measured from these fault planes varies between 80°-90°. (Candona) aff. gracilis Livental, Cyclocypris ovum (Jurine), Slickenlines, vertical or almost vertical corrugation axes Ilyocypris sp., Candona sp., Candona (Caspiocypris) sp., and the chatter marks observed on the fault planes all Cypridopsis sp., Heterocypris sp. have been determined. consistently indicate normal faulting (Figure 3b). Considering all these data mentioned above, the age of The onlapping character of the lacustrine carbonates the Yarıkkaya Formation has been accepted as Miocene- of the Yarıkkaya Formation that were deposited on the Pliocene. hangingwall block defined by the fault planes of the Çakırçal, Yarıkkaya, and Sağır faults shows that normal 3. Delta deposits faulting was active prior to deposition in the basin. The The delta deposits, described in the Yarıkkaya Formation, syndepositional activities of these faults resulted in the have been assigned to two main facies associations, development of a relatively narrow and deep depression namely Gilbert-type delta deposits and a shoal-water delta in the northern part of the basin. These high-angle normal assemblage, on the basis of their facies characteristics and faults also enabled the development of steep morphology depositional architectures. The stratigraphic positions in this northern part of the basin. The distance from the of these deltaic sequences, the morphotectonic attributes faults bordering the northern margin of the Yalvaç Basin of the basin margins where they were formed, and the and the drainage basin limits on the hinterland is 3.5-8 sedimentological features of the deltas are detailed below. km, while the difference in topographic relief between the In brief, Gilbert-type delta deposits have been identified on basin floor and the surrounding mountains was between the northern margin of the Yalvaç Basin and shoal-water 750-1000 m. delta deposits on the southern margin. The minimum basin depth during deposition of the 3.1. Northern basin margin Gilbert-type delta can be estimated from the thickness 3.1.1. Morphotectonic features of the foreset deposit and is 45 m at the fault contact and The Yalvaç Basin has a triangular-shaped geometry. Its 125 m basinward to the south (Figure 3a). Thus, it can be northern edge, which forms the narrowest part of the basin, inferred that this Gilbert-type delta was deposited on a 520
  6. ILGAR et al. / Turkish J Earth Sci SE F g. 4a NW F g. 3c F g. 3b F g. 6 F g. 5 a SW NE Bedrock Sultandağı sequence Yarıkkaya fault Neogene lacustr ne carbonates b c fo reset delta algal l m est one s d e Figure 3. (a) Panoramic view of the Gilbert-type delta sequence in the Miocene Yarıkkaya Formation adjacent to the northern basin margin and the Yarıkkaya normal fault. (b) Close-up view of the Yarıkkaya Fault showing slickenlines and almost vertical corrugation axes. (c) General view of the delta foreset, delta bottomset and the underlying undulatory algal limestones of the Yarıkkaya Formation. (d, e) Closer views of the algal limestones at the base of the Miocene Gilbert-type delta package showing semi-spherical shapes and stromatolitic mounds. The measuring stick (scale) in (d) and (e) is 1 m. Picture a is from locality 5, picture (b) from locality 6, picture (c) from locality 7, pictures (d) and (e) from locality 8 in Figure 1b. The localities of Figures 3b and 3c are also shown in Figure 3a. fault-controlled, narrow, and relatively deep basin margin. Although the Gilbert-type delta identified in the The morphology of the basin floor during the delta Yarıkkaya Formation was deposited in a relatively deep deposition appears to have been rather undulatory (Figure basin, its bottomset deposits directly rest on the algal 3c), probably as a result of syndepositional fault activity. limestones of the Yarıkkaya Formation (Figure 3c), which 521
  7. ILGAR et al. / Turkish J Earth Sci are rich in peloids, freshwater oncoids, and plant root multistorey palaeochannels of braided streams (Collinson, traces coated by algae. Dendritic algal mats generated by 1996; Miall, 1996). The coarse pebble to cobble gravels on cyanobacteria are observed in almost all limestones. These the erosional surface are channel-floor lag deposits (Miall, algal mats with semispherical shapes evidently cohered 1985; Nemec and Postma, 1993). Planar parallel and planar- to form the stromatolites (Figures 3d and 3e). Such cross stratified sandstones and conglomerates, overlying limestones, composed entirely of dendritic stromatolites the lag deposits, reflect the longitudinal, transversal, and created by cyanobacteria, are typical of the stromatolite oblique bars within the river channel (Miall, 1985; Nemec boundstone facies (LMF11), generally deposited in and Postma, 1993). Accordingly, these sediments have hypersaline lacustrine environments and indicative of a been interpreted as the bedload of powerful streams (Ilgar shore-near shore depositional environment (Clausing, and Nemec, 2005). 1990). Braided river deposits, displaying identical facies 3.1.2. Gilbert-type delta deposits characteristics to those in the delta topset assemblage have As developed on the northern margin of the basin, this also been identified on the relay ramp of the Yarıkkaya facies assemblage consists of well-bedded conglomerates normal fault at the northern basin margin (Figures 1b, and sandstones that form clinoformal wedges up to 150 4b, and 4c). These erosionally overlie the lacustrine m thick. The clinoformal architecture characterized carbonates of Yarıkkaya Formation on the fault footwall by steeply inclined foreset beds, which are overlain by block, reflecting the effects of forced regression on this horizontal topset units and underlain by subhorizontal margin. This forced regression results from a fall in bottomset deposits, is characteristic for Gilbert-type deltas relative lake level, possibly related to elevation of the basin (Figure 3a; Barrell, 1912; Colella, 1988; Postma, 1990). margin. Thus, the topographic elevation of the braided The erosional angular contact between the delta topset river deposits on the relay ramp is higher than the delta and the delta foreset deposits reflects the river-dominated topset in the basin interior, which also demonstrates the deltas (Colella, 1988). The normal regressive wedge of the postdepositional activity of the Yarıkkaya Fault. prograding Gilbert-type delta deposits covers an area of 3.1.2.2. Foreset facies approximately 4 × 6 km, on the basin margin (Figure 1b). Description: Delta foreset deposits, consisting mainly Details of the topset, foreset and bottomset facies of the of conglomerate beds and subordinate sandstone beds, Gilbert-type delta deposits are described and interpreted are inclined basinwards at up to 20° (Figures 3a and 5a), below. form thickening and coarsening-upward successions. The 3.1.2.1. Topset facies thickness of the delta foreset sequence increases basinwards Description: These alluvial facies, forming the subaerial up to 125 m. Delta foreset deposits pass tangentially into part of the delta, display erosive basal contacts with the the delta bottomset deposits in the basinward direction underlying, basinward-inclined foreset beds (Figure 4a). (Figure 3a). Conglomerate beds, tabular to mound-shaped, The facies association comprises mainly gray-light brown have a thickness-range of 20-150 cm, but are mainly 35- colored, medium-coarse pebble conglomerates, and 60 cm (Figures 5a, 5b, and 5c). They consist of granule to coarse-grained sandstones. These form fining-upward coarse pebble and cobble gravel, and occasionally contain bedsets with concave erosional bases (Figures 4a, 4b, and large amounts of coarse cobbles and boulders reaching up 4c), which are usually stacked upon one another, and to 100 cm (Figures 5b and 5c). have a thickness of 50-170 cm and a width of 3-15 m. The Conglomerates, comprising spherical-shaped and laterally discontinuous, isolated beds of coarse pebble to rounded clasts that are mostly derived from Jurassic cobble conglomerates line the erosional surfaces at the recrystallized limestones, have clast-supported texture. base (Figure 4c). These basal beds display clast-supported Intergranular space is filled with fine sand to granule clasts. texture filled with coarse sands, granules, and fine pebbles These rudites display a range of sedimentary structures. but lack stratification. Planar parallel- and planar cross- Some, especially the mound-shaped units, are massive, stratified, coarse-grained sandstones and fine pebble nongraded, or inversely graded (Figure 5c). The tabular conglomerates with a thickness of 25-60 cm overlie the beds are planar parallel stratified (Figure 5d), weakly basal gravel layers (Figure 4c). Any stratification within stratified, or normal-graded (Figure 5e). Stratification the conglomerates is created by differences in clast size in the weakly stratified conglomerates is distinguished and sorting. The gravels, mostly derived from Jurassic by grain size and/or the matrix differences. The tabular recrystallized limestones, are spherical to rod-shaped and sandstone interbeds, 5-20 cm thick, show mainly planar subrounded to rounded. Spaces between the pebbles are stratification with normal grading and consist of medium filled with medium to coarse sand and granule grains. to very coarse sand. Interpretation: These conglomerates and coarse- Interpretation: Planar parallel stratification observed grained sandstones are interpreted to represent the in fine- to medium-pebble conglomerates and sandstones 522
  8. ILGAR et al. / Turkish J Earth Sci SW NE SW NE c e4 delta topset gur Delta topset bra ded r ver channel-fill depos ts F n on ct se reset delta fo a 1m b 7 c mult storey fluv al 14 palaeochannels Mult storey fluv al palaeochannels, 6 Each channel-fill cons sts of scour-bounded, 13 fin ng-upward bedsets form ng the l ght brown colored sandy channel-floor lag and channel-bar depos ts and gravelly mudstone mult storey fluv al palaeochannels 5 12 4 l ght brown colored sandy and gravelly mudstone 11 18 3 mult storey fluv al palaeochannels mult storey fluv al palaeochannels 10 mult storey fluv al palaeochannels cons st of channel-floor lag and 17 2 channel-bar depos ts 9 16 1 Th ckness (m) 8 15 Surface of forced regress on Yarıkkaya Fm. (lacustr ne l mestones) 0 c vf m vc gr fp cp Mud Sand Gravel Figure 4. (a, b) Delta topset sequences in Yarıkkaya Formation, comprising fining-upward channel-fill deposits in angular erosional contact with delta foreset deposits. (c) Sedimentological log of the Figure 4b showing multistorey, fining-upwards fluvial channel-fills comprising channel-floor lag and channel-bar deposits. Note that the lacustrine carbonates of the Yarıkkaya Formation was truncated by a denudation surface of forced regression and subsequently covered by fluvial deposits. Note geologist in (b) to provide scale. Picture (a) is from locality 5, picture (b) from locality 9 in Figure 1b. The locality of Figure 4a is also shown in Figure 3a. 523
  9. ILGAR et al. / Turkish J Earth Sci SE NW Figs. 5b, c, d t rese a fo delt a b c HDTC LDTC debr s flow depos ts d HDTC e HDTC LDTC HDTC LDTC Figure 5. (a) Panoramic view showing the basinward inclined delta foreset sequence. (b) Delta foreset composed of debris flow, high- density turbidity current (HDTC) and low-density turbidity current (LDTC) deposits. (c, d, e) Close-up view of the debris flow, low- density turbidity current and high-density turbidity current deposits, respectively. The measuring stick (scale) in (b) is 1 m. The pictures are from locality 5 in Figure 1b. The locality of Figure 5 is also shown in Figure 3a. Note the localities of Figures 5b, 5c, 5d and 5e are shown in Figure 5a. indicate the tractional deposition resulted from mound-shaped conglomerates have been interpreted as hyperpycnal turbidity currents (Bornhold and Prior, 1990; noncohesive debris flow deposits (Nemec and Steel, 1984; Nemec, 1990). Hyperpycnal turbidity currents reflect river- Nemec, 1990), while normal graded conglomerates with generated low-density turbidity currents (Lowe, 1982). erosional bases represent deposits of high-density turbidity Massive or inversely graded, clast-supported, planar, and current deposits in delta foreset sequences (Lowe, 1982). 524
  10. ILGAR et al. / Turkish J Earth Sci 3.1.2.3. Bottomset facies villages, before the maximum transgression reached the Description: These facies, forming the toes of the foreset southern structural border of the basin. Thus, during beds and passing tangentially into basin-floor deposits, the shoal-water delta sedimentation this margin was not consist mainly of sandstones, and granule to fine pebble directly controlled by activity of the basin margin faults. conglomerates (Figure 6). Sandstone beds are tabular, 3-25 The stratigraphic sequence recognized on this cm thick (Figures 6a and 6b), and composed mainly of southern margin consists of four main facies associations medium-coarse sand, but occasionally contain fine pebble representing shoreface, foreshore, shoal-water delta, and gravel (Figures 6b and 6c). Sedimentary structures in these meandering river-flood plain environments (Figure 8a). The deposits are dominated by planar parallel stratification; base of this sequence comprises thin-bedded limestones, however, current-ripple cross lamination (revealing probably deposited in a shoreface environment (Figures 8b basinwards palaeoflow), normal grading, and rare planar and 9). The shoal-water delta complex, consisting mainly cross stratification with a set height of 5-20 cm are also of sandstones and conglomerates with a total thickness present as subordinate structures (Figures 6b, 6c, 6d, and of 10 m, gradually encroaches on to the carbonates. This 6e). Conglomerates are generally thin-bedded and planar delta complex is overlain by approximately 11 m of fine/ parallel stratified. medium sandstones and conglomerates, interpreted as Interpretation: Planar parallel stratified, normal graded alternating shoreface-foreshore deposits. The thickness of sandstones and conglomerates have been interpreted as the individual facies-elements varies between 30-120 cm low-density turbidity current deposits (Lowe, 1982) that and 15-40 cm, respectively. A second generation of shoal- bypassed the delta foreset and were transported to the water delta complex with a thickness of 13 m developed basin. on these coastal deposits (Figure 9). These delta deposits 3.2. Southern basin margin are overlain by sandy limestones, organic-rich mudstones, 3.2.1. Morphotectonic features and a white-colored tuff of 150 cm thickness (Figure The southern margin of the basin is bounded by the 9). This tuff is succeeded by a 45 m thick sequence of northern flank of the Anamas Mountains. Two main conglomerates, sandstones, and mudstones interpreted as faults, Balcı and Yakaköy, and many small faults demarcate meandering river and flood-plain deposits. These fluvial this margin (Figure 1b). The Balcı Fault, morphologically deposits are overlain by alternations of thin-bedded the most conspicuous structure in this sector, defines the limestones and marls, typical lake-background sediments. boundary between the older bedrock and the Neogene These interfingering facies association indicate the basin deposits (Figure 7a). This fault strikes N 45° E in the regressive and transgressive phases of sedimentation west, trends E-W in the eastern sector, and generally has a related to the relative lake-level changes (Ilgar and Nemec, dip of 35°-65° to the north. 2005). The shoal-water delta, shoreface, and foreshore The Yakaköy Fault, a nearby important fracture in the facies associations show that the relative increase in lake- south of the basin, is more than 20 km long with a strike level during the deposition of this sequence in the southern of N 45° E and 65° NW dip. In addition, a large number of margin of the basin was between 70-350 cm, attesting to small-scale fractures that are roughly parallel to the major a low rate of relative lake-level rise and the persistence faults have been observed. These faults form boundaries of a shallow water depositional environment that pass between the bedrock and the Neogene basin deposits. laterally into terrestrial environment for a long period. Slickenlines, almost vertical corrugation axes and chatter These facies data also show that the shoal-water delta and marks (Figures 7b and 7c), all indicating normal faulting, related environments formed the southern margin of the are observed on the fault planes of the southern basin basin for a very long time before maximum transgression margin faults, as in the northern basin-margin faults. drowned the basin (Figure 10). The lack of facies reflecting Lacustrine deposits formed in the hangingwall block deep lacustrine conditions also demonstrates the relatively onlap onto the fault planes, showing that normal faulting smooth and stable character of the basin floor, which commenced prior to the start of deposition. Moreover, enabled the southern margin of the basin to reach as far these faults maintained their activity during deposition, as the Anamas Mountains during the period of maximum providing the accommodation space in the south of the transgression of the lake water level. basin. 3.2.2. Shoal-water delta deposits The shoal-water delta deposits identified in the south This facies association, consisting mainly of sandstones of the basin were deposited approximately 10 km north of and conglomerates, gradually overlies the lacustrine the Balcı Fault, which forms the structural border of the carbonates and forms thickening and coarsening-upward southern basin margin (Figure 1b). This demonstrates successions (Figure 9). These deposits show mound- that, during the delta sedimentation, the southern basin shaped and lenticular geometry, which are 70-350 cm in margin was situated in the vicinity of Madenli and Bahtiyar thickness and approximately 50-100 m in lateral extent 525
  11. ILGAR et al. / Turkish J Earth Sci NW SE c delta bottomset PPS a b PCS PPS 2 4 CRCL PPS d PPS F g. 6c F g. 6d CRCL CRCL 1 F g. 6e e PCS 3 CRCL thickness (m) PPS PCS 0 PPS- planar parallel strat ficat on mud sand gravel PCS- planar cross strat ficat on CRCL- current r pple cross lam nat on Figure 6. Outcrop details of some delta bottomset units in the Yarıkkaya Formation. (a) Longitudinal outcrop section showing subhorizontal bottomset sandstones and fine pebble conglomerates. (b) Detailed sedimentological log of the bottomset deposits. (c, d, e) Close-up views of the sedimentary structures observed in this log. T h e locality of Figure 6 is shown in Figure 3a. T h e localities of the pictures c-e are shown in Figure 6b. (Figures 8b and 9). Lateral thinning and fining of the (Leeder et al., 1988; Postma, 1990), or a mouth bar-type sandstone beds are observed in shore parallel section. The fan delta (Dunne and Hempton, 1984; Wood and Ethridge, sandstone beds are slightly inclined (
  12. ILGAR et al. / Turkish J Earth Sci NE SW b Bedrock Anamas-Aksek autochthon sl ck enl n es & co rrug at on axes c cha tter ma Neogene bas n-fill rks a Figure 7. (a) The Balcı Fault, forming the boundary between the structural southern margin of the Yalvaç Basin, separating the Anamas- Akseki autochthon from the onlapping Neogene basin deposits. Inset photo shows the fault plane inside the trench (b, c) Slickenlines, almost vertical corrugation axes, and chatter marks on fault planes indicating normal faulting. The pictures are from locality 10 in Figure 1b. “compensational” manner (Ilgar and Nemec, 2005; Ilgar, streams, wave-ripple cross-lamination indicates reworking 2015), creates the delta complex (Figure 8b). The lateral by wave action while current-ripple cross-lamination or vertical displacement of each shoal-water delta within results from weaker frictional effluents (Ilgar and Nemec, the complex is determined to see whether the relative lake 2005; Leszczyński and Nemec, 2014; Ilgar, 2015). The well- level is at stillstand or increasing. During a period when sorted granule to fine pebble conglomerates seen in the the lake level is relatively stillstand, the deltas may prograde uppermost parts of the mouth-bar packages are interpreted basinwards and also migrate laterally to fill the available as beach deposits (Bluck, 1967, 1999), initially deposited accommodation space. When a relative rise in lake level during stream floods, then reworked by waves following the occurs, individual shoal-water deltas tend to stack upon one abandonment of the mouth bar (Ilgar, 2015). another (Ilgar and Nemec, 2005). 3.2.2.2. Distributary-channel facies 3.2.2.1. Mouth-bar facies Description: These fluvial channel-fill conglomerates Description: These deposits consist of sheet-like beds of erosionally overlie the uppermost axial element of the sandstone and conglomerate and show gently mound-shaped mouth-bar sandstones (Figures 8b and 9). This mainly depositional geometry in shore-parallel section (Figure 8b). single palaeochannel deposits are 50-150 cm thick and up to Mouth-bar packages, 70-300 cm thick and stacked upon one 5 m wide. Such distributary channel deposits mainly consist another with lateral offset, are separated by erosional surfaces of granule- and fine to medium pebble-conglomerates, and grain size differences. The gently inclined sandstone forming a fining upward succession (Figure 9). The gravel beds, 5-30 cm thick, consist of medium to very coarse sand. clasts, mainly rounded and spherical, form moderately They are mainly planar parallel stratified (Figure 9) with sorted, clast-supported textures. The isolated coarse pebble subordinate wave- and current-ripple cross-lamination on conglomerates often line the erosional bases of the channel- the upper surface of the beds. These coarsening-upwards fills. These deposits show subhorizontal or low-angle planar sandy sequences are interfingered with or capped by well- cross-stratification (Figure 9). sorted granule to fine pebble conglomerates (Figure 9). Interpretation: The coarse pebble gravel layers on the Interpretation: These sandstone packages are interpreted erosional bases are typical of channel-floor lag deposits as mouth bars and have been emplaced as a result of stream (Miall, 1985; Nemec and Postma, 1993). The subhorizontal frictional effluent of sediments supplied by rivers (Wright, or low-angle planar cross-stratified conglomerates represent 1977). Planar-parallel stratified sandstones are thought the bar deposits of relatively small fluvial distributaries to be the product of the frictional outflow of flooding (Wright, 1977). 527
  13. ILGAR et al. / Turkish J Earth Sci SW NE re 9 F gu on n fluv al sect channe l and flo F gur tuff odpla n e 8b depos t s lacus tr ne shoa carbo shor l-wa nates efac ter d e-fo elta sho resh lacu al-w o re str ater ne c delt arb a ona tes a W b E lacustr ne carbonates shoal-water delta complex shoreface - foreshore mouth-bar DC delta complex shoal-water d str butary channel mouth-bar DC mouth-bar lacustr ne carbonates Figure 8. (a) Panoramic view showing the four main facies associations identified in the southern sector of the Yalvaç Basin. (b) Field photograph and labelled interpretation (below) of the lower part of the sequence shown in (a) providing details of an identified shoal- water delta complex displaying mounded and lenticular geometry. Each small-scale shoal-water delta comprises distributary channel and mouth bar facies. Note geologist (circled) to provide scale. The pictures are from locality 11 in Figure 1b. 4. Discussion succession shows that abrupt deepening or shallowing of In Miocene time, a monotonous succession of alternating the basin did not occurred and lake water depth remained marls and thin-bedded limestones began to be deposited relatively constant during this period. The overlying in shoreface and offshore-transition environments of algal limestones, up to 32 m thick, represent stromatolite the lacustrine Yalvaç Basin (Figure 11a). This carbonate boundstone facies (LMF11) and indicate deposition 528
  14. ILGAR et al. / Turkish J Earth Sci carbonates 18 38 58 78 17 shoreface - foreshor 37 57 77 16 36 56 76 15 35 55 75 14 34 54 74 13 33 53 73 shoal-water delta shoal-water delta 12 32 52 72 11 31 51 71 mouth-bar 10 30 50 70 9 29 49 69 mouth-bar carbonates 8 28 48 68 87 shoal-water delta 7 27 67 47 86 6 26 46 66 85 mouth-bar 5 25 45 65 84 4 24 44 64 83 3 23 43 63 82 abandoned channel-fills sandy carbonates are 2 shoreface - foreshor 22 42 62 81 tuff thickness (m) 21 41 61 80 carbonates 0 20 40 60 79 Mud Sand Gravel 19 39 59 78 Figure 9. Sedimentological log showing the main facies and facies associations identified within part of the Yarıkkaya Formation in the Bahtiyar section, southern sector of the Yalvaç Basin (see the section line in Figure 8a). 529
  15. ILGAR et al. / Turkish J Earth Sci N S Ya F. ı F. rık Yarıkkaya Format on nlı lc k da ay Ba m aF Ku . Sultandağları Anamas-Aksek sequence Beyşeh r-Hoyran Nappes autochthon Figure 10. Block-diagram showing our preferred palaeogeographic model that involves contemporaneous development of Gilbert-type and shoal-water deltas on opposed margins of the Yalvaç Basin during the Miocene-Pliocene time-interval. (See text and Fig. 11 for details). in shoreline and near-shore environments within a interactions between the prevailing fluvial regime with hypersaline lacustrine basin (Clausing, 1990). its sediment load, and the physico-chemical regimes This carbonate-dominated sedimentation lasted operating within the receiving basin, which are defined until the onset of deltaic deposition on the northern by shape, size, bathymetry, and internal dynamics of the and southern margins of the Yalvaç Basin (Figure 10). basin (Elliot, 1986; Postma, 1990). If ambient water-level Especially in the north, carbonate deposition was abruptly change is neglected, the sedimentary characteristics of a halted by the intense input of coarse clastic sediments to delta system prograding within a low-energy basin vary the basin. Accompanying this change, two different delta with the type of alluvial feeder, the relief of the basin at types, in terms of grain size, sedimentary facies, spatial the river mouth and the nature and magnitude of the distribution, and architecture, were deposited on the processes operating near the river mouth (Nemec, 1990; northern and southern margins of the basin, respectively Postma, 1995). (Figure 10). The abrupt arrival of deltaic clastic sequences The coarse-grained Gilbert-type fan delta deposits on top of lacustrine carbonates represents an abrupt identified on the northern margin of the Yalvaç Basin have change in the basin dynamics. an extensive spatial distribution and include a relatively Such a change in basin character reflects control by thick (up to 125 m) delta foreset package. These delta tectonics, basin floor-basin margin bathymetric contrast, deposits accumulated just in front of the steep scarp of the eustatic sea level changes, geology of the drainage area, Yarıkkaya normal fault. The thickness of the delta foreset climate, and time (Postma, 1990). These agents determine sequence broadly reflects the basin bathymetry during the the sedimentary characteristics of the deposits by period when the delta was formed. In this case, it appears controlling sediment supply, basin subsidence, and relative that the basin floor was undulatory and the bathymetry water level changes. Coarse-grained delta depositional increased from the faulted margin towards the basin systems are influenced by synsedimentary tectonism on interior. a variety of scales (Leeder and Gawthorpe, 1987). While Despite the thickness of the delta foreset sequence footwall uplift at the basin margin due to faulting generates indicating relatively deep accommodation space, facies new sediment sources, subsidence in the basin creates recording offshore environments are not observed at accommodation space (Gawthorpe and Colella, 1990). the base of the delta bottomset deposits. Instead, the Accordingly, the type and intensity of tectonism play delta foreset sequence sharply overlies algal limestones, key roles in controlling the sediment sources, location of indicating a shallow lacustrine environment, rather than deltas, sedimentary facies, and depositional architecture an offshore setting. It is interpreted that the deposition (Leeder et al., 1988; Gawthorpe and Colella, 1990; of Gilbert-type delta on algal limestones shows an abrupt Gawthorpe and Leeder, 2000). The development of the deepening at the northern basin margin, and also an types of delta formed on basin margins is dependent on abundant coarse clastic sediment supply to the basin. This 530
  16. ILGAR et al. / Turkish J Earth Sci a carbonate depos t on n shallow lacustr ne Yalvaç Bas n N S algal l mestones F. Ya lcı rık Yarıkkaya Format on Ba ayk Sultandağları Anamas-Aksek aF sequence Beyşeh r-Hoyran Nappes autochthon . b act v ty of the bas n’s northern faults N S shorel ne sh ft ng & forced regress on Ya tecton cally controlled deepen ng rık F. F. Yarıkkaya Format on k lcı ay nlı Ba aF da . m Ku c development of the G lbert-type and shoal-water delta N S G lbert-type delta shoal-water delta Ya r F. ıkk . Yarıkkaya Format on lı F lcı ay Ba an aF md . Ku d rec procal tecton c movement on e ther s de of the bas n N G lbert-type delta progradat on and th cken ng shoal-water delta aggradat on due to small rate of fault movement S due to ncreased accommodat on space Ya . lı F r F. ıkk Yarıkkaya Format on an lcı ay md Ba aF Ku . e max mum transgress on and redepos t on of lacustr ne carbonates N S Ya . lı F r F. ıkk Yarıkkaya Format on an lcı ay md Ba aF Ku . Figure 11. Schematic interpretations of successive stages of delta deposition related to tectonic activity on opposed margins of the Yalvaç Basin during the Miocene Pliocene time interval. (Not to scale). situation indicates a sudden breakdown in the ongoing observed at the basin margin and the sudden and abundant sedimentation balance under these relatively shallow and supply of coarse clastic sediment to the basin. quiet environmental conditions. While deepening of the It is highly probable that the depositional depth basin, allowing deposition of Gilbert-type delta clastics on discrepancy between the algal limestones and the abruptly algal limestones, could occur through climatically induced succeeding deltaic clastics results from seismic activity lake level rise, this fails to explain the forced regression on the Yarıkkaya Fault (Figure 11b) and possibly also the 531
  17. ILGAR et al. / Turkish J Earth Sci Çakırçal and Sağır faults. This activity both deepened Lacustrine basins are hydrologically closed systems. the basin margin just prior to delta deposition and also Thus, asymmetrical and reciprocal changes in the uplifted the footwall, causing a fall in the local base level behaviour of different margins are characteristics for such and exposing extensive areas on the basin margin to fluvial fault-controlled lake-basins, including the amount and erosion (Figure 11b). Hence, fault activity on the northern rate of local subsidence, ultimately depend on the activity margin not only increased the accommodation space in pattern of the border faults (Ilgar and Nemec, 2005). The the basin, but also created new and abundant sediment abrupt onset of shoal-water delta deposition on top of supply to this depositional area. Detritus derived from lacustrine carbonates on the southern basin margin is best these fault-uplifted areas was transported to the basin by attributed to tectonically controlled forced regression, and braided rivers and discharged across the steep scarp of the it is evident that activity of the marginal and internal faults Yarıkkaya Fault into the relatively deep basin, ultimately and related deepening in the northern sector of the basin creating a delta with Gilbert-type architecture (Figures 11c also directly affected the southern margin. Asymmetrical and 11d). subsidence of the basin floor shifted the water mass in Simultaneously, on the south side of the main basin, the lake towards the north, causing rapid shallowing, shoal-water delta deposits, predominantly sandstones and partial exhumation, and forced regression on the southern subordinate conglomerates were accumulating (Figure margin of the basin (Figure 11b). Accordingly, enhanced 10). These shoal-water delta complexes are noticeably fluvial supply of detrital materials from the adjacent fault- thin and spatially limited by comparison with the Gilbert- uplifted areas to the southern margin of the basin led type delta deposits (Figure 10). These shoal-water delta to the development of shoal-water deltas (Figure 11c). deposits formed on a short-lived margin of the southern Stacking of the small shoal-water deltas to form delta basin (Figures 10 and 11c), located approximately 10 km packages probably resulted from ongoing activity of the north of the Balcı Fault, the structural southern boundary southern marginal faults. Consequently, the formation of the main basin. Thus, it appears that, on this southern of the shoal-water delta complex on the southern margin margin of the basin, the palaeomorphology was not of the basin was ultimately a consequence of the seesaw- directly controlled by faults. Moreover, steep slopes due to like subsidence of the basin floor produced by reciprocal faulting were not developed on this margin. movements of the basin margin faults (Figure 11d). A When the probably meandering rivers carrying their similar mechanism has been invoked by Ilgar and Nemec mixed sediment loads entered this shallow southern basin- (2005), and Akıska and Varol (2020) from lacustrine sector with its gentle gradients, they were subjected to bed deposits within other Neogene basins elsewhere in Turkey. friction, which caused more rapid deceleration and lateral During episodes of rising lake level, both the Gilbert-type expansion of the sediment-laden flows (Wright, 1977), and shoal-water delta deposits were drowned and replaced consequently leading to the deposition of the sediments by lacustrine carbonates (Figure 11e). as shoal-water deltas. These features typically display Examples of Gilbert-type deltas and shoal-water deltas a maximum thickness broadly equivalent to the water forming simultaneously on different margins have been depth and a lateral extent proportionate to this thickness. described from a variety of basins (Dunne and Hempton, Over time, these relatively thin shoal-water deltas 1984; Sohn and Son, 2004). Gilbert-type deltas have accumulated with some lateral offset to form shoal-water been identified on the steep fault-controlled southern delta complexes. The lateral and vertical displacement of margin of Lake Hazar, a pull-apart basin on the Eastern the shoal-water deltas in the delta complex sand-body is Anatolian Fault. Rivers entering this basin along its long controlled by the stillstand or relative rise in local (lake-) axis have formed a shoal-water delta by dumping their water level. During stillstands, deltas migrate laterally alluvial load on the subhorizontal basin floor in Pliocene- but a rise in relative water level leads to formation of Pleistocene (Dunne and Hempton, 1984). In the Miocene superimposed delta-packages (Ilgar and Nemec, 2005). Pohang Basin (SE Korea), among the deltas accumulated Shoal-water deltas in the southern sector of the basin on a single fault plane, a Gilbert-type delta formed on have thicknesses ranging from 70 to 350 cm, presumably the margin where the hangingwall block is deeper, while reflecting a comparable relative water level rise, ultimately shoal-water delta was deposited on the shallow margin attributable to the subsidence of the basin floor or activity (Sohn and Son, 2004). of the southern boundary faults, and especially the Balcı Fault. Moreover, the limited thickness of these shoal- 5. Conclusion water deltas demonstrates that tectonic activity around The fluvio-lacustrine Yalvaç Basin opened during the early the southern margin of the basin was insufficient in Miocene as an intramontane molasse basin, sandwiched magnitude to create the large-scale deepening required to between the Sultandağları and Anamas Mountains in the permit Gilbert-type delta development. NE sector of the Isparta Angle. The northern and southern 532
  18. ILGAR et al. / Turkish J Earth Sci basin margins are formed by normal faults, and the differing delta sequence demonstrate that this shallow lacustrine styles and rates of activity on these fractures caused the basin margin was underlain by a relatively smooth basin floor. In to evolve as an asymmetric graben and controlled both the this southern sector, a north-flowing fluvial system carrying palaeomorphology and the sedimentation styles associated detritus into the shallow, low-gradient basin deposited much with each basin margin. of its bedload near the river mouth. Thus, the gently mound- In this study, detailed appraisal of the facies present and shaped depositional geometry observed here was developed their sedimentary architecture have demonstrated that two in shoal-water deltas, which, over time, stacked upon one different delta types, Gilbert-type and shoal-water, were another with lateral offset, forming a shoal-water delta active on either flank of the basin in a certain period during complex. The thickness of these delta packages, only 70-350 Mio-Pliocene times. The co-existence of these fluvio-deltaic cm, indicates that the relative lake level rise due to tectonic facies associations within a single, relatively small trough renders a study of the Yalvaç Basin of generic interest with subsidence or to climate-change was minimal. Tectonic respect to the respective roles of a range of depositional activity around the southern margin of the basin was controls such as syndepositional tectonism, basin depth and insufficient in magnitude to create the large-scale deepening morphology of the basin floor and nature of the hinterland required to permit Gilbert-type delta development. on either flank of the compound basin. In summary, this study demonstrates and also On the northern margin of the basin Gilbert-type delta contributes to how the asymmetrical subsidence causes to deposits accumulated immediately in front of the Yarıkkaya the transgression on one margin and forced regression on the normal fault, in about 125 m water-depth. The depositional other by shoreline shifting, which results in the development depth discrepancy between the delta-front and the of different delta facies association in the same basin. underlying algal limestones demonstrates abrupt deepening just prior to the deltaic sedimentation on this northern Acknowledgments basin margin. Activity on the Yarıkkaya normal fault both This study was carried out in the scope of “Geology and uplifted the footwall and deepened the adjacent basin floor, Geodynamic Evolution of the Sultandağları” project thus, giving rise to new sediment sources and increasing the conducted between 2015 and 2016 by the Department of accommodation space. Abundant coarse-grained detritus, Geological Research of the General Directorate of Mineral derived from the basin’s hinterland was discharged across Research and Exploration (MTA). Dr. Gönül Çulha the steep scarp of the Yarıkkaya Fault into the relatively deep (MTA) is gratefully acknowledged for the determination basin, leading to creation of a major delta with Gilbert-type architecture. of ostracod fauna and Banu Türkmen-Bozkurt (MTA) for On the other hand, the shoal-water delta complex the petrographic examinations of limestones. Special thanks deposited in the southern sector of the basin was less go to Dr. Mustafa Karabıyıkoğlu, Tolga Esirtge and Dr. Şule directly controlled by the adjacent Balcı Fault. Therefore, Gürboğa for a critical reading of the original manuscript. steep slopes due to faulting did not develop on this margin. Prof. Dr. Gilbert Kelling and other anonymous referees The absence of facies reflecting deep-water conditions are much appreciated. 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