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


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.

Keywords: Gilbert-type delta, shoal-water delta, asymmetric graben, fault-margin control

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