Fluvial sedimentation in a salt-controlled mini-basin: stratal patterns and facies assemblages, Sivas Basin, Turkey

Abstract

The Sivas Basin, located on the Central Anatolian Plateau in Turkey, is an elongate Oligo-Miocene basin that contains numerous salt-walled mini-basins. Through field analysis, including stratigraphic section logging, facies analysis and geological mapping, a detailed tectono-stratigraphic study of the Emirhan mini-basin and its 26km thick sediment fill has been undertaken. Three main palaeoenvironments are recognized - playa-lake, braided stream and lacustrine - each corresponds to a relatively long-lived depositional episode within a system that was dominated overall by the development of a distributive fluvial system. At local scale, this affects the geometry of the succession and influences facies distributions within preserved sequences. Sequences affected by wedge geometries are characterized by localized channelized sandstone bodies in the area of maximum subsidence and these pass laterally to floodplain mudstone towards the diaper; several internal unconformities are recognized. By contrast, sequences affected by hook geometries display narrow and steep drape-fold geometries with no evidence of lateral facies change and apparent conformity in the preserved succession. The sediment fill of the Emirhan mini-basin records the remobilization of diapir-derived detritus and the presence of evaporitic bodies interbedded within the mini-basin, implying the growth of salt walls expressed at the surface as palaeo-topographic highs. The mini-basin also records the signature of a regional change in stratigraphic assemblage, passing from playa-lake facies to large-scale highly amalgamated fluvial facies that represent progradation of the fluvial system. The initiation and evolution of this mini-basin involves a variety of local and regional controls. Local factors include: (i) salt withdrawal, which influenced the rate and style of subsidence and consequently temporal and spatial variation in the stratigraphic assemblage and the stratal response related to halokinesis; and (ii) salt inflation, which influenced the topographic expression of the diapirs and consequently the occurrence of diapir-derived detritus intercalated within the otherwise clastic-dominated succession.