Faculté des sciences

The high-resolution stratigraphic architecture and evolution of the Burdigalian carbonate-siliciclastic sedimentary systems of the Mut Basin, Turkey

Bassant, Philip ; Strasser, Andreas (Dir.)

Thèse de doctorat : Université de Fribourg, 1999 ; no 1256.

The factors influencing the production and deposition of carbonate sediments are known. These are namely accommodation variations (eustasy and tectonics), siliciclastic sediment input, environmental changes (temperature, salinity, trophic level), nature of the producing ecologies, and the hydrodynamic regime. However, the manner in which these factors integrate through time to produce the... More

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    Summary
    The factors influencing the production and deposition of carbonate sediments are known. These are namely accommodation variations (eustasy and tectonics), siliciclastic sediment input, environmental changes (temperature, salinity, trophic level), nature of the producing ecologies, and the hydrodynamic regime. However, the manner in which these factors integrate through time to produce the diversity of stratigraphic architectures that we see is not well understood. This study addresses this question by describing the organisation of shallow marine carbonate-mixed platforms in three environmentally distinct contemporaneous settings across a basin. Chronostratigraphic correlations are made between the different sites, and this permits the comparison of coeval depositional facies and geometries, and the understanding of the factors that create the different stratigraphic architectures observed. The study interval is the Lower Miocene (Burdigalian) of the Mut Basin in Southern Turkey. This area is chosen because it presents extraordinary large-scale 3D outcrops showing depositional geometries, and such outcrops are found throughout the basin. The Mut Basin opened during the Oligocene and was partially filled by syn-extensional continental sediments. Post-extensional basin-wide thermal subsidence then occurred during the Lower Miocene, and at the same time rapid marine transgression flooded a complex relict topography, depositing shallow platform carbonates in a variety of settings, accompanied by some localised siliciclastic input. The Miocene stratigraphy of two areas was mapped out in the field. This allowed three Burdigalian margin transects to be chosen for detailed study (Dibekli, Pirinç and Alahan). Observations of the stratal geometries and the facies were combined into a high resolution sequence stratigraphic framework of retrograding/prograding sedimentary cycles in order to construct stratigraphic cross-sections of each transect. 3346m of section were logged, 400 thin sections were studied, and 145 biostratigraphic samples were analysed for nannoplankton dates (C.Müller). 10 samples were dated for planktonic foraminifers (R.Wernli). The three transects were then correlated: large-scale correlations were made by using the biostratigraphic dating, then high-resolution sequence stratigraphy and the construction of the relative sealevel curve for each site permitted correlation beyond the resolution of the biostratigraphy. The first transect (Dibekli, Silifke region) has a steep asymmetric basement graben topography, forming a narrow strait, linking the Mut Basin to the Mediterranean, where strong tidal currents are generated. Siliciclastic input is low and localised. 80m of cross-bedded bioclastic sands are deposited in a tidal regime at the base. Subsequently carbonate platforms backstep against the shallow-dipping northern flank, while platforms only develop on the steep southern flank when a firm, wide shallowmarine area is provided by a fan-delta or the shallowdipping top shoulder of the foot-wall. The energy of the environment decreases with increased flooding of the strait area. The second transect (Pirinç, Mut region) is the open northern basin margin, and shows a complete platform-to-basin transition. An isolated platform complex develops during the initial flooding, which is drowned during a time of environmental stress, possibly associated with increased nutrient levels. The platform margin then retrogrades forming largescale clinoform geometries, and progrades, before a major sea-level fall provokes slumping collapse, followed by rebuilding of the platform margin as sealevel rises again. The third stratigraphic cross-section (Alahan, Mut area) is also on the northern basin margin. Here the siliciclastic input is high due to the presence of the palaeo-Goksu River bringing in dominantly fine grained sediment from an ophiolitic hinterland. The siliciclastic depocentre migrates landwards during transgressions, creating an ecological window allowing carbonates to develop in the distal part of the delta. Carbonate production shuts down during the progradation when siliciclastics return. Environmental pressure on carbonate growth is here coupled to the relative sea-level cycles that drive the transgressions and regressions. This motif is repeated at three different scales, and possesses a distinct hierarchical organisation. The stratigraphic architecture can be broken down into four scales of cycle, each with a characteristic amplitude and period. The very large-scale cycle, (period >3.4Ma, possibly non-periodic, amplitude of 200m) is attributed to a combination of glacio-eustasy and basinwide subsidence. Large-scale cycles (average period of <570Ka, amplitudes of 100-150m), and medium scale cycles (average period <100ka, amplitudes of 18-30m) are attributed to glacio-eustatic variations, possibly driven by astronomical eccentricity cycles. The small-scale cycles (average period 10-20ka, amplitudes 3-6m) are likely to be caused by other climatic changes, or autocyclic processes. The tidal deposits in Silifke and the isolated platforms in Mut are shown to have developed contemporaneously: the dramatic difference in architectural style is due principally to the very different hydrodynamic regime, brought about by the basin topography. The chronostratigraphic framework permits the recognition of condensation and omission surfaces in the basinal and platform settings, to identify basinwide variations in sedimentation patterns, and to evaluate the relative influence of tectonism, eustatism and the environment. The exceptional quality of the outcrops with its variety of environments, and its location at the Tethys margin make this a good candidate for a reference model for Burdigalian reef and platform architectures.