Browsing by Author "Helvaci, Cahit"

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Evaporitic Sedimentation in the Southeastern Anatolian Foreland Basin: New Insights on the Neotethys Closure
(Elsevier, 2018) Yesilova, Cetin; Helvaci, Cahit; Carrillo, Emilio
We integrate stratigraphic, petrographic and geochemical analysis of subsurface data (wells) together with field surveys to study the sedimentation of a marginal Miocene sub-basin of the Southeastern Anatolian Foreland Basin (SEAFB; SE Turkey). This sub-basin, located in the Batman-Siirt region, is characterized by the existence of evaporites (carbonates, sulphates and chlorides) and alluvial detritus which were divided in the following five lithostratigraphic members, from older to younger: Lower and Upper Yaprlar; and Lower, Middle and Upper Sulha. These members deposited in an epicontinental mudflat during the Early Miocene. Both the bromine content and the sulphur and oxygen isotope composition (delta S-34(v-CDT) and delta O-18(v-SMOW)) of halite and sulphates samples, respectively, also suggest a marine origin of the precipitation brines. However, influence of geothermal fluids and dissolution-and-re-precipitation of evaporites from uplifted areas in these brines, such as the Early Miocene members and/or Triassic units, is interpreted. Comparing and integrating our results with data documented in previous works, it is here recognized that the depositional model of the studied sub-basin differs from that which explain the coeval sedimentation of units situated in the western part of the SEAFB. Moreover, our model shows some depositional and paleoenvironmental similarities with Miocene evaporites located in the Mesopotamian Foreland Basin. This work provides valuable insights on the Middle Miocene Salinity Crisis which is related to the evolution of the Neotethys closure. (C) 2018 Elsevier B.V. All rights reserved.
Isotopic Evidence for a Transition From Subduction To Slab-Tear Related Volcanism in Western Anatolia, Turkey
(Elsevier, 2014) Karaoglu, Ozgur; Helvaci, Cahit
Volcanic rocks in western Turkey show age progressive magmatism migrating from northeast to southwest that reflects a southward shift of the Aegean subduction zone during the Miocene. Slab segmentation during this period of trench-roll back is thought to have imposed source region heterogeneity trending northwest to southeast. In this study, we present new Sr, Nd, Pb and 0 isotopic analyses from the Miocene volcanic rocks of the Usak-Gure basin and compare these to previously published data. The data demonstrate a change from subduction-related sources around the Menderes Core Complex to more asthenospheric sources in the Afyon region. Isotopic compositions (Sr-Nd-Pb) of volcanic rocks from the Demirci and Selendi basins to the west and the Afyon volcanic area to the east indicate minimal upper crustal contamination. The most primitive lavas also reveal increasing K contents from west (the NE-SW-trending basins) to east (Afyon region). It is suggested that the composition of the western Anatolian volcanic rocks change from orogenic (with lithospheric mantle sources) associated to denudation of the Menderes Massif Core Complex (MMCC) to anorogenic (with asthenospheric mantle sources) in the vicinity of the Kirka-Afyon-Isparta (KAI) volcanic province with time, from Early Miocene to Quaternary. There is no asthenospheric contribution during the late Miocene onwards in the eastern margin of the MMCC, while the asthenospheric upwelling occurred only in a small area beneath the exhuming core complex. We interpret the U Usak-Gure basin to reflect a structural boundary showing a transition from a subduction-influenced metasomatized mantle source to asthenospheric mantle source volcanism driven by slab-tearing between the Hellenic and Cyprus slab segments. The Usak-Mugla Transfer Zone (UMTZ) most likely corresponds to slab-tear related westernmost faults that were induced by initiation of slab segmentation processes following the late Miocene (circa 11 Ma), and possibly since the Early Miocene. (C) 2014 Elsevier B.V. All rights reserved.
Petrographic Study and Geochemical Investigation of the Evaporites Associated With the Germik Formation (Siirt Basin, Turkey)
(Springer, 2017) Yesilova, Pelin Gungor; Helvaci, Cahit
Evaporites of the Oligocene Germik Formation are represented by massive, laminated, laminated-banded, banded, nodular and/or brecciated nodular and nodular-banded lithofacies in the Siirt-Kurtalan area. Observations of massive and laminated-banded gypsums with undulated-shaped stromatolitic algal mats, corrugated folded structures and clay-carbonate and interlayered marl in laminated gypsum levels are very important in terms of their mechanisms of occurrences. Petrographic and mineralogic examinations of secondary gypsums in the Germik Formation generally display alabastrine and porphyroblastic textures with corroded anhydrite. Structures as satin-spar, chickenwire, enterolithic and minerals as dolomite, calcite and celestite are also observed. Geochemical evaluations of gypsum and anhydrite samples belong to the lithofacies have provided an approach of their original formation and understandings the environmental impacts (such as pH, paleotemperature, paleosalinity, surface-groundwater activity and bacterial activity). Therefore, a number of analyses were conducted on the different samples for major and trace elements and the gypsum-anhydrite samples were analyzed for isotope compositions. Trace elements of these evaporites were found to be in the range of the geochemical values of the depositional environment of shallow marine evaporites. These values show also occasional influx of terrestrial waters to the environment. Moreover, the differences in the isotopic values of the lithofacies identified by the stable isotopic studies indicate the effects of temperature, salinity, biogenic activity and diagenetic processes for the formation of evaporites, under the influence of climate. Geochemical evolution of gypsum and anhydrite lithofacies of the Germik Formation leads up to muddy evaporites indicating that they were deposited in the coastal sabkha or shallow marine environments within effectively developed paleoclimatic conditions, tectonic activities, diagenetic processes and depositional system.