Stratigraphy and Geochronology of the Early Miocene Volcanic Units in the Bigadiç Borate Basin, Western Turkey

Authors: FUAT ERKÜL, CAHİT HELVACI, HASAN SÖZBİLİR

Abstract: Western Turkey has undergone an intense deformation which has given rise to two types of basins since the Late Oligocene: NE-trending and E-W-trending basins situated on the basement rocks of the Menderes Massif, the Sakarya Zone, the Lycian nappes and the Bornova Flysch Zone. One of the NE-trending basins, the Bigadiç borate basin, is located in the Bornova Flysch Zone - a 50-90-km-wide zone between the Sakarya Zone and the Menderes Massif. Lower Miocene successions are dominated by lacustrine, fluvial, evaporitic sedimentary rocks and numerous NE-trending volcanic centres which produced considerable amounts of lava and volcaniclastic detritus in the Bigadiç area. The Early Miocene basaltic to rhyolitic volcanism began 23.0 Ma and continued until 17.8 Ma in the area. The stratigraphic and geochronological data reveal that two volcanic episodes occurred, one before and one during lacustrine sedimentation. The products of these episodes are grouped into the Kocaiskan volcanites and the Bigadiç volcano-sedimentary succession, the two separated by an angular unconformity. The Kocaiskan volcanites comprise andesitic intrusions, lavas, pyroclastic deposits and extensive volcanogenic sedimentary rocks, and were mainly deposited under subaerial conditions, and underlie stratovolcanoes, the most prominent topographic highs. The Bigadiç volcano-sedimentary succession consists of volcanic and lacustrine units that were deposited in an ephemeral-lake environment. The volcanic units, comprising basaltic to rhyolitic lavas and volcaniclastic rocks, are intercalated with borate-bearing calcareous, siliceous and clayey sediments of the lacustrine units. Volcanic units in the Bigadiç area formed along NE-trending fractures that are part of a regional-scale fault zone, termed the 'İzmir-Balıkesir transfer zone'. This transfer zone, thought to have been active since the Late Cretaceous, has accommodated differential extension between the Cyclades and western Turkey. The zone was active in the Bigadiç borate basin during and after the Early Miocene, and the basin formed in a NE-trending zone of weakness under an extensional tectonic regime which caused the formation of coeval extensional detachment faults and the emplacement of syn-kinematic granites in surrounding areas.

Keywords: stratigraphy, geochronology, Early Miocene volcanism, Bigadiç borate basin, western Turkey

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