NEOGENE STRATIGRAPHY OF THE İZMİR -OUTER- BAY ISLANDS

Abstract

The volcaniclastics, derived from calcalkaline acidic-intermediate volcanism in the region during late Early Miocene, and lacustrine deposits of Middle Miocene and alkaline volcanics are exposed on Uzun Ada , Hekim Adasi, Cicek Adasi and Karantina Adasi in the Outer Gulf of Izmir. Kocadag volcaniclastics derived from Kocadag volcanism, by extruding mainly calcalkaline andesitic-dacitic products during Late Early Miocene, represents the exposed oldest rock unit. The volcaniclastic succession extending in the north of Uzun Ada is composed of pyroclastics in ignimbrite and blocky ash flow facieses, and epiclastics in volcanic mass flow (lahar) facies. Foca tuff, represented by rhyolitic ignimbrites, originated from an area around Foca and moving to an area around Uzun Ada, emplaced onto the Kocadag volcaniclastics in two main explosive stages. The Degirmentepe Member alluvial deposits composed of coarse volcanic detritus were deposited during a inactive period between the explosion stages. A K/Ar age of 16.0 Ma was obtained from a rhyolite dome, which shows lateral relationship with the correlant ignimbrites in Foca Peninsula, and so it is considered that Foca tuff emplaced onto the region at the end of late Early Miocene. Lacustrine-dominated Middle Miocene succession, which overlies the Foca tuff unconformably, differentiated as the Urla group. Urla group consists of alluvial Besiktepe Formation, the Pirnalli island volcaniclastics, which is composed of sublacustrine volcanic density-flow deposits and felsic ignimbrites, Hekim Adasi basalt comprising basic volcanics and lacustrine Urla limestone, respectively from bottom to top. Besiktepe Formation only exposed on Uzun Ada, overlies the Foca tuff with an unconformity indicating a basin margin deposition during the Middle Miocene. Pirnalli island volcaniclastic succession, which its lower boundary does not expose within the area on Hekim Adasi and Cicek Adalari, is mainly composed of epiclastics deposited by the dynamics of sublacustrine gravity flow and includes trachytic ignimbrite layers in various welding degrees. The main lithologic components of Pirnalli island volcaniclastics derived from alkaline trachytic volcanism, which was active during the Middle Miocene in Mentes Yarimadasi (peninsula). The eruption center of Hekim Adasi basalt intruding as a sill to the bottom of Urla limestone succession is on Hekim Adasi. Along the upper contact of the basalt intruding backshore deposits at the bottom of Urla limestone, the peperites occurred reflecting the interaction between molten lava and unconsolidated sediment. A K/Ar age of 14,8±0,8 Ma is obtained from the basic lavas called as trachybasalt and basaltic trachyandesite according to the major element composition. Hekim Adasi basalt, which exposed at the center of Foca Depression, can be correlated with both the Ilipinar basalt in Foca Yarimadasi the Ovacik basalt in Urla basin with respect to chrono- and lithostratigraphy. Urla limestone succession, which transgressively overlies Hekim Adasi basalt, begins with backshore mudstones, continues with foreshore deposits comprising stromatolitic oncoids and algal bioclasts and lasts with cherty limestones.