Oligocene (?)-Lower Miocene Olistostromal Sedimentary Sequence
Structurally Above the Menderes Metamorphic Core Complex and Its
Tectonic Implications, Selendi Basin, Western Anatolia
Yalçın Ersoy1, Cahit Helvacı1, Erdin Bozkurt2, Fuat Erkül1 & Hasan Sözbilir1
1
Dokuz Eylül University, Engineering Faculty, Department of Geology,
TR35100 Bornova, İzmir, Turkey
2
Middle East Technical University, Department of Geological Engineering,
Tectonic Research Unit, TR06531 Ankara, Turkey
(E-mail: yalcin.ersoy@deu.edu.tr; cahit.helvaci@deu.edu.tr; erdin@metu.edu.tr;
fuat.erkul@deu.edu.tr; hasan.sozbilir@deu.edu.tr)
Western Turkey has been experiencing continental extensional deformation, fluvial-lacustrine
sedimentation and coeval widespread volcanism since the Late Oligocene–Early Miocene.
Extension is expressed by the development of (1) NE- and (2) EW-trending basins. Timing and
origin of, and relationships between, these basins are still controversial and different models have
been proposed. This study presents new field-based data from Selendi basin, one of the NE-trending
basins located between Demirci and Uşak-Güre basins.
The stratigraphy and tectonic features of the basin fill is intensely debated. Nevertheless, two
distinct volcano-sedimentary successions separated by an angular unconformity constitute the
basin-fill. The lower succession, known as the Hacıbekir Group, consists mainly of conglomerate,
sandstone, limestone and volcanic rock intercalations. The upper succession, the İnay Group,
comprises intercalation of conglomerate, sandstone, claystone, marl and limestone that interfinger
with intermediate to felsic volcanic rocks. The basement rocks comprise metamorphic rocks of the
Menderes Massif and Neo-Tethyan ophiolitic rocks. The origin and nature of the contact between
the basement and the basin-fill is debated. There are claims that the contact is a thrust fault while
others argued for a low-angle normal fault. According to the latter model, the basin-fill as a whole
was deposited in the depocenter on the main detachment fault.
The recent field campaign confirmed the existence of the low-angle normal fault and revealed, for
first time, that hanging-wall is composed of both the ophiolites and the Hacıbekir Group. The
kinematic indicators observed in the footwall mylonites, such as asymmetrical K-feldspar
porphyroclasts, mica fishes, recrystallized and elongated quartz crystals, are consistent with a topto-NE non-coaxial ductile deformation. The ophiolites are represented by highly altered
serpentinites and ultrabasic rocks, some of which occur as isolated bodies (olistoliths?) embedded
within mainly clastic sediments of the Hacıbekir Group. The Hacıbekir Group is composed of
conglomerates, turbiditic sandstones, mudstones and limestones, intercalated with bituminous
shales. The group is also intercalated with basic-intermediate volcanic rocks and is overlain
conformably by felsic volcanic rocks, all of which named as Eğreltidağ volcanic unit.
The structural and stratigraphical data together with the contact relationships documented in the
present study is speculated to suggest that the Hacıbekir Group might have been deposited above
the low-angle fault during the top-NNNE tectonic transport of the Neo-Tethyan ophiolitic rocks
onto the Menderes Massif prior to the onset of Miocene extension in southwest Turkey. This study
also documents and records, for the first time, the presence of Oligocene (?)-Lower Miocene
olistostromal sedimentary sequence in western Turkey.
Keywords: Early Miocene, olistostromal unit, Selendi Basin, Hacıbekir Group, western Turkey