New age data from the tectonostratigraphic units of the Istranca "Massif" in NW Turkey: a correlation with SE Bulgaria

The Istranca Crystalline Complex in NW Anatolia and SE Bulgaria includes structural units that differ in lithostratigraphy, metamorphism, age and structural position. They are collectively named as the "Istranca nappes" comprising from bottom to top the Sarpdere, Mahyadag. and Dogankoy Nappes. The Sarpdere Nappe consists of Lower Triassic arkosic metasandstones with slate interlayers, followed by Middle to Upper Triassic carbonates and an alternation of Upper Triassic clastics and carbonates. The Mahyada. g Nappe comprises a low-grade metamorphic Late Paleozoic-Triassic carbonate-siliciclastic sedimentary succession. The Dogankoy Nappe includes Precambrian?-Paleozoic metasediments, intruded by Late Carboniferous-Early Permian calc-alkaline granitoids. Its Triassic cover comprises metaclastics and metacarbonates. The Istranca nappes were juxtaposed at the end of the Triassic and transgressively covered by Lower Jurassic coarse clastics, followed above by Middle to Late Jurassic carbonates, black shales and carbonate-siliciclastic sedimentary succession. The phosphate concretions in black shales yielded radiolarian assemblages indicating Late Bajocian-Early Bathonian, Early Bathonian and Early Kimmeridgian ages. These nappes and their Jurassic cover are unconformably overlain by the Cenomanian-Santonian volcano-sedimentary successions intruded by Santonian-Campanian Derekoy-Demirkoy intrusive suite. The preliminary data suggest that the Variscan basements of the Mahyadag and Sarpdere Nappes were juxtaposed prior to the Triassic and overridden by the Dogankoy Nappe of possible Rhodopean origin from S to N during the Cimmerian compressional events