CRETACEOUS RED PELAGIC CARBONATES OF NORTHERN TURKEY - THEIR PLACE INTHE OPENING HISTORY OF THE BLACK-SEA

Northern Turkey forms a part of the Rhodope-Pontide Fragment, one of T urkey's main tectono-stratigraphic units. Cretaceous geology of this r egion is characterized by the presence of a series of horsts, grabens and tilted and rotated fault blocks buried beneath the Upper Cretaceou s to Lower Tertiary volcanic material-bearing turbiditic sediments. Th ese structures and sedimentary and igneous rocks represent the most co mplete record of the opening of the oceanic Black Sea back-arc basin w hich began forming in the Aptian-Albian behind a continental extension al margin magmatic arc. This was followed by a fault-controlled syn-ri ft sedimentation and subsidence until the late Cenomanain when sea-flo or spreading in the basin and thermally-induced subsidence of the basi n margins started. Rift-drift transition in the formation of the Black Sea was marked by a drastic change in sedimentation from the depositi on of dark-coloured and organic-rich shales with limestone interbeds t o the accumulation of red pelagic carbonates and marls. Facies analyse s of these sediments reveal that the Black Sea was restricted during i ts advanced rift stage from free interchange with the Neo-Tethys to th e south and received a large amount of organic debris. Immediately aft er the onset of spreading in the late Cenomanian, the euxinic conditio ns disappeared and the water column above the southern margin of this juvenile ocean became well stirred as a gentle basinward tilting of th is margin took place. This tilting caused a wide transgression accross the margin which eliminated most of the terrigenous sediment sources, thus providing a suitable location for the deposition of the red pela gic carbonates. Such syn-breakup sequences in migratory island arcs ma y be important guides to establishing the onset of sea-floor spreading in back-arc basins, whose magnetic record is commonly poor and basin floor basements are hidden under thick sedimentary blankets.