Provenance analysis of the cretaceous clastic sequences of the central pontides

Lower Cretaceous sediments crop out over a large area of about 400 km by 200 km in the Central Pontides. They are represented by shelf sediments and deep marine turbiditic fan deposits. Lower Cretaceous sediments have more than 2000 m thickness. The provenance of Lower Cretaceous sediments were studied through paleocurrent measurements, U-Pb detrital zircon dating and geochemical analysis of detrital rutile. Paleocurrent measurements clearly show that in the eastern part of the basin, the source area was in the north. Although, it is scattered in the west of the basin, the main paleoflow directions also show northerly sources. 1348 detrital zircons from eleven samples from the turbiditic sandstones and two samples from the shelf sandstones were analyzed using LA-ICP-MS. Detrital zircon age distributions show three main peaks at Permian-Carboniferous (26%), Paleoproterozoic (17%; 1700-2200 Ma) and Archean (15%; 2750-3000 Ma); and subsidiary peaks at late-Neoproterozoic-Cambrian (9%; 525-650 Ma), Triassic (6%), late Mesoproterozoic-early Neoproterozoic (4%; 875-1100 Ma) and Silurian (2%). Four samples are from the western part (two samples from shelf sediments), four samples from the central part and five samples from the eastern part of the Lower Cretaceous basin. The age distribution of detrital zircons from Lower Cretaceous deposits show different pattern in the western, central and eastern parts. Mesoarchean, Paleoproterozoic, Permian and less prominently Triassic zircons are dominant in the eastern part. In central part Paleoproterozoic, early Neoproterozoic-late Mesoproterozoic, Triassic, Permian, Carboniferous, and Silurian zircons constitute the main populations. Archean and Paleoproterozoic rocks are unknown from the Pontides, whereas such rocks crop out widely in the Ukrainian Shield. The detrital zircon results indicate that the major source of the turbidites in the eastern and central parts of the basin was the East European Platform, which has Paleoproterozoic-Archean basement. Rare earth element abundances of detrital zircons, detrital rutile geochemistry and paleocurrent directions are consistent with the detrital zircon ages and imply a source area to the north of the Black Sea for the central and eastern part of the basin. This demonstrates that the Black Sea was not opened during the Early Cretaceous. In contrast to the eastern and the central parts of the basin, Carboniferous and Cambrian-Late Neoproterozoic zircons constitute the main population in the western part, suggesting derivation from local sources from the Istanbul Zone and the Moesian Platform. Derivation mainly from the granitic basement of East European Platform indicates that the eastern and central part of the Lower Cretaceous basin might have better reservoir potential due to petroleum geology. Although, Early Cretaceous turbidites were deposited in a fore-arc position above the active subduction zone, yet unlike typical fore-arc basin strata, the Lower Cretaceous sandstones has less than 1% of syn-depositional detrital zircons and is devoid of volcanic rock fragments. Such minor amounts of syn-depositional detrital zircons indicates that the syn-sedimentary magmatic arc, which is characteristic component of normal forearc system, is missing. This may relate to flat subduction, which may have caused this volcanic gap and produced regional uplift. The regional uplift would have resulted in intense sediment supply to the Lower Cretaceous basin. Late Cretaceous (Campanian-Maastrichtian) Gürsökü Formation, which crops out in the eastern part of the Central Pontides, is the first turbiditic sequence deposited after the opening of the Black Sea. It has a thickness about 2000 m. To deduce the provenance of this turbiditic sequence and to reveal the effects of the new tectonic setting on the provenance, we measured paleocurrents from Late Cretaceous sediments and sampled sandstones for U-Pb detrital zircon dating. We obtained 519 U-Pb detrital zircon ages from seven sandstone samples of Gürsökü Formation. 46% of the all detrital zircons is Mesozoic, 37% is Paleozoic, 11% is Neoproterozoic, 1% is Mesoproterozoic, 4% is Paleoproterozoic, and 1% is Archean. Within seven samples three samples yielded dominant peaks at Late Cretaceous (ca. 79 Ma) and a sample yielded prominent peak at Middle Jurassic (ca. 165 Ma). The detrital zircon ages of other three samples are scattered in the Paleozoic times giving prominent peaks at Carboniferous, Permian and less prominent peaks at Triassic and Silurian. Detrital zircon age distribution of Gürsökü Formation indicates that the basement rocks of the Pontides and Late Cretaceous arc volcanics, which was formed by the opening of the Black Sea, were the main sources for the Late Cretaceous basin. Archean and Paleoproterozoic zircons only represents the 5% of the all detrital zircons and scatter without an important peak. They may be recycled from older rocks. The negligible Archean and Paleoproterozoic detrital zircons in the Gürsökü Formation in contrast to the Lower Cretaceous turbidites show that East European Platform was not in a source position during Late Cretaceous. This is a reflection of the difference of tectonic setting triggered by the opening of the Black Sea. To reveal the thermal evolution of the western part of the Central Pontides, 18 samples from different units with sedimentation ages between Carboniferous to Eocene were collected from the area between Zonguldak and Cide. We performed apatite fission track analysis on 14 samples; and apatite (U-Th)/He method on 11 samples. Apatite fission track analysis of 14 samples yielded ages ranging between 46 Ma to 220 Ma and indicate that this region was uplifted gradually by the Late-Triassic-Early Jurassic, Albian and Paleocene-Middle Eocene times. This shows that this region still preserves thermochronological evidence of older orogenies, including the Karakaya Orogeny and Albian subduction events. Big differences of apatite fission track (AFT) ages in short distance points out that horst-graben structure shaped the region. (U-Th)/He dating on the apatites from samples, which are also used for AFT analysis, show evolutional history of the region in lower temperatures. Younger AFT ages with apatite (U-Th)/He ages indicate that the region was rapidly uplifted after Paleocene and two clusters have been distinguished as; Paleocene-Middle Eocene and Oligocene-Early Miocene which could be related to collision of the Pontides with the Anatolide-Tauride Block along the İzmir-Ankara-Erzincan Suture Zone.