Sedimentology of Jurassic pelagic carbonate platforms in the Ankara region (Central Anatolia-Turkey)

Abstract

The Jurassic-Lower Cretaceous successions cropping out in three different parts of Central Anatolia (Turkey) were deposited in local basins developed under tectonic influence from the Hettangian to the Oxfordian. Pelagic carbonate platforms (PCPs) were formed in the region during two separate phases in which synsedimentary faulting was active in the basins. Prior to the first tectonic phase, fluvial and tidal sediments, Hettangian-Sinemurian in age, were deposited unconformably on the low-grade metamorphic substrate of the Karakaya Complex (Upper Triassic), with Sinemurian crinoidal limestones above. At the end of the Sinemurian, the PCPs were developed as a result of fragmentation of shallow-marine platforms by normal faults. Across hanging-wall basins and footwall-blocks, the PCP facies: pelagic/hemipelagic nodular and marly Ammonitico Rosso facies, brachiopod-bearing limestone facies with hardgrounds, red fossiliferous mudstone, and calcareous Ammonitico Rosso facies containing blocks, were deposited with marked lateral thickness variations. The platform, which gradually became shallower in relation to the clastic sediments (180 m thick) filling the basin, was fragmented again at the end of the Bathonian. The opening of fractures in the platform is documented by neptunian dykes developed within the clastic facies, filled with Callovian ammonite-bearing mudstone. During the Callovian-Oxfordian, widespread ammonite-bearing red marl draped the clastic facies, although nodular pelagic and oolitic/ammonite limestones are locally present. Moreover, Oxfordian pelagic oolitic facies rest unconformably on Upper Triassic sandstone (Karakaya complex), which is locally intercalated between the metamorphics and the Jurassic cover, wherever these had been unroofed at the footwall of Jurassic normal faults. As a result of the tectonic activity during the Callovian-Oxfordian time, the Ankara region totally became a PCP in the Oxfordian. Condensed, nodular, pelagic oolitic-ammonitic limestones were deposited across the major uplifted blocks in an open-marine environment, previously called the Ankara seamount. These facies constitute an important key level in the Ankara region. Laterally continuous deep-marine sediments overlying the Late Oxfordian-Early Cretaceous PCP deposits indicate the rapid foundering of this sector.