Geology of the Eastern Pontides
The 500-km-long Eastern Pontide belt shows several common Stratigraphic features resulting from a common Mesozoic-Tertiary tectonic history. There is a heterogeneous pre-Jurassic basement comprised of Devonian? high-grade metamorphic rocks, Lower Carboniferous granodiorites and dacites, Upper Carboniferous-Lower Permian shallow-marine to terrigeneous sedimentary rocks and an allochthonous Permo-Triassic metabasite-phyllite-marble unit. The Mesozoic sedimentary sequence starts with a widespread Liassic marine transgression coming from the south. The Lower and Middle Jurassic rocks of the Eastern Pontides make up a 2000-m-thick sequence of tuff, pyroclastic rock, lava, and interbedded clastic sedimentary rock; the volcanism is proba-bly related to rifting leading to the opening of the Neotethyan Ocean in the south. The Upper Jurassic-Lower Cretaceous is characterized by carbonates, showing a transition from platform carbonate deposition in the north to pelagic carbonates and calciturbidites in the south; this indicates the develop-ment of a south-facing passive continental margin. During the Cenomanian, there was uplift and erosion throughout the Eastern Pontides. Rocks of this stage are not present, and in many localities the Senonian deposits lie uncon-formably over Jurassic carbonates and even over the Carboniferous granitic basement. This compressive event is associated with the northward emplace-ment of an ophiolitic melange over the passive continental margin of the Eastern Pontides. The obduction of the ophiolitic melange is probably caused by the partial subduction of the Eastern Pontides continental margin in a south-dipping intra-oceanic subduction zone. This was followed by the flip of the subduction polarity during the Cenomanian-Turonian, which led to the development of a Senonian volcanic arc in the outer Eastern Pontides above the northward-subducting Tethyan Ocean floor. The volcanic arc is represent-ed by >2-km-thick succession of volcanic and volcaniclastic rocks and interbedded limestones and marls. There are also intrusive granodiorite plu- tons with isotopic ages of 95 to 65 m.y. The volcanism shows a general silica enrichment, with time, ranging from basalts and andesites to dacites. The Senonian sequence in the inner Eastern Pontides is made up of a tuffaceous flyschoid series representing the fore-arc succession. The Eastern Black Sea Basin probably opened during the Maastrichtian through the rifting of the volcanic arc axis.
During the late Paleocene-early Eocene, there was north-vergent thrust imbrication of the inner Eastern Pontides with the development of a major foreland flysch basin in front of the northward moving thrust sheets. Folding and uplift occurred in the outer Eastern Pontides during this period. This compressive deformational event, the strongest Mesozoic-Tertiary orogenic phase in the Eastern Pontides, was probably caused by the collision between the Pontide arc and the Tauride microplate in the south.
Widespread calc-alkaline volcanism and shallow-marine sedimentation occurred throughout the Eastern Pontides during the middle Eocene. The middle Eocene rocks are essentially undeformed and lie unconformably over a folded and thrust-faulted basement. This major middle Eocene extensional event is probably related to an accelarated phase of opening of the Eastern Black Sea Basin. From the end of the middle Eocene onward, the Eastern Pontides stayed largely above sea level, with minor volcanism and terrigeneous sedimentation.
Figures & Tables
Regional and Petroleum Geology of the Black Sea and Surrounding Region
In 1967 and 1969, two oceanographic cruises were made in the Black Sea under the guidance of the Woods Hole Oceanographic Institute: The cruises included scientists from many countries and disciplines. Their aims were to determine the recent geological and geochemical evolution of the Black Sea, to map the shallow structure of the basin, and to study the interaction between the oxidized surface waters and the anoxic waters beneath them. The results were published 23 years ago, as AAPG Memoir 20 (Ross and Degens, 1974). During the 1969 cruise, the vessel Atlantis II collected 40 piston cores, which formed the basis of most of the subsequent geological studies that were restricted to very recent sedimentation. Speculations concerning the origin of the basin and the relationship of the geology offshore to that exposed around the margins of the Black Sea were rooted in pre-plate tectonic concepts of basin formation and were in any case hampered by a lack of relevant data (Brinkmann, 1974).
In 1976, the Glomar Challenger visited the Black Sea on Leg 42B of the Deep Sea Drilling Project and drilled and cored three deep-water sites (379, 380, and 381). Well 381 north of the Bosporus encountered sediments as old as Miocene, including some apparently deposited in shallow water (Ross, 1978).
The next major volume in Western literature to deal with the Black Sea was published a decade later, collecting papers presented two years earlier at a conference in Yalta. In this volume, a number of seismic reflection lines