Cretaceous Syn- to Postrift Sedimentation on the Southern Continental Margin of the Western Black Sea Basin
Naci Görür, 1997. "Cretaceous Syn- to Postrift Sedimentation on the Southern Continental Margin of the Western Black Sea Basin", Regional and Petroleum Geology of the Black Sea and Surrounding Region, A. G. Robinson
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The Western Black Sea Basin began opening as a back-arc basin by the rift-ing of a juvenile continental margin magmatic arc during the Aptian. Its southern continental margin succession is well exposed in the Western Pontides, Northwest Turkey. This succession consists predominantly of vol- canogenic coarse clastic rocks, shales, and carbonates with a deepening- upward character. The volcanogenic clastic rocks are mostly turbidites and mass-flow deposits in places with huge exotic blocks. The volume and nature of this clastic material were controlled by both relief of nearby sediment sources and arc volcanism, whereas the carbonates depended on ocean circulation and surface organic productivity. The Aptian to lower Cenomanian part of the succession formed during the synrift stage, whereas the rest accumulated during the postrift stage. The synbreakup stage is marked by the upper Cenomanian to Campanian sedimentary facies.
The synrift sediments commence locally with Aptian lagoonal black shales, rich in organic matter. They pass laterally and upward into an Albian unit, comprising marginal marine glauconitic sandstones succeeded by siliciclastic turbidites, marls, sandy limestones, and blue to black shales with abundant glauconite. This unit includes several levels of mass-flow deposits, comprising mostly conglomerates and olistoliths of various sizes, ranging from a few centimeters to hundreds of meters in diameter. The synrift sedi-ments end with a Cenomanian succession of blue to black shales and clayey limestones, in part with exotic blocks derived from the underlying rocks.
The postrift sediments at the base of upper Cenomanian to Campanian consist of pelagic red micrites and marls followed by mainly volcanogenic (both andesitic and basaltic) terrigeneous and carbonate turbidites and deep- water sediments, ranging from Turonian to lower Eocene. The basal pelagic carbonates rest with a slightly angular unconformity on the synrift deposits and represent the breakup facies.
Facies analyses of the rift succession indicate that the Western Black Sea Basin was isolated during its synrift stage from free interchange with the Intra-Pontide Ocean to the south, and therefore was euxinic. During the rift- drift transition in the late Cenomanian, the euxinic conditions largely disap-peared, and the water column above the arc margin of this basin became well mixed. The volcanic activity in the arc also increased in intensity soon after this transition, and largely controlled the postrift sedimentation.
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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