Geological evolution of the Central Pontides

in: Petroleum Geology of the Black Sea, Mike d. Simmons,Gabor c. Tari,Aral I. Okay, Editor, The Geological Society , London, pp.33-67, 2018

Abstract: Before the Late Cretaceous opening of the Black Sea, the Central Pontides constituted

part of the southern margin of Laurasia. Two features that distinguish the Central Pontides from

the neighbouring Pontide regions are the presence of an extensive Lower Cretaceous submarine turbidite

fan (the Çağlayan Formation) in the north, and a huge area of Jurassic–Cretaceous subduction–

accretion complexes in the south.

The Central Pontides comprise two terranes, the Istanbul Zone in the west and the Sakarya Zone in

the east, which were amalgamated before the Late Jurassic (Kimmeridgian), most probably during

the Triassic. The basement in the western Central Pontides (the Istanbul Zone) is made up of a Palaeozoic

sedimentary sequence, which ends with Carboniferous coal measures and Permo-Triassic red

beds. In the eastern Central Pontides, the basement consists of Permo-Carboniferous granites and an

Upper Triassic forearc sequence of siliciclastic turbidites with tectonic slivers of pre-Jurassic ophiolite

(the Küre Complex). The Küre Complex is intruded by Middle Jurassic granites and porphyries,

which constitute the western termination of a major magmatic arc.

Upper Jurassic–Lower Cretaceous shallow-marine limestones (the İnaltı Formation) lie unconformably

over both the Istanbul and Sakarya sequences in the Central Pontides. Two new measured

stratigraphic sections from the İnaltı Formation constrain the age of the İnaltı Formation

as Kimmeridgian–Berriasian. After a period of uplift and erosion during the Valanginian and

Hauterivian, the İnaltı Formation is unconformably overlain by an over 2 km-thick sequence of

Barremian–Aptian turbidites. Palaeocurrent measurements and detrital zircons indicate that the

major part of the turbidites was derived from the East European Platform, implying that the

Black Sea was not open before the Aptian. The Çağlayan turbidites pass northwards to a coeval carbonate–

clastic shelf exposed along the present Black Sea coast. In the southern part of the Central

Pontides, the Lower Cretaceous turbidites were deformed and metamorphosed in the Albian. Albian

times also witnessed accretion of Tethyan oceanic crustal and mantle sequences to the southern margin

of Laurasia, represented by Albian eclogites and blueschists in the Central Pontides.

A new depositional cycle started in the Late Cretaceous with Coniacian–Santonian red pelagic

limestones, which lie unconformably over the older units. The limestones pass up into thick

sequences of Santonian–Campanian arc volcanic rocks. The volcanism ceased in the middle Campanian,

and the interval between late Campanian and middle Eocene is represented by a thick

sequence of siliciclastic and calciclastic turbidites in the northern part of the Central Pontides. Coeval

sequences in the south are shallow marine and are separated by unconformities. The marine deposition

in the Central Pontides ended in the Middle Eocene as a consequence of collision of the

Pontides with the Kırşehir Massif.