A composite biostratigraphy (mainly calpionellids and foraminifera) of the Upper Jurassic–Lower Cretaceous Carbonates in Sivrihisar region (Pontides, NW Turkey): Delineation of the J-K boundary in a slope environment


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Atasoy S. G. , Altıner D.

10th ISC – International Symposium on the Cretaceous, Vienna, Austria, 21 - 26 August 2017, pp.17

  • Publication Type: Conference Paper / Summary Text
  • City: Vienna
  • Country: Austria
  • Page Numbers: pp.17

Abstract

A 748 m thick stratigraphical section was measured along the Upper Jurassic – Lower

Cretaceous carbonate sequence exposed in a tectonic klippe of the Sakarya Zone

(Pontides), north of Sivrihisar. According to the biozonation and microfacies types, two

coeval but dissimiliar rock successions, separated by a thrust fault, have been detected. The

lower succession displays a slope to basin facies and consists of the Kimmeridgian–

Berriasian Yosunlukbayırı Formation and the overlying Valanginian Soğukçam Limestone.

The deposits of this succession are characterized by the continuous pelagic background

sedimentation with taxa including calpionelldis, Saccocoma, calcareous dinocysts and

aptychi, intercalated with the calciturbidites containing platform derived clasts (benthic

foraminifera, microencrusters, worm tubes etc.). The succession commences with pelagic

deposits characterized by toe-of-slope type facies. There is an important increase in the

amount of platform-derived clasts in the Tithonian, reflecting a calciturbiditic – slope type

deposition in the succession. Latest Tithonian onward the background pelagic conditions

dominate the deposition.

This facies evolution resulted in intervals dominated by neritic or pelagic taxa or

characterized by mixed fauna. This types of bio-association gave rise to a composite

biozonation that is based on both pelagic and benthic taxa, providing the link between slope

and platform successions. This biozonation consists of the following biozones: Globuligerina

oxfordiana–Mohlerina basiliensis Zone (Kimmeridgian), Saccocoma Zone (Lower

Tithonian), Protopeneroplis ultragranulata Zone (Upper Tithonian), Crassicollaria

(massutiana subzone) Zone (uppermost Tithonian), Calpionella (alpina, Remaniella, elliptica

subzones) Zone (Lower Berriasian), Calpionellopsis (simplex, oblonga subzones) Zone

(Upper Berriasian) and Calpionellites (darderi subzone) Zone (Lower Valanginian).

Calciturbiditic deposits of the P. ultragranulata Zone are followed by pelagic deposits of the

massutiana subzone in the latest Tithonian. The Jurassic-Cretaceous boundary is located

at the base of the overlying alpina subzone defined by the acme of Calpionella alpina. The

other observed calpionellid bioevents around the J-K boundary are the stepwise last

occurences of Crassicollaria intermedia (below the boundary), Calpionella elliptalpina (at the

boundary) and Crassicollaria brevis (at the boundary) and the marked decrease in the

abundance of Calpionella grandalpina across the Tithonian-Berriasian boundary.