DEPOSITIONAL ENVIRONMENT CHARACTERISTICS OF UPPER CRETACEOUS SEDIMENTS IN HEKIMHAN BASIN (EASTERN ANATOLIA REGION, TURKEY): A BIOMARKER AND CARBON ISOTOPE APPROACH

Abstract

Biomarker analysis of soluble organic matter in the sediments such as shale has contributed to our understanding of the characteristics of the depositional environment. In this study, depositional environment properties of Upper Cretaceous sediments in Hekimhan Basin are evaluated by biomarker data and stable carbon isotope (delta C-13) values. To ascertain the biomarker features of the aliphatic fractions, GC and GC-MS analyses were realized on the studied shale and limestone samples. Investigated shale samples have generally low (< 0.5%) the total organic carbon values, however, the amount (0.115.92%) of organic matter within limestone are slightly higher than shale levels. Hydrogen index values of limestone and shale samples vary between 0.0 and 629 mgHC/g TOC and between 0.0 and 42 mgHC/g TOC, respectively. The odd-numbered and even-numbered n-alkanes ranges between the C-15 and C-27, which are among the definable components, are close to each other in terms of abundance. There are n-alkanes, especially short-chain (< n-C-21) and medium-chain (n-C-21-n-C-25) as well as long-chain n-alkanes (> n-C-25) like C-26 and C-27, in clayey limestone. The diasterane/sterane ratio in the samples is between 0.19 and 0.60. Besides, the sterane/hopane ratio being high (> 2) and C-26/C-25 tricyclic terpane ratio (0.18-0.33) shows the marine precipitation conditions. The moretane/hopane, norhopane/hopane, C-24/C-23 tricyclic terpane and C-19/(C-19+ C-23 tricyclic terpane) ratios show the carbonate depositional conditions and, the 18 a(H)-oleanane has been determined and refer to Cretaceous or younger age. The existence of C-35 homohopanes, hopane index (< 1), high C-35/C-34 homohopanes (> 0.8) also supports this result. The Ts/Ts+ Tm ratios define the marine deposition conditions (suboxic), like the decrease in C31-35 homohopane distribution and the delta C-13 values (-25.18 % and -23.48 %).