SEDIMENTOLOGICAL AND CYCLOSTRATIGRAPHIC ANALYSIS OF THE ÇAKRAZBOZ FORMATION (TRIASSIC) IN AMASRA-KASTAMONU REGION

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2021-6-21
Şen, Gül
The studied Triassic-aged successions indicate a large-scale tripartite sequence of depositional environments in the Amasra-Kastamonu region, the NW Anatolia. Sedimentological, petrographic, magnetic susceptibility, elemental geochemistry, cyclostratigraphic, and palynological analyses are carried out in order to determine the types of depositional environment where six stratigraphic sections were measured, to define the cyclic characteristics of the successions, to find out exact age of the Çakrazboz formation and to reveal prevailing paleoclimate conditions in the studied region. Based on the detailed analysis of facies, the depositional environment is characterized by a fluvial, shallow lake and deep lake from east to west. Various lithofacies are identified in the studied sections, including carbonates (limestone, marl, and calcrete) and siliciclastics (conglomerate, sandstone, siltstone, and mudstone). The fluvial environment deposits consist of meandering river sandstones and floodplain mudstones. Palustrine deposits represent the rhythmic alternation of greenish-grey and brownish-grey marlstones related to wetting and drying climate conditions and reddish-brown pedogenic calcretes related to extended pedogenesis. Lacustrine deposits are composed of carbonated mudstones, wackestones, and packstones formed under oxygenated water conditions. Based on detailed facies analysis and field observations, this study suggests that Çakrazboz formation was deposited in a balanced-fill type lake basin within the extensional regime in foreland settings. The magnetic susceptibility analysis was performed on 347 rock samples and 120 thin sections from whole measured stratigraphic sections. The presence of high magnetic susceptibility values (23,25 and 129,39*10-6 SI) in carbonates indicates a detrital entry into the lacustrine environment. Moreover, the presence of high ms values (212,49 and 160,38*10-6 SI) in brecciated limestone facies indicate that the facies exposed due to the decrease in the water level of the lake, and the extended subaerially exposure. As a result, the high magnetic values in lacustrine carbonates are linked to erosion-transportation and pedogenetic processes, and associated with bacterial origin. Elemental geochemistry method was carried out on the 152 samples from the Çakrazboz Formation were analyzed for determining CaCO3, SiO2 and organic matter. Based on elemental geochemical analysis of the Çakrazboz Formation, it is revealed that organic matter content of the mudstones, marlstones and, limestones generally varies between 0% and 3%. Furthermore, the cyclic geochemical condition is documented in successive accumulation of reddish-brown marlstone and desiccated greenish-grey marlstones. These alternations reflect fluctuations of Eh and PH conditions in lacustrine and palustrine settings. Moreover, Milankovitch-like cycles have been identified along the measured stratigraphic sections. Potential Milankovitch-like cycles have been determined along the six measured stratigraphic sections. Based on the facies analysis 3 main cycles and 11 sub-types of cycles are determined in the measured sections. Centimeter to meter-scale cycles, are generally represented by profundal to supratidal facies. A-type main cycles start with the carbonate mudstone at base, and are followed by wackestone and packstone microfacies types. B type main cycles are represented by an alternation of grey laminated mudstone and brown to green marlstone facies in Bozköy and İncigez section, and an alternation of mottled limestone and pedogenic and groundwater calcrete-like facies in Başköy-1 section. C-type main cycles are represented by sandstone and mudstone alternations in the Başköy-2 and Başköy-3 sections. The measured stratigraphic sections are commonly represented by five precession (≃19-21 ka) and one eccentricity (≃100 ka) cycles. Based on data, it is suggested that the cyclicity in these successions has similar characteristics with the cyclicity in Triassic terrestrial succession around the World. Palynological investigation was carried out to determine the age of the Çakrazboz Formation in the Amasra-Kastamonu zone. Palynological analysis was carried out on 13 samples selected from the Çakrazboz, İncigez and Başköy-1 successions. Palynological data were obtained only from samples GBK1-11, 16 and 44 of Başköy-1 succession. The studied assemblage preserves index taxa (e.g., Ovalipollis ovallis, Triradispora, and Enzonalapollenites), suggesting a Late Ladinian to Carnian age. It also indicates relatively dry climatic conditions, possibly interrupted by short periods of humid climatic conditions. As a result, detailed petrography supported sedimentology analysis suggests that Çakrazboz formation was deposited in a balanced-fill type lake basin, magnetic susceptibility analysis indicates that the high magnetic values in lacustrine carbonates are linked to erosion-transportation and pedogenetic processes, and associated with bacterial origin. Elemental geochemical analysis represents fluctuations of Eh and PH condition in lacustrine and palustrine settings. Cyclostratigraphic analysis reveals that the measured stratigraphic sections are commonly represented by precession (≃19-21 ka) and eccentricity (≃100 ka) cycles. Palynological analysis refers Late Ladinian-Carnian age for the Çakrazboz Formation. Data obtained from all analyses indicate relatively dry climatic conditions, possibly interrupted by short periods of humid climatic conditions.

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Citation Formats
G. Şen, “SEDIMENTOLOGICAL AND CYCLOSTRATIGRAPHIC ANALYSIS OF THE ÇAKRAZBOZ FORMATION (TRIASSIC) IN AMASRA-KASTAMONU REGION,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.