Środowisko sedymentacji i wczesna diageneza czarnych łupków ogniwa Passhatten (środkowy trias, Spitsbergen, Svalbard) na podstawie analizy geochemicznej

Abstract

Sedimentary environment and early diagenesis of the Passhatten Member black shales (Middle Triassic, Spitsbergen, Svalbard) in the light of geochemical analysis.A b s t r a c t. Geochemical analysis of the 50 samples of the Middle Triassic black shales (organic carbon-rich siltstones) has been carried out. The black shale samples (up to 4.92 % TOC) have been collected in the stratotype profile of the Bravaisberget Formation, west Spitsbergen. In the examined profile, the black shales occur exclusively in the Passhatten Member (the lower and middle interval of the Bravaisberget Formation). Black shale samples have been analyzed in respect to degree of pyritization (DOP), isotopic composition of pyrite sulphur (δ34S) and organic carbon content (TOC). The main goal of the research was to examine a degree of oxygenation and dynamics of a sea-bottom environment. DOP values from lower section of the Passhatten Member show wide variation, ranging from 0.29 to 0.92. In upper section of the member, the obtained DOP values show narrower range of variations, from 0.77 to 0.98. Wide variations of DOP were caused by temporary coexistence of oxic and anoxic bottom currents. In turn, narrow DOP variations indicate predominance of stratified water and anoxic bottom currents. DOP/δ34S and DOP/TOC ratios imply that syngenetic and early diagenetic pyrite precipitation during accumulation of the lower section of the Passhatten Member was controlled by availability of organic carbon and reactive iron, whereas accumulation of the member’s upper section was controlled exclusively by availability of reactive iron.