Petrography, geochemistry and sedimentology of sandstones from the Central Tertiary Basin of Svalbard – implications for diagenesis and temperature history
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Svalbard, representing an uplifted area of the north-western Barents Sea Shelf, provides a unique opportunity for studying on-shore outcrops of the shelf. The present study focuses on Paleocene-Eocene deposits of the Central Tertiary Basin (CTB), located in central-southern parts of Spitsbergen. Despite the large number of studies concerning diagenetic alteration of sedimentary rocks, parameters controlling the types and distribution of authigenic minerals are still not fully understood. Here, I systematically relate the early diagenetic minerals of the Firkanten, Grumantbyen and Aspelintoppen formations to sandstone composition and depositional environment. The identification of these diagenetic minerals integrates petrographic observations obtained from thin section analyses (optical microscopy and SEM) and XRD-analysis. Sedimentological analysis is supplemented by geochemical data, providing a better understanding of large- and small-scale variations in lithology and, thus, depositional environment. The different authigenic mineral assemblages of the sandstones of the Firkanten, Grumantbyen and Aspelintoppen formations are related to their different detrital compositions and depositional environment. The generally higher porosity values of the Grumantbyen Formation sandstones may be ascribed to the occurrence of grain coating chlorite, preventing precipitation of quartz cement at great burial depths. The relatively homogenous nature of these sandstones provides conditions favourable for precipitation of early berthierine rims. At temperatures of about 90oC (i.e. subsequent to the onset of quartz cementation) berthierine is altered into chlorite, suggesting that early diagenetic berthierine-rims, rather than later burial grain-coating chlorite, is responsible for preventing quartz cementation. The occurrence of glauconite in the lowermost part of the Firkanten Formation (Todalen Member), accompanied by abundant framboidal pyrite, may indicate the onset of transgressive conditions and deposition of the overlying marine Endalen Member. Burial diagenesis is largely influenced by the presence of early diagenetic products, in which they affect a variety of diagenetic processes. Burial diagenetic minerals, usually indicating a specific burial depth and minimum-temperature, can be used to cross-check temperatures derived from organic maturity indicators (e.g. vitrinite reflectance). The burial diagenetic signatures of the studied formations (e.g. quartz, Fe-chlorite, ankerite), suggest maximum temperatures of at least 100-110oC. Extensive quartz cementation in sandstone samples from the Firkanten and Grumantbyen formations, however, may support higher temperatures, as suggested by vitrinite reflectance measurements of coals.