Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen)

Abstract

34 S values suggests interplay of various factors in sedimentary environment. These fac− tors include oxygen concentration, clastic sedimentation rate, bottom currents and bur− rowing activity. Biological productivity and rate of dissimilatory sulphate reduction had important impact on the d 34 S variation as well. Wide variation of the d 34 S values in the studied section resulted from high biological productivity and high rate of dissimilatory sulphate reduction. Variable degree of clastic sedimentation rate and burrowing activity as well as the activity of poorly oxygenated bottom currents could also cause a co−occur− rence of isotopically light and heavy pyrite in differentiated diagenetic micro−environ− ments. Occurrence of organic matter depleted in hydrogen could also result in a wide vari− ation of the d 34 S values. Narrow variation of the d 34 S values was due to a decrease of bio− logical productivity and low rate of dissimilatory sulphate reduction. Low organic matter supply, low oxygen concentration and bottom currents and burrowing activity were also responsible for narrow variation of the d 34 S. The narrow range of the d 34 S variation was also due to occurrence of hydrogen−rich organic matter. In the studied section the major change in range of the d 34 S variation from wide to narrow appears to be abrupt and clearly associated with change in lithology. The change of lithology and isotopic values may sug− gest evolution of the sedimentary environment from high− to low−energy and also facies succession from shallow to deeper shelf. The evolution should be linked with the Late Anisian regional transgressive pulse in the Boreal Ocean.

Reconstruction of secular variation in seawater sulfate concentrations

“…The Cretaceous Western Interior Seaway was almost certainly semi-restricted throughout its existence (Adams et al, 2010). The only Mesozoic unit not to show a type II deviation, the Middle Triassic Bravaisberget Formation of Spitsbergen (Karcz, 2010;Fig. 8), was deposited in the largely unrestricted Boreal Ocean.…”

Petrography, bulk-rock geochemistry, detrital zircon U–Pb geochronology and Hf isotope analysis for constraining provenance: An example from Middle Triassic deposits (Bravaisberget Formation), Sørkappøya, Svalbard

“…1A, B) comprises marine facies of the Sassendalen Group and paralic to deltaic facies of the Kapp Toscana Group (Mørk et al, 1999). Two of the youngest units of the Sassendalen Group, the Botneheia and Bravaisberget formations, both enriched in marine organic carbon, are important petroleum source rocks in the Barents Sea (Mørk & Bjorøy, 1984;Karcz, 2010Karcz, , 2014Krajewski, 2013). Toscana Group are considered to have good reservoir potential (Worsley & Mørk, 1978;Krajewski et al, 2007;Mørk, 2013).…”

“…1B). The shale-dominated Passhatten Member comprises an organic-rich succession, which is related to deposition in a shallow-shelf environment (Karcz, 2010;Krajewski & Weitschat, 2015). The overlying sandy and silty deposits of the Somovbreen Member are described as a marine succession encompassing and covering wedges of deltaic sediments (Mørk et al, 1999;Krajewski & Weitschat, 2015).…”