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Abstract

51 samples from the Middle Triassic black shales (organic carbon−rich silt− stones; up to 4.9% TOC – Total Organic Carbon) from the stratotype section of the Bravaisberget Formation (western Spitsbergen) were analyzed with respect to isotopic composition of pyritic sulphur (34S) and TOC. Isotopic composition of syngenetic py− rite−bound sulphur shows wide (34S from −26‰ to +8‰ VCDT) and narrow (34S from −4‰ to +17‰ VCDT) variation of the 34S in upper and lower part of the section, respec− tively. Range of the variation is associated with abrupt changes in dominant lithology. Wide 34S variation is found in lithological intervals characterized by alternation of black shales and phosphorite−bearing sandstones. The narrow 34S variation is associated with the lithological interval dominated by black shales only. Wide and narrow variation of the #2;34S 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 34S variation as well. Wide variation of the 34S 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−occurrence of isotopically light and heavy pyrite in differentiated diagenetic micro−environments. Occurrence of organic matter depleted in hydrogen could also result in a wide variation of the 34S values. Narrow variation of the #2;34S values was due to a decrease of biological 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 34S. The narrow range of the 34S variation was also due to occurrence of hydrogen−rich organic matter. In the studied section the major change in range of the 34S variation from wide to narrow appears to be abrupt and clearly associated with change in lithology. The change of lithology and isotopic valuesmay 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.
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Abstract

The Bravaisberget Formation in Spitsbergen embraces an organic carbon-rich, clastic sequence that reflects a general shallow shelf development of the Middle Triassic depositional system in Svalbard . New observations and measurements of the type section of the formation at Bravaisberget in western Nathorst Land allow to present detailed lithostratigraphical subdivision of the formation, and aid to reconstruct its depositional history. The subdivision of the formation ( 209 m thick at type section) into the Passhatten, Somovbreen, and Van Keulenfjorden members is sustained after Mørk et al. (1999), though with new position of the boundary between the Passhatten and Somovbreen mbs. The Passhatten Mb is defined to embrace the black shale-dominated sequence that forms the lower and middle parts of the formation ( 160 m thick). The Somovbreen Mb ( 20 m thick) is confined to the overlying, calcite-cemented sequence of marine sandstones. The Van Keulenfjorden Mb ( 29 m thick) forms the topmost part of the formation composed of siliceous and dolomitic sandstones. The formation is subdivided into twelve informal units, out of which eight is defined in the Passhatten Mb (units 1 to 8), two in the Somovbreen Mb (units 9 and 10), and also two in the Van Keulenfjorden Mb (units 11 and 12). Units 1, 3, 5, 7 and 9 contain noticeable to abundant phosphorite, and are interspaced by four black shale sequences (units 2, 4, 6, and 8). Unit 9 passes upwards gradually into the main sandstone sequence (unit 10) of the Somovbreen Mb. The base of the Van Keulenfjorden Mb is a discontinuity surface covered by thin phosphorite lag. The Van Keulenfjorden Mb consists of two superimposed sandstone units (units 11 and 12) that form indistinct coarsening-upward sequences. The Bravaisberget Fm records two consequent transgressive pulses that introduced high biological productivity conditions to the shelf basin. The Passhatten Mb shows pronounced repetition of sediment types resulting from interplay between organic-prone, fine-grained environments, and clastic bar environments that focused phosphogenesis. The lower part of the member (units 1 to 5) contains well-developed bar top sequences with abundant nodular phosphorite, which are under- and overlain by the bar side sequences grading into silt- to mud-shale. The upper part of the member (units 6 to 8) is dominated by mud-shale, showing the bar top to side sequence with recurrent phosphatic grainstones in its middle part. Maximum stagnation and deep-water conditions occurred during deposition of the topmost shale sequence (unit 8). Rapid shallowing trend terminated organic-rich environments of the Passhatten Mb, and was associated with enhanced phosphogenesis at base of the Somovbreen Mb (unit 9). Bioturbated sandstones of the Somovbreen Mb (unit 10) record progradation of shallow-marine clastic environments. The sequence of the Van Keulenfjorden Mb (units 11 and 12) was deposited in brackish environments reflecting closure of the Middle Triassic basin in western Svalbard .
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Abstract

The Passhatten Member (Anisian–Ladinian) is the most westward exposure of the Middle Triassic sedimentary sequence of Spitsbergen. The member has an average organic carbon of 2.21 wt %. The sediments were deposited in a shallow shelf environment under conditions of high biological productivity stimulated by a well−developed upwelling system and an enhanced nutrient supply from land areas. The high biological productivity caused a high supply of organic particles to the shelf bottom. Decomposition of organic matter initiated oxygen deficiency in the bottom waters; however, bottom water dynamics on the shallow shelf temporarily replenished the oxygen. Consequently, the Passhatten Mb section is bioturbated, even in thick black shale horizons and consists of alternately spaced lithological layers with variable organic carbon content. The organic matter is dominated by type II kerogen with a mixture of strongly altered marine and/or land derived organic matter. Calculated initial hydrogen index values suggest oil−prone organic matter similar to kerogen I and II types. The organic matter is in the upper intermediate stage of thermo−catalytic alteration, close to the cata− and metagenetic boundary. Maturity indicators including Rock−Eval, Maximum Temperature, Organic Matter Transformation Ratio, Residual Carbon content, as well as the volume of methane generated suggest mature to overmature organic matter. Methane potential retained in the black shales sequence is significant. Unexpelled gas is estimated at 395 mcf/ac−ft for the examined section.
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