Studia Geologica Polonica  vol. 116 (Abstracts)

Phosphogenic facies and processes in the Triassic of Svalbard

Krzysztof P. KRAJEWSKI

Abstract
The Middle Triassic sedimentary sequence in Svalbard provides an insight into ancient organic carbon-rich, phosphogenic shelf depositional system showing broad facies associations, from coastal to deep shelf. This paper presents results of the sedimentologic, petrographic and micro- structural investigation of the phosphogenic system, with special emphasis being placed on the role of deep-water benthic microbial communities in phosphorite formation. The investigation was carried out in an attempt to explore earlier hypotheses suggesting a prominent role of gradient-type microbial mats in focusing phosphogenesis on some ancient organic-rich seafloors. The microbial mat-generated structures were recognized at recurrent levels in the Triassic phosphogenic sequence, which mark local intraformational discontinuities associated with suppressed or halted sedimentation. The horizons with well-defined microbial mat structures show high concentrations of carbonate fluor-apatite (up to 30% P₂O₅), though their contribution to the phosphorus pool of the phosphogenic facies is subordinate. Other types of phosphate, including pristine and allochthonous peloidal and nodular accumulations, dominate the phosphorite fraction of the Middle Triassic sequence, but these phosphates usually give lower burial phosphorus concentrations in the sediment. The microbial-mat generated phosphorite in the Triassic phosphogenic system of Svalbard is interpreted to have been produced by white filamentous sulphur bacteria that proliferated on sulphidic gradients during periods of substantially lowered depositional rates in the organic-rich shelf environment. These mats provided local depositional systems capable of depositing and significantly concentrating apatite in the sediment, though their development was a supplementary factor promoting seafloor phosphogenesis in the shelf environment. There is no evidence to support earlier presumptions that gradient-type microbial mats acted as an immediate and direct phosphorus source for apatite deposition at the sediment/water interface. It seems more plausible that these mats allocated the apatite concentration close to the interface due to sealing of the sediment surface to phosphate diffusion, and narrowing chemical gradients essential for the phosphorus pumps into the sediment, reactive phosphate buildup and mineral precipitation. Thus, these mats supported development of interface-linked sedimentary collectors of the formational phosphate, which, without the mats, would be partly lost from the sedimentary system or remained dispersed in the fine-grained facies.

Studia Geologica Polonica, 116: 85-109.

Isotopic composition of apatite-bound sulphur in the Triassic phosphogenic facies in Svalbard

Krzysztof P. KRAJEWSKI

Abstract
Isotopic composition of apatite-bound sulphate sulphur in the Middle Triassic phospho- genic facies in Svalbard (Bravaisberget and Botneheia formations) shows wide lateral variation, with d³⁴S values ranging from +24.9‰ to -2.0‰. After correction for likely value of coeval seawater sulphate (d³⁴S = +15.1‰ ± 1.5‰), the D³⁴S (= d³⁴S_CFA - d³⁴Scoeval evaporite) values of the Middle Triassic apatites fall in a range between approx. +10‰ and -17‰, suggesting complex and variable isotopic modifications of seawater sulphate in bottom phosphogenic environments. This variation was primarily forced by changing bottom depositional conditions along the Svalbard shelf which affected the nature, allocation, and intensity of phosphogenic processes in surficial sediments. Four bottom diagenetic systems leading to different isotopic ranges of apatite-bound sulphur are suggested: (1) rapid burial semi-closed system (D³⁴S = +10‰ to +5‰); (2) shielded interface semi-closed system (D³⁴S = 0‰ to -17‰); (3) open oxic/anoxic system (D³⁴S = +3‰ to -1‰); and (4) semi-closed anoxic system (D³⁴S = +6‰ to +4‰). The rapid burial semi-closed system and the open oxic/anoxic system dominated the formation of apatite in the shallow and deep parts of the Svalbard shelf, respectively. Seawater sulphur values of apatites formed in organic-rich deep shelf facies of the Botneheia Formation in central and eastern Svalbard reflect high original porosity of the sediment and nearsurface location of the phosphogenic zone in a thin suboxic and in the upper part of anoxic sulphidic pore environments. Heavier sulphur values of apatites formed in silty to sandy shallow shelf facies of the Bravaisberget Formation in western and southwestern Spitsbergen also reflect phospho- genesis in the suboxic/upper anoxic sulphidic pore environment, but located deeper in sediment column and characterized by limited sulphate exchange with shelf water. The semi-closed anoxic system and the shielded interface semi-closed system developed locally in bottom shelf environ- ments, reflecting peculiar, spatially restricted depositional conditions. Isotopically heavy apatitic sulphur in organic-rich facies of the Botneheia Formation deposited to the east of the Billefjorden Fault Zone in Spitsbergen reflects phosphogenesis in semi-closed anoxic system developed in surficial sediments overlain by anoxic bottom water enriched in residual dissolved sulphate. Isotopically light apatitic sulphur in microbial mat-generated phosphorite horizons occurring locally in both the shallow (Bravaisberget Formation) and deep shelf (Botneheia Formation) facies in Spitsbergen reflects phosphogenesis in nearsurface environment shielded by gradient-type microbial mats. The light apatitic sulphur originated from intense oxidation of hydrogen sulphide at a narrow oxic/anoxic interface stabilized by the growth and metabolic activity of the mat-forming communities.

Studia Geologica Polonica, 116: 111-137.

Diagenetic recrystallization and neoformation of apatite in the Triassic phosphogenic facies in Svalbard

Krzysztof P. KRAJEWSKI

Abstract
Phosphate accumulations in the Middle Triassic phosphogenic facies in Svalbard exhibit locally features of diagenetic recrystallization and neoformation of apatite. These features, seen on a microscale, reflect the development of hexagonal apatite crystal habit that replaced amorphous-like and globular micromorphologies of the original phosphatic fabric. The neoformed apatite crystals are commonly skeletal, either containing organic inclusions in crystal cores (Botneheia Formation in central and eastern Svalbard) or showing hollow or sieved cores with complex internal structure (Bravaisberget Formation in western and southwestern Spitsbergen). Detailed petrographic analysis (TLM, RLM, SEM, BEI, and XRD) of the apatite and associated diagenetic minerals, supported by the stable carbon and sulphur isotopic data (d¹³C and d34S apatite, d³⁴S pyrite, d¹³C carbonate) suggest that the development of conspicuous skeletal apatite structure was a two-stage process. The first and major stage of neoformation of apatite (carbonate fluorapatite, CFA) was associated with early diagenetic processes in the anoxic sulphidic zone in organic-rich sediment column, reflecting maintenance of phosphatic bodies at shallow depths in the subsurface environment and recrystalli- zation of original phosphatic fabric at margins exposed towards the primary and mouldic pore space. The recrystallization was preceded by early diagenetic dissolution of biogenic silica (radiolaria, sponge) and followed by precipitation of carbonate cements that originated in deeper parts of the anoxic sulphidic zone. The second stage of apatite neoformation was confined to phosphate accumu- lations occurring in the West Spitsbergen Fold Belt, and related to burial processes and thermal degradation of kerogen. Thermal degradation of organic inclusions in apatite crystal cores led to the formation of conspicuous skeletal crystals and was associated with supplementary recrystallization. The well-defined neoformed apatitic fabric is seldom observed in the Botneheia and Bravaisberget formations, suggesting that the recrystallization processes were of negligible importance during diagenesis and burial of the Middle Triassic phosphogenic sequence in Svalbard.

Studia Geologica Polonica, 116: 139-173.

Phosphorus concentration and organic carbon preservation in the Blanknuten Member (Botneheia Formation, Middle Triassic), Sassenfjorden, Spitsbergen

Krzysztof P. KRAJEWSKI

Abstract
The paper attempts to reconstruct depositional to diagenetic processes that led to the increased contents of mineral phosphorus and organic carbon in the Middle Triassic organic-rich phosphatic Blanknuten Member sequence in Sassenfjorden, Spitsbergen. The results of petrographic and geochemical analyses suggest that enhanced preservation of both organic carbon (1-8%) and mineral phosphorus (3-26% P₂O₅) in the sequence is a result of excess production, deposition, and diagenesis of indigenous marine organic matter in the shelf environment under prevailing oxygen- deficient bottom conditions. The nature of organic sources and the character of depositional environment aided the preferential preservation of oil-prone kerogen of the Type II. Diagenesis of organic phosphorus in surficial sediments led to the formation of phosphate peloids and nodules that occur dispersed in fine-grained facies (3-10% P₂O₅), or are concentrated at recurrent granular horizons due to dynamic environmental processes (12-18% P₂O₅). Maximum phosphorus concentra- tion (up to 26% P₂O₅) is noted in granular phosphorite horizons that were covered and stabilized by deep-water microbial mats during periods of diminished bottom dynamics and non-deposition. An interplay between the deposition and diagenesis of organic matter in oxygen-deficient environment, the dynamic processes leading to reworking and redeposition of sediments, and the growth and phosphatization of microbial mats accounts for the observed complexity of the phosphorus and organic carbon distributions in the Blanknuten Member sequence.

Studia Geologica Polonica, 116: 175-209.

Phosphorus and organic carbon reservoirs in the Bravaisberget Formation (Middle Triassic), Hornsund, Spitsbergen

Krzysztof P. KRAJEWSKI

Abstract
The Middle Triassic Bravaisberget Formation at Hornsund, Spitsbergen embraces a coarsening-upwards sequence which is dominated by organic-rich phosphatic silty shale in the lower part (Passhatten Member), and by fine- to medium-grained sandstone in the upper part (Somovbreen and Van Keulenfjorden members). This sequence reflects the development and maintenance of organic-prone phosphogenic shelf depositional system and the overlapped progradation of sandy facies related to land outbuilding and terrigenous sediment supply. The shallowing upward trend in the Formation is recorded by a general change of the nature and content of organic matter, from amorphous fractions of predominantly marine origin in the Passhatten Member (1.5-4 % TOC), to structured terrestrial fractions in the Somovbreen and Van Keulenfjorden Members (0.3-1% TOC). Organic matter in the Formation represents an advanced stage of thermal degradation (Ro = 2.0-2.1% for vitrinite). It is highly carbonized and overmature with respect to oil generation. Phosphate accumulations (5-30% P₂O₅) are concentrated at recurrent levels in the Passhatten Member, and at the bottom of the Somovbreen Member, where they are associated with sediment condensation, winnowing, and bioturbation. Quantitative calculations of organic carbon and mineral phosphorus show that the Bravaisberget Formation is a prominent reservoir of the two elements in the Hornsund area (524 g TOC and 64 g P per square cm). This also suggests significant expulsion of bitumen from the Formation (in a range of 300 g HC per square cm) during burial history and Tertiary tectogenesis in the West Spitsbergen Fold Belt. Organic-prone and phosphogenic environments responsible for the carbon and phosphorus concentration developed as a result of high biological productivity conditions and enhanced deposition of marine organic matter on the Middle Triassic shelf.