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Abstract

The Marhegda Bed is a carbonate-dominated Uthostratigraphic unit occurring locally at base of the Middle-Late Jurassic organic-rich sequence of the Agardhfjellet Formation in Spitsbergen, Svalbard. It has been interpreted to represent oolitic limestone facies deposited during an initial stage of Late Jurassic transgression. Petrographic, major element geochemical, and stable carbon and oxygen isotopic data presented in this paper indicate that this litho-stratigraphic unit is not a depositional limestone, but a diagenetic cementstone band originated in organic-rich sediment containing glauconite pellets and phosphatic ooids and grains. Two ep­isodes of carbonate diagenesis, including early precipitation of siderite and burial precipitation of ankerite, have contributed to the development of this cementstone. Extensive siderite precipi­tation occurred at sedimentary temperatures in nearsurface suboxic environment in which microbial reduction of ferric iron was the dominant diagenetic process. Precipitation of ankerite occurred at temperatures of about 80-100°C in burial diagenetic environment overwhelmed by thermal decarboxylation processes. Formation of ankerite was associated with advanced alteration of glauconite, dissolution of apatite and precipitation of kaolinite.
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Abstract

Ball-shaped concretions ("cannon balls") commonly occur in a marine, organic carbon-rich sedimentary sequence (Innkjegla Member) of the Carolinefjellet Formation (AptianAlbian) in Spitsbergen. The sedimentologic, petrographic and geochemical investigation of these concretions in the Kapp Morton section at Van Mijenfjorden gives insight into their origin and diagenetic evolution. The concretion bodies commenced to form in subsurface environment in the upper part of the sulphate reduction (SR) diagenetic zone. They resulted from pervasive cementation of uncompacted sediment enriched in framboidal pyrite by non-ferroan (up to 2 mol% FeCO3) calcite microspar at local sites of enhanced decomposition of organic matter. Bacterial oxidation of organic matter provided most of carbon dioxide necessary for concretionary calcite precipitation (δ13CCaCO3 ≈ -21%VPDB). Perfect ball-like shapes of the concretions originated at this stage, reflecting isotropic permeability of uncompacted sediment. The concretion bodies cracked under continuous burial as a result of amplification of stress around concretions in a more plastic sediment. The crack systems were filled by non-ferroan (up to 5 mol% FeCO3) calcite spar and blocky pyrite in deeper parts of the SR-zone. This cementation was associated with impregnation of parts of the concretion bodies with microgranular pyrite. Bacterial oxidation of organic matter was still the major source of carbon dioxide for crack-filling calcite precipitation (δ13CCaCO3 ≈ -19% VPDB). At this stage, the cannon-ball concretions attained their final shape and texture. Subsequent stages of concretion evolution involved burial cementation of rudimentary pore space with carbonate minerals (dolomite/ankerite, siderite, calcite) under increased temperature (δ18OCa,Mg,FeCO3 ≈-14% VPDB). Carbon dioxide for mineral precipitation was derived from thermal degradation of organic matter and from dissolution of skeletal carbonates (δ13CCa,Mg,FeCO3≈ - 8‰ VPDB). Kaolinite cement precipitated as the last diagenetic mineral, most probably during post−Early Cretaceous uplift of the sequence.
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Abstract

The Marhřgda Bed occurring at base of the Adventdalen Group in Sassenfjorden, Spitsbergen contains common ankerite–replaced belemnite skeletons. Petrographic, major element geochemical, and stable carbon and oxygen isotopic data indicate that the ankerite originated in a catagenic environment associated with thermal degradation of kerogenan d hydrocarbongen erationinthe sequence. It formed at maximum temperature of 150°C under burial of approx. 2 000 m, most probably during Paleogene filling and subsidence of the Central Spitsbergen Basin. Dissolution of biogenic calcite and precipitation of ankerite reflect extensive heat flow through the Adventdalen Group sequence related to the Cretaceous and Paleogene magmatic and orogenic activity in Svalbard.
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