Structurally and climatically-controlled progressive destabilisation of a multi-unit carbonate rockslide with marly interlayers over two decades (Hornbergl, Tyrol/Austria)
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Rockslide
Destabilisation
Rockslide
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ABSTRACT A calcite mass more than 1·5 km long and 20 m wide crops outs along the faulted margin of the Albian carbonate platform of Jorrios in northern Spain. The mass contains abundant dissolution cavities up to 7 m long and 1 m high, filled with cross‐stratified quartz sandstone and alternating sandstone–calcite laminae. Similar cavities are also present in a 50‐m‐wide zone of platform limestones adjacent to the calcite mass that are filled with limestone breccias and sandstone. The calcite mass has mean δ 18 O values of 19·6‰ (SMOW), whereas platform limestones have mean δ 18 O values of 24·4‰ (SMOW). Synsedimentary faulting of the carbonate margin and circulation of heated fault‐related waters resulted in replacement of a band of limestone by calcite. Soon after this replacement, dissolution by undersaturated fluids affected both the calcite mass and the adjacent limestones. Percolating marine quartz sand filled all dissolution cavities, sometimes alternating with precipitating calcite. The resulting cavities and fills, which recall products of meteoric diagenesis, are attributed to a hydrothermal origin based on their geometry, occurrence along the profile and synsedimentary tectonic relationships. The early faulting and diagenesis are related to local extensional tectonism in a large‐scale strike‐slip setting. Movements occurred during the early dispar / appenninica zone of the Late Albian.
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ABSTRACT Carbonate mineralogical studies of surface Bahamian periplatform ooze, containing pelagic calcite as well as bank-derived aragonite and magnesian calcite, were coupled with studies of the carbonate chemistry of the waters overlying these sediments. The presence of metastable aragonite and magnesian calcite in the periplatform ooze presents a unique opportunity to assess the relation between the extent of carbonate saturation in the water column and the disappearance of metastable carbonate minerals in an area of the North Atlantic Ocean where intermediate and deep waters are saturated with respect to calcite. Aragonite is present in the fine fraction of the surface sediment to a water depth of 4,800 m, but decreases sharply below 4,000 m. This is also the depth limit of the occurrence of pteropod tests and fragments in the coarse fraction of the sediment. The aragonite compensation depth should be (by extrapolation) between 5,000 and 5,200 m. The saturation depth with respect to aragonite was estimated to be located between 3,800 and 4,500 m. This corresponds fairly well with the depth of a sharp decrease of the fine aragonite as well as the depth of pteropod disappearance. Magnesian calcite in the fine fraction of the sediment decreases irregularly with depth and disappears between 3,700 and 4,000 m. The calculated saturation depth with respect to magnesian calcite (13 mole % MgCO3) is between 900 and 1,500 m. At this depth a significant drop in the concentration of magnesian calcite occurs. This decrease of magnesian calcite from 25 to 15 percent could be caused by both dissolution and the increasing distance of deeper waters from the banks. However, the higher values of specific alkalinity of the waters ranging between 750 and 2,000 m, when compared with the alkalinity values of the Sargasso Sea at the same latitude, are interpreted to be indicative that some dissolution of magnesian calcite occurs in the Bahamian basins at these water dept s. Our results show depressed levels, by 500 m or more, of the aragonite saturation depth in the water column as well as of the depth of aragonite disappearance in the surface sediment, when compared with open ocean studies in the Northwest Atlantic. In addition, our results indicate that fine magnesian calcite (12 to 13 mole % MgCO3) disappears 1,200 to 1,500 m above the extrapolated disappearance depth of aragonite. This observation is consistent with results for experimental studies, showing that aragonite is more stable in seawater than a magnesian calcite of 12 to 13 mole % MgCO3.
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Allanite
Pegmatite
Carbonatite
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The nature and distribution of carbonate minerals in selected Chernozemic soils was investigated. Soil from the Bmk, Cca and Ck horizons of two profiles was fractionated into various size fractions. Total carbonates, calcite and dolomite were determined on each fraction. The silt fraction of both parent materials was high in carbonate minerals which were primarily dolomite. The clay and fine silt fractions of the carbonate accumulation layers were high in calcite. It was evident that secondary carbonate accumulates as calcite of clay and fine silt sizes. There appeared to be periodic removal and accumulation of carbonates in the Bmk horizon, as it was low in dolomite but relatively high in calcite.
Silt
Carbonate minerals
Fraction (chemistry)
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Carbonate minerals
Replicate
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ABSTRACT There is a marked compositional contrast between hemipelagite and turbidite material of some Upper Cretaceous turbidite units of the Flysch Zone of the East Alps and of Helminthoid Flysch (Western Alps and Apennines). The first is a carbonate-poor or carbonate-free deposit (0%-5% CaCO3), whereas the directly underlying turbidite sediment contains 15% to 60% calcium carbonate. These calcite-free hemipelagites were deposited below the calcite compensation level in deep-sea troughs probably connected with subduction zones or with transform fault zones.
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