NJG83-4-03
2003
Diagenesis of the mid-Carboniferous Minkinfjellet Formation, central Spitsbergen, Svalbard
83
4
pp. 319-331
82-92394-11-3

The Moscovian Minkinfjellet Formation, deposited in the NNE-SSW striking Billefjorden Trough in central Spitsbergen, is a carbonate-dominated succession with interbedded sandstones and evaporites. The sediments accumulated in a narrow, shallow marine basin where tidal currents and high frequency sea-level changes controlled sediment distribution and depositional environments. After deposition the succession was influenced by a series of diagenetic effects, and the carbonate rocks, in particular, were altered by several different processes. Three main generations of calcite cement have been recognised from both petrographic and stable isotope studies. These are: (i) early marine fibrous cement, possibly originally aragonitic (ii) drusy spar cement formed during flushing by meteoric water, and (iii) coarse, crystalline pore-filling cement precipitated during deeper burial of the rocks. ?18O values from these cements become successively isotopically lighter, and show typical values for each of these diagenetic environments. Complete or partial dolomitization is common and widespread, presumably caused by flushing of hypersaline Mg-rich brines. The abundance of interbedded gypsum in this succession supports a seepage-reflux model of dolomitization. Chert nodules and chert layers are most common in the carbonates, both limestones and dolomites. Three mechanisms of silicification are recognised: (i) Replacement of gypsum nodules by silica. These show growth of quartz crystals toward the nodule centre, where some gypsum may still remain, (ii) Concretionary growth of finely crystalline glassy chert nucleated around organic matter, which has induced the precipitation of silica, (iii) Intergranular megaquartz formation in ooidal or peloidal limestones. In these ooid grainstones calcite spar was effectively replaced by megaquartz due to infiltration of silica-rich solutions through crystal defects or along crystal contacts. The distribution of chert in the Minkinfjellet Formation rocks was controlled by the initial accumulation of siliceous distribution of silica sponge spicules.

Quartz cement has developed in all investigated sandstones in the form of quartz overgrowth around grains. Many sandstones show pore-filling calcite cement engulfing the quartz cement, effectively reducing the original porosity.

029-196X
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