Место издания: Florence , Год издания: 2004

The Lower-Middle Cambrian Kuonamka Formation is widespread on the Siberian Platform. There are two tectonic areas of its expansion - stable part, where the thickness of the Formation is close to 30 m ( Molodo River basin) and Yudomo-Mayskaya depression, where its thickness reaches 850 m (Kachelkuan River basin). In accordance with it the level of the catagenetic transformation of the Formation in these areas is different. In the stable part of the Platform the reflectance of organic matter (in %) ranges from 0.44 up to 0.53, in the area of the Yudomo-Mayskaya depression it is equal 6.11-7.25. Phosphories in he Molodo River basin are related to the black shale bed in the lower part of the Kuonamka Formation (upper part of the Lower Cambrian), where they are represented by the coarse nodules. In the Kachelkuan River basin two types of phosphorites were investigated . One of them is represented by the fine nodules is confined to the black shale in the lower part of the Formation, another one consists of the coarse nodules - to the higher its part. The Upper Jurassic black shale Bazhenov Formation is distributed at the West Siberian Geocyneclise. The thickness of the Formation is mainly 25-30 m. The phosphorites were revealed in the drill hole core from the depth of 2920 m and underwent, of course, the deep catagenic influence, too. Phosphorites in the Bazhenov Formation are observed very rarely and have been not studied before. The content of REE in Bazhenov and Kuonamka (Molodo River Basin) Formations phosphorites, given below, is rather close (the first figure is the percentage of REE in phosphorites of the Bazhenov Formation, the second one - in phosphorites of the Kuonamka Formation, Molodo River basin): La: 188-166, Ce: 293-334; Nd: 146-194; Sm: 35-52; Gd: 36-57; Tb: 5.94-9.5; Yb: 20-25; Lu: 2.81-3.42. In the fine and coarse nodular phosphorites of the Yudomo-Mayskaya depression the percentage of REE is accordingly: La: 352-184, Ce: 726-437; Nd: 323-216; Sm: 64-51; Gd: 50-45; Tb: 7.7-6.8; Yb: 10.4-13.1; Lu: 1.27-1.24. Shale-normalized REE distribution reveals the depletion in HREE - Yb and Lu, or only Lu - in all studied samples. It doesn't correspond to the existent preposition that this effect is typical only for "old" (Vendian and Early Cambrian) phosphorites. It can be proposed that really HREE depletion corresponds to the level of the catagenic transformation of phosphorite, which is most strongly pronounced in the Yudomo-Mayskaya depression.