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Kirjeldus

From Mens, K., & Pirrus, E. (1997). Vendian. In Geology and mineral resources of Estonia (pp. 35–38). Estonian Academy Publishers: 

The Kotlin Formation. The name was introduced by Sokolov to designate the “Laminarites” Clay (Männil 1958a). The formation is spread in eastern Estonia, in a more typical form in its northeastern part, attaining a thickness of 52.6 m in the Narva borehole. In the west direction, the thickness decreases quickly, pinching out on the Tapa - Ellavere line. The formation is known only from core sections, and the interval of 109–150 m of the Meriküla core has been defined as the hypostratotype. The lower boundary is drawn at the level where multicoloured deposits turn grey. At the base of the formation, gravel and coarse-grained sand occur locally.

The dominant components of the formation are thinly laminated grey claystones with intercalating light-coloured very fine-grained sandstones or siltstones, or both. The lamination is complicated by the occurrence of dark-brown films of organic matter.

The rocks of the formation are low in sand and silt, the content of which in the upper- and lowermost parts only locally exceeds 50%. Quartz and feldspars (particularly K-feldspar) are the main detrital minerals of sand and gravel grain-size. The content of micas, including biotite and muscovite, is also notable. Their ratios depend on the type of rock. The rocks are characterised by a small content of both opaque and transparent allogenic minerals. Heavy minerals are dominated by siderite and pyrite of authigenic origin. Illite is a dominant clay mineral; the content of kaolinite ranges from 15 to 40%, the latter value being fixed in the lowermost part. Chlorite is common; in the middle part of the formation, its average content is 15–20% (Pirrus 1970).