6.3 SEDIMENTARY MODELS
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Fig. 6.27. Photograph of varved layering from an Irish Pleistocene periglacial lake. (Courtesy of J. M. Cohen.)
posed of a couplet silt and clay. The silt was deposited from summer glacial melt water.
The clay settles from suspension during winter when there was no runoff into the lake.
Lakes in arid and/or low-lying areas may lack terrigenous input, but their deposits are
still often varved. In these cases the varves consist of couplets of organic mud and lime.
6.3.2.4.2 Autochthonous permanent lakes
Kukal's second class of lake occurs in low-lying terrain in temperate and warm humid
climates. Minor amounts of fines are brought in by rivers. Carbonate sedimentation occurs away from the river mouths, both around the lake shores and in the deeper parts
of the lake. This takes the form of charophyte calcareous algal marls in the center of
the lake. Around the shores wave action may disaggregate freshwater mollusk shells
to form calcarenites. This type of sedimentation occurs in the north German lakes such