Lower Ordovician mud mounds from the St. Petersburg region, northwestern Russia

Hecker-type mud mounds are calcareous-clay buildups occurring in the Lower Ordovician (Billingen and Volkhov regional stages), condensed cool-water carbonates of northwestern Russia. The unusual feature of these buildups is the dominance of terrigenous clay in their cores. In all the buildups an initial gently-sloping mound of unconsolidated fossiliferous clay rests on a hardground surface. A layer of microsparite overlies the clay, with another hardground surface on the top. The mud mounds vary in diameter from tens centimetres to hundreds metres. Large mounds form complex multi-storey structures and contain sedimentary facies, which differ markedly from the facies of surrounding rocks. There is evidence that siliceous sponges formed these mud mounds. The Hecker-type mud mounds are moderate to cold-water sponge buildups with a unique combination of features found in their cold and warm water Phanerozoic counterparts.