Oncolite
Onkolithe are sedimentary rocks that contain Onkoide or even predominantly built up from these. The term goes back to the geologist Albert Heim, who coined it in 1916.
The products are Onkoide biogenic precipitation, for example by algae, and are irregularly rounded, often circular or elliptical shape. In them surrounding a ply constructed, constructed concentric sheath, usually made of calcium carbonate, a center which sometimes consists of sedimentary particles such as shells. The shell structure may be irregular in larger Onkoiden and is on the side of the sediment overlying interrupted. In contrast to the similar structure, usually a few millimeters in size ooids Onkolithe are several centimeters in diameter often much larger. They arise as ooids in marine and maritime areas with moving water, but with less sediment.
Widespread is the matrix of Onkolithe where Onkoide happen also calcareous, and contains other biological radicals, such as shells and Crinoidenreste. Some Onkolithe bituminous smell after breaking.
Onkoide are already known from the Proterozoic, but they also occur younger rocks. Examples of today's Onkoide are about the Maritschnegg List A of Lake Constance.
Recent or Subrecent freshwater Onkoide in Alz ( Chiemgau, Upper Bavaria)
The Alz, the outflow of Lake Chiemsee, a summer hot and winter cold and calcareous river, in its upper reaches ( Upper Alz ) between Seebruck and Altenmarkt an der Alz at some favorable points freshwater Onkoide and Onkolith formation ( Kalkflächen or limestone cliffs ) is occurring. The Chiemsee is the river as an upstream buffer in relation to water management ( quantity) and sediment management. The seasonally -related flood with high turbidity and sediment transport of the Tiroler Ache, on the one hand when the snow melts in spring, the other during long lasting periods of rain in the summer, as well as reduce the Alz reach.
Without this buffering the growth of Onkoide would not be possible, because only with a suitable flow rate, sufficient light ( low turbidity ) and sufficient heat are given appropriate training conditions. Especially in the uppermost part of section to Truchtlaching (low gradient / low flow velocity ) are documented extensive areas with Onkoiden and Onkolithen.
The on the nuclei ( nucleus) growing up from glaciofluvial gravels lime layer is formed by calcifying cyanobacteria. Layer thicknesses of several centimeters are not uncommon. By designing of labeled nuclei (growth experiment ) and the observation over several years could be demonstrated that in situ growth of the Alz - Onkoide.
Recent or Subrecent freshwater Onkoide in the Moosach bei Freising (Bavaria )
The Moosach is a left tributary of the Isar and whose source was originally in the area of the local part Moosach in Munich. In the area of Freising is the river running parallel to the river Isar in the Isar River and empties into this. Shortly before the estuary at Oberhummel, between the villages Hangenham and Asenkofen, the incidence is of recent Onkoiden ( tufa onciods ) in the booklet for international Kalkowsky Symposium " Geobiology of Stromatolites ", described in 2008. The size of the recent Onkoide is given as 2-15 cm. It is explicitly on the comparability of ratios as in other pre-alpine rivers such as Alz are observed and documented, pointed out.