Holozäne Süßwasserkalke im Alme und Aftetal südlich von Büren (Kreis Paderborn, Nordrhein-Westfalen).-
Münster. Forsch. Geol. Paläont., 69: 261 - 289.
In the Alme and Afte valley and its side valleys located at the southeastern margin of the Westphalian Cretaceous Basin, Holocene tufa deposits are widespread. Tufa develops only in hard-water creeks and small streams close to karstic springs. In wide valleys unconsolidated, chalky tufa accumulations reach up to 6 m thickness and may cover an area of up to 80 000 m2. Laterally, the carbonate sediments interfinger with fluviatile clastic and sometines also with paludal deposits. However, hard but nevertheless highly porous tufa bodies commonly deelop in small creeks with steep gradients. Due to turbulent flows or thin water films, evasion of dissolved CO2 is effectively accelarated thus resulting in predominantly inorganic calcium carbonate precipitation. Hence, fast flowing creeks, cascades and waterfalls show high tufa accretion rates. The decrease in dissolved CO2 as a result of increasing water temperature or photosynthetic activity of cyanobacteria, algae and plants seem to play a minor role in the precipitation of calcite.
Together with the tufa deposits oncoids occur in some creeks, somtimes lying colose to one another. In the main site they develop by accretion around a nucleus, e.g. plant derived fragments and pebbles up to a diameter of 25 cm. Considering the gentle slope of the creek and the low water discharge it is obvious that large oncoids were never overturned. Consequently, an in-situ growth of at least part of the ocoid population is suggested.Due to the coarse-grained substrate, sufficient diffuse light reaches the undersurface, thus allowing for continuous cyanobacterial growth.
Pollen analysis of organic rich horizons intercalated in the calcareous deposits reveals a late Preboreal beginning of the tufa precipitation. Probably, the main deposition took place during the late Boreal and Atlantic stages. However, tufa formation is still in process.
Last changes Nov. 2004 by Reinhold Leinfelder