Factors controlling growth of modern tufa: results of a field experiment - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 161

Fig. 13. Small filaments - probably encrusted diatom stalks or fibres of EPS: (a) small curved moulds within sparry
crystals, the moulds are more than 20 mm above the basement, tufa formed on Cu tablet, Z ´ zriv ´ site, between August
2002 and October 2003, arrow indicates position of tablet before its detachment; (b) moulds in sparry crystals lying
directly on limestone tablet, arrow indicates its upper boundary, L ´ ˇ ky top point, tufa on limestone tablet exposed
between August and November 2002; (c) transversal and oblique cross-sections of filament moulds; (d) pit in a
sparry crystal having been an attachment point of diatom stalk, L ´ˇ ky top point, December 2003; a, b - thin sections,
c, d - SEM images.

occurrence of both groups, Trichoptera and Chiro-
nomidae there (A. Kownacki, pers. comm., 2003).

accompanied by diatoms, cyanobacteria and their
EPS. Clotted micrite commonly hosts larval

housings.
Clotted micrite is found in nearly all studied
samples. It predominates in samples growing in
sluggish flow conditions and is rare in waterfall set-
tings (cf. Pedley 1992). If it occurs in the latter set-
tings, it forms thin laminae alternating with sparitic
ones or it fills shelter cavities, for instance between
sparite crystals. Clotted micrite never forms the
lowermost layer of tufa directly on Cu tablets.
Although the lowermost layer may be built of
minute crystals, it does not display clotted texture
or the presence of algae (Fig. 18c). Such crystals
quickly augment upward, growing up according to
the rule of competitive crystal growth (cf. Gonz ´ lez
et al. 1992).
The above observations seem to confirm the
view by Pedley (1992, 1994, 2000) that micrite in
tufa is genetically connected with micro-organisms.
Microbial origin of micrite was, for example, postu-
lated also by Jones & Kahle (1995) who studied

Carbonate precipitates and their textures

Clotted micrite. Micrite displaying clotted texture
is common in the studied tufa samples (Fig. 18).
It consists of anhedral crystals approximately a
few micrometres across on average. The crystals
agglomerate and form rugged clots several dozen
micrometres across. Micrite builds laminae up to
0.5 mm thick, which commonly fill irregularities
in its basement, such as intercrystalline porosity
of underlying sparite laminae (Fig. 18d). It also
engulfs sparite crystals (Fig. 18e). In some
samples the substrate of micrite layer is corroded
(Fig. 19a), which is better visible when the substra-
tum is composed of sparry crystals.
Diatom frustules or their moulds and cyanobac-
terial filaments abundantly co-occur with micrite
(Figs 15b & 18f ). On the surface of growing tufa
the minute carbonate crystals building micrite are