Factors controlling growth of modern tufa: results of a field experiment - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 165

Fig. 17. Larval housings: (a) larval housing within tufa composed of calcified cyanobacterial filaments, note sparry
crystals displaying competitive growth pattern which cover the housing, tufa formed on limestone tablet at L ´ ˇ ky E site
between August 2002 and October 2003; (b) housings arranged one over another, tufa formed on limestone tablet at
L ´ˇ ky E point between August 2002 and October 2003; (c) horizontally arranged housings, tufa formed at H ´ j lower
waterfall point between August 2002 and June 2003; (d) field photo of tufa at H ´ j site covered by larval housings with
meandering pattern, June 2003, coin is 22 mm across; a - SEM image; b, c - thin sections.

Moreover, they are regarded as a typical 'winter
deposits' (e.g. Pedley 1992, 1994). During the
present experiment such laminae grew at all the
studied sites. They always build the lowermost
layer deposited on the copper tablets, where they
lack biogenic components (Figs 13a, 14a & 23c).
They were formed also during the winter season,
for example at the H ´ j and Karw ´ w sites. Sparry
laminae abound in samples from waterfalls, as at
L ´ˇ ky, H ´ j and Karw ´ w (Figs 16d, 22 & 23a).
Sparry crystals commonly develop on plant debris,
especially leaves, and larval housings, and on any
other objects introduced into the tufa-depositing
environment (Fig. 23d).

during autumn and early spring (Figs 12f & 19d).
The quartz grains at that site derive from a loess
cover which is easily eroded. As carbonate is a
ubiquitous component of the nearby loess, it most
probably enters the tufa-depositing stream and is
introduced into growing deposits as well. Other allo-
genic components: clay minerals and iron oxides,
were detected by EDS analyses. Plant debris,
especially leaves and small twigs are also commonly
incorporated into the growing tufa (Figs 9b, c & 23d).

Lamination. Three types of lamination were
recorded within the tufa that grew on the tablets.
The most common type of lamination is one with
alternating laminae of sparry crystals and micrite
(Fig. 18a - d). Boundary between the laminae may
be accentuated by corrosion. Corrosion is best
visible on the upper surfaces of crystalline laminae
underlying the micritic ones. Subtle changes in
crystal size, probably in conjunction with changes
in

Detritic components. Although all the studied tufas
are autochthonous, they include some admixture of
detritic components. Fragments of redeposited tufa
are probably the most common, but they are hard
to be distinguished. Fine sand- and silt-sized
quartz grains are found at all the studied sites,
except Z´zriv´. They are particularly common at
the Karw´w site, especially in the tufa deposited

porosity are responsible for the lamination

visible within hemisphaerically layered micrite

(Fig. 19b, c).