Calcareous tufa as indicators of climatic variability: a case study from southern Tuscany (Italy) - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 265

Fig. 5. Calcareous tufa facies in the Valdelsa synthems. Association B: (a) Alternated lime-mudstone and calcareous
sands in a palustrine tufa succession (FOC Synthem, Gracciano Val d'Elsa). (b) Small phytoherms and clusters
of phytoclastic material included in the sandy facies of the calcareous tufa (FOC Synthem, Bagnoli). Association C:
(c) lithoclastic accumulation (lithoclast sands and conglomerates) with small irregularly distributed phytoherms
(CAL Synthem, Colle Val d'Elsa). (d) Section of the CAL Synthem near Poggibonsi; the contact with the Pliocene
substrate is marked by a broken line.

crystalline and shrublike vertical structures (Fig. 6b)
alternated with micritic mudstone (Fig. 6c) locally
enclosing lenses of accumulated fragments of
calcite rafts (Fig. 6d). The chalky, porous facies
made of peloidal micrite lacks any trace of macro-
phytes or faunal elements (ostracods and gastro-
pods). Some of these features are identical to those
illustrated for deposits forming in hydrothermal
systems: the crystalline and shrublike vertical struc-
tures (Fig. 7), described by Chafetz &Guidry (1999)
and the calcite rafts illustrated by Folk et al. (1985)
and described as 'paper thin rafts' by Guo & Riding
(1998). These lithologic features, unusual for the
calcareous tufa characterized by roughly bedded,
unlithified chalky deposits rich in micro- and macro-
phytes remains, could be interpreted as originated
on carbonate-rich waters flowing from hydrother-
mal springs (Capezzuoli et al. 2008).
Sedimentological and petrographic evidence
suggest that the carbonate-rich waters flowing at
Bagnoli in the Late Pleistocene Imbotroni Valley

were mixed waters in part of meteoric/karstic
origin in part cooled waters derived from a
thermal source located upstream and now covered
by younger deposits. We can speculate that a
thermal spring was building up a local small traver-
tine system, as suggested by the occurrence of
well-bedded shrub laminites and paper-thin rafts.
Laterally this water should have been mixed with
ambient temperature, low carbonate water of the
Imbotroni Creek that upstreamwas unable to precipi-
tate carbonate deposits (Fig. 1b). This new mixed,
carbonate enriched water, could precipitate tufa
deposits in ponds andmarshes of the richly vegetated
fluvial valley system. Ponds filled with carbonate
mud inhabited by ostracods and gastropods were
bounded by shallow barrages colonized by bryo-
phytes and small cyanobacterial tufts able to entrap
encrusted fragments of the surrounding vegetation.
Thus, we can speculate that the laminated
carbonates were precipitated from hydrothermal
waters perhaps comparable to the low-temperature