## 3 Types of Calcareous Tufa  
Calcareous tufa may be divided into two main groups: autochthonous tufa, deriving from in situ encrusted organisms, and allochthonous tufa, consisting of phytoclasts (encrusted fragments of plants) arenitic (microdetrital facies) and ruditic (macrodetrital facies) in size [21; 22; 23; 24; 25]. Based on the sedimentary facies, autochthonous tufa may be distinguished:  
* stromatolithic tufa, including sequences of laminae (usually 1-10 mm in thickness) formed during short depositional intervals characterized by the presence of particular encrusting microorganisms (Figure 1);
* microenvironment tufa, consisting of strata lens whose fabric reveals the structure of constructing organisms (usually mosses or algae) encrusted in growth position;
* phytodermal tufa, exhibiting a layered/lensoid organization similar to microthermal tufa but larger and composed of large, encrusted plants, usually mosses, receds, and other phanerogams (Figures 2 and 3).  
Figure 1: Stratified stromatolihic tufa in the upper basin of the Esino River (Marche, Italy).  
Figure 3: Phytoermal tufa at Romanatt Dam (Tigray, Ethiopia).  
Figure 2: Plant remains encrusted in phytothermal tufa at the Romanatt dam (Tigray, Ethiopia).  
Allochthonous tufa deposits have a typical clastic texture with fragments of incrustations on vegetal organisms sometimes providing information (e.g., clast orientation, imbrication, etc.) about their transporting flow. Fragments with an irregularly laminated cortex of calcium carbonate, often characterized by a spheroidal to oblate shape and usually referred to as oncoids [26], are common components of allochthonous tufa deposited in streams, rivers, and lakes. [PERSON] [24] attributes the spheroid shapes of grains to high competence flow, the elongated shapes to slow flow, and the irregular shapes to calm waters.  
Clastic fragments cemented by calcareous tufa are sometimes found inside terraced alluvial or slope deposits. They form mainly in the first stages of tufa deposition [27].  
Following Choquette and Pray [28], the porosity of calcareous tufa limestone may be distinguished into non-fabric porosity (produced by fracturing, karstic dissolution, and burrowing invertebrates) and fabric porosity.  
Depending upon the cohesion between the constituting crystals, calcareous tufa deposits range from soft and chalky to dense and highly indurated [6].  
Tufa deposits are affected by meteoric diagenesis soon after deposition when exposed at the surface and by burial diagenesis when overlain by more recent thick sediments [6; 25; 29].  
The principal changes caused by meteoric diagenesis are related to the dissolution/precipitation of calcium carbonate (void filling, cementation) induced by percolating rainwater or groundwater; other diagenetic effects are recrystallization, microbial micritization, bioturbation, oxidation of organic matter and sparmicritization, a term introduced by [PERSON] [30] to describe the etching action of microorganisms at or near the tufa surface [6]. Burial diagenetic effects resulting from increased lithostatic and hydrostatic pressure, heating, and the ingress of mineral-enriched solutions include compaction and porosity reduction resulting from further cementation, dissolution of the original fabric, sometimes with replacement by other minerals, and reactions between the original carbonate component and accessory minerals [6].  
The original differences in porosity combined with those due to diagenesis make the permeability of tufa deposits extremely variable.