Comment on “First records of syn-diagenetic non-tectonic folding in Quaternary thermogene travertines caused by hydrothermal incremental veining” by Billi et al. Tectonophysics 700–701 (2017) 60–79

Abstract

Billi et al. (2017) proposed a new interpretation for the origin and internal structure of thermogene travertine deposits. On the basis of evidence from two quarries located in southern Tuscany (Italy), they interpreted some travertine beds as calcite veins and argued that undulating travertine beds formed by syn-diagenetic (i.e. non-tectonic) folding that was caused by laterally-confined volume expansion caused by incremental veining. They assumed that such a process causes changes to the rock properties, including porosity reduction, rock strengthening, and age rejuvenation. The interpretations by Billi et al. (2017) challenge and question the current understanding and interpretation of thermogene travertine deposits. This understanding, based on numerous studies since the 1980s, is that these deposits form from thermal water flowing downslope, and precipitating calcium carbonate. Here, we explain how the comparison with active depositional systems is essential for the understanding the origin of structures in older, inactive travertine deposits, such as those studied by Billi et al. (2017). We further argue that the three-dimensional setting of travertine deposits should be taken into account in order to discuss the possible development of secondary structures. Indeed travertine deposition on slopes typically leads to the formation of terraced morphologies with pools bordered by rounded rims and separated from each other by steep walls. The resulting three-dimensional structures can be misinterpreted as asymmetric folds in two-dimensional views (i.e., in saw-cut walls of quarry). In this paper we debate the interpretations offered by Billi et al. (2017) and their criteria to recognise syn-diagenetic, non-tectonic folds in travertine deposits, and explain why many of their ideas are questionable.

3D reconstruction of the Lapis Tiburtinus (Tivoli, Central Italy): The control of climatic and sea‐level changes on travertine deposition

“…Generally, this lithofacies association laterally passes to laminated deposit, shrub and intraclast deposit lithofacies types. At the macro scale, the crystalline crusts appear as laminated bands developed perpendicular to the substrate with variable sizes, ranging from centimetres to decimetres, erroneously interpreted as syn‐sedimentary folds (Alçiçek et al., 2017). Petrographically, crystalline crust appears as vertical fans of dendritic crystals forming cementstone.…”

Facies and early diagenesis of rainwater-fed paleospring calcareous tufas in the Kurkur oasis area (southern Egypt)

“…Speleothem-like flowstones are very common in tufa depositional systems, and are interpreted as precipitated by fastflowing water in smooth to stepped slopes and rims of pools as a result of CO 2 -degassing from water rich in carbonates (e.g. Guo and Riding 1998;Jones et al 2000;Gandin and Capezzuoli 2014;Alçiçek et al 2017). Speleothem-like crusts are dominantly abiotically-formed, crystalline and hard, and show fine laminae (e.g.…”

Fissure Ridges: A Reappraisal of Faulting and Travertine Deposition (Travitonics)

“…Here, a transition to typical tufa fabrics can occur due to the progressive cooling of the thermal water or mixing with surface rainwater. The rapid lateral juxtaposition or shifting of all these depositional facies [21,58] is a consequence of several microenvironments (waterfalls, pools, microterraced slopes, fans, and smooth slopes) rapidly developing one upon the other and determining undulating geometries (Figure 11b,f) along the slope gradient; note that these latter are deposition-related geometries that are not the consequence of syn-diagenetic folding (i.e., non-tectonic) [59]. The vertical accretion of the slope gradient causes diversion of the water flow that therefore can give rise to non-conformities where paleosoils and karst phenomena can develop (Figure 11f,g), bounding every single depositional unit.…”