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 (original) (raw)
Related papers
Tectonophysics, 2017
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.
Fissure Ridges: A Reappraisal of Faulting and Travertine Deposition (Travitonics)
Geosciences
The mechanical discontinuities in the upper crust (i.e., faults and related fractures) lead to the uprising of geothermal fluids to the Earth’s surface. If fluids are enriched in Ca2+ and HCO3-, masses of CaCO3 (i.e., travertine deposits) can form mainly due to the CO2 leakage from the thermal waters. Among other things, fissure-ridge-type deposits are peculiar travertine bodies made of bedded carbonate that gently to steeply dip away from the apical part where a central fissure is located, corresponding to the fracture trace intersecting the substratum; these morpho-tectonic features are the most useful deposits for tectonic and paleoseismological investigation, as their development is contemporaneous with the activity of faults leading to the enhancement of permeability that serves to guarantee the circulation of fluids and their emergence. Therefore, the fissure ridge architecture sheds light on the interplay among fault activity, travertine deposition, and ridge evolution, provi...
Depositional trends of travertines in the type area of Tivoli (Italy)
Rendiconti Lincei, 2017
A study of a late Pleistocene 30 m-thick travertine borecore drilled in the northwestern sector of the Acque Albule basin (Tivoli, Rome), type area for these freshwater calcareous deposits, it is here presented. The Tivoli travertines have been traced sideways from the core into wire-cut surfaces well exposed in nearby active quarries. They normally appear well bedded and show erosional-non depositional discontinuities, recurring at a centimetric to metric-decametric scale and show a variable penetration by younger sediments (Ca-carbonates as well as reddish soils), as evidenced by their karstic and/or pedogenetic features. The microstratigraphic and sedimentologic analysis has revealed a number of lithofacies grouped in three associations, suggesting environmental domains spanning from gentle slope to shallow lake. This sedimentary organization and stable isotope (O, C) analyses carried out at cm scale have allowed to recognize a hierarchy of higher to lower frequency cycles. On these bases, it is argued and proposed that the higher frequency cyclicity is due to water table fluctuations, as a consequence of millennial-scale climatic changes that in turn may be linked to the solar perturbations (short-term cycles). The lower frequency, instead, cycles (metres to decametres in thickness) suggest medium-term (sub-Milankovitch) and longer term, precession-driven (Milankovitch) periodicities. These conclusions are supported by the correlation of the hierarchical organization of the travertine cycles with a number of radiometrically dated discontinuity surfaces outcropping along the quarry walls and traceable into the borecore. This correlation allows the cyclostratigraphic analysis to be constrained into late Quaternary geochronology and fits well within the precession and obliquity periodicities, as predicted for this time interval.
Earth-Science Reviews
Morphologically-different deposits of thermal travertines are known worldwide, but what factors controlled their morphology, volume, and growth for tens of thousands of years is only partially understood. Two main morphotypes of Quaternary thermal travertines are reconsidered here to understand the reasons for their differ- ential growth: the fissure ridge travertines of Denizli Basin, western Turkey, and the travertine plateau of Tivoli, central Italy. For comparable longevities and average vertical deposition rates, the main differences between the studied travertines are as follows: (1) volume of the travertine plateau is about one hundred times larger than each fissure ridge; (2) despite a larger volume, the travertine plateau does not produce relief, whereas the fissure ridges produce a characteristic prominent topography; (3) the travertine plateau grew primarily through lateral progradation, whereas the fissure ridges through vertical aggradation; (4) travertine deposition occurred in different environments: principally low-energy flat or shallow environments at Tivoli and high-energy inclined environments at Denizli; (5) the growth of the Tivoli plateau occurred in a subsiding basin, whereas the fissure ridges were not influenced by significant subsidence; (6) C- and O-isotope signatures from the two studied trav- ertines are different; (7) despite similar annual precipitations, the present water discharge in the Tivoli area is about ten times greater than that of the Denizli Basin. U-series ages from the two deposits are correlated with paleoclimate oscillations at regional and global scales. Geological field evidence together with paleoclimate cor- relations suggest that, in both the study cases, the main body of travertine deposits (the bedded travertine) grew preferentially when the water table was high (warm and/or humid periods). Conversely, when the water table was depressed (cold and/or dry periods), the Tivoli travertine underwent partial erosion and the Denizli ridges were cut by axial veins and lateral sill-like structures filled by banded sparitic travertine. A comparative model is proposed where the main factor driving the difference in the morphostratigraphic architecture of fissure ridges and travertine plateaus is the volume of water discharge. A high discharge rate resulted in the precipitation of CaCO3 far away from the springs, hence driving the lateral progradation of the Tivoli plateau. A reduced discharge rate caused travertine precipitation close to the springs, thus causing the vertical aggradation of the Denizli fissure ridges. Paleoclimate oscillations must have controlled the amount of fluid discharge, which, in turn, must have influenced the opening of the feeding fractures by an increased pore pressure.
Journal of the Geological Society, 2017
Travertine deposits have long been considered as powerful tools for investigating neotectonics and reconstructing palaeoseismic events. We document, for the first time, the effects of overpressured hydrothermal fluids injected within travertine deposits. We also describe tectofacies interpreted as a consequence of coseismic events. Calcite veins, banded or massive, associated with hydrofracture and fluid-escape features, promoted hydrothermal eruptions and lithoclast formation in latest Quaternary travertine exposed in two quarries near Rapolano Terme (Northern Apennines, Italy). The isotopic composition of the calcite veins confirms the hydrothermal origin of the parent fluids and their rapid ascent, as indicated by the estimated palaeo-temperatures (43-50°C). Integration of U-Th ages obtained for the calcite veins with palaeoseismic evidence from a local archaeological site built at the top of one of the analysed travertine deposits suggests that hydrofracture and fluidescape structures were associated with five main seismic events that occurred from the latest Pleistocene to the fourth century AD. In sum, the travertine tectofacies have a key role in better constraining the seismotectonic setting of a region and thus offer a powerful tool for the evaluation of seismic hazard for areas characterized by low seismicity and travertine deposition.
The notion of "anthropogenic" carbonate deposits takes into consideration human impact on continental limestones precipitated from hot (travertine) or cold (calcareous tufas, speleothems) waters. It is documented here by a geoarchaeological study of the Roman site of Jebel Oust, Tunisia, where the exploitation of a hot spring is attested from the first century AD to the end of Late Antiquity. Petrographical and geochemical analyses performed on travertine deposits offer evidence of the anthropisation of the hot spring and its associated deposits, and provide new elements about water management and bathing practices during Roman times. La notion de dépôts carbonatés "anthropiques" prend en compte l’influence des activités humaines sur la précipitation des calcaires continentaux issus d'eaux chaudes (travertins) ou froides (tufs calcaires, spéléothèmes). Elle est ici illustrée par une approche géoarchéologique du site antique de Jebel Oust, en Tunisie, où l’exploitation d’une source chaude est attestée depuis le début de notre ère jusqu’à la fin de l’Antiquité tardive. L'analyse pétrographique et géochimique des dépôts de travertin a permis de mettre en évidence une anthropisation majeure des dynamiques sédimentaires associées à la source chaude et de fournir des éléments inédits à la question de la gestion des eaux chaudes pour des pratiques thermales dans l'Antiquité romaine.
International Journal of Earth Sciences, 2009
In this paper we describe an example of travertine fissure-ridge development along the trace of a normal fault with metre displacement, located in the eastern margin of the Neogene-Quaternary Siena Basin, in the Terme S. Giovanni area (Rapolano Terme, Italy). This morphotectonic feature, 250 m long, 30 m wide and 10 m high, formed from supersaturated hot waters (39.9°C) flowing from thermal springs aligned along the trace of the normal fault dissecting travertines not older than Late Pleistocene (24 ± 3 ka). A straight, continuous fissure with a maximum width of 20 cm occurs at the top of the ridge, along its crest. Hot fluids flow from cones mainly located at the extremities of the ridge, where travertine is depositing. The travertine fissure-ridge shows an asymmetrical profile since it buries the fault scarp. The difference in height of slopes corresponds to the vertical displacement of the normal fault. Fissuring of the recent travertine deposits along the strike of the crestal fissure, as well as recent hydrothermal circulation, lead us to believe that the Terme S. Giovanni normal fault may be currently active. On the whole, the Terme S. Giovanni fissure ridge can be defined as a travertine fault trace fissure-ridge, adding a helpful example for studying the relationship between faulting and travertine deposition.
A Review and Reassessment of Travertine Classification
Géographie physique et Quaternaire, 2000
This paper provides a review of the classification of travertines with emphasis on their morphology. Three criteria are used to describe them: geochemistry, microfabric and morphology. Geochemically, travertines may be divided into two groups, the meteogene travertines, where the carrier carbon dioxide originates in the soil and epigean atmosphere, and the thermal (thermogene) travertines where the carbon dioxide comes from thermally generated sources. Many travertine fabrics are influenced by bacteria and plants. These include 'stromatolitic' forms, many oncoids, shrubs, tufts, mats and moss travertines. Morphologically, travertines are conveniently divided into autochthonous (spring mounds and ridges, cascades, barrages, fluvial and lacustrine crusts, paludal deposits and cemented rudites) and the allochthonous or clastic travertines (valley-fills, back-barrage deposits, alluvial cones). Travertine deposits often include a wide range of fabrics and morphologies in one syst...
Geomorphology
An enigmatic, c. 2 km-long and 15 m-high travertine ridge, the Colle Fiorito ridge, occurs in the northwestern sector of the Tivoli travertine plateau, central Italy. The main questions addressed in this paper concern the origin and mode of growth of this prominent ridge. The presence of active structures beneath the studied ridge is inferred by recent and past earthquakes located at shallow depths immediately beneath Colle Fiorito. To understand the surficial structure of the Colle Fiorito ridge and the travertine depositional environment, we constructed a 10 m-resolution DEM, analyzed recent and past aerial photographs, and conducted field surveys and meso- to micro-scale sedimentological analyses. To understand the ridge subsurface structure, we studied a set of 32 stratigraphic well logs available from previous works and from the local decorative stone industry, and realized a 2D electrical resistivity tomography (ERT) across the ridge. Results show a gentle antiformal structure affected by subvertical zones of strata discontinuity. The Colle Fiorito structure is interpreted as a previously-unknown fissure ridge travertine grown at the edge of the Tivoli travertine plateau, perhaps when the volumetric deposition rate reached its climax in the plateau for the abundance of fluid discharge and the rise of the water table. Such a fluid pressure may have activated the faults and fractures beneath Colle Fiorito, thus opening new pathways for the ascension of geothermal fluids toward the surface.