Factors governing travertine deposition in fluvial systems: The Bagni San Filippo (central Italy) case study

Although fossil fluvial travertines have been described, factors controlling their formation are poorly understood because of the paucity of their modern counterparts. To disclose processes affecting their deposition, a modern fluvialtravertine system at Bagni San Filippo (Siena, central Italy) was studied here and compared with other cases. Thestudied travertines occur in a valley with an ephemeral stream, but continuous water influx from hot springs onthe hillside makes part of the stream become perennial. Four sub-environments were recognized: slopes, waterfalls,pools, and channels. The first two are mainly composed of laminated crystalline crust-boundstone. In contrast, poolsare mainly covered by lime mud and some post-flood travertine-encrusted breccia, while channels commonly display abundant travertine-encrusted breccia. Many erosional features and products were also found and are largelythe result of episodic erosion triggered by heavy rainfalls and accompanied floods. The predominant erosional processes might include abrasion and plucking. Based on environment distribution, stream bed morphology, and erosional distinctions, the fluvial system was distinguished into slope-pool-waterfall and channel-pool-waterfallsubsystems. Such system differentiation is attributed to the original stream bed difference: wide beds promotedthe development of slopes, while narrow beds encouraged travertine erosion and subsequent gravel accumulationduring flood events, favoring the formation of channels. The comparison shows that fluvial travertine depositionlargely occurs within existing rivers. The relative contribution of spring water to original river water controls the deposit composition (the higher contribution ratios, the more abundant carbonate/travertine facies). Furthermore,erosion might be common and unavoidable because the reported fluvial travertines were all formed in non-arid regions. These findings suggest that fluvial travertine deposition is influenced by topography, hot spring contribution,original river bed geometry, and fluvial erosion, and might aid in the interpretation of ancient fluvial travertine systems. Additionally, downstream fluvial travertine systems might show similar characteristics to fluvial tufas, buttheir formation is clearly hot spring influenced, indicating the importance of analyzing facies distribution anddelta13C-delta18O signatures in fluvial carbonate studies.