Journal of Conference Abstracts

Hot Spring Hillslope-Depression Travertine Facies: Quaternary, Rapolano Terme, Italy

Li Guo (lg203@hermes.cam.ac.uk)¹ & Robert Riding (riding@cardiff.ac.uk)²

¹ Cambridge Arctic Shelf Programme, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

² Department of Earth Sciences, Cardiff, CF1 3YE, UK

Late Pleistocene-Recent travertines up to 40 m thick exposed in extensive quarries in the Rapolano Terme area, Tuscany, exhibit a wide range of lithotypes, facies and sequences. Vent location and the hilly local topography are key factors influencing travertine localization, character and development. Vents on hillcrests and slopes result in white travertines. Depression-fill travertines are darker due to admixture with clay- and iron-rich sediment. Steep slopes result in rapid lateral facies change between hillcrests, slopes and depressions. Rapid vertical facies change also occurs as vent activity fluctuates, as vents switch position, and as slopes prograde and depressions fill. Facies transition is further enhanced by rapid decrease in precipitation away from vents, and the feedback effect of travertine deposition itself which dams and diverts water flow. These hillslope-depression deposits also contain numerous erosion surfaces.

Hillcrest travertines continue to form at the Terme San Giovanni locality where mesothermal water (~38°C) emerging from fault-controlled vents has created a fissure ridge 240 m long and up to 10 m high. Active parts of the ridge crest are covered by small cones; inactive parts are fissured and have small pools. Deposits include crystalline crust, paper-thin raft, and shrub lithotypes. The ridge has smooth and terraced marginal slopes, dominated by crystalline crusts with small shrubs in terrace pools. At the base of the ridge there is a rapid transition to lateral flats and depressions where ridge-water collects and deposits shrub, irregular pisoid, reed, paper-thin raft, and fine-grained and organic-rich travertines.

Location of the Terme San Giovanni vents on a hillcrest provides a wide array of downslope locations for subsequent further deposition. Water channelled to nearby valley sides deposits thick crystalline crusts on valley-slopes and waterfall overhangs; locally with small pools filled by smooth spherical pisoids. On the valley floor, mixing of waters forms varied stream-fill deposits that include micritic reed, paper-thin raft, and coated bubble travertines.

Extensive quarries in older, probably Late Pleistocene-Holocene, deposits show proximal light-coloured slope and terrace travertines passing distally into darker reed mound and depression-fill travertines. Lithotypes include crystalline crust, shrub, pisoid, paper-thin raft, coated bubble, reed, and lithoclast-breccia. High precipitation rates resulted in rapid slope aggradation and progradation. Dilution by rainwater probably lowered precipitation rates in depressions, but deposition was augmented by allochthonous material eroded from upslope travertines.

Slope Depositional Systems consist of Smooth and Terrace Slope facies characterized by white crystalline crusts, with diverse additional lithotypes in terrace pools. Depression Depositional Systems have mixed light and dark travertines with horizontal to gently concave stratification. Extensive light-coloured Shrub Flat travertine is dominant; darker Marsh-Pool Facies composed of fine lithoclast and reed travertine is localized. Reed Mounds composed of mixed light and dark travertines localized by abundant reed growth, formed where spring water emerged near the bases of low angle slopes.

Distal reduction in accretion rate was the major influence on sequence development. Light coloured slope travertines interdigitate with darker depression deposits. Vertical aggradation of slope deposits, mound progradation, and filling of topographic depressions is expressed by advance and retreat of facies. Evolution from depression to slope or mound sequences is termed "steepening up". Up-sequence change from slope or mound to depression facies is termed "levelling up". Exposure surfaces associated with palaeosols are common in all facies and often constitute sequence boundaries. They are more closely spaced in depression sequences, reflecting slower and possibly also more discontinuous accumulation at sites furthest from hot springs.

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