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Mixed thermal and ambient water travertines of ancient Roman spa – Aquae Iasae - Varaždinske Toplice, Croatia

The present-day town and spa of Varaždinske Toplice (Northern Croatia) was in the 1st-4th centuries AD the site of the Roman settlement - Aquae Iasae. Thanks to the archaeological excavations and research, at the uppermost terrace of the settlement, an area (ca 6000 m² ; ; ) with Roman architectural remains has been uncovered. This consists of the baths (thermae), basilica, Capitoline temples and the forum with the stone-built pool where the sulphur-rich thermal water springs out. Water that spilled over was in Roman times drained to the south. Latest archaeological research of the Roman basilica, performed in 2004, showed that slightly downslope, above the basilica floor, 3 m thick sediment succession occurred. As they covered the floor of basilica, the age of the succession was estimated to no more than 1600 years. These deposits were considered downslope precipitates of the thermal spring run-off waters influenced by the surface water dilution but with overall travertine characteristic. XRD analysis shows simply calcite as constituent mineral. Precipitation, occurred some distance SE from spring, covering the abandoned Roman buildings and filling depressions. At the base of travertine succession thin, dense, laminated sediments of lithotype L-1 occurred. Microbial laminae, as well as faint pizoid « ; ; ghosts» ; ; , suggest bioticaly influenced precipitation in rather shallow depression with occasional subaeral conditions. Subsequent deposits represent incrustation of higher plants, possibly reeds, which resulted in thick-bedded, porous lithotype L-2 pointing to rain-diluted and cooled waters from the hot spring. Plant stems were incrusted by sparry calcitic, arboresced or ray shaped crystals. Between the plant molds paper-thin rafts are accumulated reflecting crystallization at the surface in shallow stagnant water. Abundant presence of rafts, precipitated at the water surface of still pools, creates delicate, porous « ; ; foamy» ; ; younger layers of lithotype L-3. Intercalation of L-2 and L-3 lithotypes suggest that the deposition was changing between more-or-less dry vegetated flats and shallow still pools. The remainder interlayers is represented by yellow-white sand-sized lithoclastic lithotype L-4. Particles strongly suggest primary bioticaly and abiotically precipitated calcareous deposits. Quartz particles were also noticed. Apparently, the origin of clasts includes lateral (upslope) existence of travertine crusts. Those deposits suffered mlechanical erosion or partial solution in atmospheric conditions after which the remaining particles has been transported and accumulated in nearby depression. Composition of spring water, mixing of thermal and ambient waters, volume of water as well as humid climatic effects and subaeral reworking determined the morphology of travertine lithotypes described.

travertine ; thermal water ; geoarchaeology ; Aquae Iasae ; Croatia

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