| Abstract |
Carbon dating (14C) ages ranging between 10,700 and >42,650 years BP have been reported by various investigators for organic matter (charcoal fragments included in a pumice flow deposit; paleosoil underlying the pumice deposit) from Ciomadul volcano, next to Tusnad-Bai. This is the most recent volcanic eruption in Romania documented to date, and it implies the existence of a still active magma chamber, with which a high temperature (>225ÅãC) geothermal system might be associated. The most significant hydrothermal activity in the area consists of a lineament of springs that discharge about 5 kg/s (aggegate flow rate) of sodium chloride thermal water which is CO2 rich, relatively low in TDS (1.5-4.5 g/l), and has a maximum temperature of 23oC. The Na-K geothermometer indicates that the spring waters could have originated from a hot (.300oC) reservoir of thermal fluid. At the same time, the K-Mg geothermometer points out that chemical re-equilibration becomes more advanced further south. Actual outflow temperatures of the springs and their TDS contents accordingly decrease southward. Next to the thermal springs a well has been drilled to a total depth of 1120 m. During tests it discharged 4 kg/s of sodium-bicarbonate, thermal water, with 12.5-13 g/l TDS, at a temperature of 63oC. The Na-K geothermometer indicated that, compared to the springs, the water discharged by the well originated in a reservoir separate from and cooler than (at .180oC) the reservoir which conceptually feeds the aforementioned thermal springs. The poorer results of the well (compared to the geothermometer indications of the springs), also suggest that the area of surface hydrothermal manifestations at Tusnad-Bai represents only a lateral, shallow outflow from a more distant and deeper, hot geothermal system (possibly located several kilometers to the north). In similarly mountainous areas, such settings have been previously recognized world-wide. Thermal gradients recorded in shallow (<50 m deep) boreholes provide additional evidence for the inferred setting: the 250oC/km contour extends northward from the surface discharges, over lava domes which are significantly less permeable than the pyroclastics from which the thermal springs emerge. Such a high thermal gradient recorded on a virtually impervious formation also suggests that a lateral, southward directed outflow probably occurs beneath that lava dome. |