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GEOCHEMICAL CLUES ABOUT A DEEP RESERVOIR THAT SUPPLIES SALINE FLUIDS TO GROUNDWATER OUTFLOWS IN THE SOUTH-EAST CARPATHIANS (ROMANIA)

Horia Mitrofan

Abstract

The East Carpathians Bend region includes a multitude of natural discharges of saline groundwater. Some of these outflows - such as the mud volcanoes at Beciu and a couple of springs at Lep?a - have previously been shown to be sensitive to the intense seismic activity which developed in that area. Yet no attempts have so far been made for exploring relationships that possibly existed between those brines chemical characteristics, and the local geo-structural setting: the latter consists of a stack of successively overthrust nappes, that are built up mainly of Cretaceous and Neogene flysch deposits, with the most external thrust front being partly overlain by formations deposited subsequently (since the Sarmatian) in the Carpathians Foreland. All considered brines have a Na-Cl character and their overall chemical facies is, as well, quite similar (i.e., the concentration ratios of all the main chemical constituents are comparable). In addition, the Sr+/Ca2+ ratio is - both for Beciu, and for Lep?a - very large (0.21-0.23); much larger than, for instance, the seawater Sr/Ca ratio of 0.019. Overall, this seems to indicate, besides a Sr enrichment process, a common provenance for all those saline fluids. The latter inference is however rather unexpected, if one takes into account that the Lep?a springs discharge from formations belonging to the so called ?Marginal Folds? nappe, while the Beciu mud volcanoes occur within the Foredeep deposits, and that the horizontal distance between those two sampling sites is about 60 km. Still a presumed common reservoir might be hosted by another ? so called ?Subcarpathian? - nappe: the latter is overlain - in the normal succession of the overthrust units - by the Marginal Folds nappe which hosts the Lep?a springs; while the Subcarpathian nappe most external (eastward) section is covered by Foredeep deposits from which the Beciu mud volcanoes discharge. At the same time, Mg-Li geothermometer assessments indicate that the initial temperature of the Beciu brine supplied by the inferred deep-origin reservoir could amount to 160В°C: accordingly, that reservoir depth would be 4.8 km - which is consistent with the position occupied by the Subcarpathian nappe in the concerned region.

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DOI resolver: doi.org/10.5593/sgem2018/1.1/s01.015

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