Interpretation of observed fluid potential patterns in a deep sedimentary basin under tectonic compression: Hungarian Great Plain, Pannonian Basin

Abstract

The ≈ 40 000 km2 Hungarian Great Plain portion of the Pannonian Basin consists of a basin fill of 100 m to more than 7000 m thick semi‐ to unconsolidated marine, deltaic, lacustrine and fluviatile clastic sediments of Neogene age, resting on a strongly tectonized Pre‐Neogene basement of horst‐and‐graben topography of a relief in excess of 5000 m. The basement is built of a great variety of brittle rocks, including flysch, carbonates and metamorphics. The relatively continuous Endrőd Aquitard, with a permeability of less than 1 md (10−15 m2) and a depth varying between 500 and 5000 m, divides the basin's rock framework into upper and lower sequences of highly permeable rock units, whose permeabilities range from a few tens to several thousands of millidarcy. Subsurface fluid potential and flow fields were inferred from 16 192 water level and pore pressure measurements using three methods of representation: pressure–elevation profiles; hydraulic head maps; and hydraulic cross‐sections. Pressure–elevation profiles were constructed for eight areas. Typically, they start from the surface with a straight‐line segment of a hydrostatic gradient (γst = 9.8067 MPa km−1) and extend to depths of 1400–2500 m. At high surface elevations, the gradient is slightly smaller than hydrostatic, while at low elevations it is slightly greater. At greater depths, both the pressures and their vertical gradients are uniformly superhydrostatic. The transition to the overpressured depths may be gradual, with a gradient of γdyn = 10–15 MPa km−1 over a vertical distance of 400–1000 m, or abrupt, with a pressure jump of up to 10 MPa km−1 over less than 100 m and a gradient of γdyn > 20 MPa km−1. According to the hydraulic head maps for 13 100–500 m thick horizontal slices of the rock framework, the fluid potential in the near‐surface domains declines with depth beneath positive topographic features, but it increases beneath depressions. The approximate boundary between these hydraulically contrasting regions is the 100 m elevation contour line in the Duna–Tisza interfluve, and the 100–110 m contours in the Nyírség uplands. Below depths of ≈ 600 m, islets of superhydrostatic heads develop which grow in number, areal extent and height as the depth increases; hydraulic heads may exceed 3000 m locally. A hydraulic head ‘escarpment’ appears gradually in the elevation range of − 1000 to − 2800 m along an arcuate line which tracks a major regional fault zone striking NE–SW: heads drop stepwise by several hundred metres, at places 2000 m, from its north and west sides to the south and east. The escarpment forms a ‘fluid potential bank’ between a ‘fluid potential highland’ (500–2500 m) to the north and west, and a ‘fluid potential basin’ (100–500 m) to the south and east. A ‘potential island’ rises 1000 m high above this basin further south. According to four vertical hydraulic sections, groundwater flow is controlled by the topography in the upper 200–1700 m of the basin; the driving force is orientated downwards beneath the highlands and upw...

Hidraulic potential anomaly indicating thermal water reservoir and gas pool near Berekfürdő, Trans-Tisza Region, Hungary

“…1) can be observed in the fluid potential field, in the course of a regional-scale hydrogeologic study of the Great Hungarian Plain, Eastern Pannonian Basin (Tóth and Almási 2001).…”

“…1. river, canal; 2. road; 3. lake; 4. settlement; 5, 6. location and identification code of hydraulic cross-section; 7, 8. trace and identification of seismic section; 9. areal extent of selected p(z) profiles 10. identification code of well (H -hydraulic, V -hydrochemical,intercepts the plane of the fault (Underschultz et al 2005). Conduit faults induce equipotential plumes, sacs and ridges (Tóth and Almási 2001) in the potential field. Those faults reaching the surface lead to the appearance of springs with thermal and/or chemical anomalies, and recharging aquifers at depth (Underschultz et al 2005).…”

“…The anomaly appeared as a "potential plume" on the NNW-SSE oriented (H4) hydraulic cross-section of Tóth and Almási (2001). The "plume" rises from a PreNeogene basement high (-1500 m ASL); its closest approach to the surface is at a depth of 400 m, near Berekfürdõ (Fig.…”

“…1). Tóth and Almási (2001) explained the development of this phenomenon by hypothesizing the presence of structural elements, which cut through the thick, regionally extensive low-permeability (aquitard) Neogene strata (e.g. Endrõd and Algyõ formations).…”

Soil and wetland salinization in the framework of the Danube-Tisza Interfluve hydrogeologic type section

“…Below the Algyõ Aquitard, the turbiditic siltstone, sandstone and clay series of the Szolnok Aquifer and the shaly series of the Endrõd Aquitard are characterized by hydraulic conductivities varying between 10 -9 and 10 -6 ms -1 . The Algyõ Aquitard is overlain by the Great Plain Aquifer with an assigned average K of ~10 -5 ms -1 (Tóth and Almási 2001).…”