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Hydrology and denudation rates of halite karst
.•
Journal of
Hydrology
ELSEVIER
Journal of Hydrology 162 (1994) 171-189
[2]
Hydrology and denudation rates of halite karst Amos Frumkin 1 Israel Cave Research Center, Department of Geography, The Hebrew Universityof Jerusalem, Har Hatsofim, Jerusalem, Israel
Received 29 November 1993; revision accepted 11 January 1994
Abstract
Salt karst terrains exist mainly in arid climates where rock salt outcrops may escape complete destruction by dissolution. Such is the case with Mount Sedom, on the SW shore of the Dead Sea, one of the most arid parts of Israel. Many small catchments developed over the relatively insoluble cap rock which overlies the highly soluble rock salt. The catchments were surveyed and classified. Some 57% of the surface area is drained by an underground karst system. Water samples from various points in the system were analysed, and water development was inferred. Waters in cave conduits do not reach saturation during flood flow, unless the water is ponded for at least several hours. Based on the available evidence, regional karst denudation is tentatively estimated to be about 0.5-0.75 mm year -1, occurring mainly within the rock salt.
1. Introduction
Unlike carbonate rocks, salt dissolves by a simple dissociation reaction. The simplicity o f the process allows easy determination of geological and hydrological factors affecting salt karst development. Although salt is a c o m m o n sedimentary rock, salt outcrops are rare in non-arid climates. Its extreme high solubility (360 g 1-1 at 20°C) permits total dissolution of an exposure by meteoric water. Consequently, salt outcrops appear mainly in arid environments, where they escape complete destruction. Karst water chemistry and denudation have been widely studied in carbonate rocks (Ford and Williams, 1989, pp. 42-126), but there have been few such studies in shallow salt karst. This paper discusses the types of karst water and drainage patterns in the M o u n t Sedom salt karst system. Based on these data, an estimate of the regional denudation rate and distribution is attempted.
rePresent address: Department of Geography, McMaster University, Hamilton, Ont., Canada L85 4K1. 0022-1694/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(94)02457-M
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A. Frumkin / Journal of Hydrology 162 (1994) 171-189
2. The study area Mount Sedom is a rock salt diapir, forming an elongated ridge 14 km 2 in area, rising 250 m above the Dead Sea (Fig. I). It consists of Plio-Pleistocene(?) beds of rock salt of marine origin (Zak, 1974), piercing through younger lake evaporites and clastics. Before its subaerial extrusion, the top of the rising diapir rock salt was partially dissolved by ground-water. The residual anhydrite, marls, shales, chalk and sandstone have accumulated to form a cap rock up to 50 m thick (Zak and Bentor, 1968). Late Pleistocene lake deposits partly cover the cap rock (Begin et al., 1980). The climate is now hyper-arid with an average rainfall of 50 mm year- i.
3. Runoff and the drainage system Runoff develops on Mount Sedom only during high-intensity rain events whose duration is relatively short. Between 1984 and 1991 an automatic rain recorder, three rain gauges, a stage recorder and an automatic water sampler were operated in a basin of 0.02 km 2 draining into Sedom Cave. Twelve stream flow events were recorded during this interval. Rain intensities lower than 1 mm per 10 rain did not produce any runoff; intensities between 1 and 2 mm per 10 rain gave rise to initial runoff, whereas intensities greater than 4 mm per 10 min resulted in high-discharge flow in all channels. The flood water has high concentrations of suspended load, ranging from 10% to 80% by weight (Gerson and Inbar, 1974). This, as well as high dissolved load, increases the kinematic viscosity of the water by as much as 10-20 times compared with normal water in other karst terrains. This may cause the effective minimum aperture for turbulent flow in a karst conduit to be several centimetres as opposed to 5-15 mm reported in limestone karst (Ford and Williams, 1989, p. 242). During intense rain events runoff collects upon the relatively impermeable and insoluble cap rock and flows partially into fissures and aUogenic caves. Some caves drain directly to the margins of the diapir, which are the local base level of drainage (Fig. 2). Such integrated cave systems can usually be traversed the full distance from the stream sink to the outlet. Other caves in the central areas of the mountain have no distinct outlet but terminate in a chamber above the apparent water table. The surface catchments of Mount Sedom were surveyed and classified into four groups (Fig. 3): (1) Subaerial, non-karstic basins, discharged entirely at the surface. These drain 43% of Mount Sedom area. (2) Basins draining to integrated cave systems. These number 51 catchments, occupying 31% of the area. The caves sustain fast flood flow through rock salt conduits towards the lower areas surrounding Mount Sedom, where the water reappears at the surface as springs from stream caves. (3) Basins draining to central non-integrated caves. These are 31 catchments occupying 8% of Mount Sedom area. These caves are explorable down to a point where the large passage seems to end. (4) Small catchments with sinkholes leading into caves that are too small for human
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A. Frumki~ / Journal of Hydrology 162 (1994) 171-189
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A. Frumkin / Journal of Hydrology 162 (1994) 171-189
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A. Frumkin / Journal of Hydrology 162 (1994) 171-189
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Fig. 3. Drainage types on Mount Sodom. 1--Subaerial, non-karstic runoff; 2--drainage through integrated cave systems; 3--drainage through non-integratod caves; 4--drainage into small sinkholes with no explorable cave.
176
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
entry drain 18% of Mount Sedom surface area. Hundreds of such sinkholes have been found, draining catchments whose area is usually smaller than the minimum for an enterable cave catchment (about 300 m2; Frumkin, 1992). No conduit outlets could be shown to drain the last two groups of basins, which make up 26% of the total area. The subsurface flow routes of water drained from these basins may consist of two parts: upstream, where aggressive water can dissolve the salt, a conduit develops, sometimes reaching dimensions of a traversable cave; downstream, where the large conduit ends, the fast flood flow transforms into a slow diffuse flow infiltrating through cave bottom sediments and rock fissures. During floods water backs up in the conduit passages above these points, forming temporary 'lakes' (Fig. 2). The ponded water discharges slowly into the cave bottom, where granular porosity of cave sediments, as well as tight rock fissures, restrict the flow rate.
4. Dissolution processes and chemical evolution of the water 4.1. Methods
Water samples were collected by (1) manual sampling of springs, underground reservoirs and one flood event; (2) automatic sampling of stream flow during the rising flood stage; (3) rain sampling in a Lambrecht rain recorder, protected from evaporation (Fig. 4). Samples were filtered by Whatman 42 paper within a few hours after collection. Comprehensive chemical analysis of 37 water samples (Table 1) was undertaken in the Israel Geological Survey and the Dead Sea Works by flame photometer atomic absorption spectroscopy for Na +, K +, Ca 2+, Mg2+; by titration for Ca 2+, Mg 2+, CI-, Br-, and HCO3-, by inductively coupled plasma (ICP) for SO42-, and by X-ray diffraction (XRD) for Br-. It should be noted that some ions were determined by two different methods. Reaction error (RE), defined as 100(anions-cations)/ (anions + cations) was calculated for samples in which all the ions were determined directly. Its value is less than 1.5% in all cases. In Samples SE0-12 and ST101-111, C1- was not determined directly but calculated from the ion balance, with the consequence that the reaction error is unknown. Water samples were analysed for 180 by equilibration with CO2 at the Department of Environmental Sciences and Energy Research, Weizmann Institute of Science. 4.2. Rain and flood water
Water derived directly from rainfall, such as subaerial and underground stream flow or underground reservoirs of the same origin, is referred to here as meteoric water. The evolution of such water along the flow routes within the karst system is displayed in Fig. 5. The proximity of the samples to the dissolution line for pure halite shows that it is the main mineral being dissolved. Rainwater has negligible total dissolved solids (TDS). Surface runoff water has been in contact only with cap
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rocks (TDS 4-9 g 1-1). The other samples were in contact with rock salt and have much higher TDS (85-383 g 1-1). Most of the flowing water sampled (centre of Fig. 5) did not reach saturation. This is attributed to the short residence time of this water in the caves--no more than a few minutes. On the other hand, flood water detained in
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
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Fig. 5. Classification of Mount Sedom water samples according to NaCl content and total solute concentration. Solid line shows halite dissolution in pure water. Meteoric water falls close to this line because halite is the main mineral dissolved, Meteoric water develops along the line from lower left to upper right. Spring brine water evolves differently, and includes higher concentration of Mg 2+, Ca 2+ and K + ions. Underground reservoirs originate from meteoric flood water, and not from spring water.
subsurface reservoirs for several hours did reach saturation or come very close to it (upper fight of Fig. 5). Thus, the chemical evolution of the water here is controlled by the duration of water-rock contact. Solute concentration per litre of cave flood water decreases as the discharge increases (Fig. 6), but the maximum solute loadings occur during peak flood discharge (Fig. 7). This is similar to normal behaviour of rivers in other terrains. A film of water flowing down the wall at the base of a 70 m deep shaft had 316 g l-I solutes, indicating that thin laminar flow may approach saturation after a shortduration contact with rock salt. The small thickness of the film probably allows quick diffusion of solutes into the whole volume of water. However, most of the
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
181
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Fig. 6. Rain, discharge and solute concentration for 26 December 1988 flood event in Sedom Cave. High discharge is accompanied by low solute concentration, as in surface streams and limestone karst.
flood volume was observed to descend the shaft in a free fall during high discharge, and film flow dominated only during low discharge at the end of the flood event. This may account for some of the observed change in solute concentration during the flood (Fig. 6). This evidence suggests that film flow during low discharge may be the process dominating the enlargement of salt shafts in a manner similar to that for many limestone shafts (Pohl, 1955; Brucker et al., 1972; Frumkin, 1986). On the other hand, enlargement of sub-horizontal passages was observed during high discharge: the Tlula Passage in Sedom Cave (Fig. 4) deepened and widened by 2-3 cm in each of two intense floods on 9 February 1987 and 22 March 1991; in four smaller flood events no measurable enlargement occurred. Flood flow observed in sub-horizontal cave passages was mainly turbulent. However, water-rock contact may still be poor in passages whose bottom is covered with alluvium, which is the case in most of the mature stream passages. High-discharge turbulent flow is responsible for the major part of dissolution. The chemical composition of the water was used to calculate the differing proportions of the dissolved minerals (Table 2) by solving a system of linear equations as suggested by Braitsch (1971). Water flowing within a rock salt conduit dissolved 81-98% halite (% equivalent), 1-7% anhydrite or gypsum, 0-4% carnallite and 0 - 8 % sylvite. The Na/C1 equivalent ratio is close to unity. Surface flood water flowing on cap rocks dissolved 37-57% halite, 38-58% anhydrite and gypsum, 5-6% carnallite and no sylvite. This agrees well with the bulk rock composition of Mount Sedom (Zak, 1967) and the relative solubilities of the minerals found: halite is the main karst mineral, with similar abundance in both the bedrock and in the flood water. Anhydrite (CaSO4) dissolution in caves is limited because of its lower solubility, but it dominates in the surface runoff, which is in contact with cap rocks composed mainly of anhydrite. Sylvite (KC1) and carnallite (KC1.MgCI2.6H20) are minor constituent of rock salt, but their dissolution is enhanced relative to their abundance because of their higher
A. Frumkin/JournalofHydrology 162 (1994) 171-189
182 1000
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solubility. Dolomite solubility is suppressed in Mount Sedom by the common ion effect (Lerman, 1970) as well as by the absence of biogenic CO2 in the arid environment. Halite solubility is also affected by soluble ions (e.g. Mg 2+, K +) because of the common ion effect. However, this does not normally affect flood waters, which are highly undersaturated.
4.3. Underground reservoirs Underground reservoirs are defined here as any natural ponding of meteoric water in Mount Sedom caves. Unlike the flowing water, underground reservoir waters have very high concentrations of solutes: TDS > 295 g 1-1 . Na/C1 ratio is high: 0.93-0.98. The chemistry thus is dominated by halite dissolution by flood waters that approached saturation during prolonged contact with the rock salt. The oxygen isotopic composition of the water collected during the field monitoring programme suggests that the reservoirs may be divided into two groups. Waters retained in small residual pools (as well as rain water, surface runoff and flood flow) ranged from -4.5 to -0.59/00 Standard Mean Ocean Water (SMOW) (Fig. 8). The simplest interpretation of their behaviour is that in these waters there was no
A. Frumkm/JournalofHydrology 162 (1994) 171-189
183
Table 2 Calculated composition of main minerals dissolved in Mount Sedom meteoric water samples (in % equivalent)
Sample
Water type
Site
no. 91/18 91/16 SE6 91/3 91/4 91/5 SE5 91/6 SE4 SE3 SE2 91/7 SE9 91/8 SE12 SE1 SE7 STI 11 ST110 SE11 ST101 GV 1 GV3 GV2 MIF1 91/11 91/10 91/2 91/9
Surface runoff Surface runoff Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Residual pool Residual pool Film flow Residual pool
Terminal lake Terminal lake Terminal lake
Terminal lake Residual pool Residual pool Residual pool Residual pool
Above Sedom Cave Above Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Gvishim Cave Gvishim Cave Gvishim Cave Mifrazim Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave
Halite
Anhydrite
Carnallite
Sylvite
(%)
(%)
(%)
(%)
37.1 56.7 82.6 91.8 91.9 93.0 81.5 94.1 86.3 94.1 95.4 95.6 96.8 96.1 96.1 95.5 96.4 97.5 97.6 97.7 97.8 96.9 96.9 96.9 97.5 96.9 96.9 96.9 97.1
57.6 37.7 6.9 7.1 7.0 6.2 5.9 5.1 4.6 4.6 3.6 3.6 2.6 3.0 3.0 2.5 1.7 1.8 1.8 0.9 1.0 0.7 0.7 0.7 1.5 1.9 2.0 1.9 1.8
5.3 5.6 3.0 1.1 1.1 0.8 4.4 0.8 3.3 1.4 0.9 0.9 0.7 0.9 0.9 2.0 1.9 0.7 0.7 0.0 0.7 0.3 0.3 0.3 1.1 1.1 1.1 1.2 1.1
0.0 0.0 7.5 0.0 0.0 0.0 8.2 0.0 5.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.5 0.4 2.1 2.0 2.1 0.0 0.0 0.0 0.0 0.0
measurable change in isotopic ratios as solute loads increased from 0.2 to about 300 g 1-1 . All of these waters were sampled less than 40 h after the storm events. The second group has - 0 . 5 < t5180 < 2, i.e. higher values than local rain and flood water (Fig. 8). These waters were sampled from underground terminal lakes whose volumes are some tens of cubic metres. The lakes occupy the downstream ends of stream passages in which no distinct conduit outlet was found (Fig. 9). They are perched tens of metres above the nearest base level, and the water level in each lake differs from the others by several metres, up to tens of metres. Several measurements conducted in two lakes during 1985-1988 show that their level dropped by about 20 cm year - l , accompanied by halite precipitation on the walls. Water samples collected from the lakes 3, 12 and more than 20 months after flood events had 6180 of - 0 . 4 , 0.2 and 1.9, respectively. The enrichment of 6~80 can be attributed to fractionation through evaporation of lake water (e.g. Faure, 1986, p. 432). Halite precipitation
A. Frumkin / Journal o f Hydrology 162 (1994) 171-189
184 400 (D
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Fig. 8. Water development in Mount Sedom as indicated by the relationship of total solute concentration to 8180 of the water Flood water and small pools have 8180 in the range of local precipitation. Lake water is enriched in 180, indicating evaporation.
indicates that there is saturation of the water with respect to halite. Anhydrite helictites developed on the halite crystals suggest that water continued evaporating until saturation with respect to CaSO4 was reached (Forti and Buzio, 1985). This sequence of mineral precipitation is the opposite of the order in evaporating seawater, where CaSO4 is precipitated before NaC1. The very existence of a cave in rock salt implies that salt must have been removed by liquid water, because evaporating water cannot remove it. Water may escape from the non-integrated type of cave into bedrock fissures below the alluvium at the bottom of the cave. Only water and solutes can exit in this manner, because bed load and suspended load must settle on the cave bottom. Silt and clay fractions of the load may gradually seal the cave bottom against further infiltration. This tends to extend the residence time of lake water in these caves. The walls of long-lived lakes (Fig. 9) exhibit horizontal notches with corrosion bevels and facets (Ford and Williams, 1989, p. 307), suggesting water stratification: the aggressive light water of a recent flood forms the top layer over older, heavier, saturated water. A horizontal dissolution groove is formed by the aggressive water
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
185
Fig. 9. Temporary undergroundlake at the bottom of a non-integratedcave. (Note horizontal dissolution grooves showing previouswater levels.)
layer. Similar features have been observed in gypsum caves and rarely in limestone caves (Pfeiffer and Hahn, 1972; Kempe et al., 1975). Caves lacking a conduit outlet usually contain evidence (such as horizontal silt deposits on the walls) o f previously obstructed flood water. However, the walls of most of these caves have neither notches nor halite crystals, indicating a short residence time of the water in the cave without saturation being reached. Longer residence time o f the water is believed to have taken place in three dry lakes that do exhibit notches and halite crystals. Three other caves were holding water in their terminal lakes continuously during the study period (1984-1991), and their notches and salt deposits indicate a longer residence time. The variable residence time o f lake water is attributed to variability in fissure width and alluvial sealing of cave beds, which influence infiltration rates. 4.4. Brine spring water
Small brine springs (discharging less than 2 m 3 day-i; Zak, 1967) occur along the base o f the eastern escarpment of Mount Sedom and in the channel floor of eastwarddraining caves. The soil surface around the eastern base o f the mountain remains moist all year, up to several metres above local base level. A trench dug near the outlet of Sedom Cave soon became filled by brine. Phreatic brine was also encountered in three boreholes to the west of Colonel Cave (Petroleum Services, 1979). The
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A. Frumkin /Journal of Hydrology 162 (1994) 171-189
water-level evidence suggests the existence of some kind of a brine water table sloping eastward towards the base level. Spring brines are saturated with dissolved salts (Table 1, Fig. 5). NaCI concentration is less than 87%, and the Na/CI ratio is less then 0.66. Such waters have probably dissolved other chloride minerals, apart from halite, and precipitated halite during their evolution (Sulin, 1946). Their chemical composition differs from both the Mount Sedom flood waters and other brines in the Dead Sea valley. For example, K/C1 ratio in Mount Sedom brines is 0.05-0.1, compared with 0.005-0.033 in other Dead Sea valley brines, Dead Sea water and the Mount Sedom floods. However, there appears to be a chemical variability among the various springs of Mount Sedom itself (Table 1). This evidence suggests that there is a phreatic aquifer in Mount Sedom (e.g. Milanovic, 1981, p. 121), whose flow routes are distinct from those of the integrated caves. The principal water sources of the aquifer are uncertain, but it may be partly recharged by vadose water, for instance by infiltration from subsurface lakes fed by floods. Some 26% of Mount Sedom surface is drained into sinks without a conduit outlet, thus contributing to aquifer recharge (see above). The aquifer is possibly recharged by other sources too, such as basal injection into the diapir.
5. Solutional denudation rates
Reliable outflow discharge and solute concentration data are needed for evaluating regional solutional denudation, preferably by applying mass flux rating curves to the flow duration curve. However, the water chemistry data collected in this study may not be representative of the entire Mount Sedom karst for long periods, because of the difficulties in accumulating valid sample sets in arid regions. Therefore only a rough estimate for minimal solutional denudation rate will be attempted. The minimum solute discharge (D) may be approximated by
D = a ~-~(CiAi)
(1)
where Q is the minimum outflow discharge, Ci is minimum measured solute concentration for each type of runoff and Ai is the area fraction contributing to each type of runoff. Mount Sedom receives no runoff from its surroundings. Assuming negligible longterm change of water storage within the mountain, its aggregate discharge (both subaerial and subsurface) must be approximated by the effective rain. Minimum effective rain is estimated to be 10 mm year -1 (Gerson, 1972), and the rest (80% of mean total precipitation) is lost by evapotranspiration. Effective rain multiplied by the surface area of Mount Sedom (13.7 km z) suggests that there is a minimum discharge (Q) of 137 000 m 3 year -l. Ai w a s measured by planimetry from Fig. 3 and Ci was estimated from the water chemistry data of Table 1. Subaerial runoff (43% of Mount Sedom area) is 4 g 1-1. This is believed to represent a true minimum value, because some surface water flows
A. Frumkm/JournalofHydrology 162 (1994) 171-189
187
over rock salt outcrops, considerably increasing their dissolved load. Runoff through integrated salt cave systems (31% of the area) is 85 g 1-1 . This value can be considered the minimum for most floods in the majority of the caves. However, caves with the largest catchments may experience some high-discharge floods with lower solute concentrations. Slow diffusion, where no conduit outlet was found (26% of the area) is 320 g 1-1 . This water is saturated long before it flows out from Mount Sedom. The minimum solute discharge estimated by Eq. (1) is 15 x 106 kg year- 1. Dividing this by Mount Sedom's area yields a minimum solutional denudation rate of 0.5 mm year -~. Some 99% of this denudation is attributed to rock salt dissolution, mainly along conduits. The entrenchment rate in cave conduits measured directly and by dating of cave levels varies between 4 and 25 mm year-1 (Frumkin and Ford, 1994). If a lower evapotranspiration loss of 70% is assumed, because water may be rapidly swallowed underground, a maximum solutional denudation rate of 0.75 mm year-1 is obtained. The 0.5-0.75 mm year-1 is believed to be a likely range of solutional denudation rate.
6. Vertical distribution of dissolution
The vertical distribution of solution processes in Mount Sedom may be divided into three domains: the surface; the underground conduits allowing rapid stream flow; the slow flow domain, including terminal lakes and diffuse flow. The fraction of solutional denudation contributed within each domain can be estimated by
P = Q,ACi/Z(Q,DCi)
(2)
where Qi is the discharge passing through the domain and ACi is the solute concentration added to the water within the domain. Minimum measured values of solute concentrations will be used. For the subaerial domain, AC is about 4 g 1-l and Qi is the minimum total discharge of 137 000 m 3 year -1 . Applying this to Eq. (2) suggests that about 4% of the solutional denudation occurs on the surface. For the rapid flow domain, AC is about 80 g 1-1. Assuming that all underground flow that escapes evaporation experiences at least a short rapid flow path beyond the sink, Qi is taken as about 78 100 m 3 year-l--the fraction (57%) of the total discharge that drains through the subsurface. The solutional denudation occurring in the rapid conduit flow domain estimated by Eq. (2) is about 41%. For the slow flow domain, AC is about 235 g 1-I. Some 26% of the mountain area drains by slow diffusion, suggesting that Q for this domain is about 35600 m 3 year -1. The solutional denudation occurring in the slow flow domain estimated by Eq. (2) is about 55%. These proportions contrast with vegetated soil-covered limestone karst, where most solution takes place in the soil and the uppermost bedrock (Ford and Williams, 1989, p. 116). Unlike limestone karst, where soil-generated CO2 is the important factor enhancing dissolution at the top of a bedrock-soil mass, In Mount Sedom soil-generated CO2 does not seem to play a role. On the contrary, most dissolution in Mount Sedom occurs in the lower reaches of conduits and
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A. Frumkin / Journal of Hydrology 162 (1994) 171-189
fissures, by water that passed the relatively insoluble cap rock to reach the extremely soluble rock salt mass underneathl
7. Conclusion Since its subaerial extrusion in the Early Holocene (Frumkin et al., 1991), Mount Sedom has developed a karst system which allows most of the effective rain to drain through the subsurface. The water table was probably lowered and a vadose zone developed as drawdown vadose caves evolved, in a similar manner to that in limestone allogenic karst (Ford and Ewers, 1978). Groundwater flow does occur in fissures and conduits above base level, although deeper, where joints become annealed by lithostatic pressure, the salt is fairly impervious. The dissolved loads of flood water depend primarily on the residence time of the water in contact with rock salt. Flood waters passing rapidly through integrated cave systems do not reach saturation, whereas waters infiltrating through tight fissures or held up in subsurface reservoirs reach saturation, mainly by dissolving halite. The solute-discharge relationships are similar to those in limestone and gypsum karst. A phreatic brine aquifer whose water source is not clear gives rise to small springs along the eastern side of Mount Sedom. The karst denudation rate in rock salt is estimated to be about 0.5-0.75 mm year-', which is 1-2 orders of magnitude higher than limestone denudation rates in moister terrains (Ford and Williams, 1989, p. 116). Most of the dissolution occurs within the rock salt mass under the cap rock, unlike limestone karst which favours epikarst dissolution.
8. Acknowledgements The project was financed by a grant from the research fund of the Society for the Protection of Nature in Israel. The author thanks Dr. M. Raab of the Israel Geological Survey, Y. Harash and Dr. S. Tandy of the Dead Sea Works for chemical analyses, Dr. Y. Yehieli of the Weizmann Institute of Science for isotopic analyses, the Israel Cave Research team for fieldwork assistance. Dr. A. Starinsky of the Hebrew University for useful discussion, Dr. I. Zak of the Hebrew University for advice and for collecting four samples, and Drs. D.C. Ford and S.R.H. Worthington of McMaster University for critically reading the manuscript.
9. References Begin, Z.B., Nathan, Y. and Ehrlich, A., 1980. Stratigraphy and facies distribution in the Lisan Formation--new evidence from the area south of the Dead Sea, Israel. Isr. J. Earth Sci., 29: 182-189. Braitsch, O., 1971. Salt Deposits--Their Origin and Composition. Springer, Berlin, 297 pp. Brucker, R.W., Hess, J.W. and White, W.B., 1972. Role of vertical shafts in the movement of ground water in carbonate aquifers. Groundwater, 10: 5-13.
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
189
Faure, G., 1986. Principles of Isotope Geology. Wiley, New York, 589 pp. Ford, D.C. and Ewers, R.O., 1978. The development of limestone cave systems in the dimensions of length and depth. Int. J. Speleol., 10: 213-244. Ford, D.C. and Williams, P.W., 1989. Karst Geomorphology and Hydrology. Unwin Hyman, London, 601 pp. Forti, P. and Buzio, A., 1985. Una nuova scoperta nelle grotte di Monte Sedom (Israele): le eceentriche di anidrite. Mineralogical and Paleontological notes, 46:11-16. Frnmkin, A., 1986. Speleogenesis of vertical shafts in a Mediterranean environment (Ofra, Israel) (abstract), 9th Congreso International de Espeleologia, Barcelona, 1986, Barcelona, Union Internacional de Espeleologia, Vol. 1, pp. 264-267. Frumkin, A., 1992. The karst system of the Mount Sedom salt diapir. Ph.D. Dissertation, Hebrew University, Jerusalem, 135 pp. (in Hebrew, English abstract). Frumkin, A. and Ford, D.C., 1994. Rapid entrenchment of stream profiles in the salt caves of Mount Sedom, Israel. Earth Surf. Processes Landforms, in press. Frumkin, A., Magaritz, M., Carmi, I. and Zak, I., 1991. The Holocene climatic record of the salt caves of Mount Sedom, Israel. Holocene, 1(3): 191-200. Gerson, R., 1972. Geomorphic processes of Mount Sedom. Ph.D. Dissertation, Hebrew University, Jerusalem, 234 pp. (in Hebrew, English abstract). Gerson, R. and Inbar, M., 1974. The field study program of the Jerusalem-Elat Symposium, 1974. Reviews and summaries of Israeli research projects. Z. Geomorphol., Suppl., 20:7-11. Kempe, S., Brandt, A., Seeger, M. and Vladi, F., 1975. "Facetten" and "Laugdecken", the typical morphological elements of caves developing in standing water. Ann. Sprlrol., 30(4): 705-708. Lerman, A., 1970. Chemical equilibria and evolution of chloride brines, 50th anniversary symposium. Mineral. Soc. Am. Spec. Pap., 3: 291-306. Milanovic, P.T., 1981. Karst Hydrogeology. Water Resources Publications, Littleton, CO, 434 pp. Petroleum Services, 1979. Solution mined liquid hydrocarbon storage caverns, Mount Sedom, Progress Report 2. Petroleum Services Ltd., Tel Aviv, 12 pp. Pfeiffer, D. and Hahn, J., 1972. Karst of Germany. In: M. Herak and V.T. Stringtield (Editors), Karst: Important Karst Regions of the Northern Hemisphere. Elsevier, Amsterdam, pp. 189-223. Pohl, E.R., 1955. Vertical shafts in limestone caves. Nat. Speleol. Soc. Occas. Pap., 2, 24 pp. Sulin, V.A., 1946. Waters of Petroleum Formations in the System of Natural Waters. Gostoptekhizdat, Moscow, pp. 35-96. Zak, I., 1967. The geology of Mount Sedom. Ph.D. Dissertation, Hebrew University, Jerusalem, 208 pp. (in Hebrew, English abstract). Zak, I., 1974. Sedimentology and bromine geochemistry of marine and continental evaporites in the Dead Sea basin. In: A. Coogan (Editor), Fourth Symposium on Salt, Vol. 1, Cleveland, Ohio, Northern Ohio Geological Society, Cleveland, OH, pp. 349-361. Zak, I. and Bentor, Y. K., 1968. Some new data on the salt deposits of the Dead Sea area, Israel. Symposium on the Geology of Saline Deposits, 1968, Hannover. UNESCO, Hannover, pp. 137-146.
Journal of
Hydrology
ELSEVIER
Journal of Hydrology 162 (1994) 171-189
[2]
Hydrology and denudation rates of halite karst Amos Frumkin 1 Israel Cave Research Center, Department of Geography, The Hebrew Universityof Jerusalem, Har Hatsofim, Jerusalem, Israel
Received 29 November 1993; revision accepted 11 January 1994
Abstract
Salt karst terrains exist mainly in arid climates where rock salt outcrops may escape complete destruction by dissolution. Such is the case with Mount Sedom, on the SW shore of the Dead Sea, one of the most arid parts of Israel. Many small catchments developed over the relatively insoluble cap rock which overlies the highly soluble rock salt. The catchments were surveyed and classified. Some 57% of the surface area is drained by an underground karst system. Water samples from various points in the system were analysed, and water development was inferred. Waters in cave conduits do not reach saturation during flood flow, unless the water is ponded for at least several hours. Based on the available evidence, regional karst denudation is tentatively estimated to be about 0.5-0.75 mm year -1, occurring mainly within the rock salt.
1. Introduction
Unlike carbonate rocks, salt dissolves by a simple dissociation reaction. The simplicity o f the process allows easy determination of geological and hydrological factors affecting salt karst development. Although salt is a c o m m o n sedimentary rock, salt outcrops are rare in non-arid climates. Its extreme high solubility (360 g 1-1 at 20°C) permits total dissolution of an exposure by meteoric water. Consequently, salt outcrops appear mainly in arid environments, where they escape complete destruction. Karst water chemistry and denudation have been widely studied in carbonate rocks (Ford and Williams, 1989, pp. 42-126), but there have been few such studies in shallow salt karst. This paper discusses the types of karst water and drainage patterns in the M o u n t Sedom salt karst system. Based on these data, an estimate of the regional denudation rate and distribution is attempted.
rePresent address: Department of Geography, McMaster University, Hamilton, Ont., Canada L85 4K1. 0022-1694/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(94)02457-M
172
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
2. The study area Mount Sedom is a rock salt diapir, forming an elongated ridge 14 km 2 in area, rising 250 m above the Dead Sea (Fig. I). It consists of Plio-Pleistocene(?) beds of rock salt of marine origin (Zak, 1974), piercing through younger lake evaporites and clastics. Before its subaerial extrusion, the top of the rising diapir rock salt was partially dissolved by ground-water. The residual anhydrite, marls, shales, chalk and sandstone have accumulated to form a cap rock up to 50 m thick (Zak and Bentor, 1968). Late Pleistocene lake deposits partly cover the cap rock (Begin et al., 1980). The climate is now hyper-arid with an average rainfall of 50 mm year- i.
3. Runoff and the drainage system Runoff develops on Mount Sedom only during high-intensity rain events whose duration is relatively short. Between 1984 and 1991 an automatic rain recorder, three rain gauges, a stage recorder and an automatic water sampler were operated in a basin of 0.02 km 2 draining into Sedom Cave. Twelve stream flow events were recorded during this interval. Rain intensities lower than 1 mm per 10 rain did not produce any runoff; intensities between 1 and 2 mm per 10 rain gave rise to initial runoff, whereas intensities greater than 4 mm per 10 min resulted in high-discharge flow in all channels. The flood water has high concentrations of suspended load, ranging from 10% to 80% by weight (Gerson and Inbar, 1974). This, as well as high dissolved load, increases the kinematic viscosity of the water by as much as 10-20 times compared with normal water in other karst terrains. This may cause the effective minimum aperture for turbulent flow in a karst conduit to be several centimetres as opposed to 5-15 mm reported in limestone karst (Ford and Williams, 1989, p. 242). During intense rain events runoff collects upon the relatively impermeable and insoluble cap rock and flows partially into fissures and aUogenic caves. Some caves drain directly to the margins of the diapir, which are the local base level of drainage (Fig. 2). Such integrated cave systems can usually be traversed the full distance from the stream sink to the outlet. Other caves in the central areas of the mountain have no distinct outlet but terminate in a chamber above the apparent water table. The surface catchments of Mount Sedom were surveyed and classified into four groups (Fig. 3): (1) Subaerial, non-karstic basins, discharged entirely at the surface. These drain 43% of Mount Sedom area. (2) Basins draining to integrated cave systems. These number 51 catchments, occupying 31% of the area. The caves sustain fast flood flow through rock salt conduits towards the lower areas surrounding Mount Sedom, where the water reappears at the surface as springs from stream caves. (3) Basins draining to central non-integrated caves. These are 31 catchments occupying 8% of Mount Sedom area. These caves are explorable down to a point where the large passage seems to end. (4) Small catchments with sinkholes leading into caves that are too small for human
173
A. Frumki~ / Journal of Hydrology 162 (1994) 171-189
\
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A. Frumkin / Journal of Hydrology 162 (1994) 171-189
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176
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
entry drain 18% of Mount Sedom surface area. Hundreds of such sinkholes have been found, draining catchments whose area is usually smaller than the minimum for an enterable cave catchment (about 300 m2; Frumkin, 1992). No conduit outlets could be shown to drain the last two groups of basins, which make up 26% of the total area. The subsurface flow routes of water drained from these basins may consist of two parts: upstream, where aggressive water can dissolve the salt, a conduit develops, sometimes reaching dimensions of a traversable cave; downstream, where the large conduit ends, the fast flood flow transforms into a slow diffuse flow infiltrating through cave bottom sediments and rock fissures. During floods water backs up in the conduit passages above these points, forming temporary 'lakes' (Fig. 2). The ponded water discharges slowly into the cave bottom, where granular porosity of cave sediments, as well as tight rock fissures, restrict the flow rate.
4. Dissolution processes and chemical evolution of the water 4.1. Methods
Water samples were collected by (1) manual sampling of springs, underground reservoirs and one flood event; (2) automatic sampling of stream flow during the rising flood stage; (3) rain sampling in a Lambrecht rain recorder, protected from evaporation (Fig. 4). Samples were filtered by Whatman 42 paper within a few hours after collection. Comprehensive chemical analysis of 37 water samples (Table 1) was undertaken in the Israel Geological Survey and the Dead Sea Works by flame photometer atomic absorption spectroscopy for Na +, K +, Ca 2+, Mg2+; by titration for Ca 2+, Mg 2+, CI-, Br-, and HCO3-, by inductively coupled plasma (ICP) for SO42-, and by X-ray diffraction (XRD) for Br-. It should be noted that some ions were determined by two different methods. Reaction error (RE), defined as 100(anions-cations)/ (anions + cations) was calculated for samples in which all the ions were determined directly. Its value is less than 1.5% in all cases. In Samples SE0-12 and ST101-111, C1- was not determined directly but calculated from the ion balance, with the consequence that the reaction error is unknown. Water samples were analysed for 180 by equilibration with CO2 at the Department of Environmental Sciences and Energy Research, Weizmann Institute of Science. 4.2. Rain and flood water
Water derived directly from rainfall, such as subaerial and underground stream flow or underground reservoirs of the same origin, is referred to here as meteoric water. The evolution of such water along the flow routes within the karst system is displayed in Fig. 5. The proximity of the samples to the dissolution line for pure halite shows that it is the main mineral being dissolved. Rainwater has negligible total dissolved solids (TDS). Surface runoff water has been in contact only with cap
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rocks (TDS 4-9 g 1-1). The other samples were in contact with rock salt and have much higher TDS (85-383 g 1-1). Most of the flowing water sampled (centre of Fig. 5) did not reach saturation. This is attributed to the short residence time of this water in the caves--no more than a few minutes. On the other hand, flood water detained in
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
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subsurface reservoirs for several hours did reach saturation or come very close to it (upper fight of Fig. 5). Thus, the chemical evolution of the water here is controlled by the duration of water-rock contact. Solute concentration per litre of cave flood water decreases as the discharge increases (Fig. 6), but the maximum solute loadings occur during peak flood discharge (Fig. 7). This is similar to normal behaviour of rivers in other terrains. A film of water flowing down the wall at the base of a 70 m deep shaft had 316 g l-I solutes, indicating that thin laminar flow may approach saturation after a shortduration contact with rock salt. The small thickness of the film probably allows quick diffusion of solutes into the whole volume of water. However, most of the
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
181
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flood volume was observed to descend the shaft in a free fall during high discharge, and film flow dominated only during low discharge at the end of the flood event. This may account for some of the observed change in solute concentration during the flood (Fig. 6). This evidence suggests that film flow during low discharge may be the process dominating the enlargement of salt shafts in a manner similar to that for many limestone shafts (Pohl, 1955; Brucker et al., 1972; Frumkin, 1986). On the other hand, enlargement of sub-horizontal passages was observed during high discharge: the Tlula Passage in Sedom Cave (Fig. 4) deepened and widened by 2-3 cm in each of two intense floods on 9 February 1987 and 22 March 1991; in four smaller flood events no measurable enlargement occurred. Flood flow observed in sub-horizontal cave passages was mainly turbulent. However, water-rock contact may still be poor in passages whose bottom is covered with alluvium, which is the case in most of the mature stream passages. High-discharge turbulent flow is responsible for the major part of dissolution. The chemical composition of the water was used to calculate the differing proportions of the dissolved minerals (Table 2) by solving a system of linear equations as suggested by Braitsch (1971). Water flowing within a rock salt conduit dissolved 81-98% halite (% equivalent), 1-7% anhydrite or gypsum, 0-4% carnallite and 0 - 8 % sylvite. The Na/C1 equivalent ratio is close to unity. Surface flood water flowing on cap rocks dissolved 37-57% halite, 38-58% anhydrite and gypsum, 5-6% carnallite and no sylvite. This agrees well with the bulk rock composition of Mount Sedom (Zak, 1967) and the relative solubilities of the minerals found: halite is the main karst mineral, with similar abundance in both the bedrock and in the flood water. Anhydrite (CaSO4) dissolution in caves is limited because of its lower solubility, but it dominates in the surface runoff, which is in contact with cap rocks composed mainly of anhydrite. Sylvite (KC1) and carnallite (KC1.MgCI2.6H20) are minor constituent of rock salt, but their dissolution is enhanced relative to their abundance because of their higher
A. Frumkin/JournalofHydrology 162 (1994) 171-189
182 1000
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solubility. Dolomite solubility is suppressed in Mount Sedom by the common ion effect (Lerman, 1970) as well as by the absence of biogenic CO2 in the arid environment. Halite solubility is also affected by soluble ions (e.g. Mg 2+, K +) because of the common ion effect. However, this does not normally affect flood waters, which are highly undersaturated.
4.3. Underground reservoirs Underground reservoirs are defined here as any natural ponding of meteoric water in Mount Sedom caves. Unlike the flowing water, underground reservoir waters have very high concentrations of solutes: TDS > 295 g 1-1 . Na/C1 ratio is high: 0.93-0.98. The chemistry thus is dominated by halite dissolution by flood waters that approached saturation during prolonged contact with the rock salt. The oxygen isotopic composition of the water collected during the field monitoring programme suggests that the reservoirs may be divided into two groups. Waters retained in small residual pools (as well as rain water, surface runoff and flood flow) ranged from -4.5 to -0.59/00 Standard Mean Ocean Water (SMOW) (Fig. 8). The simplest interpretation of their behaviour is that in these waters there was no
A. Frumkm/JournalofHydrology 162 (1994) 171-189
183
Table 2 Calculated composition of main minerals dissolved in Mount Sedom meteoric water samples (in % equivalent)
Sample
Water type
Site
no. 91/18 91/16 SE6 91/3 91/4 91/5 SE5 91/6 SE4 SE3 SE2 91/7 SE9 91/8 SE12 SE1 SE7 STI 11 ST110 SE11 ST101 GV 1 GV3 GV2 MIF1 91/11 91/10 91/2 91/9
Surface runoff Surface runoff Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Flood flow Residual pool Residual pool Film flow Residual pool
Terminal lake Terminal lake Terminal lake
Terminal lake Residual pool Residual pool Residual pool Residual pool
Above Sedom Cave Above Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave Gvishim Cave Gvishim Cave Gvishim Cave Mifrazim Cave Sedom Cave Sedom Cave Sedom Cave Sedom Cave
Halite
Anhydrite
Carnallite
Sylvite
(%)
(%)
(%)
(%)
37.1 56.7 82.6 91.8 91.9 93.0 81.5 94.1 86.3 94.1 95.4 95.6 96.8 96.1 96.1 95.5 96.4 97.5 97.6 97.7 97.8 96.9 96.9 96.9 97.5 96.9 96.9 96.9 97.1
57.6 37.7 6.9 7.1 7.0 6.2 5.9 5.1 4.6 4.6 3.6 3.6 2.6 3.0 3.0 2.5 1.7 1.8 1.8 0.9 1.0 0.7 0.7 0.7 1.5 1.9 2.0 1.9 1.8
5.3 5.6 3.0 1.1 1.1 0.8 4.4 0.8 3.3 1.4 0.9 0.9 0.7 0.9 0.9 2.0 1.9 0.7 0.7 0.0 0.7 0.3 0.3 0.3 1.1 1.1 1.1 1.2 1.1
0.0 0.0 7.5 0.0 0.0 0.0 8.2 0.0 5.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.5 0.4 2.1 2.0 2.1 0.0 0.0 0.0 0.0 0.0
measurable change in isotopic ratios as solute loads increased from 0.2 to about 300 g 1-1 . All of these waters were sampled less than 40 h after the storm events. The second group has - 0 . 5 < t5180 < 2, i.e. higher values than local rain and flood water (Fig. 8). These waters were sampled from underground terminal lakes whose volumes are some tens of cubic metres. The lakes occupy the downstream ends of stream passages in which no distinct conduit outlet was found (Fig. 9). They are perched tens of metres above the nearest base level, and the water level in each lake differs from the others by several metres, up to tens of metres. Several measurements conducted in two lakes during 1985-1988 show that their level dropped by about 20 cm year - l , accompanied by halite precipitation on the walls. Water samples collected from the lakes 3, 12 and more than 20 months after flood events had 6180 of - 0 . 4 , 0.2 and 1.9, respectively. The enrichment of 6~80 can be attributed to fractionation through evaporation of lake water (e.g. Faure, 1986, p. 432). Halite precipitation
A. Frumkin / Journal o f Hydrology 162 (1994) 171-189
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-5
-4
-3
re rain and surface runoff
i
i •
flood flow in caves
•
residual pools in caves
-2
-1
0
1
2
(5180 ( S M O W )
=+ terminal lakes in caves zx spring water
Fig. 8. Water development in Mount Sedom as indicated by the relationship of total solute concentration to 8180 of the water Flood water and small pools have 8180 in the range of local precipitation. Lake water is enriched in 180, indicating evaporation.
indicates that there is saturation of the water with respect to halite. Anhydrite helictites developed on the halite crystals suggest that water continued evaporating until saturation with respect to CaSO4 was reached (Forti and Buzio, 1985). This sequence of mineral precipitation is the opposite of the order in evaporating seawater, where CaSO4 is precipitated before NaC1. The very existence of a cave in rock salt implies that salt must have been removed by liquid water, because evaporating water cannot remove it. Water may escape from the non-integrated type of cave into bedrock fissures below the alluvium at the bottom of the cave. Only water and solutes can exit in this manner, because bed load and suspended load must settle on the cave bottom. Silt and clay fractions of the load may gradually seal the cave bottom against further infiltration. This tends to extend the residence time of lake water in these caves. The walls of long-lived lakes (Fig. 9) exhibit horizontal notches with corrosion bevels and facets (Ford and Williams, 1989, p. 307), suggesting water stratification: the aggressive light water of a recent flood forms the top layer over older, heavier, saturated water. A horizontal dissolution groove is formed by the aggressive water
A. Frumkin / Journal of Hydrology 162 (1994) 171-189
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Fig. 9. Temporary undergroundlake at the bottom of a non-integratedcave. (Note horizontal dissolution grooves showing previouswater levels.)
layer. Similar features have been observed in gypsum caves and rarely in limestone caves (Pfeiffer and Hahn, 1972; Kempe et al., 1975). Caves lacking a conduit outlet usually contain evidence (such as horizontal silt deposits on the walls) o f previously obstructed flood water. However, the walls of most of these caves have neither notches nor halite crystals, indicating a short residence time of the water in the cave without saturation being reached. Longer residence time o f the water is believed to have taken place in three dry lakes that do exhibit notches and halite crystals. Three other caves were holding water in their terminal lakes continuously during the study period (1984-1991), and their notches and salt deposits indicate a longer residence time. The variable residence time o f lake water is attributed to variability in fissure width and alluvial sealing of cave beds, which influence infiltration rates. 4.4. Brine spring water
Small brine springs (discharging less than 2 m 3 day-i; Zak, 1967) occur along the base o f the eastern escarpment of Mount Sedom and in the channel floor of eastwarddraining caves. The soil surface around the eastern base o f the mountain remains moist all year, up to several metres above local base level. A trench dug near the outlet of Sedom Cave soon became filled by brine. Phreatic brine was also encountered in three boreholes to the west of Colonel Cave (Petroleum Services, 1979). The
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water-level evidence suggests the existence of some kind of a brine water table sloping eastward towards the base level. Spring brines are saturated with dissolved salts (Table 1, Fig. 5). NaCI concentration is less than 87%, and the Na/CI ratio is less then 0.66. Such waters have probably dissolved other chloride minerals, apart from halite, and precipitated halite during their evolution (Sulin, 1946). Their chemical composition differs from both the Mount Sedom flood waters and other brines in the Dead Sea valley. For example, K/C1 ratio in Mount Sedom brines is 0.05-0.1, compared with 0.005-0.033 in other Dead Sea valley brines, Dead Sea water and the Mount Sedom floods. However, there appears to be a chemical variability among the various springs of Mount Sedom itself (Table 1). This evidence suggests that there is a phreatic aquifer in Mount Sedom (e.g. Milanovic, 1981, p. 121), whose flow routes are distinct from those of the integrated caves. The principal water sources of the aquifer are uncertain, but it may be partly recharged by vadose water, for instance by infiltration from subsurface lakes fed by floods. Some 26% of Mount Sedom surface is drained into sinks without a conduit outlet, thus contributing to aquifer recharge (see above). The aquifer is possibly recharged by other sources too, such as basal injection into the diapir.
5. Solutional denudation rates
Reliable outflow discharge and solute concentration data are needed for evaluating regional solutional denudation, preferably by applying mass flux rating curves to the flow duration curve. However, the water chemistry data collected in this study may not be representative of the entire Mount Sedom karst for long periods, because of the difficulties in accumulating valid sample sets in arid regions. Therefore only a rough estimate for minimal solutional denudation rate will be attempted. The minimum solute discharge (D) may be approximated by
D = a ~-~(CiAi)
(1)
where Q is the minimum outflow discharge, Ci is minimum measured solute concentration for each type of runoff and Ai is the area fraction contributing to each type of runoff. Mount Sedom receives no runoff from its surroundings. Assuming negligible longterm change of water storage within the mountain, its aggregate discharge (both subaerial and subsurface) must be approximated by the effective rain. Minimum effective rain is estimated to be 10 mm year -1 (Gerson, 1972), and the rest (80% of mean total precipitation) is lost by evapotranspiration. Effective rain multiplied by the surface area of Mount Sedom (13.7 km z) suggests that there is a minimum discharge (Q) of 137 000 m 3 year -l. Ai w a s measured by planimetry from Fig. 3 and Ci was estimated from the water chemistry data of Table 1. Subaerial runoff (43% of Mount Sedom area) is 4 g 1-1. This is believed to represent a true minimum value, because some surface water flows
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over rock salt outcrops, considerably increasing their dissolved load. Runoff through integrated salt cave systems (31% of the area) is 85 g 1-1 . This value can be considered the minimum for most floods in the majority of the caves. However, caves with the largest catchments may experience some high-discharge floods with lower solute concentrations. Slow diffusion, where no conduit outlet was found (26% of the area) is 320 g 1-1 . This water is saturated long before it flows out from Mount Sedom. The minimum solute discharge estimated by Eq. (1) is 15 x 106 kg year- 1. Dividing this by Mount Sedom's area yields a minimum solutional denudation rate of 0.5 mm year -~. Some 99% of this denudation is attributed to rock salt dissolution, mainly along conduits. The entrenchment rate in cave conduits measured directly and by dating of cave levels varies between 4 and 25 mm year-1 (Frumkin and Ford, 1994). If a lower evapotranspiration loss of 70% is assumed, because water may be rapidly swallowed underground, a maximum solutional denudation rate of 0.75 mm year-1 is obtained. The 0.5-0.75 mm year-1 is believed to be a likely range of solutional denudation rate.
6. Vertical distribution of dissolution
The vertical distribution of solution processes in Mount Sedom may be divided into three domains: the surface; the underground conduits allowing rapid stream flow; the slow flow domain, including terminal lakes and diffuse flow. The fraction of solutional denudation contributed within each domain can be estimated by
P = Q,ACi/Z(Q,DCi)
(2)
where Qi is the discharge passing through the domain and ACi is the solute concentration added to the water within the domain. Minimum measured values of solute concentrations will be used. For the subaerial domain, AC is about 4 g 1-l and Qi is the minimum total discharge of 137 000 m 3 year -1 . Applying this to Eq. (2) suggests that about 4% of the solutional denudation occurs on the surface. For the rapid flow domain, AC is about 80 g 1-1. Assuming that all underground flow that escapes evaporation experiences at least a short rapid flow path beyond the sink, Qi is taken as about 78 100 m 3 year-l--the fraction (57%) of the total discharge that drains through the subsurface. The solutional denudation occurring in the rapid conduit flow domain estimated by Eq. (2) is about 41%. For the slow flow domain, AC is about 235 g 1-I. Some 26% of the mountain area drains by slow diffusion, suggesting that Q for this domain is about 35600 m 3 year -1. The solutional denudation occurring in the slow flow domain estimated by Eq. (2) is about 55%. These proportions contrast with vegetated soil-covered limestone karst, where most solution takes place in the soil and the uppermost bedrock (Ford and Williams, 1989, p. 116). Unlike limestone karst, where soil-generated CO2 is the important factor enhancing dissolution at the top of a bedrock-soil mass, In Mount Sedom soil-generated CO2 does not seem to play a role. On the contrary, most dissolution in Mount Sedom occurs in the lower reaches of conduits and
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fissures, by water that passed the relatively insoluble cap rock to reach the extremely soluble rock salt mass underneathl
7. Conclusion Since its subaerial extrusion in the Early Holocene (Frumkin et al., 1991), Mount Sedom has developed a karst system which allows most of the effective rain to drain through the subsurface. The water table was probably lowered and a vadose zone developed as drawdown vadose caves evolved, in a similar manner to that in limestone allogenic karst (Ford and Ewers, 1978). Groundwater flow does occur in fissures and conduits above base level, although deeper, where joints become annealed by lithostatic pressure, the salt is fairly impervious. The dissolved loads of flood water depend primarily on the residence time of the water in contact with rock salt. Flood waters passing rapidly through integrated cave systems do not reach saturation, whereas waters infiltrating through tight fissures or held up in subsurface reservoirs reach saturation, mainly by dissolving halite. The solute-discharge relationships are similar to those in limestone and gypsum karst. A phreatic brine aquifer whose water source is not clear gives rise to small springs along the eastern side of Mount Sedom. The karst denudation rate in rock salt is estimated to be about 0.5-0.75 mm year-', which is 1-2 orders of magnitude higher than limestone denudation rates in moister terrains (Ford and Williams, 1989, p. 116). Most of the dissolution occurs within the rock salt mass under the cap rock, unlike limestone karst which favours epikarst dissolution.
8. Acknowledgements The project was financed by a grant from the research fund of the Society for the Protection of Nature in Israel. The author thanks Dr. M. Raab of the Israel Geological Survey, Y. Harash and Dr. S. Tandy of the Dead Sea Works for chemical analyses, Dr. Y. Yehieli of the Weizmann Institute of Science for isotopic analyses, the Israel Cave Research team for fieldwork assistance. Dr. A. Starinsky of the Hebrew University for useful discussion, Dr. I. Zak of the Hebrew University for advice and for collecting four samples, and Drs. D.C. Ford and S.R.H. Worthington of McMaster University for critically reading the manuscript.
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Faure, G., 1986. Principles of Isotope Geology. Wiley, New York, 589 pp. Ford, D.C. and Ewers, R.O., 1978. The development of limestone cave systems in the dimensions of length and depth. Int. J. Speleol., 10: 213-244. Ford, D.C. and Williams, P.W., 1989. Karst Geomorphology and Hydrology. Unwin Hyman, London, 601 pp. Forti, P. and Buzio, A., 1985. Una nuova scoperta nelle grotte di Monte Sedom (Israele): le eceentriche di anidrite. Mineralogical and Paleontological notes, 46:11-16. Frnmkin, A., 1986. Speleogenesis of vertical shafts in a Mediterranean environment (Ofra, Israel) (abstract), 9th Congreso International de Espeleologia, Barcelona, 1986, Barcelona, Union Internacional de Espeleologia, Vol. 1, pp. 264-267. Frumkin, A., 1992. The karst system of the Mount Sedom salt diapir. Ph.D. Dissertation, Hebrew University, Jerusalem, 135 pp. (in Hebrew, English abstract). Frumkin, A. and Ford, D.C., 1994. Rapid entrenchment of stream profiles in the salt caves of Mount Sedom, Israel. Earth Surf. Processes Landforms, in press. Frumkin, A., Magaritz, M., Carmi, I. and Zak, I., 1991. The Holocene climatic record of the salt caves of Mount Sedom, Israel. Holocene, 1(3): 191-200. Gerson, R., 1972. Geomorphic processes of Mount Sedom. Ph.D. Dissertation, Hebrew University, Jerusalem, 234 pp. (in Hebrew, English abstract). Gerson, R. and Inbar, M., 1974. The field study program of the Jerusalem-Elat Symposium, 1974. Reviews and summaries of Israeli research projects. Z. Geomorphol., Suppl., 20:7-11. Kempe, S., Brandt, A., Seeger, M. and Vladi, F., 1975. "Facetten" and "Laugdecken", the typical morphological elements of caves developing in standing water. Ann. Sprlrol., 30(4): 705-708. Lerman, A., 1970. Chemical equilibria and evolution of chloride brines, 50th anniversary symposium. Mineral. Soc. Am. Spec. Pap., 3: 291-306. Milanovic, P.T., 1981. Karst Hydrogeology. Water Resources Publications, Littleton, CO, 434 pp. Petroleum Services, 1979. Solution mined liquid hydrocarbon storage caverns, Mount Sedom, Progress Report 2. Petroleum Services Ltd., Tel Aviv, 12 pp. Pfeiffer, D. and Hahn, J., 1972. Karst of Germany. In: M. Herak and V.T. Stringtield (Editors), Karst: Important Karst Regions of the Northern Hemisphere. Elsevier, Amsterdam, pp. 189-223. Pohl, E.R., 1955. Vertical shafts in limestone caves. Nat. Speleol. Soc. Occas. Pap., 2, 24 pp. Sulin, V.A., 1946. Waters of Petroleum Formations in the System of Natural Waters. Gostoptekhizdat, Moscow, pp. 35-96. Zak, I., 1967. The geology of Mount Sedom. Ph.D. Dissertation, Hebrew University, Jerusalem, 208 pp. (in Hebrew, English abstract). Zak, I., 1974. Sedimentology and bromine geochemistry of marine and continental evaporites in the Dead Sea basin. In: A. Coogan (Editor), Fourth Symposium on Salt, Vol. 1, Cleveland, Ohio, Northern Ohio Geological Society, Cleveland, OH, pp. 349-361. Zak, I. and Bentor, Y. K., 1968. Some new data on the salt deposits of the Dead Sea area, Israel. Symposium on the Geology of Saline Deposits, 1968, Hannover. UNESCO, Hannover, pp. 137-146.