Field Measurements of Gypsum Denudation Rate in Kulogorskaya Cave System
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Karst and Caves in Carbonate Rocks, Salt and Gypsum – poster 2013 ICS Proceedings FIELD MEASUREMENTS OF GYPSUM DENUDATION RATE IN KULOGORSKAYA CAVE SYSTEM Nikolay Franz1, Sergey Sorokin2,3, Alexandra Alexeeva4, Irina Inshina4, Olga Novysh3, Anton Kazak3 1Arkhangels Speleological Association “Labirinth”, Arkhangels, Russia, [email protected] 2Tver State University, Zhelyabova 33, Tver, Russia, [email protected] 3Arkhangels Speleological Association “Labirinth”, Arkhangels, Russia 4Tver State University, Tver, Russia Results of measurements of karst denudation rate in natural conditions in Kulogorskaya cave system are reported. Taking into consideration relatively high rate of denudation in gypsum caves we have used simplified MEM measurement method. Observation of hydrological situation suggests that most of the dissolution in caves of the area occurs during a few days of spring flood. Here we report results of measurement at 62 points located at 6 sites for the period from 2003 to 2012. Data analysis gives us average retreat rate of 0.0448 mm/day, while point is underwater. Confidence interval for this value is 0.0379 to 0.0516 mm/day. 1. Introduction annual rainfall 560 mm. The coldest month – January (average long-term temperature -13.0 °C). The warmest Kulogorskaya cave system includes 7 caves, three of which month – July (average long-term temperature +15.4 °C). constitute the largest cave system in Russia in a gypsum, with a total length of surveyed passages more than 17.5 km. Karstifiable rocks of massif are represented by gypsum and dolomite of Permian age. Primary karst rock is gypsum, Cave system is studied by speleologists from Arkhangels, St. forming homogeneous strata ranging from 0.2 to 7 m, Petersburg, Moscow, Tver and other cities under the guidance interleaved with dolomites. Average total width of of Arkhangels Speleological Association “Labirinth”. sedimentary cover above caves is 20 m. This report presents the preliminary results of the Caverns in the massif are well represented in all measurements of dissolution of gypsum in the natural hydrodynamic zones: vertical transfer zone, zone of conditions, which are held from 2003. seasonal fluctuations of water table levels and phreatic zone. Human-passable channels are located only at the zone of water table. In karst dissolution infiltrational, condensational and floodwaters are involved. Local erosion bases for caves in Kulogorskaya cave system is Pinega River and Pinega-Kuloy channel. 3. Conditions of karst denudation The most important role in the process of dissolution of gypsum walls in the water-table zone play floodwater penetrating the karst massif through numerous small ponors laid along the Kulogorsky ledge in the area of contact between the karst rocks and the floodplain alluvium. The penetration of floodwater into massif mainly occurs in spring (April–May) when Pinega waters rise by 4–5 meters above the winter low water (in the caves in this period water rises up to 2–3 meters, completely flooding almost all horizontal passages). During this period water flows in Figure 1. Geographical position of Kulogorsky Cave Region. horizontal passages in water-table zone, which are typically dry in other seasons. Absorbed water temperature typically ranges around 1 °C. 2. Geography and geology Autumn floods in the river and caves are less affluent: the Geographically Kulogorskaya cave system is located in the rise of water in the river – up to 2–3 m, in caves – up to 1 north of the East European Plain, in the watershed of Pinega m. The inflow of water into the massif in this period is only (tributary of Northern Dvina) and Kuloi (Fig. 1). through channels of phreatic zone (flooding of caves is caused by raising of the water-table level). Reverse runoff The climate here is quite severe, with low air temperatures after autumn flood occurs on the same path in the opposite and high humidity, rainfall exceeds evaporation. The multi- direction (the horizontal filtering through the thickness of year average annual temperature is +0.2 °C. The average the alluvium in the direction of erosion basis). 185 Karst and Caves in Carbonate Rocks, Salt and Gypsum – poster 2013 ICS Proceedings Figure 2. Measurement method. The main feature of the hydrogeology of Kulogorsky massif Rate of Karst Denudation in Russian) were fitted by us only is absence of significant inverse flow of flood waters after in the passages of this zone. passing through the peak of the spring flooding. In other words, floodwaters which penetrate the caves from the floodplain, never pour back by the same horizontal 4. Methods channels. Our current understanding is that at mean water period the water from caves is unloaded by slow horizontal To study the rate of dissolution of gypsum in the natural filtering through the thickness of alluvium in the direction conditions since 2003, we have equipped 15 measurement of the local base level of erosion. sites in five caves. Almost all sites were equipped on the surfaces of the ceilings and walls of passages laid in the To determine the saturation of cave waters with gypsum, massif of white-and fine-grained gypsum with rare clusters we measure its electrical conductivity, which is directly of large gypsum crystals. proportional to the salinity of water – both according to the data of other researchers (Krawczyk and Ford 2006), and Since the expected rate of dissolution of gypsum is higher confirmed by our observations. than that of limestone, we used a simplified, compared to the classic version of the MEM method (Smith, 1977). In 2004 we conducted experiment in K-2 cave on the dissolution of gypsum powder in the water from Each SKD site includes from 1 to 4 rows of control points. underground lake (initial conductivity 2.1 mS) at natural Rows of control points may be oriented horizontally or cave temperature +2.2 °C, which gives us maximal vertically. Each row, in turn, includes from 4 to 7 observation conductivity of 2.5 mS. points located with an interval of 3 cm along a straight line between the two fixed reference points. But these values are not reachable in the natural underground water exchange, which is convincingly shown by the results of long-term observations of hydrochemical karst springs in the river Pinega: conductivity of discharged water is never more than 2.2 mS (88% of maximum mineralization). Since caves are inaccessible to humans during flood, we have used the tanks with automatic closure to determine the salinity of flood waters. These tanks have been installed by us in 2005 at a different distance from ponors. Analysis of the data showed, that conductivity of aggressive flood water, which penetrates the massif is 0.5–0.6 mS (20–24% of maximum). After passing for many hundreds of meters underground transit, these waters are saturated to 1.8–1.9 mS (72–76%). Then, in a few months of slow water level drop, conductivity of underground water reaches 2.2–2.3 mS (88–92%). Thus, during the transit time (in a few days at the peak of the spring flood) in conditions of heavy water exchange, floodwaters are saturated with the products of dissolution of sulphate rocks up to 50% approximately. Over the following months, during slow level decline, water gather additionally about 15% to reach the maximum observed mineralization of 2.2–2.3 mS, or 88–92% of the maximum possible in these climatic conditions. Since, as mentioned above, main floodwater flow occurs in the zone of water-table, we can assume that maximal denudation of cave walls happens in this zone. That is why Figure 3. Location of measurement sites. all karst denudation rate measurement sites (“SKD” for 186 Karst and Caves in Carbonate Rocks, Salt and Gypsum – poster 2013 ICS Proceedings Figure 4. Hydrograph of spring flood at Pinega river. Data from the state gauging station in Kulogory village in the immediate vicinity of the cave system (Federal State Unitary Enterprise “Centre of Russian water works inventory and state water cadaster”, 2012). Reference points are stainless steel screws fixed to the wall exceptionally high rate where identified. They are shown with plastic plugs to a depth of 50–60 mm. During the with “high rate” comment in tables 1 and 4, and they are measurement special aluminum rod of square section with not used for any calculations. test holes every 3 cm is imposed on the heads of the screws Using data from the remaining points we calculate mean (Fig. 2). retreat rate for every measurement row and site. Vernier caliper probe is inserted to each hole and distance Average retreat rate for all observation points is 0.0448 between wall surface and outer surface of the rod is measured. mm/day. The confidence interval for this value from 0.0379 On every SKD site we made a special “passport” with the to 0.0516 mm/day. tables of the measured values on each set of control points. The column “conditions” of Tables 1 and 2 provides an We also made an exact measurement of height above sea assessment of denudation conditions for each of the sites. level of each control point row. In the sites marked “Corrosion” there is no signs of water flow. “Erosion” – conditions are suitable for water flow and there are signs of such flow. 5. Results and discussion It is interesting to note that the maximum retreat rate on This report presents the results of observations on six oldest surfaces of walls and the roof was measured at the SKD-8 SKD sites equipped in 2003–2004 and affected by nine site, equipped in one of the passages of Vodnaya (Water) spring floods (2004–2012). Locations of sites are shown in Cave, where the nature of water deposits suggests flood- Fig.