Deciphering Multiphase Slope Deformation From A Cave’s Perspective

Caves are environments that often act as traps in which time appears to have stopped. Because inactive cave passages are protected from external erosive factors, marks left by past processes and events are well preserved in these passages, unlike their surface counterparts which often can be obliterated. By investigating deformed speleothems, researchers can find precise dating over a wide time range, up to 0.6 Ma.

We used two features of caves to study the faults in Kalacka cave, located in the Tatra Mountains, Poland. The Tatra Mountains form the highest mountain range of the Carpathians, reaching 2655 m a.s.l. at Gerlachovský Peak.

During the research for my Ph.D. thesis, I analyzed dislocated cave passages, i.e. structures younger than the caves. The paleostress reconstruction indicated that within such fault population, two groups can be distinguished. One can be associated with tectonic processes. The maximum compression vector for the second group was calculated to be vertical, i.e. equal to gravity. This was one of the main arguments for recognizing these faults as a result of mass movements (cf. Szczygieł, 2015). In the Kalacka cave, one of the faults was unique because it was covered by flowstone, which was cracked just above the fault, which offered the opportunity to determine the maximum age of faulting if we could date the youngest lamina cracked with speleothem.

This is what I started to work on after finishing my Ph.D. When we cut the sample in the laboratory, it turned out that the lower part showed one more episode of dislocation. Thus, with 3 cm of a sample, we managed to determine the time interval of two phases of faulting along one plane.

This methodology is based on the fact that by dating the youngest layer of the broken speleothem and the oldest layer of regrown speleothem, we are able to determine the time interval in which the destruction occurred. Speleothem dating was performed using the uranium-thorium method using mass spectrometry (ICP-MS), allowing the dating of small samples (weighing 0.1-1 g), i.e. isolated from a single lamina, giving a high resolution of measurements. The use of the mass spectrometry method allowed dating of samples > 550 ka.

Thanks to dating, we had some idea of the faults’ activity timing, but on the surface, above the cave, morphology did not point toward mass movements. So, we’ve taken a few steps to check it out. We started with a basic gemological and geological mapping. The fieldwork was focused on the determination of the localization, height, orientation, and surface relief (smoothness, flatness, etc.) of the scarps. In addition, fractures and bedding orientations within individual blocks (between scarps) and in the in situ area were measured to find the relative rotation between individual blocks and undeformed rock.

Main scarp and head of the landslide above Kalacka Cave. Image courtesy Jacek Szczygieł.

To check the link between scarps on the surface and dislocation in the cave, we applied geophysical methods. A geophysical study based on electrical resistivity tomography (ERT) and induced polarization (IP) methods was carried out over two parallel survey lines 200 and 300 m long. The well-known and widely-applied ERT method uses a direct current to measure the resistivity of rocks, as described by Ohm’s law. Different type of rocks have different resistivity, while fractures, faults, or voids (like caves) give different results.

All this data — tectonic and geomorphological data from the cave and from the surface, as well as geochronological and geophysical data — gave us enough clues to conclude that the deformation of our mountain was caused by a landslide that reached the Kalacka cave. But we also wished to know what triggered that mass movement.

Fractured flowstone with naturally detachedfragment which we sampled. First we planned to date only the youngest layer but when we cut it we found “scar” after older event. Image courtesy Jacek Szczygieł.

One of the common causes of landslides is increased rainfall. Taking advantage of the fact that speleothems are excellent carriers of paleoclimatic and paleoenvironmental information, we attempted to assess the impact of climate on the development of slope failure by employing microtexture and stable isotope (δ18O, δ13C) analyses of the flowstone under investigation. Incidentally, these studies resulted in the acquisition of the first information on middle-Pleistocene environmental conditions in the region. Such a comprehensive examination of the cave and the surrounding area allowed us to reconstruct the evolution of the research area.

Use of a speleothem permitted us to obtain paleoenvironmental information that may be directly related to the gathered ages. Just before the dilation activation, δ13C values dropped and rose back just after the movement. Although the resolution of stable isotope analysis does not allow us to decipher the direct cause of mass movement, it indicates regional environmental changes, such as precipitation intensity, which appears to be crucial in the initiation of slope failure above Kalacka Cave.

The slopes of Kalacka Turnia suffered from complex bedrock mass movements that may have begun as dilation, as pointed by faults found in the speleothem, activation of which was dated for a range of 280 ± 7 ka to 265 ± 8 ka. The second stage (the landslide above the cave) has not been dated. Nevertheless, we can conclude that the deformation took place later than 280 ± 7 ka. According to the geoelectrical data, the structures developed during the second stage reach as high as 40 m in depth.

Lack of the toe, or downslope, of the landslide can be interpreted in two ways. First, we are not sure that the landslide ever had a toe. Considering the ratio of head scarp height (up to 4 m) to the length of the entire form (ca. 250 m), and its extent from the valley bottom up to the ridge, this mass movement could be identified as a deep-seated gravitational slope deformation. Such forms are often devoid of lobes. On the other hand, the crown and head of the landslide have been preserved, and the subsurface structure contains the surface of the rupture, which may indicate that the toe could have developed there. The lack of landslide forms in the relief below the boundary of the last glacial deposits would suggest that the toe was later eroded by glacier ice.

Fractured flowstone with naturally detachedfragment which we sampled. First we planned to date only the youngest layer but when we cut it we found “scar” after older event.An intersected sample with obtained ages and marked fractures (on the left) and a microscopic view of the older dilation (on the right). Image courtesy Jacek Szczygieł.

During the final stage, the dilation was rejuvenated (post 35 ± 4 ka), and on the surface, a rockfall occurred, covering the last glacial deposits which points to its development during the latest Pleistocene or the Holocene. Moreover, crevasses and dilation extending beyond the range of the landslide may indicate its continued mass relaxation. This leads us to the conclusion that Kalacka Turnia was gradually deformed by deep-seated gravitational slope deformations.