Veronika Smolková

Late-Holocene evolution of a floodplain impounded by the Smrdutá landslide, Carpathian Mountains (Czech Republic)

Landslides affecting narrow mountainous valleys might significantly determine sedimentation dynamics of floodplains. We present here a detailed study of the sedimentary archive within a landslide-controlled impounded floodplain (Smrdutá site, Czech Flysch Carpathians) using geochronological (14C and 137Cs), sedimentological and pollen evidence. A sedimentary sequence deposited above the landslide dam points to three highly discontinuous and instantaneous depositional events dated to 4.6 and 2.0 cal. ka BP, whereas the last cycle started approximately in the 17–18th centuries and has continued to recent times. Such sedimentary pulses characterized by the duration of several decades to a few centuries originated as a consequence of the blockage and/or reduction of the valley floor width by successive long-runout landslides from a slope formed by tectonically and lithologically anisotropic flysch bedrock. Stages of mass movement activity revealed by the Smrdutá landslide correlate well with major humid late-Holocene oscillations suggesting its high sensitivity to century-scale climatic deteriorations. The character of lithological units forming individual sedimentary pulses, erosional hiatuses and sedimentary traces caused by the July 1997 extreme flood indicate a decisive role of large flood events during accretion and erosion of the floodplain-impounded section.

Open image in new windowAge and Reactivations of Catastrophic Complex Flow-Like Landslides in the Flysch Carpathians (Czech Republic/Slovakia)

Catastrophic complex flow-like landslides (CFLLs) are characterised by a deep-seated retrogressive landslide of structurally unfavourably oriented rocks and earthflows that occupy the lower slope positions and originate due to the liquefaction of material accumulated on the upper slopes. These landslides are locally important geomorphic agents of Late Quaternary mountain evolution in the Flysch Belt of the Outer Western Carpathians (Czech Republic/Slovakia). Most of the CFLLs dammed and steepened adjacent valleys. Radiocarbon dating suggests that a majority of them moved repeatedly throughout the Holocene, namely approx. every 1–2 ka. Dated events occurred during humid phases of the Younger Dryas/Holocene transition (11.5–9.4 cal ka BP), Atlantic chronozone (7.4–6.6 cal ka BP), the beginning of the Subboreal chronozone (ca. 4.6 cal ka BP) and, primarily, within the Subatlantic chronozone at ca. 2–0.8 cal ka BP (>50 % of all events). Our study suggests that slopes based on an unfavourable structural setting and affected by long-term deep-seated gravitational deformations may produce CFLLs, even if they are located in medium-high mountains. Although our chronological dataset is influenced by the erosion of older landforms, most of the dated reactivations correlate with regional increase in precipitation identified by previous palaeoenvironmetal studies.

Low-Gradient Megalandslides at the Northern Boundary of the Caucasus-Crimean Orogene: Seismically Induced?

We have detected at least 16 extremely large landslides (0.2–2.8 km3) in an ~800 km long stretch of the hilly northern foothills of the Caucasus-Crimean orogene (Russia, Ukraine). All landslides originated within tectonically inverted foredeep basins formed predominantly by the Neogene clay-rich substratum (claystones, marls and occasionally limestones). The landslides, which evolved as blockslides combined in their distal parts with earthflows, are characterized by an extremely low height to length ratio (0.05–0.08). OSL and radiocarbon dating of deposits of the selected landslides pointed to various stages of the last glacial and Holocene. The largest landslide observed in the Kuban river valley/Russia (32 km2; 2.8 km3) originated between ca 13 and 35 ka BP during the last glacial. The activity of the Crimean landslides/Ukraine was determined to two main stages: the interval between the end of the last glacial and ca 6 ka BP, and ca 1–1.5 ka BP. Although the determination of triggering factors of ancient megalandslides is difficult, we exemine the palaeoseismic hypothesis considering surface rupturing of some Late Quaternary faults in the vicinity of the landslides. Potential seismic origin of the megalandslide in the Kuban river valley is in line with the presence of nearby situated tectonically deformed late Pleistocene river terraces. Crimean and Kuban landslides are situated along the regional tectonic zones bordering the Alpine orogenic belt and the ancient Scythian platform. Taking into account semiarid continental climatic conditions in the time of the landslide formation, palaeoseismic scenario seems to be quite reliable.

The May 2010 Landslide Event in the Eastern Czech Republic

In May 2010 intensive rainfalls and consequent floods affected Central Europe, including the eastern part of the Czech Republic. In the period 15–25 May 2010, more than 150 landslides originated in the Outer Western Carpathians and their foredeep. In the case of the Girova rockslide, the May 2010 rainfall event only supplied the final triggering push. The main preconditions of slope failure were long-term, internal changes within the rock mass. As evidenced by dendrogeomorphological dating, rock strength weakening was connected to long-term (decennial-scale) creep leading to the final catastrophic movement. Many trees from the upper section of the landslide do not reveal any year of reaction wood. It could be explained by a translational, blocklike character of movement.

Karstification as a Predisposing Factor of Seismically Triggered Landslides: Case Study from the Crimean Mountains (Ukraine): Introduction to the Problem

Deep-seated gravitational deformations are significant denudational agents of rock slopes at the margins of karstified plateaus of the Crimean Mountains (CM). The CM evolved during Mesozoic–Cenozoic times as a response to the deformation between the Black Sea domain and East-European platform. The southwestern part of the area is characterized by steep, up to 1000-m-high coastal escarpments consisting of Late Jurassic limestones overlying tuff layers and weak Late Triassic flysch with sporadic small intrusions of Middle Jurassic diorites, gabbros and granites. Steep rock slopes contrast with elevated, highly karstified plateaus situated approximately 500–1300 m a.s.l. The aim of this article is to show long-term evolution of a giant rock slope failure close to the Black Sea coast in the southwestern tip of the CM near Foros Town. The failure evolved in highly anisotropic limestones overlying plastic flysch layers where the main head scarp follows a strike-slip fault. The Foros slope failure is an excellent demonstration of the significance of a preparatory stage in the evolution of large deep-seated slope deformations. Inherited and undisturbed horizontal slickensides on the sub-vertical, inactive fault surface serve as good evidence of significant extensional movement of the surface blocks away from the main headscarp. The studied deformation shows that in a relatively small area tensional (cutting) surfaces can be formed by a great variety of rock discontinuities such as the strike-slip fault, joints and steeply inclined bedding planes. The presence of well-developed, nowadays weathered, speleothems furthermore points to significant karstification that provided additional widening of spaces within rock mass. Gravitational movement destroyed and unroofed several cave systems originally presented at the former edge of a karst plateau. Our findings reveal that large rock slope failures can be added to the factors contributing to the evolution of unroofed caves. Although triggering factors of the activation of individual parts of slope deformations can be determined only hypothetically, lessons learned from widespread landslide activity during and after the 1927 Yalta earthquake and rainfall-driven landslides in the vicinity of Feodosia Town make us consider both seismic loading of slopes and high pore-pressures during heavy winter rainfalls or rapid spring snowmelt to be significant factors. Beside seismic activity, intensive Late Holocene slope processes can be attributed to intensive human activity.