Flysch Formations Research Articles

The North Penninic Bündnerschiefer and Flysch of the Prättigau (Swiss Alps) revisited

During the re-mapping of the area for the Geological Atlas of Switzerland, a significant stratigraphic unconformity was discovered in the North Penninic (Valais) Bündnerschiefer and the Flysch series of the northern Prättigau. It separates different units of the Cretaceous Bündnerschiefer from the Palaeogene Flysch. We explain this observation by a basin conversion from extension to compression, which caused the initial deformation of the Bündnerschiefer in an accretionary wedge. Interlinked return-flow has created a new heterogeneous substrate for the flysch sediments and explains the different types of unconformities. The basin conversion coincided with high-grade metamorphism in the vicinity of the the South Penninic suture and the Austroalpine units, and the increased exhumation in the Austroalpine nappe stack. Detrital zircon dating confirms also a change from European to Austroalpine detrital sources in the flysch sandstones. We discuss a palaeotectonic model leading to hP/lT metamorphism of the Bündnerschiefer in the Late Eocene (c. 42 Ma). It appears that the flysch formations were also involved, but to a lesser degree by tectonic deformation from the late Early Eocene onwards, as the pervasive folding characteristic of the Bündnerschiefer is absent. This has been followed by a phase of S-directed backfolding. During the Oligocene and Miocene, more extensive deformation occurred by SE to NW compression and finally by probable westward thrusting and folding. Our main theme is the transition from passive to active continental margins, which in Alpine plate tectonic framework corresponds to the transition to flysch sedimentation by basin conversion. Our results show that the simultaneity of the transition from extension to compression, as indicated by the accumulation of flysch, shifted in time from south to north in the Alpine Tethys.

Sedimentary Characteristics and Basin Evolution of a Compartmentalized Foreland Basin—Internal Ionian Zone, Western Greece

This study is based on the detailed sedimentological analysis of eleven sections and one well through the late Eocene–Oligocene flysch formation of the Internal Ionian Zone (IIZ) in Western Greece. The sections are spread from the northern parts of Epirus to the north and Aitolokarnania to the south. Sedimentological data combined with biostratigraphic analyses resulted in a five-stage evolutionary model for the basin. Unit I corresponds to the lower part of the examined sections, indicating the onset of clastic sedimentation. Regarding depositional environments, it is regarded as a basin plain where lobe distal fringe accumulations occur. Unit II consists almost exclusively of heterolithic facies, marking the advance of a lobe complex system. Massive sandstone facies dominate unit III and can be considered a more proximal submarine fan system. Unit IV reflects a calm period of the basin, where mud-dominated heterolithics and hemipelagic mudstones were deposited. Hemipelagic mudstone facies with intervals of heterolithics, conglomerates, and deformed and massive sandstone facies characterize unit V. The architecture resembles a slope system incised by canyons and channels. The sand-rich intervals in Units III and V could act as the most favorable reservoir levels. In contrast, the sand-rich intervals in Unit II are considered less promising due to their higher heterogeneity.

Failure mechanism in a highway embankment founded on a sloping tectonised flysch formation

A highway embankment founded on a slope in a tectonised flysch formation of the Apennines chain (Central Italy) has been affected for about 30 years by continuous slow movements that caused some fractures on the road paving thus requiring periodic maintenance works. Due to the relevance of the damaged infrastructure, different investigation and monitoring campaigns have been carried out to collect information about the properties of the involved soils, the groundwater regime and the displacement field. Field monitoring, in particular, revealed that the movements were the result of a failure mechanism affecting both the embankment and the foundation soils. In particular, the observed displacements showed a weather-induced seasonal trend featured by acceleration/deceleration stages during the wet/dry periods. In order to better understand the detected mechanical response, a numerical model accounting for a soil hydrological balance of the interested area has been developed allowing a reliable historical reconstruction of the displacement regime.

Weather induced post failure kinematics of a highway embankment founded on a marly sandstone slope

A highway embankment founded on a sloping tectonised marly-sandstone flysch formation located in the Apennines chain (Italy) has been affected for about 30 years by continuous slow movements. Given the strategic importance of the involved infrastructure, different investigation and monitoring campaigns have been carried out to get information about the properties of the involved soils and collect data about the displacements and piezometric regime. Field monitoring, in particular, reveals that the observed displacements result from a failure mechanism involving both the embankment and the foundation soils. However, significant gaps in monitoring jeopardize the possibility to assess the long-term trends in the displacements and piezometric regime and the significance of weather forcing in regulating the phenomena. To address such research questions, a procedure, easily transferable in different contexts, is proposed and applied to the test case: a simple hydrological proxy indeed permits evaluating the rate of movement featured by weatherinduced seasonal variability. Such a mechanical response has been confirmed by the results of a simplified numerical model aimed at finding out the main features of the observed kinematics accounting for a hydrological balance of the involved area.

The Red Flysch Formation in Southern Apennines: Lithological and Structural Features and Challenges in Geotechnical Characterization and Modelling

Flysch formations are widespread all around the Mediterranean basin and in many other parts of the world. Their special fabric, characterized by a strong heterogeneity due to alternations of often deformed lapideous and fine-grained layers and by a network of discontinuities, brings to light the strong limitations of the geo-engineering sciences when dealing with such complex materials. Based on the results of an investigation carried out in a long section of the Southern Apennines, the paper examines this problem focusing in particular on the role played by material fabric at different scales on the hydraulic and mechanical behavior of the deposit.

Probabilistic shear wave velocity model based on downhole test database for Apennine flysch formations

Defining a reference shear wave velocity profile (Vs) is crucial for seismic-hazard assessment and seismic response analysis. However, selecting a reference velocity profile is not straightforward, mainly because of the high variability of shear wave velocity in complex formations. This study considered 26 shear wave profiles from downhole tests carried out in the Laga Basin (Gran Sasso, Central Apennines), where the soil can be categorised as complex owing to the chaotic succession of turbidite and Quaternary continental deposits. Downhole tests and stratigraphy were used to (i) develop a probabilistic model for predicting the shear wave profiles calibrated on the collected test data and (ii) propose an outlier detection method for estimating the first fundamental frequency of the deposit from horizontal-to-vertical spectral ratio (HVSR) analyses. Specifically, the stochastic shear wave model provides the uncertainty of the first fundamental frequency estimated from downhole tests based on a simplified mechanics-based formulation. The defined bounds, were forward uncertainty propagation following a Monte Carlo approach, have been used to define the criteria for outlier detection in HVSR frequency assessments, given an approximate estimate of the equivalent depth.

Recognition of the flysch substrate using the electrical resistivity tomography (ERT) method to assess the effectiveness of the injection process

During the construction of a section of the S-7 Lubień – Rabka-Zdrój dual expressway, located in the area of the Carpathian flysch (Carpathian Flysch Belt, South Poland), damage to the embankment was observed, as well as cracks and depressions in the new pavement. An analysis of the geological and engineering conditions in the area of the road section under construction showed the existence of a complex tectonic structure of the flysch formations, a shallow groundwater table, and numerous landslides. In order to stabilize the road substrate, it was decided to carry out injections, and the locations of these injections were initially geotechnically tested. However, due to the high variability of the geological structure, the target method employed was electrical resistivity tomography (ERT), which performed the survey in two stages. In Stage I, the geoelectrical/geochemical structure of the near-surface zone was identified, and the probable causes of road damage were indicated. This stage was completed by performing the stabilization and sealing process of the ground with an injection mixture. In Stage II, studies were carried out to evaluate the effectiveness of the injection process. The ERT method effectively identified the shallow geological structure and, in particular, delineated the zone of strong fractures in the flysch and areas associated with faults. Using the electrical resistivity tomography method, it was also possible to determine the injection mixture’s approximate penetration depth and the loosening zone’s degree of filling.

Development of a Modern-Style Trench-Arc-Backarc System in the Proto-Tethys Ocean (Qilian Orogenic Belt, NW China)

Abstract The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U–Pb chronology and geochemistry on various rocks from a middle-ocean ridge (MOR)-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 531 to 477 Ma and felsic-mafic arc volcanic rocks with ages of 506 to 439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490- to 448-Ma suprasubduction zone (SSZ)-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530 to 500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500 to 490 Ma with rapid thinning of crust to ~35 km; and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~490 to 450 Ma.

Evaluation and Economics of Shale Gas Reserves in the Flysch-Eocene Formation of the Jaca Basin

The new international outlook for the gas sector suggests evaluating exploitable reserves in previously unconsidered areas including hydraulic fracturing technology. In order to estimate the amount of gas in the Jaca Flysch formation, the analysis of geological and geophysical logs and the volumetric method have been used. It has been taking into account the part of the formation likely to contain gas, the porosity (2.65%) calculated from sonic logs with Wyllie’s equation, the water saturation (35.3%) with Archie’s formula, and the initial gas formation volume factor (Bgi), estimated with MHA-P3 software with the reservoir pressure/temperature data 3600 psi/90 °C. The economic analysis of each well has been carried out based on three options, without stimulation, with 50% and 100% stimulation by fracking, and five possible construction costs (7.5–15 MM€). The impact of the use of the fracking technology on the production of the well is about 48%. The production rate and the economic impact that its exploitation would have on the domestic demand for natural gas has been analyzed, resulting in a significant contribution to the national energy mix of between 10–20% of consumption for several years.

Integrated Geomechanical and Digital Photogrammetric Survey in the Study of Slope Instability Processes of a Flysch Sea Cliff (Debeli Rtič Promontory, Slovenia)

This work presents an integrated study approach that combines the results of a geomechanical survey with data obtained using digital photogrammetry (DP), to assess slope instability processes affecting a sea cliff at the Debeli Rtič promontory (Slovenia). The investigated cliff is 4–18 m-high and is made up of an alternation of sandstones and marlstones belonging to the Flysch Formation of Trieste, which is Eocene in age. The studied cliff was subjected to localized slope failures that occurred in the past and is currently subject to frequent rock collapses, thus resulting in its partial and episodic retreat. Field evidence acquired through a traditional survey was integrated with outputs of the DP technique based on 1399 images that were collected using both a commercial unmanned aerial vehicle (UAV) and a mobile phone (MP). UAV-derived images were useful for performing rock mass structure analysis in the upper part of the investigated cliff, where the traditional survey was not possible due to hazardous operating conditions. In addition, the use of a MP was observed to be a useful tool for the rapid collection of images at the toe of unsafe marine cliff environments. This study highlights that UAV-DP and MP-DP techniques can only be effective if the outcomes obtained from the 3D model reconstruction are validated by direct measurements acquired by means of the traditional field survey, thus avoiding improper or even erroneous results while enlarging the amount of data and the area of investigation. The study approach presented herein allowed for the assessment of slope instabilities affecting the Flysch Sea cliff, whose retreat is caused by the combined action of marine erosion and slope gravitational processes.

Přirozená radioaktivita sedimentárních hornin v severovýchodní části moravskoslezského paleozoika

The paper provides detailed information on the natural radioactivity of sedimentary rocks in the northeastern part of the Moravo-Silesian Palaeozoic Basin in the Bohemian Massif. The studied area is dominated by the Lower Carboniferous flysch sediments belonging to the Horní Benešov, Moravice, and Hradec-Kyjovice formations (from botttom to the top). These three flysch formations consist of siliciclastic sediments, mainly graywackes, siltstones, silty shales, and clay shales, while conglomerates are less abundant. The overlying sediments of the Ostrava Formation (Namurian) occur on the eastern margin of the studied area. The Ostrava Formation has a character of a paralic coal-bearing molasse with alternating beds of sandstone (or arkose), siltstone and claystone. The sediments often have an elevated content of organic matter, concentrating into coal seams. The rocks of the Ponikev Formation (Devonian) occur in a narrow belt in the western part of the studied area. The Ponikev Formation consists mainly of silicites. Potassium, uranium and thorium contents were measured using a laboratory gamma-ray spectrometer in 1612 rock samples from 672 localities on five map sheets (1 : 50 000 scale). The calculated values of mass activity of 226Ra equivalent (am) demonstrate that the natural radioactivity of sedimentary rocks of all three flysch formations is lower or similar to that of the average Earth crust (about 180 Bq . kg–1). The radioactivity of flysch sediments increases with decreasing grain size from psephites (average am is 122 Bq . kg–1) to psammites (144 Bq . kg–1) and to aleurites + pelites (188 Bq . kg–1). Siltstones and claystones with a low organic matter content (226 Bq . kg–1), coaly siltstones and coaly claystones (240 Bq . kg–1), coaly sandstones and arkose sandstones (232 Bq . kg–1), and coal (202 Bq . kg–1) of the Ostrava Formation show slightly higher average am values. The relatively high radioactivity of these sediments is related to the elevated content of uranium (7.9 ppm on average).

CONTROL FACTORS OF IRON MINERALIZATION IN THE METALLOGENY OF THE LJUBIJA ORE REGION

<p>The control factors of mineralization in the Ljubija ore region are stratigraphic, lithological, magmatogenic, structural-tectonic and hypergenic. The distribution of iron ores in the Ljubija ore region is primarily controlled by the spread of the Olistostrome member of the Carboniferous Javorik flysch formation. Only that member, whether uncovered on the surface or overlain by an Upper flysch member, contains primary siderite-ankerite iron ores. In addition, the distribution of ore bodies is significantly affected by the concentration and location of mineralized limestone and limestonedolomite olistoliths. The seating of carbonate olistolitic bodies in the member is irregular, that is, chaotic. The size, shape, and morphology of these bodies and their boundaries are extremely uneven and diverse, in both primary and tectonic occurrences. Not all olistolitic bodies are mineralized. Those that are can be completely, partially or minimally mineralized. Due to all this, ore bodies and deposits “have no continuity and appear in isolation”. Therefore, there is a high exploration risk. These major irregularities are not characteristic of redeposited powdered limonite ores in Plio-Quaternary lacustrine sediments. Cimmerian lateral ruptures were important for the distribution of hydrothermal solutions and today's distribution of ore deposits and occurrences within ore fields and ore nodes. The hypergenic control factor significantly influenced the forming of iron oxide ores. In the Alpine tectogenesis, old systems of faults and joints were renewed and new created. In this respect, the most significant were transverse, open, steep SW-NE ruptures.</p>

The erosionally confined to emergent transition in a slope‐derived blocky mass‐transport deposit interacting with a turbidite substrate, Ventimiglia Flysch Formation (Grès d’Annot System, north‐west Italy)

AbstractBasal interaction beneath frontally‐emergent mass‐transport deposits has been widely documented in seismic data, but its effect on deposit heterogeneity not convincingly calibrated at outcrop. Several blocky mass‐transport deposits occur as part of the Late Eocene Ventimiglia Flysch of north‐west Italy, comprising slope‐derived marlstones, representing the original slide, and turbidite material, entrained after erosion of substrate sediments; this study reports on the best exposed. Correlation of twenty‐nine sedimentary logs tied to the hosting turbidite stratigraphy allows thickness and facies changes to be tracked over an area of ca 40 km2, spanning the erosionally confined to emergent transition. A basal erosion >55 m deep and several kilometres wide confines a marlstone megabreccia containing megaclasts of up to 1 km across, interpreted as the product of a submarine slide originated from a sector collapse of the western basinal slope. Approaching the downstream limit of this erosional confinement the marlstone megabreccia is replaced by highly deformed turbidites that, more distally, are in turn superseded by a debrite composed dominantly of turbidite material. Structural and textural characteristics suggest that the distally‐extending debrite was deposited by a forerunner debris flow formed as substrate sediments liquefied ahead of the advancing slide, whereas the deformed turbidites were accumulated at slide margins shortly before it came to a halt. Farther downstream, the debrite is a few metres thick and sits onto the undisturbed basin floor, indicating that the mass flow became emergent distally, and was sufficiently mobile (with an estimated runout in excess of a few tens of kilometres) to redistribute the material evacuated from the basal erosion (>0.5 km3). The mass‐transport deposit terminates upward into a graded marlstone conglomerate deposited by a late‐stage multiphase flow. This study provides a rare insight into facies variation in a frontally‐emergent mass‐transport deposit, showing how basal interaction with poorly consolidated substrates can result in erosional confinement and significant transformation of the parental flow.

Нерудні мінерали прожилково-вкрапленої мінералізації у відкладах Кросненської зони Українських Карпат (район нового Бескидського залізничного тунелю)

The construction of the new Beskydy railway tunnel, which revealed the indigenous deposits of the Krosno formation of the Ukrainian Carpathians, provided an opportunity to obtain new results from the study of another prospective oil and gas-bearing areas of the flysch formation within the Krosno structural-facies unit. There is a second tunnel in length in Ukraine and passes under the Verkhovyna watershed ridge of the Ukrainian Carpathians. Its section is represented by the rocks of the Krosno formation, namely the stratification of sandstones, argillites and silt stones. There are two fracture zones have been indentified here, in which developed veinle-impregnated mineralization of ore (sulfides) and non-ore (calcite and quartz type of the “Marmarosh diamonds”) minerals. Calcite and quartz formas monominerals veins and in paragenesis calcite-quartz type of the “Marmarosh diamonds” ‒ sulfides. Quartz occurs in the form of splice sand well-faceted crystals, transparent, in yellow, brown, black, green colour. Calcite of a milky, translucent colour in the form of veins, veinlets, impregnation, powder on quartz, druz. Contains significant impurities of magnesium, manganese and iron which means the presence of a rhodochrosite-magnesite-siderite component. Hydrocarbon fluids are present in all calcite and quartz crystals of the “Marmarosh diamonds” type. By chemical composition, it is methane with impurities of higher hydrocarbons (up to hexane). Sometimes the manifestations of the outflow of a black substance with the smell of oil from the cracks in the veinlet-impregnated mineralization are recorded studied rock complexes of the flysch formation of the Krosno zone of the Ukrainian Carpathians (construction area of the new branch of the Beskydy railway tunnel), which acquires an important genetic significance.

A slow and complex landslide under static and seismic action

A dam construction project for the heightening of a dam is threatened by a large landslide located in the Spanish Southern Pyrenees. A marked syncline structure, folding a flysch formation overlying a marl substratum, is the main geological feature of the unstable valley slope. The landslide is characterized by two main and superimposed slip surfaces which follow the bedding layers. The motion observed is very slow and its acceleration can be related to rainfall regimes and changes in loading condition due to excavations.The paper provides an insight into the complexities associated with stability and kinematic evolution of a landslide under static and dynamic actions. Simplified procedures are proposed to predict the complex landslide behaviour. A comprehensive hydrogeological model is developed to relate rainfall history and water pressure which incorporates available data about the anisotropic permeability of flysch and the natural drainage obtained from pumping test and insitu measurements. The stability analysis of the slope focuses on the discussion of safety factors and their practical implication not only in terms of static limit equilibrium, but also on creeping velocity. The latter was evaluated under the simplified assumption of planar landslide and invoking the strain-rate effects on residual strength. A derived analytical expression provides a relationship between Safety Factor and creeping velocity. Under such hypothesis, the increment of the strength required to avoid predicting unrealistic landslide acceleration should be imposed for a lower range of shear velocity than the usual value imposed in the laboratory shearing tests.The seismic response of the landslide is also analysed. A Newmark's approach and a stress-strain finite element (FE) analysis are compared, with special emphasis on the effect of the superimposed slip surfaces on the dynamic landslide response. To do that, the Newmark's analysis has been extended to two overlaying sliding surfaces with independent strength properties and pore water pressure. It is concluded that assuming a single deeper sliding surfaces may underestimate the co-seismic permanent displacement. The comparison of two-block Newmark's analysis with two-dimensional elastoplastic FE approach highlights the effect of geometry in the landslide motion.

New data on the age of the Emine Formation (East Balkan, Bulgaria)

The study represents new data about chronostratigraphic range of the Emine flysch Formation in its type section at the Black Sea coast near the “Irakli” touristic resort. In the highest stratigraphic levels, calcareous nannofossils of NP 10 Rhomboaster bramlettei zone have been documented, supporting early Eocene age for the formation. These new finds will shed light on the timing of cessation of the turbiditic sedimentation in East Balkan, as well as on the specific expression of the global event – the Paleocene–Eocene Thermal Maximum (PETM) in deep-water environment.

Thermal Maturity of the Grajcarek Unit (Pieniny Klippen Belt): Insights for the Burial History of a Major Tectonic Boundary of the Western Carpathians

The Grajcarek Unit of the Pieniny Klippen Belt (PKB), at the boundary between the Central (Inner) and Outer Carpathians, resulted from the convergence of the ALCAPA (the Alps–Carpathians–Pannonia) block and European plate. The strongly deformed slices of the Grajcarek Unit consist of Jurassic–Cretaceous sedimentary rocks associated with Late Cretaceous–Middle Palaeocene synorogenic wild-flysch, and sedimentary breccias with olistoliths. Maximum burial temperatures and burial depths were estimated based on vitrinite reflectance data. The vitrinite reflectance values were wide scattered through the Grajcarek sedimentary succession, especially in the flysch formations. This is attributed mainly to the depositional effects that affected the vitrinite evolution. The determined maximum burial temperatures were interpreted due to the regional compression controlled by tectonic burial coeval with thrusting and strike-slip faulting. The regional vitrinite reflectance variations might estimate cumulative displacement around the NNW–SSE and oriented the strike-slip Dunajec fault, which is a continuation of the deep fracture Kraków–Myszków fault zone.

The Geo-Hydro-Mechanical Properties of a Turbiditic Formation as Internal Factors of Slope Failure Processes

Similar to many inner areas of Southern Europe, the Daunia Apennines are affected by widespread landsliding, often consisting of slow, deep-seated movements. Recurrent acceleration of these landslides causes damage to buildings and infrastructures, severely biasing the socio-economic development of the region. Most landslides in the area of study occur within clayey units of turbiditic flysch formations, often severely disturbed by tectonic thrust and previous landsliding. The Faeto Flysch (FAE) is one of the most widespread turbiditic formations in the Daunia Apennines and is representative of the tectonised geological formations involved in slope failure. This work, by examining the landslide processes occurring at four pilot sites, aims at connecting the observed mechanisms to the geo-hydro-mechanical setup of FAE in the slopes. It is found that the soil portion of FAE consists of highly plastic clays, resulting in low intrinsic shear strength, and hence controls the initiation and progression of failure in the slopes, as such representing an internal predisposing factor to landsliding. In addition, the presence of fractured rock strata confers a high permeability at the slope scale, with respect to that of the soil matrix. This results in severe piezometric levels in the slope, which represent another internal predisposing factor to failure, and in the ability to induce significant seasonal pore water pressure oscillations down to great depths, connected to rainfall infiltration, thus triggering the recurrent acceleration of the landslides.

Assessment of the Ecological Risks of Landslide Damages in the Carpathian Region

The dynamism of the landslides within the Carpathian region of Ukraine is because of the difficult engineering and geological conditions. High landslide den sity and significant population density contribute to the fact that environmental parameters worsen and require rational management. Permanent natural factors like clay flysch formation, fault tectonics, high seismic activity, and dense network of rivers mostly facilitate the active development of landslides in the Carpathian region. However, it is triggered by extreme long-term precipitation. The numerical parameters of population density, the landslide damage coefficient, and the predictive range of landslide intensification were selected to assess the ecological risk of damages in the area. The landslide dam age coefficient characterizes the tendency of the area to landslide development, considering all the factors contributing to the landslides. Risk, as a multifunctional calculated complex, includes the calculation of damage, according to which we can assess the possibility of risk for the human being while assuming the equal distribution of the population within the study area. The integral components of the risk are calculated based on the data gathered to assess the growth of risks in the future, considering the area distribution and predictive time series of the landslide intensification. This analysis has identified engineering and geological areas having the greatest risk to human life.

Conceptual model of fracture‐limited sea cliff erosion: Erosion of the seaward tilted flyschs of Socoa, Basque Country, France

AbstractSea cliff morphology and erosion rates are modulated by several factors, including rock control that reflects both lithology and rock structure. Erosion is anticipated to preferentially exploit ‘fractures’, broadly meant as any discontinuity in an otherwise continuous medium, where the rock mass is weakest. Unpicking the direct control of such fractures on the spatial and temporal pattern of erosion remains, however, challenging. To analyse how such fractures control erosion, we monitored the evolution of a 400 m‐long stretch of highly structured sedimentary cliffs in Socoa, Basque Country, France. The rock is known as the Socoa flysch formation. This formation combines decimetre‐thick turbidites composed of repeat triplets of medium to strong calcareous sandstone, laminated siltstones and argillaceous marls. The sequence plunges at 45° into the sea with a shore‐parallel strike. The cliffs are cross‐cut by two normal and reverse fault families, with 10–100 m alongshore spacing, with primary and secondary strata‐bound fractures perpendicular to the bedding, which combined delimit the cliff rock mass into discrete blocks that are exploited by the erosion process. Erosion, and sometimes plucking, of such beds and blocks on the cliff face was monitored using ground‐based structure‐from‐motion (SfM) photogrammetry, over the course of 5.7 years between 2011 and 2017. To compare with longer time change, cliff‐top retreat rate was assessed using SfM‐orthorectified archive aerial photographs spanning 1954–2008. We show that the 13,250 m2 cliff face released 4500 blocks exceeding 1.45 × 10−3 m3, removing a total volume of 170 m3. This equates to an average cliff erosion rate of 3.4 mm/year, which is slightly slower than the 54‐year‐long local cliff‐top retreat (10.8 ± 1.8 mm/year). The vertical distribution of erosion reflects the height of sea water inundation, where the maximum erosion intensity occurs ca. 2 m above high spring‐tide water level. Alongshore, the distribution of rockfall scars is concentrated along bed edges bounding cross‐cutting faults; the extent of block detachment is controlled by secondary tectonic joints, which may extend through several beds locally sharing similar mechanical strength; and rockfall depth is always a multiple of bed thickness. Over the longer term, we explain block detachment and resultant cliff collapse as a cycle. Erosion nucleates on readily exploitable fractures but elsewhere, the sea only meets defect‐free medium‐strong to strong rock slabs offering few morphological features for exploitation. Structurally delimited blocks are quarried, and with sufficient time, carve semi‐elliptic scars reaching progressively deeper strata to be eroded. Lateral propagation of erosion is directed along mechanical weaknesses in the bedding, and large episodic collapses affect the overhanging slabs via sliding on the weak marl beds. Collapse geometry is confined to one or several triplets of turbidite beds, but never reaches deeper into the cliff than the eroded depth at the foot. We contend that this fracture‐limited model of sea‐cliff erosion, inferred from the Socoa site dynamics and its peculiar sets of fractures, applies more broadly to other fractured cliff contexts, albeit with site‐specific geometries. The initiation of erosion, the propagation of incremental block release and the ultimate full failure of the cliff, have each been shown to be fundamentally directly controlled by structure, which remains a vital control in understanding how cliffed coasts have changed in the past and will change in the future.