Limestone Sequence Research Articles

Newly-formed illite preserves fluid sources during folding of shale and limestone rocks; an example from the Mexican Fold-Thrust Belt

We combine structural, fluid-inclusion microthermometry, illite-crystallinity, X-Ray Diffraction (XRD) and O and H stable isotope analyses of authigenic illite to determine the source of local fluids interacting with rock during folding in anchizonal shales of the Mexican Fold-Thrust Belt (MFTB). A well-exposed train of mesoscopic, asymmetrical folds in a sequence of Cretaceous limestones interbedded with shale was targeted for this study. We test the hypothesis that syn-folding vein minerals and clay minerals were formed from the same fluids by comparing the δ2H composition of inclusion fluids in calcite and quartz from veins, and from illite concentrates from sheared shale layers, and the sources of that fluid.Five clay size-fractions (<0.05, 0.05–0.2, 0.2–1, 1–2, and <2 μm) were separated from eight shale samples. In the 40 clay grain-size fractions analyzed, illite, calcite, kaolinite, smectite, chlorite and minor quartz were identified by XRD analysis. Most samples show different proportions of various clay minerals, except for the finer fractions in two of the samples (BL3 and BL4) were illite is the only clay phase present. The discriminating potential of δ18O values of clay is generally masked by the abundance of calcite in all samples. In contrast, samples containing chlorite and smectite show very low values in δ2H (−75.9 to −53.9‰), while samples containing illite and kaolinite or pure illite show relatively high δ2H values (−33.1 to −50.1‰). The latter fall within the δ2H range (−39 to −49‰) determined in fluid inclusions of syntectonic veins, indicating isotopic equilibrium between water, veins fillings and illitic clay during deformation, according to fractionation factors at these temperatures (220–250 °C). The δ2H values and fluid inclusion salinities in the sampled rocks indicate that water active during folding was partly marine and partly meteoric and that the amount of such pore-water represented a small fraction of the deformed rock volume. Our study demonstrates that the H-isotopic composition of anchizonal illitic clays, commonly present in exhumed fold-thrust belts, can be used to determine the source(s) of fluids that were active during deformation.

Importance of ground properties in the relationship of ground vibration–structural hazard and land application

Abstract Vibration parameters like frequency, acceleration and particle velocity play an active role in the relationship of ground vibration–structural hazard. These parameters change depending on blasting energy and the properties of rock environment. Therefore, in the first step, rock mass properties and possible directional variation were investigated by using different geophysical methods (electrical resistivity, seismic refraction and multi-channel analysis of surface wave) and current sounding information. Each method offers different sensitivities and resolutions depending on the physical characteristics of different materials. Evaluating these as a whole increased the solubility of the research. According to seismic S- and P-wave velocities, electrical resistivity and sounding information, the study area consists of consecutive sequences of alluvium, clay limestone, tuff and limestone units. And these units show variations from place to place in the study area. In the second stage, evaluations were made according to the structural hazard standards used widely in the literature and components of velocity, acceleration and frequency obtained from blasting vibration seismograph and accelerometers. As a result, it is seen that ground vibrations show different spreading properties in different directions and different hazard risks depending on the geological structure of the region.

Biometric Analysis of Lepidocyclina (Nephrolepidina) from Baba Formation (Late Oligocene) in Bai-Hassan Well-25, Kirkuk Area, Northeast Iraq

The investigation is based on a subsurface sequences of bioclastic limestone in Kirkuk area, Northeastern Iraq. These sequences were deposited in a shallow open marine environment during Late Oligocene. Tests of larger foraminifera Lepidocyclina(Nephrolepidina) are abundant in the lower and upper parts of the Baba Formation. Biometric investigations of the Megalospheric forms of Nephrolepidina assemblages from, Bai-Hassan well- 25 section led to recognize three morphometrically defined species by the combination of three parameters, 1- the degree of embracement of the protoconch by the deuteroconch (Factor A), 2- the numbers of accessory auxiliary chambers on the deuteroconch (Factor B), and 3- (Factor C ) which expressed by the protoconch angle( ) formed by the two hypothetical line from the center of protoconch throught the outer attachment points of the deuteroconch walls with protoconch.

Measurement of rates of surface lowering of limestone in the southern Pennines: an example in the Wye catchment, Derbyshire, UK

Measurement of the differential denudation between chert and limestone in a Carboniferous limestone sequence that was cut during excavations for a railway tunnel (in 1865) has provided a means of assessing limestone dissolution rates in a valley side setting in the southern Pennines over the last ∼150 years. Dissolution rates of 0.031 mm/annum have been determined. This rate falls within the range of previously determined limestone dissolution rates in the UK, which have been described in the context of different methods of measurement and the difference in denudations rates over time.

New and revised inaniguttid Radiolaria and associated trilobites from the upper Darriwilian (Ordovician) Shundy Formation of Kazakhstan

Representatives of the family Inaniguttidae dominate a diverse and well-preserved radiolarian assemblage from Kazakhstan. The fauna was extracted from a limestone sample of the Shundy Formation, a limestone sequence that accumulated on the slope of a carbonate platform and which crops out in the Aksuran Mountains (North Balkhash Region). The family Inaniguttidae is represented in the studied assemblage by five genera and 14 species (including two new species, Triplococcus aksuranensis and Inanibigutta maletzi). The genus Triplococcus is particularly abundant, representing half of the Inaniguttids in the studied assemblage. Based on the presence of species Haplentactinia juncta, the fauna can be correlated with the upper Darriwilian Haplentactinia juncta–Inanigutta unica assemblage. Identified trilobites (Endymionia semielliptica and Porterfieldia sp. aff. P. delicata) found in the same sample also suggest a late Darriwilian age, which agrees with the age suggested previously by graptolites.

Organic maturation of the Algarve Basin (southern Portugal) and its bearing on thermal history and hydrocarbon exploration

The Algarve Basin, southern Portugal is under-explored in terms of petroleum geology. Organic maturation levels and the thermal history of this basin have been ascertained, together with their implications for future exploration. Maturity was determined using vitrinite reflectance and spore fluorescence/colour. The succession was extensively sampled (158 onshore samples and 20 samples from two offshore wells). Thermal history was modelled using 1D PetroMod®.A palynostratigraphic study of two offshore wells was also completed, showing thick marl–limestone sequences of Middle and Upper Jurassic age. Hiatuses were identified in the offshore wells within the Jurassic section and between the Jurassic and the Miocene sections.The Mesozoic rocks of the Algarve Basin lie within the oil window. Vitrinite reflectance ranges from 0.52–0.7%Rr in the Lower Cretaceous to 1–1.1%Rr in the Upper Triassic–Hettangian. Miocene rocks that unconformably overlie the Mesozoic strata are immature (0.42–0.47%Rr). Maturation levels increase with increasing age of the strata, indicating that burial was the main process controlling maturation. Thick marl–limestone sequences of the Middle Jurassic (Callovian) and the Upper Jurassic (Oxfordian) are the main source rock intervals. The Miocene successions of both offshore wells contains reworked vitrinite and palynomorphs with maturation levels similar to the Mesozoic rocks of the basin, suggesting exposure and erosion of these rocks during Miocene times.Palaeogeothermal gradients in the onshore Algarve Basin range from 52 °C/km to 24.7 °C/km with pre-Miocene exhumation estimated at 2000–2500 m. The probable age for the removed cover is Upper Cretaceous to Lower Palaeocene(?) and peak temperatures in the Mesozoic rocks were attained during latest Cretaceous–Early Palaeocene times. Modelled palaeotemperatures suggest that the Jurassic entered the oil-window at the beginning of the Cretaceous with peak oil generation in late Cretaceous time.

A new section through the Corallian Group (Oxfordian, Upper Jurassic) rocks of Calne, Wiltshire, southern England

Abstract Information obtained during the digging of a pipeline trench southwest of Calne, Wiltshire, has enabled the compilation of the first complete sequence through the strata of the Corallian Group (Oxfordian) in this area of west Wiltshire. A detailed description of this sequence of sands, clays, ooidal and coralliferous limestones which represent the Kingston and Stanford Formations is provided, and the information obtained used to provide a new synthesis of the stratigraphy of the Corallian Group in west Wiltshire. The existence of a previously unrecognized sedimentary sub-basin of the Wessex Basin is demonstrated. Previously unrecorded stratal sequences and faults have been discovered, and the existence established within the Wiltshire Corallian of a new coralliferous horizon, the Westbrook Coral Bed. The setting of this sedimentary sequence within the tectonic regime which existed in this part of the Wessex Basin in the Late Jurassic is reviewed.

Eulogy for Dieter Meischner (21 November 1934 – 24 May 2012)

Dieter Meischner was born in Braunschweig and studied geology and palaeontology at Gottingen, where he passed his examinations in record time. From 1970 to his retirement in 2000, he served as a professor and head of the sedimentary geology group in Gottingen. He was awarded the Hermann Credner Prize of the Deutsche geologische Gesellschaft (1969) and the Gustav Steinmann Medal of the Geologische Vereinigung (1996). Dieter Meischner’s scientific oeuvre covers an enormous range of topics: Palaeozoic biostratigraphy, facies and tectonics, circulation and ecology of marginal seas, palaeoecology, cold-water carbonates, modern carbonate systems as sea-level gauges, carbonate diagenesis, speleology, construction of marine sampling tools and even the detailed planning of a research submarine. For his doctoral thesis, Dieter Meischner studied the sedimentology and palaeogeography of an early Carboniferous sequence of basinal bioclastic limestones, from which he deduced the basics of limestone turbidites. In the same profiles, he established the biostratigraphy of Visean conodonts. Together with fourteen students, Dieter Meischner produced a new, precise geological map of the Kellerwald area in the eastern Rhenish Massif, based on numerous conodont datings of rocks devoid of macrofossils. These findings not only led to a revolution in large-scale palaeogeography, but also yielded complex tectonic profiles with duplex structures and evidence of long-distance tectonic transport—results which later proved indispensable in plate tectonic interpretations of the German Variscides. Dieter Meischner greatly respected the work of his own supervisor, Hermann Schmidt, who had aroused his interest both in Palaeozoic facies and in marine geology. Following H. Schmidt’s example, Meischner set-up a low-budget but highly efficient research programme, delving into the marine geology of the Adriatic, beginning at Rovinj. A small but devoted working group mapped the sea floor and studied benthic communities, hydrographic profiles and sediment cores, which documented—among other features—profiles of pH and Eh as the driving forces of early diagenesis. Did you know that there are Slovenian rose wines with a pH of 2.5? Forty years ago, research in the Yugoslavian Adriatic was an adventure that called for a lot of improvisation, in spite of Dieter Meischner’s meticulous planning. Most instruments and spare parts had to be imported and crammed into minibusses, in which driver and co-pilot occupied the front seats while two others were trying to sleep on top of the load behind—a difficult task, especially with Dieter at the wheel. After passing two frontiers with lots of bureaucracy, the cargo would be set-up in some makeshift laboratory and sampling tools launched from a hired fish trawler. The remains of the night’s catch in the stern net W. Franke (&) G. Wefer Frankfurt, Germany e-mail: w.franke@em.uni-frankfurt.de

Time constraints on the evolution of southern Palawan Island, Philippines from onshore and offshore correlation of Miocene limestones

The link between the deformation of southern and central Palawan Island, Philippines and the deformation of the adjacent offshore wedge is investigated. The wedge is a continuation of the Palawan fold and thrust belt and bounds the Borneo–Palawan Trough to the Dangerous Grounds and to Palawan Island. Key parameters for the understanding of the formation and development of this wedge are two limestone formations: The Oligocene to Middle Miocene Nido limestone and the Upper Miocene to Lower Pliocene Tabon limestone.The initiation of the thrust belt formation is constrained by the Nido limestone, which was deposited from shortly before the breakup of the eastern South China Sea (∼35Ma) until the Early Miocene. Age data available from wells offshore central Palawan gives an age of Early Miocene close to the base of the Nido limestone. While cropping out onshore north Palawan, these limestones were overthrusted by the wedge in southern and central Palawan. Seismic images show gently east dipping carbonates below the wedge. The seismic data show that these limestones are only mildly affected by the wedge formation.The end of the wedge development can be constrained by the Tabon limestone. With an age of ∼9 to ∼4Ma, this limestone sequence overlies unconformably the offshore wedge. A detailed biostratigraphic correlation of the Tabon limestone along the southwest Palawan shelf, using well data, combined with multichannel seismic data and investigations onshore southern and central Palawan, shows a time- and space-transgressive development of these limestones. They are progressively younging towards the west. We propose that the formation of the Tabon limestone is directly linked with the development of the wedge that tectonically controls the formation of this carbonate platform. This constrains the time for the final phase of the formation of the Palawan thrust belt. After the final compressional phase and wedge formation in the lower Early Pliocene the wedge underwent a phase of subsidence.Based upon the detailed correlation of these limestones we propose that the wedge did not form in the southern Palawan area prior to ∼18Ma. Using the sealing Tabon limestone as time constraint we suggest that the development of the wedge in the south Palawan area started in the lower Middle Miocene (∼15Ma) and continued developing towards the west until the upper Late Miocene to Early Pliocene (∼5Ma).After the wedge propagation stopped, the wedge front collapsed in several places due to gravitational sliding.Southern and central Palawan were uplifted above sea level during a second phase of compression in the Late Pliocene. Onshore outcrops give indications to a working spleothem since ∼1.2Ma.

Peat Water Treatment Using Combination of Cationic Surfactant Modified Zeolite, Granular Activated Carbon, and Limestone

This research was conducted essentially to treat fresh peat water using a series of adsorbents. Initially, the characterization of peat water was determined and five parameters, including pH, colour, COD, turbidity, and iron ion exhibited values that exceeded the water standard limit. There were two factors influencing the adsorption capacity such as pH, and adsorbent dosages that were observed in the batch study. The results obtained indicated that the majority of the adsorbents were very efficient in removing colour, COD, turbidity at pH range 2-4 and Fe at pH range 6-8. The optimum dosage of cationic surfactant modified zeolite (CSMZ) was found around 2 g while granular activated carbon (GAC) was exhibited at 2.5 g. In column study, serial sequence of CSMZ, GAC, and limestone showed that the optimal reduction on the 48 hours treatment were found pH = 7.78, colour = 12 TCU, turbidity = 0.23 NTU, COD = 0 mg/L, and Fe= 0.11 mg/L. Freundlich isotherm model was obtained for the best description on the adsorption mechanisms of all adsorbents.

Investigation of the weathering layer using seismic refraction and highresolution seismic reflection methods, NE of Riyadh city

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16 m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18 m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.

PRELIMINARY REPORT ON THE NEW FINDINGS OF MISSISSIPPIAN TRILOBITES IN THE BŘEZINA FORMATION (MORAVIAN KARST, CZECH REPUBLIC)

New fossiliferous layers of the Březina Formation were discovered in the valley of the Říčka Brook SSW of the village of Ochoz u Brna in 2011. Four taxa of trilobites were preliminary determined: Archegonus (Phillibole) cf. polleni (Woodward, 1894), Archegonus (Phillibole) cf. cauliquercus Brauckmann 1981, Liobole (Liobole) glabra proxima Chlupáč, 1966 and ?Spinibole sp. The newly discovered fossiliferous beds belong to the Viséan (Lower to ?Upper Viséan, approximately cu IIγ–cu IIδ). The occurrence of limestone pebbles and limestone cobbles in aleuropelitic shales was also recorded. The Late Tournaisian foraminifer Darjella monilis Malakhova, 1964 was discovered in a dark grey limestone cobble derived from the Hády-Říčka limestone sequence of the Líšeň Formation.

The latest Carboniferous–Early Permian Dorud Group of the eastern Alborz (Iran): biostratigraphy and taxonomy of smaller foraminifers

The latest Carboniferous–Early Permian Dorud Group in the Chaman‐Saver area of eastern Alborz, Iran is more than 222 m thick and includes thick sequences of oncolitic limestone, sandy limestone, sandstones and shales. The Emarat and Ghosnavi formations of this Group are dated here as latest Gzhelian to early Sakmarian Stages. During the Asselian Stage, the sea level fell abruptly and epeirogenic episodes occurred. These events generated a broad, shallow carbonate platform suitable for the growth and diversity of smaller foraminifers in the Chaman‐Saver area which, consequently, displays faunal differences with the rest of the Alborz Mountains. Three foraminiferal biozones are proposed: Nodosinelloides potievskayae–Vervilleina bradyi Zone (latest Gzhelian), Calcitornella heathi–Nodosinelloides sp. Zone (latest Gzhelian–Asselian), and Rectogordius iranicus n. gen. n. sp.–Hemigordius schlumbergeri Zone (early Sakmarian). The new taxa described herein include: Pseudovidalina iranica n. sp., P. damghanica n. sp., Rectogordius iranicus n. gen. n. sp. and Tezaquina sp. 1. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A new interpretation of the sedimentary cover in the western Siljan Ring area, central Sweden, based on seismic data

Two new reflection seismic profiles over the Paleozoic successions of the western part of the Siljan Ring impact structure show a contrasting seismic signature. The more southerly c. 10km long Mora profile reveals a highly disturbed structure, with only a few kilometers of relatively horizontally layered structures observed. However, interpretations of refracted arrivals in the data, that can be correlated to reflections, indicate the Silurian clastic rocks to be about 200m thick in the central part of the profile. Weak reflections from about 600m depth suggest a 400m thick Ordovician limestone sequence to be present. Cores from the area show a mainly shale lithology for the Silurian and only a thin sequence of Ordovician strata, suggesting a rapid thickening of the Ordovician towards the north. On the more northern c. 12km Orsa profile clear reflections from the Paleozoic successions are seen along the entire profile, except on the southernmost few kilometers. Based on interpretations of refracted arrivals, the Silurian succession appears to be considerably thinner here, and possibly absent at some locations. The Ordovician is also interpreted to be thinner in this area, with a maximum thickness of about 200–300m along most of the profile. A deeper reflection from about 2km within the crystalline basement may represent a dolerite sill. The lack of clear basement reflections on the Mora profile can be attributed to near-surface conditions and the acquisition geometry. The seismic data and recent coring in the area suggest the presence of a deeper paleo-basin towards the southwest with significantly more shales being deposited and the Paleozoic successions being severely disturbed. The shallow coring and seismic data will help form the basis for locating future boreholes for deeper drilling to study impact processes and the Paleozoic evolution of central Sweden.

LateDevonian marine anoxia challenged by benthic cyanobacterial mats

Mass occurrence of benthic cyanobacterial mats in a sequence of Late Devonian black shales and bituminous limestones of the Holy Cross Mts. (central Poland), enclosing the famous Kellwasser and Hangenberg extinction horizons, is reported. The microbiota forming the mats is compared with some modern benthic chroococcalean cyanobacteria. Similarly to their extant counterparts, the Devonian cyanobacteria must had been phototrophic and oxygenic aerobes which could, however, tolerate slightly sulfidic conditions characterizing the near-bottom waters of the Late Devonian epicontinental sea. The cyanobacterial mats successfully colonized the oxygen-deficient and H(2)S-enriched seabed otherwise unfavorable for most other benthic biota. The redox state of this sluggish Late Devonian sea, ascribed previously mostly to anoxic or euxinic conditions, is reassessed as probably pulsating between anoxic, dysoxic, and weakly oxic conditions. The redox state was dependent on the rate of oxygen production by the cyanobacterial mats, the intensity of H(2)S emissions from the decaying mat biomass, and the rate of planktonic production.

Constraints on the frontal crustal structure of a continental collision from an integrated geophysical research: The central‐western Betic Cordillera (SW Spain)

Mélange rocks outcrop widely in the central and western frontal sectors of the Betic Cordillera, as in many other collisional orogens where they form part of the accretionary wedges. Extrusion of Triassic plastic clays and evaporites was favored by the progressive accretion of the Betic External Zones, mixing rocks of different provenance and forming a synorogenic frontal mélange unit. MT data coupled with gravity data are a valid combined methodology to characterize the geometry of these mélange units, since the characterization of plastic rocks geometry is usually uncertain using seismic techniques. The results correlate well with known geological features (sedimentary basins, calcareous ranges, evaporitic rocks) and reveal the deep geometry. A resistive body, slightly dipping toward the SE, points to the continuity of the Iberian Massif below the Guadalquivir basin (2°) and the External Zones (6–8°). To the south, gravity models show the Iberian continental crust subducting below the Internal Zones with a roughly 20–35° slope. The main conductive bodies are related to the location of evaporitic rocks involved in the frontal mélange. They overlie the Iberian Massif and, southwards, the frontal Jurassic and Cretaceous limestone sequences of the External and Median Subbetics. In this setting, thick plastic rock units placed above the foreland could act as a lubricant facilitating continental subduction, and being progressively accreted toward a frontal mélange.

Late Eocene evolution of the Çiçekdağı Basin (central Turkey): Syn-sedimentary compression during microcontinent–continent collision in central Anatolia

Abstract The Central Anatolian Crystalline Complex (CACC) exposes metasediment rocks overlain by Cretaceous ophiolites and intruded by granitoids. Following late Cretaceous exhumation of its high-grade metamorphic rocks, the CACC started to collide with the Central Pontides of southern Eurasia in the latest Cretaceous to Paleocene. Here, we present the sedimentary, stratigraphic and tectonic evolution of the Cicekdagi Basin, located in the northwest of the CACC. Magnetostratigraphic dating, supported by 40 Ar/ 39 Ar geochronology, shows a late Eocene basin age. The basin fill unconformably overlies metamorphic basement in the south and ophiolites of the CACC in the north. It consists of red conglomerates, sandstones and siltstones, which overlie a sequence of nummulitic limestones. In the south, these limestones are ~ 10 m thick, are underlain by a few meters of conglomerate unit unconformably covering the CACC metamorphics. In the north, the limestones are underlain by a ~ 200 m thick sequence of volcanics and fine-grained clastics intercalating with shallow marine black shales. The upper Eocene sediments of the Cicekdagi Basin were deformed into a syn-anticline pair. Progressive unconformities in the northern flank and a rapid and persistent ~ 180° switch in paleocurrent directions from southward to northward in the southern flank of the anticline demonstrate syn-sedimentary folding. We interpret the folding to result from a southward progression of the Cankiri foreland basin as a result of ongoing collision between the CACC and the Pontides.

Investigating gravitational grabens related to lateral spreading and evaporite dissolution subsidence by means of detailed mapping, trenching, and electrical resistivity tomography (Spanish Pyrenees)

The active lateral spread of the Peracalc Range (Spanish Pyrenees) has developed on a Cretaceous limestone sequence around 250 m thick, underlain by tectonically thickened (∼2.5 km) Triassic halite-bearing evaporites and clays. Outward expansion of the Triassic sequence by ductile deformation and probably halokinesis toward the debuttressed and unloaded front of the range has been accommodated in the overlying cap rock through the development of a striking horst and graben morphostructure. Fault scarps show anomalously high height to length ratios (aspect ratio; H max / L ) compared to the values reported for tectonic faults. This retrogressive gravitational deformation has aborted a paleodrainage, expressed as wind gaps, hanging valleys, and defeated streams. The significant vertical displacement component in this rock spread is attributed to subsidence caused by interstratal evaporite dissolution, as supported by the dissolution-induced collapse and graben structures mapped at the foot of the range. To our knowledge, the rock spread of Peracalc, covering around 4.5 km 2 and with a minimum volume of 0.9 km 3 , is the largest documented landslide of the Pyrenees. The excavation of trenches and the acquisition of electrical resistivity tomography profiles provided information on the thickness and subsurface structure of the graben fills, the age of the lateral spread (older than 45 ka), an unexpected episodic kinematic behavior of the gravitational faults, and the timing of deformation events, including slumping of lake deposits.

The prehistoric Vajont rockslide: An updated geological model

This study presents the detailed reconstruction of the entire structure of the prehistoric Vajont rockslide (about 270–300millionm3 of rocks and debris) for the first time, describing the complex geometry and the characteristic superimposition of distinct rigid blocks on a very thick shear zone. The prehistoric Vajont rockslide was characterized by an enormous ‘en masse’ motion of a rigid overlying rock mass (100–130m thick) that moved downslope, sliding onto a very thick shear zone (40–50m thick, on average) made up of a chaotic assemblage of blocks, limestone angular gravel, and high plasticity clays (montmorillonitic clays).Coarse loose sediments, still exposed on the 9 October 1963 detachment surface, are always associated with large blocks made of strongly fractured rock masses (Fonzaso Formation: middle‐upper Jurassic) preserving the stratification. The blocks of stratified and folded limestone sequences appear to be ‘sheared off’ from the underlying bedrock and can be considered as displaced rock masses planed off by the motion of the overlying rigid rock mass (‘rock mass shavings’). The prehistoric Vajont rockslide was characterized by a multistage failure with a marked retrogressive evolution. The first rupture (Pian del Toc block) rapidly destabilized the upper slope, mobilizing a second rock mass block (Pian della Pozza block) that, in turn, determined the multiple rupture of the revealed shear zone material (Massalezza lobe). Even if the exact timing of the different phases is not known, the entire multistaged failure process was very rapid. At the end of the multistage retrogressive failure, the slope morphology of the northern toe of Mt. Toc was drastically changed and the large failed rock mass settled into the preexisting Vajont Valley assuming the unusual chair-like geometry.The Vajont rockslide represents a very significant example on how a complex geological situation, if not adequately analyzed and reconstructed, can lead to dangerous misinterpretations or even to erroneous engineering–geological and geotechnical models. Accurate fieldwork and modern technologies can be fundamental in solving such a very intriguing ‘geological puzzle.’

New data on the structure of the sedimentary section on the Ita Mai Tai Guyot (Magellan Seamounts, Pacific Ocean)

The geological-prospecting investigations carried out by scientists from the Yuzhmorgeologiya State Scientific Center in the eastern segment of the Magellan Seamounts of the Pacific during the cruise of the R/V Gelendzhik in 2003–2010 brought new data on the topography of the Ita Mai Tai Guyot and its Lower Cretaceous-Pleistocene sedimentary section. The guyot is characterized by its complex irregular morphology with angular outlines being complicated by numerous relief mesoforms: spurs, scarps, and volcanic cones and domes. It is shown that the sedimentary cover was formed during a long period. The analysis of the organic macro- and microfaunal remains from the sedimentary rocks made it possible to define transgressive phases in the guyot’s development: the Albian-Cenomanian, the Late Campanian-Maastrichtian, and the late Paleocene-early Eocene. The oolitic limestone sequence recovered by DSDP Hole 202 is determined to be the Albian in age. Based on the original and published data, the geological development of the guyot was analyzed in comparison with other similar structures of the Magellan Seamounts.