Degree Of Lithification Research Articles

Morphometry of the Mt Manengouba volcano: Insights into tephrostratigraphy of Eboga maars (South West Cameroon)

The GIS-based geomorphological and morphometric approaches were combined with field- and tephrostratigraphic analyses to reconstruct the history of the Mt Manengouba volcano including the Eboga maars in the southwestern part of the Cameroon Volcanic Line (CVL). The elevation, slope, relative relief, topographic position and terrain ruggedness indexes from the Digital Elevation Model (DEM, 12.5 m) were determined to constrain two main geomorphic units corresponding to the Elengoum and Eboga nested stratovolcanoes which were affected by differential erosional processes. The studied grain size, shape, vesicularity, structure, degree of lithification, sorting, thickness, grading patterns, sedimentary features, spatial distribution revealed three tephrostratigraphic units: U1 (U1-1, lithic and juvenile; U1-2 dominantly juvenile), U2 (U2-1 ash- and juvenile rich-deposits; U2-2, juvenile-scoria with few lithic) and U3 (scoria cone deposits). The total volume of ∼0.199 km3 of tephra deposits ranges the Eboga maars volcanoes within the small-volume monogenetic types. These results revealed dry/wet phreatomagmatism and strombolian activity as a contribution to the seven phases-eruptive history of the Mt Manengouba volcano: the pre-Manengouba; emplacement of Elengoum stratovolcano; collapse of Elengoum summit and formation of Elengoum caldera; emplacement of Eboga stratovolcano; the collapse of Eboga summit and formation of Eboga caldera; a phreatomagmatic phase and emplacement of Female and Male maars ending with an explosive stage associated with the formation of scoria and parasitic cones.

Retrieving planktonic foraminifera from lithified rocks, examples from the Eocene limestones and marls (External Dinarides, Croatia)

Paleoecologic (paleoclimatologic) and biostratigraphic studies of pelagic and deep-water deposits rely on the identification of planktonic foraminifera. Here we report and compare the results of planktonic foraminiferal assemblages from the Middle Eocene indurated limestones and marls collected in the External Dinarides extracted with acetic acid of different concentrations (50%, 60%, 70% and 80%) and different reaction (exposure) times. The deposits originated within the Dinaric foreland basin, have been assigned to the so-called Transitional beds and Flysch, and are characterized by different ratio of carbonate content and degree of lithification. The aim of this paper is to compare the efficiency of the laboratory procedures for obtaining isolated specimens and to evaluate the impact of preparation procedure on the quality of tests (complete test vs. secondary dissolution effects). For each acetic concentration we assessed:(1)the effectiveness of the treatment in terms of the time required for successful extraction of planktonic foraminifera, and(2)the degree of dissolution by analyses of dissolution proxies, including the weight percentage of sieved residues after disaggregation and preservation features of the tests. Our results indicate that accurate taxonomic analysis of carbonate rocks requires the use of 60% acetic acid for a shorter reaction time, and hydrogen peroxide methods for marls.

Facies model, cementation and sedimentary evolution of a 20th-century beach and beachrock: The Gorrondatxe wave and tidal coastal system (NE Spain)

Lithology, bedding, and sedimentary structures of the Gorrondatxe coastal and beach system show an upper shoreface around the low-water level and containing the coarsest grain sizes; a foreshore in the main intertidal zone, which is mainly composed of sands with minor gravels and a distinctive low-angle, seaward-dipping, planar-parallel lamination; and a backshore zone with a mixture of marine-derived sands (washover) and non-marine sands and muds (aeolian, subaerial ponds) above the high-tide water level. The facies model shows a wave-dominated, mesotidal, composite beach with a fining-upwards and aggradational trend. Over the 20th century, a beachrock developed in this area as a result of heavy-industry waste dumping (steel industry) off the coast, which later became attached to the coast. There are a variety of cements and degrees of lithification in the beachrock, suggesting that the beachrock-related processes, although coetaneous, were not similar in all these zones and that different cements developed at the same time. Sand cementation can be used to recognize such facies in the stratigraphic record or in well cores, when other kinds of data are not available. The upper shoreface sandstones are hard and well-cemented, with a whitish circumgranular aragonite cement and good intergranular porosity; the foreshore sands are medium-hard, with a brown, irregular, aragonite needle cement, which shows higher intergranular porosity; finally, the backshore sands are only lightly cemented, with a brown aragonite and rhombohedral calcite cement, with the highest intergranular porosity. SEM images show the presence of Coccus-type bacteria in all the different facies and zones; these are the best proof of metabolic activity, probably of ureolytic bacteria, in the origin of all these cements. Over the course of the 20th century, Gorrondatxe evolved from a formerly dissipative, ridge and runnel beach to an active, aggrading and prograding, reflective beach, and finally to an inactive beach prone to erosion. The change to a reflective profile and the cementation of the beachrock indicate that anthropic offshore inputs were rapidly recycled into the beachface. Another major change occurred when waste discharges ended, and the former sediments and beachrock started to be eroded to their current state.

Overburden deformation induced by active diapirism: Discrete element numerical simulations

AbstractActive diapirs are commonly observed in sedimentary basins worldwide. Factors controlling overburden deformation processes and patterns during the active rise of active diapirs, such as sediment cohesion and overburden thickness, are not well understood. We have utilised the discrete element numerical simulation method to simulate active diapirism in models comprised of sediments with varying cohesions and roof thicknesses. We found that sediment cohesion can significantly affect overburden deformation with low‐cohesion sediments favouring the generation of a broad, smooth forced fold and a symmetric graben, while high‐cohesion sediments favour the formation of master reverse faults and preferentially uplifted and rotated fault blocks in their hangingwalls in the central crest, with an overall asymmetric structural geometry. Moreover, overburden thickness does not significantly affect structural styles in the overburden. Our results also suggest that the higher the sediment cohesion, the higher the amplitude (the lower the wavelength) of the forced fold above the rising diapir becomes. Our models produced a wide spectrum of deformation structures that resemble those in nature and reveal a strong link between structural styles and sediment diagenesis in the context of active diapir‐piercing sediments with varying degrees of lithification.

Time-Sensitive Aspects of Mars Sample Return (MSR) Science.

Time-Sensitive Aspects of Mars Sample Return (MSR) Science.

Grain-Size Analysis of Ancient Deep-Marine Sediments Using Laser Diffraction

Grain-size analysis of siliciclastic sedimentary rocks provides critical information for interpreting flow dynamics and depositional environments in sedimentary systems and for analysing reservoir quality of sandstone. Methods such as sieving and thin-section analysis are time consuming and unsuited for large sample numbers. Laser diffraction particle analysis is quick and reliable for analysing 100s of samples, assuming successful disaggregation. Here, we evaluate this method utilizing samples from three siliciclastic formations in Northern Italy: the Miocene Castagnola and Marnoso-Arenacea Formations, and the Cretaceous to Palaeocene Gottero Formation, which vary in degree of lithification. We focus on: 1) methods of whole-rock disaggregation; 2) methods of subsampling sediment for laser diffraction analysis; and 3) comparison of thin-section analysis with laser-diffraction particle size analysis. Using an ultrasonic bath and a SELFRAG (high voltage selective fragmentation) as disaggregation tools, this study evaluates separation of whole, undamaged grains subsequently measured by laser diffraction analysis. We show that it is possible to disaggregate ancient, well cemented rocks using an ultrasonic bath. When disaggregating samples with the SELFRAG method, grain-size measurements become less accurate and less precise with increasing sample lithification and increased presence of cement. This is likely a combination of incomplete grain disaggregation in the SELFRAG and heterogeneity within samples. Following disaggregation, we compare sub-sampling methods using a stirrer plate versus a pipette. Both produce accurate analyses, but the stirrer method is the most reliable and replicable. A comparative small subsample method, run as one whole sample with no need for subdivision into aliquots, is found to be reliable and replicable but is more susceptible to heterogeneity within field samples. When comparing laser diffraction results to grain-size volume methods estimated from thin-section analysis, thin-section sand grains are overestimated, and clay/silt grains are inaccurate. These results provide a framework for understanding potential biases introduced through various sample preparation and measurement methods.

In-situ temperatures and thermal properties of the East Siberian Arctic shelf sediments: Key input for understanding the dynamics of subsea permafrost

Significant reserves of methane (CH4) are held in the Arctic shelf, but the release of CH4 to the overlying ocean and, subsequently, to the atmosphere has been believed to be restricted by impermeable subsea permafrost, which has sealed the upper sediment layers for thousands of years. Our studies demonstrate progressive degradation of subsea permafrost which controls the scales of CH4 release from the sediment into the water-atmospheric system. Thus, new knowledge about the thermal state of subsea permafrost is crucial for better understanding of the permafrost -hydrate system and associated CH4 release from the East Siberian Arctic Shelf (ESAS) – the broadest and shallowest shelf in the World Ocean, which contains about 80% of subsea permafrost and giant pools of hydrates. Meanwhile, the ESAS, still presents large knowledge gaps in many aspects, especially with respect to subsea permafrost distribution and physical properties of bottom sediments. New field data show that the ESAS has an unfrozen (ice-free) upper sediment layer, which in-situ temperature is −1.0 to −1.8 °C and 0.6оС above the freezing point. On one hand, these cold temperature patterns may be related to the presence of subsea permafrost, which currently primarily occurs in the part of the ESAS that is shallower than 100 m, while ice-bearing sediments may also exist locally under deeper water in the Laptev Sea. On the other hand, the negative bottom sediment temperatures of −1.8 °C measured on the Laptev Sea continental slope sediments underlying water columns as deep as down to 330 m may result from dissociation of gas hydrates or possibly from dense water cascading down from the shelf. In contrast, data collected on recent expeditions in the northern Laptev shelf, zones of warmer bottom temperatures are coinciding with methane seeps, likely induced by seismic and tectonic activity in the area. These warm temperatures are not seen in the East Siberian Sea area, not even in areas of methane seeps, yet with little seismic activity.The thermal conductivity and heat capacity of bottom sediments recorded in the database of thermal parameters for the ESAS areas mainly depend on their lithification degree (density or porosity), moisture content, and particle size distribution. The thermal conductivity and heat capacity average about 1.0 W/(m·K) and 2900 kJ/(m3·K), with ±20% and ±10% variance, respectively, in all sampled Arctic sediments to a sub-bottom interval of 0–0.5 m.

Methodology for dissolution of sediment and calcareous deposits for paleontological specimen collection and identification

The Lance Creek Formation (Late Cretaceous) is significant in the study of late dinosaurs and is a diverse formation containing both terrestrial and aquatic macrofossils. To study this important ecosystem, all of the fossils present must be uncovered and examined from Lance Creek sedimentary rocks, which exhibit variable degrees of lithification. In order to liberate the fossils, a dissolution methodology was designed to determine which solution was most effective at uncovering specimens and dissolving/disaggregating sediment. Different solutions were tested including water, a 50% Calgon© solution, and a 5% acetic acid solution. The control was a sedimentary rock sample not subjected to any solutions prior to rinsing and sieving. A small-scale dissolution (10 g of loose sediment) was performed using each solution and examined for fossils. Acetic acid was deemed the most effective solution for the dissolution of dense sandstones, and indurated sediment from the Lance Creek Formation. Large-scale disaggregation (800 g of consolidated sedimentary rock) yielded abundant terrestrial, fluvial, and marine macrofossils. Macrofossil disaggregation using these methods has the potential to yield a more diverse assemblage of contemporaneous fossils than macrofossils alone, and can therefore provide substantial insight into ecological reconstructions.•A small-scale study was done to determine which solution was the most efficient at dissolution of sediment. Acetic acid was deemed the most effective.•A large-scale experiment was done on dense sandstones using 5% acetic acid and a shaking incubator.•A large-scale experiment of indurated sediment was performed using 5% acetic acid.

Stratigraphy and petrophysical characteristics of Lower Paleocene cool-water carbonates, Faxe quarry, Denmark

The Faxe limestone quarry in eastern Denmark exposes Danian (Lower Paleocene) cool-water carbonate deposits. They constitute remnants of an apparent build-up that covers about 12 km2 today. The Danian deposits at Faxe are conspicuous due to their pronounced thickness of coral limestone relative to the regional carbonate system. In the Faxe quarry, scleractinian corals are uniquely exposed in up to 30 m high mounds. The rapid accumulation of scleractinians combined with induration of the mounds may locally have protected the limestone from Quaternary glacial erosion and created a Danian thickness anomaly at Faxe. The position of Faxe above a local fault-bounded basement high and the extent of coral limestone has been better defined by new mapping. A mapped lithostratigraphic surface in the quarry reveals the large-scale organisation of nested bryozoan mounds on three elongated ridges striking NW–SE. The main scleractinian coral mounds are located above this horizon. Data for reservoir characterisation, mainly of the bryozoan mounds, were collected as photographs of the outcrop, petrophysical and petrographical data from cored boreholes, and as ground-penetrating radar sections. Old boreholes and measured sections were used to reconstruct the build-up, and new nannofossil data allow a discussion of stratigraphy and accumulation rate. The petrophysical data show that common mound-building bryozoan packstone has higher permeability and lower capillary entry pressure than chalk, whereas less commonly occurring grain-dominated packstone and grainstone deposits from local higher-energy sites of the mound complex were found to have reduced amounts of coccolith mud, significantly higher permeability and a higher degree of lithification. Based on biostratigraphic age constraints, correlation of flint – limestone couplets and recog-nised hierarchical patterns, we develop a cyclostratigraphy for the middle Danian and suggest that cyclicity in lithology and petrophysical characteristics of bryozoan limestone are controlled by precession and eccentricity of the orbit of the Earth.

The role of microbial extracellular polymeric substances on formation of sulfate minerals and fibrous Mg-clays

A variety of Mg-, Na- and Ca-sulfates, including gypsum, and Mg-clays precipitate within the microbial mats that developed seasonally in the hypersaline playa lake El Longar (Central Spain). A multidisciplinary study of the microbial mats along a transect of the lake demonstrates that the degree of lithification of the mats increases from the soft and poorly lithified layers lying on the submerged ground to the highly crystalline mats present in the desiccated fringes.The community composition of the mats is diverse, dominated in terms of OTUs by Proteobacteria, Firmicutes and Cyanobacteria. The Proteobacteria include sulfur-cycling microbes, supporting biogeochemical cycling of this element. Cyanobacteria are the dominant EPS producers in microbial mats and Firmicutes and Proteobacteria have both been shown to play a role in EPS turnover.Several ions (e.g., Mg, S, Si, Al, Fe, Ca, Na) are bound to functional groups of EPS. The abundance of sulfate groups and carboxylic acids suggested that the EPS have a strong binding capacity for Mg2+ and Ca2+, which were higher in the submerged microbial mats. Na, Mg, K, Ca, SO42− and Si are more concentrated in the EPS matrices than in the natural brines. The concentration of ions in the EPS is sufficient to precipitate a mixture of minerals through freeze-drying, that is similar to that found in intact mats. In the EPS of the submerged mats precipitate more types of minerals, including sulfates and Mg-clays, than in the EPS of the exposed areas. These results suggest that the mineral nucleation occurred during the degradation of the mats when the EPS was progressively desiccated and likely released high amounts of ions.The present study helps to understand intrasedimentary growth of sulfates and clays in microbial mats. The presence of these mineral assemblage can be related to a potential biosignature of microbial past life on Earth and possibly also on exoplanets, like Mars.

Estimation of shear‐wave velocities in unconventional shale reservoirs

ABSTRACTAnalysis of compressional and shear‐wave sonic logs in seven organic‐rich shale reservoirs exhibiting a wide range of velocities, total organic content, thermal maturity, fluid compressibility and mineral composition indicates that compressional‐wave velocity is a strong predictor of shear‐wave velocity as is the case for conventional reservoirs. Excluding data with high water saturations, a simple linear relationship is found between the compressional and shear‐wave velocities in shale reservoirs in accordance with laboratory measurements. The relationship can be further refined for prediction purposes by local calibration or with a linear correction for total organic content. Correcting for compositional variation and fluid effects requires more complex treatment. An empirical rock physics model that explicitly incorporates these effects results in high accuracy (0.2% average mean error) in shear‐wave velocity estimation for the entire data set. The model also exhibits good precision with mean absolute error of 2%. This is significantly better than what is achieved by rock physics models that do not explicitly consider fluid properties. These results are obtained without local calibration and without requiring correction for thermal maturity, degree of lithification or the shape or distribution of the solid organic matter. Explicit consideration of mineral composition, fluid substitution effects and amount of solid organic matter was necessary in this formulation to achieve this high average prediction accuracy with near zero bias as well as good precision across all formations and without local calibration. The successful use of Gassmann's equations in this algorithm suggests that errors in shear‐wave velocity prediction in shale reservoirs due to violation of the assumptions underlying these equations are either small or self‐compensating.

Chromium isotope heterogeneity on a modern carbonate platform

Chromium isotope (δ53Cr) values of marine sedimentary rocks are increasingly utilised for monitoring changes in redox conditions in the Earth's ancient oceans and atmosphere. Carbonate rocks are of particular interest as they are relatively common in the sedimentary record and have been shown under laboratory conditions to be capable of recording the Cr isotope composition of precipitating fluids. However, little data from appropriate modern analogues is currently available to constrain the variability in δ53Cr values within a carbonate platform and slope system and assess the preservation potential of marine δ53Cr values during burial, lithification and diagenesis. In this paper we investigate the heterogeneity of δ53Cr values on a modern marine carbonate platform in the context of carbonate sediment deposition, mineralogy, and the influence of burial and early lithification. We present new δ53Cr values from carbonate sediments in the upper 200 m of ODP Leg 166 Site 1003 taken from the western margin of the Great Bahama Bank (GBB) and compare these data to modern carbonate sediments accumulating on the GBB platform-top. Shallow-water surface sediments record a range of heavy δ53Cr values from +0.77 to +1.07‰ while drill core samples record a larger range of δ53Cr values from +0.51 to +0.97‰. Down-core variability in δ53Cr values occurs systematically with changes in sediment composition and mineralogy, where more positive δ53Cr values occur in drill core samples that preserve proportionally more banktop-derived carbonate and less well developed diagenetic textures. These values decrease down-core where proportionally more pelagic carbonate exists, and with increasing neomorphism of primary aragonite to low-Mg calcite. The mineralogical dependence of δ53Cr values at Site 1003 appears to be related to variations in relative sea-level across the platform and the degree of lithification and diagenetic recrystallization with depth, rather than in response to changes in the δ53Cr composition of the overlying water column. As the GBB represents a reasonable modern analogue for many Phanerozoic and Precambrian carbonate successions, the preliminary framework presented here may have significant implications for the interpretation of vertical profiles of δ53Cr values in paleo-redox studies. In particular, smaller scale variability in ancient carbonate sections can be more simply explained by local variations in depositional environment and diagenetic regime rather than fluctuations in water column redox conditions.

Towards a new understanding of the genesis of chalk: Diagenetic origin of micarbs confirmed by clumped isotope analysis

AbstractChalk is usually thought to be a homogeneous sediment with a relatively simple early diagenetic history. Here, clumped isotope analyses of samples from a core of Campanian Maastrichtian chalk are presented, indicating that material smaller than 5 µm has a different origin than the coccolith‐dominated coarser fraction. The smallest size fraction (1 to 5 µm) of chalk is dominated by calcite particles without a distinct morphology (micarbs). Clumped isotope data of the micarbs reveals formation temperatures of 14 to 18°C which is 8 to 10°C colder than those derived from coeval coccoliths. The micarbs are interpreted as the product of calcite neoformation, precipitated in the uppermost part of the sediment column (100 metres below sea floor) and linked to early dissolution of aragonitic fossils. These findings prove that early cements can be an abundant component in chalk, and thus challenge the common notion that chalk is always largely composed of calcareous nannofossils, and differs only in terms of minor constituents and degree of lithification.

Silurian metalliferous nodules from the region of Asaritsa peak, West Balkan mountain – preliminary results

First data for polymetallic nodules and host sediments in Silurian sequence from the region of Asaritsa peak, West Balkan mountain are presented. The nodules are rich of Fe, Mn, Co and Ni. Both sediments and nodules have high content of REE. The described Silurian nodules and sediments show a number of similarities with modern deep-sea polymetallic nodule bearing sediments. Some differences such as probable depth of deposition, mechanism of nodule formation, degree of lithification are also established.

Earthquake-induced soft sediment deformation (SSD) structures from the Bilara limestone formation, Marwar basin, India

The Neoproterozoic Bilara limestone Formation of the Marwar Group, Rajasthan, India exposes metres-thick layers of soft sediment deformation (SSD) structures at different stratigraphic levels which could be traced over hundreds of metres on the outcrop scale. The SSD structures include disharmonic folds, low-angle thrusts, distorted laminae, fluidisation pipes, slump and load structures, homogeneities, diapirs, etc. Whereas SSD structures suggesting tensional stress, viz., intrastriatal graben, fluidisation, slump, etc. dominate in the lower part of the Bilara succession, features implicating compression, viz., folds, low-angle thrust are prevalent in the uppermost part. Since SSD structures are mostly confined within the algal laminites, we interpret that enhanced micritic fluid pressure below early cemented algal carbonate played a major role in laminae deformation. Depending on the degree of lithification and pore-water pressure, deformation features formed either plastically or led to diapiric injection at enhanced pore water pressure. Separated by near-horizontal underformed strata, the SSD layers, traceable over hundreds of metres, are interpreted as products of seismic shacking. Considering the time frame of the Marwar basin, i.e., the Precambrian–Cambrian transition, the SSD horizons present within the Bilara succession may hold the potential for the correlation with SSD structures reported from the time-correlative stratigraphic successions present in erstwhile adjoining tectonic terrains, e.g., China, Siberia, etc.

A petrographic study of the genesis of anomalous sections in the Bazhenov Formation, the Imilor deposit, Western Siberia

Research subject. Anomalous sections present in the oil- and bituminous-bearing Bazhenov Formation (BF), in the central areas of the West Siberian basin, are considered to have emerged due to the injection of plastic pulp masses from the sedimentation slope into weakly lithified basal deposits. The aim of this research was to evaluate the as-emerged rocks by analyzing their composition and structure at the macro- and micro- levels. Materials and methods. We investigated reference well sections in the Imilor deposit to elucidate the interaction of the rocks with sandy-clay landslide pulp, their volumetric weight, ductility or brittleness, caused by the presence or absence of a continuous mineral frame. Results. According to core and well logging data, nine regional packs were distinguished in the BF stratotype. The maximal depth of the natural hydraulic fracturing of the rocks is explained by their high bulk density. The lateral movement of the main slump mass is found to have occured on the stratigraphic level of the first and second BF packs, since these layers possessed zero-buoyancy with regard to the injected pulp under their approximate equality in terms of bulk density. The traces of such active movements were revealed in rock samples from the walls of horizontal cracks in the form of clay intrusions having a thickness of about 0.1 mm and strict orientation of all micro-crystals along the walls and the flow. It is shown that inclusions of the rocks under investigation (bazhenits) in BF are generally characterized by their initial stratigraphic sequence, which is confirmed by the sequential disintegration of proto-bazhenit plates during the process of BF up-floating on a growing sedimentation slope. During the disintegration of the lithified and non-lithified layers of radiolarits, the former crumbled due to alternating loads, while the latter - mobile radiolarites - were represented by the hydraulic fracturing liquid, along with quicksand. Conclusion. A detailed petrographic study of core samples from intact deposits of the Bazhenov and George Formations and their anomalous sections have allowed their layer-by-layer comparison to be confirmed the landslide hypothesis and to be derived the information on the degree of lithification of the BF strata during the formation of its anomalous sections.

Friction and Cohesion as Multifaceted Factors of Soil Shear Resistance

Aspects are examined of dispersive soils that are conditioned by the mechanism and magnitude of their shear resistance. From a general physicochemical perspective, we study factors not considered in the Mohr-Coulomb theory that explain its departure from experimental data-mineral and granulometric composition, morphology of structural elements, the degree of lithification, the adsorption films and hard "liners" on the surface of minerals, the effect of the disjoining pressure on the real effective stresses, the mechanism of friction, and structural changes during deformation.

Planktonic foraminiferal biostratigraphy and assemblage composition across the Cenomanian–Turonian boundary interval at Clot Chevalier (Vocontian Basin, SE France)

The Cenomanian–Turonian boundary interval is generally considered a critical time for planktonic foraminifera due to the environmental perturbations associated with Oceanic Anoxic Event 2. However, only the rotaliporids became extinct at the onset of the event, whilst several lineages evolved and/or diversified. This remarkable morphologic plasticity is often overlooked in the literature, partly because a number of stratigraphic sections have only been studied in thin-section due to the degree of lithification of the samples. Improved documentation of the morphological variability of planktonic foraminifera and better defined species concepts are required in order to improve biostratigraphy, particularly as Helvetoglobotruncana helvetica is an unreliable marker for the base of the Turonian. At the same time, detailed study of the planktonic foraminiferal response to OAE 2 demands a more profound knowledge of the assemblage composition.We present new biostratigraphic, taxonomic, and quantitative data for planktonic foraminiferal species from the Clot Chevalier section (Vocontian Basin, SE France), with the aim of (1) providing a detailed biostratigraphic analysis of the section, (2) documenting the morphological plasticity of specimens in this time interval and stabilizing species concepts, and (3) identifying promising markers to improve the resolution of the present biozonation and allow regional correlation. Samples were processed with acetic acid to extract isolated planktonic foraminifera. Assemblages were assigned to the upper Cenomanian Rotalipora cushmani Zone and to the uppermost Cenomanian–lowermost Turonian Whiteinella archaeocretacea Zone. Planktonic foraminiferal bioevents and assemblage composition identified at Clot Chevalier are compared with the well-studied Pont d'Issole section located ca. 15 km to the NE, highlighting similarities and differences in the species occurrences that may complicate the stratigraphic correlation between the two sections.The results of our study support the validity and common occurrence of species that have been misidentified and/or overlooked in the literature (i.e., Dicarinella roddai, Praeglobotruncana oraviensis, Marginotruncana caronae) and indicate that primitive marginotruncanids evolved before the onset of OAE 2, although species diversification occurred only after the event. Moreover, we believe that the first appearance of P. oraviensis might represent a promising bioevent for approximating the Cenomanian/Turonian boundary, after calibration with bio- and chemostratigraphically well-constrained sections. Finally, we describe three new trochospiral species, named “Pseudoclavihedbergella” chevaliensis, Praeglobotruncana pseudoalgeriana and Praeglobotruncana clotensis.

Eocene Paleoseismic Record of the Green River Formation, Fossil Basin, Wyoming, U.S.A.: Implications of Synsedimentary Deformation Structures In Lacustrine Carbonate Mudstones

Abstract Laterally extensive intervals containing sedimentary deformation features are identified in outcrops of lacustrine deposits of the Eocene (∼ 52–51 Ma) Green River Formation in Fossil Basin, southwestern Wyoming. Fossil Basin is the smallest of the paleo-lakes in the Green River system and is located in the Sevier Fold and Thrust Belt, which was tectonically active during deposition. Deformation structures include convolute lamination, load structures, hybrid brittle–ductile features, sedimentary dikes, microfaults, fluid-escape features such as cusps and pipes, oil-shale breccias, and mass-transport deposits. In most cases these structures are hosted by finely laminated, variably organic-rich carbonate mudstones that accumulated in a low-energy profundal lacustrine environment. Deformed intervals range from a few centimeters to 5 m in thickness and are bound above and below by undeformed strata of the same facies, which demonstrates the early post-depositional timing of deformation. The distribution of these features indicates a primary control exerted by sediment rheology, resulting from variation in grain size, clay and organic content, and degree of lithification, which in turn were governed by the depositional environment and early diagenesis. Based on their morphological attributes, lateral extent, and recurrence at different stratigraphic levels, along with their sedimentary environment and the tectonic setting of the basin, the deformation structures are interpreted to be “seismites,” i.e., the result of stresses induced by syndepositional earthquakes, and other potential trigger agents are rejected. Moreover, stratigraphic relationships indicate that these small-scale deformation features as well as the mass-transport deposits, formed on a virtually flat lake floor, ruling out instability caused by the gravitational regime. Syndepositional deformation in Fossil Basin points to tectonic activity along the nearby Absaroka and/or Hogsback thrusts and related fault systems. Seismites point to possible tectonic forcing as a mechanism for changes in bathymetry and sedimentation in the Green River Formation and, consequently, demonstrate their potential for elucidating the tectonic history of other lacustrine and marine basins.

Mélanges and other types of block-in-matrix formations in the Cantabrian Zone (Variscan Orogen, northwest Spain): origin and significance

Block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath or ahead of submarine thrust systems. These mélanges may reach up to kilometric thickness and are mostly composed of broken formations (boudinaged sequences) of late Carboniferous age and scattered ‘exotic’ blocks derived from older Palaeozoic formations. Moreover, the mélanges in the Cantabrian Zone also include subordinate debris flow deposits with a chaotic block-in-matrix fabric (olistostromes). The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated through submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones, whereas the unlithified muddy matrix underwent continuous deformation. The scattered ‘exotic’ blocks ranging in age from early Cambrian to early Carboniferous were incorporated into the mélanges as individual blocks from competent well-lithified formations, originally located in the lower part of the nappe stacks. Although the Cantabrian Zone mélanges include olistostromic intervals, most of the olistostromes of this zone occur in a different geological setting. They are usually intercalated in the normal marine deposits of the Variscan foreland basin and, in contrast to the mélanges, they are mostly related to the margins of carbonate platforms, ahead of moving nappes. Finally, other instances of olistostromes are related to slopes generated by limb rotation of growth folds, which developed on submarine wedge-top successions.