Uncemented Sediments Research Articles

Modeling the pressure sensitivity of uncemented sediments using a modified grain contact theory: Incorporating grain relaxation and porosity effects

We develop a modified grain contact theory to better describe the pressure-dependent elastic properties of uncemented sediments. The Hertz-Mindlin (HM) theory typically predicts shear moduli that are much higher than observed laboratory measurements, resulting in inaccurate estimates of the dry bulk to shear modulus ratio [Formula: see text]. The HM theory further predicts that the [Formula: see text] ratio is constant with pressure, whereas ultrasonic core measurements typically show an increasing [Formula: see text] ratio as effective pressure decreases. Laboratory data also suggest that the dry bulk and shear moduli variation with effective pressure is greater than the cube-root power law predicted by the HM theory. We introduce two new pressure-dependent calibration parameters to account for the shortcomings in effective medium theory, and we develop a new method to predict pressure-dependent elastic properties. Our calibration parameters agree with the results of published granular dynamics simulations, and they incorporate grain relaxation and porosity effects not included in existing effective medium theories. Our new model provides improved fits to laboratory data when compared with existing models, and it can be used for improved prediction of elastic properties as a function of effective pressure. Our new theory can also be used to model uncemented sediments with values of Poisson’s ratio [Formula: see text], where many existing grain contact and effective medium theories currently fail.

A new velocity-pressure-compaction model for uncemented sediments

SUMMARY Knowledge of the pressure dependence of rock properties is useful for a wide range of earth science problems, especially related to pore pressure changes caused by fluid injection or withdrawal, as often occurs in groundwater, hydrocarbon and CO2 sequestration reservoirs. A long-standing problem is that theoretical models of velocity-pressure response often do not match laboratory measurements, and alternately, empirical regressions fit to such data do not extrapolate accurately to wider pressure ranges since they have little or no physical basis. Accurate determination of the dry rock frame properties at low effective pressure is a key aspect of the problem, particularly when ultrasonic laboratory measurements are not available in this pressure range. We present a new model to describe the pressure sensitivity of the bulk and shear moduli for uncemented sedimentary rocks. Our model incorporates effects of sedimentary compaction and critical porosity, including a relationship to account for porosityanddensitychangewithpressure.Themodelistestedonlaboratorymeasurementsfor various rock samples and fits well over a wide range of pressures. The new velocity‐pressure model should be useful for improved prediction and interpretation of pressure-dependent rock properties and seismic data.

Permeability Reduction Produced by Grain Reorganization and Accumulation of Exsolved CO2 during Geologic Carbon Sequestration: A New CO2 Trapping Mechanism

Carbon sequestration experiments were conducted on uncemented sediment and lithified rock from the Eau Claire Formation, which consisted primarily of K-feldspar and quartz. Cores were heated to accentuate reactivity between fluid and mineral grains and to force CO(2) exsolution. Measured permeability of one sediment core ultimately reduced by 4 orders of magnitude as it was incrementally heated from 21 to 150 °C. Water-rock interaction produced some alteration, yielding sub-μm clay precipitation on K-feldspar grains in the core's upstream end. Experimental results also revealed abundant newly formed pore space in regions of the core, and in some cases pores that were several times larger than the average grain size of the sediment. These large pores likely formed from elevated localized pressure caused by rapid CO(2) exsolution within the core and/or an accumulating CO(2) phase capable of pushing out surrounding sediment. CO(2) filled the pores and blocked flow pathways. Comparison with a similar experiment using a solid arkose core indicates that CO(2) accumulation and grain reorganization mainly contributed to permeability reduction during the heated sediment core experiment. This suggests that CO(2) injection into sediments may store more CO(2) and cause additional permeability reduction than is possible in lithified rock due to grain reorganization.

Fluid-driven fractures in uncemented sediments: Underlying particle-level processes

Granular materials subjected to fluid flow may experience fracture formation and fluid flow localization. Current explanations for hydraulic fracture in soils fail to satisfy the inherent characteristics of granular materials: effective stress-dependent cohesionless-frictional strength. We apply complementary experimental and numerical techniques to identify the underlying particle-scale mechanisms. First, we show that the miscibility of the invading fluid with the host fluid leads to distinct localization processes that depend on the balance between particle-level skeletal forces (effective stress-dependent), capillary forces (the invasion of the interfacial membrane when immiscible fluids are involved), and seepage drag forces (associated with fluid flow velocity). Then, we identify the positive feedback mechanisms at surface defects and fracture tips that promote fracture initiation and sustain fracture propagation. These include increased porosity at the tip due to strains preferentially normal to the fracture alignment, either eased membrane invasion (immiscible fluids) or higher hydraulic conductivity (miscible fluids), and the emergence of particle-level forces that promote opening-mode particle displacement. This effective stress compatible sequence of events helps identify the parameters that govern fluid-driven fracture formation in uncemented sediments, and explain experimental observations.

An experimental investigation of the development and permeability of clay smears along faults in uncemented sediments

The generation of clay smears along faults in uncemented sediments has been studied through laboratory experiments in a newly developed high stress ring shear apparatus. The main objective is to investigate basic mechanisms involved in the deformation process of sediments during faulting and formation of clay smears. The experimental test program comprises ring shear tests on sand with embedded clay segments (sand–clay sequence) under constant effective normal stress. Visual inspection of the samples after testing, analyses of thin sections and permeability measurements across the shear zone are used to characterise geometrical continuity, thickness and sealing potential of the smear. Deformation processes such as grain reorientation, clay smear and cataclasis are identified from the tests. The complexity of the shear zone is observed to increase with the effective normal stress applied to the specimen and the number of clay segments used in the ring (multilayered sand–clay sequences). At low effective normal stress, in clay-rich sediments, clay smear is the most efficient mechanism for permeability reduction. The permeability across the smear decreases with ring rotation (or shear displacement) and effective normal stress. A maximum decrease of two orders of magnitude compared to the permeability of the surrounding sand is observed after 90° rotation under 10.5 MPa effective normal stress. Sand–sand juxtaposition shear is dominated by grain rolling causing only minor permeability reduction. At high effective normal stress, permeability measurements across clay smear and sand–sand juxtaposition yield similar values indicating that the permeability reduction is dominated by grain size reduction in the sand.

Contraction-driven shear failure in compacting uncemented sediments

Shear failure in sediments is universally linked with active boundary conditions, such as those imposed by tectonic stresses. Under conditions of no lateral strain, and in the absence of tectonic stress, soil mechanics theories predict a simple one-dimensional compaction in which sediment particles displace vertically without shear failure during pressure diffusion. Conflicting with this theory, shear failure planes are often found in sediments that formed under near-horizontal burial conditions. We investigated the effect of particle-scale volume contraction as a potential cause of shear failure in uncemented particulate materials and found that loss of particle volume under confined conditions (no external loading) resulted in pronounced lateral stress reduction, often reaching Coulomb frictional failure conditions. Shear strain localization was analytically predicted and modeled numerically, due entirely to volume loss at the grain scale. We define this mode of internally driven shear failure as “contractile” to distinguish it from that caused by external loading, and show that it can explain many natural fracture systems without invoking regional tectonics.

Inversion of Seismic Surface Wave Data to Resolve Complex Profiles

For a complex layered site, development of a shear wave velocity profile from inversion of seismic surface wave data can yield nonunique results. One class of solutions provides depth-averaged profiles that are useful but do not detail the local velocity anomalies. Simulated annealing inversion can be used to establish whether nonuniqueness is a concern for a particular case and, if so, focus results to allow a solution that is expected based on independent knowledge of site conditions. A two-step process is recommended whereby the model parameters are first optimized using simulated annealing and then fine tuned by unconstrained linearized inversion. The process also yields useful confidence measures. One example of a complex layered site where the proposed procedure is helpful is a desert profile with high-velocity cemented layers interspersed among uncemented sediments.

Effects of compaction processes on stresses, faults, and fluid flow in sedimentary basins: examples from the Norwegian margin

Abstract Modelling of sediment compaction requires that the rate limiting processes are understood. The compaction of uncemented sediments at relatively shallow burial depths should be modelled as a function of effective stress following soil mechanical principles and using experimental compaction data for calibration. In siliceous rocks chemical compaction is dominant at depths greater than 2–3 km (80–100°C). Chemical compaction should be modelled as a function of the temperature history and the mineralogical and textural composition of the sediments. The rate of chemical compaction for siliceous sediments is to a large extent a function of the quartz cementation, which is an exponential function of temperature, while the effective stress plays a minor role. In the case of carbonate sediments the kinetics of precipitation of cement is much faster and the effective stress is more important than temperature. The magnitude and distribution of effective situ stresses is a complex function of external tectonic stresses, gravitational forces and fluid pressures. Sediments undergo mechanical compaction when subjected to high effective stress and are much more compressible than basement rocks. Chemical compaction also results in a reduction in rock volume and this has a strong feedback on the in situ stresses. If the horizontal stress is greater than the vertical stress, both mechanical compaction and chemical compaction will also occur in the horizontal direction, thus relaxing in situ stresses unless there is significant basin shortening. Calculations show that relatively large in situ stress anomalies (10 MPa) may be relaxed in 5–10 ka by chemical compaction during basin subsidence. Chemical compaction may also continue during uplift; it is fundamentally different from mechanical compaction and must be modelled separately.

The effect of carbonate cementation on permeability heterogeneity in fluvial aquifers: An outcrop analog study

Spatial variations in permeability have been widely recognized as a dominant control on solute transport and petroleum migration. Outcrop studies are one means of improving our understanding of the nature of permeability heterogeneity and the underlying geologic controls. To date, these studies have either neglected the importance of cementation or investigated only the modern cemented permeability without explicitly considering how cementation has affected permeability. Two outcrops of channel sand deposits in the Plio-Pleistocene Sierra Ladrones Formation, New Mexico are investigated to quantify (1) the effect of cementation on permeability at a given location, and (2) the patterns of cementation and their impact on permeability heterogeneity. We analyzed approximately 400 spatially located samples (∼ 10 cm 3 each) from the two outcrops. Cementation is typically weak to moderate, filling less than 60% of the original pore space. Permeability and cement content are bimodally distributed and negatively correlated. Permeability reduction is investigated by measuring modern permeability, minus-cement permeability, minus-cement porosity, and volumetric cement content. Permeability is reduced locally by up to three orders of magnitude. By combining models of permeability for uncemented sediments and cemented sediments, we derive a model of cemented permeability ( k c) that depends primarily on three factors: uncemented permeability ( k u), original porosity, and cement fraction. Our results suggest that accounting for porosity reduction only may result in an overestimation of k c. Conversely, accounting for porosity reduction, increased specific surface area, and increased tortuosity appears to result in an underestimation of k c. Accounting for porosity reduction and increased specific surface area provide the best estimate of k c. Cementation is highly variable spatially and is poorly correlated with lithofacies. Variogram analysis of cementation and permeability indicates that the spatial distribution of cementation is the dominant control on the permeability correlation structure. The spatially variable cementation also results in an order-of-magnitude increase in the variance of log-permeability. Applying stochastic models of effective flow and transport properties to the results of this study suggest that cementation has a relatively small effect on effective permeability but can potentially increase dispersivity by two orders of magnitude.

Compressional and shear wave velocities in uncemented sediment containing gas hydrate

The competing hypotheses for gas hydrate formation at the particle scale in sediments describe processes of pore‐filling, frame‐building, or cementation. New measurements of compressional (VP) and shear wave (VS) velocities in fine‐grained sands subjected to low confinement and monitored during formation of tetrahydrofuran hydrate indicate that hydrate nucleates in the pore space (presumably at grain boundaries) and grows with limited impact on the sediment shear stiffness, VP, and VS until crystals begin to interact with the granular skeleton at ∼40% hydrate concentration. VS increases significantly more than VP at higher hydrate concentrations, reflecting larger changes in the specimen's shear stiffness than its bulk stiffness. The results indicate that seismic velocities and/or their ratio (VP/VS) have limited capability for locating hydrate or constraining hydrate concentrations.

Influence of burial history on microstructure and compaction behaviour of Kimmeridge clay

The paper presents the results of laboratory compaction tests on two samples of the lower Kimmeridge Clay Formation obtained from two different outcrop localities in the UK. The purpose of the testing programme is to study the compaction behaviour of an argillaceous sediment from the same parent material but with different diagenetic histories. Since most available data are for clays subjected to moderate effective stresses (less than 20 MPa), the compaction laboratory tests were carried out to as high as 120 MPa, corresponding to about 9 km burial. The microstructure of the tested materials was studied in a scanning electron microscope before and after compaction testing to evaluate the effects of diagenesis on the microstructure of argillaceous sediments. The two tested materials have, in the past, been subjected to different burial depths and degrees of chemical precipitationand cementation. Several pieces of evidence are shown of the effects of mechanical and chemical processes on the compaction behaviour of argillaceous materials. Mechanical compaction can only account for part of the porosity loss and increase in stiffness of argillaceous materials during burial. Mechanical compaction produces an anisotropic rearrangement of clay particles in uncemented sediments. Once chemical diagenesis has taken place, chemical cementation preserves the microstructure of the sediment and only minor changes in clay particle arrangements take place during further mechanical compaction, even when very high stresses are used. Also, it is seen that chemical processes reduce the permeability more strongly than mechanical compaction.

Geological perspectives on consolidation of clay‐rich marine sediments

Experimental consolidation of uncemented clay‐rich marine sediments provides information concerning their stress history. A main finding is that some of the well‐known behavior of soft sediment deformation in geotechnical applications cannot validly be extrapolated to sediments that have been subjected to higher stresses and longer times of geologic conditions. This study confirms that the yield stress of the uncemented sediment accurately reflects its previous maximum consolidation state. Furthermore, we have identified a new phase of post‐yield strain that is associated with higher values of the modified compression index (the slope of the porosity versus the logarithm of effective vertical stress) than that of elastic deformation, but with much lower values than that for primary consolidation. This post‐yield behavior is a linear, non‐elastic deformation, and is termed tertiary consolidation. Yield stress appears independent of creep time or strain rate, whereas the tertiary‐primary consolidation transition is sensitive to these parameters. During post‐yield creep (secondary consolidation) the slope of the porosity versus the logarithm of time curve, or the secondary consolidation index, is generally assumed constant. However, this is not valid for claystones at effective vertical stresses above about 1 MPa, where the secondary consolidation index increases with stress. At a given effective vertical stress, the secondary consolidation index also increases with creep times greater than about 105 s (≈28 h).

Development of a Lithocodium (syn. Bacinella irregularis)-reef-mound- A patch reef within Middle Aptian lagoonal limestone sequence near Nova Gorica (Sabotin Mountain, W-Slovenia)

A Middle-Aptian (zone of Palorbitolina lenticularis) „patch reef“ of about 40 m maximum thickness with marked morphology was analyzed at the Sabotin Mountain near Nova Gorica. It is developed above an basal unit of superficial oolites within lagoonal sediments and is overlain by shallow subtidal to intertidal sediments which reveal short time periodically subaerial exposure and early diagenetic freshwater influxes (birds eyes, vadose silt, characeans). Above these sediments within the zone of Salpingoporella dinarica finebedded to platy, laminated bituminous limestones occur. The central part of the reef structure reveals an alternation of individual lenses of Lithocodium-boundstones and of rudist-beds (up to 4 m thickness) which are separated by coarse- to medium-grained, moderately to poorly sorted bioclastic sands. These facies types show also lateral interfingering and are concentrated in the central part of the buildup where the greatest thickness can be observed and where packstones, grainstones and boundstones prevail. The neighbouring lagoonal sediments which consist of mudstones and wackestones predominantly were analyzed in the so-called Sabotin-standardprofile which is located north of the patch reef at a lateral distance of about 300 m. Three vertical profiles (A = 90 m, B = 100 m, C = 64 m thickness) were analyzed. The middle profile B of greatest thickness is taken as reference profile and documents best the vertical facies development within the patch reef directly overlying a basal unit of peloidal packstones with superficial ooids. The patch reef itself is characterized by the faunal associations within the unit rich in Lithocodium and rudists. It is overlain by a subtidal unit of peloidal mudstones with very minor biogenic allochems. An intertidal unit above rich in birds-eyes and vadose silt is followed by fine-laminated black shales which are covering the general seqeunce of interfingering patch-reef - lagoonal sediments. Using different time lines (marker horizons) for correlation it can be shown that already very early differential compaction of fine-grained uncemented sediments in comparison to the core area of the patch reef is of great importance. The greater thickness of the patch reef itself is caused by an intensive early diagenetic marine phreatic cementation within the core zone and by the early fixing of sediment by Lithocodium aggregatum (syn. Bacinella irregularis) resulting in a greater resistivity against compaction. Furthermore a lagoonal side and an more open marine side of the patch reef can be determined. Lithocodium aggregatum is the main constructing organism within the buildup investigated, beginning with the incrustation of varying substrates and biogenic particles. Finally, a dense network of encrustation-sequences is formed interfingering with the general sedimentary textures intensively and resulting in larger „biogenic-cemented“ patches within the sediment. The correlation of the three profiles A, B, and C allows to reveal the history of differential compaction of the associated lagoonal sediments in comparison to the more massive patch reef limestones. It becomes obvious that the main compaction must have occurred within the time span of about 50 m sediment-deposition overlying the patch reef. Differential compaction resulted in differences in thickness of about 10 m from the patch reef (profile B) to the more lagoonal influenced sediments (profile A) within a lateral distance of about 50 m.

Attenuation and permeability: high frequency crosswell tomography for imaging permeability structure of sediments

Imaging the permeability within sediments and rocks is one of the important problems left to advance in geoacoustics and geophysics. The Hamilton attenuation data suggests it is possible. As a U.S.–Japan joint research project we have selected this important problem. Desired permeability and porosity of test sediments are achieved with a sophisticated technique of sediment deposition called laminar sedimentation. To minimize reflections from the tank walls a wave absorber was placed between the tank wall and sediments. Placing a second hydrophone well between the source well and the receiver well, bore hole losses were physically minimized so as to assure accurate measurements of attenuation. A broadband piezoelectric source and hydrophone arrays were used to propagate and receive high frequency M-sequence acoustic waves. High resolution permeability images within various sediments are extracted from attenuation and velocity measured by high frequency (10 kHz to 80 kHz) acoustic cross-well tomography experiments and a Biot inversion. [Work supported by ONR, NIRE, and KSEC.]

Bottom‐simulating reflectors: Seismic velocities and AVO effects

We obtain the wave velocities of ice‐ and gas hydrate‐bearing sediments as a function of concentration and temperature. Unlike previous theories based on simple slowness and/or moduli averaging or two‐phase models, we use a Biot‐type three‐phase theory that considers the existence of two solids (grain and ice or clathrate) and a liquid (water), and a porous matrix containing gas and water. For consolidated Berea sandstone, the theory underestimates the value of the compressional velocity below 0°C. Including grain‐ice interactions and grain cementation yields a good fit to the experimental data. Strictly speaking, water proportion and temperature are closely related. Fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures, provided that the average pore radius and its standard deviation are known. The reflection coefficients are computed with a viscoelastic single‐phase constitutive model. The analysis is carried out for the top and bottom of a free‐gas zone beneath a gas hydrate‐bearing sediment and overlying a sediment fully saturated with water. Assuming that the bottom‐simulating reflector is caused solely by an interface separating cemented gas hydrate‐ and free gas‐bearing sediments, we conclude that (1) for a given gas saturation, it is difficult to evaluate the amount of gas hydrate at low concentrations. However, low and high concentrations of hydrate can be distinguished, since they give positive and negative anomalies, respectively. (2) Saturation of free gas can be determined from the reflection amplitude, but not from the type of anomaly. (3) The P to S reflection coefficient is a good indicator of high amounts of free gas and gas hydrate. On the other hand, the amplitude‐variation‐with‐offset curves are always positive for uncemented sediments.

High‐stress consolidation experiments and their geologic implications

Although uniaxial consolidation of sediments in basins defines one of the most common and simple of geologic stress paths, there is remarkably little information concerning lateral stresses and elastic moduli associated with this process. Short‐term relationships explored by geotechnical engineers show that ratios of effective horizontal stress (σh′) to effective vertical stress (σν′), or Ko, are constant for a given sediment over the low‐stress range, but there is no consensus as to whether these ratios remain constant at the higher stresses and longer durations of geologic conditions. A series of uniaxial consolidation tests on a silty clay and a fine sand, including several elastic unload‐reload cycles, were carried out to explore consolidation and elastic relationships to a σν′ of 35 MPa. Over this stress range, Ko remained constant for the clay (0.62) but increased slightly for the sand (0.44 to 0.53). The unload‐reload cycles showed that the vertical Young's modulus (Eν) increases markedly with consolidation stress (σc′). These cycles also demonstrated that σh′/σν′ (elastic) is greater for clay (0.35) than for sand (0.21), but this ratio shows no variation with σc′. This implies that Poisson's ratios (v) did not vary significantly with σν′ or σc′ over the stress range of the tests. Comparison with natural sediments suggests that cementation results in increases of E of up to an order of magnitude above that of an analogous uncemented sediment, but does not seem to affect v. The effect of geologic time on Ko was explored using results of sparse applicable in situ stress measurements; these suggest not only that clay‐rich sediments can support significant differential stresses, but also that stress ratios may even retain their short‐term values for 106 yr or more.

An assessment of the use of Christensen’s equation for the prediction of shear wave velocity and engineering parameters

Abstract The full wave train sonic log is widely used in civil engineering for the determination of formation dynamic elastic moduli. In some instances, the shear wave amplitude is too small to time its arrival accurately. In these situations Christensen’s equation can be used to predict the shear wave velocity from the compressional wave velocity and the formation density. In this paper the origins of Christensen’s equation are investigated in relation to both field and laboratory studies on a wide range of natural materials. It is suggested that differences between the measured and predicted shear wave velocities are lithologically based. The equation is not valid for uncemented sediments. The dynamic elastic moduli reflect these differences. Bulk modulus is least affected by large differences in shear wave velocity. There is no relationship between the predicted and the measured values for Poisson’s ratio. The use of Christensen’s equation will be improved if corrections are made locally for lithology.

Contribution of Sea Level-Induced Erosional Events in Platform/Periplatform Deposits to Lowstand Sedimentation in the Bahamas: ABSTRACT

New accumulation curves for Bahamian platform and periplatform carbonates are used to elucidate, with good time control, a regional-scale erosional event believed to be triggered by sea level regression. Magnetostratigraphic dating of cores from Little Bahama Bank and San Salvador in conjunction with nearby biostratigraphically and magnetostratigraphically dated periplatform cores collected by the Ocean Drilling Program (ODP) leg 101 (Sites 633, 630) allow comparison of periods of accumulation and erosion. These two periplatform cores (and four additional ODP cores) show an erosional gap from about 2.2 to over 3.0 Ma, whereas the platform flooding record measured from the core areas and Great Bahama Bank shows the onset of subaerial exposure at about 2.4 Ma, continuing intermittently to about 0.8 Ma. Overlap of these two profiles suggests that a regional-scale erosional event, likely slope failure as avalanching gravity flows, occurred just after initial exposure of the platform 2.4-2.2 Ma. Net accumulation curves suggest that this event occurred after a prolonged highstand capable of producing and storing large quantities of uncemented sediment. This large sediment reservoir may have loaded the platform margin and upper slope as sea level and the wave base regressed through several high-frequency events. These data suggest that initialmore » lowering of sea level triggered this regional erosion event on the platform flanks, thus moving periplatform sediment basinward, although other triggering mechanisms (i.e., tectonic) are possible. This sequence of events may contribute significantly to lowstand deposit seen in seismic profiles from pure carbonate environments.« less

The application of stress path and critical state analysis to sediment deformation

The importance of consolidation, compression and shear as deformation mechanisms in uncemented sediments is discussed and the critical state concept is introduced as a unifying model for these aspects of deformation. From the critical state and the law of effective stress, the concepts of burial and tectonic stress paths are introduced with reference to the development of growth faults in an unlithified sediment undergoing simultaneous burial and extension. It is demonstrated that particulate deformation mechanisms can be important at burial depths of several kilometres, especially if the sediment is overpressured, and that deformation of this type will influence the geometry and nature of the structures produced. A model for the geometry and spacing of growth faults developed in unlithified sediments is proposed utilizing stress path and critical state concepts.

Tidal-flat sedimentation in the Lower Jurassic of Bornholm, Denmark

Lower Lias sediments from the Danish island of Bornholm exhibit many sedimentary indications of tidal influence during their deposition. A section in the Sinemurian is interpreted as a regressive tidal flat with supratidal salt-marsh (coals and rootlet-beds), tidal flat (flaser- and wavy-bedded clays and sands), tidal creeks, storm scours and sand sheets, large scale metaripples and subtidal sand bars. The lack of calcareous fossils is probably due to post-depositional leaching in the uncemented sediments; but the scarcity of burrows and the lack of fauna may also reflect a reduced overall salinity, or the often unstable (thixotropic) substrate conditions. The presence of tidally influenced sediments on the margins of the Lias epeiric sea in Europe demonstrates that tidal effects should not be ignored when considering aspects of epicontinental sedimentation in the Mesozoic of Northern Europe.