computer analysis of regional groundwater flow and boundary conditions in the basin of mexico abstract mexico city is situated in the valley of mexico on the extensive lacustrine clays that overlay highly productive aquifers of both volcanic and sedimentary origin the valley is closed by volcanic mountains the natural hydraulic boundary conditions associated withe mountain ranges and their relationship to the important aquifers were studied using a two dimensional steady state finite element model in cross section four cross sections were analysed under hydrologic conditions existing prior to the large scale pumping of the aquifers factors such as bulk hydraulic conductivities and regional infiltration rates were obtained from field observations and the literature to assess location of the associated groundwater divides and the water table in the mountains the modeled flow patterns are consistent with the historical hydrologic records piezometric characteristics and observed surface features of the groundwater in the basin of mexico from the modeling results the groundwater recharge in the mountains is 30 50 of the mean average precipitation higher and lower rates result in a flow regime that is not compatible with field observations in general the location of the divides in the mountains is displaced towards the valley of mexico which influences the groundwater budget of the valley the water table in places is several hundred metres below ground surface in accordance with field observations of a very thick unsaturated zone before major aquifer exploitation began about 50 years ago 40 50 of the total discharge into the valley was by upward flow through the lacustrine deposits the best results were obtained using a subsurface distribution of hydrostratigraphic units based on recently published geological interpretations

the use of velocity and conductivity data for the quantification of heterogeneity a comparison hydraulic conductivities of a portion of the twin lake aquifer are derived from grain size analyses performed at 3 cm vertical sampling intervals in four boreholes spaced l m apart correlation of conductivity logs between four boreholes leads to a geometrical model which corresponds closely to the spatial structure derived from the velocity field of previous tracer tests histogram and correlogram analyses of conductivities agree closely with those of velocities and the two data sets appear equally useful for characterizing the aquifer and its spatial variability using darcy s law flow velocities and conductivities are combined to calculate a theoretical hydraulic gradient comparison to gradients observed at the site reveals agreement on a large scale however differences exist in detail because observed piezometric heads fail to reflect small scale heterogeneity since these differences are significant for contaminant transport prediction it is recommended for future studies to directly measure flow velocities from a tracer experiment rather than to reply on head measurements and on darcy s law for the calculation of transport parameters

characterization of aquifer heterogeneity by in situ sensing in analyzing groundwater flow and mass transport most commonly the heterogeneity of a geologic formation is quantified by the field mapping of the hydraulic conductivity distribution in this paper two alternative approaches are proposed according to the first nonparametric approach aquifer heterogeneity is characterized in terms of spatial variability in the concentration field generated in a natural gradient tracer test with a high energy gamma emitting radiotracer in situ sensing technology is used for mapping the concentration distribution and collected concentration versus depth profiles are correlated to produce the local scale geometric model of aquifer heterogeneities for further numerical groundwater flow and mass transport modeling in the second parametric approach heterogeneity is characterized in terms of spatial variability of the groundwater velocity rather than in terms of spatial variability of the hydraulic conductivity geostatistics is used to characterize the local scale velocity variation in terms of velocity integral scales in three orthogonal directions in the longitudinal along the mean flow and two transverse across the flow directions the velocity correlation volume is estimated as 16 68 2 0 22 m it defines the characteristic volume for the field scale groundwater flow and mass transport modeling it was concluded that the field mapping of aquifer structure by measuring concentrations and travel times of a radiotracer leads to a fundamentally new approach to the characterization of geologic heterogeneity and offers a reliable means for solving this complex problem of contemporary hydrogeology in quantitative terms

pb isotopic evidence for an early enriched crust on mars abstract martian meteorite nwa 7533 is a regolith breccia that compositionally resembles the martian surface measured by orbiters and landers nwa 7533 contains monzonitic clasts that have zircon with u pb ages of 4 428 ga the pb isotopic compositions of plagioclase and alkali feldspars as well as u pb isotopic compositions of chlorapatite in the monzonitic clasts of nwa 7533 have been measured by secondary ion mass spectrometry sims the u pb isotopic compositions measured from the chlorapatite in nwa 7533 yield an age of 1 357 81 ga 2 the least radiogenic pb isotopic compositions measured in plagioclase and k feldspar lie within error of the 4 428 ga geochron these data indicate that the monzonitic clasts in nwa 7533 are a product of a differentiation history that includes residence in a reservoir that formed prior to 4 428 ga with a value 238 u 204 pb of at least 13 4 1 7 2 and a value 232 th 238 u of 4 3 this value is more than three times higher than any other documented martian reservoir these results indicate either the martian mantle is significantly more heterogeneous than previously thought value of 1 14 vs 1 5 and or the monzonitic clasts formed by the melting of martian crust with a value of at least 13 4 therefore nwa 7533 may contain the first isotopic evidence for an enriched differentiated crust on mars

origin and movement of groundwater and major ions in a thick deposit of champlain sea clay near montreal at a site near montreal piezometers and core samples were used to assess the origin age and rate of groundwater movement in a 28 m thick deposit of champlain sea clay 3h occurs only within the upper 3 4 m which indicates that groundwater below this surficial weathered zone originated prior to 1952 concentrations of 18o na and cl in water from the deepest piezometer suggest that the deep clay was deposited in a mixture of about 33 seawater and 67 freshwater profiles of several major ions show a gradual increase in concentration with depth mathematical simulations of vertical migration of ca2 mg2 k na and cl provide close matches to the field profiles when only upward diffusion of these ions into the surficial freshwater zone is included in the model with a diffusion time of approximately 10 000 a when the downward advection rate of 0 13 cm a 1 obtained from field measurements of hydraulic gradient and hydraulic conductivity is included in the model the simulated major ion profile

numerical simulations of twin lake natural gradient tracer tests a comparison of methods the primary objectives of the twin lake experiments have been to study geologic heterogeneities and field scale dispersion and to provide data for developing and evaluating groundwater flow and transport models the experiment reported here was undertaken in 1987 1988 to expand the studies of heterogeneity and dispersion from a 40 m scale investigated in 1983 to a full 270 m scale from the point of tracer injection to the groundwater discharge zone the hydrogeologic setting the experimental and analytical methods and the interpretation of experimental data are the topics of discussion in this paper the 1983 and 1987 1988 tracer tests were used to evaluate the predictive abilities of various numerical groundwater flow and transport models the modeling is done along the mean flow direction in the vertical plane the two dimensional finite element modeling was successful in simulating the hydraulic head distribution but not the groundwater motion or the tracer transport in addition owing to the advection dominated nature of the groundwater flow the transport simulation suffered from numerical dispersion significant improvements in simulations were obtained using method of characteristics and random walk based computer models the influence of dimensionality on the transport simulation was evaluated by comparing the two and three dimensional random walk simulations it was found that over the first 40 m along the mean flow path the three dimensional simulation does not reproduce the plume migration any better than the two dimensional simulation does random walk simulations of the transport observed in the 1983 and 1987 1988 tests are also done on the basis of the advection dispersion equation and tracer test derived velocities without invoking the concept of hydraulic conductivity and darcy s law these simulations were found to be superior to simulations based on the coupled flow and transport models it was also found that the simulated hydraulic head distribution is insensitive to spatial variations in hydraulic conductivity and the simulated groundwater velocity distribution is inappropriate for transport modeling it was concluded that the advection dispersion models of the local scale transport perform better in predicting the tracer migration at the twin lake aquifer using measured velocities rather than velocities simulated by a flow model

groundwater contamination at the kesterson reservoir california 1 hydrogeologic setting and conservative solute transport shallow groundwater under kesterson reservoir merced county california was contaminated over a period of 6 years while the reservoir was used for disposal of brackish subsurface agricultural drainage water from farmlands in the san joaquin valley drainage water contained an average of 10 000 mg l total dissolved solids tds high concentrations of selenium 300 g l nitrate 37 mg l as nitrogen boron 15 mg l and lesser amounts of mercury chromium and molybdenum infiltration and lateral migration of this water created a 20 m deep plume of high tds boron rich water covering an area of about 5 km2 transport of selenium and nitrate was inhibited by biogeochemical reactions in the pond bottom sediments so that migration of these two constituents into the aquifer was limited this paper focuses on physical parameters affecting seepage and lateral migration of nonreactive solutes and on hydrogeological factors associated with selenium break through into the shallow aquifer an accompanying paper describes biogeochemical processes that limit nitrate and selenium migration white et al this issue the regional hydrogeologic system at kesterson reservoir is dominated by the presence of four lithostratigraphic units these are in order of increasing depth a fine grained surfical layer extending to depths of as much as 8 m a semiconfined sandy aquifer extending to 60 m a widespread lacustrine clay confining layer and a confined sandy aquifer within each of these units order of magnitude variations in the hydrologic properties of the sediments occur ubiquitously over lateral distances of tens of meters and vertical distances of a fraction of a meter seepage rates from the ponds into the underlying aquifer are dominated by local hydrogeologic properties and thickness of the fine grained surface layer selenium migrated into the aquifer in an area where the fine grained surface layer was thin or absent once in the aquifer local rates of migration are controlled by the small scale structure and hydrogeologic properties of the sediments the aquifer is composed of a heterogeneous mixture of sands silts and clays individual sand beds which average about 0 3 m thick do not appear to extend over lateral distances of more than 30 m the large range of velocities combined with a limited lateral extent of individual layers are expected to create a broadly dispersed plume of solutes migrating downgradient from kesterson a noninvasive electromagnetic survey of electrical conductivity distributions in the shallow aquifer appears to confirm this conceptual model for transport in the shallow aquifer

use of vertical profiles of δ18o to constrain estimates of hydraulic conductivity in a thick unfractured aquitard vertical distributions of 18o were used to constrain hydraulic conductivity k in a thick unfractured aquitard previous investigations utilizing permeability tests at a variety of scales keller et al 1989 indicated that bulk k at the site ranges from 3 5 0 11 m s to 1 2 10 10 m s advective displacements calculated on the basis of these k values lead to a wide range of predicted distributions of natural tracers after 11 000 years comparison of 18o distributions and simulated advection diffusion profiles provide a more precise constraint on advective displacement and hence on k the simulations cast in terms of a peclet number p v2t d ranging in value from 0 to 12 indicate that transport is diffusion controlled and that k is actually less than 1 2 10 11 m s p greater than 0 1 k 1 2 10 11 m s overestimates transport this result indicates that conventional permeability tests in very tight media tend to overestimate k in practical terms advective transport at the site is negligible and transport can be simulated with diffusion only p o

tritium as an indicator of recharge and dispersion in a groundwater system in central ontario the detailed distribution of tritium 3h in the recharge area of a shallow unconfined sand aquifer near sturgeon falls ontario is described at this forested shallow water table site bomb tritium has penetrated uniformly to a depth of 8 12 m indicating recharge of 15 cm year which is 16 of precipitation the zone of bomb tritium contains 3h concentrations of from 16 to 269 tritium unit tu whereas much lower values 1 tu are observed in groundwaters recharged only a few years prior to 1953 the year when significant fallout of bomb tritium began tritium distribution is accurately simulated using one and two dimensional models when the ottawa record is used for the post 1953 tritium input function when dispersion is low and when a prebomb tritium input value of 3 tu is used this prebomb value is slightly lower than that suggested by kaufman and libby 1954 based on precipitation and surface water samples from the early 1950s the simulations indicate and field data corroborate that in 1986 3h levels of more than 100 tu occur only in groundwaters recharged between 1957 and 1971 while levels in excess of 200 tu occur only in groundwaters recharged during 1961 1967 the 1960s tritium peak is observed within a narrow distinct depth zone at all locations along the flow section investigated

flow of variable density formation water in deep sloping aquifers minimizing the error in representation and analysis when using hydraulic head distributions abstract although not fully adequate freshwater hydraulic heads have been used historically to represent and analyze variable density flow in sloping aquifers in sedimentary basins the use of environmental heads is valid only for strictly vertical flow in unconfined aquifers while using variable density hydraulic heads contravenes darcy s law although the use of hydraulic head surfaces is the simplest and quickest means of flow analysis and interpretation preceding other methods such as numerical modeling it introduces some errors that should be assessed and minimized in order to provide the most accurate flow representation a first error is introduced when approximating the potential and buoyancy components along aquifer slope of the flow driving force with their projections onto the horizontal plane this error is most probably negligibly small for sloping aquifers in undisturbed sedimentary basins but may be significant for aquifers dipping at a significant angle such as in folded strata a second error is introduced when using only hydraulic heads in the representation and analysis and neglecting the buoyancy component of the flow driving force the significance of this error can be assessed by performing a driving force ratio dfr analysis there is no single or critical value of the dfr below which the error in using hydraulic heads alone is negligible and above which it is not acceptable anymore rather the decision regarding the error acceptability should and can be made on a case by case basis the dfr hence the errors in flow direction and magnitude can be minimized for any given aquifer by using an optimum reference density in hydraulic head calculations that is the areally weighted average density of formation water in that aquifer in flow analyses based on potentiometric surfaces the use of freshwater as the reference density actually maximizes the errors introduced by the neglect of the buoyancy component of the flow driving force because it is either at the low end of the density variation spectrum or outside it

saline groundwaters in the canadian shield a first overview abstract the chemical and isotopic compositions of saline groundwaters and brines in the canadian shield are described the occurrence of such waters is very widespread not related to any specific rock type or mineralization and generally can be expected at depths below 1 km the most saline brines have salinities exceeding 200 g l 1 dissolved solids and their chemistries tend to suggest that in the past saline waters penetrated the canadian shield the original chemistry of these solutions was subsequently modified through strong rock water interactions and it appears that their final chemical compositions are independent of the original chemistries stable isotope analyses indicate that these waters are not simple evaporated seawater sedimentary basin brines hydrothermal solutions or residual ore forming fluids although any of these could be a precursor of the brines secondary isotope exchange reactions and the hydration of primary silicates accompanied by the formation of clay minerals could have modified not only the primary isotopic composition but may have also profoundly influenced the chemistries of these brines to be effective these processes would have had to occur in closed systems

simulation of groundwater flow in a crystalline rock aquifer system in southern ghana an evaluation of the effects of increased groundwater abstraction on the aquifers using a transient groundwater flow model monitored groundwater level data well logs and aquifer data as well as the relevant surface hydrological data were used to conceptualise the hydrogeological system of the densu basin in southern ghana the objective was to numerically derive the hydraulic conductivity field for better characterization of the aquifer system and for simulating the effects of increasing groundwater abstraction on the aquifer system in the basin the hydraulic conductivity field has been generated in this study through model calibration this study finds that hydraulic conductivity ranges between a low of 2 m d in the middle sections of the basin and about 40 m d in the south clear differences in the underlying geology have been indicated in the distribution of aquifer hydraulic conductivities this is in consonance with the general assertion that the hydrogeological properties of the aquifers in the crystalline basement terrains are controlled by the degree of fracturing and or weathering of the country rock the transient model suggest aquifer specific storage values to range between 6 0 10 5 m 1 and 2 1 10 4 m 1 which are within acceptable range of values normally quoted for similar lithologies in the literature there is an apparent subtle decrease in groundwater recharge from about 13 of the annual precipitation in 2005 to about 10 3 of the precipitation in 2008 the transient model was used to simulate responses of the system to annual increment of groundwater abstraction by 20 at the 2008 recharge rates for the period 2009 2024 the results suggest that the system will not be able to sustain this level of abstraction as it would lead to a basin wide drawdown in the hydraulic head by 4 m by the end of the prediction period it further suggests a safe annual increment in groundwater abstraction by 5 under business as usual recharge conditions identification and protection of groundwater recharge areas in the basin are recommended in order to safeguard the integrity of the resource under the scenario of increased abstraction for commercial activities in the basin copyright 2012 john wiley sons ltd

stream functions and equivalent freshwater heads for modeling regional flow of variable density groundwater 2 application and implications for modeling strategy stream functions and equivalent freshwater heads are used to simulate steady state flow of variable density groundwater in a regional cross sectional flow model through the palo duro basin texas where fluid densities vary between 1 0 and 1 15 g cm3 centroid consistent velocities computed from the stream function solution allow a more precise interpretation of local flow patterns in cross sectional models than those from the head solution effects of significant fluid density variation on the regional groundwater flow pattern are studied by comparing simulation results that incorporate spatially varying time invariant densities with those that assume uniform density modeling shows that the regional groundwater flow pattern in the palo duro basin is not significantly affected by variations in fluid densities indicating that the topographically driven flow component dominates buoyancy forces associated with dense brines an exception is near the eastern boundary where high fluid densities cause stronger downward flow however simulated equivalent freshwater heads in the variable density model differ significantly from simulated heads in the freshwater density model which is important for model calibration in addition to the well known problems of hydraulic parameter and boundary condition uncertainties modeling strategies for regional flow systems require consideration of uncertainties associated with fluid density and evaluation of equivalent freshwater head data

zircon solubility in alkaline aqueous fluids at upper crustal conditions field evidence and thermodynamic data at ambient conditions suggest that complexing of zr with hydroxyl ions at high ph enhances the solubility of zircon we tested this hypothesis by measuring the solubilities of the assemblages zircon zrsio 4 baddeleyite z b and zircon quartz z q in neutral to alkaline fluids at 0 2 gpa and 450 750 c in neutral to alkaline fluids 0 0 1 1m naoh zircon dissolved incongruently in quartz undersaturated fluids to form baddeleyite at 450 c various na zr silicates precipitated from fluids saturated and undersaturated in quartz we observed no significant dependence of solubility on temperature the measured solubilities of zircon in pure h 2o at 600 and 750 c are only slightly higher than the procedural blank of 3 3 10 7 m zr however the measured solubility of zircon increases with increasing aqueous silica suggesting that zirconium complexes with silica in the fluid at 600 c linear regression of experimental results yielded ln zr 0 25 2 9 ln si where terms in parentheses represent molal concentrations zircon solubility also seems to increase with increasing hydroxyl concentration in the fluid enhanced solubility in silica rich alkaline fluids may cause dissolution reprecipitation of zircon and resetting of its isotopic clocks suggesting that some zircon u pb ages may correspond to fluid events

groundwater flow and solute transport in fractured lacustrine clay near mexico city a network of piezometers was installed in a surficial lacustrine clay aquitard overlying a thin saline water aquifer of volcanoclastic origin at a study site near mexico city in the basin of mexico the aquifer is underlain by additional lacustrine sediments which in turn overlie a thick regional freshwater aquifer the regional aquifer provides approximately 70 of the water supply for 20 million people in the basin of mexico in the study area major ions oxygen 18 and deuterium in the pore water of the surficial aquitard exhibit large variations with depth the nature of these variations suggests that the saline pore water is being displaced downward by infiltrating meteoric water the infiltration has been induced by strong downward hydraulic gradients imposed two to three decades ago when heavy aquifer pumping of the thin saline water aquifer began one dimensional analytical models representing solute transport in both fractured and unfractured porous media were used to simulate the geochemical profiles in the surficial aquitard the fractured porous medium model using a realistic mean hydraulic gradient and fracture spacing 1 5 m and small but significant fracture aperture 30 m provide nearly an exact match to the field data from this we infer that because of vertical fractures there is a much greater potential for downward leakage of water and contaminants through the mexico city clay into underlying aquifers than has been previously thought

sedimentation and extensional basin evolution in a miocene metamorphic core complex setting alvord mountain central mojave desert california usa the lower miocene clews formation at alvord mountain mojave desert california comprises an upward coarsening sequence of synextensional continental deposits of variable thickness this sequence pinches out to the west against the alvord mountain pre tertiary basement complex and thickens eastward to c 300 m over a distance of 7 km two stages of sedimentation are recognized in the formation during the initial stage of sedimentation the depocentre was delineated by a lacustrine system that was bounded to the west by small sheetflood dominated alluvial fans and to the north by a southerly flowing fluvial braidplain lacustrine mudstone siltstone and carbonate suggest an initially closed basin conglomerate to the west represents small sheetflood dominated fans igneous clasts and a south east palaeoflow direction suggest a proximal low relief source in the western alvord mountain area pebbly sandstone of the southward prograding braidplain has a metasedimentary provenance in the paradise range to the north the second stage of deposition was dominated by coarse conglomerate and breccia west to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the cronese hills 8 km to the east these strata reflect rapid westward progradation of sheetflood and debris flow dominated alluvial fans that advanced across the axial braidplain and lacustrine system basin development is interpreted to have been controlled by regional ne directed extension on a detachment fault associated with the central mojave metamorphic core complex 30 km to the west the alvord mountain cronese hills region was initially transported as a single hangingwall block on the e dipping detachment initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall subsequent westward progradation of alluvial fans out of the cronese hills is believed to record the propagation of a nw striking sw dipping normal fault antithetic to the low angle e dipping detachment fault

the occurrence and composition of chevkinite ce and perrierite ce in tholeiitic intrusive rocks and lunar mare basalt chevkinite ce and perrierite ce are the most common members of the chevkinite group of minerals they are dimorphs and both have the general formula a4bc2d2si4o22 where a ree y ca sr th b fe2 mn mg c ti al fe3 fe2 cr mn mg zr hf nb and d ti both have been reported from a wide range of igneous metamorphic and hydrothermal rocks types but occurrences in mafic rocks are rare with minimal chemical and crystallographic documentation chevkinite ce and or perrierite ce occur with other ti zr and ree bearing accessory phases in eight suites of tholeiitic dolerite from western australia and in lunar mare basalt 10047 they are more abundant than has been recognized previously in mafic igneous rocks and they are significant hosts of incompatible elements chevkinite ce and perrierite ce from mafic rocks have distinctive chemical compositions with higher zr than recorded in examples from most other common rock types among mafic rocks two groups are recognized based on total fe contents in electron microprobe analyses crystal structural analysis by electron diffraction indicates that the high fe group 8 wt feo is chevkinite ce while the low fe group 8 wt feo is consistent with perrierite ce and both minerals can occur within a single hand specimen a previously proposed chemical discriminant is not applicable to chevkinite group minerals from typical mafic igneous rocks and crystal structural information is required to unequivocally distinguish between the two dimorphs

fracture effects in the shallow groundwater zone in weathered sarnia area clay the sarnia area of southwestern ontario is underlain by thick deposits of clay rich glacial till from ground surface to between 4 and 6 m depth the clay till is fractured oxidized and penetrated by root holes the water table fluctuates seasonally between the ground surface and the bottom of the weathered zone water level response tests in conventional piezometers installed in augered holes in the weathered zone typically provide very low values of hydraulic conductivity 10 7 10 9 cm s which are similar to values obtained from the deeper unweathered and unfractured till the augering process creates a smeared clay zone along the borehole walls that significantly reduces measured hydraulic conductivity values in this study the weathered zone hydraulic conductivity was measured using unconventional piezometers a large diameter well and a tracer experiment between two test pits the smeared zone was removed or reduced around these installations and the measured weathered zone hydraulic conductivi

groundwater flow modeling and particle tracking for chemical transport in the southern voltaian aquifers groundwater is the major source of water supply for most uses in the rural settlements in ghana a groundwater flow model was calibrated for some aquifers of the southern voltaian sedimentary system under steady state conditions the objective was to determine estimates of the hydraulic conductivities of the different hydrostratigraphic units of the southern voltaian and the distribution of recharge from precipitation data on the stable isotopes of oxygen and hydrogen from the study area suggest that groundwater recharge in the area is from rainfall the calibrated steady state model suggests that aquifer hydraulic conductivities in the study area range from 1 19 to 6 3 m day hydrostratigraphic unit specific hydraulic conductivities are discussed the calibrated recharge ranges from 3 81e 05 m day to 6 0e 04 m day which represents 0 9 6 of the precipitation in the form of rainfall six distinct flowpaths have been defined using particle tracking the particle tracking simulation suggests travel times in the range of 380 to 5 199 years from recharge areas to discharge areas along the flowpaths identified a contaminant dropped at the recharge areas in the central sections of the model area would travel at these rates along the flowpaths assuming that advection is the dominant transport process inverse geochemical modeling indicates the dissolution of albite k feldspars and anorthite respectively along flowpaths i and iv the inverse modeling along flowpaths i and iv suggest the dissolution of albite k feldspar and anorthite respectively at 1 085e 06 3 16e 08 and 3 168e 07 mmol year

a borehole methodology for hydrogeochemical investigations in fractured rock research into the development of borehole techniques for hydrogeochemical investigations in fractured rock has resulted in the construction of a geochemical probe controlled and operated with an umbilical cable system this probe has been field tested to a depth of 600 m in a multilevel ported casing at the chalk river nuclear laboratories crnl the probe provides in situ measurements of ph redox potential dissolved oxygen temperature conductivity and pressure while allowing groundwater sample collection both at surface and downhole the multilevel ported casing preserves the subsurface hydrogeochemical zonation and facilitates sampling at various depths in the crnl borehole dilute na hco3 and na cl waters ranging in age from 500 to 12 000 years have been observed in gneissic rock at depths of 85 to 350 m however accurate hydrogeochemical characterization of the low permeability metagabbro present from 400 to 600 m has been frustrated by contamination from casing installation water

groundwater flow velocity and age in a thick fine grained till unit in southeastern wisconsin abstract piezometer nests were installed at study sites in each of five north south trending end moraines of the late pleistocene oak creek formation in southeastern wisconsin the formation is composed primarily of a fine grained glacial diamicton till and laterally continuous and discontinuous coarse grained lake and meltwater stream sediment it overlies the silurian dolomite aquifer which is a source of drinking water to rural areas the average vertical linear velocity and age of ground water in the oak creek formation were estimated by three methods darcy s law environmental isotopes including 3 h 2 h 18 o and 14 c dissolved inorganic carbon and solute transport modeling of 18 o the f 1 and metro sites in the tinley moraine showed excellent agreement among the three estimates of vertical velocity and showed the lowest velocity values 0 3 0 5 cm year 1 downward which suggests that diffusion controls vertical mass transport at these sites although the extrapolated maximum age of ground water is 35 000 years ground water below 75 m at these sites is probably not older than 15 000 years which is the maximum age of the formation estimates of velocity showed less agreement at study sites in the lake border moraine system to the east and ranged from about 0 2 to 20 7 cm year 1 maximum groundwater age could range from 213 to 6000 years higher and more variable velocities perhaps owing to thinner and more heterogeneous sediment in these areas suggest that diffusion may not dominate vertical mass transport heterogeneity and fractures may also promote the development of groundwater flow systems dominated by lateral flow because of the uncertainty about the nature of groundwater flow velocity and age in the formation east of the tinley moraine future waste disposal activity in southeastern wisconsin should be confined to the thickest parts of the tinley moraine near the present f 1 and metro sites

stream functions and equivalent freshwater heads for modeling regional flow of variable density groundwater 1 review of theory and verification steady state flow of variable density groundwater is simulated using equivalent freshwater heads and stream functions on the basis of the work by de josselin de jong 1960 1969 fluids with different fluid densities are replaced by one hypothetical fluid and singularities are introduced along interfaces where the actual fluids change densities the basic flow equation describing variable density flow written in terms of equivalent freshwater head is used to derive the corresponding stream function equation and associated boundary conditions neumann boundary conditions for the stream function equation can be determined from gradients of equivalent freshwater heads along the boundary stream functions provide a direct representation of the groundwater flow pattern and flow rates where fluid densities vary in space in comparison equivalent freshwater heads and fluid densities describe the two driving forces hydraulic gradient and buoyancy force but head gradients do not necessarily describe the flow direction of variable density groundwater in isotropic media comparison of the finite element formulations indicates that potential errors in the centroid consistent velocity calculation based on the stream function solution can be expected to be smaller than those in the velocity calculation based on the head solution for cross sectional flow models because discretization in the vertical direction is typically finer than in the horizontal direction

analytical solution of water table fluctuations above an inclined leaky layer due to ditch recharge ditch recharge is a method used for raising a water table however if the aquifer lies above a leaky layer then the leaky bed will have an effect on the height of the water table this paper attempts to provide an analytical solution of the transient state to the linearized boussinesq equation the solution proposed in this paper is intended for a generalized form of the leaky layer with the boundary condition of the water level moving according to an exponential form the results of theoretical analysis and experimental studies can provide a much better explanation of ditch recharge on an inclined leaky layer copyright 2006 john wiley sons ltd

hillslope infiltration and lateral downslope unsaturated flow lateral downslope vadose flow occurs in hillslopes with deep homogeneous soils when and why this happens has been partially answered by past experimental numerical and quasi analytical studies what has been overlooked however is that lateral downslope flow in unlayered soils is largely a drainage phenomenon driven by the changing character of the boundary condition at the soil surface numerical simulations show that when rainfall ceases the near surface head gradient in a hillslope becomes nearly parallel to the slope as a simple consequence of the surfaces becoming a no flow boundary neglecting evaporation during rainfall infiltration is nearly vertical for most soils except those with high constant anisotropy

the groundwater recharge regime of some slightly metamorphosed neoproterozoic sedimentary rocks an application of natural environmental tracers isotope data of precipitation and groundwater in parts of the voltaian basin in northern ghana were used to explain the groundwater recharge regime in the area groundwater recharge is an important parameter in the development of a decision support system for the management and efficient utilization of groundwater resources in the area it is therefore important to establish the processes and sources of groundwater recharge 18o and 2h data for local precipitation suggest enrichment relative to the global meteoric water line gmwl and indicate that precipitation takes place at a relative humidity less than 100 the groundwater data plot on an evaporation line with a slope of 5 suggesting a high degree of evaporative enrichment of the precipitation in the process of vertical infiltration and percolation through the unsaturated zone into the saturated zone this finding is consistent with the observation of high evapotranspiration rates in the area and ties in with the fact that significant clay fraction in the unsaturated zone limits vertical percolation and thus exposes the percolating rainwater to the effects of high temperatures and low humidities resulting in high evapotranspiration rates groundwater recharge estimates from the chloride mass balance cmb method suggest recharge in the range of 1 8 32 of the annual average precipitation in the form of rainfall the highest rates are associated with areas where open wells encourage significant amount of groundwater recharge from precipitation in the area in the northern parts of the study area groundwater recharge is lower than 12 the recharge so computed through the application of the cmb methodology takes on a spatial distribution akin to the converse of the spatial pattern of both 18o and 2h in the area as such the locations of the highest recharge are associated with the most depleted values of the two isotopes this observation is consistent with the assertion that low vertical hydraulic conductivities slow down vertical percolation of precipitation down to the groundwater water the percolating precipitation water thus gets enriched in the heavier isotopes through high evapotranspiration rates at the same time the amount of water that finally reaches the water table is considerably reduced copyright 2013 john wiley sons ltd

inventory of h2o in the ancient martian regolith from northwest africa 7034 the important role of fe oxides water rich martian regolith breccia northwest africa nwa 7034 was analyzed by fourier transform infrared spectroscopy and transmission electron microscopy to determine the inventory and phase distribution of h2o used herein to refer to both molecular h2o and oh structural components in hydrous minerals hydrous fe oxide phases hydromaghemite and an unidentified nanocrystalline fe bearing oxide phase observed with hydromaghemite and phyllosilicates saponite were identified as the primary mineralogic hosts for h2o with a minor contribution from cl rich apatite based on mass balance calculations and modal abundances of minerals constrained by powder x ray diffraction and petrography we can account for the entire 6000 ppm h2o measured in bulk rock analyses of nwa 7034 this h2o is distributed evenly between hydrous fe rich oxides and phyllosilicates indicating that fe oxides could be as important as phyllosilicates for h2o storage in martian surface material

origin age and movement of pore water in argillaceous quaternary deposits at four sites in southwestern ontario hydrogeologic information in the form of textural properties field and laboratory determined hydraulic conductivity values vertical hydraulic gradients from piezometer nests and major ion and isotopic contents 18o 2h 3h 13c 14c was obtained from four sites located in southwestern ontario on thick deposits of clayey till and glaciolacustrine clay at each site the piezometers were installed from the water table zone situated 1 4 m below the surface to a maximum depth between 20 and 35 m below the surface values of hydraulic conductivity determined in the laboratory by consolidometer and triaxial methods are in the order of 10 8 10 7 cm s and are similar to values calculated from rates of water level rise in piezometers mean values of average vertical groundwater velocity calculated with the measured values of hydraulic conductivity porosity and hydraulic gradient using the darcy equation for a saturated non fractured porous medium range from 0 13 to 0 26 cm yr tritiated pore waters only occur within 3 6 m of ground surface thereby indicating that groundwaters below this depth recharged prior to at least 1952 18o in the pore waters exhibit a distinctive regular shift with depth from water table values of 9 to 10 smow characteristic of present day precipitation to values between 14 and 17 at depths of 20 40 m which are characteristic of much cooler waters corrected 14c ages of the groundwater are greater than 8000 yr b p thereby suggesting that the groundwaters at depth entered these deposits during or after the formation of these deposits approximately 11 000 to 14 000 yr ago 18o 2h and cl concentration profiles were simulated with a one dimensional model for transport by advection and diffusion in a saturated porous medium reasonable agreement between the model and the field profiles was obtained with values of effective diffusion coefficients of 3 0 10 6 cm2 s for 18o 2h and cl and with groundwater velocities between 0 03 to 0 05 cm yr based on these results it is concluded that the pore water in these deposits is a mixture of late pleistocene and modern waters and that the distribution of 18c 2h and cl in these deposits is influenced predominantly by molecular diffusion apparently more so than by hydraulic flow

37cl 35cl variations in a diffusion controlled groundwater system abstract groundwaters in the low permeability quaternary glacial deposits of southwestern ontario commonly exhibit increasing cl concentrations with depth because these deposits were formed in a freshwater glacial environment and because cl is not derived from weathering of these deposits previous investigations hypothesized that upward diffusion of saline formation waters from the shallow bedrock caused this cl trend this hypothesis was tested by measuring 37 cl 35 cl ratios in the groundwaters from two typical sites and comparing the results with theoretically derived isotopic distributions produced by diffusion 37cl measurements expressed relative to standard mean ocean chloride smoc ranged from 0 8 to 2 2 and vertical profiles showed a gradual enrichment in 35cl relative to 37cl with increasing distance upward from the bedrock till interface 537cl distributions were simulated with a mathematical solute transport model incorporating the effects of kinetic isotopic partitioning a sensitivity analysis indicated that the model results were affected by the groundwater velocity the isotope enrichment factor and diffusion times agreement between the simulated and observed cl isotope profiles was obtained using a groundwater velocity of 0 an isotopic enrichment factor of 1 0012 and diffusion times ranging from 14 000 to 16 000 years based on these results we conclude that the isotopic enrichment in 35cl was caused by upward diffusion over a period of about 15 000 years this diffusion period is consistent with what is known about the quaternary history of the area

petrology depositional environments and structural development of the mineta formation teran basin cochise county arizona abstract the oligocene mineta formation of the teran basin in southeastern arizona contains approximately 1000 m of interfingering alluvial fan fluvial and lacustrine strata with minor intercalated volcanics sedimentation began prior to the middle oligocene and ended with the eruption of the oligocene miocene galiuro volcanics about 28 m y ago several related depositional facies are present in a cyclic pattern within the mineta formation a complete cycle contains proximal coarse grained alluvial fan conglomerates that fine upward into sandstone and pebble conglomerate of a distal fan or braided stream environment medium to fine grained sandstones are interpreted as sandflat deposits that lie stratigraphically above the distal fan deposits and below lacustrine sandstone and algal limestone the cycle culminates with deeper water lacustrine mudstone with intercalated thin bedded turbidite sandstone discrete cycles of bedded gypsum and mudstone are found locally within the lacustrine deposits these deposits represent a temporary playa lake phase of the mineta depositional system the mineta formation filled a half graben that developed in response to extensional stresses associated with the development of the rincon catalina metamorphic core complex paleocurrents flowed southwestward from a source area of predominantly sedimentary and volcanic rocks sedimentation was contemporaneous with extensional deformation that produced low angle normal faults and tilted the section toward the northeast maximum extension was directed about n55 e s55 w roughly parallel to the direction of paleocurrent flow parallel to the mean slip direction for low angle normal faults and perpendicular to the strike of the mineta section

constraining permeabilities in a large scale groundwater system through model calibration abstract the range of field estimates of permeabilities in rocks of the atikokan research area ara canada has been reduced by calibrating a groundwater flow model of the ara the flow model was calibrated to the recharge component of the water balance and to measured freshwater heads in boreholes a good match between field and simulated recharge and between measured and simulated heads was obtained with two different permeability depth distributions that differed by up to two orders of magnitude in places solute transport simulation of the distributions of total dissolved solids tds chlorides cl and oxygen 18 18 o in groundwater in the area was performed to further refine the permeability estimates vertical average linear groundwater velocities from the calibrated flow model were used in one dimensional solute transport models of the system to generate depth concentration profiles of tds cl and 18 o recharge midline and discharge area models of both the fracture zones and the rock mass were employed the midline area is the narrow zone that separates the recharge downward flow area from the discharge upward flow area groundwater flow is essentially horizontal in the midline area envelopes formed by the simulated profiles that were created with the velocities of one simulation case more tightly enclosed the field data tds cl 18 o than those of another simulation case indicating that velocities of the former simulation case better represent the hydrodynamics of the real system the models demonstrate that the scatter of tds cl and 18 o field data for the ara is consistent with the groundwater flow model predictions and can be explained by the complexity arising from different hydraulic regimes recharge midline discharge and hydrogeologic environments fracture zones rock mass of the regional flow system solute transport simulation of hydrochemical constituents can aid in refining and constraining permeabilities in large scale groundwater systems

regional scale groundwater flow in a canadian shield setting a three dimensional numerical analysis of a 5734 km2 watershed situated in the canadian shield has been conducted to illustrate aspects of regional scale groundwater flow in a crystalline rock setting an essential requirement of the analysis is the preservation and accurate description of the complex topography surface water drainage network groundwater salinity distribution and permeability distributions the postglacial evolution of the groundwater flow system was investigated using the finite difference model swift iii robustness was assessed by exploring the sensitivity of groundwater flow to topography variable matrix permeability distribution models pore water salinity and the dissipation of elevated initial pore pressures that result from ice that overlaid the watershed in the last glacial period groundwater flow analyses indicate that freshwater heads in all model layers are highly correlated with the complex surface topography such that the transition from zones of groundwater recharge t

numerical analysis of solute migration through fractured clayey deposits into underlying aquifers this study examines the degree to which vertically fractured clayey aquitards protect an underlying aquifer from near surface sources of contamination evidence from field studies indicates that many clayey aquitards previously assumed to be unfractured at depth have vertical fractures that actively transmit groundwater calculated groundwater velocities within these fractures can exceed several meters per day for apertures in the range of those reported to occur in clays if this is the case significant quantities of dissolved contaminants originating in near surface zones may move rapidly downward through the fracture network into underlying aquifers a two dimensional laplace transform galerkin finite element model sudicky and mclaren this issue was used to assess the importance of idealized planar vertical fractures on flow and solute transport through a 15 m thick aquitard and subsequent plume evolution within an underlying aquifer in which groundwater flows horizontally sensitivity analyses indicate that solute transport through the aquitard is strongly affected by downward flow in widely spaced deep fractures with apertures as small as 10 m and that a plume of large lateral extent and significant concentration forms within the underlying aquifer in a few tens of years for apertures of the order of 20 m or much sooner if the apertures are 50 m this finding suggests that small hydraulically active fractures that are very difficult to detect or identify within an otherwise protective clayey barrier may cause rapid and large scale contamination of groundwater beneath the barrier

rare earth element minerals in martian breccia meteorites nwa 7034 and 7533 implications for fluid rock interaction in the martian crust paired martian breccia meteorites northwest africa nwa 7034 and 7533 are the first martian rocks found to contain rare earth element ree phosphates and silicates the most common occurrence is as clusters of anhedral monazite ce inclusions in apatite occasionally zoned irregular merrillite inclusions are also present in apatite monazite bearing apatite is sometimes associated with alkali feldspar and fe oxide apatite near merrillite and monazite generally contains more f and oh f rich region than the main chlorapatite host and forms irregular boundaries with the main host locally the composition of f rich regions can reach pure fluorapatite the chlorapatite hosts are similar in composition to isolated apatite without monazite inclusions and to euhedral apatite in lithic clasts the u th total pb ages of monazite in three apatite are 1 0 0 4ga 2 1 1 0 5ga 2 and 2 8 0 7ga 2 confirming a martian origin the texture and composition of monazite inclusions are mostly consistent with their formation by the dissolution of apatite and or merrillite by fluid at elevated temperatures 100 c in nwa 7034 we observed a monazite chevkinite perrierite bearing benmoreite or trachyandesite clast anhedral monazite and chevkinite perrierite grains occur in a matrix of sub micrometer ree phases and silicates inside the clast monazite ce and nd and chevkinite perrierite ce and nd display unusual la and ce depletion relative to sm and nd in addition one xenotime y bearing pyrite ilmenite zircon clast with small amounts of feldspar and augite occurs in nwa 7034 one xenotime crystal was observed at the edge of an altered zircon grain and a cluster of xenotime crystals resides in a mixture of alteration materials pyrite ilmenite and zircon in this clast are all highly altered zircon being the most likely source of y and hree now present in xenotime the association of xenotime with zircon low u and th contents and the low yb content relative to gd and dy in xenotime suggest the possible formation of xenotime as a byproduct of fluid zircon reactions on the basis of relatively fresh apatite grains and lithic clasts in the same samples we propose that the fluid rock mineral reactions occurred in the source rocks before their inclusion in nwa 7034 and 7533 additionally monazite bearing apatite and ree mineral bearing clasts are possibly derived from different crustal origins thus our results imply the wide occurrence of hydrothermal fluids in the martian crust at 1 ga or older which were probably induced by impacts or large igneous intrusions

conceptualization of the hydrogeological system of some sedimentary aquifers in savelugu nanton and surrounding areas northern ghana groundwater resources play a pivotal role in the rural water delivery system in ghana the hydrogeological system of middle voltaian terrain was simulated using available data on hydraulic heads and boundary conditions the objective was to characterize the general groundwater flow pattern and provide local estimates of the distribution of hydraulic conductivity and recharge fields the results suggest a predominant ne sw flow direction which ties in with the general regional structural trend and indicates that the hydrogeological conditions of the rocks are controlled by structural entities created in the wake of fracturing and or weathering of the rocks whose primary permeabilities are considerably reduced because of high compaction and low grade metamorphism calibrated hydraulic conductivities range between 1 90 and 10 81 m d the spatial distribution appears to reflect the intensity of fracturing and or weathering of the rock and the proportion of the clay fraction of the weathered zone vertical groundwater recharge has been estimated to range between 0 3 and 4 1 of the annual rainfall this recharge rate is quite low and reflects the imperviousness of the thick overburden because of high clay content in some places and high compaction in others despite this apparently low recharge rate groundwater resources potential in the area appear to be high and increased abstraction from existing abstraction wells by up to 50 does not appear to register significant effects on groundwater budgets at the simulated recharge rates this suggests that the well yields are much lower than the potential of the aquifer system the apparently low yields might be associated with poor well development and the choice of inappropriate well completion materials this study recommends a monitoring system to be developed for a much more regional groundwater flow simulation under transient conditions copyright 2012 john wiley sons ltd

stable isotope imaging of a dynamic groundwater system in the southwestern sacramento valley california usa abstract a dynamic image of a shallow 45 160 m below the surface aquifer in a 25km2 area in the southwestern sacramento valley california has been obtained by 18 o 16 o and d h determinations of groundwater from municipal wells the regular summertime drawdown of the water table is strongly correlated with municipal wide increases in the 18o values particularly in wells with shallow perforation levels thus demonstrating that the high 18o water resides in the upper reaches of the aquifer and penetrates more deeply with increased discharge rates these 1 5 enrichments in the 18o values are primarily attributed to infiltration of irrigation water that has penetrated to depths of at least 80 m the increased 18o values correlate with increased deviation from the meteoric water line and with increased nitrate levels indicating that the high 18o groundwater component represents local meteoric water that has been enriched by soil evaporation processes in nitrogen fertilized flood irrigated cropland isotopic gradients provide a new method for qualitatively determining subsurface permeability zones of low isotopic gradients interpreted as high 18o plumes probably follow coarse textured deposits of ancient stream channels the 18o gradients are primarily controlled by subsurface permeability although additional influences may include 1 the heterogeneous distribution of the high 18o groundwater 2 isotopic contributions from deeper groundwater 3 variable perforation depths of the wells 4 local differences in the water table level deeper sodium bicarbonate groundwaters 300 650 m below the surface have lower 18o values to 9 4 than the more shallow magnesium bicarbonate groundwaters suggesting derivation from 18o depleted meteoric groundwaters of the sierra nevada minor mixing of the deeper groundwater into the municipal discharge occurs even though previous workers considered the deeper groundwater to be confined

application of the discrete fracture network concept with field data possibilities of model calibration and validation the uneven flow distribution observed in a migration experiment in the stripa research mine is analyzed with a discrete fracture network model data on the measured inflow distribution and trace geometry in the experimental drift are the basic sources for estimation of the model parameters direct estimates cannot provide a unique definition of input data to the model thus different combinations of input data need to be analyzed in the network model detailed measurements of the inflow distribution in the experimental drift provided an opportunity to determine the model parameters through calibration in the network model calibration was performed on the mean inflow distribution of flow and trace geometry observed in the roof of the experimental drift the simulation results show that the model can be calibrated to produce an areal flow distribution that is consistent with the measured one the uncertainties in the input parameters are thus reduced but different combinations of input data are still possible the calibration simulations show that the length of conductive fractures per area might be used as a calibration parameter simulations based on different combinations of fracture size and density but with the same length of conductive fracture traces produced similar flow distributions the validity of the calibrated model is explored by predicting the flow into the boreholes at the experimental site the resulting inflow distributions accord well with those measured in two of the three boreholes the properties of the third borehole which differ substantially from the other two could not be explained by the simulation results

record of the ancient martian hydrosphere and atmosphere preserved in zircon from a martian meteorite how the atmosphere hydrosphere and surface materials interacted on early mars is poorly understood oxygen isotopic composition of zircon grains in a martian meteorite reveals a prolonged history of exchange between martian regolith and atmosphere

fluvial conglomerates in continental red beds of the buntsandstein lower triassic in the eifel f r g and their palaeoenvironmental palaeogeographical and palaeotectonic significance abstract fluvial conglomerates occur in a variety of facies within the continental red bed sequence of the buntsandstein lower triassic in the eifel at the western margin of the mid european triassic basin the alluvial rudites are classified into channel floor lag channel bar channel top lag crevasse splay and floodplain lag conglomerates the conglomerates were predominantly deposited in scott type and in subordinate amounts also in donjek type and south sasketchewan type braided rivers the vertical distribution of clast size and frequency within the megacycles reflects the palaeotectonic sequence of uplift events in the source area resulting in supply of large amounts of coarse detritus which cause recurrences in fluvial style gradually upwards diminishing size and abundance of gravel indicate continuous lowering of source area relief during weakening endogenic activity or even tectonic quiescence after final peneplanation the palaeoenvironmental interpretation of the individual members and the palaeotectonic analysis of the conglomerate successions are combined with a reconstruction of the depositional history of the eifel buntsandstein which comprises an evolution of fluvial deposition the terminal stage of the palaeoenvironmental evolution is highlighted by a climax of regional diversification of conglomerate sedimentary milieu the horizontal distribution of clast size and abundance delineates the palaeogeographical evolution of the elongated eifel basin giving evidence of a progressive sidewards extension of the depositional area with time lateral supply from parts of the western margin and mixing of proximally derived coarse clasts with distantly delivered fine detritus result in highly inhomogeneous gravel distributions within the sedimentary region often not being characterized by gradually decreasing size and abundance of gravel in downstream direction the stepwise southwestwards shift of the marginal centre of supply of coarse gravel with time during the course of the lateral extension of the basin results in ascension of the fluvial conglomerate wedges into successively younger formations comprising consecutively middle buntsandstein upper buntsandstein lower muschelkalk middle muschelkalk and finally middle keuper the extraformational conglomerates mainly delineate the palaeogeographical framework whereas the intraclastic rudites primarily give evidence of processes operating within the depositional area reflecting erosion and reworking of overbank sequences with floodplain fines palaeosols and aeolian dune sands during lateral channel shift the brockelbank intraformational carbonate breccias to conglomerates originating from reworking of calcrete palaeosols are a peculiar facies element with particularly high palaeoenvironmental significance they document the frequency spatial extension and temporal persistence of pedogenesis within the alluvial plain the size of the extraformational clasts gives evidence of the range of current velocities during aggradation of the channels the distribution of conglomerate type and clast characteristics within cyclothems indicates the presence of stage fluctuations and variations in discharge in parts of the sequence the preservation of complete cyclothems or the multistoreying of channel bar conglomerates reflects the differential importance of the two main mechanisms controlling accumulation of sediments in the alluvial plain primary depositional restriction to suppression of formation and secondary erosional removal to destruction of finer grained channel sediments floodplain deposits and aeolian dune sands or palaeosols condensation of a series of depositional events within monotonous stacked conglomerate sequences obliterates the full range of sedimentary processes and mimics periods of long term stability of conglomerate formation waning flow mud drapes are particularly significant instruments to split up the individual genetical units of formation of fluvial conglomerates often comprising discrete flood phases separated by stage fluctuations of different magnitude the homogeneous or microcyclic composition of the coarse member reflects uniphase or multiphase aggradation of watercourses with gradually declining or abruptly waning individual flood periods the fluvial rudites thus are of high significance in the reconstruction of the depositional milieu in the eifel buntsandstein and provide a key to detailed interpretation of palaeoenvironmental processes palaeogeographical constellations and palaeotectonic events which could not have been identified to such an extent in exclusively sandy sequences