Water Table Oscillations Research Articles

Pleistocene calcretes from eastern Tunisia: The stratigraphy, the microstructure and the environmental significance

This paper is meant to study the stratigraphy, the mineralogy, the microstructure and the geochemistry of Pleistocene calcretes from eastern Tunisia in order to infer the environmental factors intervening in their formation. Samples of eight profiles of Pleistocene calcretes from eastern Tunisia were examined on the basis of a variety of techniques including Optical Microscopy (OM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), chemical analysis and Atomic Absorption Spectrophotometer (AAS) techniques. Then, the obtained data underwent a statistical analysis on the basis of Factor Analysis (FA) and Principal Component Analysis (PCA). On the basis of field missions, five different horizons have been differentiated from bottom to top of all profiles: nodular, powdery, massive Brecciated and laminar horizon. The mineralogical study shows two minerals categories inversely proportional: calcite and (quartz and the clay). It shows also shows that Palygorskite is the dominant clay mineral. The escarpment edge is capped by a limestone containing fibrous palygorskite. Finally, superficial calcrete are described: a brecciated horizon which occurs in pockets on the plateau surface. This study about eastern Tunisia revealed the occurrence of successive cycles of calcretisation. Pedogenesis, water table oscillation, sedimentogenesis and stromatogenesis are the intervening factors in the calcretisation process. During the Pleistocene, they interfered with each other according to the climatic pulsations. From the studied case, it may be noticed that the formation of each calcrete horizon is the result of a dominating process that takes place during a distinguishable stage. In the first stage, the pedogenic process is developed by palygorskite formation including authigenic replacement or formation from a precursor mineral, neoformation from the breakdown products of such minerals or neoformation from suitable solutions. In the second stage, the powdery horizon is formed in the slope of the distal zone which presents a drained environment. In the third stage, several diagenetic processes (cementing, compaction, dissolution...) contribute to the formation of the laminar and massive horizon. Since it is exposed to dryness for a long period, the massive horizon is harder and more compact. In the fourth stage, the banding of light–dark in the laminar horizons reflects a dry-wet season alternation seasons. Dark beds are formed by the stromatolitic cover were developed during the wet season, whereas light beds were developed in an extremely arid climate argued by the presence of the detrital grains. In the fifth stage, the brecciated horizon, which occupies the channels, is formed by well rolled concretions, which present a dismantling material of Early and Middle Pleistocene calcretes after the Post-Villafranchian compressive phase. Thus, calcretisation seems to have been controlled by periods of uplift and stability of the slope, given that calcrete formation might be inhibited by the activation of the sedimentation of colluvial materials as a consequence of the tectonic activity. We also suggest that groundwater and biological activity may play a significant role in the development of pedogenic, sedimentological and polygenetic calcrete cycles within the same sedimentary basin. The alternation of dry and wet climatic periods may be responsible for the calcrete genesis.

Drenagem interna como fator de diferenciação de Latossolos do Distrito Federal

O objetivo deste trabalho foi determinar a influência de características físico-hídricas dos solos em atributos diagnóstico de Latossolos. Realizou-se o levantamento de atributos morfológicos, químicos, físicos, mineralógicos e físico-hídricos de cinco perfis de Latossolos Vermelhos (LV) e cinco perfis de Latossolos Vermelho-Amarelos (LVA) petroplínticos, considerados representativos dessa classe de solos no Distrito Federal. Além da caracterização dos atributos diagnóstico, a oscilação do nível freático dos solos foi monitorada por meio de poços de observação de 2,5 m de profundidade, durante um ano. Os Latossolos estudados foram considerados semelhantes química e fisicamente. Contudo, diferiram quanto às caracterizações morfológica e mineralógica, com a presença de horizontes concrecionários e goethita nos LVA. A estabilidade da goethita nesses solos foi influenciada pela oscilação do lençol freático. Constatou-se menor condutividade hidráulica saturada e menor variação da profundidade freática nos LVA, em razão da deficiência de drenagem interna causada pela presença dos horizontes concrecionários. Os LVA apresentam, portanto, menor potencial agrícola que os LV, no Distrito Federal. As características físico-hídricas de Latossolos têm pouca influência sobre atributos diagnóstico, com exceção da mineralogia dos óxidos de ferro, que apresentam os teores de goethita aumentados em condições de baixa drenagem interna.

Relações pedomorfogeológicas nas chapadas elevadas do Distrito Federal

A caracterização pedológica e o entendimento das relações entre pedologia, geologia e geomorfologia são importantes para a compreensão da distribuição dos solos numa paisagem. O objetivo deste estudo foi avaliar as relações pedomorfogeológicas na região das Chapadas Elevadas do Distrito Federal (DF), mediante caracterização química, física, mineralógica e geoquímica dos solos de ocorrência nesse compartimento da paisagem. Foram selecionadas duas topossequências representativas da distribuição pedológica nas Chapadas Elevadas do DF, cujos solos foram formados a partir de rochas metassedimentares do Grupo Paranoá, representados por Latossolos Vermelhos (LV), Latossolos Vermelho-Amarelos (LVA) e Cambissolos (C). Os Latossolos das duas topossequências apresentaram a maioria dos atributos físicos, químicos e mineralógicos semelhantes. A variação da cor nesses Latossolos é proveniente da mineralogia diferenciada dos óxidos de Fe - hematita predominante nos LV e goethita nos LVA. No entanto, as análises por meio de ICP-AES apresentaram teores de Fe2O3 similares nos Latossolos, demonstrando material de origem (rochas metassedimentares) de composição geoquímica semelhante. A formação da goethita nos LVA foi considerada dependente da sua posição geomorfológica de desenvolvimento, nas bordas das chapadas, onde a oscilação do lençol freático proporcionou a formação de horizonte litoplíntico, com consequente deficiência das condições de drenagem. Os Cambissolos apresentaram-se quimicamente semelhantes aos Latossolos em razão do material de origem, que são rochas metassedimentares já pré-intemperizadas. O estudo das relações pedomorfogeológicas permitiu constatar que a distribuição dos solos nas Chapadas Elevadas do Distrito Federal é condicionada pela evolução geomorfológica e pela geologia da região.

Influence of Host-Rock Texture on the Morphology of Carbonate Concretions in a Meteoric Diagenetic Environment

Abstract The integrated stratigraphic-petrographic–geochemical study of Quaternary marine terraces that have undergone well-constrained sea-level and water-table oscillation(s) over geologically short time periods can provide important clues on diagenetic processes at the marine–continental interface. The Punta di i Cani marine terrace (Quaternary; Saint-Florent, Corsica) unconformably overlies the well-cemented, low-permeability arenites of the Miocene Monte S. Angelo Formation and is erosively overlain by unlithified, high-permeability Holocene continental sediments. The terrace is composed of a retrogradational, fining-upward succession, from fluvial pebbly sandstone to beach and shoreface deposits. The entire thickness of the terrace hosts concretions made of low-Mg calcite. Concretion geometry is linked to the sedimentological facies of the host sediment: (i) massive fluvial pebbly sandstone at the base of the terrace contains knobby, subhorizontal concretions somewhat resembling turbidite flute casts; (ii) very low-angle cross-bedded, well sorted beach deposits contain pipy, subhorizontal concretions; (iii) trough-crossbedded upper-shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae, locally obliterated by a patchy, isotropic cementation. Overall, the degree of cementation decreases from the bottom to the top of the terrace. Cement chemistry and morphology, as well as stable carbon and oxygen isotopic signatures, indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast. Elongate concretions are invariably parallel to the topographic gradient, consistent with the notion that the growth of concretions was controlled by advection. Smectitic vadose cement is locally present; its crosscutting relationships with calcite cement point to periodic oscillations of the water table. The results of this study (i) show that elongate concretions can be used as interstitial paleoflow indicators and (ii) suggest that concretion geometry in an advection-dominated diagenetic environment can depend on the small-scale permeability distribution, as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies.

Sources of oxygen flux in groundwater during induced bank filtration at a site in Berlin, Germany

The microbial degradation of pharmaceuticals found in surface water used for artificial recharge is strongly dependent on redox conditions of the subsurface. Furthermore the durability of production wells may decrease considerably with the presence of oxygen and ferrous iron due to the precipitation of trivalent iron oxides and subsequent clogging. Field measurements are presented for oxygen at a bank filtration site in Berlin, Germany, along with simplified calculations of different oxygen pathways into the groundwater. For a two-dimensional vertical cross-section, oxygen input has been calculated for six scenarios related to different water management strategies. Calculations were carried out in order to assess the amount of oxygen input due to (1) the infiltration of oxic lake water, (2) air entrapment as a result of water table oscillations, (3) diffusive oxygen flux from soil air and (4) infiltrating rainwater. The results show that air entrapment and infiltrating lake water during winter constitute by far the most important mechanism of oxygen input. Oxygen input by percolating rainwater and by diffusive delivery of oxygen in the gas phase is negligible. The results exemplify the importance of well management as a determining factor for water oscillations and redox conditions during artificial recharge.

Field capacity in soils with a yearly oscillating water table

An experiment was performed by measuring the field capacity (FC) in two different fields with a water table (WT) fluctuating over the year from about −2.5 to about −1 m. The basin method was adopted and the tests were repeated every second month (the interval between the last two tests was 4 months). Samples were taken to follow the moisture content of the soils at −37.5 and −12.5 cm. The depth of the WT was monitored over 400 days. The differences in the gravitational water content ( θ g ) at all FCs were analysed and found to be substantial (about 25% over 1 year in both soils). A comparison with water retention curves determined in three seasons made it possible to evaluate the matric pressure at the sampled FC levels and showed that, in one of the soils, consideration of the overburden potential adequately satisfies the equilibrium conditions, meaning that the FC corresponds to static situations at least until the WT depth sinks below a certain level. In the other soil, richer in clay and organic matter, equilibrium soil conditions were not found and the known dynamic concept of FC remains valid.

Effect of yearly oscillating water table on soil moisture retention curves

Yearly oscillating water tables (WT) are not unusual in soils subjected to climates characterised by dry seasons alternating with wet seasons when the underground WT is not very deep (50–250 cm). This implies a time succession of moisture changes along the soil profile that can modify soil properties such as the fine pore porosity. The determination of the soil water retention curve (SWRC) in two fields over three seasons (1st winter, following summer, 2nd winter) highlighted significant shifts in the SWRC. These shifts could not be explained solely in terms of changes in physical parameters (mainly surface tension, entrapped air, water continuity, apparent contact angle). The discrepancy between theoretical and experimental results is usually greater in finer soils such as those under consideration. It is proposed that the observed changes are mainly due to reversible alteration of the chemical–physical and microbiological activity of the soil as supported by the high multiple regression coefficient found between the observed seasonal shifts of the SWRC as well as by the significance of the partial regression coefficients for clay–organic matter soil content, for depth of the WT from the sampling point and their interactions, independently of temperature effects.

Field Observation of Edge Waves and Beach Cusps on the South Baltic Sea Coast

Abstract Within the two major theories of the generation of rhythmic shoreline features, the earlier one relates to edge waves, whereas the one developed later perceives shoreline rhythmic forms as effects of self-organized shoreline behavior. This study focused on identification of infragravity waves (edge waves) and periodic components of shoreline configuration and searched for their potential relationships in the mildly sloping, sandy south Baltic coast. The objective was not to suggest which theory better describes shoreline rhythmic features but was chosen for an empirical assessment of possible links between the generation of rhythmic morphological beach forms and specific infragravity waves motion at a dissipative shore. The investigated area features a gently sloping seabed with several bars inducing multiple breakers, which can be classified as a dissipative beach. Field measurements were taken during two periods lasting several months each. Long records of water table oscillations, nearshore cu...

Comment on “Influence of capillarity on a simple harmonic oscillating water table: Sand column experiments and modeling” by Nick Cartwright et al.

[1] Cartwright et al. [2005] and, in a preceding paper, Nielsen and Perrochet [2000] conducted an investigation to assess the effects of an oscillating water table on the effective porosity. Both studies present results from a thorough laboratory experiment and numerical simulations. We wish to comment on some of the findings and conclusions of Cartwright et al. [2005] with regard to the effect of capillarity and the response at very high and very low oscillation frequencies. We also present some results from a simple numerical solution to a nonhysteretic one-dimensional Richards equation model and compare these to the original data and the modeling results presented by Cartwright et al. [2005].

Reply to comment by A. G. J. Hilberts and P. A. Troch on “Influence of capillarity on a simple harmonic oscillating water table: Sand column experiments and modeling”

We thank Hilberts and Troch [2006] for their comment on our paper [Cartwright et al, 2005]. Before proceeding with our specific replies to the comments we would first like to clarify the definitions and meanings of equations (1)-(3) as presented by Hilberts and Troch [2006]. First, equation (1) is the fundamental definition of the (complex) effective porosity as derived by Nielsen and Perrochet [2000]. Equations (2) and (3), however, represent the linear frequency response function of the water table in the sand column responding to simple harmonic forcing. This function, which was validated by Nielsen and Perrochet [2000], provides an alternative method for estimating the complex effective porosity from the experimental sand column data in the absence of direct measurements of h_(tot) (which are required if equation (1) is to be used).

Transport of radionuclides in an unconfined chalk aquifer inferred from U-series disequilibria

U-series disequilibria measured in waters and rocks from a chalk aquifer in France have been used as an analog for long-term radionuclide migration. Drill core samples from a range of depths in the vadose zone and in the saturated zone, as well as groundwater samples were analyzed for 238U, 234U, 232Th and 230Th to determine transport mechanisms at the water/rock interface and to quantify parameters controlling the migration of radionuclides. Isotope measurements in rocks were done by TIMS, whereas ( 234U/ 238U) and ( 230Th/ 232Th) activity ratios in water samples were measured by multi-collector-ICP-MS. Both depletion and enrichment in 234U relative to 238U were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water-table oscillation, respectively. The correlation between ( 230Th/ 232Th) activity ratios and 87Sr/ 86Sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples. Water samples are all characterized by ( 234U/ 238U) > 1 resulting from α-recoil effect of 234Th. Groundwaters are characterized by a more radiogenic signature in 87Sr/ 86Sr than the rocks. Moreover, ( 230Th/ 232Th) activity ratios in the waters are lower than in the rocks, and increase with distance from the water divide, which suggests that Th transport is controlled by colloids formed during water infiltration in the soil. A 1-D transport model has been developed in order to constrain the U-series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time-dependent composition for the solid phase. This model permits a prediction of the time scale of equilibration of the system, and an estimation of parameters such as weathering rate, distribution coefficients and α-recoil fractions. Retardation factors of 10–35 and from 1 × 10 4 to 2 × 10 5 were predicted for U and Th, respectively, and can be used to predict the migration of radionuclides released as contaminants in the environment. At the scale of our watershed (∼32 km 2), a characteristic migration time from recharge to riverine discharge of 200–600 yr for U and 0.2–3.7 Myr for Th was obtained.

Petrography and depositional environment of the No. 308 coal seam (Upper Silesian Coal Basin, Poland)—a new approach to maceral quantification and facies analysis

Abstract Oriented block samples of coal representing the lower layer (2.1 m) of the No. 308 coal seam were collected in the Ziemowit mine. Microscope investigations were carried out using a self-developed, fast method of maceral quantification that enables obtaining a continuous petrographical profile (“microprofile”) of a seam and computer processing of the data. The contents of macerals and maceral groups distinguished along the profile were averaged using the method of moving averages to show trends of environmental changes in a mire. The facies diagram proposed is based on an assumption that oscillations of water table in the mire affect the coal petrographical composition, while stronger water influxes into the mire increase the mineral matter content of coal. The author has identified three major types of the mire environments in which plant material was deposited during deposition of the No. 308 coal seam. These are permanently inundated = planar mire (PM) with two sub-environments—PM “margin” and PM “central”; temporarily inundated = transitional mire (TM) with two sub-environments—TM “wet” and TM “dry”; and elevated = domed mire (DM) with two sub-environments—DM “progressive” and DM “regressive”.

Influence of river meandering dynamics on riparian vegetation pattern formation

The present work investigates how the dynamics of meandering rivers influence the formation of riparian vegetation patterns. To this aim, a model coupling river dynamics and riparian vegetation evolution was developed. Meandering dynamics were simulated with a fluid dynamical model using shallow water equations on an erodible bed. The riparian vegetation model takes into account some of the main actions caused by the river, i.e., water table oscillations, floods, and sedimentation. A logistic law and an exponential decay were used to model the increase and decrease in the biomass, respectively, consequent to river migration. The numerical simulation by the model highlights how river dynamics are able to induce typical vegetation patterns that are similar to some real riparian landscapes (some examples are shown). The results also show the role of (1) the relevant river‐vegetation interactions and (2) the temporal scales of vegetation evolution and river migration.

Influence of capillarity on a simple harmonic oscillating water table: Sand column experiments and modeling

Comprehensive measurements of the water table response to simple harmonic forcing at the base of a sand column are presented and discussed. In similar experiments, Nielsen and Perrochet (2000) observed that fluctuations in the total moisture were both damped and lagged relative to the water table fluctuations. As a result, the concept of a complex effective porosity was proposed as a convenient means to account for the damping and phase lag through its magnitude and argument, respectively. The complex effective porosity then enables simple analytical solutions for the water table (and total moisture) dynamics including hysteresis. In this paper, these previous experiments are extended to cover a wider range of oscillation frequencies and are conducted for three well‐sorted materials with median grain diameters of 0.082, 0.2, and 0.78 mm, respectively. In agreement with existing theory, the influence of the capillary fringe is shown to increase with the oscillation frequency. However, the complex effective porosity model corresponding to the classical Green and Ampt (1911) capillary tube approximations is shown to be inadequate when compared to the data. These limitations are overcome by the provision of an empirical, frequency‐dependent complex effective porosity model fit to the data. Using measured moisture retention parameters, numerical simulation of the data solving a nonhysteretic van Genuchten–Richards' equation type model is unable to replicate the observations. Existing results of a hysteretic numerical model are shown to be in good agreement with the extended database.

Water table waves in an unconfined aquifer: Experiments and modeling

Comprehensive measurements are presented of the piezometric head in an unconfined aquifer during steady, simple harmonic oscillations driven by a hydrostatic clear water reservoir through a vertical interface. The results are analyzed and used to test existing hydrostatic and nonhydrostatic, small‐amplitude theories along with capillary fringe effects. As expected, the amplitude of the water table wave decays exponentially. However, the decay rates and phase lags indicate the influence of both vertical flow and capillary effects. The capillary effects are reconciled with observations of water table oscillations in a sand column with the same sand. The effects of vertical flows and the corresponding nonhydrostatic pressure are reasonably well described by small‐amplitude theory for water table waves in finite depth aquifers. That includes the oscillation amplitudes being greater at the bottom than at the top and the phase lead of the bottom compared with the top. The main problems with respect to interpreting the measurements through existing theory relate to the complicated boundary condition at the interface between the driving head reservoir and the aquifer. That is, the small‐amplitude, finite depth expansion solution, which matches a hydrostatic boundary condition between the bottom and the mean driving head level, is unrealistic with respect to the pressure variation above this level. Hence it cannot describe the finer details of the multiple mode behavior close to the driving head boundary. The mean water table height initially increases with distance from the forcing boundary but then decreases again, and its asymptotic value is considerably smaller than that previously predicted for finite depth aquifers without capillary effects. Just as the mean water table over‐height is smaller than predicted by capillarity‐free shallow aquifer models, so is the amplitude of the second harmonic. In fact, there is no indication of extra second harmonics (in addition to that contained in the driving head) being generated at the interface or in the interior.

The "Calamine" of Southwest Sardinia: Geology, Mineralogy, and Stable Isotope Geochemistry of Supergene Zn Mineralization

The mining district of southwest Sardinia, Italy, is one of the classic areas where primary carbonate-hosted Zn-Pb sulfide ores are associated with a relatively thick secondary oxidation zone containing Zn (hydroxy-)carbonates and silicates, the so-called calamine, exploited until the 1970s. The extent of the capping oxidized ore zones, reaching deep below the surface, is generally independent of the present-day water table. The base of the oxidation profile containing nonsulfide Zn minerals in various uplifted blocks in the Iglesiente area can be both elevated above or submerged below the recent water table. The genesis of the ores is therefore considered to be related to fossil, locally reactivated, oxidation phenomena. The mineralogy of the nonsulfide mineralization is generally complex and consists of smithsonite, hydrozincite, and hemimorphite as the main economic minerals, accompanied by iron and manganese oxy-hydroxides and residual clays. This study places the secondary ores in the context of the tectonostratigraphic and climatic evolution of Sardinia and includes a petrographic and mineralogic study of the most abundant minerals, relating the mineralogy of secondary Zn and Pb carbonates to their stable C and O isotope geochemistry and constraining the origin of the oxidizing fluids and the temperature of mineralization. The δ 18 O VSMOW values of smithsonite are homogeneous, regardless of crystal morphology, position, and mine location (avg. 27.4 ± 0.9‰). This homogeneity points to a relatively uniform isotopic composition of the oxidation fluid and corresponding formation temperatures of 20° to 35°C. Considering the karstic environment of smithsonite formation in southwest Sardinia, this high temperature could be due to heat release during sulfide oxidation. The carbon isotope compositions of secondary Zn carbonates display considerable variations of more than 9 per mil ( δ 13 C VPDB from –0.6 to –10.4‰). This large range indicates participation of variable amounts of reduced organic and marine carbonate carbon during sulfide oxidation. The isotopic variation can be related to a variation in crystal morphologies of smithsonite, reflecting different environments of formation with respect to water table oscillations in karstic environments (upper to lower vadose to epiphreatic). The same range in δ 13 C isotope values is displayed by the calcite associated with Zn carbonates and by recent speleothems. The most reliable time span for the deposition of bulk calamine ore in southwest Sardinia ranges from middle Eocene to Plio-Pleistocene, although further multiple reactivation of the weathering profiles, peaking within the warm interglacial periods of the Quaternary, cannot be excluded.

Progression of natural attenuation processes at a crude oil spill site: II. Controls on spatial distribution of microbial populations

A multidisciplinary study of a crude-oil contaminated aquifer shows that the distribution of microbial physiologic types is strongly controlled by the aquifer properties and crude oil location. The microbial populations of four physiologic types were analyzed together with permeability, pore-water chemistry, nonaqueous oil content, and extractable sediment iron. Microbial data from three vertical profiles through the anaerobic portion of the contaminated aquifer clearly show areas that have progressed from iron-reduction to methanogenesis. These locations contain lower numbers of iron reducers, and increased numbers of fermenters with detectable methanogens. Methanogenic conditions exist both in the area contaminated by nonaqueous oil and also below the oil where high hydrocarbon concentrations correspond to local increases in aquifer permeability. The results indicate that high contaminant flux either from local dissolution or by advective transport plays a key role in determining which areas first become methanogenic. Other factors besides flux that are important include the sediment Fe(II) content and proximity to the water table. In locations near a seasonally oscillating water table, methanogenic conditions exist only below the lowest typical water table elevation. During 20 years since the oil spill occurred, a laterally continuous methanogenic zone has developed along a narrow horizon extending from the source area to 50–60 m downgradient. A companion paper [J. Contam. Hydrol. 53, 369–386] documents how the growth of the methanogenic zone results in expansion of the aquifer volume contaminated with the highest concentrations of benzene, toluene, ethylbenzene, and xylenes.

UNDERGROUND CONTAMINATION MECHANIZEM OF VOCs UNDER OSCILLATING WATER TABLE

Groundwater contaminations by VOCs (Volatile Organic compounds) are searched in many sites. These contamination are spread in both saturated zone and unsaturated zone. In these cases, mass transport is very important between these two zones, because the volatilization of VOCs from groundwater and the subsequence process are same mechanism on groundwater contamination.In this paper, the mechanism of VOCs transport in subsurface and these processes are discussed. Two types of experiments are carried out. One is the evaluation of the transport of VOCs in saturatedunsaturated zone under a soil moisture profile. Other is the estimation of the transport of VOCs under the oscillating water table condition. And a numerical model, considered with mass transfer on the interface, is applied to both saturated zone and unsaturated zone. This simulation agrees qualitatively with experimental results.

The coal-bearing, Cenozoic As Pontes basin (northwestern Spain) : geological influence on coal characteristics

Lignite deposits in the Cenozoic As Pontes strike-slip basin (northwestern Spain) were formed as a function of specific paleoclimatic conditions and tectonic evolution of the basin. During the early evolutionary stages, the presence of active normal faults and thrusts inside the basin resulted in two subbasins with distinct differences in sedimentary records, with respect to lignite seam occurrence, thickness, areal extent and lithotype development. In contrast, during the late evolutionary stages the basin was not split and a more homogeneous sedimentary record in terms of coal seam occurrence and lithotype characteristics developed. A total of 26 lignite samples, distributed along the basin infill, were analyzed by organic petrography and geochemistry. All are lignite B (ASTM). The lignites deposited during lower basin infill sedimentation (units l and 2) are dark, matrix-rich, mainly huminitic brown coals, with minor bright, liptinitic-rich coal lithotypes. The dark huminitic coals in these units show sedimentological and paleontological evidence of lacustrine influence. Persistent, relatively moderate to low Tissue Preservation Index (TPI) values and high average Gelification Index (GI) values are characteristic of these limno-telmatic coals. Lignites in the upper basin infill (Unit 4) are almost exclusively matrix-rich, huminitic brown coals. Persistent, widespread, low TPI values and variable, but low, average GI values are also characteristic of these telmatic coals. The overall coal petrological data trend (TPI, GI and widespread development or absence of liptinite-enriched lignite layers) recorded from the lower to the upper basin infill units agrees with the stratigraphic and sedimentological data, which show a trend of increasingly drier conditions. Development of bright, liptinite-enriched lignite layers was widespread during the early basin evolutionary stages and was influenced by punctuated water-table oscillations. Sedimentological, petrological and organic geochemical data point to the conclusion that, although the paleoenvironments where peat deposition took place did not undergo dramatic changes, they were affected by distinguishable variations (i.e., water hydrochemistry and groundwater-level stability), linked mostly to the evolution of basin morphology (depending mainly on tectonics) and basin water balance.

Effects of Water-Table Oscillation on Redox Property of Peat in a Floating Mat

Seasonal changes in redox potential (Eh) of peat were investigated in a floating mat which experiences a periodical inundation. In the inundated season (late December-June), Eh of the peat within 3 cm of the surface was 150-200 mV vs. NHE (standard hydrogen electrode) and it fluctuated little. It became <50 mV in July, although the mat surface emerged from water and the peat surface was in direct contact with the air. Redox potential began to increase in late July and reached its maximum in early November (...)