Water Table Oscillations Research Articles

Relationship of Clay Minerals to Depositional Environment in the Non-Marine Eocene Pontils Group, Se Ebro Basin (Spain)

ABSTRACT The Eocene sequence along the central part of the SE margin of the Tertiary Ebro Basin (Igualada area) consists of alternating marine and non-marine deposits up to 2000 m thick. Non-marine conditions persisted through the Cuisian--early Bartonian (early-middle Eocene) and resulted in the deposition of the Pontils Group (up to 900 m thick). During the late Bartonian--early Priabonian (late Eocene), marine environments prevailed in this area, leading to the deposition of the Santa Maria Group. Paleoenvironmental interpretations of the Pontils Group were obtained from sedimentology data of an earlier study. The nonmarine environments recorded in this group are: alluvial fans, flood plains, mudflats, playa-lakes, and shallow freshwater lakes. X-ray diffraction, TEM, SEM, EDX, and chemical analyses of mudstones and marls from the Pontils Group indicate that the diverse environments also can be distinguished from one another on the basis of the clay-mineral content. Illite is present in all samples of the Pontils Group. Palygorskite, with minor amounts of illite and sometimes chlorite, is the dominant clay mineral in flood plain and mud flat sediments. Palygorskite is also predominant in sabkha, playa-lake and carbonate-lagoonal sediments. This clay mineral is absent in sediments from fresh, organic-rich lakes. Kaolinite is relatively abundant in mudstones from the organic-rich lakes. Smectite is relatively abundant in the sediments from subaqueous environments but is almost always absent from the sediments which were subjected to subaerial exposure and water-table oscillations such as flood plain and mudflat deposits. According to these clay-mineral distribution trends, the organic-rich lacustrine, lagoonal, and marine sediments preserve the detrital clay-minerals. The original detrital assemblages were mostly formed by illite, associated with chlorite, kaolinite and subordinate smectite. In the environments which underwent repeated flooding and desiccation, chlorite and smectite were eventually eliminated and palygorskite was formed. Microscopic observation suggests that palygorskite was formed in mudflats and flood-plains mainly by the transformation of smectite and/or illite in a Mg-rich environment. In the sabkha facies, the abundance of palygorskite, frequently associated with smectite, dolomite, and chert, suggests palygorskite neoformation in a Si- and Mg-rich environment.

Paleosuelos de sepiolita en el neógeno de la cuenca de Madrid

The mineralogical composition from sepiolite bearing clays allows us to stablish remarkable variations among this mineral: smectite, illite, detrital grains (quartz and feldspar) and carbonates (calcite and dolomite) according to bed thickness. Sepiolite formation is related to silicification processes which control textural features, crystallinity degree (sharpness) and sepiolite aggregate size as a function of the SiO 2 /MgO ratio. Silicification is favoured by CaCO 3 and an oscillating water table with dissolution-precipitation mechanisms which give rise to hydromorphic paleosoils in the shallower zones. In these paleosoils sepiolite develops mainly on saponite bearing green clays, with glomerulae and laminated textures. In deeper zones, sepiolite appears with massive textures (grid-type) on clays, cementing when there is a high content of terrigenous minerals, both are diagenetic in origin.

Ecohydrology of Scottish peatlands

ABSTRACTMires are terrestrial ecosystems which conserve organic production in the form of peat because their soils are waterlogged. Scotland's damp climate makes it rich in mires, with fens, raised mires and blanket mires occurring widely. Intact examples are mostly treeless due to wind exposure. They have two functional layers of soil: a shallow acrotelm overlying the catotelm or peat deposit proper. The acrotelm is the main region of diagenesis and contains the water table, which lies close to the surface for most of the time. Evapotranspiration depends on water table depth and surface vegetation. In the catotelm, water transmission is described by Darcy's law with sufficient exactness to allow prediction of the water table profile. An analysis of seepage using Dupuit–Forchheimer theory predicts a hemi-elliptical profile whose curvature is governed by the water balance in dry years and which, in its turn, closely controls the overall shape of the mire. Water flow is more rapid in the acrotelm, where it creates a regular series of different soil physical regimes which are linked with distinct combinations of surface topography and vegetation arranged sequentially from the centre of a mire to its edge. Such regular arrangements are most clearly seen in the Flowe Country of E Sutherland and Caithness. Acrotelm structure also permits variation in flow rate with depth, so that the amplitude of water table oscillations is narrowly confined and the mire thus protected against both desiccation and sheet flow.

Study of pipeline stability with an oscillating water table : Moore, P J; Dight, P M Proc 3rd Australia-New Zealand Conference on Geomechanics, Wellington, 12–16 May 1980, V1, P53–59. Publ Wellington: New Zealand Institution of Engineers, 1980

A study of the required depth of cover of a pipeline to prevent flotation when the pipeline is submerged below the ground water table in sandy soil has been carried out. This study includes consideration of the shear stresses that may be mobilized in the overburden soil. Comparisons between these calculated depths of cover at which pipeline flotation may occur and those observed experimentally were made. Reasonable agreement was obtained provided the calculation was carried out in a particular way. The phenomenon known as pipeline jacking was also examined experimentally. It was found that pipeline jacking did not occur unless large voids were purposely left beneath the pipe and the overburden material was loosely placed around the pipe.

Regional groundwater motion in response to an oscillating water table

A case is made for the use of simple models of groundwater flow emphasizing the salient regional characteristics of groundwater basins and for relating them to basin-wide features of groundwater flow. As an example a simple formula for the “natural basin yield” as defined by Freeze is calculated from a model which was introduced by Tóth. The Tóth model is then extended to take seasonal fluctuations of the water table into account; the type of response of the instantaneous head distribution in the basin due to elastic storage in the aquifer may range from severe distortions of Tóth's steady-state distribution to virtually no distortion. The type of response depends on two ratios which can be easily calculated from the basin parameters.