Response of reef coral communities to sea-level rise: a Holocene model from Mauritius (Western Indian Ocean)

The fringing reef at Pointe-au-Sable (Mauritius, Indian Ocean) was used to examine the effects of Holocene sea-level rise on coral growth. This reef is about 1000 m wide and comprises a forereef slope (30 m maximum depth), a narrow reef crest and a very shallow backreef (1·5 m maximum depth). Four major coral communities were recognized, which developed within relatively narrow depth ranges: a Pachyseris/Oulophyllia community (deeper than 20 m), an Acropora‘tabulate’Faviid community (20–6 m); a robust branching Acropora community (less than 6 m) and a Pavona community (less than 10m). Three high-recovery cores show the Holocene reef sequence is a maximum of 19·3 m thick and comprises four coral biofacies which are similar to counterparts identified in modern communities: robust branching, tabular-branching, robust branching-domal and foliaceous coral facies. A minimum sea-level curve for the past 7500 years was constructed. Using distribution patterns of coral biofacies and radiocarbon dates from corals, reconstruction of reef growth history indicates that both offshore and onshore reef zones were developing coevally, aggrading at rates of 4·3 mm year^?1 from 6900 years B.P. The reef caught up with sea-level only after sea-level stabilized. Changes in coral community and reef growth rates were driven principally by increasing water agitation due to the decrease in accommodation space. Based on the composition of the successive coral assemblages, the reef appears to have grown through successive equilibrium stages.     

Timing of the Last-Interglacial High Sea Level on the Seychelles Islands, Indian Ocean

Corals from the Seychelles Islands, Indian Ocean, occur mainly as small coralline algae–vermetid remnants found in cavities adhering to the rock surface, and they rarely attain more than 2 m²in area. Samples ofGoniastreaandPoritesfrom elevations between 1.7 and 6 m above present mean sea level were dated by TIMS²³⁸U–²³⁴U–²³⁰Th techniques. The ages from well-preserved corals lie between 131,000 and 122,000 yr B.P., in agreement with most other observations of the last-interglacial sea level. Field evidence and dating from high marine limestones from two sections at La Digue Island indicate a period of coral buildup until 131,000 yr B.P., followed by a drop in sea level between 131,000 and 122,000 yr B.P.     

The effects of Holocene barrier-island evolution on water-table elevations, Hatteras Island, North Carolina, USA

 On Hatteras Island, North Carolina, USA, complex stratigraphy results from surficial sediments being superimposed on an inherited Pleistocene platform. Numerical simulations used to simulate water-table profiles on the island illustrate the influence that the complex stratigraphy has on water-table elevations. Field data consist of water-table profiles collected from a cross-island transect of wells. These profiles show unusually high water-table elevations in the south-central portion of Hatteras Island. Geophysical data are used to constrain the subsurface stratigraphy. Simple analytical solutions to the groundwater flow equation using laterally varying recharge rates cannot match mean water-table profiles. More complex numerical simulations, which do match the profiles, indicate that the elevated water levels result from a low-permeability unit that extends several kilometers along the longitudinal axis of the island. Island geomorphology and ground-penetrating radar (GPR) data suggest that this unit is a former interdunal wetland that has been buried by a series of parabolic dunes. Received, September 1999 / Revised, March 2000 / Accepted, March 2000     

Late Holocene island reef development on the inner zone of the northern Great Barrier Reef: insights from Low Isles Reef

A sedimentological and stratigraphic study of Low Isles Reef off northern Queensland, Australia was carried out to improve understanding of factors that have governed Late Holocene carbonate deposition and reef development on the inner to middle shelf of the northern Great Barrier Reef. Low Isles Reef is one of 46 low wooded island-reefs unique to the northern Great Barrier Reef, which are situated in areas that lie in reach of river flood plumes and where inter-reef sediments are dominated by terrigenous mud. Radiocarbon ages from surface and subsurface sediment samples indicate that Low Isles Reef began to form at ca 3000 y BP, several thousand years after the Holocene sea-level stillstand, and reached sea-level soon after (within ～500 years). Maximum reef productivity, marked by the development of mature reef flats that contributed sediment to a central lagoon, was restricted to a narrow window of time, between 3000 and 2000 y BP. This interval corresponds to: (i) a fall in relative sea-level, from ～1 m above present at ca 5500 y BP to the current datum between 3000 and 2000 y BP; and (ii) a regional climate transition from pluvial (wetter) to the more arid conditions of today. The most recent stage of development (ca 2000–0 y BP) is characterised by extremely low rates of carbonate production and a dominance of destructive reef processes, namely storm-driven remobilisation of reef-top sediments and transport of broken coral debris from the reef front and margins to the reef top. Results of the present study enhance existing models of reef development for the Great Barrier Reef that are based on regional variations in reef-surface morphology and highlight the role of climate in controlling the timing and regional distribution of carbonate production in this classic mixed carbonate–siliciclastic environment.     

Geodynamics of the Amirante Ridge and Trench Complex,Western Indian Ocean

The formation and evolution of the ～600 km long arcuate Amirante Ridge and Trench Complex (ARTC) is a significant geomorphic–structural feature in the Western Indian Ocean (WIO). The WIO contains evidence of at least two major magmatic episodes followed by continental rifting within the span of a little more than 20 million years. This involved the splitting of Madagascar from India at around 85 Ma and then separation between India and the Seychelles at 64–63 Ma as a possible consequence of two powerful volcanic eruptions from the Marion and Reunion hot spots, respectively. Formation and evolution of the ARTC represents this tumultuous period in the Indian Ocean, approximately between 85 and 60 Ma (Late Cretaceous–Early Tertiary).

We integrated geophysical, palaeomagnetical, and petrological data to examine three existing models that attempt to explain the formation of ARTC. In contrast, our study hints at several stages of extension and compression responsible for its formation. Our integrated data also suggest that the Carlsberg Ridge may have played a prominent role in the evolution of the ARTC that seems to have formed through a ridge-jump process after the conjugate spreading centres – Mascarene and Palitana ridges formed earlier during the India–Madagascar separation – ceased spreading because of violent eruption of the Reunion hot spot at around 65 Ma. The eruption disturbed the plumbing system of magma ascent, resulting in cessation of spreading along the conjugate spreading centres, forcing a ridge jump.

A collage of seismic refraction and reflection, free-air gravity, magnetic anomaly data, and Ar dating of rocks indicates that as the Carlsberg Ridge swept the Seychelles towards south, the crust between Madagascar and the Seychelles was increasingly compressed, with the abandoned northern Mascarene spreading centre absorbing the maximum stress. With continued compression, the western limb of the abandoned spreading ridge was thrust below the eastern limb to a limited degree. This partial subduction agrees with the gravity and seismic results. Our new study also accounts for the anomalous presence of 14 km-thick oceanic crust beneath the ARTC and its characteristic difference in petrology with other established subduction zones in the world.     

Geologic controls on porosity development in the Maynardville Limestone, Oak Ridge, Tennessee

 Understanding the geologic controls of porosity development and their relationship to the karst aquifer system in the Cambrian Maynardville Limestone is important in determining possible contaminant transport pathways and provides essential data for hydrologic models within the Oak Ridge Reservation of east Tennessee. In the Maynardville Limestone, several important factors control porosity development: (1) lithologic controls on secondary microporosity and mesoporosity are related to dissolution of evaporite minerals and dedolomitization in supratidal facies; (2) depth below the ground surface controls the formation of karst features because the most active portion of the groundwater system is at shallow depths, and karst features are rare below ≈35 m; and (3) structural controls are related to solution enlargement of fractures and faults. Received: 21 May 1996 · Accepted: 30 August 1996     

鄂尔多斯盆地西缘奥陶纪生物礁基本特征、分布规律及成礁模式

鄂尔多斯盆地西缘奥陶纪发育有生物礁。本文通过对鄂尔多斯盆地西缘野外及钻井岩心生物礁调查及室内鉴定分析,认为在乌海桌子山地区、棋探1井地区、彭阳石节子沟等地发育了较为典型的生物礁,主要造礁生物为珊瑚、层孔虫和海绵。三处生物礁发育特征有差异,北段礁体生长发育2～3期,中段发育2期生长,南段生长发育可达4期;北端及南端生物礁造礁生物个体较大,生长的水体环境相对较浅,而中段生物礁造礁生物个体较小,生长的水体相对较深。这些典型生物礁的生长、发展和消亡呈现了不同的发育特征和时空分布规律,北段生物礁体发育层位早,中、南段发育层位晚,这种分布变化规律与华北板块在早古生代处于赤道附近发生了北漂右旋的旋转漂移规律有关。通过对不同地段生物礁基本特征分析,结合δ13C和δ18O同位素分析结果,认为盆地西缘生物礁成礁模式为中央古隆起西部陆缘海背景下的开阔台地边缘带点礁-滩体组合模式,分布较为局限,认为该地区生物礁死亡原因系海平面快速上升事件导致礁体被淹死所致,也进一步推测生物礁之上存在一次海平面快速上升事件。     

The hydrodynamic and sedimentary setting of nearshore coral reefs, central Great Barrier Reef shelf, Australia: Paluma Shoals, a case study

The Great Barrier Reef (GBR) shelf contains a range of coral reefs on the highly turbid shallow inner shelf, where interaction occurs with terrigenous sediments. The modern hydrodynamic and sedimentation regimes at Paluma Shoals, a shore‐attached ‘turbid‐zone’ coral reef, and at Phillips Reef, a fringing reef located 20 km offshore, have been studied to document the mechanisms controlling turbidity. At each reef, waves, currents and near‐bed turbidity were measured for a period of ≈1 month. Bed sediments were sampled at 135 sites. On the inner shelf, muddy sands are widespread, with admixed terrigenous and carbonate gravel components close to the reefs and islands, except on their relatively sheltered SW side, where sandy silty clays occur. At Paluma Shoals, the coral assemblage is characteristic of inner‐shelf or sheltered habitats on the GBR shelf (dominated by Galaxea fascicularis, up to >50% coral cover) and is broadly similar to that at Phillips Reef, further offshore and in deeper water. The sediments of the Paluma Shoals reef flats consist of mixed terrigenous and calcareous gravels and sands, with intermixed silts and clays, whereas the reef slope is dominated by gravelly quartz sands. The main turbidity‐generating process is wave‐driven resuspension, and turbidity ranges up to 175 nephelometric turbidity units (NTU). In contrast, at Phillips Reef, turbidity is <15 NTU and varies little. At Paluma Shoals, turbidity of >40 NTU probably occurs for a total of >40 days each year, and relatively little time is spent at intermediate turbidities (15–50 NTU). The extended time spent at either low or high turbidities is consistent with the biological response of some species of corals to adopt two alternative mechanisms of functioning (autotrophy and heterotrophy) in response to different levels of turbidity. Sedimentation rates over periods of hours may reach the equivalent of 10 000 times the mean global background terrigenous flux (BTF) of sediment to the sea floor, i.e. 10 000 BTF, over three orders of magnitude greater than the Holocene average for Halifax Bay of <3 BTF. As elsewhere along the nearshore zone of the central GBR, dry‐season hydrodynamic conditions form a primary control upon turbidity and the distribution of bed sediments. The location of modern nearshore coral reefs is controlled by the presence of suitable substrates, which in Halifax Bay are Pleistocene and early Holocene coarse‐grained (and relatively stable) alluvial deposits.     

Sea-level and reef accretion history of Marine Oxygen Isotope Stage 7 and late Stage 5 based on age and facies of submerged late Pleistocene reefs,Oahu, Hawaii

In situ Pleistocene reefs form a gently sloping nearshore terrace around the island of Oahu. TIMS Th–U ages of in situ corals indicate that most of the terrace is composed of reefal limestones correlating to Marine Oxygen Isotope Stage 7 (MIS 7, ~ 190–245 ka). The position of the in situ MIS 7 reef complex indicates that it formed during periods when local sea level was ~ 9 to 20 m below present sea level. Its extensiveness and geomorphic prominence as well as a paucity of emergent in situ MIS 7 reef-framework deposits on Oahu suggest that much of MIS 7 was characterized by regional sea levels below present. Later accretion along the seaward front of the terrace occurred during the latter part of MIS 5 (i.e., MIS 5a–5d, ~ 76–113 ka). The position of the late MIS 5 reefal limestones is consistent with formation during a period when local sea level was below present. The extensiveness of the submerged Pleistocene reefs around Oahu compared to the relative dearth of Holocene accretion is due to the fact that Pleistocene reefs had both more time and more accommodation space available for accretion than their Holocene counterparts.     

Reconstruction of palaeoceanographic conditions in the South Atlantic Ocean at the last two Terminations based on calcareous dinoflagellate cysts

 Despite the increasing interest in the South Atlantic Ocean as a key area of the heat exchange between the southern and the northern hemisphere, information about its palaeoceanographic conditions during transitions from glacial to interglacial stages, the so-called Terminations, are not well understood. Herein we attempt to increase this information by studying the calcareous dinoflagellate cysts and the shells of Thoracosphaera heimii (calcareous cysts) of five Late Quaternary South Atlantic Ocean cores. Extremely high accumulation rates of calcareous cysts at the Terminations might be due to a combined effect of increased cyst production and better preservation as result of calm, oligotrophic conditions in the upper water layers. Low relative abundance of Sphaerodinella albatrosiana compared with Sphaerodinella tuberosa in the Cape Basin may be the result of the relatively colder environmental conditions in this region compared with the equatorial Atlantic Ocean with high relative abundance of S. albatrosiana. Furthermore, the predominance of S. tuberosa during glacials and interglacials at the observed site of the western Atlantic Ocean reflects decreased salinity in the upper water layer. Received: 9 November 1998 / Accepted: 26 October 1999     

Geochemistry of Cu, Co, Ni, Zn, Cd and Cr in the surficial sediments of a tropical estuary, southwest coast of India: a granulometric approach

 Geochemical characteristics of six trace metals – Cu, Co, Ni, Zn, Cd and Cr – in the bulk sediment and sand, silt and clay fractions of a tropical estuary on the southwest coast of India have been studied and discussed. In bulk sediment, the trace metal concentration is controlled mainly by the textural composition of the sample. Mud, sandy mud and sandy silt register higher concentrations of trace metals than that in sand-dominant sediments. The granulometric partitioning studies also re-affirmed the role of particle size in enriching the trace metals. The silt and clay fractions exhibit 7–8 times the enrichment of Cu and Cd compared to that in sand. The enrichment factors of Zn, Cr, Ni and Co in the silt and clay fractions, compared to that in sand, are 5–6, 4–5, 2–5 and 2–3 times, respectively. The trace metals in the sand fraction, particularly Ni and Cr, exhibit strong positive correlation with the heavy mineral content of the samples. It clearly indicates a heavy mineral pathway to the trace metals in the sand fraction. Cu and Co in silt and clay fractions exhibit a marked decrease towards the high saline zones of the estuary. This is attributed to the desorption of Cu and Co from particulate phases during estuarine mixing. Contrary to Cu and Co, the content of Zn in the clay fraction shows a marginal increase towards the estuarine mouth. This could be explained by the influx of Zn-rich contaminant discharges from Zn-smelting industries located slightly north of the estuarine mouth. The released Zn will effectively be held in the lattices of the clay mineral montmorillonite, which also exhibits a marked increase towards the estuarine mouth. The anomalously high values of Cd in some places of the Central Vembanad estuary is attributed to the local pollution. Received: 10 July 1995 · Accepted: 3 June 1996     

Application of a solute transport model under variable velocity conditions in a conduit flow aquifer: Olalde karst system, Basque Country, Spain

A model based on numerical solutions, which allows for solving the dispersion equation under variable recharge and velocity conditions, is developed to simulate solute transport in conduit flow aquifers during flow recession periods. As an example, the evolution of a tracer in the little known karst conduit that links the sinking stream of Oma valley to the Olalde spring is investigated in the karstic region of Santa Eufemia-Ereñozar (Basque Country, Spain). The model, with different hypothetical structures, allows for obtaining series of tracer breakthrough curves, which are fitted to experimental data using an optimization algorithm. These results, although they can be used to simulate the tracer evolution between the two points considered, do not allow for determining the internal structure and spatial disposition of contributions in the aquifer.     

Correlations between 29Si, 17O and 1H NMR properties and local structures in silicates: an ab initio calculation

In order to gain insight into the correlations between ²⁹Si, ¹⁷O and ¹H NMR properties (chemical shift and quadrupolar coupling parameters) and local structures in silicates, ab initio self-consistent field Hartree-Fock molecular orbital calculations have been carried out on silicate clusters of various polymerizations and intertetrahedral (Si-O-Si) angles. These include Si(OH)₄ monomers (isolated as well as interacting), Si₂O(OH)₆ dimers (C₂ symmetry) with the Si-O-Si angle fixed at 5° intervals from 120° to 180°, Si₃O₂(OH)₈ linear trimers (C₂ symmetry) with varying Si-O-Si angles, Si₃O₃(OH)₆ three-membered rings (D₃ and C₁ symmetries), Si₄O₄(OH)₈ four-membered ring (C₄ symmetry) and Si₈O₁₂(OH)₈ octamer (D₄ symmetry). The calculated ²⁹Si, ¹⁷O and ¹H isotropic chemical shifts (δ_i ^Si, δ_i ^O and δ_i ^H) for these clusters are all close to experimental NMR data for similar local structures in crystalline silicates. The calculated ¹⁷O quadrupolar coupling constants (QCC) of the bridging oxygens (Si-O-Si) are also in good agreement with experimental data. The calculated ¹⁷O QCC of silanols (Si-O-H) are much larger than those of the bridging oxygens, but unfortunately there are no experimental data for similar groups in well-characterized crystalline phases for comparison. There is a good correlation between δ_i ^Si and the mean Si-O-Si angle for both Q ¹ and Q ², where Q ⁿ denotes Si with n other tetrahedral Si next-nearest neighbors. Both the δ _i ^O and the ¹⁷O electric field gradient asymmetry parameter, η of the bridging oxygens have been found to depend strongly on the O site symmetry, in addition to the Si-O-Si angle. On the other hand, the ¹⁷O QCC seems to be influenced little by structural parameters other than the Si-O-Si angle, and is thus expected to be the most reliable ¹⁷O NMR parameter that can be used to decipher Si-O-Si angle distribution information. Both the ¹⁷O QCC and the ²H QCC of silanols decrease with decreasing length of hydrogen bond to a second O atom (Si-O-H···O), and the δ _i ^H increase with the same parameter. Received: 18 July 1997 / Revised, accepted: 23 February 1998     

On the influence of wavelength-dependent light scattering on the UV-VIS absorption spectra of oxygen-based minerals: a study on silicate glass ceramics as model substances

 The Mie scattering theory shows that the presence of randomly distributed submicroscopic inclusions with narrow size distribution and a refractive index n _i in a matrix with different refractive index n _m may give rise to a λ-dependent, band-like scattering (e.g., Kortüm 1969). Intensity and spectral position of this scattering depend on a combination of several independent parameters, such as size and number of inclusions, their form and n _i /n _m ratio (Kortüm 1969). Recently, it was confirmed that at a certain inclusion size and n _i /n _m ratio the scattering bands can contribute to the UV-edge in the electronic absorption spectra of garnets, influencing their colour (Khomenko et al. 2001). In natural minerals, however, it is impossible to differentiate between individual influence on scattering of the above mentioned parameters because of complex and interconnected variations in number, size and type of inclusions. Additionally, in most natural minerals variable amounts of transition metal ions are present. They may cause UV-VIS absorption due to ligand to metal charge transfer (LMCT) in the same region where band-like scattering may occur (Khomenko et al. 2001). At least some of these difficulties may be avoided in the case of some glass ceramics where number and size of crystalline microinclusions can be controlled by varying ceramization conditions such that fine-grained homogeneous microstructures are formed (e.g., James 1982; Petzoldt and Pannhorst 1991). Thus, glass ceramics of known composition, containing different amounts of microcrystals of specified size, may serve as unique patterns for the experimental study of effects caused by submicrocrystals on bulk properties of transparent solid materials, such as scattering, UV-VIS transparency and colour. Four types of parent glasses and a series of glass ceramic materials produced from them by heat treatment were investigated using transmission electron microscopy (TEM). These materials were also studied by transmission spectrometry in the range 35 000–20 000 cm⁻¹. Different inclusions, from five to several hundred nm in size, were observed in the glass matrices depending on their compositions and heating history. These inclusions represent two groups: early very small crystals of Ti, Zr oxides and relatively large crystals of stuffed high-quartz type or keatite. The absorption spectra of the glass ceramics show largely varying long-wavelength slopes of the UV absorption. UV-edge intensity correlates mostly with the size of the inclusions and changes drastically when larger keatite-type microcrystals are growing. Small variations in the UV edges also follow the early process of Ti-phase separation and nucleation. This may be explained by Ti depletion from the glass matrix and, thus, by reducing the measured intensity of LMCT in the first co-ordination sphere of Ti⁴⁺ ions. The different yellowish colourations of unheated glasses studied here are caused by this effect, whereas developing several hundred-nm-large keatite crystals leads to a strong scattering effect and a milky colour in glass ceramics. Received: 15 February 2002 / Accepted: 11 November 2002 Acknowledgements M. Garsche and A. Kisljuck generously provided the base glass and glass ceramic samples. F. Galbert and S. Herhing-Aghte, both at the Technical University Berlin, helped with electron microprobe analyses and refractive indices measurements, respectively. The Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, provided financial support through grant La324/35. To all these individuals and this institution our sincere thanks are due.     

Microsommite: crystal chemistry, phase transitions, Ising model and Monte Carlo simulations

Microsommite, ideal formula [Na₄K₂(SO₄)] [Ca₂Cl₂][Si₆Al₆O₂₄], is a rare feldspathoid that occurs in volcanic products of Vesuvius. It belongs to the cancrinite–davyne group of minerals, presenting an ABAB… stacking sequence of layers that contain six-membered rings of tetrahedra, with Si and Al cations regularly alternating in the tetrahedral sites. The structure was refined in space group P6₃ to R=0.053 by means of single-crystal X-ray diffraction data. The cell parameters are a=22.161?Å=√3a _dav, c=5.358?Å=c _dav; Z=3. The superstructure arises due to the long-range ordering of extra-framework ions within the channels of the structure. This ordering progressively decreases with rising temperature until it is completely lost and microsommite transforms into davyne. The order–disorder transformation has been monitored in several crystals by means of X-ray superstructure reflections and the critical parameters T _c?≈?750?°C and β?≈?0.12 were obtained. The kinetics of the ordering process were followed at different temperatures and the activation energy was determined to be about 125?kJ?mol^?1. The continuous order–disorder phase transition in microsommite has been discussed on the basis of a two-dimensional Ising model in a triangular lattice with nn (nearest neighbours) and nnn (next-nearest neighbours) interactions. Such a model was simulated using a Monte Carlo technique. The theoretical model well matches the experimental data; two phase transitions were indicated by the simulated runs: at low temperature only one of the three sublattices begins to disorder, whereas the second transition involves all three sublattices.     

Depth of the base of the Jackson aquifer, based on geophysical exploration, southern Jackson Hole, Wyoming, USA

 A geophysical survey was conducted to determine the depth of the base of the water-table aquifer in the southern part of Jackson Hole, Wyoming, USA. Audio-magnetotellurics (AMT) measurements at 77 sites in the study area yielded electrical-resistivity logs of the subsurface, and these were used to infer lithologic changes with depth. A 100–600 ohm-m geoelectric layer, designated the Jackson aquifer, was used to represent surficial saturated, unconsolidated deposits of Quaternary age. The median depth of the base of the Jackson aquifer is estimated to be 200 ft (61 m), based on 62 sites that had sufficient resistivity data. AMT-measured values were kriged to predict the depth to the base of the aquifer throughout the southern part of Jackson Hole. Contour maps of the kriging predictions indicate that the depth of the base of the Jackson aquifer is shallow in the central part of the study area near the East and West Gros Ventre Buttes, deeper in the west near the Teton fault system, and shallow at the southern edge of Jackson Hole. Predicted, contoured depths range from 100 ft (30 m) in the south, near the confluences of Spring Creek and Flat Creek with the Snake River, to 700 ft (210 m) in the west, near the town of Wilson, Wyoming. Received, May 1997 · Revised, February 1998 · Accepted, April 1998     

Formation of diapiric structure in the deformation zone,central Indian Ocean: A model from gravity and seismic reflection data

Analyses of bathymetry, gravity and seismic reflection data of the diffusive plate boundary in the central Indian Ocean reveal a new kind of deformed structure besides the well-reported structures of long-wavelength anticlinal basement rises and high-angle reverse faults. The structure (basement trough) has a length of about 150 km and deepens by up to 1 km from its regional trend (northward dipping). The basement trough includes a rise at its center with a height of about 1.5km. The rise is about 10 km wide with rounded upper surface and bounded by vertical faults. A broad freeair gravity low of about 20 mGal and a local high of 8 mGal in its center are associated with the identified basement trough and rise structure respectively. Seismic results reveal that the horizontal crustal compression prevailing in the diffusive plate boundary might have formed the basement trough possibly in early Pliocene time. Differential loading stresses have been generated from unequal crust/sediment thickness on lower crustal and upper mantle rocks. A thin semi-ductile serpentinite layer existing near the base of the crust that is interpreted to have been formed at mid-ocean ridge and become part of the lithosphere, may have responded to the downward loading stresses generated by the sediments and crustal rocks to inject the serpentinites into the overlying strata to form a classic diapiric structure.     

TEM study of a special grain boundary in a synthetic K-feldspar bicrystal: Manebach Twin

 A monoclinic KAlSi₃O₈ feldspar Manebach twin boundary was synthesized by diffusion bonding and examined using high-resolution transmission electron microscopy. The sharp (001) twin boundary is straight and free of strain. The boundary width is smaller than d₀₀₁. There is no rigid body shift observed at the twin boundary, and the feldspar structure is arranged symmetrically across (001). The twin boundary structure consists of bridged tetrahedral crankshafts, which are characteristic of the feldspar lattice. The grain boundary structure is in good agreement with the geometrical model of Taylor et al. (1934). The grain boundary composition of K_1/2H_1/2AlSi₃O₈ differs from their model. Received: 13 February 2002 / Accepted: 24 December 2002 Acknowledgements We thank M. Rühle, S. Hutt, J. Mayer, A. Strecker and U. Salzberger at MPI, Stuttgart, for their support and valuable advice in preparing TEM sections of bicrystals.     

Water resources development and management in the Cuddapah district, India

 Intensive application of surface water in command areas of irrigation projects is creating water logging problems, and the increase of groundwater usage in agriculture, industry and domestic purposes (through indiscriminate sinking of wells) is causing continuous depletion of water levels, drying up of wells and quality problems. Thus the protect aquifers to yield water continuously at economical cost, the management of water resources is essential. Integrated geological, hydrological (surface and groundwater) and geochemical aspects have been studied for the development and management of water resources in drought-prone Cuddapah district. The main lithological units are crystallines, quartzites, shales and limestones. About 91 000 ha of land in the Cuddapah district is irrigated by canal water. A registered ayacut of about 47 000 ha is irrigated by 1368 minor irrigation tanks. A total of 503 spring channels are identified in the entire district originating from the rivers/streams, which has the capacity of irrigating about 8700 ha. The average seasonal rise in groundwater level is 7.32 m in quartzites, 5.35 m in crystallines, 3.82 m in shales, 2.50 m in limestones and 2.11 m in alluvium. Large quantities of groundwater are available in the mining areas which can be utilised and managed properly by the irrigation department/cultivators for the irrigation practices. Groundwater assessment studies revealed that 584 million m³ of groundwater is available for future irrigation in the district. From the chemical analysis, the quality of groundwater in various rock units is within the permissible limits for irrigation and domestic purposes, but at a few places the specific conductance, chloride and fluoride contents are high. This may be due to untreated effluents, improper drainage system and/or the application of fertilisers. Received: 10 June 1998 · Accepted: 15 November 1998     

Groundwater as a geologic agent: An overview of the causes, processes, and manifestations

 The objective of the present paper is to show that groundwater is a general geologic agent. This perception could not, and did not, evolve until the system nature of basinal groundwater flow and its properties, geometries, and controlling factors became recognized and understood through the 1960s and 1970s. The two fundamental causes for groundwater's active role in nature are its ability to interact with the ambient environment and the systematized spatial distribution of its flow. Interaction and flow occur simultaneously at all scales of space and time, although at correspondingly varying rates and intensities. Thus, effects of groundwater flow are created from the land surface to the greatest depths of the porous parts of the Earth's crust, and from a day's length through geologic times. Three main types of interaction between groundwater and environment are identified in this paper, with several special processes for each one, namely: (1) Chemical interaction, with processes of dissolution, hydration, hydrolysis, oxidation-reduction, attack by acids, chemical precipitation, base exchange, sulfate reduction, concentration, and ultrafiltration or osmosis; (2) Physical interaction, with processes of lubrication and pore-pressure modification; and (3) Kinetic interaction, with the transport processes of water, aqueous and nonaqueous matter, and heat. Owing to the transporting ability and spatial patterns of basinal flow, the effects of interaction are cumulative and distributed according to the geometries of the flow systems. The number and diversity of natural phenomena that are generated by groundwater flow are almost unlimited, due to the fact that the relatively few basic types are modified by some or all of the three components of the hydrogeologic environment: topography, geology, and climate. The six basic groups into which manifestations of groundwater flow have been divided are: (1) Hydrology and hydraulics; (2) Chemistry and mineralogy; (3) Vegetation; (4) Soil and rock mechanics; (5) Geomorphology; and (6) Transport and accumulation. Based on such a diversity of effects and manifestations, it is concluded that groundwater is a general geologic agent. Received, December 1998 · Revised, January 1999 · Accepted, January 1999