Hydrogeological evolution of the Luni river basin, Rajasthan, western India: A review
V N Bajpai
Department of Geology, University of Delhi, Delhi 110 007, India.
e-mail: vnbajpai@yahoo.com

The Luni river basin has been evolved as a result of typical hydrogeomorphic processes of arid zone, operating under the influence of active tectonic lineaments. A detailed analysis of stream morphology in relation to geology and lineaments carried out on selected windows indicated the morphological control of the streams while flowing over the lineaments from the eastern to the western part of the basin. Typical valley lls indicated by dark green tone on digitally processed images and the pediments showing greenish white tone appear in sharp contrast and indicate respectively the graben and horst structures. A detailed identication of lineaments for the georesources and geological evaluation has been carried out. Earlier analysis carried out on Bouguer anomalies correlate with graben and horst structures in the subsurface. Quaternary sequences have been dated
from 80 ka to 3 ka indicating a range of fluvial to aeolian deposits reflecting prevailing climatic conditions. However, the changes in sediment type from coarse and mixed of all size grades to ne in a vertical litho-column warrant further studies on ne resolution stratigraphy and high resolution stratigraphy for understanding climatic variations in the region.

Palaeoenvironmental history of Bap-Malar and Kanod playas of western Rajasthan, Thar desert
B C Deotare1, M D Kajale1, S N Rajaguru1, S Kusumgar2, A J T Jull3 and J D Donahue3
1Deccan College, Postgraduate and Research Institute, Pune 411 006, India.
e-mail: prag21@vsnl.net
2Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
3Radiocarbon Laboratory, Department of Physics, University of Arizona, Tuscon, USA.

Two playas in the arid core of the western margin of the Thar desert viz., Bap-Malar and Kanod, have been investigated using palynology, geomorphology, archaeology, AMS-radiocarbon dating, stable isotopes, evaporite mineralogy and geoarchaeology. The principal objective was to obtain a reliable lithostratigraphy of the playa sediments. These are about 7m thick in the Bap-Malar and > 2:5 to 3m thick in the Kanod. AMS 14C dates of > 15 ka BP on pollen from sediment layers indicates that the Bap-Malar playa possibly existed even during the LGM. These playas were full of water during the early Holocene (8 ka BP { 5.5 ka BP) and were ephemeral during the Pleistocene- Holocene transition and early to mid to late Holocene. The playas dried almost 1000 years earlier than those occurring on the eastern margin. Pollen of graminaceae, chenopodiaceae / amaranthaceae, cyperaceae etc. and evaporite minerals like gypsum, halite in the profiles indicate that the playas were surrounded by vegetation dominated by grass and that, they remained brackish to saline even during the mid Holocene, lake full stage. Stable dune surfaces, pediments with regoliths, and gravelly channels of ephemeral streams provided a favorable geomorphic niche for nomadic human activity since  7 ka BP. Though local ecological factors have played an important role in the evolution of the playas, the winter rains, connected with northwesterly depressions, most likely played a vital role in maintaining these playas.

Calcretes in the Thar desert: Genesis, chronology and palaeoenvironment
R P Dhir1, S K Tandon2, B K Sareen1, R Ramesh3, T K G Rao4, A J Kailath3 and N Sharma2
1Central Arid Zone Research Institute, Jodhpur 342 003, India.
2Geology Department, University of Delhi, Delhi 110 007, India.
3Physical Research Laboratory, Ahmedabad 380 009, India.
4RSIC, Indian Institute of Technology, Powai, Mumbai 400 076 , India.

The calcretes in the Thar desert occur in a variety of settings, including the piedmonts, sheet-wash aggraded plains; and this study adds calcretes in regolith and colluvio-alluvial plains to the group of settings in which calcretes occur in the region. Field logs, morphological details and analytical data such as petrographic, cathodoluminescence and geochemical characteristics are
described along with a discussion on their implications. Sand dunes and sandy plains dating to < 20 ka have weakly developed calcretes. The better-developed calcrete horizons occur in pied- monts, interdunes or in areas that have sucient groundwater. Deep sections in the region show phases of calcrete development in aeolian sand aggradation at  150,  100,  60 and 27{14 ka. The extensive sheetwash plains have mature calcretes and date to mid-Pleistocene. Our studies indicate that these calcretes represent a hybrid process, where carbonate enrichment of the originally calcareous host occurred due to periodically raised groundwaters, and its differentiation into nodules occurred under subaerial environment i.e., after recession of groundwater. Deep sections also show a stack of discrete calcretes that developed in individual aggradation episodes with
hiatuses as indicated by ESR dating results. Nodules display a multiplicity of carbonate precipitation events and internal reorganization of calcitic groundmass. The process is accompanied by degradation and transformation of unstable minerals, particularly clays and with a neosynthesis of palygorskite.

The ancient calcretes are dated from the beginning of the Quaternary to  600 ka and show more evolved morphologies marked by brecciation, dissolution, laminar growth on brecciated surfaces, pisolites and several generations of re-cementation. Mica/chlorite schists and such other rocks are particularly vulnerable to replacement by carbonate. In an extreme case, replacement of quart-zose sandstone was observed also. The presence of stretches of alluvio-colluvial plains in an area
presently devoid of drainage bespeaks of occasional high-energy fluvial regime, under a semi-arid climate. The mid-Pleistocene period saw a shift towards more arid climate and this facilitated sheetwash aggradation. Finally, during the late Pleistocene, aggradation of aeolian sands indicated a progressively drier climate. However, this does not nd its reflection in stable isotope data. The amount of carbonate in the form of calcretes is substantial. The present studies indicate that aeolian dust or rainwater are minor contributors to the carbonate budget. A more important source was provided by the pre-existing calcretes in the sheetwash aggraded plains and detrital carbonate in the aeolian sediments. The original source of carbonate in the region, however, remains unresolved and will need further investigations. Electron spin resonance protocols for the dating of calcretes
were developed as a part of this study and the results accorded well with geological reasoning.


Late Quaternary stratigraphic development in the lower Luni, Mahi and Sabarmati river basins, western India
M Jain1, S K Tandon2 and S C Bhatt3
1Risoe National Laboratory, DK-4000 Roskilde, Denmark.
2Department of Geology, Delhi University, Delhi - 110 007, India.
3Department of Geology, Bundelkhand University, Jhansi, UP, India.
1email: mgj@aber.ac.uk

This study reviews the Quaternary alluvial stratigraphy in three semi-arid river basins of western India i.e., lower Luni (Rajasthan), and Mahi and Sabarmati (Gujarat alluvial plains). On the basis of OSL chronologies, it is shown that the existing intra-valley lithostratigraphic correlations require a revision. The sand, gravel and mud facies are present during various times in the three basins, however, the fluvial response to climate change, and the resulting facies associations, was different
in the Thar desert as compared to that at the desert margin; this makes purely lithostratigraphic correlations unviable. It is further shown that the rivers in the Thar desert were more sensitive to climate change and had small response times and geomorphic thresholds as compared to the desert-margin rivers. This is illustrated during the early OIS 1, when the Luni river in the Thar
desert was dynamic and showed frequent variations in fluvial styles such as gravel bedload braided streams, sand-bed ephemeral streams and meandering streams, all followed by incision during the early Holocene. The coeval deposits in Sabarmati, however, only show a meandering, floodplain-dominated river.

 Late Quaternary alluvial deposits in these basins unconformably overlie some older deposits that lack any absolute chronology. Based on the facies types and their associations, and the composition and architecture of the multistoried gravel sheets in the studied sections, it is suggested that older deposits are of pre-Quaternary age. This hypothesis implies the presence of a large hiatus incorporating much of the Quaternary period in the exposed sections.


The aeolian sedimentation record of the Thar desert
A K Singhvi1 and Amal Kar2
1Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
2Central Arid Zone Research Institute, Jodhpur 342 003, India.

A review of the aeolian sedimentary record of the Thar desert is presented. This includes a regional survey of the major dune forms, their genesis and their relationship to climate and other regional landforms. A key aspect of this work is the chronometry of the dunes using luminescence methods. Luminescence dating of sand has enabled quantication of the duration of the phases of sand aggradation and quiescence, time scales of dune migration and the dating of pedogenic carbonates. We demonstrate that the conventional wisdom of synchronicity of dune aggradation with glacial epoch is not true in the context of Thar sands and here only a short duration window of opportunity existed for dune aggradation. Luminescence ages further suggest that this window occurred during a transitional climatic regime from glacial to interglacial about 4{10 ka after the glacial epoch.
Other inferences included are that:
 the aeolian activity in the Thar began over >150 ka, resolving that Thar is not of anthropogenic origin as suggested previously;  the present spatial extent of the aeolian activity in the Thar is in a contracted stage compared to that in the geological past, which refutes the arguments on its rapid north-eastward expansion;
 the current dune migration rates in areas of signicant human-induced disturbances are much higher than during the geological past;
  the monsoon activity in the Thar varied signicantly, from being minimal during the isotopi c
marine stages 4 and 2 to being close to the present during stage 3;
 on shorter time scales the dune activities correlated with the lacustrine records of the region with a phase dierence of a few centuries and a periodicity of  1500 years;
 the sand aggradation climate in the southern margin in Gujarat gradually shrank northwards such that in general dunes older than 10 ka are seen in the extreme southern margin and dunes younger than 2 ka ages occur mostly in the western part of Rajasthan.

The vorticity and angular momentum budgets of Asian summer monsoon
P L S Rao1, U C Mohanty2, P V S Raju2 and M A Arain1
1School of Geography and Geology, McMaster University, Hamilton, ON, Canada L8S 4K1.
2Centre for Atmospheric Sciences, Indian Institute of Technology-Delhi Hauz Khas, New Delhi 110 016, India.

The study delineates the vorticity and angular momentum balances of Asian summer monsoon during the evolution and established phases. It also elucidates the dierences between these balances in the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis and the National Centre for Medium Range Weather Forecasts (NCM- RWF) analysis elds. The NCEP/NCAR reanalysis for a 40 year period (1958{97) and the NCM- RWF analysis for a three year (1994{96) period are made use of for the purpose. The time mean summer monsoon circulation is bifurcated into stable mean and transient eddy components and the mean component is elucidated.

The generation of vorticity due to stretching of isobars balances most of the vorticity transported out of the monsoon domain during the evolution period. However, during the established period, the transportation by the relative and planetary vorticity components exceeds the generation due to stretching. The eective balancing mechanism is provided by vorticity generation due to sub-grid scale processes. The flux convergence of omega and relative momenta over the monsoon domain
is eectively balacnn by pressure torque during the evolution and established phases. Neverthe- less, the balance is stronger during the established period due to the increase in the strength of circulation.

Both the NCMRWF and NCEP elds indicate the mean features related to vorticity and angular momentum budgets realistically. Apart from the oceanic bias (strong circulation over oceans rather than continents), the summer monsoon circulation indicated by the NCEP is feeble compared to NCMRWF. The signicant terms in the large-scale budgets of vorticity and angular momentum enunciate this aspect.

Observations of the atmospheric surface layer parameters over a semi arid region during the solar eclipse of August 11th, 1999
Praveena Krishnan, P K Kunhikrishnan, S Muraleedharan Nair, Sudha
Ravindran, Radhika Ramachandran, D B Subrahamanyam and M Venkata Ramana
Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, India.
e-mail: pkkrishnan−spl@yahoo.co.in pk−kunhikrishnan@vssc.org

This paper discusses the observations of the Atmospheric Surface Layer (ASL) parameters during the solar eclipse of August 11th, 1999. Intensive surface layer experiments were conducted at Ahmedabad (23210N; 72360E), the western part of India, which was close to the totality path. This rare event provided by nature is utilised to document the surface layer eects during the eclipse period using measurements of high frequency fluctuations of temperature, tri-axial wind
components as well as mean parameters such as temperature, humidity, wind speed and subsoil temperature. Analysis showed that during the eclipse period, the turbulence parameters were aected leading to the suppression of the turbulence process, the main dynamic process in the atmospheric boundary layer, while the mean parameters showed variations within the natural variability of the observational period. The spectra of the wind components and temperature indicated decrease in  spectral power by one order in magnitude during the eclipse period. The rate of dissipation of turbulent kinetic energy is found to decrease by more than one order during the eclipse period. The stability parameter showed a change from unstable to stable condition during the period of eclipse and back to unstable condition by the end of eclipse.

Enrichment characteristics of radioelements in various types of rock from Sambalpur district, Orissa, India
A A P S R Acharyulu, B Sreenivasa Murty and B K Bhaumik
Physics Laboratory, Eastern Region, Atomic Minerals Directorate for Exploration and Research, Department of
Atomic Energy, Jamshedpur 831 002, India.

Over the years a large number of rock samples were collected by the explorationists of Atomic Minerals Directorate for Exploration and Research (AMD), Department of Atomic Energy from Sambalpur district, Orissa, India. The variation of uranium enrichment with various types of rock in these samples was studied with an aim to help quick reviewing of toposheets in case of newly taken up areas, for uranium exploration. The radioelements U, Th and K determined for these samples are plotted in ternary diagrams for classifying them as being uraniferous or thoriferous with respect to various type of rocks, keeping the total natural content of radioelements (expressed by eU3O8) as a parameter.

In this study it is seen that samples of quartzofeldspathic breccia type are enriched in uranium irrespective of their further sub classication and eU3O8 content. Granites are enriched in thorium when eU3O8 < 100 ppm and are enriched in uranium when eU3O8 > 100 ppm.
A database prepared for this purpose may be useful for reviewing toposheets in future.


Influence of southern oscillation and SSTs over Nino-3.4 region on the winter monsoon rainfall over coastal Andhra Pradesh
O S R U Bhanu Kumar, C V Naidu and S R L Rao
Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, India.

An analysis of the mean monthly data of 124 years reveals that the relationship between the Southern Oscillation Index in September and the winter monsoon rainfall (WMR) over Coastal Andhra Pradesh (CAP) is variable and non-stationary. In the recent four decades, however, SOI (Sept) is negatively and signicantly correlated with CAP WMR. A similar analysis is performed using 50 years of mean monthly SSTs over Nino-3.4 region in August and September and CAP WMR to
detect a possible relationship and there is a striking positive relation between them. In both of the above cases, the September signal is more signicant in the recent four decades than for the other months and seasons for probable prediction of CAP WMR. Finally, to examine the influence of SO on the winter monsoon rainfall, a non-parametric test \Mann-Whitney Rank Statistics" test has been applied to the rainfall associated with extreme positive and negative SOI events.


Delineation of structures favourable to groundwater occurrence employing seismic refraction method { A case study from Tiruvuru, Krishna district, Andhra Pradesh
N Sundararajan, Y Srinivas, M Narasimha Chary+, G Nandakumar and A Hanmantha Chary
Centre of Exploration Geophysics, Osmania University, Hyderabad 500 007, India.
+Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet,
Hyderabad 500 016, India.

The contacts associated with an outlier in biotite gneiss and sandstones near Tiruvuru, Krishna district, Andhra Pradesh which are generally favourable for groundwater occurrence were investigated employing refraction seismic method. Results were examined by correlating the signals with local geology, bore well data and other available information in order to improve the reliability of interpretation.

Seasonal cycle of hydrography in the Bab el Mandab region, southern Red Sea
M A Al Saafani and S S C Shenoi
Physical Oceanography Division, National Institute of Oceanography, Goa 403 004, India.

The seasonal cycle of temperature - salinity variations in the Bab el Mandab region (southern Red Sea) is described using CTD data collected during four cruises spread over the period May 1995 { August 1997. A two layer system exists during early summer, winter and spring while a three layer system exists during summer. During summer, a large amount of the Gulf of Aden water intrudes into the Bab el Mandab region; up to the northern limit (14:5N). The quantity of Red Sea water that flows into the Gulf of Aden is maximum during the winter and minimum during the summer.


Folds in multilayered rocks of Proterozoic age, Rajasthan, India
A M Bhola1, B K Sharma2 and S K Ghosh
Department of Geology, University of Delhi, Delhi 110 007, India.
1e-mail: ambhola@du.ac.in
2e-mail: bksharma@du.ac.in

First phase folds F1 developed in polydeformed Ajabgarh Group rocks of Proterozoic age are studied using various geometrical methods of analysis for compatibility of homogeneous strain in both class 1{3 pairs by correlating t0 = plots with existing curves for competent layers and matching t= plots with the flattening curves for the incompetent layers.

F1 folds were initiated by the process of buckling but underwent [(2=1) = 0:2 to 0.7] for com- tent layers and R-values of 1.1 to 5 for incompetent layers. The varying flattening is also revealed by the geometry of folds. The apparent buckle shortening of folds which ranges between 49 and 67 per cent with a majority of the folds having shortening values between 50% and 55% (exclusive of layer parallel strain) and inverse thickness method strain up to 50%. Besides flattening, the fold geometry was also modified by the pressure solution. This is borne by the presence of dark seams rich in phyllosilicates and disseminated carbonaceous material offsetting limbs of buckled quartz veins in slates.