deformation of the proterozoic successions in the pranhita godavari basin south india a regional perspective abstract a number of proterozoic intracratonic basins namely cuddapah pranhita godavari pg abujhmar indravati ampani chattisgarh and khariar basins occur to the immediate west of the eastern ghats granulite belt eggb in india of these the pg basin hosts an axial outcrop of late paleozoic mesozoic gondwana rocks flanked on either side by proterozoic sedimentary successions among the latter the somanpalli group in the eastern part and the penganga and yellandlapad groups in the western part show contractional folds imbricate thrusts and cleavage these deformed rock groups and the pakhal group in the western belt include shallow to deep marine sedimentary successions stratigraphic relations indicate that the major episode of deformation in the pg basin occurred prior to deposition of the unconformably overlying sullavai group the latter is a neoproterozic undeformed fluvial sequence with some aeolian intercalation deformed successions also occur in other proterozoic basins of the south indian craton notably those in the nallamalai fold belt nfb along the eastern part of the cuddapah basin the major fold fault structures in the arcuate nfb vary in trend from nnw sse in the southern part to ne sw in the northern part at least one phase of folding and thrusting predates the 1000 ma old event in the eggb deformation associated with the grenville and later pan african tectonic mobility has imprints on the neoproterozic kurnool group in the palnad sub basin northern nfb the major structural trend in the pg basin is nw se except for the yellandlapad area in the southern part where late superposed folds trend e w the deformation along the eastern margin of the chattisgarh basin its southern extension in khariar ampani and indravati basins may also be related with the main grenville deformation in the eggb a regional mesoproterozic episode of basin closure and inversion is thus evident from the proterozoic cratonic successions west of the eastern ghats this occurred prior to a regional development of neoproterozoic successions at these sites

origin and palaeomagnetism of the mesoproterozoic gangau tilloid basal vindhyan supergroup central india abstract the stratigraphic position and claimed glacial origin of the diamictitic gangau tilloid in central india have long been controversial our study shows that the 50 m thick tilloid overlies the palaeoproterozoic bijawar group with angular unconformity and forms the base of the mesoproterozoic 1200 1400 ma vindhyan semri group the tilloid contains clasts up to 1 m across virtually all of stable siliceous lithologies like those of the bijawar group supported by a fine grained haematitic matrix glacially faceted and striated clasts were not observed the tilloid displays much disruption of beds and pebble sized clasts of lamellar and nodular chert quartz underwent brittle fracture and plastic puckering during deposition values are high for sio2 68 8 85 5 and fe2o3 7 73 20 1 and low for al2o3 1 03 5 50 and other elements the composition of the tilloid is similar to that of laterites formed on sandstone bedrock thermal step demagnetisation of 91 core samples from seven sites spanning the tilloid revealed three components a low temperature tertiary component a an intermediate temperature steep downward component b and more rarely a high temperature less steep component c haematite is the likely carrier of the remanence interpreted as chemical remanent magnetisation c may be older than b and date from near the time of deposition bedding corrected c has a direction of d 161 4 i 63 2 a95 12 7 that gives a pole at 18 2 s 93 4 e dp 15 8 dm 20 0 the pole plots near 1200 ma on the australian precambrian apparent polar wander path using the veevers et al 1991 reconstruction for east gondwanaland the sedimentology composition and geochemistry of the gangau tilloid accord with deposition by continental debris flows derived mainly from a ferruginous regolith formed on sedimentary rocks zoned chert quartz bodies may have been precipitated in shrinkage cracks and voids in the lower saturated zone of the regolith and possibly also as thin beds or crusts of silica within the drainage system the inferred ferruginous weathering in the source area favours warm and humid conditions with a dry season and precipitation of silica in the regolith implies seasonal or longer wet and dry intervals debris flow activity indicates brief episodes of abundant runoff there is no evidence of glaciation production of abundant ferric iron in the source regolith implies the presence of appreciable atmospheric oxygen by 1200 1400 ma

tectonothermal events in central indian tectonic zone citz and its implications in rodinian crustal assembly abstract the precambrian crust of central india comprising bundelkhand craton bkc in the north and bastar craton bc in the south were accreted along a ene wsw trending proterozoic central indian tectonic zone citz the citz is an ensemble of several low to medium grade supracrustal belts gneisses granitoids and a few linear tracts of granulite belts a number of crustal scale shear zones define the boundaries of the supracrustal belts and discrete terrains available data suggest polyphase tectonothermal events in citz spanning from palaeoproterozoic to early neoproterozoic each event is characterized by contractional tectonic regime and attendant metamorphism and magmatism a new plate tectonic model in contrast to the earlier one of yedekar et al geol surv india spec publ 28 1990 is proposed according to the present model a northward dipping subduction system leading to continent continent collision explains the growth and assembly of the citz the continent continent collision is tentatively placed at ca 1 5 ga which is comparable to similar tectonothermal event in chotanagpur gneissic complex further east while the suturing event predates the global grenvillian orogeny ca 1 3 1 0 ga the sausar tectonothermal event ca 1 1 0 9 ga of citz is correlatable with the latter

hindoli group of rocks in the eastern fringe of the aravalli delhi orogenic belt archean secondary greenstone belt or proterozoic supracrus tals abstract the very low grade metamorphic sequence of volcano sedimentary rocks sandwiched between the platform sediments of the vindhyan supergroup to the east and the banded gneissic complex bgc to the west in the eastern fringe of the aravalli delhi orogenic belt has remained a stratigraphic enigma in the precambrian geology of rajasthan this sequence known earlier as the gwalior series and in contemporary literature as the hindoli group has been considered by several workers as a proterozoic supracrustal unit and by some others as an archean secondary greenstone belt based purely on geological considerations u pb zircon geochronology was conducted to find an answer to this controversy on samples of felsic volcanics conformably intercalated with the hindoli sediments and hence considered contemporaneous with them zircons from a sample of massive rhyodacite gave a concordia age of 1854k7 ma though zircons from a sample of felsic tuff gave a wide range of ages between 3259 1877 ma careful consideration of the nature of the samples and their constituent zircons suggests that the hindoli group rocks represent a low grade proterozoic supracrustal cover sequence in the eastern part of the bhilwara belt broadly synchronous to the aravalli bhilwara sedimentation around 1 8 ga

1891 1883 ma southern bastar cuddapah mafic igneous events india a newly recognized large igneous province abstract a newly recognized remnant of a paleoproterozoic large igneous province has been identified in the southern bastar craton and nearby cuddapah basin from the adjacent dharwar craton india high precision u pb dates of 1891 1 0 9 ma baddeleyite and 1883 0 1 4 ma baddeleyite and zircon for two se trending mafic dykes from the bd2 dyke swarm southern bastar craton and 1885 4 3 1 ma baddeleyite for a mafic sill from the cuddapah basin indicate the existence of 1891 1883 ma mafic magmatism that spans an area of at least 90 000 km2 in the south indian shield this record of 1 9 ga mafic ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south eastern margin of the south indian shield is a widespread geologic phenomenon on earth similar periods of intraplate mafic ultramafic magmatism occur along the margin of the superior craton in north america 1 88 ga molson large igneous province and in southern africa along the northern margin of the kaapvaal craton 1 88 1 87 ga dolerite sills intruding the waterberg group existing paleomagnetic data for the molson and waterberg 1 88 ga large igneous provinces indicate that the superior and kalahari cratons were at similar paleolatitudes at 1 88 ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a limpopo like 2 0 ga deformation zone in the superior province the widespread occurrence of 1 88 ga intraplate and plate margin mafic magmatism and basin development in numerous archean cratons worldwide likely reflects a period of global scale mantle upwelling or enhanced mantle plume activity at this time

microbial mat control on siliciclastic precambrian sequence stratigraphic architecture examples from india abstract an attempt has been made to understand within a cause and effect framework the sequence building pattern in proterozoic time that witnessed non uniformiteranean microbial mat growth and epeiric sea development marine or marginal marine successions in three neoproterozoic formations the sonia sandstone in western india and the sirbu shale and upper bhander sandstone in central india have been examined here for this purpose the strikingly common feature of all three formations is vertical stacking of highstand systems tracts without intervention of any significant transgressive deposits in one instance only the transgressive systems tract is represented by a thin granular transgressive lag and in all other cases the evidence of transgression is simply marked by the presence of marine flooding surfaces the absence of transgressive strata relates to the low sea floor gradients which facilitated rapid transgressions combined with a generally low sediment supply aggradation under normal regressive highstand conditions in spite of the low sediment supply was promoted by the prolific growth of microbial mats which reduced the effects of wave and current reworking by organic binding of clastic particles

gravity field deep seismic sounding and crust mantle structure over the cuddapah basin and dhawar craton of india abstract south of 21 n the indian peninsula is characterised by very low bouguer gravity anomalies ranging from 110 mgal to 60 mgal a low gravity field is present over the eastern margin of the cuddapah basin dharwar craton as well as areas covered by the deccan traps several profiles drawn in the e w direction across the southern peninsula show that the gravity anomalies have long wavelengths of the order of several hundred kilometers this feature appears to be related to the geoid low brought out by gaposchkin and lambeck 1970 1971 on the basis of satellite gravity data deep seismic sounding investigations carried out by kaila et al 1979 along kavali udipi profile running from the east coast of india to the west coast provide a good deal of subsurface information concerning the configuration of the cuddapah basin dharwar schist belts and the moho along the entire profile various factor contribute to the gravity field the gravity field over cuddapah basin has been analysed in terms of sediments comprising the basin the moho configuration and intrusive bodies present underneath the basin in order to explain satisfactorily the low gravity field over the dharwar craton to the west of the cuddapah basin a lower density upper mantle has been inferred this feature appears to be related to processes that have taken place during the eruption of the deccan traps over the indian peninsula and the formation of the indian ocean

alluvial fan to storm dominated shelf transition in the mesoproterozoic singhora group chattisgarh supergroup central india abstract mesoproterozoic siliciclastic strata at the basal part of the singhora group central india provide an example of a pre vegetation alluvial fan and braid delta depositional system detailed process based facies and paleoenvironmental analysis have led to the identification of 15 facies types grouped under four different facies associations which record paleoenvironmental settings ranging between continental alluvial fan and distal marine shelf two formations investigated are the rehtikhol formation formed in a conglomeratic or coarse arenaceous alluvial fan braid plain and delta setting and the overlying saraipalli formation which preserved the sedimentation record of a storm infested inner to distal shelf setting the preponderance of granular and sandy sediment gravity flow products with subordinate rock fall deposits within the alluvial fan system together with a pervasive west northwesterly paleocurrent is suggestive of a gently sloping beveled source area in the east southeast distal to the inferred fan a braid plain system developed with the decrease in the depositional slope streams within the braid plain had widths up to several hundred meters the braid plain system at the margin of the marine basin developed a delta with the deposition of wave reworked fluvial dominated sediments the crest trends of wave generated bedforms within the inferred delta sediments depict the northeast southwest shoreline interpreted for the singhora sea the prodeltaic basinal part of the singhora braid delta was storm infested the inner shelf acted as a zone of throughput with high velocity storm flows eroding deep narrow scours gutter casts and is interpreted as a zone of bypass operation of geostrophic currents on the singhora shelf has been inferred from the directional relationship of gutters with the inferred orientation of the shoreline a proximal distal relationship has been established within the shelf products based on the relative dominance of erosional and depositional features the singhora alluvial fan and braid delta facies are arranged in a fining upward sequence denudation of the source area increasing bathymetry with relative rise in sea level or a combination of both is inferred both extrabasinal and intrabasinal forcings possibly contributed to the deepening upward trend

sandstone composition of the upper jurassic emuerhe formation from the mohe basin china the sandstone composition analysis carried out on the samples from three different sections western section middle section and eastern section of the upper jurassic emuerhe formation in the mohe basin northeastern china reveals that the okhotsk orogenic belt and the ergun massif basement are the source areas for the emuerhe formation the source area of the western section and the a segment of the middle section is the okhotsk orogenic belt the tectonic attributes of the okhotsk orogenic belt are those of a dissected arc the transportation distance of clastic rocks from these sections gradually lengthened and the uplift rate of the okhotsk orogenic belt gradually increased the source area of the eastern section and the b segment of the middle section is the ergun massif basement compared with the okhotsk orogenic belt the tectonic attributes and uplift rate of the ergun massif basement are different from the former the attributes of the ergun massif basement are those of basement uplift the transportation distance of clastic rocks from the eastern section and the b segment of the middle section gradually shortened and the uplift rate of the ergun massif basement gradually decreased these conclusions are consistent with the characteristics of a foreland basin thus we can draw the conclusion that the mohe basin belongs to the foreland basin

u th pb monazite geochronometry of the eastern ghats belt india timing and spatial disposition of poly metamorphism abstract the eastern ghats belt of peninsular india exposes the deeply eroded high grade parts of a composite orogenic belt that including the rayner complex in antarctica once formed an important crustal component of proterozoic east gondwana recent work established four major crustal units separated by tectonic boundaries the late archaean jeypore and rengali provinces northwest and north the late palaeoproterozoic ongole domain southwest and the meso neoproterozoic eastern ghats province east to identify the timing and regional distribution of tectonothermal events within the composite belt monazites in 66 pelitic and felsic granulite samples from 26 localities were studied with the electron microprobe chemical dating technique in most cases the monazites are polygenetic with rock and grain scale chemical and chronological heterogeneities that document growth during episodes of partial melting and significant modifications through strain and fluid induced recrystallisation and replacement in the ongole domain monazite apparent age populations show that the fabric defining high to ultrahigh grade metamorphism occurred between 1650 and 1590 ma after the emplacement of basic and felsic plutonic complexes into the supracrustal granulites at ca 1 7 ga the monazites do not record pre intrusion tectonothermal events but give evidence of later imprints of ductile to brittle deformation accompanied by hydration between 1450 and 1350 ma that are correlated with mesoproterozoic crustal extension rifting and alkaline plutonism along the eastern cratonic margin the ongole domain was not involved in grenvillian crustal reworking and pan african tectonism 520 ma was focused in shear zones especially at tectonic boundaries in the eastern ghats province monazite apparent age populations in the time span between 1260 and 500 ma witness a prolonged and polyphase tectono metamorphic evolution during mesoproterozoic and neoproterozoic early phanerozoic times apparent ages of high y monazite grains armoured in garnet of khondalites and high mgal granulites bracket the timing of early ultra high temperature metamorphism between 1250 and 1100 ma dominant and comparable apparent age populations of matrix monazites in metapelitic and felsic granulites indicate that the fabric defining granulite facies metamorphism and anatexis of the entire terrane occurred between 980 and 940 ma synchronous with the intrusion of voluminous megacrystic granitoid plutons the easternmost chilka lake domain experienced renewed pervasive deformation and high grade metamorphism and melting at ca 800 ma coeval with emplacement of massif type anorthosite and further high grade reworking during neoproterozoic tectonism 680 650 ma apparent ages of 530 470 ma recorded in matrix monazites of strongly sheared mylonitic granulites indicate that the internal segmentation and structural configuration of the eastern ghats belt and its cratonic forelands occurred during late neoproterozoic early phanerozoic times through w to nw directed intracratonic deformation focussed on major shear zones and terrane boundaries when the grenvillian eastern ghats province rayner complex terrane collided with india

basal tuffs and contemporaneity of the chattisgarh and khariar basins based on new dates and geochemistry abstracttime bracketing of thousands of meters thick unmetamorphosed fossil free sedimentary successions is a long standing challenge in indian proterozoic stratigraphy limited geochronologic ages from either the basal or the upper part of the successions prevent workers from constraining the precise age interval for individual basin fill leaving aside the issue of correlatability between different basin sequences this article deals with silicic tuff units from the basal part of two areally separated proterozoic sedimentary successions of central india the singhora group of the chattisgarh supergroup and the khariar group the singhora tuff is enriched in large ion lithophile rb ba th etc elements and light rare earth elements and in a discrimination diagram falls within the volcanic arc granite field u th pb electron probe microanalyzer geochronology of monazite and zircon grains from the singhora tuff revealed several age data clusters namely 2500 2100 1800 1500 and 1000 ma in c

ramgundam sandstone a middle proterozoic shoal bar sequence abstract much of the abundant precambrian sedimentary record of india appears to have been deposited in shallow coastal environments one example of this the ramgundam sandstone is a moderately thick 120 m unit composed of lens shaped bodies of arkosic sandstone and interbedded sandstone and shale outcrops are mostly limited to small quarry pits but sufficient exposures are present to show major bedding characteristics principal components are wave built bars and interbar fillings the bars 1 to 5 m thick and tens of meters wide contain laterally accreting sets of trough cross bedded sandstone interbar deposits are composed of horizontal to subhorizontal beds of ripple laminated sandstones and interlaminated shales siltstone and sandstone containing wavy lenticular and flaser laminae shallow water subaerial exposure and probable tidal fluctuations are recorded on the bar surfaces by flat topped ripples and interference ripples water recession marks and desiccation cracks

overview of the meso to neoproterozoic evolution of the vindhyan basin central india abstract despite its origin in a stable intracratonic basin the vindhyan supergroup central india embodies tectonic driven depositional cycles of various orders a marked change in sedimentation pattern was coupled with a transient plate margin compression in the otherwise extensional regime an unconformity laterally correlatable with a conformity that divides the supergroup in two sequences arose in consequence maximum flooding surfaces mfs within the sequences can generally be correlated with intrabasinal volcanism that drastically diminished through the initial rift to later sag stage of basin evolution concomitantly with the change in tectonic milieu paleoseismic deformation structures and related deposits also became drastically reduced in scale nevertheless metre and decametre scale depositional cycles correlate well with the subtle record of paleoseismicity intermittent tilting of the depositional substratum is demonstrable in systematic bed dip changes in metre scale fining upward depositional cycles in the rift stage in the sag stage it is reflected merely in consistent deflection of paleocurrent directions at the top of the decametre scale parasequence sets towards the top of the supergroup progressive thickening and decreasing frequency of parasequences indicates a declining rate of basin subsidence progradation despite punctuation thus resulted in basin filling

the crater facies kimberlite system of tokapal bastar district chhattisgarh india abstract discovery of diamondiferous kimberlites in the mainpur kimberlite field raipur district chhattisgarh in central india encouraged investigation of similar bodies in other parts of the bastar craton the earlier known tokapal ultramafic intrusive body located beyond the 19 km milestone in tokapal village along the jagdalpur geedam road was reinterpreted as crater facies kimberlite its stratigraphic position in the meso neoproterozoic intracratonic sedimentary indravati basin makes it one of the oldest preserved crater facies kimberlite systems ground and limited subsurface data dug tube wells and exploratory boreholes have outlined an extensive surface area 550 ha of the kimberlite the morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic epiclastic rocks that are surrounded by a 2 km wide spread of pyroclastic rocks lapilli tuff tuff ash beds and volcaniclastic breccia drill holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the kanger and jagdalpur formations and also form sills within the sedimentary sequence of the indravati basin the lapilli tuffs are commonly well stratified and display slumping base surges and lava flows occur in the southern part of the tokapal system the geochemistry and petrology of the rock correspond to average group i kimberlite with a moderate degree of contamination however the exposed rock is intensely weathered and altered with strong leaching of mobile elements ba rb sr layers of vesicular fine grained glassy material represent kimberlitic lava flows tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc serpentine carbonate matrix with locally abundant spinel and sphene garnet has not been observed and phlogopite is very rare very limited microdiamond testing two 18 kg samples proved negative however the composition of chromite grains indicate crystallization in the diamond stability field

marine to fluvial transition proterozoic upper rewa sandstone maihar india abstract the proterozoic upper rewa sandstone in central india is generally interpreted as entirely marine a detailed study in maihar madhya pradesh however reveals upward transition from marine to fluvial through a mixed facies association also bearing eolian imprints facies associations differ in stratal geometry and arrangement palaeocurrent direction and pattern as well as in grain size and sorting the fine grained marine association is dominated by tidal sheet deposits with characteristic rhythmic changes in thickness and style of cross stratification and their packages as well as local evidence of current reversals subordinate beach deposits are also present the major palaeocurrent direction is highly consistent and westward the coarse grained often granule rich sandstones of the fluvial association are embodied by four facies correlatable with flow stages and relative bed shear these braided stream deposits are preserved in vertical stacks of tabular sandbodies in response mainly to basin subsidence the palaeocurrent pattern is unimodal and northwestward but its dispersion covers 180 the rocks are poorly sorted and at places incorporate sand clasts although they are virtually mud free and mineralogically mature as in two other associations the medium to fine grained sandstones at the intermediate interval between the marine and fluvial associations are of distinct lensoid geometry they incorporate representatives from the previous two associations as well as translatent strata interdune erg deposits and possibly aeolian cross strata consequently the palaeocurrent pattern is polymodal with a spread over 270 an overall coarsening up trend in the fluvial part and thinning up trend in the tidal part imply progradation in spite of evidence of basin subsidence a high rate of sediment discharge from a perennial river system in a humid climate is suggested

facies architecture and sequence development in a neoproterozoic carbonate ramp lakheri limestone member vindhyan supergroup central india abstract outcrop based study of high resolution sequence stratigraphy intra and interfacies sedimentologic clues in neoproterozoic lakheri limestone central india reveals definite signals for relative sea level variations and paleogeographic shifts between shallow beach shoreface and distal shelf depositional cycles in this carbonate formation are bound by unconformities or disconformities of ravinement origin on outcrop scale the unconformities are sharp and irregular and marked by definite depletion in 13 c values of limestones in the study area the entire lakheri limestone succession is divided into four depositional sequences bound by type i unconformities different variants of system tracts transgressive highstand and lowstand resulting from differences in paleogeographic locations basin floor physiography and variable rates of relative sea level change constitute the sequences developed on lakheri carbonate ramp a possible forcing of intrabasinal tectonics both extension and compression on meter to tens of meter thick lakheri sequences is inferred facies impersistence both along and across depositional strike of lakheri limestone is an additional complexity in depositional architecture of carbonate ramp sequence this possibly resulted from formation of depositional slope breaks through tectonic or depositional factors three dimensional reconstruction of lowstand wedge brings out the control of rugged physiography of the basin floor imparted through patchy biohermal growth on facies distribution in this neoproterozoic ramp succession

deformation pattern in the kurnool and nallarnalai groups in the northeastern part palnad area of the cuddapah basin south india and its implication on rodinia gondwana tectonics abstract the proterozoic basins of india adjoining the eastern ghats granulite belt eggb in eastern and southern india contain both mesproterozoic and neoproterozoic successions the intracratonic set up and contractional deformation fo the neoproterozoc successions in the paland sub basin in the northeastern part of cuddapah basin and similar crustal shortening in contemporaneous successions lying west of the eggb and nellore schist belt nsb are considered in relation to the proposed geodynamic evolution of the the rodinia and gondwana supercontinents tectonic shortening in the palnad sub basin northeast cuddapah partitioned into top to westnorthwest thrust shear flexural folds and cleavage development under overall e w contraction suggests foreland style continental shortening within an intracratonic set up a thrust sheet containing the nallamalai rocks and overlying the kurnool rocks in the northeastern part of palnad sub basin exhibits early tight to isoclinal folds and slaty phylllitic cleavage which can be correlated with early mesoproterozoic deformation structures in the nothern nallamalai fold belt nfb nne ssw trending folds and cleavage affect the kurnool group and overprint earlier structures in the thrust sheet thrusting of the nallamalai rocks and the later structures may have been related to convergence of the eastern ghats terrane and the east dharwar bastar craton during early neoproterozoic greenvillian and or later rejuvenation related to pan african amalgamation of east and west gondwana

morphology internal structure and mechanics of small longitudinal seif dunes in an aeolian horizon of the proterozoic dhandraul quartzite india the excellently preserved metre scale linear bedforms in an aeolian horizon of the proterozoic dhandraul quartzite india show oppositely dipping strata arranged in a zigzag pattern the strata are dominantly of translatent type deposited by along crest migrating ripples preserved on the flanks of dunes the bedforms thus may be interpreted in a morphodynamic sense as longitudinal seif dunes in order to determine the regional palaeoflow pattern the migration directions of ripples preserved at the top of sheet sandstones that are associated with the dune cross strata and internally show subhorizontal translatent strata were measured a directionally varying flow with a mean direction nearly parallel to the mean axial trend of the dunes is indicated the kinematics of the dunes were thus largely the result of alternate operation of two oblique flow components each of which was deflected at a dune crest into an along crest flow on the downwind flank of the dune the deflected flow formed along crest migrating ripples which in turn deposited climbing ripple strata alternate deposition on the two opposite flanks resulted in near vertical accretion of the dunes as is indicated by the zigzag pattern of stratal arrangement

proterozoic platform sequences of peninsular india implications towards basin evolution and supercontinent assembly abstract a group of less disturbed and unmetamorphosed proterozoic platformal sediment package lying over deformed and metamorphosed archean to paleoproterozoic basement in several parts of peninsular india is informally termed as purana basins in indian stratigraphy the trend shift in the understanding of these basin fills in last two decades often supported by international collaborations has made significant advancements in respect to unconformity and sequence development relationship with accretion and break up of the supercontinents initiation and evolution of the basins in different tectonic set ups nature of sediment provenance paleoclimate and paleo weathering patterns and basin water chemistry geophysical backups encompassing the basins occasionally extending up to the craton and adjacent mobile belts helped in refining the understanding on the basement structure and tectonic forcings the present paper is an attempt towards the collation of the database generated in last two decades in order to present the status of current understanding on different purana basins with regard to interactions of physical chemical and biological parameters of the proterozoic earth

basement structure of the cuddapah basin from gravity anomalies abstract an attempt has been made to understand the complex nature of gravity anomalies over the cuddapah basin and their relationship with geology structure and tectonics the gravity anomalies could delineate the approximate boundaries of the basin which is characterized by a steep gravity gradient on its eastern margin and gentle gradients on all its other sides prominent gravity lineaments which probably represent faults separate sub basins within the main basin previous studies have indicated that the southwestern gravity high is related to a basic body and that the macherla high is thought to be due to a greenstone belt the gravity high near kurnool also appears to be due to a greenstone belt which might extend into the basin beneath the sedimentary rocks with the exception of these highs the gravity picture of the cuddapah basin is a broad low of about 35 mgal in amplitude regression analysis between residual gravity and basement depths inferred from aeromagnetic interpretation and deep seismic sounding dss data suggest a probable density contrast of about 0 07 g cm 3 between the basement and the cuddapah rocks p wave velocities from dss data support this view with a negative density contrast of about 0 05 0 10 g cm 3 between the basement and the sedimentary rocks from the residual gravity the basement structure map of the cuddapah basin has been prepared by using an effective density contrast of 0 07 g cm 3 the results indicate that the thickest pile of sedimentary rocks about 10 km occurs in the nallamalai sub basin at many locations the basement depths obtained in the present analysis corraborate those obtained from the analysis of other data

shell bioerosion in high latitude low energy environments firths of clyde and lorne scotland abstract the agents of bioerosion in the firths of clyde and lorne include endolithic algae cliona phoronids acmaea virginea lepidopleurus asellus and regular echinoids which directly and indirectly cause shell disintegration and the production of calcareous sediments algae and acmaea are very effective in shell erosion but are confined to the photic zone where over 90 of the shells have suffered from their attack other bioerosion agents are not depth restricted but individually affect a lesser number of shells the photic limit approx 30 m is estimated from the disappearance of the deepest growing algae conchocelis and ostreobium the density of algal boring is high due to the low energy at the bottom it is shown that algal borings and acmaea grazing are also important as palaeobathymetric tools

trace element and nd isotopic evidence for sediment sources in the mid proterozoic vindhyan basin central india abstract ancient sediments reflect the compositions of old eroded continental crustal material and preserve geochemical signatures of paleo tectonic and volcanic events geochemical study of such sediments provides insight into tectonic events that shaped the face of the earth nd isotopes are best suited for such studies because rare earth elements ree specifically sm and nd are not fractionated during post depositional processes such as erosion transportation and diagenesis of organic poor sediments and hence reflect the source composition of these sediments sedimentation in the proterozoic vindhyan basin of central india with a 4 km thick succession of unmetamorphosed and mostly undeformed sediments lasted over 1000 ma from 1630 ma up to the precambrian cambrian boundary these sediments provide an ideal opportunity to study the evolution of ancient continental crust as well as the tectonic evolution of peninsular india the occurrence of porcellanites in the lower part of the basin indicates active volcanism during deposition of the vindhyan sediments these tuffs show large ion lithophile element lile enrichment as well as depletions in nb and ta in primitive mantle normalized plots characteristic of an arc volcanic source in combined trace element ratio plots these volcanic rocks indicate mixing between mantle derived melts and the continental crust and overlap with the composition of global arcs their initial nd at 1630 ma ranging from 5 7 to 0 9 is similar to those observed in andean type continental margin arcs trace element concentration patterns as well as nd isotopic compositions of the clastic and non clastic sediments of the vindhyan basin from one of the type localities of the vindhyan stratigraphy in the son valley are vastly different from those of the bundelkhand granite which is the basement for these sediments indicating that these granitic rocks were not a major source of sediments for the vindhyan basin our nd isotopic study clearly indicates that sedimentation in the vindhyan basin which lasted over 1 ga was not continuous the boundaries between the semri and kaimur groups as well as the rewa and bhander groups are characterized by abrupt changes in the nd isotopic compositions indicating changes in provenance with sediments in the kaimur and bhander groups being derived from more juvenile sources compared to the semri and rewa groups respectively this inference is also supported by changes in t dm and f sm nd for the sediments across these boundaries the strikingly different trace element patterns of the rewa group sediments compared to the underlying kaimur sediments indicates that sediment sources also changed across the kaimur rewa boundary multiple changes in provenance in the vindhyan sediments indicate that this basin was tectonically active during the entire span of sedimentation while there is some uncertainty in determining the exact sources of sediments for the vindhyan basin we argue on the basis of our geochemical in particular nd isotopic ratios data and available paleocurrent data that majority of the sediments were derived from a now extinct andean type arc source in the south that resulted from a southerly dipping subduction prior to the collision of the bhandara and bundelkhand cratons according to this reconstruction the vindhyan basin was initiated as a foreland basin after the collision of these two cratons

facies associations and depositional environment of the proterozoic carbonate hosted microbanded manganese oxide ore deposit penganga group godavari rift basin india the late proterozoic penganga group pranhita godavari valley south india is a sequence of siliciclastic carbonate sediments that was deposited in an intracratonic rift basin the nonvolcanogenic sedimentary succession containing listric basin marginal faults shows abrupt facies changes from craton to trough with rich resources of stratabound microbanded manganese oxide ore hosted within the trough facies the cratonic sediments of the basin are mature siliciclastics with clear imprints of tide dominated shallow subtidal deposition the siliciclastics show rapid lateral transition into a thick sequence of nonstromatolitic and micritic ribbon limestone containing sediment gravity flow deposits in the basal part and a persistent horizon of bedded manganese ore deposit in the upper part the limestone is overlain by a thick sequence of basinal shale facies through an interbedded sequence of limestone and shale uniformly thick laterally persistent parallel sided and thin bedded limestone shale rhythmites and parted limestone rhythmites characterize the ribbon limestones except for widespread thin parallel lamination structures formed by waves tidal currents and storm induced currents are totally absent the limestone is brown and argillaceous in the basal part and changes upwards to siliceous and pyritiferous light gray to dark gray varieties and finally to carbonaceous black limestone the physical attributes of the limestone suggest complete drowning of the carbonate interval in response to higher sea level caused by tectonic subsidence and a deep water slope and basinal environment of carbonate deposition under continued transgression the bedded manganese oxide ore interstratified with chert simulating iron formation typically shows micron scale to millimeter scale thin parallel laminae of almost uniform thickness texture and composition the manganese ore and chert laminae chiefly have a cryptocrystalline to microcrystalline fabric consisting of grains of clay size to fine silt size todorokite and birnessite are the most abundant sedimentary manganese oxides they were locally transformed to braunite and bixbyite during diagenesis the centimeter thick laminated chert beds are analogous to microbanded chert mesobands of the iron formations and are composed of microcrystalline quartz the sedimentary attributes of the ore chert sequence suggest deposition from suspension fallout in a low energy environment penecontemporaneous deformation of the ore deposit indicates slow downslope movement of the semiconsolidated sediments over the basin floor the sedimentology and stratigraphy of the penganga group suggest a sediment gravity flow influenced deep water base of slope basinal environment of manganese accumulation during the thermal subsidence stage of basin development within an extensional rift setting on the basis of sedimentological evidence and parallel studies done on iron formations a possible hydrothermal source of manganese is also considered this represents a significant departure from the prevalent views of high energy shallow water shelf environments of manganese deposition and terrigenous sources of metal in cratonic basins hosting several important manganese deposits and thus it has important implications for genesis of manganese ores

pb sr and nd isotope systematics of uranium mineralised stromatolitic dolomites from the proterozoic cuddapah supergroup south india constraints on age and provenance abstract the pb sr and nd isotopic compositions were determined on uranium mineralised and barren stromatolitic dolomite samples from the vempalle and tadpatri formations of the cuddapah supergroup in southern india subsamples from individual handspecimens were analyzed to minimize the potential of heterogeneous initial pb compositions all of the u mineralised samples gave pb pb ages close to 1800 ma but with high mswd values presumably due to local redistribution of u and pb in the samples the best pb pb age of 1756 29 ma is interpreted as the time of u mineralisation and as a minimum age for carbonate sedimentation and dolomitisation within the cuddapah supergroup the sr isotopic compositions of the samples vary with the mineralised samples being more radiogenic even the lowest 87 sr 86 sr of the barren sample exceeds the seawater 87 sr 86 sr about 1800 ma ago on the other hand the nd systematics of the samples does not appear to have been disturbed by processes subsequent to carbonate sedimentation the calculated initial nd isotopic compositions suggest a dominant continental crustal source that was at least 600 ma old at the time of deposition of the carbonate sediment

c o sr and pb isotope systematics of carbonate sequences of the vindhyan supergroup india age diagenesis correlations and implications for global events abstract the vindhyan supergroup of central india records a substantial portion of proterozoic time and contains some of the most disputed precambrian cambrian fossil discoveries despite the significance of these fossil finds many issues including reliable geochronology remained unresolved similarly the geochemical aspects of the sedimentary sequences and their implications to the evolution of proterozoic oceans have been studied only at a reconnaissance level here we report the results of the pb pb dating for the rohtasgarh limestone from the lower vindhyan supergroup giving an age of 1601 130 ma an estimate that is in accord with the recent u pb zircon dating of the lower vindhyan supergroup by rasmussen et al geology 30 2002 103 and ray et al geology 30 2002 131 sr isotope stratigraphy of carbonates from the upper vindhyan supergroup suggests a mid neoproterozoic 750 650 ma age for these formations this in turn argues for a long hiatus between the deposition of the lower and upper vindhyan supergroups our results conflict with the reports of cambrian age small shelly fossils and ediacaran fossils from the upper vindhyan rocks at the same time they suggest that the alleged trace fossils in the lower vindhyan rocks must be of paleoproterozoic age the most common 18 o values for calcitic components from limestone formations of the supergroup range from 10 to 7 similar to paleo neoproterozoic carbonates worldwide the modes of 13 c in the lower and upper vindhyan supergroups are 0 2 and 3 2 respectively again consistent with the worldwide dataset the secular patterns of carbon isotope trends do not support the earlier assumptions that the carbonate sequences at the southern margin correlate with those at the western or northern margins of the vindhyan basin c and sr isotope data provide new calibration points for the seawater evolutionary curves particularly for the paleoproterozoic for strontium isotopes the data from the lower vindhyan supergroup suggest that seawater departed probably less rapidly from mantle values than was previously believed

slope controlled seismic deformation and tectonic framework of deposition koldaha shale india abstract penecontemporaneous deformations with remarkable lateral continuity at selected stratigraphic levels record frequent seismicity in the meso proterozoic koldaha shale vindhyan supergroup central india their consistent bipolar orientation reflects the master control of basinal tectonics boudinage small scale conjugate faults and bidirectional joint planes indicate east northeast west southwest basinal extension detailed study of a 27 km strike parallel stretch between chorhat and shikarganj against a broader perspective reveals existence of multiple nw se elongated subbasins stratigraphic and sedimentologic criteria point to inheritance and subsequent sustenance of steep western and gentler eastern flanks of the sub basins it is also indicated that the perennial supply of sediment took place from the east while the western slope contributed only during tectonic seiches the relatively large scale faults are correlatable with the steeper and the smaller deformation structures to the gentler flank the nnw oriented half graben structures that were reactivated time and again is elicited previous geophysical studies inferred roughly e w elongated rift valley in the granitic basement and the present observations indicate that the rifting continued at least up to the time of koldaha shale deposition the half graben could then be guided by cross faults resulted from dextral shear it is imperative then the basin interior sedimentation and deformation patterns were more closely controlled by the cross faults rather than by the basin margin faults only a set of slump folds in a 10 m thick interval indicates that the northern margin of the main basin was relatively steeper this is further corroborated by concentration of large fans in mineralogically immature sediments and southerly paleocurrent at the northern fringe of the basin in contrast the vindhyan sediments commonly belong to orthoquartzite carbonate association a regional northerly slope of the vindhyan basin floor towards the relatively steeper flank provides an explanation for the general northwesterly paleocurrent even though the eastern flank of the sub basins sloped wsw this integrated study of the physical aspects of the sedimentary pile against the background information of basement tectonics recognize rifting with a dextral shear component as the overriding factor controlling the early part of evolution of the vindhyan basin

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

proterozoic rifting in the pranhita godavari valley implication on india antarctica linkage abstract the pranhita godavari pg valley a major lineament within the south indian cratonic province that preserves sediment dominated deposits spanning from mesoproterozoic to mesozoic appears to be a key element in supercontinent reconstruction the sedimentary basins of the valley include a thick succession of early mesoproterozoic to late neoproterozoic rocks the godavari supergroup which is unconformably overlain by the late palaeozoic mesozoic gondwana sequence the godavari supergroup is internally punctuated by several regional and interregional unconformities into a number of unconformity bound sequences having group level and subgroup level status the lithostratigraphic attributes of the succession indicate multiple events of fault controlled sedimentation marked by transgression and regression as well as uneven rates of uplift and subsidence of the basin floor in an extensional tectonic regime the amplitude of translation of the unconformity surfaces across the base level attests to collective role of tectonic movement and sea level changes in building the stratigraphic framework of the valley the stratigraphic framework and depositional systems such as fan and fan deltas together with local outburst of felsic volcanism further indicate repeated rifting of the craton geochronologic data indicate that the rift basin started to open in early mesoproterozoic concomitantly with the breakup of the mesoproterozoic supercontinent during which the india and east gondwana fragments were separated the spatial variation in the declivity of the unconformity surfaces and the trend of thickness variation of the unconformity bound sequences point that the basin deepened and opened towards southeast to join an ocean that developed between the south indian craton and east antarctica the contractional deformation structures preserved in several lithounits were produced under ne sw directed regional compression during late neoproterozoic basin inversion

the purana basins of southern cratonic province of india a case for mesoproterozoic fossil rifts abstract since its cratonization in the palaeoproterozoic southern peninsular india witnessed the development of a number of large intracratonic sedimentary basins traditionally referred to as purana basins spanning in age from late palaeoproterozoic through neoproterozoic the localization of these intracratonic basins namely pranhita godavari pg chattisgarh and cuddapah basins is apparently controlled by pre existing sutures and or weak zones the pg basin and the cuddapah basin host unconformity bound thick sediment dominated successions attesting to several cycles of fluvial shallow marine to shelf slope basin sedimentation deposition was punctuated by block uplifts resulting in local hiatuses and or volcanic upheavals leading to intercalation of thin but persistent basaltic flows and acid tuffs and ignimbrites basin margin deep faults apparently played a role in the facies distribution in these basins based on these features we propose that these basins initiated as continental rifts which however never opened up into a full fledged ocean basin but links with open seaway are evident from frequent occurrence of deposits representing tidal and storm influence particularly in upper part of the chattisgarh succession spatial distribution of facies and sediment thickness in the cuddapah and pg basins suggest that an open seaway existed to the east of the south indian cratonic province during the mesoproterozoic while similar criteria point to the existence of an open seaway north of the chattisgarh basin development history including nature of inversion suggest that the southern cratonic province of india existed as a single large continental mass since the mesoproterozoic in spite of episodes of supercontinent build up and fragmentation involving india and east gondwana during the proterozoic

stratigraphic architecture of the proterozoic succession in the eastern chattisgarth basin india tectonic implications abstract the basin fill succession of the proterozoic eastern chattisgarth basin comprises a sandstone shale conglomerate dominated shallow marine assemblage and an assemblage of limestone shale pyroclastics deposited in off shelf slope base of slope environments the proximal and distal lithologic assemblages developed as lateral equivalents and show signatures of sea level fluctuations amenable to chronostratigraphic correlation the stratigraphic architecture of the proximal assemblage is characterized by cyclic deposition on different scales the largest order cyclicity is represented by alternating sandstone sequences and mudstone dominated sequences of the order of several tens of metres the smallest order cyclicity is represented by metre scale units punctuated aggradation cycles pac bounded by isochronous surfaces the lowermost large scale sandstone sequence the lohardih sandstone unconformably overlies the highly deformed rocks of the gneissic basement complex the sequence was deposited as alluvial fan fan delta complexes controlled by basin margin faulting the upper two sandstones the daihan sandstone and the kansapathar sandstone represent two different phases of major sand accumulation in fan or river deltas their rapid emplacement over muddy shelf deposits along erosional surfaces signaled abrupt sea level fall and basinward migration of facies belts generation and accumulation of very large volumes of sand and their rapid influx into the basin accompanied by major changes in depositional motif and in the circulation system in the basin collectively indicate that larger order sand depositing cycles were caused by tectonic uplift of the cratonic basement the smallest order cycles the pacs on the other hand may be attributed to glacioeustacy though tectonic control cannot be ruled out the sand deficient mud dominated large scale sequences are attributed to major transgression during periods of relative tectonic quiescence and subsidence of the cratonic hinterland episodic tectonic uplift and derivation of sands from similar quartzo feldspathic sources suggest repeated rifting of the cratonic basement and expansion of the rift into a major marine basin

earth s earliest microbial mats in a siliciclastic marine environment 2 9 ga mozaan group south africa this study provides evidence for the existence of filamentous cyano bacteria forming sediment stabilizing mats in shelf environments at 2 9 ga the oldest known occurrence of microbial mats in siliciclastic rocks the mesoarchean mozaan group south africa features fine grained quartzites of an ancient shallow shelf environment these sandstones contain wrinkle structures which in thin section reveal filamentous textures forming carpet like microbial mat fabrics the textures resemble the trichomes of modern cyanobacteria chloroflexi or sulfur oxidizing proteobacteria mineralogical geochemical and isotopic analyses acre consistent with a biological origin of the filament like textures carbon filaments with biogenic isotopic signatures 1 3 c 24 2 0 5 are closely associated with hematite goethite and chert minerals which may derive from the former presence of oxygen within the microbial mats detrital quartz zircon and rutile in the mats could indicate baffling trapping and binding of the bacterial communities

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

forced regressive wedges on a neoproterozoic siliciclastic shelf chandarpur group central india abstract outcrop based sequence analysis across the transition from the chaporadih to the kansapathar formation of chandarpur group chattisgarh supergroup central india resolved a long standing uncertainty on occurrence of depositional discontinuity through undoubted identification of a forced regressive wedge and associated intraformational unconformity process based facies analysis identified eleven different facies types that are grouped into five different facies associations paleoenvironments range from shelf shoreface foreshore and beach tide influenced estuary and delta bounded between the surface of marine erosion at its base and subaerial unconformity marked by paleosol development at its top the coarse grained wedges of shoreface sandstone is interpreted as forced regressive in origin the basinward downstepping of two such regressive wedges bears evidence for lowering of wavebase in connection with relative fall in sea level the valley created during the falling sea level was filled by sediments in lowstand transgressive and highstand depositional motif the aggradational to weakly progradational bedsets of foreshore beach sediments record the history of early slow rise in base level with steepening in the rate of relative sea level rise the coastline transformed from wave dominated to tide dominated wide variability in the balance between sediment supply and accommodation is recorded in the facies types and stacking motif of transgressive estuarine sections at different studied locations the highstand tidal delta is a supply driven normal regressive product at stable or marginally rising base level

the nature of the archean upper crust as revealed by the geochemistry of the proterozoic shales of the kaladgi basin karnataka southern india abstract major trace and rare earth element geochemical studies of the shales belonging to the 1 8 ga proterozoic kaladgi supergroup of southern india shows that they are derived from strongly chemically weathered archean upper crust this has resulted in depletion of na2o cao sr and u in these shales the shales of lower stratigraphic intervals are enriched in mgo fe2o3 cr ni co and sc relative to the upper shales the upper shales by contrast show enrichment in elements such as ba th and zr these characteristics and the barium cobalt ratios show that the early sediments were derived from a more mafic provenance and the contribution of granitoids increased towards the later stages of kaladgi sedimentation rare earth element ree modelling shows that the average kaladgi shale represents a contribution from a crust with a mafic to felsic rock ratio of 60 40 the post depositional introduction of potassium and lree in these shales has been inferred based on the overall geochemical characteristics

microbially originated wrinkle structures on sandstone and their stratigraphic context palaeoproterozoic koldaha shale central india abstract wrinkle structures on sandstone beds are abundant and present in diverse forms in the palaeoproterozoic koldaha shale vindhyan basin central india the inferred palaeogeography of sedimentation ranges from offshore marine to terrestrial the wrinkled sandstones belonging to the relatively deeper marine part good preservation of ultramicroscopic microbial elements within these sandstones strongly supports a microbial mat origin of the wrinkles which was facilitated by development of an oxygen depleted condition immediately beneath the sediment water interface as depicted in early pyrite growth later impermeable shale encasing the sandstone beds arrested further scope for destruction of the microbial elements trapping of fine detritus by microbial filaments is documented by the invariable occurrence of a silty clay veneer on the wrinkled surfaces and by its characteristic texture wrinkle morphology has been classified and an attempt has been made to understand the origin of each of the varieties and to visualize the processes that acted upon the microbial mats to produce the wrinkle structures in their specific palaeogeographic context the general palaeogeographic bias in occurrence of the sandy wrinkles is well reflected in their concentration in the lower part of the highstand systems tracts hst s one superposed above the other comprising the koldaha shale the smaller scale palaeogeographic control is indicated by compartmentalization of two wrinkle varieties in the upper and the basal intervals of the hst s and overlapping occurrence of other varieties in the medial interval with apparent preference for either the upper or the lower levels

proterozoic braided ephemeral fluvial deposits an example from the dhandraul sandstone formation of the kaimur group son valley central india abstract the dhandraul sandstone formation of the middle proterozoic kaimur group the vindhyan supergroup son valley india has been reinterpreted to represent a product of braided ephemeral fluvial environment the previous interpretation was shallow marine barrier bar evidences in favour of braided river deposition of the dhandraul sandstone are a cosets of low angle planar cross bed sets are closely associated with cosets of parallel laminated sets with particle streaming on bedding plane surfaces b vertical recurrence of planar laterally impersistent erosional surfaces that are invariably carpeted by red coloured intraformational shale pebble conglomerate c profuse development of ripple marks d sedimentary structures indicative of critical to supercritical flow conditions and e consistent palaeocurrent patterns

stable isotope geochemistry of sedimentary carbonates from the proterozoic kaladgi badami and bhima groups karnataka india abstract a reconnaissance isotope survey has been carried out on sedimentary carbonate formations from the middle and late proterozoic 1 0 to 2 0 ga kaladgi badami and bhima groups of karnataka southern india while the 13 c values of the kaladgi dolomites and limestones 0 3 1 2 o pdb are consistent with an interpretation of this sequence as a normal marine carbonate facies as previously inferred from geological and sedimentological evidence the markedly increased average of 3 4 0 5 o pdb for the dolomite members of the overlying badami and bhima groups suggests genetic relationships to a primary hypersaline evaporitic environment the 18 o values of the two carbonate provinces are very similar 21 7 1 3 o and 21 1 1 2 o smow respectively displaying the 18 o deficiency typical of precambrian carbonates