outcrop analog for mixed siliciclastic carbonate ramp reservoirs stratigraphic hierarchy facies architecture and geologic heterogeneity grayburg formation permian basin u s a abstract the grayburg formation late permian guadalupian is a shallow marine succession flanking the delaware and midland basins of texas and new mexico u s a the grayburg exemplifies the facies heterogeneity imparted by cyclic interbedding of siliciclastic and carbonate rocks from lithologically diverse inner ramp to outer ramp facies assemblages high resolution correlation and mapping of laterally continuous grayburg strata exposed in the brokeoff mountains new mexico allow the stratigraphic architecture facies distribution and lateral variability to be characterized in detail this study provides an outcrop analog for stratigraphically equivalent subsurface reservoirs and comparable carbonate ramp reservoirs that accumulated during periods of low amplitude sea level fluctuations vertical and lateral facies successions in the grayburg record four hierarchical scales of cyclicity the entire grayburg is a composite sequence that initiated with transgression of the san andres platform and culminated with subaerial exposure followed by a major basinward shift in deposition this third order cycle contains four high frequency sequences defined by transgressive outer ramp facies overlain by aggradational ramp crest to inner ramp facies capped by an unconformity each high frequency sequence contains several composite cycles intermediate scale cyclic successions the high frequency fifth order cycles constitute the smallest scale upward shoaling facies successions that can be recognized and mapped comprising the basic correlation entity to delineate lithofacies bodies lateral heterogeneity in the grayburg reflects both systematic facies transitions and interwell scale meters to hundreds of meters variability due to geologic complexity larger scale systematic facies changes that reflect primary environmental and or depositional controls e g water depth platform position accommodation trends can be characterized using well and or seismic data abetted by appropriate depositional models interwell heterogeneity due to geologic complexity however is difficult to recognize from subsurface datasets appropriate outcrop analogs provide information on lateral facies dimensions and heterogeneity architecture that is essential for constructing more realistic three dimensional reservoir models rather than oversimplified models based on lithofacies correlations forced between wells by linear interpolation an understanding of geologic heterogeneity exhibited in outcrop analogs is crucial for geoscientists involved with characterizing and modeling subsurface heterogeneity

inferring tectonic provenance of siliciclastic rocks from their chemical compositions a dissent abstract chemical compositions of siliciclastic sedimentary rocks are commonly used to infer their tectonic provenance we have tested the universal applicability of the underlying principle in a small but controlled study expecting 100 confirmation of the practice a comparison is made between the chemical composition of the 1480 ma butler hill granite in an uplifted cratonic block of the st francois mountain igneous complex and that of a small 1 m thick regolith body a weathered granite sample and the basal quartz arenites of the 520 ma lamotte formation immediately above the regolith the results show that in plots of k 2 o na 2 o vs sio 2 al 2 o 3 the regolith and sandstone samples correctly plot in the passive margin field although the weathered granite plots in the arc field in plots of th sc zr 10 and la th sc the results plot in the passive and active continental margins and their extensions in other common plots to discriminate tectonic provenance e g sio 2 vs k 2 o na 2 o fe 2 o 3 mgo vs al 2 o 3 sio 2 fe 2 o 3 mgo vs tio 2 sc cr vs la y a few points plot in the passive margin field but scatter into and outside of other fields of tectonic provenances the chondrite normalized ree distributions show variable degrees of negative eu anomalies with flat hree conforming to a felsic source the lree distributions show both positive and negative ce anomalies that can be ascribed to the variability of redox conditions during weathering and diagenesis of the original siliciclastic sediments the variability of the eu anomaly was likely affected by post erosion processes in addition to whatever was inherited from the parent rocks we conclude that chemical compositions can provide good clues but are neither strong indicators nor unique identifiers of their tectonic provenance rather they indicate a dominantly felsic or dominantly mafic or a mixed set of source rocks

compositional evolution of siliciclastic sediments recording the tectonic stability of a pericratonic rift mesozoic kutch basin western india abstract this study presents a combined petrographical and geochemical investigation to track the compositional changes in the mesozoic sedimentary record during the evolution of the pericratonic rift basin of kutch the mesozoic sequence of the proven petroliferous basin of kutch representing 3000 m thick siliciclastic and carbonate deposition within a time frame of 70 ma consists of jhurio jhumara jhuran and bhuj formations in ascending order of succession the dominance of arkosic sandstones and the abundance of zircon and monazite indicate predominantly felsic source rocks the concentration of individual trace elements such as th sc zr v ni and ratios like th sc cr v y ni th co la sc support the predominantly felsic source rock the lree enrichment and the negative eu anomaly corroborate the predominantly felsic source however concentrations of v ni and ti reveal the mafic input in older stratigraphic units while gdn ybn indicates dominant post archean source across the mesozoic succession younger stratigraphic units reveal contributions from the archean rocks from nagar parkar and aravalli highland regions the cia values suggest intermediate to intense weathering while the icv data record a shift from immature sediments in the older formations to mature and recycled types in the younger formations the high value of zr sc as well a high content of hf reflects the addition of zircon by recycling or by erosional unroofing of older continental rocks major oxide sio2 na2o and k2o and trace elements la th and hf bear the passive margin setting for the kutch basin during the mesozoic the decrease in feldspar content in sandstones increase in the concentration of zr and hf in shale and a decrease in values of icv from older to younger stratigraphic units corresponds to the compositional evolution related to increased stability of the kutch basin during the transition from early syn rift to post rift

palaeoweathering composition and tectonics of provenance of the proterozoic intracratonic kaladgi badami basin karnataka southern india evidence from sandstone petrography and geochemistry abstract petrographic and geochemical data on the sandstones of the proterozoic intracratonic kaladgi badami basin southern india are presented to elucidate the palaeoweathering pattern and composition and tectonics of their provenance the kaladgi badami basin hosting the kaladgi supergroup occupies an e w trending area the supergroup unconformably overlies archaean basement ttg gneisses granites and greenstones comprises a cyclic arenite pelite carbonate association and is divided into the bagalkot and badami groups the immature arkosic character of the basal saundatti quartzite member bagalkot group containing fresh and angular feldspars along the northern margin of the basin suggests that during the initial stage of deposition this part of the basin received sediments from a restricted uplifted and less weathered source dominated by k rich granites occurring to the north in contrast the saundatti quartzite along the southern margin displays a mostly mature quartz rich character with less abundant but severely weathered feldspars and higher sio2 and cia but lower al2o3 tio2 rb sr ba k2o k2o na2o zr ni and zr cr this is interpreted in terms of a tectonically stable considerably weathered mixed source archaean gneisses granites and greenstones along the southern fringe of the basin the highly mature quartz arenite muchkundi quartzite member also of the bagalkot group occurring higher up in the succession exhibits minor but severely altered feldspars and higher sio2 na2o cia cr and ni with lower k2o al2o3 tio2 and k2o na2o this reflects that with the passage of time the source evolved to a uniform extensively weathered tectonically stable peneplained provenance which consisted of less evolved ttg gneisses and greenstones this was followed by closure deformation and upliftment of the basin hosting the bagalkot group and subsequent deposition of the badami group sandstone members of this younger group cave temple arenite and belikhindi arenite range widely in mineralogy quartz arenite to arkose and chemistry including cia and point to a source that varied from uplifted less weathered k rich granites to less evolved peneplained ttg gneisses and greenstones or even bagalkot sediments variable alteration of feldspars in the kaladgi sandstones and severe depletion of ca na and sr in the associated shales indicate a humid tropical tropical and subtropical climate facilitating chemical weathering

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

seismites in the lokapur subgroup of the proterozoic kaladgi basin south india a testimony to syn sedimentary tectonism abstract syn sedimentary deformation structures ssds can indicate disturbance caused by autogenic or allogenic processes affecting the depositional system such structures are prolific in the proterozoic shallow marine sediments of kaladgi basin especially in the basal part of the succession lokapur subgroup in both clastics and carbonates widespread distribution lateral traceability of the deformed horizons across several hundreds of meters their occurrence between undeformed beds and proximity to nw se to e w trending faults and shears are used to attribute a seismic origin to some of these ssds release of accumulated stresses in the basin floor undergoing extension caused the episodic seismic activity producing the seismites these seismites record the syn sedimentary tectonism that controlled facies distribution patterns in the mesoproterozoic bagalkot group in conjunction with sea level fluctuations on one hand this is a contribution towards recognition of seismic events during the mesoproterozoic a period from which few seismites have been recorded on the other hand recognizing that the crustal processes that operated in brittle regimes during the creation of this extensional supracrustal basin around 1800 ma were comparable to those operating in present day extensional systems opens a window of understanding how the proterozoic platform basins hosting thick shallow marine sequences evolved

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

estimating carboniferous sea level fluctuations from gondwanan ice extent the magnitude of carboniferous sea level fluctuations is not well known but the estimates are important for constraining cyclothem models in this paper we apply area volume relations developed for quaternary ice sheets to estimate the glacio eustatic component of sea level fluctuations in the carboniferous utilizing the best estimate of gondwanan ice area at the time of its greatest extent we arrive at a figure of 60 15 m for the isostasy corrected glacio eustatic component this value is nearer the lower limit of sea level estimates for cyclothem fluctuations from other sources

outcrop analog for cyclic shelf reservoirs san andres formation of permian basin stratigraphic framework permeability distribution geostatistics and fluid flow modeling vertical and horizontal transects were sampled from core and outcrop of the san andres formation at lawyer canyon guadalupe mountains new mexico to assess permeability variation in a geologic framework of upward shallowing carbonate cycles and to show the potential effect these variations have on viscous dominated flow behavior in analogous reservoirs these cycles occur in a ramp crest facies tract are 3 13 m 10 45 ft thick and contain both vertical and lateral variation of lithofacies thicker cycles consist of a basal dolomudstone which is overlain by burrowed dolopackstone and capped by bar flank ooid peloid dolograinstone and bar crest ooid dolograinstones in vertical transects permeability is extremely variable about the mean yet upward increasing trends coinciding with the succession of lithofacies typify a given cycle semi variance analysis shows permeability to be uncorrelated vertically at distances greater than 5 5 m 18 ft which is the average cycle thickness suggesting that the cycles may equate to a fluid flow unit in a reservoir semi variance analysis of measurements collected along a horizontal transect within bar crest dolograinstones of a single cycle shows permeability is uncorrelated at distances greater than 3 6 m 12 ft this correlation distance appears to be controlled by alternating porous and tightly cemented zones that formed during dolomitization vertical and lateral variogram models were fit to the spatial parameters to generate a variety of conditionally simulated permeability fields fluid flow simulations show viscous dominated flow behavior is compartmentalized by both the individual cycles and groups of cycles the basal dolomudstones are potential baffles to flow crossover between cycles but poorly developed cycles i e those that are mud rich and lack well developed bar flank and bar crest facies result in the greatest compartmentalization of fluid flow within a succession of cycles

wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western united states a comparison panfish j t and peterson f 1988 wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western united states a comparison in g kocurek editor late paleozoic and mesozoic eolian deposits of the western interior of the united states sediment geol 56 261 282 wind directions for middle pennsylvanian through jurassic time are predicted from global circulation models for the western united states these predictions are compared with paleowind directions interpreted from eofian sandstones of middle permsylvanian through jurassic age predicted regional wind directions correspond with at least three quarters of the paleowind data from the sandstones the rest of the data may indicate problems with correlation local effects of paleogeography on winds and lack of resolution of the circulation models the data and predictions suggest the following paleoclimatic developments through the time interval studied predominance of winter subtropical high pressure circulation in the late pennsylvanian predominance of summer subtropical high pressure circulation in the permian predominance of summer monsoonal circulation in the triassic and earliest jurassic and during the remainder of the jurassic influence of both summer subtropical and summer monsoonal circulation with the boundary between the two systems over the western united states this sequence of climatic changes is largely owing to paleogeographic changes which influenced the buildup and breakdown of the monsoonal circulation and possibly owing partly to a decrease in the global temperature gradient which might have lessened the influence of the subtropical high pressure circulation the atypical humidity of triassic time probably resulted from the monsoonal circulation created by the geography of pangaea this circulation is predicted to have been at a maximum in the triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

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

geochemistry of loess paleosol sediments of kashmir valley india provenance and weathering abstract middle to late pleistocene loess paleosol sediments of kashmir valley india were analyzed for major trace and ree elements in order to determine their chemical composition provenance and intensity of palaeo weathering of the source rocks these sediments are generally enriched with fe 2 o 3 mgo mno tio 2 y ni cu zn th u sc v and co while contents of sio 2 k 2 o na 2 o p 2 o 5 sr nb and hf are lower than the ucc chondrite normalized ree patterns are characterized by moderate enrichment of lrees relatively flat hree pattern gd cn yb cn 1 93 2 31 and lack of prominent negative eu anomaly eu eu 0 73 1 01 average 0 81 paas normalized ree are characterized by slightly higher lree depleted hree and positive eu anomaly various provenance discrimination diagrams reveal that the kashmir loess paleosol sediments are derived from the mixed source rocks suggesting large provenance with variable geological settings which apparently have undergone weak to moderate recycling processes weathering indices such as cia ciw and pia values 71 87 83 83 and 80 57 respectively and a cn k diagram imply weak to moderate weathering of the source material

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

integrated characterization of carbonate ramp reservoirs using permian san andres formation outcrop analogs the san andres formation permian guadalupian of the permian basin is representative of carbonate ramp reservoirs in that it has highly stratified character complex facies and permeability structure and generally low recovery efficiencies of 30 of original oil in place the approach used here to describe carbonate ramp reservoirs such as the san andres formation produces detailed reservoir models based on integration of sequence stratigraphic analysis petrophysical quantification through definition of rock fabric flow units and fluid flow simulation synthesis of these subdisciplines clarifies which aspects of the geologic petrophysical model are most significant in predicting reservoir performance and ultimately in understanding the location of remaining oil satur tion the san andres formation crops out along the algerita escarpment a long oblique dip continuous shelf to basin exposure in the central guadalupe mountains these outcrops provide a unique opportunity to study lateral relationships in geologic and petrophysical structure analogous to those occurring between wells in subsurface reservoirs on the basis of sequence stratigraphic analysis three scales of cyclicity are recognized depositional sequences high frequency sequences and cycles examination of the cycles in two detailed window areas provides a practical scale for petrophysical quantification and fluid flow simulation an understanding of cycle position within the high frequency sequence framework also provides predictive information petrophysical analysis revealed six rock fabric groups dominated by intergranular separate vug or dense intercrystalline pore types comparison of these rock fabric groups with facies descriptions produced a rock fabric flow unit model that honors the geologic structure of the cycle and sequence framework permeability data were averaged within rock fabric flow units using a geometric mean approach based on fine scale fluid flow modeling of deterministic and stochastically generated permeability fields two dimensional black oil fluid flow models illustrate that 1 major differences in sweep efficiency and fluid flow performance are predicted when linear interwell interpolations are compared with actual interwell scale geologic structure as determined by outcrop geologic and petrophysical mapping 2 an understanding of static geologic petrophysical conditions provides only a partial understanding of reservoir performance defined by the interaction of these static properties and dynamic properties of fluid flow interaction within the flow unit architecture and 3 because of the orderly distribution of high and low permeability facies within cycle stacks of high frequency sequences this larger scale of geologic description can give a reasonable first order approximation of fluid flow patterns and early breakthrough end page 181

geochemistry of the neoarchaean high mg basalts boninites and adakites from the kushtagi hungund greenstone belt of the eastern dharwar craton edc implications for the tectonic setting abstract volcanic rocks of the kushtagi hungund greenstone belt from the eastern dharwar craton edc show extreme diversity due to the presence of high mg high fe high mg fe basalts high mg dacites and andesites boninitic affinities and sodic plagioclase quartz rich felsic rocks adakites with na2o 4 6 wt the adakites drummond and defant 1990 martin 1999 and high mg dacites andesites are being reported for the first time from greenstone belts of the dharwar craton trace elements in these different types of volcanic rocks have distinct characteristics for example the basalts have 1 al2o3 tio2 16 pm value of 17 2 la yb n 1 3 3 low order negative anomalies of nb nb zr zr hf hf and ti ti 4 zr nb 4 10 and 5 ce nb 3 4 two samples of andesites having mg enrichment and al2o3 tio2 ratios of 31 35 are interpreted to have boninitic affinities other dacites and andesites are characterized by 1 high mg 8 10 wt 2 high al2o3 tio2 22 27 3 zr nb 8 14 4 la yb n 2 7 5 negative nb nb zr zr hf hf and ti ti anomalies and 6 sc lu 67 172 the adakites are characterized by 1 a narrow range of sio2 67 70 wt 2 enriched na2o 4 6 wt with very high na2o k2o 2 5 7 7 3 low mg 0 38 0 48 4 high la yb n 13 31 with fractionated patterns no eu anomaly and gd yb n 2 4 5 very high al2o3 tio2 50 80 and 6 enriched ni cr and co total 50 ppm these adakites resemble 3 0 ga tonalite trondhjemite gneisses of the dharwar craton which are interpreted as products of partial melting of the basaltic slab the available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting

weathering and mineralogical variation in gneissic rocks and their effect in sangrumba landslide east nepal sangrumba landslide is one of the largest and the most active landslides in nepal himalaya geologically the landslide belongs to the higher himalaya and consists of pre cambrian biotite garnet and sillimanite gneiss with some quartzite the present paper aims at describing various degrees of rock weathering and their effect in sangrumba landslide field study followed by mineralogical geochemical and geotechnical analyses of the collected rock and soil samples from the landslide zone were used in characterizing weathering degree the gneisses are intensely weathered while quartzite is unweathered petrographical and x ray diffraction analyses showed that the rocks in the landslide zone had undergone weathering process with the formation of different types of clay minerals as kaolinite vermiculite smectite and chlorite this was further confirmed by the scan electron microscope and energy dispersive x ray analyses these clay minerals drastically reduced the rock strength facilitating the extensive failure of the sangrumba landslide the major and trace element composition of the rock and soil samples was calculated from the xrf analyses the geochemical analyses and weathering indexes of rocks showed that they are significantly weathered and had a major influence in the formation of the sangrumba landslide in addition mechanical strength measurement of rock soil showed that the strength drastically decreases as the weathering intensity increases rainfall followed by the rock type are the most dominant parameters influencing the weathering process which leads to the formation of large landslide as the present one these findings can be used in other areas with similar geological and topographical conditions

pennsylvanian to jurassic eolian transportation systems in the western united states peterson f 1988 pennsylvanian to jurassic eolian transportation systems in the western united states in g kocurek editor late paleozoic and mesozoic eolian deposits of the western interior of the united states sediment geol 56 207 260 the direction of sediment transport in eolian sandstones of permsylvanian to jurassic age was interpreted from crossbedding resultants vector means obtained from studies of eolian rocks in the western u s supplemented by data from the few eolian units of eastern north america these were compiled from the published or unpublished theses hterature from unpublished fidd data contributed by colleagues or from measurements made for this study in addition new paleogeographic maps were compiled to evaluate the influence of geographic features on the atmospheric circulation patterns that are inferred from the crossbedding studies regionally the crossbedding indicates northeasterly northerly or northwesterly winds present coordinates from pennsylvanian through most of middle jurassic time a rather abrupt change in wind directions occurred in late middle jurassic time late part of the cauovian age when westerly wind patterns developed by the late jurassic the winds shifted to southwesterly calculations of the consistency factor vector mean strength made from region wide analyses of the resultants indicate fairly unidirectional winds from the permsylvanian through the early jurassic middle jurassic circulation was more varied judging from crossbedding studies in the lower part of the entrada sandstone crossbedding in upper jurassic eolian rocks of wyoming and south dakota yielded a random pattern but upper jurassic rocks farther south on the colorado plateau and adjoining areas show a return to a fairly unidirectional pattern comparing the resultants with their reconstructed paleogeographic setting shows surprisingly little influence of major geographic features on overall circulation patterns however the greatest amount of local variation occurred at or near highly indented shorelines where the temperature contrast between land and water produces local wind currents that may vary appreciably from regional circulation patterns although they do not cause noticeable horizontal deflections in wind patterns small and low topographic highs appear to be able to promote the development of a dune field if a source of sand is available and if streams do not enter the growing dune field

a combined petrographical geochemical provenance study of the newland formation mid proterozoic of montana a provenance study was conducted on the mid proterozoic newland formation in which petrographical features of sandstones and geochemical characteristics of shales were integrated to arrive at an internally consistent interpretation sandstones of the newland formation are typically arkosic sands and arkoses with very well rounded quartz and feldspar grains and only minor amounts of extrabasinal rock fragments the predominant feldspar types are k spar and microcline feldspar grains are smaller than quartz grains and feldspars show little alteration due to weathering detrital modes of newland sandstones qfl diagrams indicate that they were derived from a stable cratonic source these petrographical features imply a source area dominated by granites and granitoid gneisses semi arid to arid climate tectonic quiescence and overall peneplain conditions shales of the newland formation are dominated by illite quartz silt and fine crystalline dolomite they have small la th rations relatively large hf contents and small contents of cr co and ni all indicative of derivation from crust of granitic composition small tio 2 al 2 o 3 ratios also suggest source rocks of granitic composition the average chemical index of alteration cia for newland shales is 71 8 which in light of the probable granitoid source indicates modest amounts of chemical weathering relatively large sio 2 contents and large k 2 o na 2 o ratios reflect derivation from stable cratonic areas and tectonic quiescence thus in general the petrography of sandstones and geochemistry of shales provides the same provenance clues for the newland formation one notable discrepancy between the two approaches is that the sandstones indicate an arid to semi arid climate with very minor chemical weathering whereas the cia of the shales indicates at least modest amounts of chemical weathering this indicates on one hand the need to better calibrate the cia with a large variety of muds from modern climatic settings and on the other hand the possibility that this discrepancy is due to transport segregation

sandstone composition and provenance of the rotliegend of the ne german basin abstract the ne german basin contains an approximately 2 5 km thick rotliegend sedimentary section subdivided into 4 formations these were deposited following the cessation of the late carboniferous early permian volcanic episode and a ca 20 ma period of erosion and non deposition initial deposition was confined to two areas but with increasing thermal subsidence the basin extended to the south petrographic 190 sandstones and geochemical 48 sandstones and mudstones analyses suggested that the sands are quartz rich and are derived largely from cratonic or recycled sources volcanic and sedimentary rock fragments form most of the labile grains the succession was deposited in a relatively stable tectonic environment with sediment input derived largely from the lower and upper palaeozoic strata of the variscan hinterland and the upper carboniferous and lower permian volcanics volcanic input increases with younger stratigraphic units this is interpreted as resulting from the unroofing of a more basic source

precambrian basin margin fan deposits mesoproterozoic bagalkot group india abstract three dimensional facies variability in coarse clastic sedimentary rocks breccia conglomerates and coarse grained sandstones at the base of the ramdurg formation suggests terrestrial scree and fans giving way downslope to fluvial sediments along the margin of the mesoproterozoic bagalkot basin in india in consistency with the basic tenet of precambrian alluvial sedimentation fluvial architectural elements and field location specific consistency in channel flow direction support an invariably braided pattern for the rivers although the architectural element packaging pattern does show distinct changes downslope with channel belts thinning and becoming more regular in their geometry with possible persistence of a semi arid climate downslope change in flow durability in the channels was controlled primarily by water discharge depending on position of the channels with respect to the mean level of the water table basin margin slope intersection confined between an unconformity below and a granular lag succeeded by thoroughly wave featured sandstone and argillite carbonate above the coarse and poorly sorted clastic sedimentary rocks of the basal ramdurg are interpreted as a base level lowstand product for which the sedimentation rate exceeded the rate of space creation for sediment accumulation consequently the fan succession as a whole is coarsening upward fluvial sections nonetheless fine upward as the depositional slope gradient became progressively reduced with aggradation of channels tectonics related slope variation along and across the basin margin devoid of vegetation dictated the sediment distribution and sequence building pattern primarily eventual termination of this basin margin depositional system was caused by later enhancement in the rate of base profile rise

tectonic setting and provenance of the neoproterozoic uinta mountain and big cottonwood groups northern utah constraints from geochemistry nd isotopes and detrital modes abstract the neoproterozoic uinta mountain group was deposited in an east trending intracratonic rift bounded on the north by an active fault system and opening into a shallow sea on the west where the big cottonwood group was deposited in an estuary although this rift may have been associated with the early stages in the breakup of rodinia it was not an aulacogen geochemical nd isotope and detrital mode studies indicate that uinta mountain group sediments were derived from mixed archean and paleoproterozoic sources with the former dominating big cottonwood group sediments appear to have been derived predominantly from paleoproterozoic sources the archean sediment source is the wyoming craton and source rocks comprised dominantly granites enriched in th u y zr hf and ree the relative abundance of enriched granite implied by sedimentary rocks of the uinta mountain group indicates that the wyoming craton is anomalous compared to other archean cratons cia values and a cn k relationships in shales of the uinta mountain and big cottonwood groups indicate high degrees of weathering of sources probably in subtropical to tropical climates supporting a near equatorial location for southwestern laurentia at about 800 ma differences in the nd isotopic composition between the big cottonwood group and neoproterozoic sedimentary rocks in western utah and northeast nevada suggest a northwest striking uplift in northwest utah possibly ancestral to the paleozoic toole uinta arch

age geological setting and paragenesis of heavy rare earth element mineralization of the tanami region western australia metasedimentary rock hosted heavy rare earth element hree mineralization occurs as numerous orebodies distributed across a large district of the tanami region of central australia close to a regional unconformity between archean metasedimentary rocks of the browns range metamorphics brm and overlying proterozoic birrindudu group sandstones the orebodies consist predominantly of quartz xenotime and minor florencite and occur along steeply dipping structures within a stockwork of hydrothermal veins and breccias paragenetic stages of the mineralization include 1 a pre ore stage of a greenschist facies overprint of detrital minerals including quartz alkali feldspar plagioclase and muscovite aligned in the premineralization foliation 2 syn ore quartz and white mica alteration associated with a multistage mineralization of the ore minerals primarily in breccias and veins and 3 a post ore stage of veining and brecciation forming several generations of quartz plus hematite barite anhydrite and pyrite in situ u pb dating of xenotime from several deposits prospects yielded an age range for mineralization of 1 65 to 1 60 ga this timeframe lacks local magmatism or orogeny and is significantly younger than the ca 1 72 ga 40ar 39ar age of the pre ore muscovite far field stresses associated with the distal isan and liebig orogenies are invoked as drivers of large scale fluid flow and fault re activation in the region we propose that ore formation was achieved via fluid leaching of ree from the brm followed by fluid mixing in fault zones especially in the vicinity of the unconformity between the brm and overlying birrindudu group sandstones this mineralization style shares many features with unconformity related u deposits and there is significant potential for discovery of further ree orebodies of this style especially in the vicinity of regional unconformities in intercontinental sedimentary basins

were the hydraulic parameters of precambrian rivers different abstract palaeohydraulic estimates have been made using time tested standard empirical formula involving channel dimensions especially channel depth in four indian proterozoic fluvial formations these are the mesoproterozoic ramdurg younger mesoproterozoic muchkundi and neoproterozoic cave temple arenite in karnataka and upper rewa sandstone in central india the basic data came from direct measurement of decompacted thicknesses of completely preserved channel fills or bedforms and more commonly of cross sets the result in combination with data derived by others in a similar way from a few other precambrian fluvial formations helps to provide a new insight into the palaeohydraulics of precambrian rivers river gradients appear to have been steeper during the precambrian the possibly continuous range of variation in river gradients from alluvial fans to the plains as a whole shifted to higher values during the precambrian epoch the likely reason is a greater possibility of sediment bypassing on vegetation free land concomitant downstream increase in discharge was enhanced in precambrian rivers due to lack of withdrawal of groundwater through biogenic activity precambrian rivers were commonly ephemeral but downstream the ephemerality index was reduced and some rivers might have been perennial where they debouched into standing bodies of water the precambrian river deposits studied generally had smaller width and depth with respect to phanerozoic and modern rivers possibly due to unstable substratum causing surface run off tended to be impelled along multiple independent minor channels

petrography and geochemistry of lower carboniferous greywacke and mudstones in northeast junggar china implications for provenance source weathering and tectonic setting abstract northeast junggar occupies an important position that links east junggar and chinese altai numerous magmatic and sedimentary rocks of the paleozoic in this area recorded the final amalgamation processes of east junggar and chinese altai this study analyzes the petrological and geochemical characteristics of sandstones and mudstones from the early carboniferous nanmingshui formation in northeast junggar the provenance and tectonic setting of these clastic rocks are discussed petrography indicates that the composition and texture maturity of the sandstones are low the components of the sandstones are mainly volcanic fragments 61 87 feldspars 9 30 and monocrystalline quartz 2 18 with a few polycrystalline quartz and other minerals slice observation indicates that the majority of the volcanic fragments of sandstones are basic intermediate volcanic rocks with a few dacite and felsic plutonic fragments the detrital modes of the sandstones reflect that these sandstones are derived from undissected arcs a low to moderate chemical index of alteration and the al2o3 cao na2o k2o diagram reflect a low to moderate weathering degree in the source area trace and rare earth element e g la th hf sc cr co and eu contents and their ratios suggest that the source rocks of the clastic rocks are intermediate basic rocks with some felsic rocks compared with sandstones the source rocks for mudstones are more felsic the petrography and geochemistry characteristics of the clastic rocks suggest that the proximal dulate arc should be the primary source area mixing calculations based on ree data suggest that approximately two thirds of the sandstone fragments are intermediate basic volcanic rocks the contents of the major and trace elements as well as the stratum features of the clastic rocks manifest that these clastic rocks resemble sedimentary rocks in a back arc basin the formation of this back arc basin is caused by the southward subduction of the zaysan erqis ocean

models for generation of carbonate cycles computer modeling provides a quantitative approach to a better understanding of actual carbonate cyclic sequences to model carbonate cycles the authors can use water depth dependent sedimentation rate for each facies an initial lag time linear subsidence tidal range and period and amplitude of sea level oscillation about a horizontal datum tidal flat capped cycles up to a few meters thick result from low amplitude sea level oscillation of a few meters and short lag times nonerosive caps reflect sea level lowering being balanced by subsidence and basinward migration of the shoreline not exceeding tidal flat progradation rate when higher amplitude sea level oscillations occur the tidal flats are abandoned on the inner shelf during sea level fall because seaward movement of the strandline outpaces progradation rate of flats increased amplitude also results in sea level falling faster than flats can subside so that disconformities with thick vadose profiles develop high amplitude 100 m or more oscillations result in incipient drowning of platforms and juxtaposition of deep water facies against shallow water facies within cycles sea level falls before the platform can build to the sea level highstand and the shoreline migrates much more rapidly than tidal flats can prograde thus cycles are disconformity bounded and lack tidal flat caps 10 references