A. H. M. Ahmad

Geochemistry and detrital modes of Proterozoic sedimentary rocks, Bayana Basin, north Delhi fold belt: implications for provenance and source-area weathering

Bayana Basin, sited along the eastern margin of the north Delhi fold belt of the Aravalli Craton, contains an ∼3000 m-thick sequence comprising one volcanic and seven sedimentary formations of the Delhi Supergroup. The sedimentary units are the Nithar, Jogipura, Badalgarh, Bayana, Damdama, Kushalgarh, and Weir formations in order of decreasing age. Petrographic study of the sandstones as well as major and trace elements (including rare earth elements) and bulk-rock analyses of the shales and sandstones allow the determination of their provenance, source-rock weathering, and basinal tectonic setting. The sandstones are quartz rich and were derived mainly from exhumed granitoids typical of a craton interior. Geochemical patterns of the sandstones and shales are similar. However, trace element abundances are low in sandstones, probably due to quartz dilution. The coarser clastic Damdama and Weir sandstones, which occur at higher stratigraphic levels, have strikingly low trace element concentrations compared with the underlying Bayana and Badalgarh sandstones. All samples show uniform LREE-enriched patterns with negative Eu-anomalies (Eu/Eu* = 0.16–0.23) and are similar to those of post-Archaean Australian shales (PAAS). However, the (La/Yb) n ratios (averages 11–18) of all the sedimentary units are higher than those of PAAS, except for the Bayana Sandstone, which has low values (average 6.77). The chemical index of alteration (70–78) and the plagioclase index of alteration (87–97) values and the A–CN–K diagram suggest moderate to intense weathering of the source area. The provenance analyses indicate that basin sedimentation was discontinuous. It received input from a terrain comprising granitoids, mafic rocks, sedimentary sequences, and tonalite-trondhjemite-granodiorite (TTG) suites. The Nithar and Badalgarh sandstones received input from a source consisting predominantly of granitoids. The succeeding Damdama and Weir sandstones received debris from granitoids and TTG in different proportions. The Kushalgarh shale was possibly derived from a source consisting granites and mafic rocks with a TTG component. The pre-existing sedimentary formations also contributed intermittently during the different phases of sedimentation. Bulk-rock geochemical data suggest Mesoarchaean gneisses and late Archaean granites of BGC/BGGC (Banded Gneissic Complex/Bundelkhand Granitic Gneiss Complex) basement as possible source terrains. These data indicate deposition in a continental rift setting. The coeval formation of many rift-related Proterozoic sedimentary basins in the BGC/BGGC terrain suggests that the North Indian Craton underwent major intracratonic extension during Proterozoic time, probably triggering the break up of Earths first supercontinent.

Integrated petrographic, mineralogical, and geochemical study of the upper Kaimur Group of rocks, Son Valley, India: Implications for provenance, source area weathering and tectonic setting

The upper Kaimur Group (UKG) of the Vindhyan Supergroup in central India, primarily consists of three rock types-Dhandraul sandstone, Scarp sandstone and Bijaigarh shale. The present study aims to reconstruct the parent rock assemblages, their tectonic provenance, mineralogy, weathering intensity, hydraulic sorting and depositional tectonic setting. Samples from the UKG rocks representing the Dhandraul sandstone, Scarp sandstone and Bijaigarh shale were studied using a combination of petrographic, mineralogical, and geochemical techniques. Texturally, medium to coarse grained UKG sandstones are mature and moderate to well sorted. Deficiency of feldspars in these sandstones indicates that the rocks are extensively recycled from distant sources. Their average modal composition for Scarp (avg. Qt99 F0.2L0.8) and Dhandraul (avg. Qt99 F0.1L0.8) sandstones, classifies them as quartz arenite to sub-litharenite types, which is consistent with geochemical study. Major element concentrations revealed that sandstones have high SiO2, K2O < Na2O, and low Fe2O3, which are supported by the modal data. On the other hand, sandstone samples are enriched in most trace elements such as Ce, Sr, V, Sc and Zr and depleted in U and Th. The CIA values (43.17–76.48) of the UKG rocks indicate low to moderate weathering, either of the original source or during transport before deposition, which may have related to low-relief and humid climatic conditions in the source area. Further, petrographic and geochemical interpretations indicate that they are derived from craton interior to quartzose recycled sedimentary rocks and deposited in a passive continental margin. Therefore, granitic and low grade metamorphic rocks of Mahakoshal Group and Chotanagpur granite-gneiss, situated on the southern and south-eastern side of the Vindhyan basin are suggested as possible provenance for the UKG rocks.

Facies controlled porosity evolution of the Neoproterozoic Upper Bhander Sandstone of Western India

Abstract The Upper Bhander Sandstone is dominantly composed of quartzarenites. The basal and top portions are sandstones, with the middle section comprising thinly bedded shales with interlayer silt and sandstone units. The sandstone units are composed of several varieties of quartz, feldspar, micas, rock fragments and heavy minerals. The Upper Bhander Sandstone was deposited in a transgressive phase and later modified by tidal processes and wave- and storm-dominated processes in a tide-influenced Barrier Beach Complex of the shallow marine environment. This study reveals that, during mechanical compaction, a rearrangement of grains took place and point and long contacts were formed. The early silica cementation and shallow burial resulted in high primary porosity. This phase was followed by chemical compaction and the replacement of silica cement by iron cement (Fe-cement) under the deep burial phase of these sandstones. Dissolution of Fe-cement and feldspars resulted in secondary porosity development. Quartz overgrowths are better developed on coarse- to medium-sized grains than on fine-sized grains. These observations suggest a progressive compaction, which initiated at the sediment–water interface and continued till deep burial diagenesis in a rapidly subsiding basin. The existing optical porosity of the Upper Bhander Sandstone is 4% and the minus cement porosity is 18%.

Facies analysis and depositional environment of the Jurassic, Jumara Dome sediments, Kachchh, western India

The study deals with the depositional environment of Jumara Dome sediments. The Jumara Dome is an important outcrop of Bathonian to Oxfordian sediments amongst the Kachchh Mainland exposures. On the basis of facies analysis three associations have been documented, namely, G-1 consisting of low energy facies comprising of cross-bedded sandstone, massive sandstone, grey shale and thin bedded sandstone, bioclastic — lithoclastic grainstone, bioclastic — lithoclastic packstone, microbioclastic packstone/wackestone, bioturbated laminated wackestone to mudstone and pelagic lime mudstone; G-II consisting of moderate energy facies comprising of laminated sandstone and grapestone or agglutinated grainstone; G-III consisting of high energy facies comprising of interbedded gypsiferous shale and sandstone/siltstone, oolitic grainstone to conglomerate and bioclastic grainstone. The facies associations reflect an ideal shallowing upward sequence representing slope, bioclast bar, lagoon and inner shelf. Presence of wide range of facies indicates that the rocks of the studied area were deposited during the fluctuating sea level, interrupted by the storms, in the shallow marine environment.

The Effects of Diagenesis on the Reservoir Characters in Ridge sandstone of Jurassic Jumara dome, Kachchh, Western India

Ridge sandstone of Jurassic Jumara dome of Kachchh was studied in an attempt to quantify the effects of diagenetic process such as compaction, cementation and dissolution on reservoir properties. The average framework composition of Ridge sandstone is Q80F17L3, medium-to coarse grained and subarkose to arkose. Syndepositional silty to clayey matrix (3% average) is also observed that occurs as pore filling. The diagenetic processes include compaction, cementation and precipitation of authigenic cements, dissolution of unstable grains and grain replacement and development of secondary porosity. The major cause of intense reduction in primary porosity of Ridge sandstone is early cementation which include silica, carbonate, iron, kaolinite, illite, smectite, mixed layer illite-smectite and chlorite, which prevents mechanical compaction. The plots of COPL versus CEPL and IGV versus total cement suggest the loss of primary porosity in Ridge sandstone is due to cementation. Cements mainly iron and carbonate occurs in intergranular pores of detrital grains and destroys porosity. The clay mineral occurs as pore filling and pore lining and deteriorates the porosity and permeability of the Ridge sandstone. The reservoir quality of the studied sandstone is reduced by clay minerals (kaolinite, illite, smectite, mixed layer illitesmectite, chlorite), carbonate, iron and silica cementation but on the other hand, it is increased by alteration and dissolution of the unstable grain, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to serve as a reservoir is strongly related to sandstone diagenesis.

Diagenetic features of Jurassic Fort Member sandstone, Jaisalmer formation, western Rajasthan

Jaisalmer Formation consists of 360m thick succession of medium to coarse grained sandstones with interbeds of shale, claystone and occasional lignite that rest over Lathi Formation, is the basal part of the Jaisalmer basin. The rocks are exposed amidst desert, low mounds and shallow stone quarries. Sandstones were deposited in shallow marine to deltaic environments. The studied sandstones consist of abundant quartz followed by feldspar, mica, chert, rock fragments and heavy minerals. The study mainly deals with identification of various diagenetic features such as compaction, cementation and porosity evolution. During mechanical compaction rearrangement of grains took place and point, long and suture contacts were formed. The sandstones are cemented by iron oxide, silica overgrowth, carbonate and clay. Porosity has developed due to dissolution of iron, carbonate cement and feldspar grains. Dissolution and alteration of feldspar, lithic fragments and pressure solution were the main source of quartz cements. The sandstones show good amount of existing optical porosity with an average of 7.19%. Porosity reduction is mainly due to early stage of mechanical compaction and subsequent pervasive calcite and iron oxide cementation. Further, porosity reduced due to deposition of clay cement.

Petrographical and geochemical signatures of Jurassic rocks of Chari Formation, Western India: implications for provenance and tectonic setting

The sandstones of the Ridge and Athleta members of Chari Formation (Callovian-Oxfordian) exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium to coarse grained, poorly to well sorted, sub-angular to sub-rounded, and show low to medium sphericity. These sandstones were derived from a mixed provenance including granites, granite-gneisses, low and high-grade metamorphic, and some basic rocks of Aravalli range and Nagarparkar massif. The petrofacies analysis reveals that these sandstones belong to the continental block and recycled orogen tectonic regime. The studied sandstones are modified by paleoclimate, distance of transport, and diagenesis. Mineralogically and geochemically, sandstones are classified as quartzarenite, subarkose, arkose, sublithic arenite, and wacke, respectively. The A-CN-K ternary plot and CIA, CIW, PIA, and ICV values suggest that the similar source rocks suffered moderate to high chemical weathering under a hot-humid climate in an acidic environment with higher

Provenance and weathering history of Archaean Naharmagra quartzite of Aravalli craton, NW Indian shield : Petrographic and geochemical evidence

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