Estimation of dolomite formation: Dolomite precipitation and dolomitization

Reactive-transport models are developed here that produce dolomite via two scenarios: primary dolomite (no CaCO₃ dissolution involved) versus secondary dolomite (dolomitization, involving CaCO₃ dissolution). Using the available dolomite precipitation rate kinetics, calculations suggest that tens of meters of thick dolomite deposits cannot form at near room temperature (25-35°C) by inorganic precipitation mechanism, though this mechanism will provide dolomite aggregates that can act as the nuclei for dolomite crystallization during later dolomitization stage. Increase in supersaturation, Mg⁺²/Ca⁺² ratio and CO₃^-2 on the formation of dolomite at near room temperature are subtle except for temperature.This study suggests that microbial mediation is needed for appreciable amount of primary dolomite formation. On the other hand, reactive-transport models depicting dolomitization (temperature range of 40 to 200°C) predicts the formation of two adjacent moving coupled reaction zones (calcite dissolution and dolomite precipitation) with sharp dolomitization front, and generation of >20% of secondary porosity. Due to elevated temperature of formation, dolomitization mechanism is efficient in converting existing calcite into dolomite at a much faster rate compared to primary dolomite formation.     

Stratigraphy, sedimentology and bulk organic geochemistry of black shales from the Proterozoic Vindhyan Supergroup (central India)

Four organic-rich shale units of the Proterozoic Vindhyan sedimentary succession have been scanned to reveal their origin and hydrocarbon potential. The wavy-crinkly nature of the carbonaceous laminae is suggestive of a microbial mat origin of the shales. These shales are thus different from Phanerozoic black shales which typically exhibit planar laminae. The hydrocarbon potential of the black shale units has been evaluated by Rock-Eval pyrolysis. Total organic carbon content of many of the shales exceeds 1%. The meanT _max for the black shales translate to a vitrinite reflectance range of 2.05-2.40% Rm based on standard conversion techniques. These shales have reached the catagenetic stage near the beginning of anthracite formation.     

Deciphering potential groundwater zone in hard rock through the application of GIS

Remote Sensing and Geographic Information System has become one of the leading tools in the field of hydrogeological science, which helps in assessing, monitoring and conserving groundwater resources. It allows manipulation and analysis of individual layer of spatial data. It is used for analysing and modelling the interrelationship between the layers. This paper mainly deals with the integrated approach of Remote Sensing and geographical information system (GIS) to delineate groundwater potential zones in hard rock terrain. The remotely sensed data at the scale of 1:50,000 and topographical information from available maps, have been used for the preparation of ground water prospective map by integrating geology, geomorphology, slope, drainage-density and lineaments map of the study area. Further, the data on yield of aquifer, as observed from existing bore wells in the area, has been used to validate the groundwater potential map. The final result depicts the favourable prospective zones in the study area and can be helpful in better planning and management of groundwater resources especially in hard rock terrains.     

Origin and geochemical characterization of the glauconites in the Upper Cretaceous Lameta Formation,Narmada Basin,central India

This study presents geochemical characteristics of glauconites in estuarine deposits within the Maastrichtian Lameta Formation in central India. Resting conformably over the Bagh Group, the Lameta Formation consists of ~4-5 m thick arenaceous, argillaceous and calcareous green sandstones underlying the Deccan Traps. The sandstone is friable, medium-to coarse-grained, well-sorted and thoroughly crossstratified, and contains marine fossils. Detailed petrography, spectroscopy and mineral chemistry indicates unique chemical composition of glauconite with high K_2O, MgO, Al_2O_3 and moderate TFe_2O_3. Glauconite is formed by the replacement of K-feldspars, initially as stringers in the cleavages and fractures of feldspars. Incipient glauconite subsequently evolves fully, appearing as pellets. Fully-evolved glauconite pellets often leave tiny relics of K-feldspar. XRD exhibits characteristic peak of 10A from basal(001)reflection of glauconite, indicating the "evolved" character. The K_2O content of glauconites in the Lameta Formation varies from 5.51% to 8.29%, corroborating the "evolved" to "highly-evolved" maturation stage.The TFe_2O_3 content of glauconite varies from 12.56% to 18.90%. The PASS-normalized-REE patterns of glauconite exhibit a "hat-shape" confirming the authigenic origin of glauconites. The slightly-negative to slightly-positive Ce anomaly value and the moderate TFe_2O_3 content of glauconite agree well with a suboxic,estuarine condition. The replacement of K-feldspar by the glauconite contributes towards the high K_2O content. Compositional evolution of glauconites in the Lameta Formation is similar to those observed in many Precambrian sedimentary sequences.     

Paleogene stratigraphy of Kutch,India: an update about progress in foraminiferal biostratigraphy

The Paleogene sections of Kutch are the reference for the regional chronostratigraphic units of India. The ages of these dominantly shallow marine carbonates are mainly based on larger benthic foraminifera (LBF). The taxonomic revisions of the LBF and the progressively refined shallow benthic zonations (SBZ) have necessitated the present study on updating the stratigraphy of the area. The sedimentation in Kutch commenced with the deposition of volcaniclastics in terrestrial environments in the Paleocene. The marine transgression in SBZ 5/6 deposited finer clastics and carbonates, designated as Naredi Formation, in early Eocene. There is no evidence of marine Paleocene in Kutch. A major hiatus spanning SBZ 12 to SBZ 16 was followed by the development of a carbonate platform and deposition of Harudi Formation – Fulra Limestone during the Bartonian, SBZ 17. The hiatus corresponds to a widespread stratigraphic break in Pakistan and India to Australia, referred as the ‘Lutetian Gap.’ The Maniyara Fort Formation is assigned to SBZ 22 B and SBZ 23, and its age is revised to Chattian. Climate played a major role in building up of the Paleogene stratigraphic succession of Kutch, the carbonates formed during the warming intervals and the stratigraphic gaps were in the intervening cooling periods.     

Satellite detection of earthquake thermal infrared precursors in Iran

Stress accumulated in rocks in tectonically active areas may manifest itself as electromagnetic radiation emission and temperature variation through a process of energy transformation. Land surface temperature (LST) changes before an impending earthquake can be detected with thermal infrared (TIR) sensors such as NOAA-AVHRR, Terra/Aqua-MODIS, etc. TIR anomalies produced by 10 recent earthquakes in Iran during the period of Jun 2002–Jun 2006 in the tectonically active belt have been studied using pre- and post-earthquake NOAA-AVHRR datasets. Data analysis revealed a transient TIR rise in LST ranging 2–13°C in and around epicentral areas. The thermal anomalies started developing about 1–10 days prior to the main event depending upon the magnitude and focal depth, and disappeared after the main shock. In the case of moderate earthquakes (<6 magnitude) a dual thermal peak instead of the single rise has been observed. This may lead us to understand that perhaps pre-event sporadic release of energy from stressed rocks leads to a reduction in magnitude of the main shock. This TIR temperature increment prior to an impending earthquake can be attributed to degassing from rocks under stress or to p-hole activation in the stressed rock volume and their further recombination at the rock–air interface. A precise correlation of LST maps of Bam and Zarand with InSAR-generated deformation maps also provides evidence that the thermal anomaly is a ground-related phenomenon, not an atmospheric one.     

Characterization of an island aquifer from tidal response

Growing demand for potable water for various needs has lead to indiscriminate exploitation of groundwater resources, particularly, in the terrain where surface water resources are negligible. One such area is an island where groundwater is the only source of fresh water. Groundwater is the prime source of fresh water on most of the atolls in the world. Groundwater on these islands is in the form of thin fragile floating lens and is often vulnerable to overexploitation, draught, tidal waves, tsunami and cyclone resulting in seawater ingress. Sustainable development of this meager source of fresh groundwater for a longer time becomes a more difficult task on small atolls with a large population depending on this vital resource. To develop a sustainable management scheme and identify the vulnerable part of aquifer, characterization of the aquifer system on islands is imperative. Groundwater on an atoll is extremely vulnerable to seawater mixing through natural as well as human activities. One such natural process is the tides of the ocean. The response of sea tide to the water table on the island offers valuable data as well as cost-effective means to characterize an aquifer system. Such characterization is vital for the management of groundwater resources on an atoll. The obtained results have compared well with the parameters obtained through a conventional pumping test. Therefore, the use of tidal response to the water table, which can easily be recorded, provides a rapid and cost-effective means to characterization of the aquifer system on the island.