• Helga De Wall

      Articles written in Journal of Earth System Science

    • Paleosol at the Archean–Proterozoic contact in NW India revisited – Evidence for oxidizing conditions during paleo-weathering?

      Manoj K Pandit Helga de Wall Narendra K Chauhan

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      A number of fine-grained sericite bearing pelitic,schistose lithologies occur along the Archean (Banded Gneiss Complex)–Proterozoic (Aravalli Supergroup)contact (APC)in the Udaipur valley in NW Indian craton.These Al-rich lithologies (subsequently metamorphosed)have been described as ‘paleosols ’,developed over a 3.3 Ga old Archean gneissic basement and are overlain by Paleopro- terozoic Aravalli quartzite.The paleosol was developed between 2.5 and 2.1,coincident with the globally recognized Great Oxidation Event (GOE).In previous studies these paleosol sections were interpreted to have developed under reducing environment,however,the finding of a ‘ferricrete ’ zone in the upper part of Tulsi Namla section (east of Udaipur)during the present study (in addition to earlier reported lithologies) has led to an alternative suggestion of oxygen-rich conditions during paleosol development.The Tulsi Namla paleosol section shows all the features characteristic of a complete paleosol section described from other Archean cratons.The paleosol includes sericite schist with kyanite as the prevalent Al-silicate in the lower part of profile while chloritoid and Fe-oxides typify the Fe-rich upper part.Alumina has remained immobile during the weathering process while Fe and Mn show a decrease in the lower part of the section and an abrupt rise in the upper part,in the ferricrete zone.The field and geochemical data indicate that the Tulsi Namla section is an in situ weathering profile and at least the upper part shows evidence of oxidizing conditions.

    • In situ gamma radiation measurements in the Neoproterozoic rocks of Sirohi region, NW India

      Lars Scharfenberg Helga De Wall Stefan Schöbel Alexander Minor Marcel Maurer Manoj K Pandit Kamal K Sharma

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      Natural gamma ray measurements using a portable device were performed at 157 sites in the area around Sirohi town and Sindreth village in Rajasthan (NW India). This region comprises sedimentary rocks, metasediments, granites and gneisses that bear characteristic GR dose values and U/Th ratios corresponding with their specific geological history. A-type Malani granites and rhyolitic derivates, also referred as high heat production granites, show distinct differences as compared to the S-type Erinpura and Balda granites, most prominent in a high Th content of the former (up to 90 ppm). Sedimentary rocks in the Sirohi and Sindreth area are variable in their signatures reflecting their variable source rocks. In the area between the Balda and Paladi villages, northeast of Sirohi, measurements in vicinity of a N–S running shear zone, have shown U enrichment up to 8 ppm. This shear zone has been synkinematically mineralized with quartz and shows evidence of fluid infiltration into the host rocks in the vicinity of the shear zone. Erinpura granites have been altered due to fluid activity and show a light depletion of K (3.96%) and Th (20.11 ppm) as compared to the unaltered rocks (K, 4.06; Th 24.46 ppm). Enrichment of U (with a mean value of 13 ppm) has also been recorded in the lower clastic unit of the Sindreth Basin, especially within gritty conglomerates wherein migration and precipitation along fault planes is proposed.

    • Continental rift-setting and evolution of Neoproterozoic Sindreth Basin in NW-India

      Stefan Schöbel Kamal K Sharma Thorsten Hörbrand Theresa Böhm Ines Donhauser Helga de Wall

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      The Neoproterozoic Sindreth Basin, NW India, and its surrounding area represent a half graben structure situated between the undeformed Malani Igneous Suite (MIS) in the west and a corridor of coeval Cryogenian ductile deformation, anatexis and granite intrusion in the east. The main lithologies observed in the basin are conglomerate, fanglomerate, debris flow and lake deposits derived from a nearby continental provenance, intercalated with concurrent mafic and felsic lava flows. Based on geological traverses across the strike of the basin, we propose a three-fold classification comprising Lower Clastic Unit and an Upper Clastic Unit and a Bimodal (basalt–rhyolite) Volcanic Unit separating the two. Tilting due to basin inversion and faulting has been observed; however, the rocks are unmetamorphosed and show undisturbed primary sedimentary features. The stratigraphic record of the basin is characteristic for deposition and magmatism in a fault-related continental setting. Implications of the findings have been discussed in the context of Neoproterozoic crustal dynamics in NW India. This study provides conclusive evidence for a continental setting for Sindreth Basin evolution and contests the recent models of active subduction setting (either back-arc basin or accretionary sediments over a subduction zone).

    • Differences in natural gamma radiation characteristics of Erinpura and Malani granites in NW India

      Lars Scharfenberg Sebastian Jandausch Lina Anetzberger Anette Regelous Kamal K Sharma Helga De Wall

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      In NW India, large volumes of exposed Neoproterozoic basement rocks are formed by two magmatic suites, Erinpura granites as a late thermal event with respect to the $\sim$1 Ga Delhi Orogeny and the younger Malani igneous suite (770–750 Ma). Average uranium and thorium equivalent concentrations (in ppm) inferred from spectroscopic gamma radiation survey are higher in Malani rocks (Th 47.33 ppm and U 6.95 ppm) as compared to the Erinpura granites (Th 33.55 ppm and U 4.77 ppm). These values are considerably above the granite world average (Th 14.8 $\pm$ 13.2 ppm; U 3.93 $\pm$ 3.27 ppm). High U (up to 19 ppm) and Th (up to 88 ppm) in some Malani granites and a constant Th–U ratio of 7 points to a high degree of fractionation of the felsic magma. Higher radioelement concentration in the east (Mirpur granite) as compared to the west (Jaswantpura granite) is substantiated by geochemical data. Areas to the west and east of the Sirohi frontal thrust show differences, most likely a consequence of anatexis in the eastern sector. A high linear correlation between inductively coupled plasma mass spectrometry and gamma-ray data underlines the suitability of in-situmeasurements for the determination of U and Th concentrations during a field survey providing basic information for future petrogenetic and risk-hazard studies in this granitic terrain.

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