• AMRITA MUKHERJEE

      Articles written in Journal of Earth System Science

    • Soft-sediment deformation structures in a Permo-carboniferous glacio-marine setting, Talchir Formation, Dudhi Nala, India

      H N BHATTACHARYA AMRITA MUKHERJEE

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      The Permo-carboniferous glacio-marine Talchir Formation of Dudhi Nala, West Bokaro Coal Basin, India, hosts soft-sediment deformation structures that originated from liquefaction and concomitant fluidization of unconsolidated sediments at or close to the sediment–water interface. Since liquefaction of water saturated sediments may be initiated by different endogenic or exogenic triggering agents, identification of the trigger needs careful analysis of the deformation structure, as well as depositional mechanism and environment of deposition of the host sediments. In-depth analysis of the studied liquefaction- and fluidization-induced soft-sediment deformation structures and sedimentary attributes of the host sediments of the studied succession unequivocally stand against the role of strong wave and tidal action, rapid sediment loading, mass-flow and subaqueous slides related shear stress, and water or gas seepage as the triggering agent. However, development of the soft-sediment deformation structures, close to sediment–water interface, closeness to syn-sedimentary fault, flowage along the tilt towards the fault, confinement within undeformed beds, episodic character and close similarities with structures that are formed during recent earthquakes, are consistent with seismic triggering for liquefaction. The studied shallow marine sediments record a sequel of climatic amelioration, glacial retreat, shallow marine sedimentation, glacio-isostatic rebound and related syn-sedimentary faulting and development of soft-sediment deformation structures.

    • A reappraisal of the Jharol Formation in the context of stratigraphy of Aravalli Supergroup, Rajasthan, India

      BHATTACHARYA H N AMRITA MUKHERJEE

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      The enormously thick sedimentary succession exposed between Rishabhdev Lineament and rocks of the Delhi Supergroup constitutes Jharol Formation of the Aravalli Supergroup in Udaipur region of Rajasthan. The whole sedimentary succession is named Jharol Formation and designated as deep water turbidites. A detailed field examination indicates presence of two distinctly different litho successions juxtaposed across a crustal level fault along which brecciated ultramafics are tectonically emplaced. The sedimentary successions exposed immediate west of the Rishabhdev Lineament are deep water sediments with distinct facies types characterizing inner to outer lobe stacked turbidites. These turbidites are similar to the turbidites of the Udaipur Formation and Tidi Formation exposed east of the Rishabhdev Lineament. However, the sedimentary succession exposed west of these deep water sediments across the aforesaid fault contains facies types that represent continental to shallow marine deposits. This basin marginal sediments continue up to the contact with rocks of the Delhi Supergroup. Such litho-association suggests that a separate formation status of the sediments lying west of the Rishabhdev Lineament should be dropped and the turbidites of the Udaipur Formation should be considered as the youngest formation of the Aravalli Supergroup.

    • Mixed siliciclastic–carbonate debrite–turbidite deposits in Paleoproterozoic Aravalli Supergroup, Zawar, Rajasthan, India: Implications on the Aravalli Basin evolution

      AMRITA MUKHERJEE H N BHATTACHARYA

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      The Paleoproterozoic Aravalli Supergroup exposed in the Zawar Pb–Zn mineralized belt of Rajasthan, India contains a mixed siliciclastic–carbonate horizon (Mochia Formation), which hosts the mineralization. The mixed sediments are underlain by a siliciclastic turbidite unit (Mandli Formation) and overlain unconformably by a fluvio-deltaic- to storm-influenced siliciclastic platformal deposit (Bowa Formation). Detailed facies analysis of the mixed siliciclastic–carbonate succession reveals that the sediments were deposited in a slope-base to basin plane environment by sediment density flow processes. Petrography of the siliciclastic–carbonate sediments reveals that micritic carbonates dominate over siliciclastic components in the compositionally mixed layers. The preponderance of micritic carbonate over siliciclastics indicates the sediments were sourced by an adjacent platform during a sea-level highstand. The sedimentation pattern in the Zawar section of the Aravalli Basin was primarily controlled by the change in sea level under the influences of syn-sedimentary tectonics. The study further reveals the implication of the sedimentary succession of the Zawar section in the stratigraphy of the Aravalli Basin.

    • A reappraisal of the Debari Formation – in the context of stratigraphy and basin evolution of the Paleoproterozoic Aravalli Supergroup, Rajasthan, India

      AMRITA MUKHERJEE H N BHATTACHARYA

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      The Debari Formation plays a vital role in constructing the stratigraphy of the Aravalli Supergroup. However, the Debari Formation occupies different positions in the stratigraphic schemes proposed by various workers, which created a continuing controversy. This problem might be linked to a lack of knowledge on detailed field relationships, underutilisation of the well-preserved sedimentary attributes and available detrital zircon ages of the polymictic conglomerate–sandstone–mudstone association that constitute the Debari Formation. Moreover, the same association has given different names in different sections.The present outcrop scale observations on the field relationships reveal that these sediments overlie the syn-rift volcanic associated fluvio-deltaic Delwara Formation and stromatolitic (phosphatic) carbonate bearing platformal Jhamarkotra Formation (${\sim}$1.7 Ga) across a basin-wide top-rift unconformity. The detrital zircon geochronology of the sediments lying below and above the unconformity suggests a nearly 20–50 Ma time lapse. Facies attributes of the polymictic conglomerate–sandstone–mudstone association(${\sim}$1.6 Ga) point to deposition under ephemeral channels in steeper braid plain to storm-induced tidal flat environments. These sediments represent the initiation of a post-rift passive margin basin, which ultimately evolved into quickly subsiding deep half-grabens in between basement highs during rapid collapse at the rift to drift transition. These deep basins received turbidite sediments (${\sim}$1.5 Ga) of the Udaipur Formation. However, equivalent sediments are named differently as Tidi Formation and Jharol Formation, exposed in other sections of the Aravalli basin. On the strength of the field studies, in-depth sedimentary facies analysisand available detrital zircon geochronology, a two-fold stratigraphy and a tentative basin evolution model have been proposed for the Aravalli Supergroup.

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