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<p>Fresh biostratigraphic studies on the drill-core samples from the key boreholes of SPN-A, UJN-D-A, and UJN-ST-A in the Ganga Basin have shown the abundant occurrence of distinctive Ediacaran and Early Paleozoic acritarchs in the Madhubani Group, which till now have been largely assigned a Meso-Neoproterozoic age. Ujhani Formation, the lowermost unit of the above succession, with mixed carbonate-siliciclastic rocks, yielded age diagnostic acritarchs belonging to <i>Lophosphaeridum rarum</i>, <i>Dictyotidium birvetense</i>, <i>Cristallinium cambriense</i>, <i>Cymatiosphaera crameri</i>, and <i>Eupoikilofusa squama</i> zones that range from Late Ediacaran–Early Cambrian to Late Cambrian–Tremadocian. The succeeding Tilhar Formation, with thick limestone and shale succession, is marked by distinctive Late Ordovician and Early to Middle Silurian acritarchs attributable to <i>Eupoikilofusa striatifera</i>, <i>Helosphaeridium citrinipeltatum</i>, and <i>Dictyotidium stenodictyum</i> zones. Meanwhile, Karnapur Formation, the uppermost lithounit of the Madhubani Group with siltstone/shale rocks, recorded age diagnostic Late Silurian acritarchs referable to the <i>Helosphaeridium latispinosum</i> and <i>Polyedryxium multifrons</i> zones. Fresh record of these acritarch assemblages, which worldwide characterize the latest Neoproterozoic and Early Paleozoic rocks, categorically suggests a younger age of Late Ediacaran (ca. 550 Ma) to Late Silurian (ca. 418 Ma) for the Madhubani Group, and corroborates the earlier interpreted similar age for this succession. The above-determined fresh age strongly repudiates the existing views of the Late Mesoproterozoic–Early Neoproterozoic age for the Madhubani Group and negates the prevailing notion that it is the subsurface northern continuation of the latest Paleoproterozoic/Mesoproterozoic–Neoproterozoic Vindhyan rocks in the Ganga plains. Newly generated biostratigraphic and lithological data from the above-studied boreholes suggest that the Pre-Tertiary Sequence (Ganga Supergroup) of the Ganga Basin is represented by two discrete successions of quite different geological ages. The lower metasedimentary succession (Bahraich Group), with thick phyllites/slates (Sarda Formation) and orthoquartzites with minor phyllites/slate intercalations (Avadh Formation), includes two distinct Late Paleoproterozoic–Early Mesoproterozoic acritarch assemblages of <i>Tappania plana</i> zone (Late Statherian, ca. 1650–1600 Ma) and <i>Navifusa granulatus</i> zone (Early Calymmian, ca. 1600–1550 Ma), and is inferred to be somewhat older than the Lower Vindhyan rocks of Son Valley. The two metasedimentary litho-units of the Bahraich Group broadly correlate with the Chandpur Phyllite and Nagthat Quartzite (Jaunsar Group) lithounits of the Garhwal–Kumaon Lesser Himalaya rather than the Semri rocks (Lower Vindhyan) of Vindhyan Basin. The upper unmetamorphosed succession, representing Ujhani, Tilhar, and Karnapur formations of the Madhubani Group with Late Ediacaran to Late Silurian (ca. 550–418 Ma) acritarch assemblages, undoubtedly younger than the Neoproterozoic Upper Vindhyan succession. A major non-depositional hiatus of ca. 1000 myr (ca. 1550 to 550 Ma) is identified between the Bahraich and Madhubani groups, which largely covers the Vindhyan sedimentation period and indicates the absence of Vindhyan or its age-alike rocks in the Ganga Basin. The latest aeromagnetic/seismic and biostratigraphic data also reveal that the Pre-Tertiary Sequence of the Ganga Basin is entirely different from the Vindhyan sequence of Son Valley and deposited in a fairly separate basin, which mostly covers the Ganga plains and has its typical depositional history. Ganga Basin is opined to be detached from the Vindhyan Basin by an east–west running subsurface Paleoproterozoic basement high of Bundelkhand–Allahabad Ridge.</p>

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Late Neoproterozoic–Early Paleozoic acritarch assemblages from the Madhubani Group, with remarks on the age and status of the Pre-Tertiary Sequence (Ganga Supergroup) of the Ganga Basin, Northern India

  • 期刊论文

  • 发表日期：

    2025年9月25日

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  • Bijai Prasad,
  • Ramson Asher

摘要

Fresh biostratigraphic studies on the drill-core samples from the key boreholes of SPN-A, UJN-D-A, and UJN-ST-A in the Ganga Basin have shown the abundant occurrence of distinctive Ediacaran and Early Paleozoic acritarchs in the Madhubani Group, which till now have been largely assigned a Meso-Neoproterozoic age. Ujhani Formation, the lowermost unit of the above succession, with mixed carbonate-siliciclastic rocks, yielded age diagnostic acritarchs belonging to Lophosphaeridum rarum, Dictyotidium birvetense, Cristallinium cambriense, Cymatiosphaera crameri, and Eupoikilofusa squama zones that range from Late Ediacaran–Early Cambrian to Late Cambrian–Tremadocian. The succeeding Tilhar Formation, with thick limestone and shale succession, is marked by distinctive Late Ordovician and Early to Middle Silurian acritarchs attributable to Eupoikilofusa striatifera, Helosphaeridium citrinipeltatum, and Dictyotidium stenodictyum zones. Meanwhile, Karnapur Formation, the uppermost lithounit of the Madhubani Group with siltstone/shale rocks, recorded age diagnostic Late Silurian acritarchs referable to the Helosphaeridium latispinosum and Polyedryxium multifrons zones. Fresh record of these acritarch assemblages, which worldwide characterize the latest Neoproterozoic and Early Paleozoic rocks, categorically suggests a younger age of Late Ediacaran (ca. 550 Ma) to Late Silurian (ca. 418 Ma) for the Madhubani Group, and corroborates the earlier interpreted similar age for this succession. The above-determined fresh age strongly repudiates the existing views of the Late Mesoproterozoic–Early Neoproterozoic age for the Madhubani Group and negates the prevailing notion that it is the subsurface northern continuation of the latest Paleoproterozoic/Mesoproterozoic–Neoproterozoic Vindhyan rocks in the Ganga plains. Newly generated biostratigraphic and lithological data from the above-studied boreholes suggest that the Pre-Tertiary Sequence (Ganga Supergroup) of the Ganga Basin is represented by two discrete successions of quite different geological ages. The lower metasedimentary succession (Bahraich Group), with thick phyllites/slates (Sarda Formation) and orthoquartzites with minor phyllites/slate intercalations (Avadh Formation), includes two distinct Late Paleoproterozoic–Early Mesoproterozoic acritarch assemblages of Tappania plana zone (Late Statherian, ca. 1650–1600 Ma) and Navifusa granulatus zone (Early Calymmian, ca. 1600–1550 Ma), and is inferred to be somewhat older than the Lower Vindhyan rocks of Son Valley. The two metasedimentary litho-units of the Bahraich Group broadly correlate with the Chandpur Phyllite and Nagthat Quartzite (Jaunsar Group) lithounits of the Garhwal–Kumaon Lesser Himalaya rather than the Semri rocks (Lower Vindhyan) of Vindhyan Basin. The upper unmetamorphosed succession, representing Ujhani, Tilhar, and Karnapur formations of the Madhubani Group with Late Ediacaran to Late Silurian (ca. 550–418 Ma) acritarch assemblages, undoubtedly younger than the Neoproterozoic Upper Vindhyan succession. A major non-depositional hiatus of ca. 1000 myr (ca. 1550 to 550 Ma) is identified between the Bahraich and Madhubani groups, which largely covers the Vindhyan sedimentation period and indicates the absence of Vindhyan or its age-alike rocks in the Ganga Basin. The latest aeromagnetic/seismic and biostratigraphic data also reveal that the Pre-Tertiary Sequence of the Ganga Basin is entirely different from the Vindhyan sequence of Son Valley and deposited in a fairly separate basin, which mostly covers the Ganga plains and has its typical depositional history. Ganga Basin is opined to be detached from the Vindhyan Basin by an east–west running subsurface Paleoproterozoic basement high of Bundelkhand–Allahabad Ridge.