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<p>Petro-geochemical analyses on representative borehole samples from the Girujan Clay Formation in the Upper Assam Shelf, NE India are carried out to elucidate the provenance, tectonic setting, source area weathering, and redox conditions. Geochemical classification indicates that most samples are shale, with a few categorized as wacke. The sandstone is petrographically characterized by a quartz-rich framework (both monocrystalline and polycrystalline), significant potash and plagioclase feldspars with accessory micas and lithic fragments. Geochemical signatures suggest a felsic source, deposited in an active marginal setting. The chondrite-normalized REE pattern aligns with the upper continental crust, enriched in LREE and depleted in HREE, with an average LREE/HREE ratio of 9.57. Ratios such as Eu/Eu* (~ 0.98), La/Sc (~ 1.61), Th/Sc (~ 1.17), La/Co (~ 1.20), Th/Co (~ 0.85), and Cr/Th (~ 13.36) suggest derivation from felsic rock source. High field strength elements (Zr ~ 228.2 ppm, Nb ~ 11.8 ppm) are enriched compared to UCC and PAAS due to their association with heavy minerals, while transitional trace elements (Co ~ 17.6 ppm, Sc ~ 12.46 ppm, V ~ 123.53 ppm) are depleted, further indicating a felsic source. Weathering indices (CIA ~ 73.75, CIW ~ 82.25, PIA ~ 79.88, ICV ~ 1.36) and the A-CN-K plot suggest moderate to intense weathering under climatic conditions ranging from arid/semi-arid to warm-humid subtropical. Redox-sensitive geochemical parameters (U, U/Th, V/Cr, Ni/Co, Cu/Zn) indicate deposition under oxic to dysoxic conditions.</p>

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Petrography and geochemistry of the Girujan Clay Formation in the Upper Assam Shelf, NE India: insights into provenance, tectonic settings, source area weathering, and redox conditions

  • 期刊论文

  • 发表日期：

    2025年10月13日

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  • Nishanta Sahariah,
  • Roshmi Boruah,
  • Dipankar Buragohain,
  • Farha Al Rosid,
  • Bhargab Rajbongshi,
  • Manash Protim Phukan

摘要

Petro-geochemical analyses on representative borehole samples from the Girujan Clay Formation in the Upper Assam Shelf, NE India are carried out to elucidate the provenance, tectonic setting, source area weathering, and redox conditions. Geochemical classification indicates that most samples are shale, with a few categorized as wacke. The sandstone is petrographically characterized by a quartz-rich framework (both monocrystalline and polycrystalline), significant potash and plagioclase feldspars with accessory micas and lithic fragments. Geochemical signatures suggest a felsic source, deposited in an active marginal setting. The chondrite-normalized REE pattern aligns with the upper continental crust, enriched in LREE and depleted in HREE, with an average LREE/HREE ratio of 9.57. Ratios such as Eu/Eu* (~ 0.98), La/Sc (~ 1.61), Th/Sc (~ 1.17), La/Co (~ 1.20), Th/Co (~ 0.85), and Cr/Th (~ 13.36) suggest derivation from felsic rock source. High field strength elements (Zr ~ 228.2 ppm, Nb ~ 11.8 ppm) are enriched compared to UCC and PAAS due to their association with heavy minerals, while transitional trace elements (Co ~ 17.6 ppm, Sc ~ 12.46 ppm, V ~ 123.53 ppm) are depleted, further indicating a felsic source. Weathering indices (CIA ~ 73.75, CIW ~ 82.25, PIA ~ 79.88, ICV ~ 1.36) and the A-CN-K plot suggest moderate to intense weathering under climatic conditions ranging from arid/semi-arid to warm-humid subtropical. Redox-sensitive geochemical parameters (U, U/Th, V/Cr, Ni/Co, Cu/Zn) indicate deposition under oxic to dysoxic conditions.