299-7 Geochemical Insights from the Muthi (Mizoram) and Sitakund (Chittagong) Limestones of the Eastern Himalayan Foreland Basins: Rare Carbonate Windows into a Siliciclastic Realm

Session: Geochemical Studies of Sediments (Posters)

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Abstract:

The eastern Himalayan foreland belt is overwhelmingly consisting of Tertiary siliciclastic deposits, rendering the occurrence of marine carbonates within this sequence both rare and geologically significant. Two isolated Miocene limestone units—the Muthi Limestone in the Tripura-Mizoram Fold Belt (TMFB), India, and the Sitakund Limestone in the Eastern Fold Belt of the Bengal Basin, offer exceptional opportunities to investigate depositional chemistry, early diagenetic alteration, and tectonic influences within a convergent basin system.

This study employs an integrated geochemical approach combining major oxide, trace element, and rare earth element (REE) analyses with normative mineralogical modeling to characterize the origin and post-depositional history of these two carbonate formations. The fossiliferous, calcite-rich Muthi Limestone is distinguished by high CaO, low MgO, and moderate detrital input. Notably, it exhibits strong positive europium anomalies (Eu/Eu* > 5) and moderate cerium enrichment (Ce/Ce* > 3), suggestive of suboxic depositional conditions and potential hydrothermal fluid interaction. In contrast, the Sitakund Limestone is chemically purer, with low siliciclastic dilution, flat REE profiles, and Ce/Ce* and Eu/Eu* values near unity, indicative of oxidizing conditions within a restricted, saline platform environment.

Multivariate statistical tools, including principal component analysis (PCA) and trace element discrimination diagrams (e.g., La/Sc vs. Cr/Th; V/Cr vs. Ni/Co), effectively differentiate the two units based on their geochemical signatures. Normative mineralogical reconstructions further reveal a calcite-dominated assemblage in both formations, accompanied by minor quartz, illite, and hematite, reflecting differing degrees of early diagenesis. These contrasting geochemical fingerprints illustrate the influence of variable tectonic settings, sedimentary regimes, and fluid-rock interactions on Miocene carbonate evolution.

Collectively, the results highlight the utility of isolated carbonate bodies as multi-proxy archives and provide new constraints on paleoenvironmental gradients and tectono-sedimentary processes across the eastern Himalayan fold-thrust belt.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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