Model Answer
0 min readIntroduction
Stratigraphy is the branch of geology concerned with the layering of rocks (strata) and their chronological relationships. The Gondwana Supergroup represents a vast accumulation of sedimentary rocks deposited in a series of rift basins across India during the late Paleozoic and Mesozoic eras (Permian to Cretaceous). These basins formed due to the breakup of the supercontinent Gondwana. The Gondwana succession is economically significant due to its substantial coal reserves, but coal formation wasn’t uniform throughout the entire period. Understanding the interplay between stratigraphy, depositional environments, and geological processes is crucial to explaining the uneven distribution of coal seams within this supergroup.
Stratigraphy of the Gondwana Supergroup
The Gondwana Supergroup is broadly divided into three main groups: Talchir, Barakar, and Gondwana. These groups are further subdivided into formations based on lithological characteristics and fossil content.
- Talchir Formation (Permian): Characterized by coarse-grained sediments like conglomerates, sandstones, and shales, indicating a high-energy fluvial environment. Glacial deposits are also present, suggesting a cold, glacial climate during the early Permian.
- Barakar Formation (Lower Gondwana – Permian-Triassic): Predominantly composed of sandstones, shales, and coal seams. This formation represents a transition from glacial conditions to a warmer, humid climate, favoring the development of extensive swampy forests.
- Gondwana Formation (Upper Gondwana – Triassic-Jurassic): Consists of sandstones, shales, and thick coal seams. This formation represents a period of sustained warm and humid climate, leading to prolific coal formation.
- Rajmahal Formation (Jurassic-Cretaceous): Characterized by basaltic lava flows interbedded with sedimentary rocks, marking the onset of Deccan Volcanism.
Depositional Environments
The Gondwana basins were formed by rifting and were filled with sediments derived from the surrounding highlands. The depositional environments varied over time and space, influencing the type of sediments deposited and the potential for coal formation.
- Fluvial Systems: Dominated the Talchir and parts of the Barakar formations, characterized by braided rivers and channels depositing coarse sediments.
- Deltaic Environments: Present in the Barakar and Gondwana formations, where rivers entered the basins, forming deltas and distributing sediments across floodplains.
- Swampy Forests: Extensive swampy forests developed in the lower-lying areas of the deltas and floodplains, providing the organic matter necessary for coal formation. These were particularly prevalent during the Barakar and Gondwana formations.
- Lacustrine Environments: Lakes formed in some areas, contributing to the deposition of fine-grained sediments.
Why Certain Intervals are Rich in Coal Seams
Coal seam formation requires specific conditions, and these conditions were not consistently present throughout the Gondwana succession. Several factors explain why certain intervals are richer in coal seams:
- Paleoclimate: Warm and humid climates, as during the Barakar and Gondwana formations, promoted the growth of lush vegetation, providing abundant organic matter. The Talchir period, being glacial, had limited vegetation.
- Tectonic Activity & Basin Subsidence: Continuous subsidence of the basins created accommodation space for sediment accumulation and preserved organic matter from oxidation. Active rifting also created favorable conditions for peat accumulation in localized areas.
- Sediment Supply & Preservation: A balanced sediment supply was crucial. Too much sediment would bury the organic matter too quickly, preventing peat formation. Too little sediment would lead to oxidation. The Barakar and Gondwana formations experienced a balanced sediment supply.
- Water Table Level: A high water table in the swamps prevented oxidation of the organic matter, promoting peat accumulation.
- Volcanic Activity: While the Rajmahal formation marks the onset of volcanism, earlier minor volcanic events may have contributed nutrients to the swamps, enhancing vegetation growth.
The Barakar and Gondwana formations, with their favorable climatic conditions, tectonic settings, and sediment supply, represent the most prolific coal-bearing intervals within the Gondwana Supergroup. The Talchir formation, with its glacial environment, lacks significant coal deposits.
Conclusion
The Gondwana Supergroup represents a complex geological record of the breakup of Gondwana and the evolution of depositional environments in India. The uneven distribution of coal seams within this supergroup is a direct result of the interplay between paleoclimate, tectonic activity, sediment supply, and preservation conditions. Understanding these factors is crucial for exploring and exploiting the remaining coal resources in India and for reconstructing the paleogeographic and paleoclimatic history of the region. Further research focusing on high-resolution stratigraphic studies and paleoenvironmental reconstructions will be vital for optimizing coal exploration strategies.
Answer Length
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