Model Answer
0 min readIntroduction
The Vindhyan Supergroup represents a vast sedimentary sequence deposited in the Proterozoic eon, forming a prominent geological feature across central India. It’s renowned for its thick, well-bedded sandstones, shales, and limestones, crucial for understanding the Precambrian geology of the Indian subcontinent. The Lower Vindhyan Group, the basal part of this supergroup, is particularly significant due to its age, lithological characteristics, and the insights it provides into the early stages of Vindhyan basin formation. Understanding this group is vital for reconstructing the paleogeographic and tectonic evolution of the region.
Geological Setting and Stratigraphic Position
The Vindhyan Supergroup, spanning approximately 1.6 to 0.57 billion years (Ga), overlies the older Bundelkhand Granite and is unconformably overlain by the Deccan Traps. It is broadly divided into four groups: Lower Vindhyan, Kaimur, Rewa, and Bhander. The Lower Vindhyan Group represents the oldest phase of Vindhyan sedimentation, marking the initial stages of basin development. It rests directly on the eroded surface of the Bundelkhand Granite and is characterized by a distinct stratigraphic sequence.
Lithology and Composition
The Lower Vindhyan Group is primarily composed of the following lithological units, in ascending order:
- Bijawar Group: Characterized by conglomerates, quartzites, and shales. The conglomerates indicate the initial filling of the basin with coarse clastics derived from the surrounding highlands.
- Sirsi Group: Predominantly composed of sandstones, shales, and minor limestones. These sandstones are typically fine-to-medium grained and exhibit cross-bedding, suggesting deposition in a fluvial environment.
- Semri Group: Known for its distinctive red sandstones and shales, often containing ripple marks and mud cracks, indicating shallow water depositional conditions.
Age and Paleontology
Determining the precise age of the Lower Vindhyan Group has been challenging due to the scarcity of fossils. However, based on lithological correlations and limited biostratigraphic evidence (stromatolites and trace fossils), it is generally assigned to the Mesoproterozoic era, specifically between 1.7 to 1.6 billion years ago. The absence of abundant fossils suggests a relatively low oxygen environment during deposition. Stromatolites, fossilized microbial mats, are found in some limestone layers, providing evidence of early life forms.
Geographical Distribution
The Lower Vindhyan Group is primarily exposed in the Vindhyan Range of central India, extending across the states of Madhya Pradesh, Uttar Pradesh, Rajasthan, and Gujarat. Key areas of outcrop include:
- Madhya Pradesh: Around Panna, Satna, and Rewa districts.
- Uttar Pradesh: In the Mirzapur and Sonbhadra districts.
- Rajasthan: In the Bundi and Kota regions.
- Gujarat: Limited occurrences in the eastern parts of the state.
Economic Significance
The Lower Vindhyan Group holds significant economic importance due to its building materials:
- Sandstone: The massive sandstones of the Semri Group are extensively used as building stones, paving materials, and for monuments. The red sandstone from this group is particularly prized for its aesthetic appeal.
- Limestone: The minor limestone occurrences are used in cement production.
- Clay: Shales are used in the manufacture of bricks and tiles.
- Diamondiferous Pipes: The region is also known for diamondiferous kimberlite pipes that intrude the Vindhyan rocks, notably in the Panna region of Madhya Pradesh.
Tectonic Implications
The deposition of the Lower Vindhyan Group is linked to the stabilization of the Indian shield and the initiation of rift-related sedimentation. The presence of conglomerates at the base suggests erosion of the underlying granite and the initial development of a rift basin. The subsequent deposition of sandstones and shales indicates a transition to a more stable sedimentary environment.
Conclusion
The Lower Vindhyan Group represents a crucial chapter in the geological history of India, providing valuable insights into the Precambrian evolution of the subcontinent. Its distinctive lithology, stratigraphic position, and economic significance make it a subject of continued geological research. Further studies, including detailed paleontological investigations and geochronological dating, are needed to refine our understanding of its age and depositional environment. The group’s contribution to building materials and diamond resources continues to be economically important for the region.
Answer Length
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