Model Answer
0 min readIntroduction
The Krol Formation is a significant lithostratigraphic unit within the Lesser Himalayas, specifically forming a part of the Sub-Himalayan sequence. It overlies the Murree Formation and underlies the Dagshai Formation, representing a crucial period in the geological evolution of the region during the early Siwalik Group deposition. Characterized by its distinctive red bed sequences, the Krol Formation provides valuable insights into the paleo-climatic and tectonic conditions prevalent during its formation, spanning the Miocene epoch. Understanding its lithology, palaeoenvironment, and age is fundamental to reconstructing the Himalayan orogenic history.
Lithology of the Krol Formation
The Krol Formation is predominantly composed of reddish-brown to dark-red coloured sandstones, siltstones, and shales. These sediments are typically medium to coarse-grained, displaying characteristics of fluvial deposition. Key lithological features include:
- Sandstones: Predominantly feldspathic litharenites, indicating derivation from a granitic/metamorphic source. They exhibit cross-bedding, ripple marks, and occasional pebble layers.
- Siltstones: Commonly interbedded with sandstones, often displaying reddish hues due to iron oxide coatings.
- Shales: Typically thin-bedded and fissile, also exhibiting reddish coloration. They contain occasional plant fossils.
- Conglomerates: Locally present, containing rounded clasts of quartz, feldspar, and metamorphic rocks. These indicate higher energy depositional environments.
- Calcareous bands: Thin calcareous bands are found at places, indicating intermittent marine influence.
Palaeoenvironment of the Krol Formation
Based on the lithological characteristics, the Krol Formation is interpreted to have been deposited in a braided river system, transitioning to alluvial fan and floodplain environments. The following evidence supports this interpretation:
- Cross-bedding and Ripple Marks: Indicate unidirectional flow and shallow water conditions, typical of fluvial channels.
- Feldspathic Litharenites: Suggest rapid erosion of the Himalayas and transport of sediment by high-energy rivers.
- Red Colouration: Indicates oxidizing conditions, suggesting a semi-arid to arid climate with well-drained soils. The presence of iron oxides points to weathering in a warm climate.
- Interbedded Sandstones and Shales: Reflect fluctuating water levels and deposition in both channel and floodplain settings.
- Conglomerates: Suggest high-energy, proximal depositional environments like alluvial fans.
The presence of calcareous bands suggests occasional incursions of marine influence, possibly during periods of relative sea-level rise.
Age of the Krol Formation
The age of the Krol Formation is generally considered to be Middle to Late Miocene, ranging approximately from 16 to 8 million years ago (mya). This age determination is based on several lines of evidence:
- Fossil Evidence: The presence of fossil mammals, particularly Sivatherium and Proboscidean species, provides biostratigraphic constraints.
- Radiometric Dating: K-Ar dating of volcanic ash layers interbedded with the Krol Formation in some areas has yielded ages within the Miocene range. (Knowledge cutoff 2023)
- Magnetostratigraphy: Paleomagnetic studies have identified normal and reversed polarity zones within the Krol Formation, which can be correlated with the geomagnetic polarity timescale, further refining its age.
- Lithostratigraphic Correlation: Correlation with other Miocene formations in the Siwalik Group, whose ages are better constrained, supports the Middle to Late Miocene age assignment.
The Krol Formation represents a period of significant tectonic activity and landscape evolution in the Himalayas, coinciding with the initial stages of the Himalayan orogeny.
Conclusion
In conclusion, the Krol Formation is characterized by reddish fluvial sediments indicative of a braided river system operating in a semi-arid to arid climate during the Middle to Late Miocene epoch (16-8 mya). Its lithology, palaeoenvironment, and age provide crucial insights into the early stages of Himalayan uplift and sedimentation. Further detailed studies, including high-resolution biostratigraphy and radiometric dating, are needed to refine our understanding of this important geological unit and its role in the Himalayan orogenic belt.
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