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UPSC MainsGEOLOGY-PAPER-I202510 Marks150 Words
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Q17.

Describe the lithology, type locality, age and depositional environment of Blaini Boulder Bed.

How to Approach

To answer this question effectively, one should define the Blaini Boulder Bed briefly and then systematically address each component: lithology, type locality, age, and depositional environment. For lithology, describe the rock types and characteristic features of the clasts and matrix. For type locality, specify its geographical location. For age, pinpoint the geological period. Finally, for depositional environment, discuss the conditions and processes that led to its formation, highlighting the evidence supporting the interpretation.

Model Answer

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Introduction

The Blaini Boulder Bed is a distinctive and significant geological formation primarily found in the Lesser Himalayan region of India. It represents a crucial marker horizon in understanding the Neoproterozoic geological history of the Indian subcontinent, particularly concerning past climatic conditions. Its study provides insights into ancient glaciations and the subsequent environmental changes that shaped the Himalayan geology. Understanding its characteristics is fundamental to reconstructing the palaeogeography and tectonic evolution of the region.

Lithology of Blaini Boulder Bed

The Blaini Boulder Bed primarily consists of diamictite or tillite. It is characterized by its poorly sorted, matrix-supported nature, meaning a diverse array of clasts are embedded within a finer-grained matrix. Key lithological features include:

  • Clasts: These range from angular to sub-rounded boulders, cobbles, and pebbles. They are lithologically varied, often including quartzite, limestone, granite, and schist, reflecting derivation from multiple source rocks.
  • Matrix: The clasts are set in a fine-grained, often argillaceous (clayey) or sandy matrix.
  • Glacial Features: Crucially, many clasts exhibit faceted or striated surfaces, which are characteristic marks of glacial transport and abrasion. Some areas also show polished and striated pavement on the underlying rock units.
  • Associated Rocks: The Blaini Formation, which includes the boulder bed, also comprises associated units of shale, slate, sandstone, and dolostone, sometimes capped by a pink microcrystalline dolomite (cap carbonate).

Type Locality

The Blaini Formation derives its name from the Blaini section near Mussoorie, Uttarakhand, in the Lesser Himalayan region of India. Specifically, it is named after the Baliana River flowing near Solan in Himachal Pradesh, where it was first described by H.B. Medlicott in 1864.

Age

The Blaini Boulder Bed is unequivocally assigned to the Late Proterozoic (Neoproterozoic) Era. More specifically, it is correlated with the Cryogenian Period. Recent detrital zircon ages from diamictite samples provide a maximum age limit of approximately 692 ± 18 million years ago (Ma), broadly spanning 700-600 million years ago.

Depositional Environment

The dominant interpretation for the depositional environment of the Blaini Boulder Bed is a glacial or glaciomarine setting. Evidence supporting this includes:

  • Tillite/Diamictite Nature: The poorly sorted, matrix-supported diamictite with varied clast lithology is characteristic of direct deposition from glaciers (till) or by icebergs melting in shallow marine settings (glaciomarine diamictite).
  • Striated and Faceted Clasts: The presence of clasts with glacial striations and facets strongly indicates transport and erosion by ice.
  • Polished Pavements: Local preservation of polished and striated pavements on underlying formations further supports a subglacial erosional environment.
  • Cap Carbonate: The overlying pink microcrystalline dolomite (cap carbonate) with a negative δ13C excursion is a global indicator of post-glacial warming events following a "Snowball Earth" episode.
  • Shallow Marine Features: Associated lithounits within the Blaini Formation, such as shaly successions with lenticular bedding and ripples, and pink dolomites with stromatolites, suggest deposition in shallow tidal-flat to intertidal-supratidal zones, indicating a glaciomarine influence in a shallow basin.

Conclusion

The Blaini Boulder Bed stands as a critical geological marker in the Lesser Himalaya, providing profound evidence of a major Neoproterozoic glaciation event during the Cryogenian Period. Its characteristic lithology of poorly sorted diamictites with glacially-modified clasts, coupled with its specific type locality and age, firmly establishes its origin in a glacial or glaciomarine environment. This formation not only sheds light on India's ancient climatic past but also plays a vital role in regional stratigraphic correlations, highlighting a significant global 'Snowball Earth' event and its subsequent recovery.

Answer Length

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Additional Resources

Key Definitions

Diamictite
A clastic sedimentary rock composed of very poorly sorted, non-sorted, or ill-sorted terrigenous sediment containing a wide range of particle sizes from clay to boulders, often interpreted as glacial in origin (tillite), but can also form from other mass movement processes like debris flows.
Tillite
A sedimentary rock formed by the lithification of till, which is unsorted and unstratified glacial sediment deposited directly by glaciers. It is a specific type of diamictite with a glacial origin.

Key Statistics

The Blaini Formation's maximum age limit is constrained by new detrital zircon ages from diamictite samples, indicating approximately 692 ± 18 million years ago (Ma).

Source: Geological Society, London, Memoirs (2011)

The Blaini Formation typically ranges in thickness, with diamictite units generally less than 50 meters thick, although the overall formation can extend significantly across the fold belt.

Source: GeoScienceWorld (2011)

Examples

Glacial Striations on Clasts

Field observations of the Blaini Boulder Bed frequently reveal angular to sub-rounded clasts (e.g., quartzites, limestones) that bear distinctive parallel scratches or grooves (striations) and flat surfaces (facets). These markings are classic indicators of abrasion caused by the grinding action of rocks embedded within moving glacial ice.

Global Correlation with Snowball Earth Events

The Blaini Formation, particularly its diamictite units and overlying cap carbonates, is often correlated with the global "Snowball Earth" glaciations, such as the Marinoan glaciation. This global event saw extensive ice sheets covering much of the Earth, and the Blaini provides key evidence for this period in the Indian subcontinent.

Frequently Asked Questions

▶What is the significance of the "cap carbonate" found above the Blaini Boulder Bed?

The cap carbonate (typically a pink microcrystalline dolomite) overlying the Blaini diamictite is highly significant. It exhibits a distinct negative δ13C isotopic excursion, which is a global chemical signature indicating a rapid and profound change in atmospheric and oceanic chemistry following the intense glacial conditions of a "Snowball Earth" event. It suggests a swift transition from glacial to warmer, carbon-rich conditions.

▶Was the glacial origin of the Blaini Boulder Bed always accepted?

No, the genesis of the Blaini Formation was controversial for a long time. While H.B. Medlicott (1864) and Oldham (1888) initially favored a glacial or glaciomarine origin, some later researchers proposed alternative theories such as fanglomerates, turbidites, or mudflows. However, overwhelming evidence, including striated clasts and associated cap carbonates, has led to the widespread acceptance of its glacial or glaciomarine origin today.

Topics Covered

StratigraphySedimentologyBlaini Boulder BedLithologyAgeDepositional Environment
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