Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

J. N. Malik ¹, Aniruddha S. Khadkikar ², S. S. Merh ²

Affiliations
1 Department of Geology, M.S. University of Baroda, Vadodara 390002, IN
2 Department of Geology, M,S. University of Baroda, Vadodara 390002, IN

Abstract

The late Quaternary continental succession exposed along the cliff-like banks (15-35 m) of the Mahi river basin comprises sediments of fluvial and aeolian origin and provides an opportunity to assess palaeoclimatic changes in western India. Seven different facies are recognized in the exposed sediment succession. The trough cross-stratified conglomerates (Gt) formed due to downstream migration of trains of sinuous-crested dunes in shallow channels, whereas the formation of planar cross-stratified gravels (Gp) is attributed to downstream accretion of the avalanching front of a mid-channel bar, The horizontally stratified sands (Sh₁ ) represent overbank deposits, while similar facies (Sh₂ ) owes its origin to infrequent sheet flooding events. The trough cross-stratified sands (St) appear to be a product of low-stage river processes. The 'loess-like' massive silts (Sim) are of aeolian origin. The calcic palaeosol facies (P) are represented by vertisols at the base and brown soils and calcic red soils towards the top indicating changing palaeoenvironments.

The overall sediment succession was built through three major climate-controlled aggradation phases. Aggradation phase 1, which is represented by gravel-bed rivers and coexistent vertic palaeosols, indicates a significantly wetter climate. Aggradation phase 2 points to a comparatively drier period during which suspended-load rivers existed. Within this phase, red soil formation suggests a climatic amelioration event around 40 ka B.P. to 23 ka B.P. Aggradation phase 3 reflects a period of extreme aridity and is manifested as the capping aeolian facies. This phase is thought to be in tandem with the Last Glacial Stage event. Stabilization of these dunes may have taken place around 6 ka B.P. as suggested by the Thar Desert palynological and lake records.

The modern Mahi channel flows due ssw. However, the palaeocurrent trends of the exposed sediment bodies reveal that the ancient drainage network in the northern sections (upper reaches) flowed due SW-SSW and in the southern sections (lower reaches) SSE, SSW and W. The present day course of Mahi is relatively younger and follows structural lineaments that developed during the Holocene (after 6 ka B.P.). The aIluvial sedimentation record is unrelated to the modern Mahi river, but is a product of ancient late Quaternary rivers which now stand either fully or partially destroyed.

Keywords

Quaternary, Palaeoclimate, Deserts, Palaeosol, Calcrete, Gujarat.

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Aniruddha S. Khadkikar ¹

Affiliations
1 Geology and Palaeontology Group, Agharkar Research Institute, G.G. Agarkar Road, Pune - 411 004, IN

Abstract

Cross-Bedding is a common sedimentary structure in the global stratigraphic record with planar and trough cross-Bedding occurring more frequently than hummocky Cross-Bedding. Whereas the latter is typical to marine environments the former is observed both in aeolian and Sub-Aqueous environments (fluvial and marine). A fourth variant of Cross-Bedding is described here that is characterized by Convex-Up structures in the depositional dip direction and Hump-Like structure across the depositional dip direction. In plan view the structure has beds that show radial fanning out dip azimuths with dip amounts varying between 25° and 30°. This structure is termed here as 'hump crossbedding' owing to its hump like geometry across the depositional dip direction. These bedding geometries may be reconciled with sediment migration along the front of a parabolic dune. Since parabolic dunes do not form in subaqueous environments, hump Cross-Bedding becomes an essential aid in distinguishing aeolian from shoreface deposits in coastal environments. The recognition of this structure becomes invaluable towards reconstructing depositional environments and deducing their palaeoenvironmental significance from Quaternary deposits such as those found in the Bahamas, Bermuda and Saurashtra.

Keywords

Aeolian, Aeolianite, Cross-Bedding, Miliolite, Stratification, Southeastern Saurashtra Coast, Gujarat.

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Aniruddha S. Khadkikar ¹

Affiliations
1 Geology and Palaeontology Group, Agharkar Research Institute, G G Agarkar Road, Pune - 411 004, IN

Abstract

Coastal aeolianites occur throughout the world both in the Sub-Tropical and Extra- Tropical regions and act as valuable repositories of both past climate changes due to their sensitivity to meteoric diagenesis. Palaeowind directions (northeast to north northeast) based on measurements of these Cross-Beds suggest that the formative winds were the Indian summer monsoon winds. Stratigraphic and architectural analyses reveal that the coastal aeolianite deposits (Miliolite)have three Terra Rossa type soils over the past ∼200 ka. This points to the fact that coastal aeolianites respond sensitivity to the changing intensities of the Indian monsoon rainfall. Periods of increased rainfall led to weathering which eventually formed Terra Rossa like soil while prolonged periods of reduced rainfall but strong winds led to the formation of parabolic dunes. As strong monsoon winds are required both for precrpitation and wind transport, this suggests, that the Indian summer monsoon winds were strong and intense throughout the past -200 ka. These results validate recent results from a coupled Ocean-Atmosphere GCM that predict stronger summer monsoon winds during glacial periods.

Keywords

Palaeoclimatology, Monsoon, Aeolianite, Quaternary, Terra Rossa.

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Aniruddha S. Khadkikar ¹, D. A. Sant ¹

Affiliations
1 Department of Geology, M.S. University of Baroda, Vadodara 390 002, IN

Abstract

No Abstract.

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