• MAHALE V P

      Articles written in Journal of Earth System Science

    • Bathymetry and sediments on the carbonate platform off western India: Significance of Halimeda bioherms in carbonate sedimentation

      Purnachandra Rao V Mahale V P Chakraborty B

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      Bathymetry across the carbonate platform off western India indicated small-size pinnacles and their lateral coalescence into 2 -6-m high mounds landward, and linear elongated carbonate ridges and troughs, mounds and banks up to a height of 20-m seaward of the platform. Seismic data indicated that these mounds were transparent with no rigid internal structure and can be defined as bioherms. The sediments were abundantly aragonite faecal pellets, Halimeda grains and ooids and their radiocarbon ages ranged from 11 to 7.5 ka BP. It appears that the growth of Halimeda bioherms on the platform was facilitated by intense upwelling during the early Holocene. The terrigenous sediments brought by rivers were deposited in the inner shelf and have not affected the growth of bioherms. It is estimated that the platform comprises at least 1.85 Gt of mass CaCO$_3$ accumulated during the early Holocene and comparable to those on the Great Barrier Reef. Halimeda bioherms produce abundant carbonate sediments and their growth period represents a geological carbonate sink and release of high CO$_2$ to the atmosphere. Detailed shallow seismic studies and sediment cores are needed to quantify the exact mass content of CaCO$_3$ and model climate change during the early Holocene.

    • Tectonic appraisal of the Mid-Thane Creek of Mumbai, India: An integrated geophysical approach

      JACOB J PITCHIKA V K DUBEY KM MISHRA A KUMAR P KUMAR S MAHALE V P CHAUBEY A K

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      Integrated geophysical surveys, comprising marine magnetic, high resolution shallow seismic and single-beam bathymetry were conducted to assess subsurface tectonics of the Mid-Thane Creek (MTC) of Mumbai. The bathymetry in the intertidal zone of MTC varies drastically due to periodic dredging, with maximum depth up to 6.4 m and a minimum of $\sim$−1.6 m. High resolution shallow seismic sections up to the depth of $\sim$35 m from the sea-floor are generated to analyze the neotectonic activity of the creek. Imprints of deep-seated lineaments are recognized from magnetic anomaly map of the MTC. To delimit lateral extent of the lineaments/faults, results of several derivative methods including tilt derivative and standard Euler deconvolution are merged with the selected crest value of the horizontal derivative. To estimate depth to the source, Euler deconvolution, tilt derivative, analytic signal, and source parameter imaging method have been used. However, the depth estimation for the lineaments/faults is highly discrepant for this region, because of the complex tectonics associated with the periodic emplacements of Deccan flood basalt. To confine the top and bottom boundary of this highly magnetized basaltic layer, we have carried out spectral analysis considering 18 windows of 2000 $\times$ 2000 m with an overlap of 500 m. The average depth to the top and bottom of the source body estimated using spectral analysis is consistent with the depth estimated from the derivative filters. This confirms that the lineaments identified by the derivative filters may embed in the basaltic layer of MTC. The most prominent lineament interpreted from the seismic and magnetic data, in the central region of MTC is inferred as the marine analogue of Alibagh–Uran Fault passing through the mainland of Alibagh and Uran close to Mumbai city.

      $\bf{Highlights}$

      $\bullet$Acquired, processed and interpreted high resolution shallow seismic, marine magnetic, single beam bathymetry data in the Mid-Thane creek of Mumbai, India.

      $\bullet$Neo-tectonic and deep-seated tectonic elements are identified using the seismic and magnetic data.

      $\bullet$Identification of the faults/lineaments and source depth estimation are performed using total horizontal, analytic signal, tilt derivative, Euler deconvolution and source parameter imaging derivative methods.

      $\bullet$Thickness of the flood basalt where the faults/lineaments are embedded is estimated using spectral analysis.

      $\bullet$Interpreting the marine extension of Alibagh–Uran fault zone in the Mid-Thane creek of Mumbai, India.

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