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CHAPTER-1 WESTERN OFFSHORE BASIN- STUDY
AREA
GEOLOGY
Western offshore basin also known as the Bombay Geologic Province, is a divergent
passive continental margin basin, located on the continental shelf off the west coast of
India. The basin is bounded by the Nagar-Parkar Ridge on the north and Deccan
Syncline on the east, Vengurla Arch on the south and the 2000m bathymetric contour
on the west.
The area includes-
1) the outer shelf of Western India k/a Bombay Shelf
2) Bombay High
3) Dahanu, Panna and Surat depressions
4) Cambay & Narmada Deltas
5) Cambay & Kutch Grabens
6) Saurashtra or Kathiaw Peninsula
Structurally, the province consists of a deformed and rifted portion of the western
Indian plate passive margin. Significant features include the uplifted structures of the
Bombay High and Saurashtra Peninsula, failed rifts forming the Kutch and Cambay
Grabens, and the Narmada lineament.
GEOLOGIC HISTORY & TECTONICS
The primary structural and stratigraphic features of this basin are attributed to the
events associated with the plate movements that occurred from Late Paleozoic time to
the present.
The western continental margin of India developed by Mesozoic rifting and has
subsided and undergone further tectonic modification during India’s northward
movement and collision with Asia. Segmentation of the margin has apparently been
controlled partly by inheritance of Precambrian structures, across one of which different
rates of subsidence and degrees of stretching have occurred. At about 25–20 Ma,
following a widespread erosional unconformity, the rate of subsidence greatly increased
both on the continental shelf and, in particular, seaward of the shelf edge. This
increased stretching occurred during reorganization of the plates in the region of the
Indian Ocean and coincided with greater resistance to underthrusting of India beneath
Asia.

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Permian to Middle Jurassic- Indian plate was located in the Southern Hemisphere
between the African, Antarctic, and Australian plates and comprised part of southern
Gondwana.
The present day Indus Basin was a shallow continental shelf accumulating shales,
sands, and carbonates at that time.
Late Jurassic- The carbonate-dominated shelf environment persisted intermittently on
the western part of the shelf through the Late Jurassic.
This is evidenced by the interbedded shales and thick limestones of the Patcham, Chari,
and Katrol Formations in the Kutch area and the Springwar and Sulaiman Limestone
Groups in the area that is now the Indus Basin.
The Kutch Graben, which formed near the intersection of three orogenic trends during
the separation of the Indian and African plates, is filled with Jurassic to Recent
sediments.
Early Cretaceous - During Early Cretaceous time, the Indian plate drifted northward
entering warmer latitudes. In the Kutch area, shallow-marine shales and sandstones
were being deposited, and, by Late Cretaceous time, regressive sandstones such as the
Bhuj, Lumshiwal, and Pab Formations on the western Indian Shelf were being
deposited.
Late Cretaceous-The Indian plate continued drifting northward toward the Asian plate,
the seafloor of the Bengal Basin began to form, and flysch accumulated on all sides of
the Indian plate.
Northward plate movement continued during the Late Cretaceous and a transform fault
became active along the Ninety-East Ridge.
Rifting between Madagascar and the Seychelles initiated formation of the Mascarene
Basin. Extensional faulting occurred as the western Indian plate sheared southward
relative to the main plate. Counter clockwise rotation of the Indian plate was initiated,
and the Seychelles plate began to break away from the Indian plate, forming the Somali
Basin.
Late Cretaceous time also brought to western Indian intense volcanism, expulsion of
the Deccan Trap basalts and further rifting, which began, and then failed, leaving the
Cambay and Kutch Grabens floored with the Deccan Trap Basalts.
Palaeocene- From Late Cretaceous to Middle Paleocene trap deposits and basal sands
continued to accumulate on the Indus and Bombay shelves. Oblique convergence of the
Indo-Pakistan plate with the Afghan and other microplates resulted in wrench faulting
and development of regional arches.

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Continued northward movement and counterclockwise rotation of the Indian plate
slowly closed the Tethyan Sea along the northern and north-western plate boundaries.
Regional uplift and rising mountain ranges on the Eurasian plates to the north and west
created a new sediment source, and the prevailing sediment transport direction of
south to north was reversed.
Eocene-Miocene- From Eocene through middle Miocene time, carbonate-platform
buildup occurred intermittently on the shelves around much of the Indian plate. A
trench formed along the subduction zone as the Indian plate began to slip beneath the
Eurasian plate.
The Eurasian plate shed large volumes of sediments into the trench as subduction
continued. This terrestrial sediment influx from the rapidly rising mountain ranges,
exceeded carbonate buildup rates on late Miocene platforms and smothered carbonate
reef development along the shelf areas. The former shelf areas along the collision zones
were either subducted or became emergent fluvial deltaic environments.
The proto Indus, Narmada, Ganges, Brahmaputra, Megna, Chindwin, and Irrawaddy
Rivers developed extensive deltas as the Himalayas and other ranges continued to shed
sediments at a high rate. To the northwest of the study area, crustal shortening
continues along the Indian-Eurasian plate boundary.