Factors Controlling Sediment Supply and Mechanism of Transport into Deepwater Tertiary Godavari Basin, East Coast of India

Sudipta Dasgupta, Neeraj Sinha, Venkatrathnam Kolla, Shashank S. P. Tripathi, and Arijit Chattopadhyay
Reliance Industries Limited, RCP, Thane-Belapur Road, Navi Mumbai, India

The delivery mechanism of Tertiary deepwater clastic sediments in the offshore Godavari Basin is controlled by several factors, viz. width of continental shelf, sea-level fluctuations, tectonic events influencing the basin-margin and/or the hinterland areas, location of the canyons on the shelf and upper slope, presence of longshore currents, seasonal fluvial flux and direct sediment delivery mechanisms. Continental shelf near Godavari river mouth is relatively narrow (ca. 15-30 km). This helps the focused dispersal of sediments from delta front to the deepwater system. During lowstands, rivers bypassed the narrow shelf, delivered sediments directly to the shelf-edge, upper slope and/or canyon heads, which subsequently act as conduits feeding sediments into the deepwater system. Relative basin margin uplifts by diastrophic events caused possible rejuvenation of incised valleys and enhanced incision of canyons by gravitational collapse on upper slope. Much of the sediment flux is from the Godavari River itself, with annual discharge of ca. 96.5 km³yr^-1. Godavari River and its tributaries, on their way to the Bay of Bengal, drain Deccan Flood Basalt terrains that contribute fine grained sediments; and the Precambrian Eastern Ghat Mobile Belt rocks and fluvial Gondwana sediments contribute the coarse clastics. Along the east coast of India, the longshore sediment transportation rate (ca.1.0×10⁶ m³yr^-1) is significant enough to carry considerable amount of sediments to the canyons feeding the deepwater system, especially during lowered sea levels. Similarly, episodic events like storms, tsunamis etc. may contribute sediments to the deepwater system.

3D Seismic datasets, wire-line logs and cores and well-cuttings data reveal depositional patterns and sediment composition on the slope and basin-floor. Tertiary deepwater Godavari Basin received sediments primarily by cohesive and noncohesive sediment-gravity flows. Fed essentially through canyons, dispersal of sediments in deeper water has been through the formation of channels, channel-avulsions, overbanks, lobes and crevasse splays and washover fans. Channels are primarily filled with cohesive muddy debris flow deposits, coarse massive clastics – gravels to medium-grained sands – deposited by the noncohesive dense sediment-gravity flow and also with intervals of thinly bedded / laminated fine-grained sand–silt–clay intercalations, deposited by dilute turbidity flows. Thin beds are also present in overbanks and levees associated with channels, which reflect spilling of dilute flows from channels. Channel breaching occurred frequently at channel bends and deposited crevasse splays. Splay deposits contain coarse-grained thick and fine-grained thin beds. Washover fans form at channel bends through flow-stripping and appear sand-prone.

AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina