A Study of the Morphological Features and Bore Hole Cuttings in Understanding the Evolution and Geological Processes in Mahanadi Delta, East Coast of India

Mahanadi delta is a classical arcuate delta along the east coast of India formed around the confluence of river Mahanadi with the Bay of Bengal in Orissa state. It was formed during the Upper Pleistocene-Holocene period overlying a dark grey shell bearing sequence with a distinct unconformity. The subaerial part of the delta has been the product of sedimentation under fluvial and fluvio-marine environments where a variety of depositional energy systems were operative e g river flow, sea waves, tides, littoral drift and wind. A variety of geomorphological features e g , river channels, point bais palaeo-channels, natural levees, flood plains, beaches, palaeo-beach ridges, tidal flats, tidal swamps, tidal creeks, spits, bars, lagoons and aeolian sand dunes have been observed and studied. These morphologic features have distinct relation between energy conditions of deposition, sediment make up, topographic level and land use. Three shell bearing marine horizons have been observed in most borehole cuttings of which the bottom most horizon is of pre Pleistocene age while the rest two are within the deltaic sequence. The bottom most marine horizon is characterised by the presence of iron silicate oolite sand Oliva-Glycimeris shell association. Delta building has taken place by shifting of distributary channels and fluctuation of sea level. Four fluvial and marine stages have been identified e g , Kathjodi-Burdha-Pracli- Debi fluvial stages interacting with four corresponding strand lines. In general the sea was receding during the delta formation except a short transgression during the second stage of the deltaic evolution. The marine marginal part of Mahanadi Delta shows three distinct morphologic makeup (1) the southwestern sector is characterized by thick wide sandy beaches, (2) the central sector by narrow beach and alternating sequence of parallel paleo-beach ridges and tidal swales and (3) the northern sector by tidal flats and mangrove swamps. This is explained by the role of the prevalent wind in shaping coastal geomorphology. This aspect is very typical of Mahanadi Delta.