The Mahanadi delta in the east coast of Peninsular India has an arcuate shape, and covers an area of about 9, 000km2 with a maximum width of 140km. Among the important geomorphic features recognized on the delta are active distributary channels, dead or defunct channels, ancient beach ridges, coastal sands, tidal flats, swamps, and spits. The sediments brought by the Mahanadi River are distributed by riverine and marine agents to give rise to a broad deltaic plain. Waves, winds and littoral currents have played an important role to rework the deltaic sediments and have been responsible for the development of the sea-marginal transition zone of the delta plain. Based on the disposition of the geomorphic features, four major stages are recognized in the evolution of the delta.
The Upper Pleistocene Kioroshi Formation is distributed in the northern part of Chiba Prefecture, central Japan. The formation consists of mainly coastal and shallow marine deposits which accumulated during the last interglacial period. The deopositional system of beach-shoreface, formed during the high-stand, can be recognized in the upper part of the Kioroshi Formation. The Pleistocene beach facies is used as a marker for the paleo-highstand sea-level. The mean uplifting and tilting rates of the study area (20×30km) since 125kyr. B. P. were estimated by comparing the present elevations of beach facies with an inferred initial form of the beach, deduced from the Holecene strand-plain.
The early Pliocene Lower Shigarami Formation is well developed in the Takafu syncline of the northern Fossa Magna region. This formation is divided into the Gonda conglomerate and sandstone member in the southern part of this district and the Takafu mudstone member in the north. The both members show interfingering relationship each other. It has been stated that the former member deposited at fan delta environment and the latter at off-shore. In this study details of the depositional conditions in the two members were clarified as follows. 1. The Lower Shigarami Formation is composed of 11 sedimentary facies, which were formed under a fan delta system on the shelf and a channel system on the continental slope. 2. These sedimentary systems are descriminated as two stages; namely, Stage 1 and Stage 2. At the Stage 1, a fan delta prograded from the southeast and turbidites in the north of this district was supplied from a fan delta on the east. And at the last of Stage 1, outer shelf environment extended to all over this area. At the Stage 2, the sedimentary environment in the south changed from outer shelf to delta front due to fan delta progradation at the early, and delta plane extended to the north at the later.