Stratigraphic Pattern and Character of Skeletal Sand Bodies in Lacustrine Deltas
Mei Zhichao I'm Jinyan (Department of Geology, North an)
Large lacustrine deltas are mainly formed in the shore area of fresh or low salinity lakes. They are usually interpreted as the type of Mississippi river-dominated deltas because of low wave energy and lacking of tidal effect. The development of a complete progradational sequence is used as an important criterion for the identification of such deltas in subsurface geological study, and the river mouth bars are considered as an essential genetic facies which marks the site of major accumulation of sand deposits and represents a favourable fades zone for hydroncarbon accumulation. Based on observation and analysis of modern and ancient lacustrine deltaic deposits, the authors suggest that the marine Mississippi delta can not be used as a universal model for lacustrine deltas. The majority of lacustrine deltas in the subaerial basins bearing oil and gas of China are formed in freshwater lakes. The bed load is translated into the lake far off the river mouth instead of constructing conspicuous river-mouth bars, because the density of river water ladden with sediments is usually greater than that of lake water and river can maintain a high speed after entering the lake. Even though any thin river-mouth bars are ever formed,they are likely to be eroded by latter river action which sometimes cuts deep into the basement of the lake. The skeletal sand bodies of lacustrine deltas are therefore composed mainly of river channel or distributary channel sands. The lacustrine deltas in the Oidos basm and other oil-bearing basins of China are subdivided into two basic types on the basis of their stratigraphic character, namely shallow-water platform delta with an incomplete progradational sequence and deep-water basin delta with a fairly complete and thick progradational sequence. The former is formed in the shallow shore area with a stable bassement. It progrades lakeward quickly and has a wide distribution with banded channel sand bodies. The latter is the product of delta prograding onto the deep basin filled with thick argillaceous deposits, and its extent of distribution is often limited due to destruction by slumping, occurring chiefly in podiform river-channel sand bodies thickened as the result of subsidence. The deltaic system in large lake basins passes in space from the shallow-water into the deep-water type and in time from the deep-water into the shallow-water type as the basin becomes increasingly filled.