Geosynclinal Facies a sedimentary facies marked by great thickness, a generally argillaceous character and few carbonate rocks. sediments deposited at different stages of geosynclinal growth are characterised by specific features and can be grouped into different facies. They are pre-orogenic, pre-flysch, flysch and molasse. Most basinal successions now exposed has passed from a basement (pre-orogenic) phase through an extending, starved (pre-flysch) phase, as the basin deepened, to a deep marine clastic (flysch) phase and finally to continental clastic (molasse) phase. However, since the tectonic settings of basins are extremely variable, there may be considerable variations in detail.
The Bengal Geosyncline is an example of synorogenic sedimentation that started in the Permian Period when Gondwanaland was disrupted and several depressions were formed in the basement. During the Permian and Mesozoic, Gondwana sediments with coal of continental facies were accumulated. In the Jurassic to early Cretaceous, Indian subcontinent was subjected to extensive movements with widespread basaltic flow and initial uplift of the himalayas and the subsidence of the bengal basin. During the Cretaceous period, argillaceous and arenaceous deposits accumulated on the stable shelf in fluvial, tidal flat, deltaic and lagoonal environments while in the foredeep and mobile belt sedimentation possibly took place in deep marine environment and turbidites possibly played an important role. During the Palaeocene to Eocene time, Jaintia Group of sediments was deposited on the stable shelf and in the deep basin and in the mobile belt marine shale similar to Disang Group of assam was deposited. In the Oligocene, barail group of sediments was deposited under marine to estuarine environment in the foredeep basin and in the mobile belt. During the Miocene, deep basin featured subsidence and marine transgression and Surma and Tipam Groups were deposited in deltaic to shallow marine and continental environments. The Late Miocene was marked by global eustatic regression and is followed by the deposition of the Pliocene Dupi Tila and Dihing groups in fluvial to deltaic facies changing seaward to marine shale and silts by a Pliocene marine transgression. During the Quaternary, a number of small deltas are formed in the basin as a result of the usual glacio-eustatic oscillations on a general regression. [ASM Woobaidullah]
See also geological evolution.