Comparative Sedimentology of Barrier-Lagoon in Carboniferous in Southwestern Inner Mongolia Autonomous Region

The barrier-lagoon systems(BLS)are widely distributed in the southwesten Inner Mongolia Autonomous Region during Carboniferous Period. The lagoon deposits of different stages in the systems are superimposed mutually,forming various sequences,with a great thickness. This is hard to be formed and preserved in the Holocene. According to sedimentological characteristics and depositional sequence,the transgressive,regressive,stationary and locally transgressive barrier lagoon systems in the carboniferous deposits of the studied area have been recognized, which are similar to the types in Holocene. It confirms that the stratigraphic models of barrier-lagoon systems summarized in the study of Holocene are helpful for recognition and study of the analogies in the geological time. However,the barrier-lagoon systems of the Carboniferous Period in the studied area have been formed in broad space for a long time,they are usually compound,such as transgressive-locally transgressive,stationary-locally transgressive BLS. In addition,the stationary BLS is characterized by both underlying and overlying strata to be terrestrial in Holocene,but the counterparts in the carboniferous deposits of the studied area are characterized by superimposition of more than ten barrier-lagoon sequences almost in the same area for a long time under conditions of continuous tectonic subsidence and stabilization of coastline. The transgressive barrier-lagoon systems occur in transgressive sequence and have lagoon deposits exceeding the coincident barrier sands in thickness. The formation model for BLS has been established.