Abstract: According to field geological section survey and in door thin-section identification, polymictic conglomerate of the 1st member of the Yingcheng Formation contain 4 types of cementation: magma, micrite, tuffaceous and sand and politic. Magma cementation is a main cementation type in the Songliao Basin. The gravels in the polymitictic conglomerate are quite complex. Most of gravels are granite derived from the base of the Songliao Basin and from Permian metamorphic rocks. In-situ volcanic rocks are secondary in abundance. The gravels are poorly sorted. Their psephicity is from moderate to fine, with sub-round and sub-angular shapes being mostly common. From base to active volcanic center, the amount of the mexotic gravels of polymictic conglomerate is decreasing, and the in-situ volcanic gravels are increasing gradually. Matrix composition is also mulriple. At bottom it is argillo arenaceous, changed into trffaceous upwords and became magmatic at the top. Unconsolidated or semi-consolidated sand-gravel deposits accumulated in some sediment environments such as alluvial fan, braided river and so on, were carried by pyroclastic flow or lava flow during volcanic eruption periods to form mixtures with pyroclastic flow, lava flow and waters. After cooling and compaction, the polymitic conglomerates were formed as a result. Since the conglomerates were often found near the bottom of the volcanic sequence at the fault basin brink, it reveals that the origin of conglomerate is relative both to tectonic activity and volcanic activity, and they are special products at the early stage of the volcanic cycle.

Key words: Songliao Basin, Yingcheng Formation, polymictic conglomerate, magma cementation