Sequence-paleogeography and coal accumulation of Early Cretaceous in Sanjiang basin, northeastern China

Abstract：The Early Cretaceous coal measurements in Sanjiang basin have been studied coal-accumulation, sedimentology and sequence stratigraphy models. Within the research using area, the main coal-bearing strata are the Chengzihe and Muling formations. Siltstones, fine sand- stones, mudstones and coals dominate the Chengzihe formation, while the Muling formations is dominated by mudstones, siltstones and fine to medium grained sandstones. Five sedimentary facies were identified namely alluvial fan, braided river, meandering river, delta and lacustrine. The main coal-accumulating environment occurs at the transition between upper and lower delta plain facies. Five sequence boundaries are recognized including regional unconformities, erosion surfaces resulting from river incision and reverse surfaces. The sequence boundaries have been used to subdivide the Early Cretaceous coal measures into four third-order sequences. The main coals were initially formed under lowstand system tracts and early transgressive system tracts but subsequently developed under middle and late transgressive system tracts and maximum lake flooding surfaces. Sequence II is identified as most favourable for coal accumulation. ThePalaeogeographic lithofacies analyses indicate that facies have source directions from the southwest, from the east. the delta plain facies are predominant. These northwest and northeast with water invasion