| Based on the cores, well-logging, outcrops and palaeontological data and the previous researcher's results in Hailaer Basin, this paper outlines the petrological and sedimentological features of this area, as well as its offering interpretations of its stratigraphic sequence, the pattern of coal-accumulation and the thick coal seams.The main results of the study are as follows:(1) five main rock types(conglomerate, sandstone, siltstone, mudstone and coal) were recognised in the Lower Cretaceous of this area, which can be further divided into 13 lithofacies. From these, five different sedimentary systems can be identified: alluvial fan, river, braided river delta, fan delta and lake. The alluvial fans developed predominantly in the Tongbomiao Formation, the fan deltas in the Nantun Formation, the lake in the Damoguaihe Formation and the fan(braided river) deltas and limnetic facies occurred mainly in the Yimin Formation. The main coal-forming environments in the Hailaer Basin were found to be the limnetic facies and fan(braided river) deltas.(2) the sedimentary sequences in the Hailaer Basin were all found to be similar to one another, comprising, from bottom to top: coarse clastic rocks, coal-bearing clastic rocks and lacustrine mudstone, lacustrine mudstone and coal-bearing clastic rocks. These sedimentary sequences show the synchronization in tectonic evolution of each depression in Hailaer Basin, which makes it possible to compare with each other under sequence stratigraphy in Hailaer Basin. In this context, on the basis of the regional stratigraphic unconformity, the erosional base of the incised valley filled by sandstone and conglomerates, the reversal surfaces of the vertical facies and palaeontological discontinuous surfaces, six third-order sequences are identified in the Hailaer Basin. In these, Sq? corresponds to the Tongbomiao Formation. Sq? to n1 of the Nantun Formation, Sq? to n2 of the Nantun Formation, Sq? to d1 of the Damoguaihe Formation, Sq ? to d2 of the Damoguaihe Formation, and Sq ? to the Yimin Formation.(3) the Tongbomiao Formation(Sq?) developed during the initial subsiding stage, when depressions in Hailaer Basin were isolated from each other. In this period, the depositional environment was dominated by alluvial fans charcterised by variegated coarse clastic rock without the development of coal. The Nantun Formation developed during a rapidly subsiding stage, in which lacustrine transgression occurred for the first time in the Hailaer Basin. There are significant differences in sequence characteristics between the east and west of the basin, as a result of differential tectonic settlement in the various individual depression. To the west of the Cuogang uplift, depressions, such as the Chagannuoer depression, subsided more rapidly, so the boundary between Sq I and Sq II is less obvious. The depositional environments in this region comprise fan deltas, braided river deltas and lakes. In contrast to this, the subsidence was intermittent on the eastern side of the Cuogang uplift, resulting in a clear sequence boundary characterized by the erosional base of an incised valley at the bottom of n2 of the Nantun Formation. The Damoguaihe Formation was formed during a strong extensional stage, characterised by large-scale lake transgression in d1. At this time, the depositional environment of every depression was lacustrine, resulting in the deposition of black mudstone, which is an important hydrocarbon source rocks. Subsequently, during Sq V(d2 of the Damoguaihe Foramtion), the tectonic framework inherited the former. In the north-eastern part of the Hailaer Basin, there was a balance between change in accommodation space and sediment supply rates, so the depositional environments were characterized by fan(braided river) deltas and shallow lakes. In contrast, in the west of the basin, the depressions remained as deep lakes. The Yimin Formation developed during a basin shrinkage stage, in which depressions to the east of Cuogang uplift became interconnected with one another, resulting in the deposition of coal-bearing strata. The overall depositional environment at this time was lmnetic.(4) coal seams developed variously and complicatedly under the framework of sequence stratigraphy from alluvial plains to lacustrine deposit. Stratigraphic analysis indicates the following: ? in the alluvial plains, the highstand system tracts are well-developed which included several fourth-order sequences characterized by gradually thinning coal-seams and thickening sandstones. ? in the upper delta plain, the sandstones of every fourth-order sequence's gradually thin while the coal seams thicken. ? in the lower delta plain, the changes in sandstones and coal seams in each fourth-order sequence's less obvious. ? in the lacustrine facies, lowstand system tracts, transgressive system tracts, and highstand system tracts are well-developed, with coal seams usually found at the base of the transgressive system tracts in the fourth-order sequences.(5) Coal-accumulating environment of Sq? was limnetic facies formed as a result of the lakes preceding or between the fans becoming shollow, and becoming connected by swamps at the edges of the depressions. In this way, aconsiderable thickness of coal seam developed on a large scale under stable conditions at the edges of the Yimin and Hulunhu Depressions. The coal-accumulating environments of Sq III were the fan and braided river delta plains at the edges of the depressions, with coal seams forming between the fans.(6) the number 2 coal seam of the Zhalainuoer Coal Mine is identified as occurring at the beginning of the transgressional sequence tract in Sq VI. Based on the coal facies analysis, the thick coal in the TST was characterized by: ? the coal consists of several transgressive type of coal-forming cycles and the boundary between coal-forming period containing high concentrations of fusinite and alongside low ash, low gelification index(GI), and a low ratio between vitrinite and inertinite at the center of the coal mine. While at the edge of the coal mine, the boundary is marked by the erosional base of incising river. ? The Number 7 coal seam of the Mianduhe Coal Mine is located at the highstand sequence tract in Sq?, and consists of several regressive coal-forming cycles. Every cycle was continuous with each other, then formed the thick coal seam in this highstand sequence. |
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