APPROACHES TO SEQUENCE STRATIGRAPHY OF CONTINENTAL FORELAND BASINS:AN EXAMPLE FROM THE EARLY CRETACEOUS NORTHERN TARIM BASIN

The continental sequence stratigraphy of continental basins is an important branch for the further development of sequence stratigraphy in the future.The theory and methodlogy of sequence stratigraphy will be greatly enriched and improved by the studies of the nature of sequence boundary,internal architectures,controlling factors and models for the se-quence stratigraphy of various continental basins in distinctive tectonic settings.In the pre-sent paper,four sequence boundaries,i.e.one supersequence (second-order) and three third-order sequences are determined for the sequences of an Early Cretaceous continental foreland basin in the northern Tarim Basin.Three systems tracts are recognized as well,in-cluding the alluvial systems tract (AST),lacustrine expanding systems tract (EST)and la-custrine contracting systems tract (CST).Furthermore,the main characteristics and con-trolling factors of these sequences in the continental foreland basin are described in more detail. The research demonstrates that the tectonic movement is the chief factor controlling the formation and development of the sequences in the study area.The formation of the su-persequence boundary and its nature tend to be governed by higher-rank regional tectonic movements,where the third-order sequence boundaries are the result of the interaction of tectonism,denivellation,sediment supply and climate,of which tectonism is more impor-tant and significant.When the tectonic movement and climate remain stable,denivellation, sediment supply and palaeotopography(slope)may act as the main controls on the deposi-tion of the sedimentary systems and stacking patterns of the parasequences within the sys-tems tracts.The inhomogeneity of the tectonism in the northern and southern parts of the foreland basin is responsible for the large discrepancies in the nature of the sequence boundaries and the characteristics of the systems tracts.For example,the third-order se-quence boundaries appear to be discordant in the zone near the South Tianshan fold-thrust block (i.e.on the northern side),while they are conformable in the zone of the southern foreland uplift.The incompleteness of the development of systems tracts within a sequence (a third-order sequence bounded by two sequence boundaries is only made up of one or two systems tracts within the extent of the whole basin) reflects the complexity and particular-ity of the continental sequences.