Abstract:

Factors controlling sequence development include topography of lake basin, basement subsidence, lake lev- el fluctuation and supply of sediments. Analyzing synthetically the changing characteristics of these factors with time and space, the sequence stratigraphy in the terrestrial lake basin with slope-break belt has been simulated numerically. Setting an altimetric point to be the sedimentary base level, those located higher than the base level would be apt to be eroded away, while those lower than the base level would be apt to be filled with sediments. Grain sizes of sediments would depend on the distance from the sedimentary base level, the larger the distance,and the finer grain size would be. Simulation results indicate that within a complete sequence cycle,lowstand system tract would mainly distribute below the slope-break belt with filling up characteristics. Accompanying the rising of lake level, transgressive system tract would onlap the salient located above the slope-break belt.Because the increase of accommodation space became slower, high stand system tract would prograde into the basin and deposit mainly coarse sediments, but erosion occurred on the salient above the slope-break belt. Junggar basin is a downwarped basin with foreland characteristics,which has been controlled by extrusion stress in Jurassic, and the differential elevation and subsidence of the basement would result in the formation of slope-break belt at the margin of the basin. The whole Jurrasic is a second-order sequence which can be subdivided into four third-order sequences, namely A,B,C and D sequences,respectively. The second-order sequence is lithologically characterized by coarse-fine-coarse,and constitute an integral episode of tectonic subsidence, which coincides very well with the computer modeling results. The coincidences of the third-order sequences are also relatively well, but the symmetry and integrity are not as good as those of the second-order sequence. The four third-order sequences consist,vertically,of coarse-fine-coarse cycles,or retrograding first and then prograding,and are,horizontally,characterized by decreasing propertion of sand-prone facies first and then increasing.

Key words: Junggar basin, sequence stratigraphy, slope-break belt, mathematic model, computer simulation