MICRO-STRATIGRAPHY AND PALEOGEOGRAPHIC RECONSTRUCTION OF UPPER TRIASSIC LACUSTRINE ZHANGJIATAN SHALE BED, ORDOS BASIN, CENTRAL CHINA

Shale laminae contain both structural (e.g., thickness and bedding type) and textural (e.g., grain size, distribution, type, and orientation) attributes that can be used to interpret paleoenvironment and paleogeography. Five lamina types are identified in thin section study of 22 shale core samples of lacustrine Zhangjiatan Shale Bed in a well in Ordos Basin: 1) Fining-upward laminae with an erosional or sharp base. They may represent A and B horizons in a “muddy” Bouma sequence. 2) Coarsening upward silty laminae with silt content of 7-35%. The laminae as a whole show an overall finning and thinning upward trend. The inverse grading is probably caused by “clay rheology” of Hampton (1975); the overall trend indicates a decrease in sediment supply. 3) Flaser, wavy, or lenticular laminae, as transitions between pure silty and clayey laminae. A sample shows a typical Bouma sequence with an erosional base and fines upward and changes gradually from flaser to lenticular beddings. 4) Laminae rich in rip-up clasts. The clasts are flattened light-colored muddy lenses, which had been redeposited with organic-rich darker mud. 5) Laminae containing hexagonal, calcite-replaced sands. The euhedral grains occur in clayey matrix and are likely of a volcanic-ash origin. In addition, laminae can be classified on the basis of grain size proportions. SEM and XRD analyses of each type provide more information. A microstratigraphy at a mm to sub-mm scale is established on the basis of structural, textural, and compositional variations through lamina counting under microscope. Stacking of microstratigraphies of individual thin sections along well logs with interpreted lithofacies and depositional systems shows a stratigraphic trend in a 1-10s m scale and are used for comprehensive environmental interpretations. The study interval overall was deposited during a lake regression, as indicated by upward decrease in clay minerals and increase in detrital silt and sand, plant remains, dewatering structures, and algal cysts. However, regression is punctuated by small-scale transgressions and volcanic-ash deposition. Microstratigraphic analysis, combining with XRD and SEM analyses are an effective method to reconstruct high-resolution paleogeography and will be experimented in the future.