(21) Analysis of grain fabric in turbidite sandstone using electron backscatter diffraction in the SEM
Takuya Ishimaru and Hajime Naruse (Kyoto University, Japan)
Grain fabric in sandstone has been investigated for reconstruction of paleocurrent directions and their depositional processes. However, direct measurements of three dimensional properties of grain fabric was difficult because sand grains are too small to observe its three dimensional morphology, and therefore grain.fabric analysis has been conducted mostly on two dimensional thin sections of sandstones. To this end, this study employed the method of electron backscatter diffraction (EBSD) in the SEM, which is the method to measure the three dimensional orientation of crystallographic axis in a cross.section quantitatively. Measured sandstones are turbidite beds collected the Upper Cretaceous Izumi Group distributed in SW Japan. The sandstone shows so.called traction carpet structures (spaced stratification) which horizontal inverse.graded bands thicker than parallel lamination. All thin.sections were cut parallel to bedding surface. As a result, the orientation of long axes of the measured sand grain represents two modes in contour map: a(t)b(i) (flow.transverse) fabric and a(p)a(i) (flow.aligned and upstream imbricated) fabric. This result coincides with the two dimensional analysis of grain fabric of traction carpet deposit that was reported by Hiscott and Middleton (1979). Thus, it is suggested that the EBSD method is effective to measure three dimensional orientation of grains in a sandstone.