Textural and mineralogical characteristics of sand grains of Oyo River Deposit at Yogyakarta, Indonesia: implications on sedimentation and origin

Oyo river deposit is one of the potential resources for small scale sand mining in Yogyakarta. The deposit extends along the upstream to downstream area of the river. A detailed study on textural and mineralogical characterization is aimed for better understanding of Oyo River sedimentation and provenance. Ten sediment samples were analyzed by using granulometry method, these include grain size, morphological and mineralogical analysis. The result shows that the average of grain size changes from coarse sand (0.88 mm) in the upstream to medium sand (0.37 mm) in the downstream. Sorting value is dominated by moderately sorted (0.7lφ - 0.94φ), skewness is dominated by very fine skewed (0.3φ - l.l0φ), and kurtosis is dominated by extremely leptokurtic (2.8φ - 6.5φ). Meanwhile for the grain shape is dominated by oblate and bladed, sphericity changes from elongate to subequant, roundness is dominated by subangular and subequant. Mineralogical composition consists of magnetite, hematite, pyroxene, amphibole, quartz, feldspar, and lithic fragment. Plotting result on triangular QFL (Quartz Feldspar Lithic-fragment) diagram shows that the provenance of Oyo River sand deposit is coming from dissected arc tectonic setting.