Grain size analysis for the sandstones from the Middle Jurassic Ma'ao Formation in the Jiyuan Basin, Henan

About 80-m thick medium-to fine-grained sandstones are recognized in the lower member of the Middle Jurassic Ma'ao Formation in the Jiyuan Basin, Henan, and previously were considered as the feldspathic quartz sandstones in many literatures. The authors in the present paper redetermined them as lithic quartz sandstones. These sandstones become progressively smaller in mean grain size (Mz) and better in sorting. Moreover, the standard deviation (σ) and skewness (SKI) for the sandstones suggest the typical features of the fluvial deposits. The frequency curves display asymmetrical bimodal or trimodal patterns, and the probability cumulative curves display the gradations from "low oblique four-part" patterns to the " high oblique two-part" patterns. Multiphase floods during the deposition of the sandstones are responsible for the partial oscillation of the mean grain sizes and standard deviations. In addition, the occurrence of the PQ, QR and RS intervals of the tractional currents indicated in the CM patterns also shows the classical patterns of the meandering stream deposits.