Fig. 7

Photomicrographs of sandstones in the Wutonggou low-order cycle. a A nonundulatory monocrystalline quartz grain (Qnu) with embayment, a potassium feldspar with Carlsbad twinning, and a plagioclase with albite twinning. Sample TD110, lower Taodonggou section; b A polycrystalline quartz grain with sutured quartz crystals, indicating its metamorphic origin. The blue arrow points to an elongate muscovite grain. S15–36, lower Zhaobishan section; c A slightly clay coated chert grain. NTR39–17, upper North Tarlong section; d A volcanic lithic fragment with felsic texture. The phenocrysts are mainly feldspar grains. TD110, lower Taodonggou section; e A volcanic lithic fragment with microlitic texture, an angular mudrock fragment, and a volcanic lithic with lathwork texture showing large feldspar laths. These grains suggest sedimentary and volcanic origins. TD108, lower Taodonggou section; f A volcanic lithic fragment with microlitic texture and a volcanic lithic with vitric texture, suggesting a volcanic origin. TD168, upper Taodonggou section. All micrographs are taken under cross-polarized light. Scale bar is 1 mm long in all photos. See Table 1 for the abbreviations of grain categories