Fig. 4

a Photomicrograph of flat-pebble conglomerates. Pebbles (P) of conglomerate, Sample 10 (Fig. 2), comprise bioclastic wackestone and are surrounded by micrite and siliciclastic matrix (M). Most bioclasts are fossils of small shelly fauna (blue arrow) and archaeocyaths (red arrow). Small shelly fauna are mainly chancelleries and hyolithids; b Photomicrograph of archaeocyathan fossils in pebbles, taken under plane-polarized light (PPL). Dailycyathus sp. (left); Nochoroicyathus sp. (right); c Photomicrograph of silty/sandy micrite, taken under cross-polarized light (XPL). Silty/sandy micrite, Sample 17 (Fig. 2), mostly comprises neomorphic spar and angular but etched siliciclastic grains, such as feldspar (red arrow) and quartz (white arrow); d–f Photomicrographs (PPL) of the oolite show that ooids become larger while siliciclastic grains become less abundant upsection, and are cemented by calcite spar. Among these three photos, ooids in photo d (red arrow) are smallest, approximately equal to silicate clasts were not only deposited in the space between ooids (red arrow), but also were the nuclei of ooids (blue arrow). Sample numbers of these three photos are respectively 26, 27 and 28 in Fig. 2