Abstract
The sedimentary environment of reddish clastic rocks in the lower most horizon of the Lower Triassic Hiraiso Formation, which is in lower member of the Inai Group, South Kitakami Terrane, is reviewed. The sequence at the Kawabukuro quarry, in Toyoma City, Miyagi Prefecture, consists of several fining-upward successions of varicolored conglomerate, sandstone, and reddish sandy mudstone with desiccation cracks and carbonate nodules. The sedimentary feature of this section is interpreted as a deposition in the fluvial environment at the lower part and in the distributary channel affected by a tidal current at the upper part. A part of the reddish sandy mudstone shows an altered vitro-clastic texture under microscope. The immediate covering of the reddish sandy mudstone in the form of a wave ripple is suggestive of the ferric alternation of the fine volcanic material in the hinterland not by weathering after deposition. Meanwhile, the existence of rip-up clasts of the reddish mudstone in the massive sandstone indicates redeposition following the erosion of the mudstone, which was once deposited as mudstone strata, under a high flow regime condition.
XRD analysis of the reddish sandy mudstone reveals the formation of hematite as the material responsible for the reddish color of the mudstone. The whole rock geochemistry of the reddish sandy mudstone is characterized by high-Fe2O3/Al2O3 and equivalent (K2O+Na2O)/Al2O3 ratios in contrast to the mudstone of the Upper Permian Toyoma Formation. The existence of the carbonate nodules scattered in the reddish sandy mudstone with small-scale bioturbation indicates weak soil formation in semi-arid environments.
