Discovery of serpentinite bearing conglomerate in the Lower Yezo Group, Hokkaido, northern Japan and its tectonic significance

Evidence of solid tectonic intrusion and following subaqueous exposure of the serpentinite in the Lower Yezo Group (Lower Cretaceous) is reported. The Yezo Group comprises the lower part of the Yezo Super-group which consists of the Early Cretaceous to Earliest Paleogene fore-arc basin-fill sediments. Several conglomeratic beds containing serpentinite clasts are found in the Kamiji Formation of the Lower Yezo Group in the Teshio Nakagawa area, northern Hokkaido. This Formation mainly consists of alternation of sandstone and mudstone, pebbly mudstone and thick-bedded sandstone, all formed as sediment-gravity-flow deposits. Subangular to subrounded serpentinite clasts are frequently found as granules in pebbly mudstones and lag of thick-bedded sandstones, which are sporadically associated with calcareous fragments of marine fauna such as crinoid, belemnite and Neithea. Although later clay-forming alteration and carbonatization obscured most original texture of the serpentinite, remaining serpentine minerals reveal its ultra-basic origin. Oolitic limestones with some detrital chromian spinels and serpentinite nuclei sometimes coexist with the serpentinite clasts in the conglomerate. Chemical composition of detrital chromian spinels in the sandstone suggests their derivation from depleted mantle peridotite. Abundant serpentinite clasts strongly suggest proximity of the source rock body in view of the extremely low durability of serpentinite against abrasion during transportation and sedimentation. Serpentinite particles associated with ooids and detrital chromian spinels in oolitic limestone also imply that the serpentinite body cropped out in a shallow marine environment. Bioclasts, serpentinite, carbonate clasts and other detritus were mixed and episodically flowed down into the deeper part of the basin to form the above serpentinite bearing conglomerate, suggesting solid serpentinite intrusion and simultaneous exposure in a Lower Cretaceous forearc basin floor.