Abstract
Shallow marine deposits of the Koguchi Member of the Early to Middle Miocene Kumano Group in southeastern Kii Peninsula consist of muddy sediments with varying proportions of coarser interbeds. The coarser interbeds which are made up of fine-grained sandstone and siltstone are interpreted as storm deposits, judging from the following sedimentary features. 1. Thin beds of sandstone and siltstone alternate with layers of generally bioturbated mudstone. 2. The beds of sandstones range in thickness from 1 to 15cm and exhibit marked evenly parallel lamination, and crude normal graded bedding, and have carbonaceous laminae at times. 3. The beds of sandstone and siltstone have a sharp lower contact and grade upward into mudstone, and have trails of organisms and sole marks such as flute cast and load cast on soles of beds, and burrows oblique to the bedding plane in beds. 4. The discontinuous low-angle cross lamination, ripple lamination, ripple-drift cross lamination and hummocky cross stratification (HARMS et al., 1975) are observed above the parallel lamination in beds. 5. Some sandstone beds show internal structures of graded rhythmite (REINECK and SINGH, 1972). 6. The alternating beds of sandstones and mudstone occurs in horizon between the offshore mudstone facies (Shikiya Member) and the shoreline facies (Miyai coal-bearing Member). 7. Sandstones are almost feldspathic (to quartzose) arenite and maturer mineralogically and texturally than those of turbidite facies association with a large amount of matrix, composite quartz and lithic rock fragments.
On the viewpoints of mineralogical and textural maturity, it is possible to distinguish between storm sandstone and turbidite.
