PLANAR CROSS-BEDDING ASSOCIATED WITH RIP CURRENTS OF UPPER CRETACEOUSFORMATIONS, NORTHEAST JAPAN

The sedimentary structure produced by rip currents is mainly character ized by seaward-dipping planar cross-bedding. Field observations from the Upper Cretaceous (Santonian) Kunitan and Taneichi formations in no rtheast Japan show that the occurrence of the cross-bedded units exten ds laterally from the ancient beachlines but is restricted in vertical extent. The planar cross-set bed of the Kunitan Formation overlies ve ry coarse-grained, parallel-laminated and trough cross-bedded sandston e beds of longshore current origin and underlies a pebbly conglomerate d bed that was probably formed by a storm-generated bottom current. Ab ove is a zone of alternated beds of bioturbated and parallel-laminated or hummocky, cross-stratified, fine-grained sandstones, representing either lower shoreface or transitional depositional environments. The planar cross-bedding of the Taneichi Formation is intercalated with th e underlying trough cross-bedding and with the overlying bioturbated o r parallel-laminated, very fine-grained sandstone bed which also inclu des hummocky cross-stratification. Both planar cross-set beds of the T aneichi and Kunitan formations are present in the succession which fin es upward, suggesting an increasing depth of deposition. Such field oc currences indicate that bedforms produced within deeply incised rip ch annels which extend towards the sea can be untouched by ravinement and remain well preserved during transgression.