K. Yagishita, PLANAR CROSS-BEDDING ASSOCIATED WITH RIP CURRENTS OF UPPER CRETACEOUSFORMATIONS, NORTHEAST JAPAN, Sedimentary geology, 93(3-4), 1994, pp. 155-163
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.