We conducted a paleomagnetic study to establish magnetostratigraphy of the upper Pliocene Shinzato Formation of the Shimajiri Group and the upper Pliocene to lower Pleistocene Chinen Formation at Urizun S, Urizun N, and Golf Links sections in Chinen Village, southern Okinawa-jima. The Chinen Formation, consisting of calcareous sandstone and sandy limestone, represents a transitional facies from the mud-dominant Shimajiri Group to the Pleistocene reef complex deposits of the Ryukyu Group. We collected samples using orientated plastic cubes and then demagnetized them progressively with alternating field to extract primary components. The samples from 5 horizons in unit B of the Urizun N section show reversed polarity. Considering previous calcareous nannofossil and planktonic foraminiferal biostratigraphic investigations in the same sections, we correlated these horizons to the subchron C2r1r. Remanents from other horizons were quite unstable and/or completely remagnetized by the present magnetic field. The Olduvai subchron was not detected in these sections.

View full abstract

Bioclasts comprise considerable parts of seafloor sediments of shallow marine tidal sandridges in the Itsuki Nada Sea, off Matsuyama City, western part of the Seto Inland Sea, Japan. Characteristics of the bioclasts and patterns of their distribution are analysed in order to detect their sources of supply.
The bioclasts in the area studied are significantly concentrated on the sandridges, and are composed mostly of barnacle's plates and molluscan shells. The former mostly consists of disarticulated and abraded plates of a large barnacle Megabalanus rosa, and is abundant in the western part of the so-called Oozu Sandridge. Its content among the bottom sediment becomes higher toward the caldron which has been formed by tidal erosion and is situated upstream of the flood tidal current. In addition, the Oozu Sandridge is covered with sand- to granule-sized clastics and there is no rockground that can provide substrate for such barnacles to attach. This suggests that the barnacle's plates were produced in the caldron where rocky substrate exposed, and transported to the sandridge by flood tidal currents. On the other hand, the latter is dominated by disarticulated shells of a byssally attached semi-infaunal, muddy sand-dweller of Modiolus comptus. Although most of the shells are strongly abraded, well-preserved ones having periostracum are found from two areas: East and southwest of the Oozu Sandridge, where muddy sand bottoms, generally preferred by the species, are well-developed. These are interpreted as the potential sources of the bivalve shells.
It is thus considered that the bioclasts apparently concentrated on the sandridges are not to have been produced there, but transported and gathered to the sandridges from various surrounding environments due to tidal currents.

View full abstract

Three-dimensional geometry and preservation processes of Middle Pleistocene tidal sand bar deposits were studied in the wall of a pit where macrotidal tide-dominated delta succession in the Ba Mieu Formation, southern Vietnam, is exposed. The delta succession at this site was divided into units A, B, and C in ascending order. Units A and B are interpreted as upward-fining tidal sand bars that migrated seaward. Unit C consists of upward-fining tidal flat deposits those onlap units A and B. As a consequence of successively weakened tidal current during the progradation of the delta and tidal flat, the tidal sand bar deposits were buried under stacked tidal flat deposits. The lower part of the tidal sand bar deposits were partially reworked by tidal currents while the upper part was not modified. After the reworked sands were deposited as mixed flat deposits on the tidal sand bars, muddy tidal flat spread and covered the tidal sand bars. As a result, the upward-convex geometry of the tidal sand bars was preserved in he tidal flat deposits.

View full abstract