Stratigraphy and Geologic Structure of the Pre-Neogene Strata in the Central Ryukyu Islands

FUJITA Hiroshi

Journal of science of the Hiroshima University. Series C, Geology and mineralogy

The stratigraphy and geologic structure of the pre-Neogene strata in the northwestern Okinawa islands and the northern Amami islands were investigated in order to clarify the sedimentary and tectonic history of Central Ryukyu during the late Paleozoic to Paleogene period.
The pre-Neogene strata of Central Ryukyu form an asymmetrically folded structure in each of the tectonic units separated by reverse or thrust faults dipping generally toward the northwest, and as a whole they form an imbricate structure. The pre-Neogene strata in the northwestern Okinawa islands are classified into the following formations roughly from northwest to southeast: Iheya, Izena, Maedake and Ie formations (Permian to early Cretaceous ? ), Dana and Moromi formations (late Cretaceous ? ), Nakijin formation (Carnian ) and Gusukuyama formation (Oxfordian to Tithonian ), Motobu formation (Permian to Triassic ? ) and Yonamine formation (Valanginian to Barremian ), Wakugawa formation (late Albian to Cenomanian ?), Nago formation (Cenomanian to Santonian ?) and Kayo formation (early middle Eocene ). The pre-Neogene strata in the northern Amami islands are divided, from west to east, into the Yuwan formation (Oxfordian to Berriasian), Yuwandake formation (Valanginian to middle Albian), Odana formation (late Albian to Cenomanian), Naze and Ogachi formations (Cenomanian to Santonian ?) and Wano formation (Eocene). Excepting the Dana, Moromi, Kayo and Wano formations, the pre-Neogene strata become generally younger from northwest or west to southeast or east in both districts. These pre-Neogene strata are characterized by the development of subaqueous slumping or sliding. Especially, the Yonamine, Yuwan and Yuwandake formations comprise a large amount of olistostromes in which olistoliths were derived
chiefly from the northwestern area. The polarity in geological age toward the southeast and the inferred source of olistoliths may suggest that the sedimentary basins of these pre-Neogene strata migrated southeastward during late Jurassic to late Cretaceous time.
In the pre-Neogene strata, "slump structures" and tectonic structures are observed. The tectonic structures are classified into I, flexural slip folds ?; II, reverse faults and asymmetric folds with cleavages; III, thrust faults and associated folds; IV, strike slip faults; V, normal, lag and reverse faults. The tectonic structures I, II, and III take part in the asymmetrically folded and imbricated structure parallel to the extension of the pre-Neogene strata, while the tectonic structures IV and V cut it obliquely. The formation of the asymmetrically folded and imbricated structure in the pre-Neogene strata may be closely related to the migration of the sedimentary basins.

eng

HIROSHIMA UNIVERSITY

[ISSN] 0075-4374

[NCID] AA00706718