Quaternary geohistory inferred by seismic stratigraphy of a carbonate province in an active margin, off Miyako Island, south Ryukyus, Japan

Abstract

High resolution seismic reflection profiles acquired at western offshore of the Miyako Island in South Ryukyus have revealed the distribution, stratigraphy and structure of the Quaternary marine carbonate deposits beneath the shelf and the shelf slope. Above the Plio-Pleistocene siliciclastic basement, five seismic units, Unit 1 to Unit 5 in descending order, were identified and correlated to the Pleistocene Ryukyu Group and Holocene sediments. A major tectonic event between ca. 1.7 Ma and 1.2 Ma deformed the Plio-Pleistocene siliciclastic basement (Unit 6). A subsequent lowstand allowed to deposit siliciclastic basal sequence (Unit 5) of the Ryukyu Group. This lowstand phase was followed by faulting which resulted in extensive erosion at the upthrownside of NE trending major normal fault on the northwestern shelf slope. Subsequent overall transgression with fluctuation of sea level enabled to grow and deposit coral reefs and the associated carbonates (Unit 4). They are correlated to the Riukiu Limestone on land, which was deposited intermittently between ca. 1.2 Ma and 0.4 Ma. After the ample transgression represented by Unit 4, tectonic movement culminated with the progress of upheaval of the Ryukyu Arc, as evidenced with a considerable uplifting as well as faulting of the Riukiu Limestone on Miyako and Irabu Islands. Later than 0.4 Ma, carbonate sequences younger than the Riukiu Limestone on land were deposited successively as Units 3 and 2 which represent shelf margin systems with reef developments. Thus, sedimentation and distribution of the depositional sequences in the Pleistocene carbonates here have been largely dependent on relative sea level changes closely related to vigorous neotectonic movements, which were closely related to the evolution of the Ryukyu Arc system. The depositional style of the Quaternary carbonates in the active island arc system (Ryukyus) seems to manifest a considerable difference from that in a passive margin.