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 I 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.

230Th/234U and 14C dating of a lowstand coral reef beneath the insular shelf off Irabu Island, Ryukyus, southwestern Japan

“…In this project, the distribution, stratigraphy and structure of marine carbonate sediments, correlated with the Pleistocene Ryukyu Group and Holocene deposits, were revealed based on high‐resolution seismic reflection profiles and three submarine sediment cores drilled at wells CR7, 8 and 9 (Fig. 2; Obata & Tsuji 1992; Honda et al . 1993; Sagawa et al .…”

“…2001). Obata and Tsuji (1992) reconstructed long‐term tectonic trends shifted at approximately 0.4 Ma from subsidence phase to uplift based on sequence architecture and configuration of internal reflections of seismic units in the Ryukyu Group. Furthermore, calcareous nannofossil and sedimentological studies with fossil coral assemblages of core sediments in 13 wells, including CR7, 8 and 9, revealed 13 lithological cycles, which could be attributed to glacio‐eustatic changes at 1.5–0.3 Ma in the middle Pleistocene of the Ryukyu Group (Sagawa et al .…”

“…2001). Obata and Tsuji (1992) found isolated mound‐shape structures of two different growth stages beneath the outer edge of the insular shelf in the seismic profiles and interpreted these structures as representing ‘reefal facies’ based on their configuration and internal chaotic reflections. The younger ‘reefal facies’ was estimated to have formed during marine oxygen isotope stage (MIS) 2 (Shackleton & Opdyke 1973; Martinson et al .…”

“… Present‐day carbonate sedimentary facies and distribution of mounded structures within Unit 2 (black triangles) offshore of the western Miyako Islands, southwestern Japan (Obata & Tsuji 1992; Tsuji 1993). There are seven sedimentary facies identified by Tsuji (1993): (i) reef facies; (ii) near‐reef facies; (iii) muddy–sandy mud facies; (iv) rhodolith and large foraminiferal gravel facies; (v) bryozoan sand facies; (vi) planktonic foraminiferal sand facies; and (vii) planktonic foraminiferal muddy sand facies.…”

Subsidence of the Miyako‐Sone submarine carbonate platform, east of Miyako‐jima Island, northwestern Pacific Ocean

“…Siliciclastic deposits corresponding to the upper Miocene–lower Pleistocene Shimajiri Group, which is exposed on Okinawa‐jima and Miyako‐jima islands, were not observed by us, but previous workers reported rocks of this group around the study area (Aiba & Sekiya ; Tsuburaya & Sato ; Kimura et al . ; Obata & Tsuji ). Park et al .…”