Pleistocene reef development in the southwest Ryukyu Islands, Japan

Abstract

Calcareous nannofossil and sedimentological studies of the Pleistocene Ryukyu Group, based on 13 cores and surface outcrops on Irabu-jima and Shimoji-jima, the Ryukyu Islands, southwestern Japan, suggest that these sediments were deposited at 1.5–0.3 Ma and unconformably overlie the Pliocene Shimajiri Group. The Ryukyu Group consists mainly of reef complex deposits, which accumulated in a wide range of depositional environments, from shallow reef flat to deep insular shelf. Carbonate lithofacies representing 10 depositional environments were delineated by comparison with the present-day marine sediments and biota around the Ryukyus. Five coral assemblages were defined by species composition and the morphology of fossil forms within the coral limestone, each indicating a particular environment. The Ryukyu Group in the study area is composed of 13 lithologic units. A complete succession of units commences with coral limestone (LST or TST) grading upward into rhodolith and larger foraminiferal limestones (TST) overlain by coral limestone (HST). Episodic subaerial exposure and subordinate karstification commonly occurred after deposition. The study area subsided during the deposition of units 1–12 and then was uplifted during deposition of unit 13. Reef development may have been dominated by obliquity or precessional cycles (sixth- and seventh-order cycles, respectively) for the first 0.5 million years (∼1.5–1 Ma), after which it may have responded to fifth-order (eccentricity) cycles.