沖繩本島南部琉球石灰岩之沉積歷史

Abstract

石灰岩是碳酸鹽沉積岩的一種，主要由海洋生物(例如：珊瑚、有孔蟲和軟體動物等)的骨骼碎片所組成，礦物成分為方解石和霰石。因現地生長的海洋生物與光合作用有關，對於海水面變化相當敏感，故碳酸鹽岩的岩相可作為海水深度的重要指標，亦可指示現地的沉積環境。Sagae et al. (2012) 利用沖繩本島南部系滿地區的野外露頭調查及16口岩心資料，建立了沖繩本島南部更新統琉球石灰岩(Ryukyu Group)的層序分層。本研究基於前人的石灰岩岩相分類及分層，利用新的野外露頭調查、新的岩心分析及薄片觀察結果，並考慮地層側向、垂直向的變化以及斷層位置，繪製出研究區域內琉球石灰岩各層的水平分布，再加上沖繩本島的大地構造作用、區域不整合面的形成時間及鄰近各島地層的定年結果，進一步探討琉球石灰岩之沉積歷史與全球海水面變化的關係。 琉球石灰岩由下而上可分為系滿(Itoman)層、那霸(Naha)層和港川(Minatogawa)層，其中前人推測系滿層和那霸層最下方的Unit 1為同時異相關係，初始沉積時兩者同時形成於不同的水深環境，之後Unit 1隨著相對海平面上升而部分不整合地逐漸覆蓋在系滿層之上。系滿層主要由分布相當區域性的珊瑚藻石灰岩(coralline algal limestone)所組成，經由薄片分析判斷珊瑚藻石灰岩的種屬，推測其沉積環境約為水深20公尺左右的塊礁(或稱補綴礁，patch reef)，受到陸源沉積物及出露地表的成岩作用的影響而呈現紅褐色。那霸層沉積水深分佈範圍廣(0-150公尺)，可依據岩相再細分成四個unit，由淺水相的珊瑚礁石灰岩和深水相的紅藻球(rhodoliths)、Cycloclypeus-Operculina、碎屑型石灰岩交錯沉積而成，岩性的變化反映出水深的周期變化。本研究發現其淺水相和深水相的週期變化與代表全球平均海平面變動的δ18O和MIS(深海同位素階)相似，透過鈣質超微化石指準層、不整合面與低海水面時期進行對比，推測那霸層的Unit 2-4可對應到MIS 20-15。雖然琉球石灰岩整體的沉積環境緩慢地向上變淺，但那霸層內加積和退積的週期以及沒有明顯的侵蝕面指示出納積空間(accommodation space)持續向上增加。港川層不整合地覆蓋在那霸層之上，由淘選度良好的碎屑性石灰岩所組成，水深在50公尺以內，顯示沖繩本島在那霸層沉積後，受到構造作用影響而開始向上抬升至今。前人研究依據野外露頭的兩個侵蝕面將港川層再細分成三層，本研究認為可對應到三個海水面相對較高的階段，進一步透過抬升速率的逆推及鄰近島嶼的定年結果，推測港川層沉積於MIS 5、7和9。 前人研究認為分布於琉球群島上的石灰岩形成於黑潮流入沖繩海槽、並改變區域沉積環境及海洋條件的時間約為1.7-1.4 Ma。故本研究推測分布於沖繩南部的琉球石灰岩亦於此時形成：系滿層和那霸層Unit 1約沉積於1.7-0.8 Ma；Units 2-4約沉積於0.8-0.6 Ma (MIS 20-15)；港川層形成於0.4 Ma之後，推測可能為MIS 5、7和9，但確切的形成年代仍需更多資訊方能確定。

Limestone consists mainly of skeleton and shells of marine calcifiers and their fragments. Therefore, the biotic composition of limestones is generally sensitive to sea level fluctuations as well as by various environmental factors, such as sea-surface temperature, salinity, nutrient level and terrigenous input and, thus, their lithofacies can be used as an important indicator of sedimentary environments. Because Okinawa Island is located near the boundary between coral-reef and non-coral-reef regions in the present-day northwestern Pacific, the Quaternary limestones (Ryukyu Group) in this island record response of coral reefs to local and global environmental changes such as glacioeustasy and increased/decreased seawater temperature. We studied succession and spatial configuration of the limestones by field and drillcore observations and analyzed their biotic composition and diagenetic products by outcrop and thin section observations. By integrating our data and those in previous studies, we reconstructed the sedimentary history of the Ryukyu Group relating to global climate and sea levels as well as regional sedimentary environments and tectonic movements. The Ryukyu Group is divided into the Itoman, Naha, and Minatogawa formations in ascending order. The deposition of the Ryukyu Group began after the warm Kuroshio Current had flowed into the Okinawa Trough in the Early Pleistocene. The initial period of the deposition of the Ryukyu Group is characterized by contemporaneous sedimentation of the Itoman Formation and the lower part of the Naha Formation, but in different environments (reef and off-reef environments, respectively). The Itoman Formation consists mainly of coralline algae with low species diversity that formed in a shallow reef setting (20–50 m water depth). Subsequently, the main part of the Naha Formation was deposited in coral reefs and associated fore-reef and shelf environments within a wide depth range (0–150 m). This formation is divided into four lithologic units and can be correlated with marine isotope stages (MIS). Units 2–4 are correlated to MIS 15–20 and the unconformity between Units 3 and Unit 4, which can be traced throughout the Central Ryukyus, is formed during the low sea stand at MIS 16. During the deposition of the Naha Formation, Okinawa Island and their environs were subsided to produce accommodation space for reef deposits forming the stratigraphic architecture of aggradation and retrogradation. The Minatogawa Formation abuts the Naha Formation with a sharp boundary and is considered to be deposited during the highstand at MIS 5, 7, or 9. The occurrences of this formation and Holocene deposits and erosional notches indicate that southern Okinawa Island has been continuously uplifted after the deposition of the Naha Formation.