5:15 PM - 6:45 PM
[BBG01-P05] Insights into the Late Middle Pleistocene Reef Formation of Minatogawa and Climatic Changes in the Ryukyu Islands
Keywords:Minatogawa Formation, Ryukyu Group, Quaternary reefs, Okinawa Island, Corals, Depositional environments
The Middle to Late Quaternary period, spanning the last 770 ky[Microsof1] , is pivotal for investigating rapid sea-level fluctuations and climate changes, as well as their impact on ice sheet stability. Despite the significance of this era, uncertainties remain regarding the timing and amplitude of sea-level changes prior to the Last Interglacial (Marine Isotope Stage 5; 130-80 ky). Quaternary fossil coral reefs hold information on sea-level and climate changes that can help bridge these gaps. Our research focuses on the Minatogawa Formation (MF), whose age has been determined to be Middle to Late Pleistocene, and which is located in southern Okinawa Island and unconformably overlies the Lower to Middle Pleistocene Naha Formation (NF). Our study aims to understand the 3D facies architecture of the MF, reconstruct the relative sea-level history, and examine the response of reef ecosystems to millennial-scale environmental changes.
From 11 boreholes drilled onshore along two transects (T1 and T2) perpendicular to shore (T1: holes[MH2] 1 to 6, T2: Holes 1B to 5B) and one borehole (C) drilled between T1 and T2, a total of 188 meters of core sections were recovered between 2021-2022, offering insights into the formation depositional environments and diagenetic history.
The thickness decreases inland and varies from 23.7 to 0 m along T1, and from 20.75 to 0 m along T2. The NF was penetrated at all sites and primarily consists of bioclastic packstone[MH3] and coral floastone (only hole 2B). At the most distal site along T1 (Hole 1)[MH4] [LF5] site, a massive siltstone of the Shimajiri Group was recovered beneath the NF. The material retrieved at the top of the holes consists of 1–3 m thick reddish modern soil, also found in dissolution cavities within Pleistocene limestones.
The MF consists of well sorted detrital limestone and coral limestone with varied subfacies. Well sorted detrital limestone is a grainstone ranging from very fine to very coarse-grained, dominated by large benthic foraminifera (LBF), and exhibits moldic porosity and dissolution vugs, sometimes with modern soil. Coral limestone subfacies framestone has a bioclastic packstone or grainstone matrix, with in-situ and fragmental crustose coralline algae (CCA), moldic porosities, and dissolution vugs, containing massive corals such as Porites or Merulinidae, and few branching corals (Acropora). Floatstone has a grainstone matrix rich in LBF, with laminar, branching, and submassive coral clasts. Rudstone features a grainstone matrix with branching and laminar corals. Coral grainstone, is poorly sorted and rich in LBF, includes massive, branching coral, and CCA clasts, akin to packstone. Packstone has a muddy matrix dominated by LBF and massive coral/CCA fragments, with mollusks, echinoids, and dissolution vugs.
On T1, a shift from well-sorted grainstone and framestone to coral limestone with rudstone and floatstone is visible from holes 1 to 4. Grainstone is prominent at higher elevations on the transects 1 and 2, while framestone and floatstone/rudstone are common at lower and intermediate hole depths. Massive corals are prevalent in framestone at proximal sites with few laminar corals; branching corals are found more in floatstone and framestone at distal sites, along with laminar corals. On T2, LBF grainstone forms the surface sediment from holes 5B to 1B and lies beneath framestone at Hole 1B. Framestone at holes 1B, 2B, and C has 2 sediment infill stages: carbonate mud, likely from regressions, fills secondary porosity, altered by bioerosion or cementation, and coarser grainstone maybe associated with transgressions. Massive corals mainly populate framestone at Hole 1B, while branching corals are scattered in floatstone and packstone (holes 3B-4B).
Paleosols and sharp boundaries between MF units and on the NF indicate sea-level changes. The MF showcases shifts in marine paleoenvironments, necessitating further work on bioassemblages, microfacies, and isotope geochemistry for comprehensive insights.
From 11 boreholes drilled onshore along two transects (T1 and T2) perpendicular to shore (T1: holes[MH2] 1 to 6, T2: Holes 1B to 5B) and one borehole (C) drilled between T1 and T2, a total of 188 meters of core sections were recovered between 2021-2022, offering insights into the formation depositional environments and diagenetic history.
The thickness decreases inland and varies from 23.7 to 0 m along T1, and from 20.75 to 0 m along T2. The NF was penetrated at all sites and primarily consists of bioclastic packstone[MH3] and coral floastone (only hole 2B). At the most distal site along T1 (Hole 1)[MH4] [LF5] site, a massive siltstone of the Shimajiri Group was recovered beneath the NF. The material retrieved at the top of the holes consists of 1–3 m thick reddish modern soil, also found in dissolution cavities within Pleistocene limestones.
The MF consists of well sorted detrital limestone and coral limestone with varied subfacies. Well sorted detrital limestone is a grainstone ranging from very fine to very coarse-grained, dominated by large benthic foraminifera (LBF), and exhibits moldic porosity and dissolution vugs, sometimes with modern soil. Coral limestone subfacies framestone has a bioclastic packstone or grainstone matrix, with in-situ and fragmental crustose coralline algae (CCA), moldic porosities, and dissolution vugs, containing massive corals such as Porites or Merulinidae, and few branching corals (Acropora). Floatstone has a grainstone matrix rich in LBF, with laminar, branching, and submassive coral clasts. Rudstone features a grainstone matrix with branching and laminar corals. Coral grainstone, is poorly sorted and rich in LBF, includes massive, branching coral, and CCA clasts, akin to packstone. Packstone has a muddy matrix dominated by LBF and massive coral/CCA fragments, with mollusks, echinoids, and dissolution vugs.
On T1, a shift from well-sorted grainstone and framestone to coral limestone with rudstone and floatstone is visible from holes 1 to 4. Grainstone is prominent at higher elevations on the transects 1 and 2, while framestone and floatstone/rudstone are common at lower and intermediate hole depths. Massive corals are prevalent in framestone at proximal sites with few laminar corals; branching corals are found more in floatstone and framestone at distal sites, along with laminar corals. On T2, LBF grainstone forms the surface sediment from holes 5B to 1B and lies beneath framestone at Hole 1B. Framestone at holes 1B, 2B, and C has 2 sediment infill stages: carbonate mud, likely from regressions, fills secondary porosity, altered by bioerosion or cementation, and coarser grainstone maybe associated with transgressions. Massive corals mainly populate framestone at Hole 1B, while branching corals are scattered in floatstone and packstone (holes 3B-4B).
Paleosols and sharp boundaries between MF units and on the NF indicate sea-level changes. The MF showcases shifts in marine paleoenvironments, necessitating further work on bioassemblages, microfacies, and isotope geochemistry for comprehensive insights.