Presentation Information
[O12-P20]Estimating the formation and marine environment of Kikaijima Island from fossil assemblages
*Sota Tsujimura1,2 (1. Kikai High School, Kagoshima, 2. KIKAI Institute for Coral Reef Sciences)
Keywords:
Ryukyu Group,Shimajiri Group,Kikai Island,Fossil
Kikaijima is an island that is part of the Amami Islands of Kagoshima Prefecture. Kikaijima is an island that has uplifted approximately 200 m over the past 100,000 years. The island formed by the deposition of coral reef limestone while the island was uplifting. The basement rocks are represented by the Shimajiri Group, which are primarily composed of sand and mud that accumulated and cemented on the seafloor. The Chinen Formation lies above the basement, and represents a shallow-sea environment during the transition to coral reef formation. The coral reef limestones deposited above the Chinen Formation are classified as the Ryukyu Group and consist primarily of limestone containing fossil corals and other organisms. The Ryukyu Group is composed of the Hyakunodai Formation (400,000–850,000 years ago) and the Wan Formation (40,000–150,000 years ago). This study focuses on the Hyakunodai Formation, which is composed of Middle Pleistocene coral reefs. The objective is to reconstruct the marine environment of that time and clarify the conditions for coral reef formation and environmental changes by analyzing the coral fossils.
The site for collecting fossil samples is in Shiomichi, located in the northeastern part of Kikai Island. Since the stratigraphy of Shiomichi has been described in previous research (Matsuda et al. 2023), this outcrop was selected as the study site for this research. At Shiomichi I investigated the stratigraphy starting from the boundary between the outcrop and the exposed road. Coral fossil samples were collected for analysis. A rock hammer, chisel, and flat chisel were used for collection. The collected samples were cleaned at the Kikai Institute for Coral Reef Science. The fossil specimens were prepared using a rock hammer and a rock cutter to observe the morphology of the coral polyps. The processed coral fossils were examined using a microscope to observe the morphology of the individual corals, measure the inner diameter of the coral polyps, and count and examine the septa, to identify them to the genus level.
I investigated 6 m of outcrops in the study location. The lower 2 m is the Shimajiri Group sediments. Above that is 40 cm of the Chinen formation. The upper is the Hyakunodai Formation limestone. We searched for coral fossils within the Hyakunodai Formation. One coral fossil was collected at 4 m , and two fossils were collected at 5 m. Genus-level identification of these samples revealed that all three fossils belonged to the genus Favia sp.
These results indicate that the lower part of the Hyakunodai Formation contains corals of the genus Favia. A review of the distribution range of the genus Favia in “Coral of the World” revealed that it is primarily found in shallow tropical and temperate zone seas. According to NOAA’s SST (Sea Surface Temperature) isobath maps, the distribution of Favia corresponds to surface temperatures ranging from 20°C to 31°C. Based on this, during the Middle Pleistocene, when the Hyakunodai Formation of the Ryukyu Group formed, the waters around Kikaijima were in oceanic conditions conducive to coral reef development. Moving forward, we aim to increase the number of samples and conduct comparisons with other sites to reconstruct the coral reef formation processes and environmental changes on Kikaijima.
The site for collecting fossil samples is in Shiomichi, located in the northeastern part of Kikai Island. Since the stratigraphy of Shiomichi has been described in previous research (Matsuda et al. 2023), this outcrop was selected as the study site for this research. At Shiomichi I investigated the stratigraphy starting from the boundary between the outcrop and the exposed road. Coral fossil samples were collected for analysis. A rock hammer, chisel, and flat chisel were used for collection. The collected samples were cleaned at the Kikai Institute for Coral Reef Science. The fossil specimens were prepared using a rock hammer and a rock cutter to observe the morphology of the coral polyps. The processed coral fossils were examined using a microscope to observe the morphology of the individual corals, measure the inner diameter of the coral polyps, and count and examine the septa, to identify them to the genus level.
I investigated 6 m of outcrops in the study location. The lower 2 m is the Shimajiri Group sediments. Above that is 40 cm of the Chinen formation. The upper is the Hyakunodai Formation limestone. We searched for coral fossils within the Hyakunodai Formation. One coral fossil was collected at 4 m , and two fossils were collected at 5 m. Genus-level identification of these samples revealed that all three fossils belonged to the genus Favia sp.
These results indicate that the lower part of the Hyakunodai Formation contains corals of the genus Favia. A review of the distribution range of the genus Favia in “Coral of the World” revealed that it is primarily found in shallow tropical and temperate zone seas. According to NOAA’s SST (Sea Surface Temperature) isobath maps, the distribution of Favia corresponds to surface temperatures ranging from 20°C to 31°C. Based on this, during the Middle Pleistocene, when the Hyakunodai Formation of the Ryukyu Group formed, the waters around Kikaijima were in oceanic conditions conducive to coral reef development. Moving forward, we aim to increase the number of samples and conduct comparisons with other sites to reconstruct the coral reef formation processes and environmental changes on Kikaijima.