The whole history of sedimentary process of the Early Triassic bedded cherts

2018 Fiscal Year Final Research Report

• Project/Area Number: 16K05572
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Geology
• Research Institution: Utsunomiya University
• Principal Investigator: Aita Yoshiaki 宇都宮大学, 農学部, 教授 (90167768)
• Project Period (FY): 2016-04-01 – 2019-03-31
• Keywords: ニュージーランド / アローロックス島 / オルアテマヌ層 / 前期三畳紀 / 層状チャート / 堆積相解析 / 生物源堆積相 / 遠洋性粘土相

Outline of Final Research Achievements

The Early Triassic bedded chert sequences from Unit 2b to Unit 4 of the Oruatemanu Formation on Arrow Rocks, New Zealand are unique because the Early Triassic bedded chert was very rare elsewhere during the Chert Gap period. The SEM observations on the acid-etched chert slabs to unravel the sedimentary facies and microstructure of a single bedded chert resulted four microfacies recognized. Facies G1-3: pelagic facies with densely packed radiolarian shells (>70%), Facies G2: a chert matrix with a high biosiliceous component (30-70%), Facies G3: a chert facies with a few biosiliceous component (10-30%), and Facies E: a pelagic clay matrix with a low content (1-10%) of biosiliceous shells and spicules. Biogenic facies with abundant radiolarian shells (G1+G2+G3) indicates enhanced flux change up to 30-70% within the Units 2b-3 during the Griesbachian and mid Dienerian, while biogenic facies (G2+G3) ratio gradually decreased to 4.7-7.7% and pelagic clay facies dominated in the Unit 4.

Free Research Field

地質学・古生物学

Academic Significance and Societal Importance of the Research Achievements

本研究の成果に基づいて，P-T境界で海洋生物の96％が絶滅した後，前期三畳紀の海洋生態系が回復途上にある時期において，動物プランクトンである放散虫がどのようにP-T境界を生き残り，前期三畳紀の海洋において放散虫が繁栄することのできた唯一のレフュージ海域が南半球のニュージーランド海域に存在したことを明らかにしたことは学術的意義が大きい。前期三畳紀の層状チャート(Griesbachian - Dienerian)において放散虫殻を多産する生物源シリカ相が卓越するが，Dienerian中期からSpathianにかけて生物源シリカ相が次第に減少することから海洋環境の悪化が推察される。