Millennial-scale reworking of tephra in alluvial to shallow marine settings: Distinguishing pseudo-isochrons from genuine ones

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• 作者：Kyoko S. Kataoka^a ; ^{kataoka@gs.niigata-u.ac.jp" class="auth_mail" title="E-mail the corresponding author} ; Atsushi Urabe^a ; Yoshitaka Nagahashi^b
• 关键词：Tephra ; Pumice ; Reworking ; Resedimentation ; Crypto-tephra ; Isochron
• 刊名：Quaternary International
• 出版年：2016
• 出版时间：18 March 2016
• 年：2016
• 卷：397
• 期：Complete
• 页码：173-193
• 全文大小：4084 K

文摘

Primary tephra layers can provide isochrons in a stratigraphic sequence. Such isochrons are powerful tools in terms of correlation and chronology of strata, and they constrain a chronological framework for geology, geomorphology, archaeology and palaeo-environmental studies in the Neogene to Quaternary. However, reworking processes can re-distribute tephra materials in both time and space, so that a given tephra can form a new discrete layer later in the depositional sequence, or it can be distributed in low concentrations (no visible layers, i.e., crypto-tephras) over a range of different stratigraphic horizons. The present study, based on sedimentological, petrographical, and geochemical analyses of reworked tephras in Japan, evaluates the duration and persistence of reworking and the tephra mobilisation and delivery to watersheds. Furthermore, it discusses how a unique isochron can be constructed from tephra distribution in a sedimentary sequence. Examples presented here are from fluvial and deltaic sequences in the Japanese Quaternary, which contain several visible tephra layers and a certain amount of tephra materials from multiple sources at different stratigraphical horizons. In the Niigata Plain, 5.4 ka Numazawako tephra material continued to be reworked for more than 4000 years after the eruption of Numazawa volcano, and was re-deposited as reworked pumice grains consisting of a “visible” tephra layer in delta front deposits. Mixed tephra materials originating from an earlier eruption of Numazawa volcano and eruptions from other calderas were also identified. In the Tsugaru Plain, post-2.5 ka subsurface deposits have >10% glass shards content throughout the sequence. Glass shards were derived from several different tephra materials, such as those from the most recent eruption of Towada volcano (AD 915), and earlier caldera forming eruptions (30 ka, 15 ka). Some of the glass shards probably originated from erosion of exposed Pliocene pyroclastic bedrock. Thus, tephra reworking processes in alluvial to shallow marine settings can continue long after an eruption has ended, and can persist even during inter-eruptive (i.e., background sedimentation) periods. As tephra reworking involves complicated processes, careful observation and interpretations are necessary to evaluate whether tephra materials can truly provide an isochron for the strata in which they are found to exist.