10:45 AM - 11:00 AM
[HQR05-07] Change in depositional environment of upper half of the early Pleistocene Inagi Formation, northern edge of the Tama Hills, central Japan
Keywords:Kazusa Group, Tama Hills, Inagi Formation, sedimentary facies, barrier-island system
The Tama Hills, distributing in the southern Kanto District is composed by the late Pliocene–early Pleistocene Kazusa Group. Depositional environment of the Inagi Formation, upper Kazusa Group has been estimated as delta front (Oka et al., 1984) or foreshore (e.g., Takano, 1994), which is distinguished from lower formations in Kazusa Group at the Tama area.
At an exposure on the northern edge of the Tama Hills, an important key tephra, NG-YR (ca. 1.4 Ma, Suzuki and Murata, 2011), has been reported. Above the NG-YR, a northwestward dipping cross-bedded sand formed by landward paleo-current were identified by Shirai and Imamura (2013). They and Kato and Shirai (2013) estimated that the cross bed was formed by flooding tidal current at tidal inlet or flood tidal delta within barrier-island system. Additionally, we described depositional environment of the upper half of the Inagi Formation, above the NG-YR tephra.
Cross beddings in the cross-bedded sand layer above the NG-YR dip not only northwestward but also southeastward, so we infer that the sand layer was formed at tidal inlet. Development of northwestward dipping gravity faults in the cross-bedded sand layer suggests depositional environment of the sand layer may be lagoon-side of tidal-inlet.
Above the northwestward dipping cross-bedded sand, 8m-thick alternation of pumiceous sand and tuffaceous mud overlies, which often show HCS succession. The alternation changes gradually bioturbated, but considerably sorted fine to medium sand with >30 m thick, which is interpreted as being deposited on bay-head-delta front. Bioturbated silt layer with <1 m thick overlying bioturbated fine to medium sand is interpreted as being deposited in saltmarsh on bay-head delta top.
References
Kato and Shirai (2016) JpGU2016 HQR15-P10.
Oka et al. (1984) 1:50,000 Geological Map of the Tokyo-Seinambu District. Geol. Survey of Japan, 148 p.
Shirai and Imamura (2013) JpGU2013 HQR23-P04.
Suzuki and Murata (2011) Geol. Soc. Japan, 117, 379–397.
Takano (1994) Geol. Soc. Japan, 100, 675–691.
At an exposure on the northern edge of the Tama Hills, an important key tephra, NG-YR (ca. 1.4 Ma, Suzuki and Murata, 2011), has been reported. Above the NG-YR, a northwestward dipping cross-bedded sand formed by landward paleo-current were identified by Shirai and Imamura (2013). They and Kato and Shirai (2013) estimated that the cross bed was formed by flooding tidal current at tidal inlet or flood tidal delta within barrier-island system. Additionally, we described depositional environment of the upper half of the Inagi Formation, above the NG-YR tephra.
Cross beddings in the cross-bedded sand layer above the NG-YR dip not only northwestward but also southeastward, so we infer that the sand layer was formed at tidal inlet. Development of northwestward dipping gravity faults in the cross-bedded sand layer suggests depositional environment of the sand layer may be lagoon-side of tidal-inlet.
Above the northwestward dipping cross-bedded sand, 8m-thick alternation of pumiceous sand and tuffaceous mud overlies, which often show HCS succession. The alternation changes gradually bioturbated, but considerably sorted fine to medium sand with >30 m thick, which is interpreted as being deposited on bay-head-delta front. Bioturbated silt layer with <1 m thick overlying bioturbated fine to medium sand is interpreted as being deposited in saltmarsh on bay-head delta top.
References
Kato and Shirai (2016) JpGU2016 HQR15-P10.
Oka et al. (1984) 1:50,000 Geological Map of the Tokyo-Seinambu District. Geol. Survey of Japan, 148 p.
Shirai and Imamura (2013) JpGU2013 HQR23-P04.
Suzuki and Murata (2011) Geol. Soc. Japan, 117, 379–397.
Takano (1994) Geol. Soc. Japan, 100, 675–691.