5:15 PM - 7:15 PM
[HQR05-P03] Identifying unconformity between the Pliocene and Pleistocene and a study on stratigraphic reconstruction in the eastern Shimokita Peninsula, Aomori Prefecture, Japan.
Keywords:Katchi Formation, Pliocene, Late Pleistocene, stratigraphy, tephra, age
1.Introduction
The Pliocene to Early Pleistocene strata, mainly sandy layers, is widely distributed in the eastern Shimokita Peninsula of Aomori Prefecture. In the northern part of the Kamikita Plain, this stratum is called the Katchi Formation (F.) [1]. A geological survey on the southern bank of Lake Takahokonuma revealed that the Katchi F. can be divided into upper and lower parts by a tilt unconformity. We also confirmed four layers of tephra, two in the upper layer and two in the lower layer. As a result of dating these layers, it is considered that the tilt unconformity has a time gap of about 1 Ma. These identifications suggest that the stratum previously considered to be Katchi F. may be subdivided into two layers in the vicinity of this survey site. We consider that these findings are important for reconstructing the stratigraphy of the Pliocene to Early Pleistocene strata and elucidating the history of the geological structure development; and therefore, we hereby report them below.
2. Survey area and geological overview
We conducted outcrops and boring investigation, and double dating of zircon grains in tephra samples using the fission track (FT) method and the U-Pb method [2] were carried out at the study area. Dating was commissioned by Kyoto Fission Track Co. Ltd., and they used a laser ablation inductively coupled plasma mass spectrometer.
The stratigraphy of the study area consists of, from the bottom up, the Takahoko F. (the Miocene), the Katchi F., marine terrace deposits that correspond to Marine Isotope Stage (MIS) 5e (123-130 ka) [3], and eolian cover deposits, we will mainly discuss the facies and structure of the lower and upper parts of the Katchi F. ( LK and UK ) .
The LK consists of sandstone and often contains thin layers of mudstone and conglomerate in addition to two layers of tephra. Tentatively, we name the lower tephra TKN1 and the upper tephra TKN2, after the survey site.
The UK consists mainly of alternating layers of sand and silt, accompanied by a layer of rounded gravel. In addition, there is poor lateral continuity, and multiple levels of poorly sorted sand gravel mixed with silt and humus deposits can be seen. Tephra is sandwiched between the humus deposits near the base and middle of the UK. Tentatively, we name the tephra near the base TKN3, and the tephra near the middle TKN4.
The Takahoko F. and the LK form an asymmetric synclinal structure.
The thicker strata in the UK can be traced roughly horizontally between outcrops.
3. Dating
Dating was carried out on TKN1, TKN2, TKN3, and TKN4. As a result, ages of 3.9±0.4 Ma, 3.8±0.4 Ma (FT; 1σ), and 4.0±0.1 Ma (U-Pb; 2σ) were obtained from TKN1, and 3.7±0.3 Ma (FT; 1σ) was obtained from TKN2, 1.3±0.2 Ma (FT; 1σ) and 2.2±0.0 Ma (U-Pb; 2σ) were obtained from TKN3, 0.5±0.1 Ma (FT; 1σ) and 0.4±0.0 Ma (U-Pb; 2σ) were obtained from TKN4. Due to the difference in zircon closure temperatures between the two methods, the FT age is used for TKN3. Therefore, the depositional age of TKN3 is estimated to be about 1.3 Ma.
4. Considerations and future developments
From the above, LK corresponds to the Pliocene and UK to the lower to middle Pleistocene.
In the future, if a survey of the Katchi F. (especially the key layer), which is widely distributed from this study site to the southeastern coast of Mutsu Bay and west of Lake Ogawara, reveals that strata boundaries with age gaps are distributed in the study site, it may be necessary to redefine the Pleistocene UK as a separate formation independent of the Katchi F.
[1] Kudo et al. (2021) Geological map of Japan 1:200,000, Noheji (2nd edition). [2] Iwano et al. (2020), A technical note. Isl. Arc, 29, 1-10. [3] Lisiecki and Raymo (2005) Paleoceanography, 20, PA1003, doi:10.1029/2004PA001071., [4] Danhara and Iwano (2017) Japan Assoc. Quat. Res. 2017 Meet., Abstr., P-10.
The Pliocene to Early Pleistocene strata, mainly sandy layers, is widely distributed in the eastern Shimokita Peninsula of Aomori Prefecture. In the northern part of the Kamikita Plain, this stratum is called the Katchi Formation (F.) [1]. A geological survey on the southern bank of Lake Takahokonuma revealed that the Katchi F. can be divided into upper and lower parts by a tilt unconformity. We also confirmed four layers of tephra, two in the upper layer and two in the lower layer. As a result of dating these layers, it is considered that the tilt unconformity has a time gap of about 1 Ma. These identifications suggest that the stratum previously considered to be Katchi F. may be subdivided into two layers in the vicinity of this survey site. We consider that these findings are important for reconstructing the stratigraphy of the Pliocene to Early Pleistocene strata and elucidating the history of the geological structure development; and therefore, we hereby report them below.
2. Survey area and geological overview
We conducted outcrops and boring investigation, and double dating of zircon grains in tephra samples using the fission track (FT) method and the U-Pb method [2] were carried out at the study area. Dating was commissioned by Kyoto Fission Track Co. Ltd., and they used a laser ablation inductively coupled plasma mass spectrometer.
The stratigraphy of the study area consists of, from the bottom up, the Takahoko F. (the Miocene), the Katchi F., marine terrace deposits that correspond to Marine Isotope Stage (MIS) 5e (123-130 ka) [3], and eolian cover deposits, we will mainly discuss the facies and structure of the lower and upper parts of the Katchi F. ( LK and UK ) .
The LK consists of sandstone and often contains thin layers of mudstone and conglomerate in addition to two layers of tephra. Tentatively, we name the lower tephra TKN1 and the upper tephra TKN2, after the survey site.
The UK consists mainly of alternating layers of sand and silt, accompanied by a layer of rounded gravel. In addition, there is poor lateral continuity, and multiple levels of poorly sorted sand gravel mixed with silt and humus deposits can be seen. Tephra is sandwiched between the humus deposits near the base and middle of the UK. Tentatively, we name the tephra near the base TKN3, and the tephra near the middle TKN4.
The Takahoko F. and the LK form an asymmetric synclinal structure.
The thicker strata in the UK can be traced roughly horizontally between outcrops.
3. Dating
Dating was carried out on TKN1, TKN2, TKN3, and TKN4. As a result, ages of 3.9±0.4 Ma, 3.8±0.4 Ma (FT; 1σ), and 4.0±0.1 Ma (U-Pb; 2σ) were obtained from TKN1, and 3.7±0.3 Ma (FT; 1σ) was obtained from TKN2, 1.3±0.2 Ma (FT; 1σ) and 2.2±0.0 Ma (U-Pb; 2σ) were obtained from TKN3, 0.5±0.1 Ma (FT; 1σ) and 0.4±0.0 Ma (U-Pb; 2σ) were obtained from TKN4. Due to the difference in zircon closure temperatures between the two methods, the FT age is used for TKN3. Therefore, the depositional age of TKN3 is estimated to be about 1.3 Ma.
4. Considerations and future developments
From the above, LK corresponds to the Pliocene and UK to the lower to middle Pleistocene.
In the future, if a survey of the Katchi F. (especially the key layer), which is widely distributed from this study site to the southeastern coast of Mutsu Bay and west of Lake Ogawara, reveals that strata boundaries with age gaps are distributed in the study site, it may be necessary to redefine the Pleistocene UK as a separate formation independent of the Katchi F.
[1] Kudo et al. (2021) Geological map of Japan 1:200,000, Noheji (2nd edition). [2] Iwano et al. (2020), A technical note. Isl. Arc, 29, 1-10. [3] Lisiecki and Raymo (2005) Paleoceanography, 20, PA1003, doi:10.1029/2004PA001071., [4] Danhara and Iwano (2017) Japan Assoc. Quat. Res. 2017 Meet., Abstr., P-10.