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[HCG25-P05] A type of gravelly hyperconcentrated flow deposit distributed in the upper reaches of the Abe River, central Japan
Keywords:hyperconcentrated flow deposit, traction carpet, gravel layers, orientation, Abe River
Introduction
Hyperconcentrated flow (HCF) is often expressed simply as intermediate flow between water flood and debris flow, however various characteristics (grain size, grading, orientation and so on) of “hyperconcentrated flow deposit” have been reported by lots of workers.
In the upper reaches of the Abe River, stratified conglomerate estimated as HCF deposit (Shirai et al., 2020) is exposed. As a result of measurement of gravel orientation and others, the conglomerate may be regarded as a type of traction carpet, bedload of HCF deposit.
Outline of the studied deposit
The “Oya-Kuzure (Oya collapsed cliff)” located on headstream of the Abe River supplied vast volume of rock debris during the early 18th century which buried valleys of the upper reaches of the Abe River (e.g., Machida, 1959). The stratified conglomerate which occupies a part of the mass flow deposits consists of unconsolidated pebble and cobble layers with decimeter- to meter-order thickness. Absence of intrabed stratification and dispersing large boulders (outsized clast) have been often utilized as indicators of HCF deposits (e.g., Smith, 1987).
Gravel orientation
Description of the stratified conglomerate and detailed method of the gravel orientation measurement are omitted here because which have been introduced in Shirai and Utsugawa (2021). Gravel orientation of more than 200 samples are measured from each division (upper and lower halves of cobble layers, upper and lower halves of pebble layers). In cobble layers, gravels of long-axis transverse to flow direction are popular. Whereas, in pebble layers, gravels of long-axis parallel to flow direction are popular but gravels of long-axis transverse to flow direction are also considerably popular.
Discussion and conclusions
(1) Sohn (1997) referred that stratified deposit with coarsening- and fining-upward intervals are produced by temporal fluctuation of grain size (in traction carpet). However, our results of grading and gravel orientation show that cobble layers quite differ from pebble layers. It is more adequate to interpret that a couplet of a pebble layer (frictional region) and overlying cobble layer (collisional region) consist of a traction carpet at least, in our studied site.
(2) Sohn (1997) also referred that a traction carpet is formed by gradual aggradation. Whereas, our result show that frictional region (pebble layer) are probably transported and deposited en-masse and long-axis transverse gravels in the lower half of pebble layer reflect difference in velocity between frictional region and underlying static bed.
(3) Smith (1986) referred that larger clasts were transported by traction and smaller clasts were deposited from suspension in gravelly HCF based on bimodal distribution of long-axis orientation and that HCF deposits often exhibit normal grading. However, his photographs (Fig. 2 in Smith, 1986) show high possibility that his normally graded units were couplet of collisional region of a traction carpet and frictional region of overlying traction carpet. If our inference is valid, larger clasts and smaller clasts almost imply components of collisional region and frictional region, respectively.
(4) It is important to divide frictional region and collisional region for understanding traction carpet in gravelly HCF depoits. The outcrop of stratified conglomerate in the upper reaches of the Abe River will offer us significant knowledge on traction carpet of HCF deposit.
References
Machida (1959) Geographical Review of Japan 32, 520–531. (in Japanese with English abstract)
Shirai and Utsugawa (2021) Program and Abstracts, Annual Meeting of The Sedimentological Society of Japan, 45–46. (in Japanese)
Shirai et al. (2020) The Quaternary Research, 59, 17–29. (in Japanese with English abstract)
Smith (1986) Geological Society America Bulletin, 97, 1–10.
Smith (1987) Journal of Sedimentary Petrology, 57, 613–629.
Sohn (1997) Journal of Sedimentary Research, 67, 502–509.
Hyperconcentrated flow (HCF) is often expressed simply as intermediate flow between water flood and debris flow, however various characteristics (grain size, grading, orientation and so on) of “hyperconcentrated flow deposit” have been reported by lots of workers.
In the upper reaches of the Abe River, stratified conglomerate estimated as HCF deposit (Shirai et al., 2020) is exposed. As a result of measurement of gravel orientation and others, the conglomerate may be regarded as a type of traction carpet, bedload of HCF deposit.
Outline of the studied deposit
The “Oya-Kuzure (Oya collapsed cliff)” located on headstream of the Abe River supplied vast volume of rock debris during the early 18th century which buried valleys of the upper reaches of the Abe River (e.g., Machida, 1959). The stratified conglomerate which occupies a part of the mass flow deposits consists of unconsolidated pebble and cobble layers with decimeter- to meter-order thickness. Absence of intrabed stratification and dispersing large boulders (outsized clast) have been often utilized as indicators of HCF deposits (e.g., Smith, 1987).
Gravel orientation
Description of the stratified conglomerate and detailed method of the gravel orientation measurement are omitted here because which have been introduced in Shirai and Utsugawa (2021). Gravel orientation of more than 200 samples are measured from each division (upper and lower halves of cobble layers, upper and lower halves of pebble layers). In cobble layers, gravels of long-axis transverse to flow direction are popular. Whereas, in pebble layers, gravels of long-axis parallel to flow direction are popular but gravels of long-axis transverse to flow direction are also considerably popular.
Discussion and conclusions
(1) Sohn (1997) referred that stratified deposit with coarsening- and fining-upward intervals are produced by temporal fluctuation of grain size (in traction carpet). However, our results of grading and gravel orientation show that cobble layers quite differ from pebble layers. It is more adequate to interpret that a couplet of a pebble layer (frictional region) and overlying cobble layer (collisional region) consist of a traction carpet at least, in our studied site.
(2) Sohn (1997) also referred that a traction carpet is formed by gradual aggradation. Whereas, our result show that frictional region (pebble layer) are probably transported and deposited en-masse and long-axis transverse gravels in the lower half of pebble layer reflect difference in velocity between frictional region and underlying static bed.
(3) Smith (1986) referred that larger clasts were transported by traction and smaller clasts were deposited from suspension in gravelly HCF based on bimodal distribution of long-axis orientation and that HCF deposits often exhibit normal grading. However, his photographs (Fig. 2 in Smith, 1986) show high possibility that his normally graded units were couplet of collisional region of a traction carpet and frictional region of overlying traction carpet. If our inference is valid, larger clasts and smaller clasts almost imply components of collisional region and frictional region, respectively.
(4) It is important to divide frictional region and collisional region for understanding traction carpet in gravelly HCF depoits. The outcrop of stratified conglomerate in the upper reaches of the Abe River will offer us significant knowledge on traction carpet of HCF deposit.
References
Machida (1959) Geographical Review of Japan 32, 520–531. (in Japanese with English abstract)
Shirai and Utsugawa (2021) Program and Abstracts, Annual Meeting of The Sedimentological Society of Japan, 45–46. (in Japanese)
Shirai et al. (2020) The Quaternary Research, 59, 17–29. (in Japanese with English abstract)
Smith (1986) Geological Society America Bulletin, 97, 1–10.
Smith (1987) Journal of Sedimentary Petrology, 57, 613–629.
Sohn (1997) Journal of Sedimentary Research, 67, 502–509.