ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

Relation between mean altitude of mountain and its mean
Quaternary uplift rate is expressed by a function. It shows
a static relation between uplift and denudation resulting
from concurrent tectonics and denudation through the
Quaternary. Ratios of denudation to uplift show that J apanese
mountains have been considerably denuded, over
30% for some mountains. Coupling of two functions; one
expresses the relation between contemporary denudation
rate and mean altitude dispersion of drainage basin and
the other expresses the relation between mean altitude dispersion
and mean altitude of drainage basin, indicates
that denudation rate increases with mean altitude, in harmony
with landslide occurrence increasing in volume with
altitude dispersion. So the earth materials in the area with
a large altitude dispersion are inferred to be highly stressed
by active tectonic movements. Denudation processes may
be a process to release the stress stored in earth materials.
A sequence of changes in mean altitude resulting from
concurrent uplift and denudation simulated based on the
above relations indicates that the landform development is
divided into three stages; the developing, culminating and
declining stages. Japanese mountains show various stages
from the earliest to latest substages of the developing stage.
The stages depend on the uplift rates by tectonics which
reflect the plate motion and geological structures. The higher
the uplift rate is, the more advanced the stage is, except
for the three mountains in Central Japan whose stages
seem to be nearly the culminating stage, being independent
of uplifting.
TORU OKAMOTO
Fossil periglacial phenomena on karst since the Last Glacial
Stage in Akka Karst, Northeastern Japan
Tohoku Research Center, Forestry and Forest Products
Research Institute, Ministry of Agriculture, Forestry and Fisheries
Shimo-Kuriyagawa 72, Morioka, Iwate 020-01, Japan
The karstic areas in Japan are mostly covered with eolian
deposits, for example, tephras and long-range eolian dust
transported from inland China. These eolian deposits are
used as not only important time-markers but also implications
of environmental changes. It is probably useful to
provide more detailed information of surface environment
in karstic area than in other rocks, because superficial deposits
have well remained in dolines developed in karst.
Furthermore, most sediments in caves have remained essentially
unaltered since initial deposition. The purpose of
this paper is to clarify fossil periglacial phenomena since
the Last Glacial Stage in Akka Karst, northeastern Japan.
The Akka Limestone is distributed in the northeastern
part of the Kitakami Mountains. The Akka Limestone,
which has 50 km wide in N-S, 1-4 km wide in E-W and
NNW-SSE strike, is a member of the Akka Formation
(700 m thick) which consists of bedded limestone, alternating
beds of limestone and chert, and chert in upward sequence.
The Omoto River, Akka River, and Osanai River
flow eastward from non-limestone area of the Kitakami
Mountains and dissect the Akka Karst into blocks of karstic
plateau up to 400-700 m above sea level. More than
100 caves have been recognized in Akka Limestone. The
climate is characterized by warm humid summers and cold
wet winters. The mean annual precipitation and temperature
are about 1,300 mm and 10°C, respectively. Precipitation
is evenly distributed throughout the year with much
rainy season July to September. Snow often falls during
December to March.
In the study area the limestone bedrock is overlain by several
late Quaternary tephras which mostly erupted from
Towada caldera and Iwate volcanoes. The tephra overlying
a slope indicates that the slope has been stabilized since
the tephra fell. On the other hand, absence of a tephra
which is usually found at the bottom of dolines or the foot
of slopes implies slope instability after the falling of tephra.
In order to estimate the changes of slope stability, tephrochronological
study was undertaken at many exposures
and pits. The identified marker tephras in the study area
have been as follows: To-a (Towada-a: 915 AD), To-Cu
(Towada-Chuseri: 5.5 ka BP), To-Nb (Towada-Nanbu: 8.6
ka BP), To-H (Towada-Hachinohe: 12-13 ka BP), Iw-Od
(Iwate-Oide: 35-40 ka BP), To-OP I (To-Okoshi I, 60-70
ka BP).
Pleistocene tephras are observed on the limestone plateau.
At exposures on the edge of the plateau it is found that
To-OP I is unconformably covered with loamy soil included
To-H, and that buried chert and limestone breccia
layer with loamy matrix is directly overlaid by Iw-Od which
covered with slope deposits containing limestone and
chert clasts. The former was probably formed before To-H
fall. The latter is considered to be solifluctional deposits,
because it has terrace-like forms and angular materials well
oriented parallel to the former slope direction. Thin humus
horizon also developed in the uppermost part of this
breccia layer. Therefore, formation of breccia layer was
probably ceased before Iw-Od fall.
In 'the Central Kitakami Mountains two periods of mass
movement by periglacial processes are confirmed during
the Last Glacial Stage. The first period was in the early Last
Glacial, around 50 ka BP, and the second was in the late
Last Glacial, between 30 and 10 ka BP. At such times, involution
and solifluction lobes were formed in the
Northern Kitakami Lowland area.
The formation age of breccia layer is in accord with first
period of mass movement around 50 ka BP. Unconformity
was formed before To-H fall. This formation probably
agrees with second period of mass movement between 30
and 10 ka BP. On the steep slopes there are not Pleistocene
tephras at all. At some exposures on gentle slopes, it is
often observed that greenish black soil derived from limestone
bedrock is directly covered with To-Cu. This indicates
that Pleistocene tephras had already been denuded
before To-Cu fall. Therefore, it is considered that most of
Pleistocene tephras on the slopes in Akka Karst were re-
291