SIGNIFICANCE OF T H E SEDIMENTS FILLING
AN UPLIFTED CAVE A T TSUKIJMO--SAN ,
KANONJI CITY, KAGAWA PREFECTURE
Minoru
S A I ~ O
Introduction and Acknowledgements
Unexpectedly during observations on the coastal region in .the 1101th of Kanonji City, Kagawa Prefecture, t h e writer discovered a n uplifted cave filled with sediments of aqueous origin, but questionable as t o marine or terrestrial rhese sediments a s described later, are stratified, fine grained except for the granules or pebbles of hard rocks, and have yielded no megascopic fossils whereby their marine or ter
-
restx~al origin could be settled Because sediments similar to the ones filling the uplifted cave have not been found in t h e area of Tsukumo hill or Kanonji City, the deposits seem to be worthy of particular attention, which is the purpose of the present articleBefore proceeding, the writer thanks Professor Kotora HATAI of the Institute of Geology and Paleontology, Tohoku University, for his suggestions concerning the present work
Locality of the Uplifted Cave
T h e uplifted cave is situated a t about three kilometers north of Ranonji City, Kagawa Prefecture at the southern foot of Tsukumo-san (152 meters high above sea-level) near to the junction with the long beach stretching southwards to the mouth of the Saida River (Fig 1) Tsukumo-san which b ~ e a k s th e rather simple shore-line extending northwards from Kanonji City, consists of sanukitic rocks (andesitic) and tuff breccia These rocks are traversed by many joints having trends of northwest to southeast T h e cave is developed along these joints which are thought to have been their embryos (Fig 2).
The Uplifted Cave
The uplifted cave incised in the sanukitic rocks along previously formed joints has very irregular walls, which points to that the duration of erosion following the development was not a short one T h e cave is about four meters in height measured from about one and a half meters above the Recent normal high tide level, three meters in width a t its exposed basal part, about two meters in width where a raised wave cut notch is developed (Fig.
4),
and about two to two and a half meters in depth At the lowest exposed part of the sediments filling the cave there i s found another excavation or sea cave which measures one and a half meters in height of its entrance and three meters in width a t base (Fig 4 ) , being Recent i n geological ageAlthough the uplifted cave (Fig 2) was developed along previously formed joints, the irregularity of i t s walls in contact with the sediments filling the cave and moderate width of the cave all point to that the amount of erosion prior to deposition of the sediments must have been considerable, although the unconformity separating the sediments from the country rocks is a plastered one Regardless of such characteristic features i t is also noteworthy that the sedim ents filling the cave are lacking in large angular rocks derived directly from the country rocks and the granules and pebbles were not subjected to transportation wor thy o f mention
That the cave had been uplifted is evident from the development and features of the sediments filling the cave, and from the position of an uplifted wave cut notch at more than two meters height measured from about one and a half meters above the Recent normal high tide level
From the sediments filling the cave i f a n be assumed that the top of the cave was or may have been originally a t or near water-level, which would be about five and a half meters lower than a t present, or the water-level was that amount higher. This may be good evidence for fluctuation in water-level since the cave was made in the sanukitic rocks
Sediments Filling the Uplifted Cave
The details of the sediments filling the uplifted cave show the sequence given in F i g , 3, and the contact of them with the country rocks is a well plastered (Fig. 6).
The sediments filling the cave are very characteristic in several important features such a s , 1 ) the small pebbles and granules intercalated in the silty sediments a r e cemented with coarse to medium grained sand, 2) the granules, pebbles and breccia are all angular and comprise pitch-stone, sanukitic andesite and rarely also of granite (none show evidence of aqueous erosion), 3) vague cross-bedding structures, laminated very fine sandy silt, and very fine sand with laminations, 4) the pebbles, breccia or even granules show only random orientation, rather sporadic distribution in the sediments (Fig. 5 ) , are not water worn. do not form a conglomerate, and their matrix may be of very fine laminated sand, vex y fine sandy silt or they may be cemented with coarse to medium grained sand, and, 5 ) stratification of the sediments with nearly horizontal dip is characteristic from bottom to top (Fig 3).
These outstanding features of the sediments filling the cave strongly suggests 1 ) that they a r e not marine in origin (no shell fragments could be found)
,
2) that they were deposited under tranquil sedi- mentary conditions without the influence of currents strong enough to produce current bedding, 3 ) that neither weathering or erosional agencies were strongly active during deposition of the sediments because their products a r e not preserved in the sediments, 4) the prevailing physical conditions must have been mild if not moderate because such are reflected in the lithological features of the sediments, and, 5) that gradual changes were taking place during t h e depositional phase From the sediments and their features just mentioned and interpretation inferred from them, i t is thought that these sediments are terrestrial or lacustrine but not marine in origin.Terrestrial Origin of the Sediments
To explain the terrestrial origin of the sediments filling the cave i t must first be pointed out that the general geology of t h e area should be considered.
It was already pointed out by SAITO(') (1960) that terrestrial sediments of Pliocene age a r e distributed from Ehime Prefecture via Kagawa Prefecture to as far north as the northern part of the Chita Peninsula in Aichi Prefecture. These sediments which comprise the Pliocene lake deposits in Kagawa Prefecture was studied in detail by S A I I O ( ~ ) (1962) who not only described the distribution and characteristics of t h e respective stratigraphic units classified by him but also discussed the geological age and problems relating to the correlation of the units with other areas in the Japanese Islands, particularly of Southwest .Japan.
From the distribution of those deposits, area a s well as thickness of the sediments and the respective heights above sea level at present a s related with geological structure, i t can be inferred that not only the larger part of Kagawa Prefecture was flooded but also that the depth of the lake must have been either considerable or that subsidence was more or less equal o deposition. And, such phenomena resulted in the cave becoming filled by sediments during gradual subsidence of t h e sedimentary basin whereby fine grained sediments were laid down in layers with intercalated sporadic angular rocks of granule to pebble size with random orientation This latter condition is not common among sediments of marine origin whereas they are not rare in those of terrestrial or lacustrine origin
Significance of the Sediments Filling the Cave
The sediments filling the uplifted cave are considered to be a relic of the Pliocene lake deposits of extensive distribution a s already mentioned because of the reasons already mentioned. These sediments are only several meters thick in lateral extension and about four meters in vertical scale, but are well stratified and contain no angular conglonlerate in situ or large angular rocks derived directly from the country rocks. The contact of the mentioned sediments with the country rocks is plastered and irregular
Such features of the sediments under consideration and discussed in earlier lines of this article, all point to that there was considerable erosion and weathering and probably also collapse of the cave roof and removal of those products before deposition of the stratified sediments had commenced, and even deeper and more extensive erosion after the sediments were uplifted The cave is thought to have developed under both subaerial and subaqueous conditions by weathering and erosion along joint planes and assisted in part by the removal of the waste products.
That sediments identical with or even similar to the ones filling the cave have not been found in t h e neighborhood or in the area of Kanonji City and Tsukumo-san, point to that they have been completely removed from the area by erosion subsequent to their formation. T h i s period of erosion ranged from post-Pliocene to Recent, even though there was fluctuation of the sea-level several times as noted from the terraces in different heights, the marine sedimets distributed below (down to about 10 meters depth) the level of the alluvial plain, and from t h e development of wave cut notches a t different heights along the rocky coasts.
From the sediments filling the uplifted cave as mentioned above, it is to be expected that similar ones will be found in other areas both bordering the Setouchi Sea and on the islands offing the coasts of Kagawa and Ehime Prefectures Although no fossil evidence was found from the sediments filling the uplifted cave, the various features and/of lithological evidence appear to be sufficient for the above mentioned conclusion
(Received June 13, 1962)
References
(1) SAIIO, M
,
BANDO, Y : Plio-Pleistocene (2) - -- : The geology of Kagawa and Northern strata of the Inner Zone of Shikoku, Japan (in Ehime Prefectures, Shikoku, Japan, Mem FacJapanese), S c i . R e p Tohoku
Unzv
, 2nd Ser , Agr,
K a g a w a Unzv , (101, 1-74 (1962). Spec Vol,
(4),
576-582 (1960)Explanation of Plate
Fig 1- Distant view of the Tsukumo hill, (152 meters high above sea-1evel)seen from the south Figs. 2,4- Situation of the uplifted cave and sequence of the sediments filling the cave
Fig. 3- An enlarged portion of the lower part of Fig 2
Showing the stratification of the sediments with nearly horizontal dip is characteristic Fig 5- An enlarged portion of the part shown by the pick in the right lower half of Fig 2
Fig 6- T h e relic of t h e mentioned sediments observed in the dent on the surface of the sanukitic rocks and the contact of those sediments with the country rocks is a well plastered
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