No. 3.] 101

30. " Nappes " and "Klippes " in Central .

By Takumi NAGAO.

Department of Geology and Mineralogy, Hokkaido Imperial University, .

(Comm. by H. YABE, M.I.A., Mar. 13, 1933.)

The fact that the geological structure of the Ishikari coal-field in the central part of Hokkaido is very complicated, owing to Mesozoic and Tertiary deposits being intensely plicated, has been well known for a long time to the Japanese geologists. There the imbrication of strata by overthrusts, overfolds and recumbent folds are common and Cretace- ous rocks are at places underlainby or thrust over the Palaeogene Ishikari Series and the Neogene Poronai Beds.1) One of the most conspicuous tectonic features in this district is related to " Klippes." The uniqueexample of " klippes " hitherto well known in detailed structure is that in the northern field of the Oyubari coal-mines. This Oyubari " klippe " of which the real nature was unveiled for the first time by Professor H. Yabe and Dr. H. Imai, consists of an inverted rock-sheet of the Upper Ammonites Beds and Hakobuchi Sandstone lying flat on the beheaded isoclinal strata of the Lower Ishikari and Poronai. Later studies by the present writer and his associates now dis- closed the occurrence of several other " klippes " in the eastern part of the Ishikari coal-fields ; from north to south they are :

1) Mesozoic and Tertiary deposits of the Ishikari coal-field are tabulated below : 102 T. NAGAO. [Vol. 9, 1. Maruyama "klippe " (4 x 1.5 km. in axial lengths) situated to the southwest of the Oyubari " klippe," and consisting of the Upper Ammonite Beds and Hakobuchi Sandstone in normal order. The rock- sheet lies almost horizontally on Tertiary strata in a recumbent fold. 2. A small " klippe," elliptical in outline (2 x 0.7 km.), situated at a little north of the Noborikawa coal-mines, consists of an inverted Hakobuchi Sandstone with its brow bent downwards andunderlain by much disturbed and even inverted strata of the Lower Ishikari and Poronai. 3. Noborikawa " klippe," an elongate-elliptical " klippe " (5.5 x 1.6 km.), lying at the south of the Noborikawa coal-mines, consists of the Upper Ammonite Beds and Hakobuchi Sandstone in normal order and thrust on the faulted and steeply inclined strata of the Cretaceous, Lower Ishikari and Poronai. 4. A small subcircular one (0.5 x 0.8 km.), lying along the Nakaho- bets, a tributary of the Mukawa,Iburi province, consists of inverted Hakobuchi rocks resting on recumbent strata of the Hakobuchi and Poronai. All these "klippes " and the Oyubari " klippe " are situated close to the upturned (overfolded ?) autochthonous Cretaceous zone lying to the east, and the present writer considers the former as remnants of a formerly extensive "nappe (Decke) " parautochthonous in the sense of the Swiss geologists. This " nappe " (restored) hereafter to be called the Ishikari " nappe," may have extended over a considerable area forming a narrow zone in the eastern part of the Ishikari coal-field. A more interesting fact worthy of notice is the occurrence of two other " klippes " of very ancientrocks of the Kamuikotan System and associated serpentine in the next western zone of, as well as in its northern and southern extension beyond the limit of the Ishikari coal- field. 1. At the west of Momijiyama along the Yubari-gawa, there is a large mass of serpentine, more than 0.5 km. wide, exposed on the left bank of the river and unconformably overlain by the Wakkanappe Beds ; the two rock-formations form a mass which is surrounded by the Momijiyama Beds in a synclineand no doubt rootless as far as observed. Numerous huge blocks of serpentine and Kamuikotanrocks are also found in a small side-valley on the opposite bank, and this peculiar occurrence of exotic blocks can be explained only by assuming them as having been derived from the same rock-sheet once extended to this side of the river. No. 3.] " Nappes " and " Klippes " in Central Hokkaido . 103

2. In the vicinity of Osachinai along the Saru-gawa, Hidaka Province, there is a flat-lying sheet just of the same kind as the pre- ceding one, 2 km. by 2 km. in the areal extension, and resting upon the Momijiyama beds in a very disturbed and nearly vertical position. These two occurrences of huge exotic masses amid the Neogene formations have for a long time been left as puzzles. They can not obviously be blocks primarily deposited in the Neogene formations at the places where they are now found and later set free from them by denudation ; nor can they be boulders carried down by certain trans- porting agencies from their original sites far distant. Thenthe most credible explanation of this remarkable feature is that the masses are also erosion-relics of some rock-sheets tectonically transported from elsewhere over a considerable distance. The Kamuikotan System with associated serpentine is a complex characteristic of central Hokkaido ; it extends from the drainage area of the Uriu-gawa, one of the large tributaries of the Ishikari-gawa, due southwards to the eastern part of the Ishikari province where it culminates in the Yfibari Mountain. The two klippes of the Kamuikotan rocks and serpentine, cited above, lie to the west of this zone, and one of them 20 km. and the other 9 km. distant from it. So far as the present writer is aware, the Wakka- nappe Beds are always underlain by the next older, coal-bearing Yubari in the Ishikari coal-field ; hence that the former lies uncon- formably upon the Momijiyama serpentine mass is an evidence giving strong support to the belief of the far distant origin of thelatter, possibly from the outside of the coal-field. Consequently it is quite natural to assume the former existence of the second "nappe " over an extended surface of central Hokkaido, which judged from its posi- tion was built later than and pushed over the first, Ishikari " nappe." The younger one may be called for a while the Ezo " nappe." In the area along the Ishikari-gawa and lying between Oyu.bari and Shimizusawa the Ishikari and Poronai beds are in isoclinal folds and interrupted by several important thrust-planes. The Maruyama " Klippe ," stated before, which lies in the eastern part of this area, rests, according toMr. Saito, upon four small recumbent folds piled one upon another. It is not quite impossible to regard these minor folds as nothing but digitations in the brow of a large nappe forced to a plication in its forward movement. The above described structure may be best interpreted as due to the westward push of these strata especially under the influence of the two overlying rock-sheets (the Ezo and Ishikari) in forward movements which played an important role of " traineaux ecraseurs." Outside the area, presumably once 104 T. NAGAO. [Vol. 9, overlain by the Ezo "nappe," the Neogene and Palaeogene formations are thrown only in a number of open folds and are nowhere imbricated. The Kamuikotan Zone is, the present writer believes, a rock sheet thrust upon the younger deposits lying to its west. In the drainage area of the Uriu-gawa, there is a large serpentine mass, 10 km. and 16 km. in the axial lengths, which is closely related to the Kamuikotan Zone. Mr. Yamaguchi lately brought forth evidences that this mass, at least in its western half, lies with a nearly horizontal thrust-plane upon the steeply inclined strata of the Ishikari and Kawabata, and the under- lying Ishikari is exposed in several " windows." Mr. Sait6 likewise observed along the western foot of the Yubari-mountain an overthrust of the Kamuikotan rocks and serpentine upon the inverted Cretaceous. The complicated geological structure of central Hokkaido is no doubt a product of continued orogenic action of a long geological range. We have now good evidence for assuming that the building of the Ishikari "nappe " certainly took place very late in the Tertiary Period, as early suggested by Messrs Yabe and Imai ; or it is, more precisely expressed, post-Oiwake and pre-Takikawa, for, while formations younger than the Poronai are not involved in this disturbance in the Ishikari coal-field, there is found no trace of abrupt structural difference within the whole Neogene complexes from the Poronai to the Oiwake. The Ezo " nappe " which overrides the Ishikari " nappe " is certainly younger in origin, but also pre-Takikawa. Along the western border of the Kamuikotan zone, Mr. Otatsume observed the upper part of the Poronai and Momijiyama successively overlapping inverted Cretaceous on one hand and overlain by the Kamuikotan overfolded from the east on the other. The earliest phase of the orogeny therefore dates back to the Poronai epoch or, it is older. The early phases of this orogeny are apparently indicated by the first and rather abrupt appearanceof large and small block of serpen- tine and ancient rocks in certain horizons of the Momijiyama. The continued crustal movements, possibly consisted of the upheaval of an ancient massif and first building as well as the subsequent westward advance of some of the " nappes," was certainly accompanied by an intense denudation that went on side by side with the sinking of the epicontinental sea in the front where took place a thick accumulation of the sediments of the Momijiyama-, Kawabata- and Oiwake Beds. The former two formations, in which conglomerate layers arepre- dominating, recall strongly the Molasse of the Alps in the nature of deposition.