Conodonts from the Kamiiso Limestone and Consideration of its Geological Age

Abstract

The so-called Matsumae Group which is widely distributed in the southwestern part of Hokkaido, is correlated with the Hidaka Group of the central part of Hokkaido. Recently, Middle Carboniferous and Upper Jurassic fossils were discovered from some of the limestones interbedded in the Matsumae Group. Namely, (1) HASHIMOTO and SHIMADA (1960) discovered a Diphyphyllum-like coral from the limestone near Oshamanbe. This coral probably indicates the Middle Carboniferous age. (2) HASHIMOTO and IGO (1961) reported Hikorokodium sp., Milleporella sp., Thecosmilia sp., Montlivaltia? sp. and Thanznasteria? sp. from a limestone interbedded in the same limestone. These fossils may be correlated with the fauna of the Upper Jurassic Torinosu Group. (3) MINATO and YAMAMOTO (1961) studied the crinoid stems, pelecypods, gastropods, hydrozoans, smaller foraminifers and calcareous algae found from the Kamiiso Limestone. Among those fossils, in particular, the calcareous algae was identified with Mesophyllum, a genus which is known only from the post Tertiary. However, they considered that the limestone that yielded Mesophyllum and other fossils is probably Upper Jurassic in age from lithologic and stratigraphic stand points. (4) MINATO and KONOYA (1963) reported Fusulinella sp. and Chaetetes sp., both which indicate the Middle Carboniferous from the limestone of the Matsumae Group at Ohira-yama in Kaminokuni-mura. And (5), the Middle Carboniferous fossils discovered by YOSHIDA and YAMAGUHI (1967) from several localities near the northeastern part of Daisengen-dake were studied paleontologically by MINATO and ROWETT (1967). (Nos. (1) - (5) refer to the same numbers in Fig. 1)
About ten years ago, one of the present writers, SAKAGAMI collected a small block of black muddy limestone with abundant and very small pelecypod fossils from the Kamiiso Limestone in the Garo quarry. Dr. Ichiro HAYASAKA suggested that the small pelecypods may be a species of Inoceramus because the shells show the prismatic layer-like structure characteristic of the genus.
In the summer of 1968, the writers collected limestone rock samples at intervals of about 50 m. in the Garb quarry along the Garonosawa River. These samples were subjected to a study of their conodont fauna and 22 species (10 species indeterminable) were discriminated from 11 localities. All of the identified conodont species are known from the Middle to Upper Triassic of North America and Europe, and in particular, Epigondolella abneptis and Epigondolella bidentata indicate the Late Triassic age.
In the number of individuals in a species, Epigondolella abneptis is predominant, namely 104 individuals were found from about 1kg. of rock sample from Loc. 09, but at Loc. 003 and Loc. 21 none were recognized. In the constitution of species from each locality, 18 species (including 6 indeterminable species of Hindeodella) were found from Loc. 004, 5 from Loc. 02, 4 from Loc. 09, 3 from Loc. 03, 2 from Loc. 08 and 1 species from each of the other 6 localities.
Recently, Triassic conodonts were reported from some localities in the Far East. Of them, ISHII and NOGAMI (1966) reported 8 species of conodonts from Bukit Kechil, west Malaysia, and amount them three, Epigondolella abneptis (as a confer), Enantiognathus ziegleri and Hindeodella triassica are common to the species from the Kamiiso Limestone. NOGAMI (1968) studied the Triassic conodonts from Timor, Malaysia and Japan and recognized 7 conodont assemblages.