GEOLOGICAL OBSERVATIONS IN THE OPDAL-SUNNDAL-TROLLHEIMEN DISTRICT By OLAF HOLTEDAHL With 20 figures in the text. Abst rac t. Based on observations made during an excursion together with Prof. T. Barth in the Opdai-Sunndai district in the north-western part of Southern Norway in 1936, the conciusion has been arrived at that rock masses of the said district, hitherto generally regarded as beionging to the Archæan, must represent younger rocks, Iargeiy Eocambrian feidspathic sandstone (sparagmite) in highiy metamorphic facies. The present contribution gives the results of more comprehensive fieid investigations undertaken in 1937 with the object of settling more de finiteiy the question of the age, and aiso of the structurai conditions, of the rock-compiexes. In a following paper, Prof. Barth deais with the petroiogical and chemical character of rocks from the area. Contents. P age In troduction . .. .. .. .. .. 29 Å The Road Sections lbu-Gjøra .. .. ............ ......... .... ...... 34 The Mountain District between Driva River and Gjevii Vann .... ........... 41 Å The District North-East and South-East of ibu . .. ............. ...... ... 45 General Remarks on the Structure of the Districts deait with above ... .. ... 46 The District North of Gjevil Vann . .. .... ........ ... ....... .... .... 48 Remarks on Structural Correiations .......... .. .. .... ... ........ .... 51 Summary of Resuits . .. 52 Introduction. During a short sta y in the Overhalla district of Namdalen (somewhat north of the Trondheimsfjord) in June 1935 I took the opportunity of making a couple of small excursions to the Grong district somewhat further east in Namdalen, where on geological maps of Norway the boundary line between the (Cambro-Silurian) Trond- 30 OLAF HOLTEDAHL heim schists, to the east, and the supposed Archæan 1 to the west, is drawn, in a N. S. direction. Mr. S. Foslie, Norwegian Survey Geologist, who in later year has carried out geological surveying in the district in question, had kindly pointed out to me, beforehand, the site of the said division line near Grong railway station. Being rather familiar with the structural conditions at the base of the Cambro-Silurian as they appear on the south-eastern side of the zone of deep-seated Caledonian deformation, stretching from western Finnmark in N.N.E. to the Stavanger Bergen region in S.S.W., I was struck by the complete absence of any basal unconformity in the Grong district. As a matter of fact it was, at any rate just from looking at the rocks in the field, very difficult to locate, exactly, the boundary plane between the coarse mica schist of the Trondheim series and the underlying gneisses. The rocks in the critical transitional zone were relatively fine­ grained quartz-feldspar-mica rocks with schistosity paraBel to that of the overlying and underlying masses. Another fact that struck me, was the augen-gneiss character of the upper part of the lower crystalline complex, as seen for a rather considerable N. S. distance, below the Trondheim schists. It was evident that if the rocks below the schists were primarily of Archæan age, then they had been so totally changed during the Caledonian period of orogeny that nothing was left of the Archæan structure. At that time I had undertaken to write, together with Prof. E. B. Bailey, the chapter on the Caledonides of north-western Europe for a new Regional Geology to be published by the Akademische Verlagsgesellschaft in Leipzig and it was therefore to me a most important matter to have seen personally the critical zone between the supposed Archæan and the overlying sediments of the Caledonian geosyncline also in other districts along the west side of the synclinal t Cf. e. g. "Geologisk oversiktskart over det sydlige Norge" (l: l mill.), Norges Geologiske Undersøkelse, 1915, and "Geologisk i:iversiktskarta over Norden. Skala l: l mill. Under medverkan av de nordiska låndernas geologiska under si:ikningar sammanstålld av A. Gavelin och N. H. Magnusson", Stockholm, 1933. In his paper "Hitteren og Smølen. Et bidrag til den norske fjeldkjedes geologi", Norsk Geologisk Tidsskrift, 2, No. 10, 1913, p. 27 (map. p. 19) J. Schetelig has proposed a Caledonian age for the gneisses and granites of the area in question, and for the coastal districts S. W. of the o uter part of the Trondheimsfjord. His ideas have not, however, been generally accepted. GEOLOGICAL OBSERVA TI ONS 31 areas of central and southern Norway. To this end I visited the Opdal district (cf. Fig. l) in jul y 1936. This district was chosen because I knew that very fine road sections existed in the critical belt, and because the information previously given by K. O. Bjørlykke 1 indicated a transitional character of the boundary zone between the metamorphic (Eocambrian) Sparagmite (sandstone) series and the gneisses below it, with a conformable succession of rocks, and finally because on a previous excursion to the district in question (for physiographical studies), I had noticed quartzitic rocks in a mountain south of the western part of Gjevil Vann, where Archæan rocks have been marked in the previously mentioned maps of 1915 and 1933. It should also be mentioned that Tornebohm in the map of his classical paper of 1896 on Central Scandinavia2 and in his "Geologisk oversiktskarta over Skandinavien" (l: l mill.), 1908, has marked "Åre" or "Seve" schists (which he regarded as metamorphic rocks belonging to the Sparagmite formation) in parts of the Driva valley west of the boundary line drawn in later maps, and further that Carstens3 has reported that he has followed the sparagmites of Opdal north­ wards into Trollheimen. The visit in Opdal was part of an excursion in various districts of Southern Norway undertaken together with Prof. Tom. Barth. He had recently worked with the "migmatization" of sedimentary rocks in the Appalachian zone of Eastern America, and was therefore particularly interested in the problems pointed to above. Two days work in the Opdal Sunndai district showed us that the gneiss complex below the well stratified Sparagmite series has a very close structural connection with the latter, and that highly metamorphic rocks, that in all probability were of Sparagmitian age, occurred rather far to the north-west together with coarse gneisses and gneiss­ granites. The writer has published a few data concerning the in­ vestigations mentioned above in a paper read in Helsingfors in l Det centrale Norges fjeldbygning, Norges Geologiske Undersøgelse, no. 39, 1905, p. 398 (with geological section; cf. also map p. 387). In the coloured map of Southern Norway accompanying the book, Bjørlykke has not marked sparagmite so far to the west as in the small map. l Grunddragen af det centrala Skandinaviens bergbyggnad. Kgl. Svenska Vet.-Ak. handlingar, vol. 28. 2 Av Trondhjemsfeltets geologi. Nyere undersøkelser. Norsk Geologisk Tids­ skrift, 7, 1923, p. l. 32 OLAF HOLTEDAHL \( SUNNDAL , � OPDAL �1. �3. 5. �l. 4. 6. Fig. l. (See explanation next page.) GEOLOGI CAL OBSERVA TI ONS 33 August 19361 and in the contribution to the new regional geology referred to above.2 In the summer of 1937 the writer took up a more systematic investigation of the geology of the said Opdai-Sunndal district, making excursions also in the more central parts of the Trollheimen Mountain district. I was during the field work accompanied by my son, Hans Holtedahl, who continued the l Trekk av det skandinaviske fje!lkjedestrøks historie. Nordiska naturforskarmiitet i Helsingfors 1936, p. 135. Here papers by Wegmann and Backlund, emphasizing the fundamental importance of Caledonian metamorphism and migmatization in the north-western part of Southern Norway, are referred to. 2 Bailey and Holtedahl, Northwestern Europe. Caledonides. Regionale Geologie der Erde, Vol. Palaeozoische Tafeln c: 2, "- • { l f und Gebirge, Leipzig, 1938, p. 21. See also Pl. \i li Il, Il. � -7 l ! l l Manuscript and illustrations were made ready for print "' 1 1 11 l l l during the later part of 1936. -.J 'li ,, l Fig Geological observations in the Opdal-Sunndal- Troll­ l N . l. li heimen district and 1937. The map area is marked in o.iJ 1936 � li: the inserted small map, where also the boundary line between l l l t l l what has generally been regarded as Archæan rocks (to the :i! 11 l west) and younger (Sparagmitian and Cambro-Silurian) ones has been drawn. l. Schist complex of supposed Cambro-Silurian age. Mainly ,, mi ca- and hornblende-schist, often garnetiferous; Iocally gneiss­ ,,,�/, l like. lntercalations of psammitic, and basic intrusive rocks. l l l S =soapstone. In the extreme east less metamorphic rocks (phyllitic and chloritic schists etc.) Highly metamorphic : ( 2. psammitic rocks, of supposed Eocambrian (Sparagmitian) age. Mainly quartz-feldspar-muscovite flagstone, commonly gneiss-Iike. Intercalations of mica schist and amphibolite. Locally quartzitic rocks. 3. Augen-gneisses (and other gneisses connected with them) occurring as layers mainly in group 2. 4. Light coloured granite or gneiss-granite or western localities. 5. The basal gneiss complex of the Lønset anticline. Augen-gneiss, banded gneiss, gneiss granite (rich in epidote). Furthermore: highly schistose gneissic rocks of uncertain origin, occurring together with quartzites in the northern part of the map area. 6. Red basal grani te of the Troll hetta area; schistose especially in upper part. Fig. 2. Somewhat simplified and generalized section through the mountain masses just north of Driva River. Norsk geo!. tidsskr. 18. 3 34 OLAF HOLTEDAHL surveying alone, after I had left the area. In the greater part of the map area of Fig. l the observations made are of a rather scattered, preliminary character. However, as they show features of consider­ able general interest, I have found that a short report at the present moment was not out of place. As photographs tell more about the structural character of rocks than many words, a considerable number, taken by me in the field (mostly in 1937), have been reproduced for the present article.