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The Siljan Ring of Paleozoic, Central Sweden: a Posthumous Ringcomplex of a Late Precambrian Dala Porphyries Caldera

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The Siljan Ring of Paleozoic, Central Sweden: a Posthumous Ringcomplex of a Late Precambrian Dala Porphyries Caldera G E O L O G l E E N M IJ N B O U W 45e JAARGANG PAGINA 125-136 MEI 1966 THE SILJAN RING OF PALEOZOIC, CENTRAL SWEDEN: A POSTHUMOUS RINGCOMPLEX OF A LATE PRECAMBRIAN DALA PORPHYRIES CALDERA M. G. RUTTEN l) SUMMARY Alrhough situated a:bout halfway between ero­ The Dala Porphyries, recently mapped by sional outliers of the epicontinental Paleozoics H j e m u from a volcanic province of late l q i s t, cover of the Fennoskandian Shield to the South Precambrian age in central Sweden, attaining a (250 km) and the geosynclinal Paleozoic of minimum volume of volcanics of 150 km3. The Siljan ring of early paleozoics, distinguished from the early the Caledonian foldbelt to the north (1 50 km), Paleozoic epicontinental cover of the Fennoskandian campare fig. l, the Siljan Paleozoics are quite Shield by its chieker facies, with reef limestones, and distinn from both. by its peculiar, strong tectonization, occupies ring a Although of epicontinental character, the Sil­ 45 km in diameter and 5 km to 10 km wide, to the east of the Dala Porphyries. jan Paleozoics differs from the ,stable interior" lt is postulared that .he Siljan ring structure Pa-leoz:oic cover Jurther south in two ways. The developed as a result of posthurnous movements at first regards its stratigraphy, where the Cambrian the site of the ring-complex surrounding a caldera is absent, but a part of the Ordavieian is deve­ from which the Dala Porphyries had erupted earlier. loped in a much thicker facies, with two reef CONTENTS horizons. The seeond regards its strong tecto­ Summary . 125 Introduction . 12 5 nization, through block faulting, tilting and re­ The early Paleozoic epicontinental cover of lated movements. The strike of this tectoniza­ the Fennoskandian Shield . 126 ion, peculiarly, is tangentional to, and follows The Siljan Paleozoics . 127 the ring structure closely. From the Caledonian Tectonics of the Siljan ring . 130 Paleozoics the Silan Paleozoics differ in being of Siljan as a meteoric crater? . 130 The Dala Porphyries . 130 epicontinental, not of geosynclinal facies, and Dala Porphyries and volcanocecconics . 132 in having only suffered the above mentioned The Dala Granite . 133 peculiar type of block faulting tectonics, and History of the Dala Porphyries and the Siljan no folding or nappe formation. Paleozoics . 136 The Dala Porphyries, of unknown, bur late Pre­ INTRODUCTION carobrian age, cover an area of some 1500 km2, The Siljan Paleozoics, mainly consisring of stra­ southwest, west and norrh of Siljan. They extend ta of Ordavieian and Silurian age, form a ring up ro, and disappear underneath of, the eastern like srrucrure situated entirely within the Pre­ border of the Caledonian fold helt. They carobrian of Central Sweden. The ring is between are overlain by the shallow syncline of the 5 km and l O km wide, and has an inner diame­ Jornian Sandstones, of younger, :hut equally l31te ter of 45 km. Precambrian age (fig. 9, c), hut else have not 1) Geologisch lnstituut, Rijks Universiteit te Utrecht suffered much tectonization since their forma­ Holland. tion. 126 The Dala Porphyries have recently been in s o n, ed. 1960). But i t ma y weil have occurred part ioterpreted as ignimbrites (Hj e l m q u i s t in rhythms of even shorter duration. For exam­ 1956), a view I should like not only ro subscribe ple, work now in progress on the island of to, bur to extend. Most of the Dala Porphyries Öland seems to indicate the existence of such show the massive, uniform structure, which is oscillations between each single bed in the well best explained as formed through fluid ash known Orthoceras Limestene of the Limhata flows. Only a very minor part shows the cha­ Stage of the Ordovician. Each bed of this series racteristic turbulent laminar flow of a highly shows signs of karst erosion on ·its upper surface, viscous lava. indicating a regression after its deposition. This The thickness of the Dala Pohrpyries is diffi­ indicates a play of rhythmic transgressions and cult to estimate, owing to the fact that a thick regressions of the order of each single bed, glacial cover leaves less than l % of the area some 10 cm to 20 cm in thickness, over an exposed. In the few post-glacial fluviatile river area of at !east 40 km in diameter. canyons which cut through the porphyries, indi­ During the early Paleozoic there has evidently vidual flows are, however, seen w have mini­ reigned on the southern part of the Fenno­ mum thicknesses varying from 20 m to 30 m. skandian Shield, a horizontaliry which we have, An estimated minimum thickness of 100 m for from our present environment, the utmost diffi­ the total series seems rherefore to be very culty to concieve. conservative. The actual thickness may weil be lt is, however, the kind of horizontality which considetable higher. we also find in other sedimentary environments. Even at a thickness of 100m, the now preser­ The, now almost classic, example being that ved volume of the Dala Pophyries would be of of the Coalmeasure eyelothems (cf. Wa n - the order of 150 km3, a figure which cortesponds l e s s, 1963). Anodher striking example has weil with other acid ignimbritic eruptions. more recently been described from the Belgian To my knowledge, the Siljm ring of Paleozoics Visean limestenes by M i c h o t and coworkers and the Dala Porphyries have not been earlier (M i c h o t, et al., 1963, P ide t, 1963). brought into a eausal relationship 2. I must Such horizontaliry seems normal for most therefore stare which characters of these seemingly shaHow sea sedimentation basins in the geologic so different crustal elements point to their com­ past, but which is quite different from our mon origin. I will begin with a short analysis of present environment. the Siljan Paleozoics, and t'hen consicler the The resason for this lies in the fact that relevant characteristics of the Dala Porphyries. we now live in a period which is both inter­ Only after that a synthesis of the history glacial and post-orogenetic. This has resulted of both areas can be tried. Bur before being on the one hand in the drowning of all shelves able to understand in what way the Siljan Paleo­ by the eustatic sea leve! rise since the last glacia­ zoics differ from the normal development in tion. Whereas on the other, the Quaternary is the eipicootinental cover furhter South, we must characterized by strong vertical crustal move­ understand the main features of that area. ments, which have not yet by far been equalized throug'h erosion, peneplanation and sedimenta­ THE EARLY PALEOZOIC EPICONTINENTAL tion. COVER OF THE FENNOSKANDIAN SHIELD The more normal circumstances during geolo­ This region is characterized by deposits formed gic history were, however, those of the much on an extremely wide, and extremely horizontal, longer periods that were both non-glacial and completely peneplained, epicontinental platform. a- teet on i c (R u t t e n, 195 3). Durin g s uch Over this 'tihe most shallow of seas transgress­ periods the horizontality mentioned above was ed over thousands of square kilometers, only normally arrived at, because crustal movement.s to withdraw afrer a short time over comparable were easily offset by emsion and sedimentation, distances. and hence an equilibrium situation was normally This play of transgressions and regressions installed. lt is, however, difficult to visualize such has already been extensively described for inter­ circumstances from our present experiences, a vals comprising stages, or even parts of stages, thing always ro keep in mind, when applying where the existance of diasrems and periods the concept of actualism (L a f i t t e, 1949; of non-deposition can be proved paleontological­ R u t t e n, 1949). ly ( cf. T h o r s l u n d i n M a g n u s- For rhe epicontinental cover of early Paleo­ zoic on the Fennoskandian Shield this horizon­ 2) A s k l u n d has, in 1936, short! y mentioned the tality implies that we have to a:bstract ourselves possibility of a volcanotectonic origin of the Siljan also from our present notions of a coastline, of ring, hut failed to indicate where the volcanics erup­ ted from this structure were to be found. near-coastal and the like. The coast at any time 127 Fig. l - Index map of part of Skandinavia. l: Mesozoic. 2: Epicontinental Paleozoic c over. 3: Eastern border of Caledonian fold belt. 4: Siljan Paleozoics. 5: Oslo Graben. 6: Dala Porphyries. 7: Dala Granite. 8: Jornian Sandstone. 9: Earlier Precambrian. formed no eonstant line across rhe shield, nor 1960. A major difficulty lies in the fact that can it be said that for instance Central Sweden a !arge part of the structure is covered by was farther from the coast rhan Sourhern Sweden, Lake Siljan and by several other lakes. Moreover, Instead, the coast line swept back and forth as professor Thorslund writes: ,Almost every­ over most of the area, if not over all of it, w'here the beautiful Paleozocic rocks are hidden with everyrhythm of regression and transgression. by a dirty Quaternary cover". Good exposures This is atrested by the fact that not only the are mostly limited to the quarries opened in various paleontological stages, bur also many of the reef limesrones, and to occasional road and the sedimentation breaks, so patiently worked out railway cuts. It follows that, whereas the by the Swedish stratigraphers, can be followed srratigraphy of the Siljan Paleozoics is known in over most, or even all of the area.
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