NORSK GEOLOGISK TIDSSKRIFT 47

PERMIAN SEDIMENTS, LAVAS, AND FAULTS IN THE KOLSÅS AREA W OF

BY

JOHANNES A. DONS & EMIL GYORY (Mineralogisk-Geologisk Museum, Sarsgt. l, Oslo 5)

Abstract. One of the most famous excursion areas in the Oslo Region has been remapped. Numerous new observations are given including a borelog of the Permian sedimen�ry sequence (the group) below the lavas.

CONTENTS Page Abstract ...... 57 lntroduction ...... 57 Sandstone ...... 60 The sub-Permian peneplane...... 62 The Permian sediments below the lavas (The Asker Group) ...... 62 The Kolsås Formation ...... 63 The Tanum Formation ...... 64

Basalt B1• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . • • • • • • . • • • • • 68 The sediments between B1and RP1lava ...... 68

Rhomb-porphyry lava RP1 • • • • . • • • • • . • • • • • • • • • • • . • • • • • • • • • • . • • . . • • • 70

Rhomb-porphyry lava RP11 ••••••••••••••••••••••••••••••••••: . • • • • 72

Basalt lava B3 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 7 2 Rhomb-porphyry dikes...... 73 The ring dikes ...... 7 4 Other dike ...... 7 4 Faults ...... 75 References...... 77

Introduction

Kolsås is a mes4-like hill situated 10 km W Df the centre of Oslo. In very generalterms, Kolsås can be regarded as a part of the scarp of the Krokskogen lava plateau. The nearly horizontal Permian lavas (rhomb-porphyry above basalt) are resistant to erosion and form the 58 JOHANNES A. DONS & EMIL GYORY often vertical cliffs. The underlying sediments (Permian sandstone and shale above Cambro-Silurian limestone, shale, and sandstone) are weaker and form the slopes and the surrounding lower ground, which has an elevation of about 100 m. The highest point of Kolsås is 379 m above sea level. Except for the very steep sides, most of the hill is wooded and critical points are therefore often not exposed for examination. As a suburban railway provides transport to the foot of this hill, Kolsås has become a very popular place for Sunday walks. Excursions to Kolsås are regularly arranged for students in geology and physical geography, and the Geological Museum has also arranged public geological excursions which, on each occasion, have been atten­ ded by several hundred persons. The Schools Service of the N atural History Museums of the University of Oslo has prepared a Kolsås ex­ cursion for schools with claf:s sets of relief models and specimens, as well as work-sheets with questions to be answered during the excur­ sion. Excursion guides to Kolsås have been published by ScHET�LIG & KrÆR (1918), FOYN (1952), and HOLTEDAHL & DONS (1957). The last systematic mapping of Kolsås was performed about 50 years ago on the scale l :25,000 and was published on the scale 1 :100,000. (BROGGER & SCHETELIG map 'Kristiania' 1917, published by Norges Geologiske Unders6kelse). The Geological Map of Oslo and District (HoLTEDAHL & DoNs 1957), scale l :50,000 was, in the case of the Kolsås area, based mainly on the old l: 25,000 maps. As part of a field course for students in geology, led by the senior author (J.A.D.), the Kolsås area was remapped during a few days in each of the years 1962-63-64. The basic aim of this course was training in field methods. The students worked in pairs and, as 6-8 pairs were working simultaneously, it was impossible for the leader(s) to control every observation. Supplementary information has been obtained from three boreholes and an east-west tunnel (see Fig. l) cut in the northern part of Kolsås (in connection with a water supply project for the Bærum district). Borehole No. l, driven 50.5 m down, only penetrated rhomb-porphyry lava RP1. No. Il started in a dike (18.2 m) and cut through 1.5 m of RP1 and 14.7 m of basalt B1 without reaching the bottom of the lava at a total depth of 34.4 m. No. Ill was 64 m deep and gave a core containing 13.6 m of RPv 21.4 m of B11 and 19.4 m of the Permian sedimentary sequence (see Fig. 3 column g). Two dikes (total core length 9.6 m) were also included in No. Ill. PERMIAN SEDIMENTS, LAVAS, AND FAULTS 59 � -N- �

\'�J

' 11:s Bosatt 83 RPw-- Rhomb porhyry RP11

� Rhomb porphyry RP,

� Ring dike l / Rhomb porphyrll sy..,itic li.-j {rp cmd dirabrase dik..

up� Frault with di�racement in m /down

tww Top of bQsall B, in m co.•l

,. ' ,. Ill - 4"...... ØOl Thicknns of a, in m Thicknns of srandston• 11 :Strratigrlll)hic columns ol lletw. a,rand RP, In cm Permirall l Q - g ••

Fig. l. Geological map of Kolsås. 60 JOHANNES A. DONS & EMIL GYORY

Some additional observations have been made in tunnels under the southern part of the hill. The topographic maps which were used during the field course, and which also served as the basis of the map (Fig. l) and blockdiagram (Fig. 5), are quite recent ones prepared by Bærum Kommune on the scale l: 5,000, contour interval 2 m. The block diagram (Fig. 5) has been drawn with the aid of Perspek­ tomat P-40 (F. Forster Apparatbau, Schaffhausen, Switzerland) which is a specialized pantograph. This apparatus easily produces blocks of normal axionometry, i.e. the projection rays are parallel to and perpendicular to the plane of projection. The angle of projection can be chosen between go and 60 °. The study program which forms the basis for this paper was designed by the senior author (].A.D.), who also drafted the manu­ script. Both authors have made individual appraisals of the data obtained. Figures, maps, and block diagrams were all drawn by Emil Gyory. Fig. 2 is one of the several stereopairs prepared by Wideroe's Flyveselskap as supplementary material for the teaching of geography in schools.

Ringerike Sandstone The uppermost part of the Cambro-Silurian, stage 10, the Ringerike Sandstone, underlies Kolsås. It is composed of thick beds of sandstone interlayered with thin beds of shaly sandstone or shales. The upper part of the Ringerike Sandstone preserved in Kolsås has a brownish red colour. At a lower level, as at Toppås on the southern side of Kolsås, the sandstone is greyish or greenish grey. In the lowland east of Kolsås, a grey conglomerate 3-5 m thick with calcareous pebbles was found. The stratigraphic position of this conglomerate is not known with certainty, but it must be low down in the Ringerike Sand­ stone formation. During the Caledonian orogeny, the Ringerike Sandstone was folded, so as to fotm shallow -syndines and ærtidines. Th-e sguthern limb of one such syncline is situated south of Kolsås. The core of the syncline underlies the central and northern parts of the hill where the sandstone beds are nearly horizontal. Construction of fold axes based PERMIAN SEDIMENTS, LAVAS, AND FAULTS 61

Fig. 2. Stereomodel of Kolsås. Photo Wideroe's Flyveselskap A/S, 3/8/1947. Altitude 3,160 m. Scale l: 15,000 on about 50 strike and dip readings on bedding-planes shows a nearly horizontal fold axis with a trend N 50 ° in the southern and western parts of the map area. In the northeastern area, the dip of the bedding planes is shallow and to the west. These readings do not fit in with the fold pattern of the rest of the area. Most probably, the fold axis turns north, and it may also have developed a southerly plunge. The syncline can be traced southwest from Kolsås for a distance of 25 km. 62 JOHANNES A. DONS & EMIL GYORY

Here it has been cut off by the Glittrevann cauldron. In good exposures, it can be seen that the relief of stress during folding took place princi­

pally in the thin shaly beds interbedded with the sandstone. The · observed rapid variations in thickness of sandstone beds may also, to some extent, be of tectonic rather than depositional origin.

The sub-Permian peneplane At Kolsås the sub-Permian peneplane separates Ringerike sandstone (below) from Permian red shales and sandstone (above). Both the resemblance in lithology above and below the peneplane and the very low angle of unconformity make it difficult to locate the peneplane exactly. On the southern slope of Kolsås, where the footpath leads up to the top, a ledge which is an exhumation of this peneplane has been developed at an elevation of 210 m above sea level. The angular unconformity here amounts to less than 10 °. Such a topographic feature is not often developed, since the contact is usually covered by scree. The peneplane is not marked on the map (Fig. 1), and in the block diagram the lower boundary of the Permian sediments is con­ structed by using the average of observed thicknesses.

The Permian sediments below the Javas (The Asker Group) Dr. G. Henningsmoen has studied this sequence for many years and has measured a great number of stratigraphic sections in the central part of the Oslo Region. He divides the sequence from bottom to top into three formations: Kolsås Formation, Tanum Formation, and Skaugum Formation. In a forthcoming paper, he will define these formations precisely. Nevertheless, he has kindly permitted us to use his nomenclature in the present study. The name Asker Group is introduced here, and is used to include the three formations men­ tioned above. Asker is the name of the township and parish west of Oslo and Bærum, where all three formations occur. The Asker Group, which in Kolsås has a total thickness of 23-35 m, is remarkably rich in calcareous material. The Kolsås Formation (more than 10 m thick) consists of red shales PERMIAN SED.IMENTS, LAVA$, AND .fAULTS 63 and red and green siltstone with scattered nodules of limestone and one or two 'limestone conglomerates'. The Tanum Formation (7-15 m thick) is composed of alternating beds of quartz conglomerate and light grey, often calcareous, sandstone. In more southwestern areas, at a distance of 8-10 km from Kolsås (Semsvik-Bergsmarka, etc.), these two formations are remarkably reduced. Here, however, a third unit, the Skaugum Formation of mainly pyroclastic sediments, overlies the quartz conglomerate with a thickness of more than 20 m. B. Playle reports (personal communication) volcanic conglomerate in one or two horizons, volcanic sandstone, and shaly sandstone. Vitric tuff has also been identified. The fossils found by HoLTEDAHL (1931) in Semsvik in the lower part of the Skaugum Formation (or upper part of the Tanum Formation) indicate that this sequence, which heralds the volcanic activity, is of lower Permian (or uppermost Devonian) age. At Billingstad, a locality between Kolsås and Semsvik, a boring was undertaken in 1964. A preliminary unpublished report by G. Henningsmoen (Fossil-Nytt 1965) shows that here the Kolsås Formation is 16.5 m thick, the Tanum Formation 17 m, and the Skaugum Formation 6-7 m thick, which means slightly greater thicknesses than at Kolsås. No complete section of Permian sediments is exposed on Kolsås, either in natura! exposures or in borings. Similarly, the tunnels inspected either did not cross the whole section, or else the conditions in them did not allow detailed studies to be made. The method of combining several short sections to give the complete sequence is not valid for this area because of the great variations in lithology observed within very short distances. During the field courses, students mea­ sured about fifteen shorter or longer sections, five of them are repro­ duced in Fig. 3 (Columns a, b, d, e, f). Column g in the same Figure shows the results from borehole Ill west of Knabberud, and the data for column c has been given by G. Henningsmoen. The Kolsås Formation has a presumed thickness of 10--15 m and is mostly composed of red-coloured shales (and sandstones), often with calcareous fragments. They alternate locally with green sandstone and shales. The upper boundary of the Kolsås Formation is drawn at the top surface of the red shales. Future work may show that it would be better to include in the Kolsås Formation the overlying 1.8 m of green shales and limy beds found in bon•hole III. 64 JOHANNES A. DONS & EMIL GYORY

Column g (borehole Ill) shows:

Red shale, finely laminated with a few thin beds containing limestone fragments ...... 4. 30 m Red sandstone with flakes of red shale and fragments of limestone . . 0.60 m Red shale, finely laminated ...... 0. 46 m lrregular limestone fragments in matrix of red shale, no bedding. 'Limestone pebble conglomerate' 0.10 m

Red shale, finely laminated ...... 1.14 m

The borehole ended here at a depth of 64 m without reaching Ringerike Sandstone. In the lower part of the Kolsås Formation (maybe in the equivalent to the lowest bed reported above) fragments of plant fossils (Cordiates ?) were found in a small quarry near the path which leads up to the top of Kolsås from the south. The find of these fossils was made by Mr. A. Lervik during an excursion in 1965 led by G. Henningsmoen. A slab of sand y red shale containing plant fossils of Neuropteris type was collected by Mr. H.-F. Grorud in 1964. It was not found in situ, but the material most probably came from the same quarry as mentioned above (written communication, Fossil-Nytt 1966, by G. Henningsmoen 15/12/1965). The red shales weather so easily that all small brooks running down the hillsides are red coloured after heavy rain. The exposures of red shales are so poor that detailed stratigraphic studies will be difficult. The Tanum Formation is composed of light grey calcareous sand­ stones and conglomerate(s), and has a thickness varying from 7 to 15m (see Fig. 3). Much of the calcite is thought to be present as detrital grains. Conglomerate is dominant in the southern part of Kolsås (columns c and d), while sandstone is more dominant both on the western and eastern sides. Here the conglomerate is developed in two or three beds with sandstone or dark grey shale between. Columns a, b, and g, as well as adj acent sections not included in Fig. 3, contain, above the upper quartz conglomerate (and in a a bed of sandstone), a multicoloured shaly bed. Dark blue-grey often changes to pale red or yellow in flames as 'flaser bedding'. This bed, which has a maximum thickness of l m, seems to contain considerable amounts of tuffaceous material and may be correlated with the pyroclastic series of sediments southwest of Kolsås (Skaugum Formation). PERMIAN SEDIMENTS, LAVAS, AND FAULTS 65

---0

c o

·-

E c... o

.._

sill 6.8m

c o

? cu 15 KEY E c... o • Basalt 81 .._ 1:·:·:1 Cong lomerate

 Limestone

! j Grey or green sandstone

"_" m "_" shale �

li!!!!!!@l Red sandstone

!l "_,, shale

Fig. 3. The Permian sedimentary sequence, the Asker group, in Kolsås. Localities are shown in Fig. 1. 66 JOHANNES A. DONS & EMIL GYORY

The usual fragment size of the conglomerate is in the range 'apple' to 'pea', though fragments over 20 cm in diameter have been found. The conglomerate is classified as oligomictic, because quartz and quartzite (often coated or pigmented throughout by hematite) are the main fragmenta! components. Pebbles of quartz mica schist, limestone, and shale have also been found. The quartz and quartzite pebbles are subrounded to rounded and have a medium to high spheric­ ity. Some conglomerate beds are rich in limestone fragments. The ratio quartzfcalcite in the matrix is subject to local variations. Cross­ bedding, usually of the 'torrential cross lamination' and 'trough cross stratification' types, is beautifully developed in the conglomerates and coarse sandstones. As the calcite-richbeds are more easily removed by weatheringthan the coarser quartz-rich parts, natura! 'tunnels' develop in the cross-bedded parts of the conglomerate. A waterfall issues from where a shaft was driven through the con­ glomerate in southern Kolsås. Especially near the point represented by column a (Fig. 3), at the western side of Kolsås, the 'tunnels' are occupied by foxes that have extended them deeper into the hillside. The inclination of foresets has been measured, and the maximum value obtained was 33°. The axes of the troughs are probably in an E-W direction; this would also be the current direction.

In section a, the Tanum Formation contains the following (see Fig. 3, column a):

Grey and reddish shale (Skaugum Formation ?) 0.10 m Coarse grey sandstone ...... 0.90 m Not exposed ...... 0.50 m Quartz conglomerate, mostly well-sorted. Beds with small pebbles alternate with beds containing large pebbles. In lower part, matrix

rich in calcite ...... 4.40 m Grey sandstone ...... 0.15 m Quartz conglomerate with sandstone beds, cross-bedding . 2.15 m Cross-bedded, well laminated sandstone 0.30 m Calcareous sandstone . . 0.20 m Light coloured sandstone 0.40 m Calcareous sandstone . . 0.10 m At this point the reporter had to move a few metres to the side to find an exposed continuation of the sequence. Green sandstone with red patches . . . . 1.00 m Transitional zone to underlying red shales . . . . 0.10 m PERMIAN SEDIMENTS, LAVAS, AND FAULTS 67

The Tanum Formation present in borehole Ill (Fig. 3, column g) is reported below without thickness corrections, which should be applied due to the fact that the borehole is vertical and the beds have a NW dip of 10-20 °. A sill has most probably not disturbed the se­ quence.

Light grey shale, faint lamination (Skaugum Formation ?) . . • • • 0.60 m Grey calcareous sandstone, faint lamination, irregular and gradational border to the shale above ...... 1.60 m Quartz conglomerate, matrix rich in calcite decreasing size and density of pebbles downwards ...... 0.90 m Impure limestone, light grey, nodular bedding 0.15 m Light green shale, finely laminated . . . .. 0.25 m Impure limestone, light grey, nodular bedding 0.20 m Grey calcareous sandstone, layering faint, partly with light nodules and dark groundmass ...... 1.30 m Dark grey calcareous sandstone, irregular lamination (flaser bedding) and containing undulating 0.5 cm thick beds of light green argillite . 0.75 m Grey calcareous sandstone, well laminated, darker grey near bottom . 0.50 m Graded bedding in argillite with 8 cm thick units . 0.20 m Dark grey fine-grained sandstone, faint lamination ...... 0.35 m Grey green argillite, finely laminated ...... 0.80 m Dark grey fine-grained sandstone, faint lamination; in lower part (0.25 m) more coarse-grained, calcareous, and grey-green 0.95 m Sill6.8 m Light grey (in lower part white) sandstone, faint lamination 1.85 m Conglomerate, red and grey quartz(ite) pebbles l cm in diameter 0.60 m Grey-green argillite, micro cross-lamination( ?) 0.50 m Grey impure limestone, nodular bedding ...... 0.30 m Grey-green argillite, micro cross-lamination(?) . . • . . . . . 0.30 m 'Conglomerate', matrix red shale, few irregular fragments of limestone,

diameter maximum 2 cm ...... 0.10 m Grey-green argillite, micro cross-lamination( ?) • . . . . . • • . . 0.50 m A mixture of the sediment above and red shale which follows below . . 0.10 m

The lowest 1.8 m may in future be included in the Kolsås Formation. It is believed that the limestone beds, the impure limestones showing nodular bedding, and 'limestone pebble conglomerate' may prove to be marker horizons in future work on the Asker Group. The Kolsås Formation is thought to represent a flood plain deposit or shallow lake deposit. The material of the Tanum Formation may come from two different sources. The calcareous part could be directly (as detrital grains) or indirectly (as chemical biochemical deposits) derived from the Silurian limestone which, at that time, most probably 68 JOHANNES A. DONS & EMIL GYORY

was exposed at the surface. The pebbles of red and white quartzite, pegmatite quartz, quartz mica schists, etc. are believed to come from a Precambrian area, e.g. the Telemark area to the west. This material may have been transported stepwise. The remarkable change in lithol­ ogy from the Kolsås Formation to the Tanum Formation may be the result of a change in topography, and it is tempting to assume that the Tanum Formation reflects fault movements connected with the beginning of volcanic activity. The trough cross stratification makes one think of fluvial deposition and deltas, but beach or nearshore marine conditions must also be considered until more regional knowl­ edge has been gained.

Basalt B1 The bluish black, non-porphyric type of basalt is the most common one in Kolsås. Types containing augite andfor plagioclase phenocrysts have also been found. The thickness of B1 varies in Kolsås between 20 and 30 m as indicated in Fig. l. Some of these values have been obtained by direct chain measurements, others are estimated by the use of map, barometer, etc. Extreme values such as 14 m and 34 m may be wrong owing to locally difficult field conditions. The thickness generally decreases from Sto N. West of Semsvik, at Bergsmarka and Glasåsen, B1 is lacking, even though nearby exposures show thicknesses of 15-29 m. Usually, the lowermost part of B1 in Kolsås is not vesicular, but locally agglomeratic (composed of fragments of basalt). The top layer is nearly everywhere porous and vesicular, and contains irregular, narrow veins of hematite coloured sandstone or tuff. The erosion of B1 before the emplacement of RP1 has therefore been negligible, or very local in the Kolsås area. No signs have been found which could support the idea that B1 in Kolsås is composed of more than one flow.

The sediments between B1 and RP1 lava From several localities in the Oslo Region, a bed of quartz-rich sedi­ ments has been reported to occur between B1 and RP1, e.g. Grefsen­ åsen 2 m thick (HoLTEDAHL 1935), 0.5-2 m thick (SÆTHER 1946), Sonsterud at Tyrifjorden 0.5-1 m thick (DoNs 1956a), Lier 1.5-3( ?) m thick (Playle, personal communication). Even though PERMIAN SEDIMENTS, LAVAS, AND FAULTS 69

mm 6 4 2 2 3 cm l l l l l l' .l Fig. 4. The border relationship between RP1lava above and sediment below. A is a thin section (between crossed nicols) taken from the specimen shown in photograph B. From a tunnel in the southem part of Kolsås. The sediment contains detrital quartz, some feldspar, and fragments of the underlying B1lava.

Schetelig reported as early as 1916 that irregularities in the upper surface of this basalt at Kolsås were filled with tuffaceous sandstone (containing windblown sand), Kolsås has frequently been described as an area where such a sediment is lacking. This is due to the fact that the sediment is not developed where the footpath leads up to Kolsås from the south. Here the RP1lava fillscracks in the B1 (which, however, is impregnated with red sandstone). In all other localities at Kolsås, a layer of sediments can be observed. The thickness varies from place to place (see the map Fig. 3) reaching its maximum (l m) at the southern end of Kolsås. On the western side of the hill, an undulating upper surface was observed for a distance of some tens of metres with an amplitude of 30 cm and wavelength of l m. There seems to be no 70 JOHANNES A. DONS & EMIL GYORY relationship between the thickness of B1 and the thickness of the overlying sedimentary rock, which could be expected if the sediment had been deposited in depressions on the lava surface. The deter­ minations of the thickness of Bv however, are not sufficiently reliable to allow us to be adamant on this point. Variations in the petrographic character of the sediments are frequcnt and considerable. Sandstone, red, brown, green, or dark grey in colour, is the most usual rock. It often contains angular frag­ ments of vesicular B1 and/or quartz pebbles. Some feldspar grains of the size and shape typical for the rhomb-porphyry have also been observed. Cross-bedding is sometimes developed. The uppermost part of the bed is often more fine-grained and shaly, being composed of calcareous and pyroclastic material. One section has 5 cm of fissile thin-bedded shale of different colour shades above 70 cm of white quartz sandstone. The feldspar fragments seem to indicate that the sediment was either formed after the RP1 lava had been emplaced in the vicinity or during a possible explosive phase heralding the RP-lava outflows. The cross-bedding is of a type developed in water, although a wind­ blown mode of emplacement has often been mentioned as possible for these particular sediments (DoNs 1956a).

Rhomb-porphyry lava RP1 RPv the 'Kolsås type', has a very typical and constant appearance wherever observed in the Oslo Region, from Larvik in the south to Hadeland in the north, a distance of 160 km. At Kolsås, the maximum preserved thickness of RP1 is calculated to be 140 m, which means that erosion may have removed just the uppermost part of RP1 in northem Kolsås. Subject to local variation within the present area are the orientation of the boat-shaped feldspar phenocrysts, the vesiculation, the amount of epidote, and the colour of the groundmass. Orientation. An areal study of the orientation of feldspar pheno­ crysts has long since been desirable as an aid to establish the direction of flow and the mode of emplacement of the rhomb-porphyry lavas. The observations now made on Kolsås seem to indicate that such a study would be fruitful. PERMIAN SEDIMENTS, LAVAS, AND FAULTS 71

Over great areas, no preferred orientation is seen in natura! rock exposures, at !east not by an untrained eye. Locally, however, a parallel arrangement can be very pronounced and constant for several tens of metres. The directions measured on subhorizontal surfaces are shown in the map (Fig. 1). Together, they form a curve opening to the E. The number of observations may easily be increased. In the core of borehole I, which was sunk on the northem slopes of Kolsås, near the western bend of the tunnel, two zones (one 2 m below the surface and 60 cm thick and the other 42 m below the sur­ face and 1.35 cm thick) contain a very irregular and abnormal pattem of phenocrysts. These zones may be the upper and lower boundaries of a single stream of RP 1 lava 40 m thick. In this stream, the longer axes of the phenocrysts have a well-defined 10° plunge. As the di pof the lava bed due to faulting is in the range 10-20°, the phenocryst may show horizontal flow. In borehole HI, there is a corresponding irregu­ larity 8 m above the bottom of RP 1• At Tyrifjord the uppermost 20 m of RP1 seems to be a separate flow (DoNs 1956a). Turbulent flow structures are often seen, for instance, at the view­ point of southem Kolsås. Where the bottom of the flow is exposed, an orientation of pheno­ crysts parallel to the undersurface is always observed in the lowest 0.5-1 m (see Fig. 4). Vesiculation. In borehole Ill, a gradual increase in vesiculation is observed downward through the lowest 80 cm. This is a general fea ture observed everywhere in Kolsås, although the thickness of the vesicular layer is variable. The vesicles are filled with calcite, epidote, and chalcedony. Coalblend found in RP1 in Asker (PLAYLE 1960) and Sonsterud (DoNs 1956b) has not been observed. Epidote. The amount of epidote replacing the feldspar phenocrysts and the minerals in the groundmass and filling vesicles is very great in some parts of Kolsås. The geographic distribution is uneven and is not confined mainly to the northern part, as would be expected if the epidotization were connected with a plutonic body situated below the Bærum cauldron (see below). Colour of the groundmass. On weathered surfaces, the groundmass is dark brown and the phenocrysts are light grey. On fresh surfaces the groundmass is commonly still dark brown while the phenocrysts have a darker grey tone. Sometimes, however, fresh surfaces of RP1 have 72 JOHANNES A. DONS & EMIL GYORY a dark grey groundmass as, for instance, on northern Kolsås. This colour change could be due to a change of hematite to magnetite in the contact aureole which was shown by SÆTHER {1945) to affect a narrow belt outside the cauldron. Further studies, however, may show that this colour change has not such a regular distribution. Mechanical deformation of RPv according to scattered observations elsewhere in the Oslo Region, also mak es the RP1 change colour from brown to dark grey.

Rhomb-porphyry lava RP11 The appearance of RP11, the 'Gaupehaug' type in Kolsås, is highly variable. In many places, the typical 'dean' rhomb-porphyry gives place to irregular breccia-like rocks. These seem to be primary rock types formed during the emplacement of the lava. Other features may be ascribed to breaking up of RP11 by the following eruption of Ba. Some parts of the RP11 area just north of the ring dike will naturally be mapped as breccia, as, for instance, near Knabberud by SÆTHER {1945). Whether this is an explosion breccia or friction breccia along the cauldron border is not known. Sæter considers it to be older than the ring dike. In the lava succession at Krokskogen, W and NW of Kolsås, RP11 is overlain by a thin flow of RP12, which again is followed by a con­ siderable outpouring of Ba. SÆTHER (1945) and 0FTEDAHL (1953) say that RP12 is not developed in this southern part of the Bærum caul­ dron. The present authors, however, look upon some of the RP rock types near Steinskogen as possible RP12 lava. The irregular distribu­ tion of RP11/Ba can, to a great extent, be ascribed to faulting, but some of the border relations indicate an irregular topographic surface onto which Ba was poured out. Basalt veins cutting RP11 have been observed.

Basalt lava Ba In road cuts east of Steinskogen, there are rapid variations between pyroxene basalt, plagioclase basalt, non-porphyric basalt, and fine­ bedded tuff. The bedding planes show a northerly dip of 20°. o K o L s A s

A

Leg end

Basalt B 3

porphyry RPn

RP1

Basalt B1

Permian sedimen\s

Ringerike sandstone O' Ring

'R''""''h porphyry dike Projection angle: 15°

Fig. 5. Block diagram of Kolsås. PERMIAN SEDIMENTS, LAVAS, AND FAULTS 73

Rhomb-porphyry dikes

Two N-S striking, apparently vertical, dikes of rhomb-porphyry were found W of Kolsås. They have not been reported previously. Both are of the dark bluish grey type with few and large boat-shaped pheno­ crysts. Such dikes are relatively rare in the area around Oslo.

The eastern dike, 30-60 m thick, can be followed continuously for a distance of 2.4 km in the direction Nl0°W, from Kolsberg farm in the south to Glittenberg farm in the north. Here it disappears under loose deposits 850 m south of the ring dike which borders the cauldron. Inside the cauldron, a corresponding dike occurs 800 m farther east. It follows the same direction for a distance of 3 km, from a point very near the ring dike in the south to the farms named Helgerud and Byrud in the north. In his description of this rhomb-porphyry dike inside the cauldron, BROGGER (1933) said 'Whether this dyke continues (south­ wards) across Kolsås has not been investigated'. It seems reasonable to believe that its continuation is marked by the rhomb-porphyry dike now found at the western side of Kolsås. The age of the rhomb-porphyry dike compared to the ring dike cannot be determined on field evidence, but it is assumed to be older than the ring dike. It seems improbable that a horizontal eastwards movement of 800 m has taken place during the cauldron subsidence, which here amounts also to about 800 m. Most probably, the rhomb­ porphyry lava was injected along parallel fissures, in an 'en echellon' pattern. We have here a case where the same dike can be studied as to texture, mineralogical composition, etc. at two different levels in the crust, with 800 m difference in altitude. The western rhomb-porphyry dike is found in a road section at the junction of Brynsveien and Glitterrudveien just W of Kolsås station. Its continuation to the N may be a rhomb-porphyry dike described by BROGGER (1933) from the riverbed (Bærumselven) near Hammersbråten l km farther N. In the cultivated area S of Kolsberg farm, both dikes are covered for a distance of 2 km. Farther S, two closely spaced rhomb-porphyry dikes are found west of Bærum Hospital. The eastern one has a thick­ ness of 12-15 m, the western one 4 . 5 m (dikes Nos. 752 and 751 in SÆTHER 1947). The thickest one seems to find its continuation in the RP dike which can be followed for a distance of 4 km southwards 74 JOHANNES A. DONS & EMIL GYORY crossing the islands of Nesoy, Bronnoy, and Langåra. Its total length would then be 13 km. The thinner western dike disappears near the main railway line at Kampebråten E of Sandvika, but has a possible continuation in a dike described by BROGGER (1933) as No. 8 Geitung­ holmen-Bråten.

The ring dike The fault zone bordering the Bærum cauldron was injected by a ring dike during the cauldron subsidence. Near Steinskogen farm, the dike tapers out, and its continuation must be sought in a serniparallel dike 25-100 m to one side. The maximum thickness within the mapped area is 120 m. The composition varies from felsite porphyry in the central and western part of the area to syenite porphyry in the extreme eastern part. In its western part, where a ditch has been blasted along the ring dike for a distance of several hundred metres, the rock contains large amounts of pyrite, chalcopyrite, epidote, fluorite, and muscovite. Considerable hydrothermal (and pneumatolytic) activity must conse­ quently have taken place after, or during, the emplacement of the ring dike. The source may be the plutonic rock of nordmarkitic composition which is assumed to underly the Bærum cauldron. A gravimetric investigation (SMITHSON 1961) indicated that the top of such a pluton is to be found 3 km below the present surface and its base at 9 km.

Other dikes The classification of dike rocks is difficult without a great reference collection or elaborate studies of thin sections. The students, therefore, placed the dikes in two or three groups, mainly according to colour. Dark fine-grained dike-rocks were called diabases, while light dikes were grouped as syenitic or mænaitic.

Most of the dikes found in RP1 were vertical dia bases, 1-6 m thick. Except for 2 or 3 dikes, they all follow a NS trend, which is the usual direction for diabases in the lowland of Oslo, Bærum, and Asker. Some of the dikes were injected along fault zones. Bifurcations of dikes were reported at three localities. Xenoliths of RPv with a diam­ eter up to 20 cm and often lense shaped, were found in three different dikes. On the eastern slopes of Kolsås, as well as in RP 11 N of Kolsås, there are a great number of dikes. As the bedrock is much covered PERMIAN SEDIMENTS, LAVAS, AND FAULTS 75 and the exact topographic location of exposures is difficult, no serious attempt was made to produce an accurate and complete dike map. One great syenitic dike follows the eastern hillside. In its southern part, it forms a dike, but E of Dælivatn it bends over and becomes a sill in its northern continuation up to section d (Fig. 1). A syenitic dike is also observed in, or parallel to, the fault plane west of Knabberud. Earlier dike studies (SÆTHER 1947, DoNS 1952) have shown that diabases were not limited to a certain period of the igneous activity of the Oslo Region, but are ubiquitous. Only two intersections of dikes were observed in the Kolsås area. l. A rhomb-porphyry dike at the road junction Glitterudveienf Brynsveien cuts a syenitic dike. 2. A diabase (not indicated in the map) is reported to cut the western part of the ring dike. This is in agreement with Sæther's observations farther east, where he reports two cross-cutting protero­ bases.

Faults Kolsås is transected by numerous faults, most of them having a direction approximately N-S. They are well marked in the topography as narrow gulleys. Brecciated material along the faults is cemented by calcite and in some cases also quartz. The largest fault; on the western side of Kolsås, can be traced southwards from the lavas down into the Ringerike sandstone, where a tunnel crosses the 3 m wide vertical breccia zone containing calcite and considerable amounts of montmorillonite. Also, tunnelwork in the northern part of Kolsås met with difficulties when several fault zones filled with calcite and this swelling clay material were encountered. The displacements by faulting are visible at great distances as abrupt changes in the altitude of the geological boundaries exposed in the steep cliffs. No slickensides have been observed which could give direct information concerning the sense of movement. The fault planes are all nearly vertical. As their strike is N-S and the dip of the lithological units is very shallow and to the NW, there must have been very remarkable displaeements parallel to the strike of the faults to explain the observed pattern as strike-slip faults. Some of the faults, e.g. one on the southeastern side of Kolsås, continue southeastwards into the Cambro-Silurian lowland, where the beds have a steeper dip 76 JOHANNES A. DONS & EMIL GYORY and the direction of fault movements are therefore more easily con­ trolled (KIÆR 1908). Strike-slip faults would here have caused much greater displacements than observed. The values given in Fig. l are vertical slips. Most of the blocks bordered by faults have bedding planes dipping to the NW. Near sections d and e, the beds are nearly horizontal. This suggests that slight rotational movements have locally taken place during the blockfaulting. In some cases, a western block has subsided relative to the adjacent eastern block; in other cases, the opposite movement has taken place. The faults nearly compensate each other, so that a general map picture would be the same whether or not the faults were taken into considera­ tion. Compensating faults are also found in the area just SW of Kolsås (Slependen, Skaugum) as shown in a schematic block diagram by KIERULF (1884). The subsidence along the ring fault bordering the Bærum cauldron has a magnitude of 700-800 m. This can be calculated when the total thickness of the lava beds from RP1 to RP11 is known, because RP1 and RP11 are found at the same level on either side of the ring dike. When following the border RP1/B1 northwards and approaching the ring dike, the dip increases remarkably. At Knabberud, the dip is about 30° northwards, and this would seem to be the result of a drag­ ging down of the rock bodies adjacent to the cauldron (see Fig. 5). Along the western side of Kolsås, the altitude of the RP1/B1 contact has been determined at several localities (see Fig. 1). In a longitudinal section the above-mentioned down warping near the ring dike is clearly seen. There are also some indications of a fault parallel to, and about 500 m S of, the ring dyke, with a relative subsidence (40 m ?) of the northern block. SCHETELIG, in his excursion guide (1918), indicated this fault, but assumed the opposite sense of movement. It is very difficult to prove the existence of this fault other than by the marked cliff feature bordering northern Kolsås. Faults cutting the ring dike have not been observed. However, some faults followed from the south to points near the ring dike seem to correspond in direction to faults within the cauldron. There is thus no conclusive evidence of the age relationship of intrusion and faulting. Earlier studies make it probable that the faults are the youngest. PERMIAN SEDIMENTS, LAVAS, AND FAULTS 77

REFERENCES

BRoGGER, W. C. 1933. Om rombeporfyrgangene og de dem ledsagende forkast­ ninger i Oslofeltet. NOYges geol. Undersok. 139, 51 pp. BROGGER, W. C. & SCHETELIG, J. 1917. Rektangelkart 'Kristiania', scale l :100,000. NOYges. geol. Undersok. DoNs, J. A. 1952. Studies on the Igneous Rock Complex of the Oslo Region. XI. Compound volcanic neck, igneous dykes and fault zone in the Ullernåsen­ Husebyåsen area, Oslo. Skr. Norske Vid.-Akad. Oslo. I. Mat.-Naturv. Kl. 1952. No. 2, 96 pp. DoNS, J. A. 1956a. Putestrukturer, sandstensganger, kullblende etc. i rombe­ porfyrlava ved Sonsterud, Tyrifjorden. NOYsk. geol. Tidsskr. 36, 5-16. DoNs, J. A. 1956b. Coal blend and uraniferous hydrocarbon in . NOYsk geol. Tidsskr. 36, 249-66. F6YN, S. 1952. 'Geologi' in Ekskursjonsbok for Oslo og omegn. Cappelen, Oslo, pp. 182-217. HoLTEDAHL, O. 1931. Jungpalaozoische Fossilien im Oslogebiete. NOYsk geol. Tidsskr. 12, 323-39. HoLTEDAHL, O. 1935. Tectonic disturbances connected with plutonic bodies in the Oslo Region. Am: jour. Sei. 29, 504-17. HoLTEDAHL, O. & DoNS, J. A. 1957. Geological guide to Oslo and District. Text to 'Geologisk kart over Oslo og omegn' (scale l : 50,000), published 1952. Skr. NOYske Vid.-Akad. Oslo. I. Mat.-Naturv. Kl. 1957, 86 pp. KrÆR, J. 1908. Das Obersilur im Kristianiagebiete. Vid.-Selsk. Skr. I. Mat.­ Naturv. Kl. Vol. 2. 1906, 595 pp. KIERULF, T. 1884. Dislokationerne i Kristianiadalen. I. Nyt. Mag. f. Naturv. 28, 171-97. OFTEDAHL, C. 1953. Studies on the Igneous Rock Complex of the Oslo Region. XIII. The Cauldrons. Skr. Norske Vid.-Akad. Oslo. I. Mat.-Naturv. Kl. 1953. No. 3. 108 pp. PLAYLE, B. 1960. Contribution to the mineralogy of Norway. No. 4. New finds of coal blend. Norsk geol. Tidsskr. 40, 65-7. SÆTHER, E. 1945. Studies on the Igneous Rock Complex of the Oslo Region. Ill. The southeastern part of the Bærum-Sørkedal cauldron. Skr. NOYske Vid.­ Akad. Oslo. I. Mat.-Naturv. Kl. 1945. No. 6, 60 pp. SÆTHER, E. 1946. Ibid. VII. The area of lavas and sediments in Nittedal. Skr. Norske Vid.-Akad. Oslo. I. Mat.-Naturv. Kl. 1946. No. 6, 34 pp. SÆTHER, E. 1947. Ibid. VIII. The dykes in the Cambro-Silurian lowland of Bærum. Skr. NOYske Vid.-Akad. Oslo. I. Mat.-Naturv. Kl. 1947. No. 6, 60 pp. ScHETELIG, J. & KrÆR, J. 1918. Exkursion til Bærum, SteinshOiden og Kolsaas. FOYh. Skand. NaturfOYsk. 16 mote. 1916. PP· 879-81. SMITHSON, S. B. 1961. A regional gravity study over the Permian Bærum caul­ dron of the Oslo Region. Norsk geol. Tidsskr. 41, 209-21.

Accepted for publication June 1966