CAULDRON SUBSIDENCES, RING-STRUCTURES, AND MAJOR FAULTS IN THE SKIEN DISTRICT, NORWAY

TOM VICTOR SEGALSTAD

Segalstad, T. V.: Cauldron subsidences, ring-structures, and major faults in the Skien district, Norway. Norsk Geologisk Tidsskrift, Vol. 55, pp. 321-333. Oslo 1975.

Permian cauldrons and ring-structures new to the Skien district of the Oslo Region are described. The Vealøs and Skrehelle cauldrons are mostly com­ posed of monzonitic (larvikite) and syenitic rocks (nordmarkites) with xeno­ liths of volcanic rocks (B1, RP1, etc.). The Mykle ring-structure is composed of monzonite (larvikite) with granite (ekerite) as the central pluton. All three have ring-dikes of syenitic composition. Total field aeromagnetic maps show the well known cauldrons of the Oslo Region as characteristic curved magnetic anomalies. Corresponding anomalies in the Skien district confirm the cauldron hypothesis concerning this district. Major faults active in Pre­ cambrian and Permian times may have determined the location of the cauldrons in the Oslo Region.

T. V. Segalstad, Institutt for geologi, Universitetet i Oslo, P.O. Box 1047 Blindern, Oslo 3, Norway.

The Permian volcanism in general and the cauldrons of the Oslo Region in particular have caught the interest of many geologists during the last century. A summarized description has been given by Oftedahl (1953, 1960a). Later work on cauldrons in the Oslo Region has been presented by Oftedahl (1967), Naterstad (1971), and Larsen (1971). The investigations in the Oslo Region have so far shown remarkably few cauldrons in its southem part. The reason is that the area has not been thoroughly studied since the work of Brøgger & Schetelig (1923); they were not fully aware of the cauldrons as a structural feature. Recent field work in this area, however, has shown the existence of several circular subsided areas of different kinds, though they are not so nicely preserved as the well known cauldrons of the Oslo Region.

Terminology Caldera is applied to large surface depressions, more or less circular in form, associated with volcanos (Reynolds 1956). Cauldron is used to denote the space occupied by a down-faulted block of rocks circumscribed by a ring-fracture (Reynolds 1956). It is an inclusive term for all volcanic subsidence structures regardless of shape or size, depth of erosion, or connection with the surface (Smith & Bailey 1968). Cauldron subsidence is the process whereby a portion of the roof of a magma chamber sinks along ring faults, accompanied by magmatic stoping 322 T. V. SEGALSTAD and sometimes by squeezing up of magma along the fault to form a ring-dike (Gary et al. 1972). Ring-structure is in this paper a general term for a circular subsided or uplifted block of rocks, circumscribed by a ring-fracture and in some cases a ring-dike, where one has no knowledge about the direction of movement. The ring-structure may represent a 'cauldron root' or the initiation of a cauldron formation. Plutonic dome is applied in this paper to a structure where a plutonic body has displaced the roof rock by uplifting.

Geology of the Skien district The areal distribution of the rocks of the Skien district is given in Fig. l. Overlying the Precambrian gneisses, we find a sequence of marine Cambro­ Silurian shale and limestone ending with the Ringerike continental sandstone (partly Ludlow, partly Downtonian). Concordant Permian sandstone overlies the Ringerike sandstone. Above a quartz conglomerate follow the B1 lava flows (alkaline basalt) described by Oftedahl (1952). The lavas of the Skien district are now being reinvestigated and the results will be published later. The B1 basalts are overlain by latite flows, rhomb porphyries (RP). The eight lowest RP flows occur within the Skien district. Among the Permian plutonic rocks of the Skien district, the monzonites (larvikite-kjelsåsite) predominate. Foyaite (lardalite) underlies some smaller areas in southwest while syenite (nordmarkites) and ægirinefarfvedsonite gra­ nite (ekerite) are exposed over larger areas of the district. Several gabbroic rock types (Oslo-essexite) occur in some feeders of early central volcanos. In addition there are many diabase and rhomb porphyry dikes, faults, and joints which are not shown on the map (Fig. 1). The major fault through the southem Norwegian Precambrian rocks (Bugge 1928) has also displaced the Permian basalt. For a more thorough description of the Oslo Region Permian rocks the reader is referred to Brøgger (1933), Barth (1944, 1954), Oftedahl (1960a), and Dietrich et al. (1965). The Permian geological history of the Skien district may be outlined as follows: Deposition of Permian sandstone with conglomerate concordant above the Permian peneplain atop the Cambro-Silurian sediments. Extrusion of B1 alkali basalt lava. Larvikite emplacement by plutonic stoping. Extrusion of rhomb porphyry lava. Faulting. Continuation of movement on 'The Major Fault'.

Fig. l. Geological map of the Skien district. In central parts based on field work by the author, in the outer parts based on Kiær (1906), Brøgger & Schetelig (1923), Rohr­ Torp (1971) (northem part), and Oftedahl (1960b) (southeastern part). CAULDRON SUBSIDENCES, RING-STRUCTURE AND FAULTS 323

- Rhomb porphyry \.-.. Fault liil Basalt 0 Ekerite l...... l Permian sandstone & conglomerate Nordmarkite

ETIJ. Ringerike sandstone � Lardalite

Marine Cambro-Silurian sediments Larvikite - Kjelsås i te IIIIIIJI] §� fJlm/ Precambrian gneisses m Oslo-essexite, volcanic neck 324 T. V. SEGALSTAD

Fig. 2. Total field aeromagnetic map of the Skien district. lsomagnetic lines with interval 100 gammas, from Nor. Geol. Unders. sheet Skien (1974). Same scale as Fig. l.

Subsidence of the Vealøs cauldron. Nordmarkite emplacement by plutonic stoping. Subsidence of the Skrehelle cauldron. Ekerite' intrusion. CAULDRON SUBSIDENCES, RING-STRUCTURE AND FAULTS 325

Fig. 3. Syenitic ring-dike in larvikite seen in centre of the photograph due to different way of cleavage. Mykle ring-structure, east of Fjellvann.

Formation of the Mykle ring-structure. Fracturing and faulting.

Mykle ring-structure

The Mykle ring-structure is situated some 20 km north of Skien. Along the border of the ring-structure there is a depression in the terrain which can readily be seen on aerial photos and topographic maps. Outside this depres­ sion the larvikite has an angular surface expression and is overgrown by spruce trees, while on the inner side the larvikite has a rounded surface ex­ pression and supports pine trees. The larvikite within the depression is full of cracks; near the depression the rock may be crumbled between the fingers. The circular depression has been followed in the field for approximately 15 km to the NNW and to the E from Fjellvann. Along the depression near Fjellvann a ring-dike is found; occasionally it sends apophyses into the adjacent rocks. The ring-dike is usually 4-8 m thick, but locally it is absent. The ring-dike is a non-porphyric mesoperthitic syenite with some quartz, plagioclase, and alkali amphibole, and may well have been derived from the central ekerite pluton. Studies of thin sections of larvikite on both sides of the ring-dike show that the larvikite on the inner side of the ring-dike is recrystallized. 326 T. V. SEGALSTAD

The Mykle ring-structure may possibly be a cauldron root connected with the stoping of the ekerite pluton. Intrusion of the ekerite caused a block of overlying larvikite to subside into the ekerite magma, though the extent of the subsidence is difficult to estimate. It may have exceeded 1500 m if com­ pared with the larger cauldrons in the Oslo Region. The present land surface is believed to be near the top of the ekerite. Many dikes and smaller outcrops of ekerite are found in the larvikite within the Mykle ring-structure. The reason for not finding any extrusive rocks derived from the ekerite within the ring-structure is perhaps that the ekerite formed an 'underground cauldron subsidence' (Franke! 1967) with an intrusion which did not reach so high that surface volcanism could take place.

Skrehelle cauldron

The Skrehelle cauldron contains a huge metamorphosed body of rhomb porphyry 'floating' in syenite porphyry and nordmarkite at the mountain Skrehelle north of Skien. The rhomb porphyry (RPt. and younger flows) shows, at its northeastem border, transition into larvikite, as Brøgger (1890) has described from Tønsberg. In the western part it is fou�d 150 m above the Ringerike sandstonejquartzite. The fact that the B1 basalt is lacking between the Ringerike sandstone and the rhomb porphyry indicates that a subsidence has taken place. The vertical subsidence of the cauldron may have been 1500 m, corresponding to the apparent thickness of the basalt. The ring-dike is sometimes up to 500 m thick and appears most often to be of nordmarkitic composition. However, to the south it is developed as syenite porphyry. The diameter of the Skrehelle cauldron is approximately 7 km, which equals the size of the Hillestad cauldron.

Vealøs cauldron

The Vealøs cauldron is situated east of Skien with the mountain Vealøs as a characteristic point near the best preserved portion of the ring-dike. The Vealøs cauldron is composed mainly of larvikite and nordmarkite. Basalts and some huge xenoliths of contact metamorphic rhomb porphyry (RP1, RP2 and higher types) (Brøgger 1890, Oftedahl 1952) are found in the southwestem part. Rhomb porphyry rocks (RP8) are found in the northem part; it is not known whether these are younger or older than the subsidence. The ring fault has displaced the B1 basalt in the southwestem part, and a nordmarkite-syenite ring-dike of variable thickness has intruded along the ring fault. The subsided part of the basalt is contact-metamorphosed plagio­ clase basalt, while the basalt outside the cauldron is a pyroxene basalt in the lower part and pyroxene-plagioclase basalt in the upper part, metamor- CAULDRON SUBSIDENCES, RING-STRUCTURE AND FAULTS 327

Fig. 4. Rhomb porphyry breccia in syenitic matrix. Vealøs cauldron, near Skifjell.

phosed near the ring-dike. At the outer border of the ring fault near Skifjell the phenocrysts of the basalts are more or less squeezed out by strain. Similar features are described from the Langesundsfjorden area (Brøgger 1890) and in the Grefsenåsen area at the southem border of the Nittedal cauldron (Holtedahl & Dons 1966). Within the ring-dike at Skifjell there is an outcrop of RP1. brecciated at the contact against the ring-dike (Fig. 4). Along the ring-dike that bisects the basalts there are some small copper claims that were worked at the beginning of the century. Copper mineralization is also found in basalt at and near the southem border of the Nittedal cauldron. The original diameter of the Vealøs cauldron has probably been approxi­ mately 15 km (based on the aeromagnetic anomaly). The size compares well with the other larger cauldrons in the Oslo Region, for instance, the Glitre­ vann cauldron. The vertical subsidence is difficult to estimate from the pres­ ent data, but may have been of the magnitude of 1500 m as for the Glitre­ vann cauldron. 328 T. V. SEGALSTAD

l l l l l \ l \ N l I l

O km CAULDRON SUBSIDENCES, RING-STRUCfURE AND FAULTS 329

Aeromagnetic interpretation The 'Total field aeromagnetic maps' from Norges Geologiske Undersøkelse (1963-1967, 1973, 1974) sh�w the well known cauldrons of the Oslo Region as characteristic curved magnetic anomalies (Fig. 5). Corresponding ano­ malies in the Skien district help to confirm the existence of cauldrons in this district. The 'Total field aeromagnetic map' of the Skien district (Fig. 2) shows relatively high and variable magnetic intensity over the area of larvikite and nordmarkite. However, the circular area of larvikite and ekerite around the Mykle lake shows distinctly lower magnetic intensity. (The terrain effect on the magnetic measurements is believed to be very little or negligible.) The Skrehelle cauldron does not show the typical anomalous pattern on the aeromagnetic map. This is also the case for the Hillestad cauldron. It is probably due to their relative small size. The diameter of the Vealøs cauldron is based on the extent of its aero­ magnetic anomaly. This cauldron is probably one of many in the southern larvikite massif, as can be interpreted from several corresponding anomalies on the aeromagnetic map (Nor. Geol. Unders. 1973, 1974). This suggestion is corroborated by corresponding depressions seen on the topographic maps in tnese areas.

Ring-structures north of Oslo The aeromagnetic maps covering the district north of Oslo (Nor. Geol. Un­ ders. 1963-1967) show some other areas with similar structures. Several curved igneous structures north of Oslo mapped by Sæther (1962) may well be interpreted as cauldrons and ring-structures (Holtedahl 1935, 1943). B. T. Larsen (pers. comm.) is now reinvestigating this area, and has identified the Svarten cauldron, Heggelia cauldron, and Oppkuven cauldron. We might also regard a structure at Kampen west of the Bærum cauldron as a cauldron remnant, and some structures in the same area may be interpreted as ring­ structures; Slottet, Langlia, Kikut, and Stryken. The aeromagnetic maps show more or less the same type of anomaly for the Kikut and Stryken ring­ structures as for the Mykle ring-structure. The Stryken ring-structure (indicated as a hypothetical cauldron on a map by Oftedahl (1970)) shows a ring-dike of felsite porphyric composition, sometimes developed as an ignimbrite-like rock (in the western part by Sæther (1962) called Fjellsjøhøgda porphyry). The ring-dike cuts mostly through rocks of syenitic composition belonging to the nordmarkite group.

Fig. 5. Total field aeromagnetic map, from Nor. Geol. Unders. sheet Skien (1974) and Oslo (1973). lsomagnetic lines with interval 100 gammas. Showing from top: Finne­ marka plutonic dome, Glitrevann cauldron, Drammen cauldron, Sande cauldron, and Hillestad cauldron. 330 T. V. SEGALSTAD

Table 1. Average ore content of some abundant plutonic Oslo rocks.

Norm Mo de No. of Rock Source of ore Magne- Hem a- samples Ilmenite tite tite

Kjelsåsite 4.8 3.1 2.3 7 Barth 1944

Larvikite 2.5 2.3 1.8 7 Barth 1944

Nordmarkite 2.3 1.8 1.1 0.4 8 Barth 1944

Ekerite 0.9 0.3 0.5 0.4 6 Barth 1944

Ekerite 0.4 0.8 0.9 0.5 7 Dietrich et al. 1965

The central pluton is represented by an ekerite as in the Mykle ring-structure. Just north of the Kikut mountain we find another possible ring-structure with ekerite as the central pluton, and felsite porphyric ring-dike cutting through nordmarkite. The same kind of magnetic anomaly may be seen west of Hurdalen where we also might expect a possible ring-structure with an ekerite central pluton quite similar to the Mykle ring-structure. From these examples it seems that ring-structures with ekerite in the core, and having an obviously lower magnetic intensity than the surrounding rocks, are the remnants of structures formed by overhead stoping of ekerite. The reason for the lower magnetic intensity is that the ekerite has a lower mag­ netite content than the other plutonic rocks in the Oslo Region (Table 1). The heat and stress forced upon a subsided block of rocks increased the number of micro cracks and caused a demagnetization (Grant & West 1965) of the rocks overlying the ekerite as seen on the aeromagnetic map.

Fault - cauldron relations in the Oslo Region The main fault in the Skien district is Precambrian and is described by Bugge (1928), among others. It enters the Oslo Region near Porsgrunn. The fault must also have been active during the Paleozoic because it faults the Cambro-Silurian sediments and the Permian sediments and basalts. In the basalts, one can follow a 4 m thick quartz breccia along the fault. The rela­ tive vertical motion along the fault during post-Silurian time has been esti­ mated at approximately 950 m by Brøgger (1890). As seen in Fig. 6 the cauldrons Iie along the same main directions as the major faults and the basalt volcanic necks. Most workers on the Oslo Region believe that the origin of the region was controlled by a system of old major Precambrian fault zones. Younger quartz breccias within and outside the Oslo Region indicate that the faults have also been active in the Paleozoic (Naterstad 1971, Ramberg 1971, Holtedahl 1974). The aeromagnetic map (Nor. Geol. Unders. 1971) shows, along the old fault lines, several small- CAULDRON SUBSIDENCES, RING-STRUCTURE AND FAULTS 331

� -- Major faults

\._ • .". Plutomc dames

• Rtng-structures (cauldron roots ) :• • • :

.�- _,' Cauldrons

' .... - •• Basalt vølcan'c necks ...,,

f"'f/JfJ Precambnan

N I

km 28

... \ l l l l

-- _,."'

Fig. 6. Volcanological map of the Oslo Region. Partly based on Oftedahl (1970). 332 T. V. SEGALSTAD scale anomalies similar to those of the volcanic necks of the Oslo Region. Touret (1970) has shown that there has been volcanism about 300 million years ago along the major fault at Degernes some 70 km from the south­ west tip of the Oslo Region. Barth (1970) and Am (1973) have also shown that there has been volcanism of assumed Permian age along the same fault near Kristiansand. In the Skien district as well as in other parts of the Oslo Region the main faults often apparently stop at the borders of a cauldron, forming a right angle between the fault and the ring-fracture of the cauldron. These main faults have probably played a considerable role in the location and origin of volcanos and thereby in the formation of the cauldrons.

Acknowledgements. - This paper is based on field work during parts of summers 1972, 1973, and 1974 for the cand. real. thesis at the University of Oslo. The field work was financed by Mineralogisk-Geologisk Museum, Oslo. Special thanks is due to Head-Curator J. A. Dons who suggested the study and for his help and encouraging discussions during the work. He also most kindly commented on the manuscript. The author also wishes to thank Dr. W. L. Griffin for correcting the English text, and cand. mag. B. T. Larsen for several helpful discussions. January 1975

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