Outline description of the excursion:
August 15: The bus leaves at 08.30 from the Bus Terminal in central Oslo. It takes us through the Sveconorwegian terrain of Östfold east of the Oslo Fiord, with strong folding, metamorphism, and magmatism in the interval 1.1-0.9 Ga. The terrain was subsequently eroded to ita crystalline basement. The details of the relatively subdued topography were sculptured by Pleistocene glaciations.
Between the international boundary at Halden and the Swedish city of Uddevalla we pass through the coastal province of Bohuslän that shares the geological age and structure of Östfold. The topography is characterized by low hills of granite and gneiss, rounded by ice-sculpturing and frequently bared of all soil and vegetation.
Going eastward between Uddevalla and Vänersbor we cross this younger segment of the Proterozoic basement. At Vänersborg, where we have our lunch, the hilly topography of the deeply eroded basement shifts into a smooth peneplain, on which Early Palaeozoic marine sediments were first deposited and later, for the most part eroded away.
East of Vänersborg we cross a major relic of Cambrian and Ordovician shale and limestone that was protected from erosion by an overlying heavy sill of dolerite of latest Carboniferous to earliest Permian age. Between this Palaeozoic outlier and Lidköping the flatness of the sub-Cambrian peneplain is the dominant feature. The only major hills that rise above the peneplain are remnants of flatlying Cambrian to lowermost Silurian sandstone, shale, and limestone, most frequently with a protective cover of dolerite.
The objective of the visit to the area is to see a collection of Middle Ordovician meteorites that will be shown to us by Mario Tassinari. The meteorites, frequently in sizes of 2-5 cm but also larger, are embedded in open-marine calcareous deposits that once covered much of northern Europe. This lithology is collectively called orthoceratite limestone. Preservation of the meteorites was favoured by a low-energy environment with early lithification of the sea-bed. However, in spite of preservation of the morphology, structure, and some diagnostic aspects of the chemistry, the mineralogy of the meteorites has been altered diagenetically.
The Siljan impact structure: The structure has its name from the largest lake in the province of Dalarna. The land about the lake is famous for its scenic beauty, ancient folklore, and monuments over historic importance. The existence of the impact has been proved by the presence of i. a. shocked quartz grains, shatter cones, and impact melt. The structure consists of a 35 km wide, roughly circular central uplift that consists mainly of Proterozoic, post-Svecofennian granites and in the west and north, an outer fringe of Neoproterozoic sedimentary and volcanic rocks. The topography of this central uplift is fairly low, the highest points reaching little over 200 m above the lake level. This uplift is surrounded by a zone of varying width (10 km at the most) of well-preserved Ordovician and Silurian sedimentary rocks, referred to as annular trough. The most recent published date for the impact is 377 Ma, which is late Middle Devonian. At this time the final, Scandian stage of the Caledonian orogeny had advanced its front far eastward into the present Scandinavian Peninsula. The afternoon will be spent in the annular trough.
|Stop 1. Kinnekulle: This hill dominates the landscape. It is another erosion relic with horizontal and undisturbed, Cambrian to Silurian beds, all protected by a tiny cap of dolerite at the top. The Middle Ordovician orthoceratite limestone is studied at the Torsberg quarry at Österplana, where the meteorite collection was brought together. The specimens come from several beds. They are quite rare and are only discovered in the course of the manual treatment of the rock for commercial, decorative purposes. A brief visit will also be paid to the abandoned quarry at Hällekis, where the steep wall exposes a section through the entire orthoceratite limestone, spanning from the Lower to the lowermost Upper Ordovician. From the same place there is a magnificent view of the large lake. From Kinnekulle we proceed to Mariestad, where we expect to arrive at about 17.30. We check in at Hotel Vänerport. Supper will be at 19.00.
August 16: Departure from Hotel Vänerport at 08.30. The northward journey takes us through the towns Kristinehamn, Filipstad, Hällefors, Kopparberg, Djurås and Leksand to Rättvik, where we stay overnight. The bedrock along the road is exclusively crystalline basement of Proterozoic age. The geological history of the region goes back to the Svecofennian orogeny about 1.9-1.8 Ga. We have lunch at Kopparberg, a center of the Bergslagen mining province since the Middle Ages.
After check-in at Hotel Stiftsgården in Rättvik we start out for the field at 14.30.
Stop 2: Sjurberg, Siljan shore (National UTM reference - in the following merely referred to as UTM - 6754650/1458855). Lower to Middle Ordovician orthoceratite limestone showing identity of facies with Stop 1, as well as perfect preservation of original structure.
Stop 3: Kallholn quarry (UTM 6741100/1484000). Large lime operation in Upper Ordovician reef-like limestones overlain with sedimentary unconformity by Lower Silurian dark shale with limestone spheroids. The succession is faulted and folded on a relatively large scale, and dips are partly steep.
Stop 4: Kårgärde (UTM 6774850/1436600). Section of stratigraphically complete Lower and Middle Ordovician orthoceratite limestone, with west-facing bedding that is raised to vertical orientation. The contact against the stratigraphically underlying, Neoproterozoic volcanite to the east is on record as sedimentary. We are at the western edge of the central uplift. Return to the hotel scheduled at 18.30. Supper at the hotel from 19.00.
August 17: Departure from the hotel at 08.30, return scheduled at 18.30. Picnic lunch.
Stop 5: Gesundberget (UTM 6751000/1430250). An isolated hill of Proterozoic Dala Granite rising to 514 m above sealevel (354 m above the lake) and offering a grand view of the impact area.
Stop 6: Trollberget (UTM 6767950/1448875). Near the center of the central uplift. Granodioritic Dala Granite with dikes of impact melt.
Stop 7: Styggforsen (UTM 6765820/1466800). Eastern marginal zone of the central uplift in picturesque waterfall setting. A well-preserved and steeply raised Lower Silurian succession that gets younger westwards. Dark shale with limestone spheroids is stratigraphically overlain by a very regularly banded limestone-shale formation. To the west there follows a thick, granite-based, and quartz-healed breccia that separates the Silurian from the Dala Granite of the uplift. The locality is taken as an instance of impact-driven reactivation of a tectonic zone in the basement.
Stop 8: Dalhalla (formerly Unkarsheden) (UTM 6759350/1462075). A classic quarry in the Upper Ordovician reef-like limestone, now in successful use as a famed open-air stage for major musical events. Rocks, stratigraphy, and fossil content are excellently preserved, although the bedding partly stands on edge.
Stop 9: Amtjärn (UTM 6758/1460475). Abandoned quarry, now under Nature Conservancy legislation, with east-dipping reef and near-reef facies that is richly fossiliferous. Supper at the hotel from 19.00.
August 18: Departure for Östersund at 08.30. We pass Mora, Orsa, Sveg, Ytterhogdal, and Svenstavik, with a coffee break in the old village of Älvros to the west of Sveg. The bedrock is Proterozoic, mainly deep basement rocks. The country is hilly, forested, and sparsely populated. We will have lunch at the Golf Club in Lockne at 13.00.
|The Lockne open-sea impact: The impact happened in the earliest Late Ordovician at about 455 Ma. The stratigraphy shows the impact products to be intercalated at a low level of the lower Upper Ordovician Dalby Limestone Formation. There are no air-deposited backfall deposits but instead there is ubiquitous evidence of very forceful watery deposition of impact products as immediate consequence of the formation of the crater. This deposition is referred to as resurge. Mathematic modeling based on the structural and stratigraphic findings indicates that the local depth of water must have been at least 500 m though considerably less than 1 000 m when the impact happened. The structure consists of a 7 km wide inner crater, blasted into crystalline rock and surrounded by a brim of crystalline ejecta that is up to 2 km wide in the west but much narrowerin the east.
Stop 10: Nordanbergsberget, Skanska quarry (UTM 6993300/1449000). Just outside the northern margin of the inner crater at Lockne. About 1.8 Ga old Revsund Granite is overlain by Cambrian basal conglomerate and Cambrian black shale most of which is strongly squeezed. it is in its turn overlain wheresoever observable in the quarry, by basal conglomerate and more or less crushed Revsund Granite, This complete overturning is typical of many major ejecta masses. We are near the inner margin of the brim of crystalline jecta that surrounds the inner crater. There are pockets of varoious lithologies of impact breccia.
Fig. 3. - Generalized geological features of the Lockne
area, with stops 10-20. Lower Palaeozoic bedrock
(except crystalline ejecta) indicated by diagonal pattern.
Vertical pattern indivates crystalline ejecta. Toothed
lines show overthrusts, the teeth directed toward the overthrust nappe, illustrates the continuity and relative stability of the marine situation throughout much of Baltoscandia. We are 6 km from the inner crater,
and this part of thesedimentary succession is
Stop 11: Crater track (UTM 6987125/1445250 - 6987375/1445680). The track forms a loop along the south flank of the so-called Tandsbyn Gully, a southwest-directed cut through the brim of crystalline ejecta. In the bottom part of the "Gully" the crystalline ejecta are directly overlain by resurge gravel and sandstone that is in its turn overlain by fossiliferous Dalby Limestone. Higher on the flank of the "Gully" the resurge deposit is absent and the Dalby Limestone rests directly on the ejecta. We then continue the interrupted bus journey to Östersund, Best Western Hotel Gamla Teatern, where we are scheduled to arrive at 18.00. Supper at 19.00.
August 19: Departure at 08.30. Return to the hotel scheduled for 17.00. Picnic lunch.
Stop 12: Rödberget, Brunflo (UTM 6997900/1453000). Middle Ordovician orthoceratite limestone of same facies as at Stop 1, Stop 2, and Stop 4. The outcrop
Stop 13: Hälle at Lundbomsberget, Brunflo (UTM 6996535/1453365). View over the Lockne crater from the north. The bedrock at the locality is the middle part of the orthoceratite limestone.
Stop 14: Church Community Center, Brunflo (UTM 6998225/1450190). The lawn behind the Community Center is interrupted by excellent outcrops of monomictic breccia of big blocks to small nodules of orthoceratite limestone. The breccia is interpreted as a result of the stress from the expanding water crater. We are 9 km from the center of the impact.
Stop 15: Crater museum at Ångsta. A modern museum introduces the geology of the area, the Lockne crater, and its bedrock. The impact process is illustrated by a modern advanced animation.
Stop 16: Handsjö road (UTM 6987500/1447115 - 69859960/1447185). The north-south gravel road cuts, from north to south, through the southern margin of an outlier of the lowermost Caledonian overthrust nappe in the area that covers much of the interior of the crater, the nodular Dalby Limestone that forms the upper part of the post-impact crater fill, the margin of the inner crater, the inner part of the ejecta brim, and rubbly to sandy resurge deposit that rests on the crystalline ejecta.
Stop 17. Hällnäset (UTM 6985600/1451220 - 6984640/1450360). Section at and just outside the margin of the inner crater. The margin is covered by coarse sandy resurge deposit at a shore outcrop, which gives a fine view over the Lockne Lake. Further outcrops show crystalline ejecta breccia with varying lithologic components, resurge deposit from very coarse blocky to gravelly and, ultimately, sandy, overlain by fossiliferous Dalby Limestone. After returning to the hotel at five p.m. we will have supper at the Arctura Restaurant from which there is a 100 km long field of view over the are we are visiting.
August 20: Departure from the hotel at 09.00.
Stop 18: Ynntjärnen (UTM 6988060/1443700). Railway section showing orthoceratite limestone, part of it strongly stirred during water crater development, crystalline ejecta, rubbly as well as coarsely sandy resurge deposits, and post-impact Dalby Limestone. The section shows how strongly this westerly segment of the cratered area was affected by Caledonian thrusting from the west. Lunch at the Golf Club.
Stop 19. Klocksåsen (UTM 6984285/1444425). Cambrian and Lower Ordovician outside the inner crater, in immediate vicinity to the brim of crystalline ejecta. Relatively spared by the impact but overlain by small, isolated masses of crystalline ejecta. Fine view of the forested sub-Cambrian peneplain that spreads widely to the east of the erosional boundary of Caledonian overthrusts.
Stop 20. Kajan (UTM 6985615/1442040). 5 km west of the margin of the inner crater. Middle Ordovician orthoceratite limestone with undisturbed bedding, overlain by breccia of the same limestone, strongly stirred and with a few rare clasts of granitic ejecta. The outcrop offers a fine panorama to the west, where the hilltops of the Caledonides form the horizon. The excursion ends at the hotel at 14.30. The only flight from Östersund on Saturdays is at 16.00 arriving 17.00 at Stockholm Arlanda airport, our bus will take those who want this connection to Östersund airport. The hotel is 600 m from Östersund Central Station. There is a train att 17.00, arriving Stockholm Arlanda airport at 22.16. There is another train at 16.15 arriving Trondheim (Norway) at 20.13, from where a plane leaves for Oslo at 06.10 on the following morning, with a few flights on the same route following after it during the day.