The Gallejaur
Structure
Excursion guide
by
Map showing the Gallejaure area.
Karta över Gallejaureområdet.
Excursion sites in the Gallejaure area.
Exkursionslokaler i Gallejaureområdet.
COPYRIGHT:
Bilden på försättsbladet är framställd med hjälp av SGU:s flygmagnetiska databas
Bilden på sidan 9 är framstäld med hjälp av SGU:s regionalgravimetriska databas.
Copyright för ovanstående Sveriges Geologiska Undersökning.
För kartillustrationer i detta dokument gäller: Ur Vägkartan © Lantmäteriverket Gävle 2002.
Medgivande M2002/2680.
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Informationsblad om terrestriska meteoritimpakter
Robert Liljequist, Ecominas.
English summary see page 6.
Varje naturligt fast föremål som faller ned från rymden på jorden benämns meteorit. Det uppskattas
att ungefär 500 ton kosmiskt stoft varje dag kommer in i jordens atmosfär. Endast omkring 500
meteoriter större än 10cm i diameter överlever varje år passagen genom luften. Som mest hittas och
tillvaratas 5 meteoriter årligen. Antalet kända meteoriter från olika delar av världen uppgår idag till
drygt 2 200. Flera meteornedfall har bevittnas – enbart i USA har man iakttagit över 760 nedfall.
Det första meteornedfallet i Sverige från vilket stenar blivit tillvaratagna, inträffade nyårsdagen
1869 i trakten av Hessle, 3 mil utanför Uppsala. Nedfallet finns livfullt beskrivet i litteraturen. Den
andra oktober 1951 för en 300 gram tung meteorit ned i Århus i Danmark. Vid nedslaget gick den
sönder i fyra bitar.
Det äldsta med säkerhet bevittnade nedfallet ägde rum i Frankrike den 18 november 1492, men det
dröjde ända fram till mot slutet av 1700-talet innan vetenskapen accepterade det faktum att det föll
ned stenar från rymden.
Mer eller mindre väl bevarade kratrar på jordens yta minner om forna meteoritnedfall. Det kanske
bästa exemplet på en stor cirkulär krater med en väl utbildad rand är Barringer Crater (tidigare känd
som Meteor Crater) från Canyon Diablo i Arizona. Diametern på denna krater är 1 200m och den
når ned till 180 meters djup. Randzonen höjer sig 46m ovan den omgivande platån. Bergartsfragment
ligger utspridda utanför kratern över en radie av 10km från kraterns centrum. Tusentals järnfragment
har uppsamlats från omgivningarna, sammanlagt till en vikt av 20 ton. Ett av fragmenten finns
utställt på den mineralogiska avdelningen på Riksmuseet i Stockholm
Vetenskapsmän menar att en meteorit som varit i stånd att bilda en krater av detta format måste ha
splittrats och delvis förångats vid nedslaget. I Australien har man funnit 4 välbildade kratrar där
järnfragment påträffats. Totalt har endast 8 kratrar eller kratergrupper med järnmeteoriter hittats.
Dessutom är omkring 45 kratrar utan meteoritfragment kända. Längre fram beskrivs stora strukturer
på jordytan - utan bibehållna kratrar - som uppkommit genom nedslag av större kroppar.
Mycket sällan har människor skådat nedfall som resulterat i en krater av mätbar storlek. En av
dessa var den sibiriska ”Sikhote Alin” som föll den 12 februari 1947. En gångformad krater på 28m
i diameter bildades och det största järnfragmentet vägde 1 800 kilo. Sovjetiska vetenskapsmän har
räknat ut att det rörde sig om en asteroid som färdades med en hastighet av 40 kilometer i sekunden.
I Brasilien inträffade den 13 augusti 1930 ett starkt ljus- och ljudfenomen i Amazonas. ”Solen blev
blodröd och ett mörker spred sig över allt och alla” berättar ett ögonvittne. Kratern besöktes för
första gången av forskare i juni 1997. Kratern uppmättes till 1.2km i diameter men endast kratervallen
i hård röd lera återstod.
Den största kända enskilda meteoriten är Hobameteoriten (se bild) som hittades i sydvästra Afrika.
Den väger 66 ton och mäter 3x3x1m i omfång. De i storlek fem nästföljande järnmeteoriterna
väger ungefär hälften av Hobarmeteoriten. Den största kända stenmeteoriten väger en tredjedels
ton. De katalogiserade meteoriterna som upplockats från jordytan visar sig bestå till 94% av
stenmeteoriter, till 4.5% av järnmeteoriter och till 1,5% av stenjärnmeteoriter.
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Excursion Guide
Meteoriterna utgör de äldsta prov vi känner till från vårt solsystem. Åldersbestämningar som har
utförts med hjälp av isotopförhållande mellan uran och bly, rubidium-strontium och kalium-argon
pekar mot en gemensam ålder. Bildningstiden uppskattas till 4,5 miljarder år och anses vara
födelsetiden för vårt solsystem. Det anses troligt att meteoriterna stammar från skilda delar av den
yttre rymden, men man vet att de flesta asteroider ligger i ett bälte mellan planeterna Mars och
Jupiter. Omkring 2 100 asteroider med en storlek över 1 kilometer anses kretsa i jordnära banor.
När en asteroid med en hastighet av cirka 30 kilometer/sekund kolliderar med jorden uppstår en
krater som är cirka 20 gånger större i diameter än asteroidklumpen. Händelsen i Tungaska den 30
juni 1908 orsakade en förstörelse över en yta som täckte en radie av 10km. Nedslagskroppen
exploderade innan den nådde jordytan men mindre bitar åstadkom små kratrar på 10-tals meters
diameter. Det kan vara intressant att veta att ozonhalten i atmosfären minskade med 30% som en
följd av händelsen i Tungaska.
Vid ett större nedslag sker under ett mycket kort förlopp olika processer som kan indelas i tre faser.
•
•
•
Under en kompressionsfas som varar några sekunder förångas den inkommande meteoritkroppen
medan berggrunden delvis smälter och delvis krossas. Trycket orsakar förändringar i
kristallfaserna i olika mineral. Diaplektiskt glas bildas och högtrycksförändringar i kvarts ger
upphov till coesit och stishovit, kol övergår till diamant, och uppsmältning och gasbildning äger
rum. Graniter smälter vid 60 Gpa (Gigapascal), basiska bergarter vid 80 Gpa och sedimentära
bergarter vid 30 Gpa.
Under en urgröpningsfas (excavation phase) som varar cirka 100 sekunder bildas en fiktiv
krater (transient crater) samtidigt som material kastas ut ur kratern, ofta ballistiskt. Kaotiska
materialströmmar sker utmed kraterytan (ground surge).
Under en påföljande modifikationsfas, som varar minuter till timmar, sker de slutliga
avlagringarna och förändringarna i och omkring kratern. Smältan och så kallade klastiska breccior
blandas, en central dom bildas, kraterranden rasar in, smälta med bergartsfragment injekteras i
kraterbotten och hela strukturen deformeras. Resultatet blir en komplicerad kollapsstruktur med
en centralt upplyft bergartsparti i mitten, omgivet av en sänka och ytterst terassbildningar.
Idag har man identifierat och övertygande bevisat förekomsten av mellan 150 och 200 kända
impaktkratrar på jorden. Av dessa har endast 10% identifierats under den Prekambriska tiden, en
era som omfattar omkring 88% av jordens ålder. Denna brist på impaktkratrar jämfört med det
enorma antalet kratrar vi kan se på alla våra närliggande himlakroppar beror på huvudsakligen två
faktorer. För det första eroderar och förstörs mycket av nedslagets utkastade material och kraterform
under de långa tidsrymder som följt nedslaget. Jordens yta är extremt dynamisk jämfört med våra
grannar i solsystemet. För det andra är det mycket få geologer som tillägnat sig de senaste
decenniernas framsteg inom impaktforskningen och därför inte vet hur man skall läsa berggrunden
när den uppvisar tecken på att ha varit utsatt för de förstörelseeffekter som uppstår vid ett stort
nedslag. Det är knappast en överdrift att hävda att den moderna forskningen kring planetära och
terrestriska meteornedslag har orsakat en vetenskaplig revolution lika stor som när
kontinentaldriftsteorin accepterades. Då liksom nu möts nya idéer – hur välgrundade de än må vara
– av stor skepsis och ofta av smädande uttalande från de forskare som inte förmår sig ta del av nya
forskningsrön. Det är lättare att behålla sin världsbild intakt.
I Sverige har, främst genom Frans E Wickmans pionjärsinsatser, flera nedslagsstrukturer kunnat
påvisas och verifieras: Siljan (ca. 55km i diameter), Dellen (20km), Lockne (>7km), Mien (7km),
Granby (3km), Tvären (2km), Hummeln (1km), Björkö (9km), och Åvike (9.5km) Flera av dessa
har slagit ned i det Ordoviciska havet för ungefär 450 miljoner år sedan (Lockne, Granby,Tvären).
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Ett par stora nedslagsstrukturer har nyligen upptäckts och är på väg att verifieras: Gallejaur (ca.
50km)och Doubblon (kanske 90km).
De hittills största nedslagstrukturerna finns i Sydafrika (Vredefort – ca 250 km diameter) och i
Kanada (Sudbury – ca 200km diameter). Bägge dessa stora nedslagsstrukturer har associerats med
malmbildning och stora delar av världens produktion av nickel och guld och i mindre utsträckning
koppar och platinametaller har dessa himlakrockar att tacka för sin tillkomst.
Gallejaurstrukturen uppmärksammades redan på 1960-talet genom att den centrala delen utgörs av
en mycket magnetisk och tung kropp. Tyngdkraften svarade dåligt mot den specifika vikten av
ytbergarten – en granitliknande bergart som kallas Gallejaursmonzonit. Den kraftigaste magnetismen
ligger i en ring runt monzoniten och bergarten, som bland annat blottas i gamla Skellefteälvsfåran,
diagnosticerades (på grund av sin gröna färg och välkristalliserade korn av amphibol) som en
andesitbergart och benämndes Vargforsandesiten. De karterande geologerna hade svårt att förstå
hur denna vulkaniska bergart kunde vara så bemängd med granitfragment. Historiskt sett har de
flesta bergartssmältor som uppstått genom meteornedslag tolkats som vulkaniska bildningar: i Mien
kallades de ryolit, i Dellen och i Lappajärvi benämndes de andesit och i Gardnos i Norge tolkades
de som vulkaniska explosionsbreccior.
Ett annat fenomen omkring Gallejaur är förekomsten av s.k. authigene brecciering, bergarterna har
spruckit upp och formar en puzzleliknande bildning, där man nästan tror sig kunna placera tillbaka
bitarna till en hel bild. Detta fenomen är mycket utbrett och återfinns inte i övriga delar av
Skelleftefältet. Den märkligaste fyndplatsen är Mensträsk och här myntades på 20-talet begreppet
Mensträskbreccia. Mellanrummet mellan puzzlebitarna har fyllts ut med karbonat och detta tolkades
som en urgammal vittringsrest i berggrunden. Denna typ av uppkrossning är vanlig i de bergarter
under kratern som drabbats av chockvågor. Identiska företeelser finns i Rieskratern vid Nördligen
i södra Tyskland, under den lilla kratern i Gardnos i Norge och vid Åvike utanför Sundsvall.
I hela Skelleftefältet – från Skellefteå i öster till Kristineberg i väster – är bergarterna ganska ordentligt
omvandlade (metamorfoserade) och ligger uppresta på kant. Så är inte fallet med bergarterna kring
Gallejaur. De ligger i ett mer eller mindre horisontellt läge och pålagrar de äldre skelleftevulkaniterna
och sedimenten. Detta observerades redan på 50-talet av observanta geologer som Erland Grip och
Gunnar Kautsky. Erland Grip tyckte det var märkvärdigt att de yngre sedimenten, som ligger
avlagrade i Gallejaurstrukturen, befann sig i ett så väl bevarat skick.
Sammanfattande kan man – inte utan fog – säga att bergarterna kring monzoniten mitt i
Gallejaurstrukturen består av så gott som uteslutande sönderkrossade bergarter och bergarter med
olika typer av fragment (polymikta breccior). Och detta förklaras enligt min uppfattning bäst av att
en stor asteroid krockade med denna del av jordskorpan för drygt 1 850 miljoner år sedan.
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Excursion Guide
The Gallejaur Structure, Northern Sweden.
Robert Lilljequist, Ecominas
Rocks, interpreted as impactgenerated lithologies, occur in a large area surrounding the Gallejaur
magnetic structure at latitude 65o10´/longitude 19o30´ in northernmost Västerbotten County in northern Sweden. These rocks comprise a variety of different types of breccias: authigenic/autochthonous
monomict breccias from the underlying rock units (monomictly brecciated basement),
parautochthonous, monomict breccias, and polymict melt breccias. No shock metamorphic minerals have yet been identified.
The Gallejaur structure is located in the central part of the Skellefte mining district in the Precambrian
Baltic Shield. The ca. 1.9 Ga old Skellefte district is an extensively mineralised, mainly felsic,
submarine volcanic belt. The predominant lithologies are acid to mafic volcanic and volcanoclastic
rocks, interbedded with, and overlain by, graphitic schist and greywacke. The rocks, which in the
present article are described as impact-generated, have been called the Vargfors Group and overlie
the Skellefte volcanics and sediments with an angular unconformity. In general, the older rocks are
deformed and folded, which results in a more or less vertical position, whereas the younger rock
sequence is flat lying and undeformed outside of the regional shear zones. The youngest rocks in
the area, intruding the Skellefte district supracrustals and probably the impact-generated rock, are
A/I-type granitoids belonging to the Revsund-Adak granite suite, which have been dated at ca. 1.80
to 1.78 Ga.
The highly magnetic ring around the centre of the structure is interpreted as an impact melt body
with varying amounts of more or less absorbed clasts of various basement lithologies. Density
measurements indicate a mafic composition, implying that the original rocks were andesitic to
basaltic. The gravity anomaly of the central rise region, above +15 mgal (150 gu), is high. This
gravity high is surrounded by an encircling gravity low of about –20 mgal amplitude. The central
uplift area is between 10 and 12 km in diameter according to the gravity and magnetic anomaly
maps, which corresponds to a final crater diameter of 50-60 kilometres, and a transient cavity of
about half that size.
In the centre of the Gallejaur ring structure a fine-grained crystalline rock of monzonitic composition is found, which is loaded with rounded clasts of volcanic character. The origin is not yet established, but the lack of deformation and location within the supposed central uplift indicate that the
monzonitic rock could have an origin as crystallised impact melt. The Gallejaur monzonite has
been dated at 1873 +/- 10 Ma, which is taken as the age of the impact event. The monzonite and an
underlying porphyritic rock seem to be differentiates of one melt body.
In a northwest-southeast zone across the structure occur water-deposited immature sediments (pelitic,
arkosic and conglomeratic), which are interpreted as fill.
The original impact structure has been affected by later deformation events, which are mainly
represented by shearing, faulting and erosion.
The enigma is that we could have an impact event within an active volcanic region. Most of the
fragments (clasts) are of volcanic origin. This has influenced the geologists to interpret all of the
monomict breccia rocks as volcanic or volcanoclastic. The magnetic rocks around the Gallejaur
“monzonite” are referred to as Vargfors andesite. However, if we avoid looking only at isolated
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outcrops and instead observe the distribution of the different rock types, we will find that their
formation from volcanic events is difficult to prove.
The geology of the area surrounding the Gallejaur melt sheet is very different from what we can
observe in the eastern and western parts of the Skellefte district.
The intention with this guide is to apply a different view on the geology and considerate a new
model for the formation of the rocks within an about 25km radii from the centre of the Gallejaure
magnetic and gravimeter anomaly. This might result in new genetic interpretations of several of
the ore deposits within the structure, which in its turn might lead to the discovery of new mineral
deposits.
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Location map of the Gallejaure structure
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GRAVITY MAP GALLEJAURE.
Gravity map over mapsheet 23J Norsjö. Red is gravity high and blue represents gravity low areas.
The gravity hight in the southwest corner is caused by a ultramafic intrusion.
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Magnetic lineaments around the Gallejaure structure and assumed crater margin.
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Brecciated target rocks, allogenic (polymict breccia), impact melt and infill crater sediments
in the Gallejaure Structrue
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The Gallejaur Structure
The Holmtjärn Mine and Granbergsliden
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At Holmtjärn the outcrop 100m to the north consist of cm-sized pieces forming an autochthonous
breccia, weakly mineralised. At the outcrop 60m to the SE a volcanic rock with dm-sized rounded
clasts is exposed (monomict?).
Interpretation: The breccia formations present in the Holmtjärn area indicates that this is a huge
brecciated block either downslided from the central uplift area or drawn in from the outer crater
wall. The sulphide mineralisation is supposedly formed before the impact event but then fractured
into several parts and enriched by hydrothermal solutions after the impact event.
Alternative interpretation: Volcanic and hydrothermal breccia forming a favourable environment
for ore deposition.
At the outcrops at Granbergsliden a monomict breccia with plastically deformed clasts of dm-size
is exposed.
Interpretation: Basal impact breccia with exposed to high temperature and pressure.
Alternative interpretation: A subaqueous mafic volcanic centre consisting of a fire fountain deposit and hyaloclastites with pillow lobes and closely packed pillow lava.
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Excursion Guide
How to get there: From road 365 take the road to Kusfors-Svansele. After 2,8km turn north at the
sign G2G HOLMTJÄRN and follow the road straight to the end (1.6km), passing the old mine and
the restored new mine. 100m north of the end of road there is a small outcrop (about 2 square
meters).
To get to the outcrop at Granbergsliden, turn of the road to Holmtjärn after 600m and drive to the
road end (1.6km) on a gravel road damaged by car tracks. From road end walk 300 meters south.
Co-ordinates: Holmtjärn: 1676972E-7228326N and 1677030E-7228172N
Granbergsliden: 1677684E-7226595N
Plastically deformed monomict breccia. Granbergsliden 1677684E/7226595N
Mineralised authigene breccia at Holmtjärn Mine. 1676972E/7228326N
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The Gallejaur Structure
Rackejaurgruvan and Rackejaurheden
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At the northwestern corner of the Rackejaur open pit there are exposures of a polymict breccia with
granite clasts (Kautsky 1957, p.48) in a granite detritus matrix.. Kautsky describes densely packed
small granite clasts of 3-8cm size. This breccia continues eastwards onto the massive sulphide ore
body, but with increasing shearing of the breccia towards the deposit. Grip (1951,p.39) mentions
granite clast-bearing rock in drill-cores from drill-holes penetrating the massive orebody. The granite clasts are of Jörn type, and the problem to solve is how they were transported 20 kilometers from
their source.
Interpretation: The polymict breccia is formed by fall-in ejecta related to the crater formation.
Alternative interpretation: Kautsky interpretes the “conglomerate” as fluviatil deriving from nonexposed granite massive.
At Rackejaurheden an outcrop with autochthonous and monomict matrix supported breccia can be
seen to overly feldspar porphyry volcanic rocks. A gradational transition from practically undeformed
volcanic rock to autochthonous brecciated rocks to monomict breccia with up to half-meter large
clasts can be seen.
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Excursion Guide
Interpretation: The brecciation forms part of the crater floor resulting from an impact even.
Alternative interpretation: subvolcanic intrusion/lava with a texture that indicate an “in-situ
hyaloclastite” fragmentation of the lava through cooling.
How to get there: From Malå to Rackejaur the distance is 25 km. The Rackejaurheden locality lies
further 3 km north of the mine.
Co-ordinates: Rackejaur: 1657200E-7230900N.
Rackejaurheden: 1657555E-7233954N
Grip, E., 1951: Geology of the sulfide deposits at Mensträsk and a comparison with other deposits in the Skellefte district. SGU Ser C No 515, 55p
Kautsky, G., 1975: Ein Beitrag zur Stratigraphie und dem Bau de Skelleftefeldes. SGU Ser C, No 543.
Polymict breccia at the Rackejaure Mine. 1657200E/7230900N.
Authigene (Autochthonous) brecca in bolder at Rakkejaureheden
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Monomict breccia at Rakkejaureheden. 1657600R/7233950N.
The Gallejaur Structure
Mensträsk Breccia
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The Mensträsk breccia has since long puzzled the geologists. G Kautsky (1957) refers to the
Mensträsk conglomerate and felsite breccia. Kautsky observed its flat-lying position with weak
undulations on top of felsitic altered volcanic rocks. The breccia grades upward into a monomict to
a polymict breccia with clasts of felsite, quartz porphyry and sediments. Both Kautsky and Grip
(1951) interpret the breccia as a sedimentary weathering breccia. The breccia matrix is often filled
with carbonate material.
Interpretation: The Mensträsk breccia is interpreted as an autochthonous breccia formed in the
upper parts of the crater floor from the Gallejaur impact event. The autochthonous brecciation has
a very widespread distribution around the Gallejaur Structure but has not been found in other parts
of the Skellefte district. The brecciation is practically identical to similar crater floor breccias at e.g.
the Ries Crater in Germany.
Alternative interpretation: The Mensträsk breccia was formed as a weathering breccia thus indicating a major hiatus in the Skellefte district. The flat position indicate a major angular unconformity.
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How to reach the outcrops: From Malå to Bäverhult (42km), continue the road 365 towards
Glommerträsk for 6km to the Rakkejaur road to the left (west). Follow the Rakkejaur road for 6km
and park at M located at a topographic height. Here lie several boulders with autochthonous
breccia with acid volcanic fragments in a jig-saw puzzle structure with carbonate infilling. The
fragments are from cm to dm scale in size.
Co-ordinates: 1666905E and 7221755N
At 1.2km E of the boulder locality, following the road towards road 365, a foot-path starts just to the
left of a row of dustbins. Following the foot-path 150m occurs local bedrock/large boulders with
carbonate-cemented autochthonous breccia. Co-ordinates: 1667336E and 7220753NContinuing the foot-path further 400m to the north, crossing a small stream, flat outcrops occur on
both sides of the path. To the right acid volcanic rocks, slightly mineralised. To the left (east) the
same rocks overlain or grading into Mensträsk breccia. Co-ordinates: 1667607E and 7220925N.
Grip, E., 1951: Geology of the sulfide deposits at Mensträsk and a comparison with other deposits in the Skellefte district. SGU Ser C, No 515, 52pp.
Kautsky, G., 1957: Ein Beitrag zur Stratigraphie und dem Bau des Skelleftefeldes. SGU Ser C, No 543.
Boulder of Mensträsk breccia, 1666905E/7221755N. The matrix is partly filled with carbonates.
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Mensträsk breccia with carbonate
infilling. 1667336E/7220753N.
Autochthonous breccia S of Mensträsk, same coordinates as above.
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The Gallejaur Structure
Autochthonous breccia at Stenheden
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Autochthonous brecciation has partly affected the outcrops at Stenheden. The brecciation overprints the silicified acid volcanic rocks. At the outcrop local jig-saw puzzle structures can be appreciated.
Interpretation: Brecciation of the original crater floor, affecting the target rocks.
Alternative interpretation: Some kind of hydrothermal brecciation.
How to get there: From road 365, turn off at the road signed MAURLIDEN 5. After 1450m turn
right on a small forest road, and continue 500m. On the left (east) side a small outcrop is seen ca
50m from the road.
Co-ordinates: 1672197E-7223313N
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Autochthonous (authigene) breccia similar to what is seen at the Stenheden outcrop
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The Gallejaur Structure
Monomict breccia SE of Maurliden
➔
A monomict breccia is seen with plastically deformed clasts of dm-size, some with vesicles. The
rock is similar to the rocks at Granbergsliden. The weathering has partly obliterated the structures.
Interpretation: The monomict breccia is thought to have been originated as more or less in-situ
rotated up-heated clasts close to the bottom of the impact crater.
Alternative interpretation: Subaquatic fire fountain deposits close to synvolcanic faults.
How to get there: Drive to the Maurliden mine (3.5km from road 365), and turn right just before
the mine entrance. After 1.5km turn right on a forest road and follow the road 1.9km. Here goes a
road to the right, uphill a forest clearing. Park and walk the small road for 100m.
Co-ordinates: 1675437E-7220278N.
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Monomict breccia with plastically deformed clasts. Outcrop southeast of Maurliden.
1675437E/7220278N
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The Gallejaur Structure
Monomict breccia at Tistelmyran
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Monomict breccia is seen on a clearing to the left (north) of the road to the Maurliden Mine. At the
first outcrop a monomict to autochthonous breccia is exposed with up to 0.5m large rounded clast.
At the second outcrop the monomict clasts are separated by a greenish matrix material.
Interpretation: The monomict breccia is thought to have been originated as more or less in-situ
rotated clasts.
Alternative interpretation: Subaquatic hyaloclastites with pillow lobes (not earlier observed in
acid volcanic products).
How to get there: The outcrops are located 2 kilometres from road 365, about 50m from the road
to the Maurliden mine. Beware of mine trucks!
Co-ordinates: 1673037E-7223660N and 1673426E and 7223544N
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Monomict brecca at Tistelmyran, W of the Maurliden Mine. 1673037E/7223660N.
Monomict clasts separated by greenish and brownish material. 1673426E/7223544N.
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The Gallejaur Structure
➔
Polymict breccia at Maurträsket
A polymict breccia is exposed at several small outcrops with mm-dm lithic clasts and mineral
clasts, angular to subrounded in shape. The clasts are mainly from volcanic rocks. In order to
protect the surface the moss vegetation has be uplifted and then replaced.
Interpretation: The polymict breccia has been formed through downfall of earlier expelled ejecta
or through a re-sedimentation of downfall ejecta.
Alternative interpretation: Volcanic debris from various sources.
How to get there: Drive to the Maurliden mine (3.5km from road 365), and turn right just before
the mine entrance. Continue the road to just south of Maurträsket. The outcrop lies 50m uphill the
road.
Co-ordinates: 1675089E-7221917N.
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Polymict breccia at Maurträsket 16750890E/7221917N
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The Gallejaur Structure
Monomict breccia N of the Mensträsk Village
➔
Monomict breccia with clasts of acid volcanic rock. The clasts are up to 2dm in size and angular to
subrounded in shape.
Interpretation: The breccia has been formed close to the crater floor from bedrock of local provenance. The formation is thought to have been originated as more or less in-situ rotated clasts.
Alternative interpretation: Volcanic brecciation through eruption under sea level.
Note: The monomict breccia has a very similar appearance at several localities distributed extensively, and mostly just above autochthonous brecciated bedrock grading into unaffected rocks. The
formations appear to be in a sub-horizontal position.
How to reach the outcrops: Drive north on road 365 to the signs indicating Rope Line and
Bäckerudden. Follow this road 900meter, then turn to the right (northwards) between houses and
barns. Follow the small gravel road 600m, just before a yellow-framed house (end of road).
Co-ordinates:
1669808E-7224409N and 1669887E-7224346N
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Monomict breccia at Mensträsk, to the E of the road. 1669890E/7224346N.
Monomict breccia at Mensträsk, to the west of the Road.1669800E/7224400N.
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The Gallejaur Structure
Långtjärn
➔
This locality is included in the excursion guide as an example of breccia occurring in the northwest
sector of the Gallejaur Structure. Outcrops in this area are scarce. Drill-holes 2-5 km south of the
locality penetrates flat lying sequences of clast-rich breccia. At the Långtjärn outcrop a polymict
clast-bearing rock occur with up to half a meter large clasts.
Amygdaloid lava is represented as large clasts. It appears that some of the clasts are plastically
deformed.
Interpretation: Polymict impact breccia n.d.
Alternative interpretation: Volcanic breccia or volcanoclastic debris formation.
How to get there: From the road Sandträsk-Lidmyrliden take the gravel road to Långtjärn. After
1km the road crosses over some small outcrops. The road can be in bad shape.
Co-ordinates: 1663315E-7244152N
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Polymict breccia at Långtjärn with deformed clasts. To the left a large clast of amygdaloid lava.
1663315E/7244152N.
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The Gallejaur Structure
➔
Högheden
At the road there is an exposure of a polymict breccia with volcanic and granitic clasts in a dark
matrix. A weak hydrothermal alteration and pyrite affect the rock and lichen growth has partly
obliterated the rock texture. The clasts are best seen on a wet surface.
Interpretation: Polymict impact breccia n.d.
Alternative interpretation: Volcanic breccia or volcanoclastic debris formation.
How to get there: From the road 365 (“old Military road”), turn left and drive 4km, passing the
Högheden settlement.
Co-ordinates: 1669400E-7243845N
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The Gallejaur Structure
Storgrovan turbidites
➔
Polymict calcareous breccia and unsorted sedimentary debris are exposed at this locality.
The carbonate-rich breccia is mostly covered by rich vegetation. The unsorted sediment is rich in
schist fragments, up to 1dm in size and angular in shape. The metamorphic grade is low.
Interpretation: The rocks are interpreted as crater infill sediments.
Alternative interpretation: The turbidites have been formed in a basin as epiclastic debris flows.
How to get there: The locality can be reached by driving the road on the north side of Skellefte
River (from road 365) 6.5km. The outcrops are located at a curve on the road, close to a river bay.
Co-ordinates: 1678418E-7223642N
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Storgrovan turbidite with cm to dm large clasts in an unsorted sediment with anguslar clasts.
1678418E/7223642N.
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The Gallejaur Structure
➔
Kölen turbidites and suevite (?)
➔
At the first location a rock with white clasts of cm to dm size is exposed. Streaky, shard-like fragments are common and the rock looks similar to an ignimbrite. The textures are best seen on a wet
surface. At the second locality turbidites in a flat position are outcropping. Shale and gritty unsorted layers are represented in the outcrops.
Interpretation: The rock is formed as a suevite with flattened melt clasts. The rocks in the second
locality are interpreted as crater infill sediments.
Alternative interpretation: The rock is originated as a volcanic ignimbritic rock. However the
environment is subaquatic, and ignimbritic textures are not common in formations formed under
water. The turbidites in the second locality are formed in a basin as epiclastic debris.
How to get there: From the road to Mensträsk drive Road 365 1.2km northwards, and turn into a
forest road in quite bad shape with many up sticking stones. After 1.7km turn to the right at the
sharp curve. Just after the curve is a flat outcrop with white clasts and flattened fragments.
To reach the other outcrop continue the road for about 1km, turn left(south) and continue further
1km to the end of the road (this part can be wet and difficult to access. The turbiditic sediments are
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outcropping on a forest clearing 150m SSE of road end.
Co-ordinates: 1669670E-7225183N and 1668236E and 7224947N
Suevite or Ignimbrite? Outcrop at Kölen 1669670E/7225183N
Turbidite with
Excursion Guide
both unsorted and well layered beds. 1668236E/7224947N
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The Gallejaur Structure
➔
Flat-lying turbidites at Vargforsdammen
Tubidite sediments with clay clasts in a near horizontal position.
Interpretation: The sediments have been formed as in-fill sediment within the crater depression.
Rapid infill of the topographic depression combined with slumping and sliding contribute to a very
unsorted product. If the impact took place in an aquatic environment the influx of sediments after
the impact event would be very rapid and in part chaotic.
Alternative interpretation: Sedimentary turbidity debris from various sources.
How to get there: Follow the road from Nicknoret, at road 365, down along the present water
course at about 7 kilometres.
Co-ordinates: 1677657E-7222528N.
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Turbidites with pelitic and psammitic layers and broken pelitic beds. Southern side of the
Vargfors Dam. 1677657E/7222528N
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The Gallejaur Structure
Rörmyrberget
➔
Just beside the road is a flat outcrop with a mafic rock (amphibolite). A 10-20cm wide dyke with
dark amphibolite phenocrysts cuts the deformed amphibolite.
Interpretation: Impact melt injection dyke
Alternative interpretation: mafic volcanic dyke
How to get there: From the Glommersträsk road 365, turn left after the bridge over the Skellefte
River. After 7.4km turn at the sign RÖRMYRVÄGEN and follow the road for 1.2 kilometres.
Co-ordinates: 1668408E-7233280N
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Melt injection dyke in deformed and metamorphic amphibolites, Rörmyrberget.
1668408E/7233280N.
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The Gallejaur Structure
➔
Jörn Granite with dyke
The road exposes a number of small and poorly available outcrops along the ditch. This is, however,
one of the few outcrops where the Jörn granite lies in close contact with the Gallejaur “supracrustal”
rocks. Jörn Granite is intruded by a dark fine-grained rock with amphibole phenocrysts, similar to
the rocks around the Gallejaur “monzonite” The contact is outcropping, showing clasts of granite in
the dark rock.
Interpretation: Impact melt injection dyke into the Jörn Granite, demonstrating that this was part
of the crater floor.
Alternative interpretation: Mafic dyke
How to get there: Follow Bredträskvägen (stems out from road 365) after 7km from the bridge
over the Skellefte River. Follow Bredträskvägen 4.7km, passing a wooden bridge, to a small cottage.
Co-ordinates: 1677500E-7230900N
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Melt injection dyke in Jörn granite, Bredträsket. Jörn granite in the lower part of the photo and a
clast of Jörn granite in dark melt rock 10 cm NW of the compass. 1677500E/7230900N.
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The Gallejaur Structure
➔
Treholmsfors
At the outcrops close to the river a greyish to grey green rock is outcropping, rich in angular to
subrounded lithic and mineral clasts. Unfilled cavities (vugs) are also common. The rock is polymict
and include granite clasts and clasts with concentric structure. Upstream the size and number of
clasts increase. Hundred meters upstream the first locality there is a polished outcrop with a greenish rock with amphibole phenocrysts and “ghost” clasts. This type of rock has been called Vargfors
Andesite.
Interpretation: Impact melt with clasts
Alternative interpretation: Volcanoclastic debris or clastic debris flow.
How to get there: From the Glommersträsk road 365, turn left after the bridge over the Skellefte
River. After 1.4km follow the road sign TREHOLMEN to the left. Drive 100m and continue straight
ahead on a track that leads to a boulder stockpile platform with signs of a car tipping over the edge.
From here walk 100m downstream to a small stream flowing into the Skellefte river course.
Co-ordinates: 1672137E-7227770N
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Granite clast in melt rock. Treholmfors. 1672210E/7227838N.
Breccia clast in melt rock. Treholmfors. 1672107E/7227838N.
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The Gallejaur Structure
➔
Skellefte River
At this locality we can study perlitic textures on the polished outcrops, melt rock with cm-size
clasts and clast-rich rock with 90% dm-sized clasts, mainly of granite.
Interpretation: Impact melt with clasts
Alternative interpretation: Sedimentary formed rocks or Vargfors Andesite (according to existing maps).
Note: The perlitic texture must have been formed from cooling of a glassy matrix - which exclude
a sedimentary origin. An andesite has not these types of inclusions. Other alternative interpretations are welcomed.
How to get there: Follow the asphalt road, starting towards W from road 365, about 1.9km, turn left towards the
river on a gravel road. After 1 300 meters from the start of the gravel road, park the vehicle at the red signs.
Follow the red marked pegs down to the outcrops at the river.
Co-ordinates: 1670635E - 7229154N (perlitic textures); 1670588E-7229155E (clast-rich with
granite)
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Perlitic texture from hydration of glassy rock 1670634E-7229154N.
Melt rock with cm size polymict clastsincluding granite 1670610E-7229155N
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Melt rock rich in subrounded granite clasts in dark matrix 1670580E-7229155N
The Gallejaur Structure
The Gallejaur “monzonite”
➔
The Gallejaur “monzonite” occupies the central part of the Gallejaur Structure. This central part
has a high gravity anomaly that far exceeds the spec gravity of the “monzonite” At the locality the
typical crystalline rock is exposed, a relatively fine-grained intrusive-like rock poor in quartz but
rich in dark inclusions of cm to dm size.
Interpretation: Recrystalized impact melt from a slowly cooled melt pod in the central part of the
central uplift.
Alternative interpretation: Differentiated magma in a caldera structure grading into heavier rock
at depth.
How to get there: Park the vehicle at a road entrance 8.5km from the bridge over the Skellefte
River on the road 365. Follow the old abandoned road 100m N and then follow the red marking
through a dense vegetation and over a small stream.(a 100m walk).
Co-ordinates: 1674546E-7234875N
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Gallejaure “monzonite”, rich in mafic clasts 1674546E-7234875N
Gallejaure “monzonite” from SKB drillhole.
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The Gallejaur Structure
➔
Gallejaur Dam exposure
This is the only locality where part of the supposed central uplift is exposed. The road outcrop is
dominated by a dioritic rock in which dykes and irregular massed of fine-grained dark material
(pseudotachylites) occur. The relationship between the two rock types is best seen when the
surface is wet. Pseudotachylites are either formed as friction melt or melting taken place as a
result of pressure release.
Interpretation: At larger impact structures the central part is uplifted as a central peak. Geological investigations of central uplifts of terrestrial complex craters shows that they are composed of
deformed and fractured rocks that originally underlay the transient crater (Melosh 1989). Grieve
et al. (1981) has shown that the stratigraphic uplift of the crater’s center (h) is related to the final
crater diameter D by: h=0.06D1.1 where all distances are in kilometres. The rocks in the outcrop
at Gallejaur Dam are interpreted as forming part of a central uplift.
Alternative interpretation: The rock is part of a fault zone where the friction has created
pseudotachylitic melt.
Reference:
H.J Melosh, Impact cratering - a geological process. Oxford Monographs on Geology and Geophysics No 11, 1989.
Grieve, R.A.F. et al., Constraints on the formation of ring impact structures, based on terrestrial data. In Multiring Basins (Eds. PH Schultz and
RB Merill), Proc. Lunar Planet. Sci. Conf 12A, pp.37-57.
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How to get there: Follow a gravel road eastwards from Dragnäs at road 365 to Glommersträsk.
Drive through the old Village of Gallejaure at the border between the Västerbotten and
Norrbotten Provinces. The exposure is covered by metal net to prevent the falling out of lose
rock.
Co-ordinates: 1669897E-7233338N
Pseudotachylitic dyke in diorite.
Gallejaure village 1669800E-7223500N.
Breccia dyke with fine grained melt
rock with diorite clasts
Gallejaure village 1669800E-7223500N.
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Oblique view of the lunar far side photographed from Apollo spacecraft.
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Photo from lunar far side photographed from Apollo spacecraft.
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Schematic illustration of the formation of complex craters with either (a) central peaks or (b) peak
rings. Uplift of the crater floor begins even before the rim is fully formed. As the floor fises further,
rim collapse creates a wreath of terraces surrounding the crater. In smaller craters the central uplift
“freezes” to form a central peak. In larger craters the central peak collapses and creates a peak ring
before motion ceases.
Schematically illustrates the for- mation of a complex crater and ils central peak or peak ring. Most
of the details of this figure are de- rived directly from the geologic study ofterrestrial complex
craters. One interpretative feature in- cluded in this illustration, however, is the depic- tio n of the
floor uplift beginning beJare the rim has been completely excavated. A fully formed parabolic
transient crater thus never develops be- cause the floor begins to rise almost as soon as the crater
has stopped growing in depth. This type of collapse has been observed in laboratory-scale im- parts
into strengthless materials (liquid water) and has appeared in recent numerical simulations of the
excavation of large impact craters.
Figuren illustrerar hur en komplex krater bildas och den centralt upplyfta höjdtoppen. De flesta av
detaljerna i bilden baserar sig på studier av kratrar på planeten jorden. Det antas att upplyftningen
av de centrala delarna sker redan när kratern gröps ur. Såväl den centrala höjden som kanterna av
kratern kollapsar nästan omedelbart. Detta har kunnat simuleras i laboratorieskala.
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Hoba meteoriten i norra Namibia. Världens största meteoritklump på 80 ton, bestående av järn och
nickel. Foto R. Lilljequist 1993.
The Hoba meteorite in northern Namibia. The world’ s largest nickel-iron meteorite. The weight is
80 tonnes.
Några av Gibbonsvärmens 72 meteoriter. De väger mellan 150 och 350 kilo vardera. Meteoritsvärmen
slog ned i Namibia och styckena har hittats längs ett 300 km långt stråk. Foto R Lilljequist 1993
Some of the 72 meteorites from the Gibbon swarm in Namibia.
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Map showing the Gallejaure area.
Karta över Gallejaureområdet.
Excursion sites in the Gallejaure area.
Exkursionslokaler i Gallejaureområdet.
COPYRIGHT:
Bilden på försättsbladet är framställd med hjälp av SGU:s flygmagnetiska databas
Bilden på sidan 9 är framstäld med hjälp av SGU:s regionalgravimetriska databas.
Copyright för ovanstående Sveriges Geologiska Undersökning.
För kartillustrationer i detta dokument gäller: Ur Vägkartan © Lantmäteriverket Gävle 2002.
Medgivande M2002/2680.
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