the gallejaur structure - georange · excursion guide page 4 meteoriterna utgör de äldsta prov vi...

The Gallejaur Structure

Excursion guide

by

Excursion Guide

Map showing the Gallejaure area. Karta över 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.

Excursion sites in the Gallejaure area. Exkursionslokaler i Gallejaureområdet.

För kartillustrationer i detta dokument gäller: Ur Vägkartan © Lantmäteriverket Gävle 2002.Medgivande M2002/2680.

Excursion Guide

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 uppskattasatt ungefär 500 ton kosmiskt stoft varje dag kommer in i jordens atmosfär. Endast omkring 500meteoriter större än 10cm i diameter överlever varje år passagen genom luften. Som mest hittas ochtillvaratas 5 meteoriter årligen. Antalet kända meteoriter från olika delar av världen uppgår idag tilldrygt 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 nyårsdagen1869 i trakten av Hessle, 3 mil utanför Uppsala. Nedfallet finns livfullt beskrivet i litteraturen. Denandra oktober 1951 för en 300 gram tung meteorit ned i Århus i Danmark. Vid nedslaget gick densönder i fyra bitar.

Det äldsta med säkerhet bevittnade nedfallet ägde rum i Frankrike den 18 november 1492, men detdröjde ända fram till mot slutet av 1700-talet innan vetenskapen accepterade det faktum att det föllned stenar från rymden.

Mer eller mindre väl bevarade kratrar på jordens yta minner om forna meteoritnedfall. Det kanskebästa exemplet på en stor cirkulär krater med en väl utbildad rand är Barringer Crater (tidigare kändsom Meteor Crater) från Canyon Diablo i Arizona. Diametern på denna krater är 1 200m och dennår ned till 180 meters djup. Randzonen höjer sig 46m ovan den omgivande platån. Bergartsfragmentligger utspridda utanför kratern över en radie av 10km från kraterns centrum. Tusentals järnfragmenthar uppsamlats från omgivningarna, sammanlagt till en vikt av 20 ton. Ett av fragmenten finnsutställt 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 hasplittrats och delvis förångats vid nedslaget. I Australien har man funnit 4 välbildade kratrar därjärnfragment påträffats. Totalt har endast 8 kratrar eller kratergrupper med järnmeteoriter hittats.Dessutom är omkring 45 kratrar utan meteoritfragment kända. Längre fram beskrivs stora strukturerpå 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 avdessa var den sibiriska ”Sikhote Alin” som föll den 12 februari 1947. En gångformad krater på 28mi diameter bildades och det största järnfragmentet vägde 1 800 kilo. Sovjetiska vetenskapsmän harräknat 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 blevblodröd och ett mörker spred sig över allt och alla” berättar ett ögonvittne. Kratern besöktes förförsta gången av forskare i juni 1997. Kratern uppmättes till 1.2km i diameter men endast kratervalleni 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 järnmeteoriternaväger ungefär hälften av Hobarmeteoriten. Den största kända stenmeteoriten väger en tredjedelston. De katalogiserade meteoriterna som upplockats från jordytan visar sig bestå till 94% avstenmeteoriter, till 4.5% av järnmeteoriter och till 1,5% av stenjärnmeteoriter.

Excursion Guide

Meteoriterna utgör de äldsta prov vi känner till från vårt solsystem. Åldersbestämningar som harutförts med hjälp av isotopförhållande mellan uran och bly, rubidium-strontium och kalium-argonpekar mot en gemensam ålder. Bildningstiden uppskattas till 4,5 miljarder år och anses varafödelsetiden för vårt solsystem. Det anses troligt att meteoriterna stammar från skilda delar av denyttre rymden, men man vet att de flesta asteroider ligger i ett bälte mellan planeterna Mars ochJupiter. 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 enkrater som är cirka 20 gånger större i diameter än asteroidklumpen. Händelsen i Tungaska den 30juni 1908 orsakade en förstörelse över en yta som täckte en radie av 10km. Nedslagskroppenexploderade innan den nådde jordytan men mindre bitar åstadkom små kratrar på 10-tals metersdiameter. Det kan vara intressant att veta att ozonhalten i atmosfären minskade med 30% som enföljd 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 meteoritkroppenmedan berggrunden delvis smälter och delvis krossas. Trycket orsakar förändringar ikristallfaserna i olika mineral. Diaplektiskt glas bildas och högtrycksförändringar i kvarts gerupphov till coesit och stishovit, kol övergår till diamant, och uppsmältning och gasbildning ägerrum. Graniter smälter vid 60 Gpa (Gigapascal), basiska bergarter vid 80 Gpa och sedimentärabergarter vid 30 Gpa.

• Under en urgröpningsfas (excavation phase) som varar cirka 100 sekunder bildas en fiktivkrater (transient crater) samtidigt som material kastas ut ur kratern, ofta ballistiskt. Kaotiskamaterialströmmar sker utmed kraterytan (ground surge).

• Under en påföljande modifikationsfas, som varar minuter till timmar, sker de slutligaavlagringarna och förändringarna i och omkring kratern. Smältan och så kallade klastiska brecciorblandas, en central dom bildas, kraterranden rasar in, smälta med bergartsfragment injekteras ikraterbotten och hela strukturen deformeras. Resultatet blir en komplicerad kollapsstruktur meden 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 kändaimpaktkratrar på jorden. Av dessa har endast 10% identifierats under den Prekambriska tiden, enera som omfattar omkring 88% av jordens ålder. Denna brist på impaktkratrar jämfört med detenorma antalet kratrar vi kan se på alla våra närliggande himlakroppar beror på huvudsakligen tvåfaktorer. För det första eroderar och förstörs mycket av nedslagets utkastade material och kraterformunder de långa tidsrymder som följt nedslaget. Jordens yta är extremt dynamisk jämfört med våragrannar i solsystemet. För det andra är det mycket få geologer som tillägnat sig de senastedecenniernas framsteg inom impaktforskningen och därför inte vet hur man skall läsa berggrundennär den uppvisar tecken på att ha varit utsatt för de förstörelseeffekter som uppstår vid ett stortnedslag. Det är knappast en överdrift att hävda att den moderna forskningen kring planetära ochterrestriska meteornedslag har orsakat en vetenskaplig revolution lika stor som närkontinentaldriftsteorin 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 nyaforskningsrön. Det är lättare att behålla sin världsbild intakt.

I Sverige har, främst genom Frans E Wickmans pionjärsinsatser, flera nedslagsstrukturer kunnatpåvisas 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 dessahar slagit ned i det Ordoviciska havet för ungefär 450 miljoner år sedan (Lockne, Granby,Tvären).

Excursion Guide

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 iKanada (Sudbury – ca 200km diameter). Bägge dessa stora nedslagsstrukturer har associerats medmalmbildning och stora delar av världens produktion av nickel och guld och i mindre utsträckningkoppar och platinametaller har dessa himlakrockar att tacka för sin tillkomst.

Gallejaurstrukturen uppmärksammades redan på 1960-talet genom att den centrala delen utgörs aven mycket magnetisk och tung kropp. Tyngdkraften svarade dåligt mot den specifika vikten avytbergarten – en granitliknande bergart som kallas Gallejaursmonzonit. Den kraftigaste magnetismenligger 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 enandesitbergart och benämndes Vargforsandesiten. De karterande geologerna hade svårt att förståhur denna vulkaniska bergart kunde vara så bemängd med granitfragment. Historiskt sett har deflesta bergartssmältor som uppstått genom meteornedslag tolkats som vulkaniska bildningar: i Mienkallades de ryolit, i Dellen och i Lappajärvi benämndes de andesit och i Gardnos i Norge tolkadesde som vulkaniska explosionsbreccior.

Ett annat fenomen omkring Gallejaur är förekomsten av s.k. authigene brecciering, bergarterna harspruckit upp och formar en puzzleliknande bildning, där man nästan tror sig kunna placera tillbakabitarna till en hel bild. Detta fenomen är mycket utbrett och återfinns inte i övriga delar avSkelleftefältet. Den märkligaste fyndplatsen är Mensträsk och här myntades på 20-talet begreppetMensträskbreccia. Mellanrummet mellan puzzlebitarna har fyllts ut med karbonat och detta tolkadessom en urgammal vittringsrest i berggrunden. Denna typ av uppkrossning är vanlig i de bergarterunder kratern som drabbats av chockvågor. Identiska företeelser finns i Rieskratern vid Nördligeni 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 ordentligtomvandlade (metamorfoserade) och ligger uppresta på kant. Så är inte fallet med bergarterna kringGallejaur. De ligger i ett mer eller mindre horisontellt läge och pålagrar de äldre skelleftevulkaniternaoch sedimenten. Detta observerades redan på 50-talet av observanta geologer som Erland Grip ochGunnar Kautsky. Erland Grip tyckte det var märkvärdigt att de yngre sedimenten, som liggeravlagrade i Gallejaurstrukturen, befann sig i ett så väl bevarat skick.

Sammanfattande kan man – inte utan fog – säga att bergarterna kring monzoniten mitt iGallejaurstrukturen består av så gott som uteslutande sönderkrossade bergarter och bergarter medolika typer av fragment (polymikta breccior). Och detta förklaras enligt min uppfattning bäst av atten stor asteroid krockade med denna del av jordskorpan för drygt 1 850 miljoner år sedan.

Excursion Guide

The Gallejaur Structure, Northern Sweden.

Robert Lilljequist, Ecominas

Rocks, interpreted as impactgenerated lithologies, occur in a large area surrounding the Gallejaurmagnetic structure at latitude 65o10´/longitude 19o30´ in northernmost Västerbotten County in north-ern Sweden. These rocks comprise a variety of different types of breccias: authigenic/autochthonousmonomict breccias from the underlying rock units (monomictly brecciated basement),parautochthonous, monomict breccias, and polymict melt breccias. No shock metamorphic miner-als have yet been identified.

The Gallejaur structure is located in the central part of the Skellefte mining district in the PrecambrianBaltic 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 volcanoclasticrocks, interbedded with, and overlain by, graphitic schist and greywacke. The rocks, which in thepresent article are described as impact-generated, have been called the Vargfors Group and overliethe Skellefte volcanics and sediments with an angular unconformity. In general, the older rocks aredeformed and folded, which results in a more or less vertical position, whereas the younger rocksequence is flat lying and undeformed outside of the regional shear zones. The youngest rocks inthe area, intruding the Skellefte district supracrustals and probably the impact-generated rock, areA/I-type granitoids belonging to the Revsund-Adak granite suite, which have been dated at ca. 1.80to 1.78 Ga.

The highly magnetic ring around the centre of the structure is interpreted as an impact melt bodywith varying amounts of more or less absorbed clasts of various basement lithologies. Densitymeasurements indicate a mafic composition, implying that the original rocks were andesitic tobasaltic. The gravity anomaly of the central rise region, above +15 mgal (150 gu), is high. Thisgravity high is surrounded by an encircling gravity low of about –20 mgal amplitude. The centraluplift area is between 10 and 12 km in diameter according to the gravity and magnetic anomalymaps, which corresponds to a final crater diameter of 50-60 kilometres, and a transient cavity ofabout half that size.

In the centre of the Gallejaur ring structure a fine-grained crystalline rock of monzonitic composi-tion is found, which is loaded with rounded clasts of volcanic character. The origin is not yet estab-lished, but the lack of deformation and location within the supposed central uplift indicate that themonzonitic rock could have an origin as crystallised impact melt. The Gallejaur monzonite hasbeen dated at 1873 +/- 10 Ma, which is taken as the age of the impact event. The monzonite and anunderlying 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 mainlyrepresented by shearing, faulting and erosion.

The enigma is that we could have an impact event within an active volcanic region. Most of thefragments (clasts) are of volcanic origin. This has influenced the geologists to interpret all of themonomict 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

outcrops and instead observe the distribution of the different rock types, we will find that theirformation from volcanic events is difficult to prove.

The geology of the area surrounding the Gallejaur melt sheet is very different from what we canobserve 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 newmodel for the formation of the rocks within an about 25km radii from the centre of the Gallejauremagnetic and gravimeter anomaly. This might result in new genetic interpretations of several ofthe ore deposits within the structure, which in its turn might lead to the discovery of new mineraldeposits.

Excursion Guide

Excursion Guide

Location map of the Gallejaure structure

Excursion Guide

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.

GRAVITY MAP GALLEJAURE.

Excursion Guide

Excursion Guide

Magnetic lineaments around the Gallejaure structure and assumed crater margin.

Excursion Guide

Brecciated target rocks, allogenic (polymict breccia), impact melt and infill crater sediments in the Gallejaure Structrue

Excursion Guide


The Holmtjärn Mine and Granbergsliden

At Holmtjärn the outcrop 100m to the north consist of cm-sized pieces forming an autochthonousbreccia, weakly mineralised. At the outcrop 60m to the SE a volcanic rock with dm-sized roundedclasts is exposed (monomict?).

Interpretation: The breccia formations present in the Holmtjärn area indicates that this is a hugebrecciated block either downslided from the central uplift area or drawn in from the outer craterwall. The sulphide mineralisation is supposedly formed before the impact event but then fracturedinto several parts and enriched by hydrothermal solutions after the impact event.

Alternative interpretation: Volcanic and hydrothermal breccia forming a favourable environmentfor ore deposition.

At the outcrops at Granbergsliden a monomict breccia with plastically deformed clasts of dm-sizeis exposed.

Interpretation: Basal impact breccia with exposed to high temperature and pressure.

Alternative interpretation: A subaqueous mafic volcanic centre consisting of a fire fountain de-posit and hyaloclastites with pillow lobes and closely packed pillow lava.

➔

➔

Excursion Guide

Plastically deformed monomict breccia. Granbergsliden 1677684E/7226595N

Mineralised authigene breccia at Holmtjärn Mine. 1676972E/7228326N

How to get there: From road 365 take the road to Kusfors-Svansele. After 2,8km turn north at thesign G2G HOLMTJÄRN and follow the road straight to the end (1.6km), passing the old mine andthe restored new mine. 100m north of the end of road there is a small outcrop (about 2 squaremeters).To get to the outcrop at Granbergsliden, turn of the road to Holmtjärn after 600m and drive to theroad 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-7228172NGranbergsliden: 1677684E-7226595N

Excursion Guide


Rackejaurgruvan and Rackejaurheden

At the northwestern corner of the Rackejaur open pit there are exposures of a polymict breccia withgranite clasts (Kautsky 1957, p.48) in a granite detritus matrix.. Kautsky describes densely packedsmall granite clasts of 3-8cm size. This breccia continues eastwards onto the massive sulphide orebody, but with increasing shearing of the breccia towards the deposit. Grip (1951,p.39) mentionsgranite clast-bearing rock in drill-cores from drill-holes penetrating the massive orebody. The gran-ite clasts are of Jörn type, and the problem to solve is how they were transported 20 kilometers fromtheir 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 non-exposed granite massive.

At Rackejaurheden an outcrop with autochthonous and monomict matrix supported breccia can beseen to overly feldspar porphyry volcanic rocks. A gradational transition from practically undeformedvolcanic rock to autochthonous brecciated rocks to monomict breccia with up to half-meter largeclasts can be seen.

➔

➔

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-situhyaloclastite” fragmentation of the lava through cooling.

How to get there: From Malå to Rackejaur the distance is 25 km. The Rackejaurheden locality liesfurther 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

Excursion Guide

Mon

omic

t bre

ccia

at R

akke

jaur

ehed

en. 1

6576

00R

/723

3950

N.

The Gallejaur StructureMensträsk Breccia

The Mensträsk breccia has since long puzzled the geologists. G Kautsky (1957) refers to theMensträsk conglomerate and felsite breccia. Kautsky observed its flat-lying position with weakundulations on top of felsitic altered volcanic rocks. The breccia grades upward into a monomict toa 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 filledwith carbonate material.

Interpretation: The Mensträsk breccia is interpreted as an autochthonous breccia formed in theupper parts of the crater floor from the Gallejaur impact event. The autochthonous brecciation hasa very widespread distribution around the Gallejaur Structure but has not been found in other partsof 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 indi-cating a major hiatus in the Skellefte district. The flat position indicate a major angular unconformity.

Excursion Guide

➔➔

➔

Excursion Guide

How to reach the outcrops: From Malå to Bäverhult (42km), continue the road 365 towardsGlommerträsk for 6km to the Rakkejaur road to the left (west). Follow the Rakkejaur road for 6kmand park at M located at a topographic height. Here lie several boulders with autochthonousbreccia with acid volcanic fragments in a jig-saw puzzle structure with carbonate infilling. Thefragments 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 theleft of a row of dustbins. Following the foot-path 150m occurs local bedrock/large boulders withcarbonate-cemented autochthonous breccia. Co-ordinates: 1667336E and 7220753N-

Continuing the foot-path further 400m to the north, crossing a small stream, flat outcrops occur onboth sides of the path. To the right acid volcanic rocks, slightly mineralised. To the left (east) thesame 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.

Excursion Guide

Mensträsk breccia with carbonateinfilling. 1667336E/7220753N.

Autochthonous breccia S of Mensträsk, same coordinates as above.

Excursion Guide


Autochthonous breccia at Stenheden

Autochthonous brecciation has partly affected the outcrops at Stenheden. The brecciation over-prints the silicified acid volcanic rocks. At the outcrop local jig-saw puzzle structures can be appre-ciated.

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 turnright on a small forest road, and continue 500m. On the left (east) side a small outcrop is seen ca50m from the road.

Co-ordinates: 1672197E-7223313N

➔

Excursion Guide

Autochthonous (authigene) breccia similar to what is seen at the Stenheden outcrop

Excursion Guide


Monomict breccia SE of Maurliden

A monomict breccia is seen with plastically deformed clasts of dm-size, some with vesicles. Therock 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-siturotated 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 beforethe mine entrance. After 1.5km turn right on a forest road and follow the road 1.9km. Here goes aroad to the right, uphill a forest clearing. Park and walk the small road for 100m.

Co-ordinates: 1675437E-7220278N.

➔

Excursion Guide

Monomict breccia with plastically deformed clasts. Outcrop southeast of Maurliden. 1675437E/7220278N

Excursion Guide


Monomict breccia at Tistelmyran

Monomict breccia is seen on a clearing to the left (north) of the road to the Maurliden Mine. At thefirst 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-siturotated clasts.Alternative interpretation: Subaquatic hyaloclastites with pillow lobes (not earlier observed inacid volcanic products).

How to get there: The outcrops are located 2 kilometres from road 365, about 50m from the roadto the Maurliden mine. Beware of mine trucks!

Co-ordinates: 1673037E-7223660N and 1673426E and 7223544N

➔➔

Excursion Guide

Monomict brecca at Tistelmyran, W of the Maurliden Mine. 1673037E/7223660N.

Monomict clasts separated by greenish and brownish material. 1673426E/7223544N.

Excursion Guide


Polymict breccia at Maurträsket

A polymict breccia is exposed at several small outcrops with mm-dm lithic clasts and mineralclasts, angular to subrounded in shape. The clasts are mainly from volcanic rocks. In order toprotect the surface the moss vegetation has be uplifted and then replaced.

Interpretation: The polymict breccia has been formed through downfall of earlier expelled ejectaor 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 beforethe mine entrance. Continue the road to just south of Maurträsket. The outcrop lies 50m uphill theroad.


➔

Excursion Guide

Polymict breccia at Maurträsket 16750890E/7221917N

Excursion Guide


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 tosubrounded in shape.

Interpretation: The breccia has been formed close to the crater floor from bedrock of local prov-enance. 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 exten-sively, and mostly just above autochthonous brecciated bedrock grading into unaffected rocks. Theformations appear to be in a sub-horizontal position.

How to reach the outcrops: Drive north on road 365 to the signs indicating Rope Line andBäckerudden. Follow this road 900meter, then turn to the right (northwards) between houses andbarns. Follow the small gravel road 600m, just before a yellow-framed house (end of road).

Co-ordinates: 1669808E-7224409N and 1669887E-7224346N

➔

Excursion Guide

Mon

omic

t bre

ccia

at M

enst

räsk

, to

the

E o

f th

e ro

ad. 1

6698

90E

/722

4346

N.

Excursion Guide

Monomict breccia at Mensträsk, to the west of the Road.1669800E/7224400N.

Excursion Guide


Långtjärn

This locality is included in the excursion guide as an example of breccia occurring in the northwestsector of the Gallejaur Structure. Outcrops in this area are scarce. Drill-holes 2-5 km south of thelocality penetrates flat lying sequences of clast-rich breccia. At the Långtjärn outcrop a polymictclast-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 plasticallydeformed.

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. After1km the road crosses over some small outcrops. The road can be in bad shape.


➔

Excursion Guide

Polymict breccia at Långtjärn with deformed clasts. To the left a large clast of amygdaloid lava.1663315E/7244152N.


Högheden

At the road there is an exposure of a polymict breccia with volcanic and granitic clasts in a darkmatrix. A weak hydrothermal alteration and pyrite affect the rock and lichen growth has partlyobliterated 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 theHögheden settlement.


Excursion Guide

➔

Excursion Guide


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 inschist 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 SkellefteRiver (from road 365) 6.5km. The outcrops are located at a curve on the road, close to a river bay.


➔

Excursion Guide

Storgrovan turbidite with cm to dm large clasts in an unsorted sediment with anguslar clasts. 1678418E/7223642N.

Excursion Guide


Kölen turbidites and suevite (?)

At the first location a rock with white clasts of cm to dm size is exposed. Streaky, shard-like frag-ments are common and the rock looks similar to an ignimbrite. The textures are best seen on a wetsurface. At the second locality turbidites in a flat position are outcropping. Shale and gritty un-sorted layers are represented in the outcrops.

Interpretation: The rock is formed as a suevite with flattened melt clasts. The rocks in the secondlocality are interpreted as crater infill sediments.

Alternative interpretation: The rock is originated as a volcanic ignimbritic rock. However theenvironment is subaquatic, and ignimbritic textures are not common in formations formed underwater. 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 aforest road in quite bad shape with many up sticking stones. After 1.7km turn to the right at thesharp 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 further1km to the end of the road (this part can be wet and difficult to access. The turbiditic sediments are

➔➔

Excursion Guide

Suevite or Ignimbrite? Outcrop at Kölen 1669670E/7225183N

Turbidite with both unsorted and well layered beds. 1668236E/7224947N

outcropping on a forest clearing 150m SSE of road end.

Co-ordinates: 1669670E-7225183N and 1668236E and 7224947N

Excursion Guide


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 veryunsorted product. If the impact took place in an aquatic environment the influx of sediments afterthe 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 watercourse at about 7 kilometres.


➔

Excursion Guide

Turbidites with pelitic and psammitic layers and broken pelitic beds. Southern side of theVargfors Dam. 1677657E/7222528N

Excursion Guide


Rörmyrberget

Just beside the road is a flat outcrop with a mafic rock (amphibolite). A 10-20cm wide dyke withdark 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 SkellefteRiver. After 7.4km turn at the sign RÖRMYRVÄGEN and follow the road for 1.2 kilometres.


➔

Excursion Guide

Melt injection dyke in deformed and metamorphic amphibolites, Rörmyrberget. 1668408E/7233280N.

Excursion Guide


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 tothe rocks around the Gallejaur “monzonite” The contact is outcropping, showing clasts of granite inthe dark rock.

Interpretation: Impact melt injection dyke into the Jörn Granite, demonstrating that this was partof the crater floor.

Alternative interpretation: Mafic dyke

How to get there: Follow Bredträskvägen (stems out from road 365) after 7km from the bridgeover the Skellefte River. Follow Bredträskvägen 4.7km, passing a wooden bridge, to a small cot-tage.


➔

Excursion Guide

Melt injection dyke in Jörn granite, Bredträsket. Jörn granite in the lower part of the photo and aclast of Jörn granite in dark melt rock 10 cm NW of the compass. 1677500E/7230900N.

Excursion Guide


Treholmsfors

At the outcrops close to the river a greyish to grey green rock is outcropping, rich in angular tosubrounded lithic and mineral clasts. Unfilled cavities (vugs) are also common. The rock is polymictand include granite clasts and clasts with concentric structure. Upstream the size and number ofclasts increase. Hundred meters upstream the first locality there is a polished outcrop with a green-ish rock with amphibole phenocrysts and “ghost” clasts. This type of rock has been called VargforsAndesite.

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 SkellefteRiver. After 1.4km follow the road sign TREHOLMEN to the left. Drive 100m and continue straightahead 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.


➔

Excursion Guide

Granite clast in melt rock. Treholmfors. 1672210E/7227838N.

Breccia clast in melt rock. Treholmfors. 1672107E/7227838N.

Excursion Guide


Skellefte River

At this locality we can study perlitic textures on the polished outcrops, melt rock with cm-sizeclasts 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 exist-ing maps).Note: The perlitic texture must have been formed from cooling of a glassy matrix - which excludea sedimentary origin. An andesite has not these types of inclusions. Other alternative interpreta-tions are welcomed.

How to get there: Follow the asphalt road, starting towards W from road 365, about 1.9km, turn left towards theriver 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 withgranite)

➔

Excursion Guide

Perlitic texture from hydration of glassy rock 1670634E-7229154N.

Melt rock with cm size polymict clastsincluding granite 1670610E-7229155N

Excursion Guide

Mel

t roc

k ri

ch in

sub

roun

ded

gran

ite

clas

ts in

dar

k m

atri

x 16

7058

0E-7

2291

55N

Excursion Guide


The Gallejaur “monzonite”

The Gallejaur “monzonite” occupies the central part of the Gallejaur Structure. This central parthas a high gravity anomaly that far exceeds the spec gravity of the “monzonite” At the locality thetypical crystalline rock is exposed, a relatively fine-grained intrusive-like rock poor in quartz butrich in dark inclusions of cm to dm size.

Interpretation: Recrystalized impact melt from a slowly cooled melt pod in the central part of thecentral uplift.

Alternative interpretation: Differentiated magma in a caldera structure grading into heavier rockat depth.

How to get there: Park the vehicle at a road entrance 8.5km from the bridge over the SkellefteRiver on the road 365. Follow the old abandoned road 100m N and then follow the red markingthrough a dense vegetation and over a small stream.(a 100m walk).


➔

Excursion Guide

Gallejaure “monzonite”, rich in mafic clasts 1674546E-7234875N

Gallejaure “monzonite” from SKB drillhole.

Excursion Guide


Gallejaur Dam exposure

This is the only locality where part of the supposed central uplift is exposed. The road outcrop isdominated 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 thesurface is wet. Pseudotachylites are either formed as friction melt or melting taken place as aresult of pressure release.

Interpretation: At larger impact structures the central part is uplifted as a central peak. Geologi-cal investigations of central uplifts of terrestrial complex craters shows that they are composed ofdeformed and fractured rocks that originally underlay the transient crater (Melosh 1989). Grieveet al. (1981) has shown that the stratigraphic uplift of the crater’s center (h) is related to the finalcrater diameter D by: h=0.06D1.1 where all distances are in kilometres. The rocks in the outcropat 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 createdpseudotachylitic 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 andRB Merill), Proc. Lunar Planet. Sci. Conf 12A, pp.37-57.

➔

Excursion Guide

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 andNorrbotten Provinces. The exposure is covered by metal net to prevent the falling out of loserock.


Breccia dyke with fine grained melt rock with diorite clastsGallejaure village 1669800E-7223500N.

Pseudotachylitic dyke in diorite.Gallejaure village 1669800E-7223500N.

Excursion Guide

Oblique view of the lunar far side photographed from Apollo spacecraft.

Excursion Guide

Photo from lunar far side photographed from Apollo spacecraft.

Excursion Guide

Schematic illustration of the formation of complex craters with either (a) central peaks or (b) peakrings. 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 formation of a complex crater and ils central peak or peak ring. Mostof the details of this figure are de- rived directly from the geologic study ofterrestrial complexcraters. One interpretative feature included in this illustration, however, is the depic- tio n of thefloor uplift beginning beJare the rim has been completely excavated. A fully formed parabolictransient crater thus never develops be- cause the floor begins to rise almost as soon as the craterhas stopped growing in depth. This type of collapse has been observed in laboratory-scale im- partsinto strengthless materials (liquid water) and has appeared in recent numerical simulations of theexcavation 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.

Några av Gibbonsvärmens 72 meteoriter. De väger mellan 150 och 350 kilo vardera. Meteoritsvärmenslog 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.

Excursion Guide

Hoba meteoriten i norra Namibia. Världens största meteoritklump på 80 ton, bestående av järn ochnickel. Foto R. Lilljequist 1993.

The Hoba meteorite in northern Namibia. The world’ s largest nickel-iron meteorite. The weight is80 tonnes.

Excursion Guide

Excursion Guide

Map showing the Gallejaure area. Karta över 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.

Excursion sites in the Gallejaure area. Exkursionslokaler i Gallejaureområdet.

För kartillustrationer i detta dokument gäller: Ur Vägkartan © Lantmäteriverket Gävle 2002.Medgivande M2002/2680.

the gallejaur structure - georange · excursion guide page 4 meteoriterna utgör de äldsta prov vi...

Documents