LØSMASSE

Norges Geologiske Undersøkelse (NGU) har gjennom prosjektet ‘Tanadalen, kvartærgeologisk detaljkartlegging’ finansiert av NGU og Norges vassdrags- og energidirektorat (NVE) kartlagt Tanadalen mellom Sirbmá og elvemunningen ved Tanadeltaet i målestokk 1:20 000. Kartleggingen ble utført i perioden 2016-2019. Målet med prosjektet var å produsere et kvartærgeologisk kart i 1:50.000 eller bedre med beskrivelse og eventuelle anbefalinger til videre skredfarevurderinger med fokus på kvikkleireskred. Informasjon som er relevant for andre skredtyper inkluderes også. Kartleggingen viser at de sentrale deler av dalføret er dominert av delta og elveavsetninger, og at marine avsetninger, inkludert hav- og fjordavsetninger, for det meste ligger langs dalsidene og i sidedaler. Lokale forekomster av breelvavsetninger er også kartlagt. Kartet viser flere områder med spor etter skred av forskjellig størrelse og type, både leirskred som kan ha involvert kvikkleire, fjellskred, steinsprang og snøskred. Enkelte av disse skredene er kjent fra historiske skredhendelser. Marin grense (MG) i det kartlagte området ligger mellom ~65 m i nord og ~75 m ved Tana Bru. Det er usikkert hvor langt sør (opp i dalen) havet stod på slutten av siste istid, men marine avsetninger er funnet nært Storfossen og det er mulig at havet stod sør for Storfossen. Kvartærgeologiske kart (løsmassekart) er grunnlagskart som gir oversikt over løsmassenes utbredelse i landskapet, ulike landformer, og hvordan disse dannes. Slike kart kan brukes som et viktig utgangspunkt for kartlegging av områder som potensielt kan utsettes for skred, da de viser de generelle egenskapene til løsmassene, i tillegg til spor etter aktiv erosjon og tidligere skred. Kartene kan dermed fortelle litt om hvor utsatt et område er for slike hendelser.

Den kvartærgeologiske kartleggingen på kartblad Porsgrunn ble utført i 1980-1985,aqv NGU og Fylkeskartkontoret i Telemark. Strandforskyvning og vegetasjonshistorie er undersøkt og beskrevet av Kari Henningsmoen. Beskrivelsen til kartblad Porsgrunn er delt i to hoveddeler. Den generelle delen gir en kort innføring i kvartærgeologi med oversikt over løsmassenes dannelse og en generell definisjon og beskrivelse av det kvartærgeologiske kartet. I den spesielle delen er det gitt en oversikt over berggrunn, landformer og isbevegelser før de enkelte løsmassetyper beskrives. Isavsmelt- ningen omtales ganske nøye og brerandtrinn og marine strandlinjer er forsøkt rekonstruert. Til slutt gis det en vurdering av løsmassenes egnethet til forskjellige formål med spesiell vekt på sand- og grusressursene. Orientering om utførelse og metoder som har vært brukt under arbeidet med kartet er gitt i Tillegg. Det er også tatt med en del eksempler på anvendelse av løsmassene.

The superficial deposits occurring within the map-sheet area are classified according to their genesis and described. Till is the dominating sediment, but considerable glacifluvial deposits also occur. Glacier movement directions have been reconstructed in seven phases and subphases. The two youngest subphases correlate with events during the deglaciation. The older phases are considered to span most of Weichselian time. Of the eight glacial events known from the Porsangerfjord area, three occur within the map-area: The Outer Porsanger, Korsnes and Repparfjord events, with estimated ages about 13,500 - 11,500 years B.P. A broad marginal zone was produced during the Outer Porsanger event, extensive glacifluvial deposits are related to the Repparfjord event. Radiocarbon datings are lacking, suggested ages are based on correlation with dated events elsewhere.

The superficial deposits within the map-sheet Stiklestad are classified and
mapped according to their genesis. Ice movements and the course of deglaciation
have been reconstructed. The shore-level displacement has been investigated.
Several radiocarbon datings are presented.

The superficial deposits within the area of map-sheet Siljan, southeastern Norway, are classified and described. Most of the hilly areas are dominated by exposed bedrock or a thin and discontinuous cover of till. In the main valleys, beow the marine limit (c. 160-180 m a.s.l.), fine-grained marine sediments and fluvial deposits dominate. Glaciofluvial deposits exist in Siljandalen and Lågendalen as well as in the tributary valley Hærlandsdalen. Ice movements and the course of deglaciation are reconstructed. The oldest ice movements were towards the south, later turning towards SSE and SE. Features associated with gacial events marking small advances or halts of the ice-front during the deglaciation in late Younger Dryas and early Preboreal time have been detected within map-area. The Geiteryggen Ice-marginal deposits are correlated with the Ski Ice-marginal deposits in the Oslofjord region and dated to c. 10 000 years B.P. The mapped area was ice-free 9,850-9,800 years B.P. Short accounts are given on shorelines and shore-level displacement. The potential for different utilization of the superficial deposits is also briefly discussed.

The superficial deposits occuring within the map-area are described and classified according to their genesis. In the mountains a discontinuous or thin cover of till dominates with peat in the depressions. In the lowland areas glaciomarine sediments were deposited at a sea-level situated c. 155 m above the present one. The deglaciation of this area took place during the Bølling and Allerød chronozones. During the Younger Dryas the glaciers andvanced to the Tautra Moraines, situated some 20 km to the cast of this map-area. This led to an intense calving of the glaciers in the Trondheimsfjord. Drifting icebergs caused a marked increase in the content of coarse-grained rock fragments in the glaciomarine sediments. During the shoreline displacement the sediments below the upper marine limits were influenced by fluvial erosion and slides. Short account are given on the potential for different utilization of the mapped superficial deposits.

The superficial Quaternary deposits occuring within the map-sheet area have been classified and mapped according to their genesis. Fine-grained marine deposits dominates in the main valley below the highest postglacial sea-level. c. 1973 m a.s.l. Numerous glaciofluvially ice-contact deposits are situated in the main valley too, but only the nothernmost was built up to the sea level (true deltas). During the postglacial isostatic uplift, the rivers retransported the glaciofluvial deposits and most of this material was deposited as regression-deltas at lower levels. Above the highest sea level quaternary deposits are almost lacking except for scattered till.
Løsmasse og arealbruk: The utilization and evaluation of the superficial deposits with regards to cultivation, forestry, waste disposal, engineering, groundwater, sand and gravel resources and nature conservation, are discussed. A field trip guide to the area is presented.

The superficial deposits occuring within the map-area are described and classified according to their genesis. In the mountains a discontinuous or thin cover of till dominates with peat in the depressions. In the lowland areas glaciomarine sediments were deposited at a sea-level situated c. 180 m above the present one. In Frosta and Skatval these sediments are overlain by till, deposited by a glacial advance during early Younger Dryas, reaching just to the west of the map-area. Marginal moraines from a late Younger Dryas glacial advance can be traced more or less continuously from Hegra in Stjørdal toowards Hoklingen and Alstahaug. During the shoreline displacement the sediments below the upper marine limit were influenced by fluvial erosion and slides. Short accounts are given on the potential for different utilization of the mapped superficial deposits.

The superficial deposits occurring within the map-area are described and classified according to their genesis. In the mountains till and organic deposits dominate. In the lowland areas glaciofluvial and glaciomarine sediments were deposited at a sea-level situated c. 180 m above the present one. During the shoreline displacement these sediments were strongly influenced by fluvial erosion and slides. The ice recession vas interrupted by a glacial advance during the first half of the Younger Dryas glacial advance ca be traced less continuously from the lakes Store Dragstsjøen - Foldsjøen towards Frigården, Lånke, and Hegra in the Stjørdal valley. Radiocarbon datings from Frosta, Levanger and Verdal indicate that these marginal deposits were formed during late Younger Dryas. Short accounts are given on the potential for different utilization of the superficial deposits.

The superficial deposits occurring within the area covered by the map-sheets are classified genetically and described. Most of the deposits are tills covering wide areas in the southeast, where there are also many deposits of glaciofluvial origin. Fluvial deposits are deposited mainly in the Jakobselva valley, along the Russian border. Along the shore there are few superficial deposits, except for small marine deposits. Brief descriptions are given of the lithology and grain-size distribution of all the superficial deposits. A model for the sequence of geological events during the Quaternary era is proposed based on earlier work and recent field mapping. It is suggested that the deglaciation of the coastal area of Sør-Varanger took place about 12,500 - 10,000 years B.P.