NGU Report 2012.005 Tropical Weathering in Norway, TWIN Final
Total Page:16
File Type:pdf, Size:1020Kb
NGU Report 2012.005 Tropical Weathering in Norway, TWIN Final Report 2 Geological Survey of Norway NO-7491 Trondheim, Norway REPORT Tel.: 47 73 90 40 11 Telefax: 47 73 92 16 20 Report no.: 2012.005 ISSN 0800-3416 Grading: Open Title: Tropical Weathering In Norway, TWIN Final Report Authors: Client: Odleiv Olesen, Dag Bering, Marco Brönner, Einar Dalsegg, Karl Fabian, Ola Geological Survey of Norway Fredin, Jomar Gellein, Berit Husteli, Christian Magnus, Jan Steinar Rønning, and Norwegian Petroleum Terje Solbakk, Jan Fredrik Tønnesen & Jon Arne Øverland Directorate County: Commune: Vestfold, Vest-Agder, Sogn & Fjordane, Nord-Trøndelag, Nordland, Finnmark Map-sheet name (M=1:250.000) Number of pages: 188 Skien, Mandal, Florø, Ulsteinvik, Trondheim, Svolvær, Andøya Price (NOK): 645,- Fieldwork carried out: Date of report: Project no.: Person responsible: 2009-2011 02.05.2012 3307.00 Summary: The Geological Survey of Norway and the Norwegian Petroleum Directorate established the TWIN Project (Tropical Weathering In Norway) in 2009 to improve the understanding of deep weathering on mainland Norway as well as offshore. All known reports of deep weathering in Norway have been reviewed and registered in a GIS database. We have selected four representative locations for more detailed studies in southern Norway (Kjose, Lista, Vågsøy-Stad and Inderøya) and another four locations in northern Norway (Vestvågøya, Hadseløya, Andøya and Hamarøya). Geophysical studies (2D resisitivity and refraction seismic) reveal fairly continuous saprolite layers with a thickness up to 100 metres in Vestfold (Kjose), Lofoten- Vesterålen, Hamarøya and Varangerhalvøya. The results also show that the degree of weathering varies extensively over short distances. An offshore petroleum reservoir consisting of weathered basement rocks is most likely equally heterogeneous. The deep weathering in the Lofoten-Vesterålen and Hamarøya occur most likely on the basement surface of rotated fault blocks, whilst the weathering in southeastern Norway and eastern Finnmark seems to occur in a stable and more distal part relative to the Mesozoic rift basins. The remnants of deeply weathered basement on the mainland of Norway occur as accumulations of clay minerals along structurally defined weakness zones and locally as up to c. 100 m thick continuous saprolite layers. Weathering extending down to more than 200 m depth is observed in fracture zones. XRF analysis and mass balance calculations (degree of leaching) commonly show a high degree of mineral alteration with bulk leaching of main elements (Si, Al, Na and K) in the range of 30-65%. The pervasive alteration of large volumes of bedrock is indicative of chemical weathering caused by percolating acidic groundwater in a humid climate, possibly during tropical to sub-tropical conditions in the Mesozoic. Subsequent erosion and denudation have largely removed the evidence of deep weathering on the mainland. The deep weathering process can also explain some of the enhanced concentrations of heavy metals and REE in some Norwegian tills. Understanding these processes is consequently a key to a successful mineral exploration programme as well as an efficient planning of new tunnels and other rock constructions in Norway. Keywords: Geofysikk (Geophysics) Kontinentalsokkel (Continental shelf) Tolkning (Interpretation) Magnetisk metode (Magnetic Berggrunnsgeologi (Bedrock Dypforvitring (Deep weathering) method) geology) Paleomagnetisme (Palaeomagnetism) Elektrisk metode (Electrical method) Fagrapport (Scientific report) CONTENTS 1 INTRODUCTION ............................................................................................................... 7 1.1 Saprolite – deep tropical weathering ......................................................................... 8 2 GEOPHYSICAL METHODS ........................................................................................... 19 2.1 Resistivity and induced polarisation........................................................................ 19 2.1.1 Acquisition ................................................................................................ 19 2.1.2 Inversion.................................................................................................... 20 2.2 Refraction seismics.................................................................................................. 21 2.3 Amager method ....................................................................................................... 22 2.4 Palaeomagnetic dating............................................................................................. 23 2.4.1 Sampling.................................................................................................... 24 2.4.2 Sampling procedure for weathered rocks at Hadseløya............................ 24 2.4.3 Palaeomagnetic measurements.................................................................. 26 3 GEOLOGICAL AND GEOCHEMICAL METHODS...................................................... 27 3.1 Geochemical and mineralogical analyses................................................................ 27 3.1.1 Introduction ............................................................................................... 27 3.1.2 Sampling locations .................................................................................... 28 3.1.3 Analytical methods.................................................................................... 28 3.1.4 Results ....................................................................................................... 29 3.1.5 Conclusions ............................................................................................... 30 3.2 Remote sensing mapping of deep-weathered bedrock ............................................ 30 4 REVIEW OF PREVIOUS DEEP-WEATHERING REPORTS........................................ 33 4.1 Geophysical data ..................................................................................................... 35 4.2 Profiles..................................................................................................................... 36 5 SOUTHERN NORWAY ................................................................................................... 37 5.1 Lista ........................................................................................................................ 37 5.2 Kjose........................................................................................................................ 47 5.2.1 Geochemistry, clay mineralogy and granulometry ................................... 48 5.2.2 Electric resistivity measurements.............................................................. 50 5.3 Vågsøy – Stad – Inderøya........................................................................................ 55 5.3.1 Vågsøy....................................................................................................... 55 5.3.2 Stad............................................................................................................ 66 5.3.3 Inderøya..................................................................................................... 71 6 NORTHERN NORWAY................................................................................................... 77 6.1 Introduction ............................................................................................................. 77 6.2 Geological framework of the Lofoten-Vesterålen-Hamarøya region ..................... 78 6.3 Vestvågøya .............................................................................................................. 79 6.3.1 Resistivity profiling................................................................................... 81 6.3.2 Seismic profiling ....................................................................................... 83 6.3.3 Comparison of seismic and resistivity profiling and conclusions............. 85 6.4 Hadseløya ................................................................................................................ 87 5 6.4.2 Palaeomagnetic sampling.......................................................................... 91 6.5 Sortland ................................................................................................................... 94 6.5.1 AMAGER results ...................................................................................... 94 6.5.2 Resistivity profiling................................................................................... 95 6.6 Hamarøya ................................................................................................................ 98 6.6.1 Resistivity profiling................................................................................... 99 6.7 Andøya .................................................................................................................. 100 6.7.1 Resistivity profiling................................................................................. 102 6.7.2 Palaeomagnetic sampling, Ramså........................................................... 108 6.7.3 Seismic profiling, Staveheia,................................................................... 111 6.8 Conclusions from Lofoten-Vesterålen and Hamarøya.......................................... 117 7 REGIONAL GEOMORPHOLOGY AND DEEP WEATHERING ............................... 120 8 DISCUSSION.................................................................................................................