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Marco Brönner1, Odleiv Olesen1, Einar Dalsegg1, Dag Bering2, Ola Fredin1, Bart Hendriks1, Christian Magnus2, Karl Fabian1, Terje Solbakk2, Jon Arne Øverland2, Jan Steinar Rønning1 & Thomas Møller1

1) Geological Survey of Norway (NGU), NO-7491 Trondheim2) Norwegian Petroleum Directorate (NPD), P.O. Box 600, NO-4003 Stavanger

Ha

Deep weathering in the Lofoten-Vesterålen area

– first results from the NGU-NPD TWIN Project

~14 m

9001 - near Oppeid

0.17 SI *10-3

0.56 SI *10-3

~20 m

from www.Norgei3D.no

INTRODUCTION

Deep weathering in Norway commonly occurs alongstructurally defined weakness zones and thus can play adecisive role in the genesis of the relief of a landscape.Glacial erosion and various tectonic processes with faultblock rotation and uplift have often been inferred toexplain the relief of the landscape in Lofoten andVesterålen, but especially recent studies have shown thatthese processes are not sufficient to explain themagnitude and variation in relief. The numerous soundsand islands not only in the Lofoten-Vesterålen area, butalong the Norwegian coast could to a large extent beconditioned by exhumation and erosion of weatheredbasement. Furthermore, remnants of weatheredbasement have an increasing impact on society andeconomy in Norway as they can be responsible, on theone hand, for rock avalanches and tunnel hazards butalso have a considerable potential for groundwater andhydrocarbon reservoirs.In the joint NGU-NPD project ‘Tropical Weathering In

Norway’ (TWIN), both institutes want to investigate thisphenomenon with the aim of gaining a better knowledgeof the processes related to deep weathering in Norway.The development of new mapping and dating techniquesfor weathered rocks is considered as an important factorwithin this project.

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from www.Norgei3D.no

from www.Norgei3D.no

HAMARØYA

VESTVÅGØYA

ANDØYA

Resistivity measurementson Andøya

Sampling of fresh and weathered basement rocks for paleomagnetic dating at the Ramså field on Andøya

Profile 2Profile 2

Profile 3

HADSELØYA

from www.Norgei3D.no

NØSV

Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 mainly reflects the rugose top of a probably weathered shallow gabbro intrusion. The top basement on Profile 1 shows deep trenches and appears craggy, which indicates deep weathering. The weathered basement rock is of clay-rich material (Fig. 1b), cropping out on the beach and in the river bed in the southern Ramså field .

Quarry with deeply weathered mangerites discovered in the 1980s(Photo: P.-R. Neeb)

Unweathered low magnetic basement rock with its deep weathered analog aside.

A c. 20 m thick weathered basement rock is exposed in a quarry and resistivity

measurements indicate a thickness of locally more than 100 m. The weathered

rock is of grainy structure accompanied by large core-stones which are also partly visible in the resistivity data.

The texture of the primary rock is well preserved, which confirms the in-situ character of the outcrop.

Core-stone within the weathered basement rock

In the 1980s an open quarry with deeply weathered mangerite was discovered by NGU (Fig. 3b). The

material was used by local farmers to construct and maintain gravel roads. Resistivity data indicate a

significant irregular shaped top basement with large weakness zones, likely to be due to weathered

basement. Further sites with deeply weathered rocks were observed near by (Fig. 3a). Here one sample

was obtained from a gravelly weathered locality. The sample was measured for porosity and permeability,

giving values of 24.3 % and 291 mD at 20 bar NCP/ 20C.

weakness zones oflow resistivity

3b

STATUS

During fieldwork in the summer of 2009,known sites with deep weathering wererevisited on Andøya and Vestvågøya and newsites were found on Hamarøya and Haseløya.Two sites on Hadseløya and Hamarøya wereopen quarries with observable thicknesses ofc. 14 and c. 20 m of deeply weathered rocks.The texture of the primary crystallinebasement rock was very well preserved onboth sites. The rocks on Hadseløya appearedto be weathered rather homogeneously withonly a few core-stones, whereas on Hamarøyathere is a ‘conglomerate’ of core-stones withweathered rocks in between. Samples fordating and chemical and petrophysicalanalysis were collected from all the sites. Forthe major four locations, resistivitymeasurements were also carried out.All observed major deep-weathering locationsin the Lofoten-Vesterålen area occur in ratherflat areas, where erosion is considered to beminimal. Considering a generally muchgreater amount of erosion in the mountains,deep weathering is likely to have had aconsiderable impact on the topography of theNorwegian landscape.

The grainy product of the

deep weathering at an open

quarry on Hadseløya is seen

as a c. 14 m thick pile of

rather homogenous material

with only a few remaining

core-stones (Fig. 2a). The

texture of the primary rock is

well preserved, which

confirms the in-situ character

of the outcrop. Resistivity

profiling points to even larger

amounts of weathered rock

with thicknesses of locally

more than 100 m and a much

wider geographic extend. For

paleomagnetic dating,

oriented samples were

collected from both fresh and

weathered rocks (Figs. 2b &

2c)

3a

2a2c

2b

The project involves mineral-ogical, chemical and petro-physical characterisation ofknown occurrences of deepweathering, as well as thedevelopment and testing ofAr/Ar and paleomagnetic agedating techniques together withgeomorphological and geo-physical mapping tools. Thestudy will provide a connectionbetween geomorphologicalelements present along theNorwegian coast both onshoreand offshore.

4a

4b

4c

1a

1b

Mesozoicsedimentsbasement

Mesozoicsediments

W E

WE

NE

SW

NW SE

WE

fresh bedrock

weathered gabbro ?

fresh bedrock

deep-weathered basement

from www.Norgei3D.no

Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3 Resistivity Profiles 1 & 2 clearly show the flanks of the Mesozoic basin, whereas Profile 3

Profile 1

1c

Mesozoicsediments

Deep-weatheredbasement ?