Garnet compositions in Scottish and Norwegian basement terrains

Garnet compositions in Scottish and Norwegian basement terrains:

a framework for interpretation of North Sea sandstone provenance

Andrew Mortona,b,*, Claire Hallswortha, Bruce Chaltonb

aHM Research Associates, 100 Main Street, Woodhouse Eaves, Leics LE12 8RZ, UKbDepartment of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK

Received 31 December 2002; accepted 10 January 2004

Abstract

Detrital garnets have proved to be useful discriminators of North Sea sandstone provenance because they show a wide range of

potential compositions and are relatively stable during burial diagenesis. It has been less easy to use garnet data to provide a direct link

between sediment and source because of the lack of a comprehensive database of garnet compositions in basement terrains forming the

North Sea hinterland. This paper presents garnet compositional data from river sediments sourced from northern Scotland and Norway,

and demonstrates the presence of marked variations in garnet compositions from different regions related to fundamental differences in

basement lithology. The value of the river sediment database is demonstrated by comparing it with garnet compositions in Paleocene

sandstones from the North Sea and Møre basins, enabling the establishment of direct links between source and sediment. Paleocene

sandstones in the Gannet Field area on the southwestern margin of the Central North Sea were derived from the Dalradian of the

Grampian Highlands, those in the northern North Sea were derived from the Moine/Dalradian rocks of Shetland, and those along the

Møre Basin margin were derived from western Norway. By contrast, Paleocene sandstones in the Nelson Field (central North Sea) and

in the Beryl Embayment (northern North Sea) have a garnet component that cannot be readily traced back to any basement terrain in

either Scotland or Norway. This component, which was ultimately derived from granulite-facies metasediments or charnockites, is

interpreted as being recycled from Triassic sandstones, which were themselves derived from an exotic source, probably to the west of

the British Isles.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Detrital garnet; Provenance; Paleocene; North Sea; Norway; Scotland

1. Introduction

Identification of provenance is crucial in the under-

standing of sandstone depositional systems. Establishing

the location of the source area places important con-

straints on sediment transport pathways and intrabasinal

sand body distribution, and the nature of the sediment

source has a strong influence on porosity and permeability

characteristics. Variations in provenance provide a basis

for discrimination of sand bodies and can also be used to

establish a correlation framework at both local and

regional scales. Sandstone provenance is, therefore, a

key issue in exploration for hydrocarbons in clastic

sediments.

Heavy mineral assemblages are known to be sensitive

indicators of sediment provenance (see review by Mange

and Maurer (1992)), and have been extensively used in

investigations of North Sea sandstones from the Devonian

to the Tertiary (Allen & Mange-Rajetzky, 1992; Hallsworth,

Morton, & Dore, 1996; Morton & Berge, 1995; Preston

et al., 1998). However, determination of sandstone prove-

nance directly from heavy mineral data is not straightfor-

ward. A variety of processes that operate during the

sedimentation cycle may overprint the original provenance

signal, the most important being weathering, hydrodyn-

amics and diagenesis (Morton & Hallsworth, 1999).

Weathering causes modification of source rock mineralogy

at source (prior to incorporation into the transport system)

and during periods of exposure on the floodplain during

transport (alluvial storage). Hydraulic processes during

transport and deposition strongly affect relative abundances

0264-8172/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpetgeo.2004.01.001

Marine and Petroleum Geology 21 (2004) 393–410

www.elsevier.com/locate/marpetgeo

* Corresponding author. Tel.: þ44-150-989-1150; fax: þ44-150-989-

1235.

E-mail address: [email protected] (A. Morton).

http://www.elsevier.com/locate/marpetgeo

of minerals with different hydraulic behaviour (controlled

by grain size, density and shape). Diagenesis selectively

removes unstable minerals at the site of deposition, both at

an early stage (through circulation of acidic meteoric pore

waters) and during burial (through circulation of elevated

temperature pore waters). These processes, either acting

singly or in combination, may cause the present-day heavy

mineral suite to be profoundly different to that provided by

the original source lithologies.

To counteract the effects of these overprinting processes,

Morton and Hallsworth (1994) recommended two comp-

lementary approaches for identification of provenance-

sensitive parameters from heavy mineral assemblages:

determination of ratios of minerals with similar hydrodyn-

amic and diagenetic behaviour, and quantification of

varietal characteristics shown by a single (ideally stable)

mineral group. Types of varietal data include optical and

geochemical attributes of mineral populations. Optically

determined varietal data include tourmaline colour and

morphology (Krynine, 1946; Mange-Rajetzky, 1995),

zircon morphology (Poldervaart, 1955; Lihou & Mange-R-

ajetzky, 1996), and apatite roundness (Allen & Mange-R-

ajetzky, 1992; Morton, Spicer, & Ewen, 2003). Several

mineral groups are conducive to geochemical characteris-

ation (Morton, 1991), including pyroxene (Styles, Stone, &

Floyd, 1989), amphibole (von Eynatten & Gaupp, 1999),

tourmaline (Henry & Guidotti, 1985) and opaque minerals

(Basu & Molinaroli, 1991; Grigsby, 1990).

The mineral group that has been most widely used as a

provenance indicator in NW European sediments is garnet.

This is because garnet is one of the most abundant detrital

heavy minerals found in sandstones from this region, and

because detrital garnet has a wide range of potential

compositions. In consequence, garnet geochemical studies

have proved particularly useful in characterising, dis-

tinguishing and correlating sand bodies on the basis of

their provenance (Morton, 1985; Haughton & Farrow, 1988;

Hallsworth & Chisholm, 2000; Hutchison & Oliver, 1998;

Tebbens, Kroonenberg, & Vandenbergh, 1995).

Although garnet data have proved useful in discriminat-

ing sandstones with differing provenance, it is more difficult

to trace garnet compositions directly back to their source

regions. One of the main reasons for this is the lack of a

comprehensive database on garnet compositions in base-

ment rocks. The most likely ultimate sources of the garnets

in the North Sea and other parts of the UK continental shelf

are the basement rocks of Scotland and Scandinavia.

Consequently, knowledge of the compositions of garnets

supplied by these lithologies would place powerful

constraints on the provenance of the sediments in the

adjacent basins. To some extent, this could be achieved by

compiling published garnet geochemical data from the

basement rocks themselves (Hutchison & Oliver, 1998).

However, data from metamorphic basement rocks are by

necessity selective, and may be biased towards more

unusual lithologies. Therefore, compilations of published

data are unlikely to provide a comprehensive picture of

garnet geochemical populations in individual terrains. A

more suitable approach, therefore, is to analyse garnets from

rivers draining the main basement terrains, because river

sediments represent averaged samples of the rocks out-

cropping in the drainage basin. The only potential danger

inherent in this approach is the possibility that the detritus

may not be representative of the drainage basin, for example

by the introduction of exotic material during repeated Ice

Age glaciations. The possible effects of glacial contami-

nation can be minimised by avoiding taking samples from

lowland areas, where the introduction of exotic material is

most likely. Mearns (1989), in a study of the Sm–Nd

isotopic compositions of river sediments around northern

Scotland, adopted this approach, and demonstrated that

river sediment compositions directly reflect source charac-

teristics without any apparent contamination effects.

Garnet geochemical data have been acquired from 51

rivers in northern Scotland and 43 rivers in Norway, the

total data set comprising 4653 individual garnet analyses.

Garnet compositions from individual samples have been

presented by Hallsworth (1991), Morton and Hallsworth

(1992) and Morton (1998). For the purposes of this paper,

data from rivers draining individual structural or tectono–

stratigraphic blocks have been combined. This has enabled

the characterisation of garnet compositions supplied by

individual basement terrains, thereby placing large-scale

constraints on sand provenance in the North Sea and

adjacent areas.

Garnet compositions were determined using a Link

Systems energy-dispersive X-ray analyser attached to a

Cambridge Instruments Microscan V electron microprobe.

Data reduction used the ZAF4 programme. Count time was

30 s. Analyses were conducted on grain surfaces. Results

from poorly oriented or rough grain surfaces were identified

by low analytical totals and/or deviations from ideal

stoichiometry, and were discarded.

2. Garnet compositions in Scottish river sediment

The pre-Devonian geological framework of northern

Scotland comprises four main blocks (Fig. 1): the Scottish

Highlands south of the Great Glen Fault (area S1), the

Scottish Highlands north of the Great Glen fault and east of

the Moine Thrust (area S2), the Scottish Highlands west of

the Moine Thrust (area S3), and the Shetland Isles (area S4).

2.1. Area S1

The Scottish Highlands south of the Great Glen Fault

consists largely of Dalradian metasediments with wide-

spread Caledonian granites. Basic intrusions are also present

locally. There is a wide range in metamorphic grade of the

Dalradian, from greenschist to upper amphibolite facies

(Strachan, Smith, Harris, & Fettes, 2002).

A. Morton et al. / Marine and Petroleum Geology 21 (2004) 393–410394

Garnets from rivers draining this area are predominantly

Fe–Mn rich (Fig. 2), with variable Ca and low Mg contents

(Field B, as shown in Fig. 2). Approximately 97% of

the garnets from this area fall in Field B, with a small

number (1.3%) in Field C (high Ca, high Mg garnet). There

is virtually no representation of Field A (high Mg, low Ca

garnet). Only a small number (c. 1.5%) of the garnets

contain significant numbers of Fe3þ-rich (andraditic) garnet.

Fig. 1. Summary geological map of northern Scotland, showing location of river sediment sample sites and areas S1–S4, as discussed in text.

A. Morton et al. / Marine and Petroleum Geology 21 (2004) 393–410 395

Most of the published garnet compositions from

Dalradian metasediments fall within the range of garnet

compositions in rivers draining the Dalradian (Fig. 3).

However, garnets from the Huntly–Portsoy area (Fig. 1) are

relatively Mg-rich and Ca-poor (Ashworth & Chinner,

1978), thereby falling into Field A. This group is scarce in

the Dalradian-sourced river sediment data set, indicating

that garnets with such compositions form an insignificant

proportion of the overall garnet budget.

2.2. Area S2

The Scottish Highlands north of the Great Glen fault

and east of the Moine Thrust comprises mainly Moine

metasediments with some Caledonian granitic intrusions.

As with the Dalradian, the metamorphic grade of

the Moine ranges to upper amphibolite facies (Strachan

et al., 2002).

Garnets from rivers draining the Moine (Fig. 2) are

closely comparable with those from the Dalradian, being

Fe–Mn rich with variable Ca. Mg contents are low, rarely

exceeding 20%. As with the Dalradian, the vast majority of

the garnets fall in Field B, with over 97% of the population

falling in this area (Fig. 2). A small number fall in Field C,

but none fall in Field A. The Moine-derived garnets have a

slightly wider range of Ca contents than those from the

Dalradian, with Ca frequently up to 40%. This may reflect

lithological differences between the Moine and Dalradian,

presumably implying that the Moine contains a greater

proportion of calcareous metasediments. No andradite-rich

garnets were found in the rivers from area S2.

There are some allochthonous Lewisian blocks within

the Moine; garnets from rivers draining such areas were not

included in the Moine data compilation. Garnet assem-

blages from rivers that drain both Moine metasediments and

allochthonous Lewisian blocks are more diverse than those

Fig. 2. Garnet compositions in river sediments sourced from Scottish basement terrains. Andraditic (Fe3þ) grains have been omitted. Number of samples

per region are: Dalradian 11 (541 analyses), Moine 17 (850 analyses), Lewisian (excluding South Harris) 11 (534 analyses), South Harris 4 (200

analyses), Loch Maree Group 2 (100 analyses), Shetland 6 (297 analyses). XFeþMn, XMg, XCa are molecular proportions of Fe þ Mn, Mg and Ca, with all

Fe determined as Fe2þ. Filled circles-XMn , 5%, open circles-XMn . 5%. Small ternary plot shows the compositional zones for Field A, Field B and

Field C garnets.


that exclusively drain the Moine (Fig. 4), in that they

include a larger number of garnets that fall in Field C (high-

Ca, high-Mg). The increase in Field C garnets is predictable

given the nature of garnet assemblages draining the

Lewisian west of the Moine Thrust (area S3), as discussed

in the subsequent section.

2.3. Area S3

The area to the west of the Moine Thrust consists of

Lewisian basement with patchy Torridonian sedimentary

cover (Fig. 1). Data from rivers draining the Torridonian

have not been included in this paper. The majority of the

Lewisian outcrop consists of quartzofeldspathic gneisses,

but basic intrusions are widespread. The basic intrusions

include the Scourie dykes, believed to have been intruded

at c. 2400 Ma, and a suite of younger dykes dated as c.

2000 Ma (Fettes, Mendum, Smith, & Watson, 1992; Park,

Cliff, Fettes, & Stewart, 1994). The Lewisian also

includes larger areas of basic gneisses and some

metasedimentary rocks. The South Harris Igneous Com-

plex forms the largest outcrop of basic gneisses, and is

adjacent to the high-grade metasedimentary rocks of the

Leverburgh and Langavat belts (Fettes et al., 1992). The

Loch Maree Group consists of metasediments (Park et al.,

1994), but these are at lower metamorphic grades (middle

to upper amphibolite facies) than the Leverburgh and

Langavat belts. In view of the lithological variability

within the Lewisian Complex, rivers that drain the Loch

Maree Group and those that drain the South Harris

Igneous Complex and adjacent Leverburgh and Langavat

belts are considered separately from those that drain more

typical Lewisian gneisses.

Garnets from rivers draining typical quartzofeldspathic

Lewisian gneisses provide more diverse garnet assem-

blages than those from the Moine or Dalradian (Fig. 2).

Although garnets rich in Fe and Mn, with low Mg and

variable Ca (similar to those of the Moine and

Dalradian), are abundant, garnets with high Mg and

high Ca are also common. In consequence, approximately

80% of the garnets derived from typical Lewisian

quartzofeldspathic gneisses fall in Field B, and 20% in

Field C. The high Mg, high Ca garnets (Field C) are

probably derived from the basic gneisses, since they

correspond closely to published garnet compositions from

both the ‘Older’ and ‘Younger’ Basics (Fettes et al.,

1992) (Fig. 5). Although garnets are scarce in the

quartzofeldspathic gneisses (Fettes et al., 1992), the

more abundant low Mg, Fe þ Mn rich garnets are

probably derived from the intermediate to acidic gneisses,

together with the granites or pegmatites, a number of

which are known to be garnet-bearing (Fettes et al.,

1992). Garnets in intermediate to acidic gneisses from the

Western Gneiss Region of Norway are of this compo-

sition (see later discussion).

Garnet populations in river sediments from South

Harris are markedly richer in Mg than those from the

typical Lewisian (Fig. 2). This reflects the greater

abundances of metabasic rocks in the South Harris

Igneous Complex compared with the typical Lewisian,

and compares closely with published data on garnet

compositions from various units within the complex

(Fig. 5). In addition to the dominant grouping of high

Mg, high Ca garnets, there is also a small but significant

number of high Mg, low Ca garnets. These are closely

comparable to garnets from high-grade metasedimentary

units within the Lewisian of Tiree (Fig. 5) and are

Fig. 3. Published garnet data from the Dalradian (area S1), compiled from

Ashworth and Chinner (1978), Atherton (1969), Chinner (1960), Droop and

Harte (1995), Harte and Graham (1975), Phillips et al. (1993) and (1994)

and Sturt (1962). Key as in Fig. 2.

Fig. 4. Garnet compositions in river sediment sourced from both Moine

metasediments and tectonically emplaced Lewisian gneisses (sample RS38,

area S2). Key as in Fig. 2.


considered to represent the contribution made by the

metasedimentary rocks of the Leverburgh and Langavat

belts, adjacent to the South Harris Igneous Complex.

Field C garnets form 76% of the population from South

Harris, with fields A and C both being small but

significant components.

By contrast, the lower-grade metasediments of the

Loch Maree Group provide uniformly low Mg, variable

Ca garnets, very similar in overall aspect to the

populations supplied by the Moine and Dalradian

metasediments, with Field B garnets forming 94% of the

population (Fig. 2).

2.4. Area S4

The basement rocks of the Shetland Isles are

mainly Moine/Dalradian metasediments of varying

grade (up to upper amphibolite facies). There is one

major Caledonian granite intrusion, as well as an

ophiolite body (the Unst ophiolite) and a small Lewisian

outcrop.

Garnets from Shetland river sediments fall into the same

field as defined by Moine, Dalradian and Loch Maree Group

provenances. They are Fe–Mn rich, with low Mg and

variable Ca, and over 95% of the garnets fall into Field B

(Fig. 2). Although garnet assemblages are closely compar-

able to those derived from the other low-moderate grade

metasedimentary terrains of Scotland, Shetland river

sediments are distinctive in that they contain high

abundances of chloritoid (derived from the metasediments)

and many contain chrome spinel (sourced by the Unst

ophiolite). Since chloritoid and chrome spinel are scarce in

sediments sourced from other Scottish basement areas, the

combination of abundant chrome spinel, chloritoid and

Field B garnets is a clear indication of a Shetland

provenance.

Fig. 6. Summary geological map of southern and mid-Norway, showing

location of river sediment sample sites and areas N1–N6 as discussed in

text. S ¼ Stordalselva

Fig. 5. Published garnet data from the Lewisian (area S3), compiled from

Cartwright (1992) and Fettes et al. (1992). Filled circles are garnets from

metabasic gneisses, filled squares are garnets from metapelitic gneisses.

XFeþMn, XMg, XCa are as in Fig. 2.


3. Garnet compositions in Norwegian river sediment

Garnet data from the Norwegian river sediment data set

have enabled the identification of six separate regions, each

with a characteristic garnet assemblage. These areas

comprise the Lofoten Isles (N1), Mid-Norway (N2), north-

ern West Norway (N3), West Norway (N4), southern West

Norway (N5) and South Norway (N6), as shown in Fig. 6.

3.1. Area N1

The Lofoten Isles is believed to represent the continu-

ation of the Fennoscandian Shield west of the Caledonian

nappes (Skar, 2002). They mainly consist of high-grade

orthogneissic rocks of predominantly intermediate-acidic

composition intruded in two separate phases, one in the

Arcahaen at c. 2.6 Ga, and one in the Early Proterozoic at c.

1.8–1.7 Ga (Griffin et al., 1978; Jacobsen and Wasserburg,

1978), together with subordinate metavolcanic and metase-

dimentary rocks.

Garnet assemblages supplied by the basement rocks on

Lofoten have a distinctive bimodal character (Fig. 7). One

component falls in Field C, comprising garnets with

relatively high Ca and Mg, and forms 11% of the

population. The other falls in Field B, and comprises

garnets with very low Mg, high Mn and moderate to high

Ca, forming over 88% of the population. The assemblage

supplied by the gneisses of the Lofoten area therefore has

some affinities with those of the Lewisian orthogneiss

terrain of NW Scotland, although the low-Mg group is more

abundant than in typical Lewisian gneisses, and the

Lewisian garnet population lacks the distinctive bimodal

character of the Lofoten population.

3.2. Area N2

The Caledonian nappe domain dominates the mid-

Norwegian area (N2). The nappes in this region belong

principally to the Upper and Uppermost Allochthon

(Stephens et al., 1985). The Uppermost Allochthon

Fig. 7. Garnet compositions in Norwegian basement terrains. Andraditic (Fe3þ) grains have been omitted. Number of samples per region are: Lofoten (area N1)

4 (200 analyses), Mid-Norway (area N2) 6 (300 analyses), northern west Norway (area N3) 8 (400 analyses), west Norway (area N4) 12 (600 analyses),

southern west Norway (area N5) 6 (290 analyses), south Norway (area N6) 7 (341 analyses). Key as in Fig. 2.


comprises mainly metasedimentary rocks, including pelitic,

psammitic and calcareous schists and gneisses, whereas the

Upper Allochthon is more variable in lithological terms,

including metasediments similar to those of the Uppermost

Allochthon together with metavolcanic, ultramafic and

eclogitic rocks (Stephens, Gustavson, Ramberg, & Zachris-

son, 1985). The area also contains Caledonian granites,

especially towards the south, plus tectonic windows

exposing orthogneisses believed to represent the western

continuation of the Fennoscandian Shield beneath the

Caledonian nappes (Skar, 2002).

The garnet assemblages from this region show compara-

tively little variation, with Field B garnets dominant (Fig.

7). Field B garnets comprise over 94% of the population,

with Field C forming 3% and Field A 2%.

3.3. Area N3

Area N3 (northern west Norway) has a generally similar

geological framework to mid-Norway (area N2), predomi-

nantly consisting of the Caledonian nappe domain. In this

region, the Upper Allochthon forms the main part of the

exposed Caledonian nappes, again consisting mainly of

metasediments with a variety of subordinate lithologies. The

area also includes relatively large exposures of the

underlying basement, especially along the coast in

the northern part of the area. Gneisses, granites and gabbros

(Gee, Guezou, Roberts, & Wolff; 1985) are the main

constituents of this area of basement, which belongs to the

Southwest Scandinavian Domain (Gaal & Gorbatschev,

1987).

Garnet assemblages from this area reflect its lithologi-

cally heterogeneous nature, with garnets falling in both

Field B (79.5%) and Field C (14%). Again, Field A garnets

are comparatively scarce, forming ,5% of the population.

Field C garnets are more prevalent in rivers from the

northern part of area N3, reflecting the greater exposure of

crystalline basement rocks in this part of the region.

3.4. Area N4

Area N4 (West Norway) is dominated by the large area

of crystalline basement known as the Western Gneiss

Region, which forms part of the Southwest Scandinavian

Domain (Gaal & Gorbatschev, 1987). The region also

includes areas of the Caledonian nappe domain, although

in this area the Middle Allochthon forms the main

exposure of the Caledonides. The lithology of the Middle

Allochthon contrasts with that of the Upper and Upper-

most Allochthon, consisting mainly of Precambrian

gneissic rocks as opposed to metasediments (Bryhni &

Sturt, 1985). Both the Western Gneiss Region and the

rocks of the Middle Allochthon (such as the Jotun Nappe)

contain high-grade basic gneisses (including eclogites),

and locally pyroxenites and peridotites are present (Qvale

& Stigh, 1985).

The garnet assemblages from this area are distinctive, in

that they are dominated by garnets with high Ca and

moderate to very high Mg. This population reflects the

widespread distribution of high-grade basic gneisses and

eclogites. Garnets with very high Mg are typical of the

pyroxenites and peridotites that occur locally in parts of

western Norway. This type of garnet is scarce in most of the

river sediments analysed, but some samples (notably from

Vadheim, on the north side of Sognefjord) are rich in this

component. The range of garnets in the river sediments from

western Norway matches closely the overall range of garnets

analysed from the metamorphic rocks themselves (Krogh,

1980; Medaris, 1980, 1984; Mysen & Heier, 1972), as shown

in Fig. 8. It also demonstrates the relationship between garnet

composition and host rock composition in this gneiss terrain,

with Mg contents being highest in the ultramafic rocks,

moderate in the mafic rocks, and lowest in the intermediate-

acidic rocks and associated granites and pegmatites. This

pattern suggests that the low Mg garnets that dominate the

Lewisian-derived river sediments were sourced from the

quartzofeldspathic gneisses and associated granitic rocks.

3.5. Area N5

Rivers draining a relatively small region of southern west

Norway, between Stavanger and Flekkefjord, are distinctive

in containing large numbers of high Mg and low Ca garnets.

The majority of the garnets from this area (54%) therefore

fall into Field A, with virtually all the other garnets falling in

Field B (Fig. 7). Area N5 is dominated by the Rogaland

anorthosite norite complex, which comprises anorthosite

Fig. 8. Published garnet data from western Norway, from Krogh (1980),

Medaris (1980) and (1984) and Mysen and Heier (1972). Filled circles

represent ultramafic rocks (peridotites and pyroxenites), open circles

represent eclogites, other mafic gneisses, and quartz–biotite gneisses, and

filled squares represent granitic gneisses, tonalitic gneisses, hybridised

gneisses and pegmatites. XFeþMn, XMg, XCa are as in Fig. 2.


plutons intruded by the Bjerkreim–Søkndal layered intru-

sion, together with minor garnetiferous metapelites, all of

which have undergone granulite facies metamorphism

(Maijer et al., 1981; Moller, Andriessen, Hebeda, Jansen,

& Verschure, 2002). Charnockites are a significant

component of the intrusive complex (Duchesne & Wilmart,

1997). Similar type A garnets occur in river and beach

sediments from southern India, derived from charnockitic

and high-grade metasedimentary rocks (Sabeen, Ramanu-

jam, & Morton, 2002), as well as in high-grade Lewisian

metasedimentary rocks of South Harris (Fig. 2) and Tiree

(Cartwright, 1992). The type A garnets characterising the

river sediments from area N5 therefore confirm that high

Mg, low Ca garnet is derived from charnockites or

granulites-facies metasedimentary rocks. In this instance,

the scarcity of garnet in the charnockites themselves

suggests that the metasediments sourced the majority of

the garnets in river sediments from this area.

3.6. Area N6

Area A6 (south Norway) mainly consists of the South-

west Scandinavian Domain, comprising autochthonous

granites and gneisses formed mainly during the Gothian

orogeny (c. 1.75 – 1.50 Ga) and subjected to further

deformation, metamorphism and granite intrusion during

subsequent orogenic events, especially the Sveconorwegian

(Gaal & Gorbatschev, 1987). The basement rocks are

locally overlain by metasedimentary rocks belonging to the

Telemark Suite. Area N6 also includes relatively small

outcrops of the Caledonian nappes.

Garnets are relatively scarce in the river sediments

from this area, probably reflecting the preponderance of

acidic basement gneisses. The garnets that are present

are predominantly Fe-rich, with variable Mn and Ca,

and low Mg, and therefore the majority (.80%) fall in

Field B. These garnets are typical of intermediate to acidic

gneisses, as shown in Fig. 8. South Norway also has the

highest proportion of Fe3þ-rich garnets in the study,

suggesting a significant amount of sediment may have

been supplied from skarns.

4. Discussion

In the Scottish river sediment suite, there is a clear

distinction between the composition of garnet assemblages

derived from low-moderate grade metasedimentary terrains

and those from the high grade orthogneiss basement. Low-

moderate grade metasediments (such as the Moine of the

Northern Highlands, the Dalradian of the Grampian High-

lands, the Moine/Dalradian of Shetland, and the Loch Maree

Group of the Lewisian) all provide garnet assemblages

dominated by types falling into field B. Field B garnets

comprise between 88.1% and 95.4% of the populations,

with a small field C component (4.6–10.2%), and very few

in field A (,1%). Field C garnets are more abundant in

sediment derived from high-grade Lewisian orthogneiss

terrains, with c. 20% of the garnets from typical quartzo-

feldspathic gneiss sources and .75% of the garnets from

the metabasic-dominated South Harris Igneous Complex

being of this type. Nevertheless, Field B garnets are the

dominant influence in assemblages derived from the typical

quartzofeldspathic Lewisian gneisses.

The results from the Norwegian rivers are consistent with

those obtained from Scottish rivers, in that areas dominated

by low to moderate grade metasediments supply garnet

assemblages dominated by Field B types, whereas orthog-

neiss terrains supply larger numbers of Field C types.

Supply of Field C garnets from orthogneiss terrains is

strongly controlled by the distribution of basic gneisses

within the crystalline basement. For example, garnet

asemblages from west Norway are dominated by Field C

garnet because of the widespread distribution of metabasic

gneisses and eclogites, together with less common pyrox-

enites and peridotites. By contrast, south Norway supplies

very few garnets of this type, because of the predominantly

acidic nature of the orthogneisses. Assemblages from this

area, therefore, are dominated by Field B garnets.

In general, Field A garnets, which have a high-grade

metasedimentary or charnockitic source (Sabeen, Ramanu-

jam, & Morton, 2002), are uncommon in river sediments

draining the basement terrains of both Scotland and

Norway. They are extremely scarce in sediment derived

from low-moderate grade metasedimentary terrains (such as

the Moine or Dalradian), and in the typical quartzofelds-

pathic Lewisian gneisses. They are slightly more common

in rivers from South Harris, where they are interpreted to be

derived from the high-grade metasediments of the Lever-

burgh and Langavat belts. The only area in either Scotland

or Norway to supply assemblages dominated by Field A

garnets is the small district south of Stavanger, in southern

west Norway.

4.1. Application in provenance analysis of the North Sea

Paleocene

The value of the garnet geochemical database from the

basement terrains of Scotland and Norway can be evaluated

by comparing garnet compositions with those in the

Paleocene submarine fan systems of the North Sea and

Møre Basin areas. A detailed picture of transport pathways

in these submarine fan complexes has been established

(Reynolds, 1994), and the locations of the sediment source

areas are now comparatively well established (see review by

Bowman, (1998)).

It is therefore, possible to directly compare garnet

compositions in the submarine fan sands with those in the

proposed catchment areas. This has been achieved using

data from sandstones covering a wide range of geographic

locations and stratigraphic positions, including the Forties

Fm in the Nelson Field (UK well 22/6a-9), the Andrew Fm


Fig. 9. Configuration of submarine fan systems of the Andrew (a) and Forties (b) Fms, adapted from Reynolds (1994). (a) also shows the distribution of the mid-

Paleocene submarine fan system on the Møre Basin margin, taken from Roberts et al. (1999). The inclusion of the Møre Basin margin fan system on this map is

not meant to imply that it is synchronous with the Andrew Fm. Locations of wells discussed in the text are also shown,


and Forties Fm in the Gannet Field (UK wells 21/30-13

and 22/21-5S1), the Maureen Fm and Heimdal Fm in

well 9/13-1, located in the Beryl Embayment (data from

Morton, Hallsworth, & Wilkinson, 1993), the Heimdal Fm

in well 3/12-1, located in the East Shetland Basin (data from

Morton et al. (1993)) and the Paleocene of Norwegian well

6306/10-1, located on the margin of the Møre Basin (data

from Morton and Grant (1998)). These well locations are

shown in Fig. 9, together with the distribution of, and

proposed sediment transport routes for, the submarine fan

sands of the Andrew and Forties Fms (Reynolds, 1994),

together with the distribution of the mid-Paleocene sands on

the Møre Basin margin (from Roberts et al., 1999).

4.2. Forties Fm, Nelson Field area

The submarine fan sandstones comprising the Forties

Fm in the Nelson Field area are interpreted as being

derived from the northwest, with an entry point located on

the southeastern part of the Orkney–Shetland Platform

(Fig. 9). Garnet assemblages in Forties Fm sandstones of

well 22/6a-9 are heterogeneous, with compositions falling

in Fields A, B and C (Fig. 10). There are only minor

differences in the relative abundance of garnets falling onto

these three fields (30–46% in Field A, 28–44% in Field B,

18 – 32% in Field C), and consequently the garnet

assemblages from 22/6a-9 form a well-defined cluster on

the ternary diagram comparing the relative abundances of

these three garnet groups (Fig. 11).

In comparison with the river sediment database (Fig. 11),

the Forties Fm garnet assemblages in 22/6a-9 appear to have

components derived from both Lewisian (including typical

orthogneiss and basic gneisses similar to those in South

Harris) and Scottish metasedimentary basement sources

(Moine of mainland Scotland, Dalradian of mainland Scot-

land, Moine/Dalradian of Shetland, Loch Maree Group).

However, the large Field A group cannot be modelled as

having a Scottish basement source, and indicating that the

Forties Fm garnet assemblage includes a recycled com-

ponent that was ultimately derived from beyond northern

Scotland. One possibility is that the Field A garnets were

recycled from the Old Red Sandstone (ORS), which forms a

large part of the outcrop on the Orkney–Shetland Platform.

However, garnet data from the ORS of the Orkney–Shetland

Platform appear to rule this prospect out, since the

assemblages are dominated by Field B garnets, with minor

Fig. 10. Representative garnet compositions from Paleocene submarine fan sands in the North Sea and Møre Basin. Well locations are shown in Fig. 9. Key as

in Fig. 2.


Field C and scarce Field A (Fig. 11). Garnet data from the

ORS are therefore compatible with a Moine/Dalradian or

similar low to moderate grade metasedimentary source. It is,

however, important to recognise that the garnet geochemical

dataset on the ORS of the Orkney–Shetland Platform is

limited in size, the data shown in Fig. 11 being based on just 2

samples, and that this interpretation may have to be modified

as further data become available.

At this stage, it is considered more likely that the large

Field A component in the Forties Fm was recycled from

Permo–Triassic sandstones on or adjacent to the Orkney–

Shetland Platform. Garnet populations from the Foula Fm

of the Strathmore Field, located on the Atlantic margin of

the UK continental shelf, to the west of Orkney (Fig. 10),

are almost exclusively composed of Field A types (Fig.

11). The ultimate source of the Foula Fm Field A garnets

is interpreted as high-grade (granulite facies) metasedi-

ments or charnockites, by analogy with the garnet

compositions found in the areally restricted high-grade

metasediments in the Lewisian, and with the garnets in

sediments of southern India, which can be tied back

directly to such source lithologies (Sabeen et al., 2002).

Although there is a small region in southern west Norway

(area N5) that presently provides such garnets, it is

unlikely on paleogeographic grounds that this area

supplied the Field A garnets of the Foula Fm, and a

source to the west of Britain therefore appears most

likely.

Permo–Triassic sandstones in the northern North Sea

(such as those of the Lewis Fm of the Beryl Field) also have

a high field A garnet component (Fig. 12), although types B

and C are more common than in the Foula Fm. The Forties

Fm garnets could therefore be a mixture of recycled Foula

Fm, together with Lewisian (including both typical acid-

intermediate orthogneisses and basic gneisses) and Moine/

Dalradian material, the latter possibly recycled from

Fig. 11. Relative abundances of Field A, Field B and Field C garnets in Paleocene submarine fan sands from the North Sea and Møre Basin margin, compared

with garnets in river sediments draining the basement rocks of northern Scotland and western and southern Norway, garnets from the Foula Fm (Triassic) of

well 204/30-2 (Strathmore Field, west of Orkney) and Old Red Sandstone of the Orkney–Shetland Platform. Well locations are shown in Fig. 9.


the ORS. Alternatively, the similarity between the Forties

Fm and Lewis Fm garnet assemblages suggests the former

could have been recycled from Permo–Triassic sandstones

with garnet characteristics similar to those of the Lewis Fm.

4.3. Andrew and Forties Fms, Gannet Field area

The Forties Fm sandstones in the Gannet Field area are

interpreted as belonging to a discrete submarine fan

unrelated to the main northwesterly derived system that

deposited the sandstones in the Nelson Field area (Arm-

strong, 1987; Reynolds, 1994). The fan system in the

Gannet area is considered to have a source to the west,

possibly located in the Grampian Highlands of Scotland

(Fig. 9). Previously published garnet data from the Gannet

Field area supported a difference in provenance compared

with the main Forties fan system (Morton, 1987), and the

new data from well 21/30-13 reinforces this interpretation.

The garnet assemblages from the Forties Fm in 21/30-13 are

dominated by Field B types (Fig. 10). Consequently they

plot close to the Field B pole on the ternary diagram in Fig.

11, closely comparable to average Dalradian- and Moine-

derived garnet assemblages. The river sediment data

therefore provide strong support for derivation of the

Forties Fm sandstones from the adjacent Grampian High-

lands of northern Scotland, which are dominantly composed

of Dalradian metasediments. A possible alternative source

would be recycled ORS, providing the ORS was derived

from similar low-moderate grade metasediments.

According to Reynolds (1994), the Andrew Fm sand-

stones in the Gannet area lie at the distal end of a

northwesterly derived submarine fan complex (Fig. 9)

with a similar transport system to that of the main Forties

fan. However, garnet data from most of the samples

analysed from the Andrew Fm in well 22/21-5S1 are

closely comparable to those found in the Forties Fm in 21/

30-13 (Figs. 10 and 11). On this basis, it seems most likely

that the majority of the Andrew Fm sandstones in the area

were deposited by a local, discrete fan system sourced from

the Dalradian of Scotland. However, sandstones in the upper

part of the Andrew Fm in 22/21-5S1 have markedly

different garnet assemblages (Fig. 10), containing high

abundances of garnets in all three compositional fields. On

the ternary diagram in Fig. 11, garnets from the upper part of

the Andrew Fm plot in a similar position to those of the

Forties Fm of the Nelson Field. The close similarity between

the upper Andrew sandstones in 22/21-5S1 and those of the

Forties Fm in 22/6a-9 suggests that the upper part of the

Andrew succession in the Gannet area represents the distal

portion of the northwesterly-sourced fan system, consistent

with the configuration of the Andrew submarine fan system

proposed by Reynolds (1994). A similar source, involving

recycled Permo–Triassic plus Lewisian and Moine/Dalra-

dian components, is proposed.

4.4. Beryl embayment

The Paleocene submarine fan sandstones in the

Beryl Embayment (Fig. 9) were sourced from

Fig. 12. Composition of garnets from the Lewis Fm, Beryl Field area, UK northern North Sea (Preston et al., 1998, 2001), compared with garnets derived from

Scottish and Norwegian basement terrains. Key as in Fig. 2.


the Orkney – Shetland Platform directly to the west

(Reynolds, 1994). Garnet data from well 9/13-1 demonstrate

a significant evolution in the nature of the sediment source

area supplying the fan sandstones (Morton et al., 1993). The

garnet populations in the lower part of the succession are

dominated by the Field A component, with subsidiary Field

B and scarce Field C. The main source of these sandstones is

therefore recycled Permo–Triassic, with subsidiary Moine/

Dalradian (or recycled ORS) and a minor Lewisian

component. Towards the top of the submarine fan sandstone

succession, there is a marked change in mineralogy, with a

major increase in Field B garnets and a corresponding

decrease in Field A. This change corresponds with the

appearance of chloritoid in the heavy mineral assemblages,

and was interpreted by Morton et al. (1993) as indicating the

onset of direct sourcing from the metamorphic basement of

Shetland. The river sediment garnet dataset provides

support for this view, the observed trend in garnet

composition being consistent with a switch in source from

recycled Permo–Trias to the Moine/Dalradian metasedi-

ments of Shetland. The trend shown in Fig. 11 also hints at a

slight concomitant increase in the involvement of Lewisian-

type sources.

4.5. East Shetland basin

The Paleocene submarine fan sandstones in the East

Shetland Basin (Fig. 9) were sourced from the Orkney–

Shetland Platform directly to the west (Reynolds, 1994).

The presence of chloritoid in the heavy mineral suites is an

indication of direct sourcing from the Moine/Dalradian of

Shetland (Morton et al., 1993). Garnet data are consistent

with this view, with the assemblages being dominated by

Field B types, subordinate Field A, and scarce Field

C. Comparison with the river sediment data set indicates

that the majority of the sediment was derived from the

Shetland metasedimentary basement, with a minor amount

of Permo–Triassic recycling, and little evidence for a

contribution from Lewisian basement.

4.6. Møre Basin margin

The Paleocene sandstones along the Møre Basin margin

lie immediately offshore from the western Norway area, as

described above (Fig. 9). The garnet assemblages in well

6306/10-1 (Morton & Grant, 1998) are markedly different

from those found in the submarine fan sandstones sourced

from Scotland and the Orkney–Shetland Platform, with

most of the garnets falling into Field C, a subsidiary number

in Field B, and poor representation of Field A (Fig. 11).

Comparison with the river sediment data set (Fig. 11) shows

that the garnet assemblages in 6306/10-1 are most closely

comparable to the assemblages found in South Harris and

western Norway. Taking the geographic constraints into

account, therefore, the garnet data provide evidence for a

firm provenance link between the Paleocene sandstones of

the Møre Basin margin and the basement rocks of western

Norway, with the assemblages indicating derivation from

high-grade basic gneisses. The average western Norway

garnet assemblage is richer in the Field B component than

the sandstones in 6306/10-1, indicating heterogeneity in the

garnet compositions in this region related to the distribution

of metabasic rocks. Garnet assemblages in some of the river

sediments from this area are more closely comparable to

those of 6306/10-1. For example, the garnet assemblage

from Stordalselva (S on Fig. 6) has 76% Field C garnets,

20% Field B garnets, and 4% Field A garnets, almost

identical to one of the two samples from 6306/10-1. This

may indicate that the Paleocene sandstones on the Møre

Basin margin were supplied by relatively small scale fluvial

systems tapping a small part of the area.

5. Concluding remarks

Linking sediment to source requires the development of a

comprehensive picture of source area characteristics to

enable comparison with the sedimentary products of such

regions. One possible approach is to establish a mineral-

chemical database to link the compositions of sediments

with their potential source regions. This paper has illustrated

the efficacy of this approach, by establishing the geochem-

ical composition of garnet populations provided by the

basement terrains of Scotland and Norway and comparing

them with garnet populations in Paleocene submarine fan

sandstones in the North Sea and Møre Basins. The study has

shown that using this approach, garnet compositions can be

directly tied back to adjacent source areas. For example,

there are close similarities between garnet assemblages in

the Andrew and Forties Fm sandstones of the Gannet Field

area and those provided by the Dalradian metasediments of

the adjacent Grampian Highlands, between Heimdal Fm

sandstones of the Beryl Embayment-East Shetland Basin

areas and the Moine/Dalradian metasediments of Shetland,

and between the Paleocene sandstones on the Møre Basin

margin and the high-grade basic gneisses of western

Norway. The study has also shown that a substantial

proportion of the garnets in North Sea Paleocene sediments

cannot be tied back to a Scottish basement source,

indicating the involvement of an exotic component. This

exotic component is believed to be recycled from Permo–

Triassic sandstones such as the Foula Fm, which is found

along the Atlantic margin and was evidently sourced by a

high-grade metasedimentary or charnockitic terrain to the

west of Britain. Recycled Permo–Triassic garnets form a

significant proportion of the assemblages in the Forties Fm

of the Nelson Field, the Maureen and Heimdal sandstones of

the Beryl Embayment, and some assemblages from the

Andrew Fm in the Gannet Field area.

There are two possible ways of acquiring mineral-

chemical data on potential source areas. One is to use

published information on mineral compositions from


exposed lithologies in the catchment area. The other

approach, adopted in this study, is to generate the dataset

using present-day river sediments. We believe the latter

approach provides more representative information on

source area characteristics, since published mineral-chemi-

cal data in source rock lithologies are by necessity selective,

whereas river sediments provide an unbiased average

sample of catchment lithology. The problems of using

published information on garnet compositions are illustrated

by the Dalradian dataset. The published Dalradian garnet

data were used by Preston et al. (2001) to interpret the

provenance of Triassic Lewis Fm sandstones in the northern

North Sea (UK Quadrant 9). However, the field of Dalradian

garnet compositions generated from published information

(Fig. 3) overemphasises the abundance of garnets falling

into Field A when compared with the river sediment data

(Fig. 2). The published data set suggests that field A garnets

form c. 8% of the Dalradian garnet population, whereas the

river sediment data suggest only 0.2% are of this type.

Comparison of the garnet assemblage in the Lewis

Formation with the river sediment data set (Fig. 12) shows

that neither Dalradian- and Moine-type metasedimentary

sources, nor the Old Red Sandstone of the Orkney–Shetland

Platform, were dominant sediment contributors, casting

doubt on the interpretation of a source comprising Shetland

Islands basement rocks and the Old Red Sandstone (Preston

et al., 2001).

As well as providing specific information on the types of

garnet provided by the various basement blocks around the

North Sea, the data presented in this paper also provide

more general information on the nature of garnet assem-

blages supplied by different metamorphic basement lithol-

ogies. There are close similarities between garnet

assemblages derived from all of the low-moderate grade

metasedimentary terrains analysed in the study, such as the

Dalradian of the Grampian Highlands, the Moine of the

Northern Highlands, the Moine/Dalradian of Shetland, the

Loch Maree Group of the Lewisian Complex, and the

metasediments of the Uppermost Allochthon and Upper

Allochthon of mid-Norway. Low-moderate grade metasedi-

ments therefore consistently provide garnet assemblages

dominated by the Field B component. However, intermedi-

ate to acidic gneiss terrains, such as those of the Lewisian

and the southern Norway area, also provide garnet

assemblages dominated by Field B. Consequently, Field B

garnet assemblages cannot be used unequivocally as

indicating derivation from low-moderate grade metasedi-

ments. Nevertheless, it should be possible to use other

features of the heavy mineral assemblages to distinguish

between these possible provenances. For example, the

presence of minerals such as staurolite and kyanite is

diagnostic of a metasedimentary source. Furthermore,

intermediate-acidic gneiss terrains tend to be relatively

garnet-poor, and consequently provide sand with low

garnet:zircon ratios (low GZi in the terminology of Morton

and Hallsworth, 1994).

Field C garnets are supplied by high-grade gneiss terrains

containing an appreciable basic gneiss component, such as

the Lewisian, the crystalline basement of Lofoten, and the

western Norway area. The abundance of Field C garnets

appears to vary in proportion to the abundance of basic

gneisses in the source area, since the greatest numbers of

Field C garnets occur in sediments derived from the South

Harris Igneous Complex and western Norway. Field C not

only includes garnets of basic gneiss origin, but also those

from ultrabasic rocks such as peridotites and pyroxenites.

Such lithologies tend to supply garnets with very high Mg

contents (Medaris, 1980, 1984), and it is therefore likely that

Field C could be further subdivided to help evaluate the

relative contributions from basic and ultrabasic gneisses.

Field A garnets are, for the most part, scarce in the

basement terrains of Scotland and Norway. Data from other

areas, such as southern India, suggest that such garnets are

derived from high-grade (granulite facies) metasediments or

charnockites (Sabeen et al., 2002). This interpretation is

supported by the presence of Field A garnets in the river

sediments of South Harris, interpreted as being derived from

the Leverburgh and Langavat metasedimentary belts within

the Lewisian Complex, and by published data on garnets

from similar high-grade Lewisian metasediments from

Tiree. The abundance of Field A garnets in river sediments

from southern west Norway suggests similar rocks occur at

outcrop in this district.

There are some limitations regarding the application of

garnet geochemical studies of river sediment in linking

sediment to source. The most obvious is that the method

described herein only provides an insight into the prove-

nance of the garnet, and not the sandstone as a whole. This is

not a major limitation in a North Sea context, where heavy

mineral assemblages tend to be garnet-dominated. How-

ever, it is likely to be a more serious problem in garnet-poor

regions, and alternative methods should be considered in

such cases. One possible alternative is to concentrate on

tourmaline, which is a stable mineral and whose compo-

sition is known to vary according to its paragenesis (Henry

and Guidotti, 1985).

Another limitation is that although river sediment

provides a good guide to the nature of the source area at

the present day, source area characteristics may have evolved

through time (for example, through unroofing). This is

exemplified by the mineralogy of the Paleocene submarine

fan sandstones in the Nelson Field and Beryl Embayment,

which were derived from the Orkney–Shetland Platform, an

area presently dominated by Old Red Sandstone. Garnet

geochemical evidence, however, indicates that the ORS was

not a major contributor to the Paleocene fan sandstones, with

Permo–Triassic recycling being of much greater import-

ance. Evidently, Permo–Triassic sandstones were more

widely distributed on the Orkney–Shetland Platform during

the Paleocene than they are at the present day. By

contrast, there is a close correspondence between garnet

assemblages in basement-derived Paleocene sandstones and


the present-day metamorphic hinterland, such as the Forties

Fm of the Gannet area and the adjacent Dalradian basement,

the Heimdal Fm of the Beryl Embayment-East Shetland

Basin and the adjacent Shetland Isles, and the Paleocene

sandstones of the Møre Basin margin and western Norway.

Unroofing, therefore, does not appear to have caused any

significant evolution in basement characteristics around the

North Sea, at least from the Paleocene to the present day. In

the case of western Norway, this can be taken back at least as

far as the Jurassic, since garnet assemblages in parts of the

Brent Group sequence in the Oseberg Field (Hurst and

Morton, 1988) are dominated by the Field C component,

similar to present-day river sediments in the adjacent

basement area.

Finally, the relationship between garnet compositions in

source areas and their sedimentary product may be obscured

by burial diagenesis. Although garnet is relatively stable

during burial, it does undergo dissolution by high tempera-

ture porefluids (Morton and Hallsworth, 1999). To some

extent, the stability of garnet is controlled by its compo-

sition, with high-Ca garnets being less stable than low-Ca

garnets (Morton, 1987; Smale and van der Lingen, 1989). In

consequence, Field C and high-Ca Field B garnets are less

stable than Field A and low-Ca Field B garnets. The

application of the river sediment data set in the interpret-

ation of provenance of deeply buried sandstones may,

therefore, significantly underestimate the importance of the

basic gneiss component and, to some extent, the low-

moderate grade metasedimentary component. Caution is

therefore required when interpreting compositional data

from garnet assemblages that have been modified by

dissolution during deep burial, a process that can be

diagnosed by the presence of well-developed etch facets

on garnet grain surfaces (Morton et al., 1989).

Acknowledgements

Research into the heavy mineral suites and garnet

geochemistry of the Paleocene sandstones in the Gannet

Field was funded as part of NERC grant

NER/T/S/2000/01367. We are grateful to Euan Mearns for

providing some of the Norwegian river sediment samples, to

Shell UK Exploration and Production Ltd for permission

to publish garnet data from the Gannet and Nelson fields, to

Mary Turton (CASP) for assistance with drafting, and to

Andrew Whitham and Robert Knox for constructive

comments on an earlier draft of the manuscript.

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Garnet compositions in Scottish and Norwegian basement terrains