ISSN 0365-4850

ACTA NATURALIA ISLANDICA

Facies analysis of the Breidavfk Group sediments on Tjornes, North Iceland

J6n Eirfksson

ICELANDIC MUSEUM OF NATURAL HISTORY Reykjavik 1985 ACTA NATURALIA ISLANDICA

PUBLISHED BY THE ICELANDIC MUSEUM OF NATURAL HISTORY (NATTURUFRAEDISTOFNUN ISLANDS)

The Museum published two volumes of Acta Naturalia Islandica in the period 1946-1971, altogether 20 issues. From 1972 each paper has appeared under its own serial number, starting with no. 21.

ACTA NATURALIA ISLANDICA contains original articles dealing with the botany, geology, and zoology of Iceland.

ACTA NATURALIA ISLANDICA is published preferably in English, and appears at irregular intervals.

ACTA NATURALIA ISLANDICA may be obtained:

1) on basis of institutional exchange from the Icelandic Museum of Natural History, P.O.Box 5320, 125 Reykjavik, Iceland.

2) by purchase (including mailing costs) from Snaebj6rn J6nsson, The English Bookshop, Hafnarstraeti 4, 101 Reykjavik, Iceland.

EDITORIAL BOARD: Erling 6lafsson (ed.) Bergth6r J6hannsson Sveinn P. Jakobsson

ODDI hf. Facies analysis of the Breidavik Group sediments on Tj 6rnes, North Iceland

J6N EIRfKSSON Science Institute, University of Iceland, Jardfraedahus Hask6lans, 101 Reykjavik

Abstract. Thick Quaternary sediments of the Breidavik Group are intercalated between lava flows and tuffs on Tj6rnes, North Iceland. The vertical column of strata displays a rhythmic character. The present paper contains a model for sedimentation during glacial-interglacial cycles in a volcanic environment. The facies record is analyzed with reference to observations at the margins of modern glaciers, and fourteen separate glaciations are indicated by tillite horizons. The tillites are typically associated with kame and outwash conglomerates, followed at first by glaciolacustrine and glaciomarine mudrocks and diamictites, and then by deltaic and/or bar-lagoon assemblages, fluvial sediments and subaerial lava flows. Depositional environments are reconstructed for several steps in the geological history of the Breidavik Group. The oldest tillite has been dated at ca. 2 Ma. Several of the tillite beds are succeeded by marine sediments with fossils, especially in the lower part of the sequence. Facies analysis of the lithologies and fossils leads to the conclusion that the Breidavik Group cycles reflect glacial-interglacial cyclicity with a frequency of the order of one glaciation every 100-150 thousand years. During the first million years the deposition took place in a transitional environment while the area was subsiding, but the latter part of the geological record reflects terrestrial deposition and volcanism. CONTENTS

INTRODUCTION...... 3 THE BREIDAVIK GROUP. GEOLOGICAL SETTING AND RESEARCH METHODS .. 4 FACIES MODELS FOR GLACIAL SEDIMENTATION 6 SEDIMENTOLOGICAL CHARACTERISTICS OF THE BREIDAVIK GROUP...... 11 Tillites 11 Lodgement tillite...... 12 Flow tillite " '" ...... 15 Kame conglomerates 16 Review ofkame genesis ,...... 16 Distribution ofkames in the Breidavfk Group 17 Size and shape ofthe Breidavfk Group kames 17 Internal structure and genesis ofthe Breidavfk Group kames...... 19 Outwash and lacustrine mudrocks and conglomerates 21 Marine mudrocks and sandstones 28 Nonfossiliferous sandstones and conglomerates 30 Volcanic tuffs and lava flows of the Breidavik Group 30 INTERPRETATION OF THE BREIDAVIK GROUP 32 Palaeoenvironmental reconstruction ofglacial-interglacial cycles 35 The Furuvfk Formation 35 The Horgi Formation...... 38 The Fossgil Mernber 41 The Svarthamar Member 43 The Mana Formation...... 47 The Grasafjoll and Husavfk Formations 51 CONCLUSIONS 53 ACKNOWLEDGEMENTS 53 REFERENCES 54 3

INTRODUCTION tectonically active zones of intensive uplift, but Quaternary sections on land are generally local basins may develop in collision related 'incomplete. Repeated glaciations have tended tectonic settings (Mitchell and Reading 1978). to devastate the record of the immediate More extensive basins are likely to develop geological past. Therefore the story of continen­ along transform faults, subduction zones and tal glaciations within a given region must often divergent plate margins. Plate tectonic proces­ be pieced together from short, isolated sections ses can be assumed to operate independently of from various localities. This task is difficult in climate, and it may be assumed that the present the absence of reliable dating techniques for configuration of plate margins and processes pinning down absolute ages of individual gla­ have prevailed throughout the Quaternary. cial-interglacial cycles, and the history may be Thick Quaternary sequences of sediments may incomplete. The problem is particularly acute in probably be expected to be associated with regions where proximal glacial environments some plate margins. Unfortunately, plate mar­ have prevailed during glacials throughout the gins in the Northern Hemisphere tend to be Quaternary. Frequent erosional unconformities either mountainous areas of intensive relief and characterize many sequences from such erosion, such as the Rockies, the Alps, and the environments. Deposits from the glacio-marine Himalayas, or submarine ridges or trenches. environment, while promising a better con­ Although divergent plate margins exist on tinuity, are generally not accessible for mapping land elsewhere in the Northern Hemisphere and dating except through geophysical profil­ (California Bay, Afar), the only one to have ing. Although raised marine sediments often been subjected to Quaternary glaciations is yield important evidence about deglaciation located in Iceland, where the axial zones of patterns, they span only a very limited time rifting and volcanism feature several char­ relative to a full glacial-interglacial cycle. acteristics that have contributed towards the It has been suggested that deep sea records production and conservation of the excep­ provide a yardstick, against which the fragment­ tionally thick Quaternary sequence. Processes ary continental records might be placed (Shack­ associated with plate margins in Iceland were leton 1977). The deep sea sediments have con­ reviewed by Palmason and Saemundsson (1974) tinued to accumulate outside the glacial and by Saemundsson (1978, 1980). The key environments, irrespective of glacial events. elements are the relatively constant (over Oxygen isotope curves derived from deep sea thousands of years) rate of pouring out of volca­ sediments show recurrent changes of sea-water nics within the zones, coupled with subsidence properties, that have been interpreted as reflec­ and related tilting of flanking rock units towards ting global ice-volume changes (Olausson 1965, the axial zones. These elements are significant Shackleton and Opdyke 1973). Although the for the conservation potential of the geological oxygen isotope curves and the stages defined record because the resistant lava flows form a for them may evolve as a global framework for shield against future erosion and weathering of the Quaternary cyclicity in ice volume changes, any underlying sediments, and the subsidence they do not offer many clues to local or regional reduces the likelihood of erosion as well as patterns of glaciation. However, the accumulat­ reducing the available energy for erosion of a ing evidence from the deep sea floor of the particular rock unit if erosional circumstances North Atlantic constitutes a challenge to those do arise. In the absence of sediments, facies of who investigate Quaternary sections on land. volcanic rocks may yield important information The records must be compared and explana­ about the physical conditions during eruptions. tions sought if discrepancies exist. Piling up of hyaloclastite ridges and table moun­ Most of the land masses of the Northern tains indicates subglacial or subaqueous condi­ Hemisphere are tectonically stable shields, tions whereas ice free environments on land where the conservation potential of sediments is permit lavas to flow freely and crystallize with­ typically low. This is also true of mountainous out brecciation. 4

• (J

Fig. 1. Tectonic map of the North Atlantic showing the location of Iceland with respect to spreading ridges (Reykjanes, Kolbeinsey and Mohns), fracture zones (Tj6rnes and Jan Mayen), and adjacent continental margins (dotted lines). Plateau basalt areas shown in black.

When the plates move away from the rifting The location of Iceland within a sensitive zones, erosion and weathering supersede the climatic zone and the favourable tectonic constructive processes, and thick tilted sequ­ environment enhance the potential of Quater­ ences are exposed in tectonically distal fjords nary sections in Iceland. and valleys. Clearly, the tectonic and volcanic processes at work in Iceland provide a conserva­ THE BREIDAVIK GROUP. tion factor which makes sections there promis­ GEOLOGICAL SETTING ing for the study of actual glacier ice variations AND RESEARCH METHODS throughout the Quaternary. Remanent rock Tj6rnes Peninsula (Fig. 2) is a tectonically magnetism and K/Ar dating provide absolute active horst bounded in the east by the axial ages for the lava sequences. zone of rifting and volcanism in bxarfj6rdur, Iceland is located in the North Atlantic di­ and in the south by the Husavik faults which rectly in the path of the high altitude westerly belong to the Tj6rnes Fracture Zone (Fig. 2), jet streams (Fig. 1). The climate is cold-temper­ along which the plate boundary across North ate and maritime, and over 10% of the island's Iceland is shifted westward to continue along area of 103,000 km2 are covered by glaciers. the Kolbeinsey Ridge (Saemundsson 1974). The present day climatic conditions are consi­ Tj6rnes exposes a basal unit of Tertiary flood dered to be analogous to Quaternary intergla­ basalts (Thoroddsen 1902, Aronson and cial conditions, and any changes in the latitude Saemundsson 1974, Albertsson, 1978) followed or in the shape of the path of the low depress­ by over 1000 m thick sediments and lavas of ions across the northern hemisphere are very Pliocene and Quaternary age (Bardarson 1925, likely to be reflected in the mass balance of Askelsson 1941 and 1960, Einarsson, T. 1957a glaciers or an ice sheet in Iceland. and 1957b, Th. Einarsson et a1. 1967). 5

66° ~====~(Q Voladalstorfa .--- Knarrarbrekkutangi j:~====%~~~

HedinshOfd i

a 3km

Fig. 2. Index map of Tjornes (after Eirfksson 1981b).

During Postglacial time, lavas have flowed intermingled with sedimentary units throughout along the eastern and southern margins of the the Breidavik Group. While the Pliocene Tjornes horst. Lava flows predominate in the Tjornes basin may have formed along a trans­ uppermost part of the Tjornes sequence and are form fault well away from volcanic zones, lavas 6 began to flow into the basin in upper Gauss studies of selected Member units of the Bieida­ time (Albertsson 1976), and since then lava vik Group. flows appear to have had access to the Tj6rnes A vertical section through the Breidavik area whenever enviwnmental factors such as Group rocks exhibits a distinct repetitive pat­ sea level or ice cover permitted. Palaeocurrent tern of alternating sedimentary and volcanic data from the Pliocene Tj6rnes sediments rocks (Fig. 3). The latter component consists (Strauch 1963) indicate that the sediment mainly of basaltic lava flows and a few tuff source at that time was to the squth of Tj6rnes, layers. Several lava flows usually occur together and the same is true for the Breidavik Group in a succession, followed by sedimentary rocks (Eiriksson 1979). associations. The Furugerdi Member, which is The purpose of this paper is to describe and the lowest Member unit, begins with a diamic­ interpret some of the sedimentary rocks of the tite bed, followed at first by conglomerates, Breidavik Group on Tj6rnes. The results are sandstones and siltstones, and then by sand­ based on field investigations that began in 1975 stones and conglomerates, capped by lava while I was studying at the University of East flows. Younger units, although they vary in Anglia, Norwich (Eiriksson 1979), and on later their completeness, do conform to the same research at the Science Institute of the Uni­ pattern of vertical facies sequence. versity of Iceland, Reykjavik. Lithostratigraphy and research history of the Tj6rnes sequence were discussed in recent review papers (Eiriks­ FACIES MODELS FOR GLACIAL son 1981a, 1981b). The Breidavik Group is an SEDIMENTAnON over 600 m thick sequence of mainly sediment­ A simple model for glacial conditions where ary rocks. Apart from the youngest member ice reaches the sea during glacial maxima is unit, all the rocks are lithified and crop out in presented as a framework for discussing the coastal cliffs and along brook gullies. In central geological evidence. The model produces an Tj6rnes there are good outcrops in moun­ ideal sequence and covers a cycle in the recur­ tainous valleys. Some of the more resistant rent change from interglacial through glacial units, such as lava flows, can be traced on flat back to interglacial conditions, a change that is terrain, especially where soil erosion has been a major and well established feature of the severe. Quaternary ice age. Three geographically adja­ The main objectives of the field work were to cent zones are defined within the model, zone establish the regional stratigraphy, describe A which is constantly below sea level, a tran­ rock units in terms of sedimentary facies, and to sitional zone B, which is subjected to alternat­ analyze vertical and lateral facies relationships. ing emergences and submergences, and zone C, Facies definitions were entirely descriptive and which is terrestrial and constantly above sea based on field observations of textural and level. Zone B is invaded by depositional or structural properties. Photographs of steep erosive processes typical for zones A or C coastal cliffs were used to analyze inaccessible whenever relative sea level changes. Possible and indistinct coarse grained bedding struc­ courses of events through an interglacial-gla­ tures. cial-interglacial cycle are illustrated in Figs. 4 In the following account it is attempted to and 5, and may be briefly summarized as fol­ unravel the glacial history of Tj6rnes by means lows: Interglacial conditions (1) are interrupted of both lithological and stratigraphical lines of by a eustatic drop of sea level (2) due to research. A genetic classification of the increased ice volumes on land outside the area, sedimentary rocks is presented. Some units, and the lower part of zone B emerges. Even­ especially the marine mudrocks and sandstones, tually, a glacier advances over the area (3) and have been classified more broadly than the gla­ isostatic depression of the crust begins. Glacial cially derived rocks. The genetic classification conditions (4) take over until the glacier of the rocks forms the basis of a hypothetical retreats from the area (5). A concurrent reduc­ reconstruction of palaeoenvironments in terms tion of ice volumes elsewhere leads to a eustatic of a few simple facies models. The application rise of sea level and submergence of zone B (5, of these models may be tested further by Fig. 4), the crust being still depressed by ice. On detailed sedimentological and palaeontological the other hand, a relative rise of sea level may 7

cu ,E c:; .!II:: • o ulll N FORMATION GROUP/ L1THOZONE .ccu'-1Il o I-c:; co Husavik

Grasafjoll 1000- c:llm~$lE$~l •

Breidavik Group

Threngingar

Horgi Furuvik

Hoskuldsvik lavas

~QJ t-Qe: '_1~N Vl Tjornes beds

L~(jEND: ~ Sandstone and mud rock with lignite seam [[]] Lava flows I...... 1 Tillite .. Normal polarity ale:V)IQJ ~~ ~ Reversed polarity

Kaldakvisl lavas

Fig. 3. Composite stratigraphical column' of Breidavik Group strata. 8

+ + + + + + + + + =~~~~p~~~~"~'~'~~7...... ++++++++++++++++++++++++++++-+~ L + + + + + + + t ++++++++++++ + + +++++++++++++++++++++++

2 ~ + - ...-.---I +++++ ~ -t-+++++++++ ~~~~~~~~~~~~~~~!~:~~:~~~~~~~~2~~;~~~\:-,~+ +++++++++++++ t ++++++++++++++++++ '""""" -+ +++++++++++++++++++++++

3

+++++++++++++++ ++++++++++++++++++++

5

:..::.--~-- + + + + + + + + + + + +++++++++++++++ t=====::,d~~+:;;::]

-A ~I( B )If---- C

7

A r + + + + + + + + + + -t + + + + + + + + + + +++++++++++++ -t +++++++++++++++++ ++++++++++++++++++++ +++++++++++++++++++++ LEGEND: """'Ll Lodgement till

~ Beach features % Outwash ~ ... Marine sand IT' Subaerial lava flows =2::.=-4'::-& ~ Glacio - marine diamicton iiiiIlfii'f Ice

~ Bedrock Fig. 4. Moc;lel (a) for accumulation of lithofacies in Zones A, Band C. See discussion in text. 9

1 + + + + + + ...'... +, + + + + + + + + + + ~ + + + + + + + + + ~~~~~~~~~ L + + + ++++++++++++++++++ + + + + +, + + + + + + + + + + + +++++++++++++++++++++++++

+ + + + + + + + + + ~2;_~_~_~-~~~-~~~~~~~~~~:~:~~~~~~~+~=+~:+~~+~+~++++++++++++++++ t .. ,',. ++++++++++++++++ + + + +++++++++++++++++

3

+++++++++++++++ +++++++++++++++++

4

5

+ + + + + + + + + + + + +++++++++++++ -k:=:;;;~';;;:+3

-A ---">1 ~ B ) loi;-- C

7

;;~~iij~~~~-r + + + + + + " + + + + + +,-r ++ +' ++ t ,'~ '. :'.'. "b. , '.' . +++++++++++++ ~",,-z>~"'=. . + + + + + + + + + + +. + + + + + + ++++++++++++++++++++ ++++++++++++++++++++++ LEGEND: i\ Ll4 Lodgement till

Beach features ~~x Outwash, kames, lake mud

Marine sand II Subaerial lava flows ,,-; Glacio- marine diamicton L..±.-. Bedrock

]::::::::::{:~ Ice Fig. 5. Model (b) for accumulation of lithofacies in Zones A, Band C. See discussion in text. 10 be delayed either by rapid isostatic rebound of beyond the glacier margin is drowned by a lake. the area or by a delayed eustatic rise of sea Rhythmites are deposited distally over proximal level, in which cases the glacier would retreat ice-marginal fans. Major deltaic deposition is on "dry" land (5, Fig. 5). Highest relative sea likely to take place at the mouths of lateral level is reached during late-glacial conditions streams producing upwards coarsening sequ­ (6, Figs. 4 and 5). Isostatic rebound has begun, ences or alternating coarse/fine units. A distinc­ and the conditions revert back to interglacial tion based on the thermal regimes of glaciers (1). was made between a subglacial/proglacial land Many problems face anyone who attempts to system and a supraglacialland system. Temper­ interpret ancient sequences of diamictites and ate glaciers produce a subglaciallproglacialland related rocks. During the past two decades, system, where the sediment association consists however, considerable progress has been made of lodgement till associated with outwash sedi­ in the study of processes at present-day glaciers, ments. Subpolar or polar glaciers, on the other which is a key factor in genetic classification of hand, lead to a supraglacialland system, where old glacial sediments and understanding of the sediments are mainly lodgement till, flow­ facies types and relationships. Nevertheless, tills, and meltout till, associated with kamiform there are relatively few features that are unique outwash and proglacial outwash sediments. to glacial sediments, and some of the classifica­ Glaciomarine sediment associations were subdi­ tion criteria require laboratory techniques that vided into proximal and distal ones. The proxi­ are not applicable to lithified sediments. Con­ mal glaciomarine sediment association may clusions about the origin of these sediments are resemble the deposits of the glaciolacustrine commonly based on a wide range of observa­ valley environment strongly in fjords and coas­ tions that cannot be standardized because of tal areas. Coarse grained submarine fans are outcrop limitations and heterogeneity of sedi­ deposited at the glacier margins by freshwater ments in the glacial environment. In addition to currents, and the fines settle rapidly as the criteria which point to a particular mode of freshwater plumes mix with the saline sea deposition, the vertical sequence of sediment­ water. Turbidity currents and slumping are also ary facies can be strongly diagnostic of the important processes in this environment. Mel­ origin. ting ice-bergs may release coarse debris which Present-day glaciers fall into three broad forms erratics in the fine grained bottom mud categories defined by topographic and climatic close to the ice margin, but may produce pebbly factors. The first type consists of valley glaciers diamicton further away. The distal glaciomarine terminating within the valley system. The environment enjoys much slower sedimentation second type features glaciers or an ice sheet rates than the proximal one, and a slow terminating on flat lowlands or broad, flat val­ accumulation of suspended sediment and leys, and in the third type the ice reaches the sea iceberg-dropped detritus produces a fairly mas­ to form an ice shelf or an ice cliff. Boulton and sive glaciomarine diamicton. Deynoux (1981) have summarized criteria, In the general model developed for the which allow recognition of genetic varieties of Breidavik Group, the area delineated by the till, and reviewed definitions of sediment upper and lower limits of relative sea level is associations and land systems in the glacial particularly important for evaluating the environment. Two main types of glacial implications of the geological record, because a environments are identified, a glacioterrestrial section through a wholly terrestrial cycle or a environment and a glaciomarine one. Boulton wholly marine one can only be expected to and Deynoux differentiated between four types serve as an onloff gauge for glacier advance­ of associations on land. A glaciated valley land retreat cycles in lithological terms. An access to system and sediment association is characte­ the transitional zone is more likely to yield rized by coarse grained supratill in close associa­ information about sea level changes, which are tion with lateral kame terraces, scree and clearly very important when the implications of solifluction material, as well as fluviatile sedi­ changes of the extent of glaciers or ice sheets ments. A glaciolacustrine valley land system are to be evaluated. Only major, world-wide was coined for conditions which arise when a climatic fluctuations and resulting growth or valley glacier retreats and the whole valley floor reduction of ice volumes, can be expected to be 11 accompanied by major eustatic changes. Con­ defined to cover sediments and rocks which versely, minor climatic changes, although cap­ display certain grain size characteristics. able of causing glacier advances and deposition The main criteria used for the subdivision of of till within the terrestrial zone, are unlikely to the Breidavfk Group diamictites were the size cause major changes in either absolute or rela­ and shape of the diamictite beds, grain size tive sea level. characteristics, petrography of the clasts, and structures. The combined effects of the landscape on Tjornes Peninsula and the tectonic dip of the SEDIMENTOLOGICAL lower part of the Breidavfk Group rocks limit CHARACTERISTICS OF the exposures of individual diamictite (and THE BREIDAVIK GROUP other) beds to a few hundred metres to a few Very refined classification schemes exist for kilometres. This poses a limit to the scale on tills and related sediments of the glacial which the overall shape and size of diamictite environments (Boulton 1972, 1976a, Dreimanis bodies can be assessed. Where bed thickness 1976, Boulton and Deynoux 1981). does not vary systematically within outcrops, it More and more tools are becoming available is assumed that the diamictite is sheet shaped. for the study of Quaternary environments and Irregularities in substratum topography may their products. Many of these tools and the sometimes cause a local thinning or disappear­ methodology are based on the study of either ance of diamictite beds without affecting such active glacial environments, deposits of the last an assumption. The thickness of lodgement till glaciation where morphological data are often is extremely variable in general, and may available, or on theoretical grounds. The stu­ perhaps be related to topography (Flint 1971). dent of old Quaternary sections does not always Diamictite sheets in the Breidavfk Group vary have access to some of the information necces­ in thickness from a few tens of centimetres to sary for advanced classification, and the tens of metres. methodology for the study of hard rocks such as A bimodal or polymodal grain size distribu­ the Breidavfk Group rocks differs from that of tion in a diamicton where one of the major easily dispersed aggregates (d. discussion in modes is in the silt range strongly suggests Eyles et al. 1983). glacial origin. Smalley (1966) suggested that In the study of the Breidavfk Group glacial glacial crushing of sand grains would produce a sediments, the main emphasis was on estab­ bimodal distribution with a major silt compo­ lishing the local stratigraphy and on field nent. Dreimanis and Vagners (1971) concluded descriptions of textural properties, sedimentary from studies of till in Ontario, Canada, that structures and fossils, as well as vertical and crushing and abrasion during glacial transport lateral facies relationships. The terminology by was responsible for an increase in the silt mode Boulton and Deynoux (1981) is adhered to in of tills away from source areas. Post-depositio­ classifying the glacial sediments. nal crushing due to subglacial shearing and dilatation was suggested as an alternative possibility for silt enrichment by Boulton et al. Tillites (1974). The term diamictite was used in the field to Because of the petrological uniformity of Ice­ describe rocks which "do not have an intact land, the petrography of the diamictites on framework of gravel clasts but display, instead, Tjornes would seem unlikely to hold important a dominant matrix of fine grained materials in clues about their origin. Nevertheless it is well which the larger clasts are imbedded or "float"" known that the bedrock piled up since the onset (Pettijohn 1975). The term was introduced by of widespread glaciations within volcanic zones Flint et al. (1960) as a lithified equivalent of in Iceland is characterized by hyaloclastites, diamicton, which the same authors defined as a breccias and pillow lavas. This change in volca­ nongenetic term for "any nonsorted or poorly nic rock facies should be reflected in sediments sorted terrigenous sediment that consists of derived from erosion of bedrock. The composi­ sand and/or larger particles in a muddy matrix". tion of clasts was noted in the field in order to The terms diamicton and diamictite are thus assess bedrock composition in source areas. In 12 practice, the composition of some of the diamic­ tens of centimetres to tens of metres, and are tites was found to be intimately related to local generally interpreted as lodgement tillites. bedrock composition. Distinct glacial striae have been found at four The form of clasts was not studied systema­ localities underlying such sheets (Horgi Forma­ tically. Pebble and boulder roundness was tion, Svarthamar Member, Raudsgja Member, assessed visually in the field, and striations were and Skeidsoxl Member), and they are wide­ noted whenever observed. Their genetic signifi­ spread beneath sediments of the youngest cance is somewhat restricted, however, as the Member of the Breidavik Group. In addition, striae may be inherited from an earlier phase of large scale planing of underlying strata indicates sedimentation if the clast has been recycled. glacial erosion even though striations are mis­ The same is probably true of clast form. sing or cannot be observed because of exposure The structural properties of the diamictites relations. These erosional features represent an proved to be an important diagnostic feature. It important facies. was possible to identify different types of When beds immediately above a diamictite diamictites on the basis of variations in sheet are identified as flow tillites, kame and sedimentary structures. Bands (sedimentary or outwash conglomerates, or glaciomarine or gla­ shear), folds and faults were used to differenti­ ciolacustrine diamictites, the stratigraphical ate between lodgement till, meltout till and flow relationship strongly suggests a lodgement tillite till by Boulton (1976a) and Boulton and interpretation for the diamictite bed. A meltout Deynoux (1981). According to Boulton, flow tillite could occupy a parallel stratigraphical tills may show both sedimentary and shear ban­ position, and a differentiation of the two gene­ ding. The folds are asymmetric overturned or tic varieties may be problematic. The meltout flat-lying isoclinal folds with synclinal and anti­ process does not, however, lead to any struc­ clinal fold noses preserved. Gentle folding and tures within the till (Boulton et al. 1974, ct. also high angle folds are common in sediments Boulton 1976a). beneath flow tills. Meltout tills tend to be mass­ Grain size distribution was originally used to ive but may incorporate englacial ice tunnel classify conglomerates as diamictites. Although deposits. They are probably difficult to disting­ a high silt content in the matrix does not in itself uish from flow tills in some cases (Haldorsen constitute a proof of a particular mode of and Shaw 1981). Lodgement tills contain no deposition, it does indicate a glacial origin. sedimentary banding or stratification of any However, a lodgement till which becomes recy­ kind unless subtill sediments have been tecto­ cled by a nonsorting process, e.g. a mass flow, nically incorporated in the till. Shearing of would still retain a high silt content. Generally, recently deposited till or subtill sediment may lodgement till is expected to be richer in silt lead to streaked out lenses or shear bands. than supraglacially derived till. There is no evi­ Lodgement till folding reflects subglacial shear­ dence of a crushing process during supraglacial ing and is characterized by isoclinal folds with transport, and supraglacial debris is therefore flat lying axial planes. Anticlinal fold noses are, likely to retain the grain size distribution pro­ according to Boulton (1976a), rarely preserved. duced by weathering, mechanical or other, in Synclines facing up-glacier are common, but source areas and during transport. A depletion fold noses are often streaked out. of fines may locally take place through sorting The diamictites of the Breidavik Group are by meltwater. The deposits classified as flow classified here as lodgement tillite, flow tillite, tillites were generally found to have a silty glaciomarine diamictite, and glaciolacustrine matrix, similar to that of the lodgement tillites, diamictite. The genetic classification is based on although internal variations and occasional sort­ the examination of textural and structural prop­ ing were observed in the flow tillites. erties, and on stratigraphical relationships. The lithology of clasts in some diamictites These factors are discussed below under sepa­ was seen to follow the lithology of the substra­ rate headings for each diamictite variety. tum closely. For example, a lateral change in clast composition occurs within a distance of a Lodgement tillite few hundred metres in the lowest diamictite of Extensive sheet shaped diamictites in the the Breidavik Group (Furugerdi Member, Fig. Breidavik Group vary in thickness from a few 6). Clasts of sedimentary origin only make up a 13

2,0 m

1,0

o Fig. 6. Tillite (2) immediately above scoriaceous lava (1) is enriched in scoriae. Upper part of tillite (3) reveals folds and sorted lenses. significant proportion of diamictites where they beneath the grooved plane there is another overlie erosional surfaces cutting across plane within the marine sandstone. The latter sedimentary rocks. Subglacial erosion (shearing grooves are on a much smaller scale, the ampli­ and plucking) of jointed sedimentary rocks and tude of the grooves and ridges is only 1-2 cm. lava flows is considered to explain this close The grooves are parallel to the larger grooves relationship, which is compatible with a lodge­ (flutes) above. This lower plane of grooves is ment tillite interpretation of such diamictites. interpreted as resulting from shear movement The fabric of the diamictites was generally within the topmost part of the sands caused by not studied. Although its genetic significance the overriding glacier ice (Fig. 8). has been illustrated repeatedly (Flint 1971, The fluting process envisaged by Boulton is Boulton 1971), the availability of other evi­ obviously not confined to particular planes in a dence was considered to render a substantial lodgement till, but is likely to be initiated fabric sampling programme unneccessary. whenever a boulder or any protruding obstacle Two types of structures in some Breidavik becomes lodged or is overridden by moving ice. Group diamictites support an identification as The lodgement of boulders is expected to hap­ lodgement tillite. Basal grooves were observed pen at random in a vertical sense as lodgement on the sole of the Torfh6ll Member diamictite till accumulates. This random distribution may (Fig. 7), and grooved planes were observed be altered during deposition by the condition within the Husavik Formation diamictites. The that the lodgement of one boulder may form an grooves are identified as flute molds. The prob­ obstacle to the path of the next one, and a lem of flute genesis was recently examined by boulder cluster will result. Such clusters should Boulton (1976b), who noted that flutes are also be randomly distributed. The splitting up almost omnipresent on the surface of modern of diamictites along nearly horizontal planes as lodgement tills. Boulton's general conclusion in the case of the Breidavik Group diamictites is was that flutes are formed by the squeezing up therefore not predicted from the fluting pro­ of deformable subglacial sediment into low cess. It is suggested that the internal groove pressure areas in the ice in lee of rigid obstruc­ planes were either caused by subglacial shearing tions such as boulders. This is in line with a within recently deposited till, or that the lodge­ hypothesis of flute genesis put forward by ment was intermittent so as to allow fluting at Schytt (1959). Fluting of the substratum of the certain levels during non-deposition. Torfh6ll diamictite indicates that the underlying The second type of structures in the Breida­ marine sandstone was overridden by glacier ice vik Group diamictites consists of bands of silt­ before lithification took place. Some 30 cm enriched diamictite, which give the sediment a 14

Fig. 7. Flute molds on the sale of a tillite bed in Stapavik. Geological hammer beneath a topographic Iowan the till-substratum contact.

Fig. 8. Grooved shear plane in sub-tillite sediment (Stapavik). 15 cm long biro points to plane. 15

Fig. 9. Faceted boulder (1.8 m long) in lodgement tillite in Fossgil.

streaked look, especially in matrix dominated ites were seen to originate at large boulders. It diamictites. The bands are generally lighter col­ is suggested that banding can result, when boul­ oured than the adjacent rock, and sometimes ders become lodged beneath glaciers and are have a weaker resistance to weathering. The then subjected to shearing of material against lateral extent of these bands varies from a few them. This would involve crushing of material tens of centimetres to a few metres or more. for a period of time at a certain level in the They tend to be undulating rather than flat, lodgement till, which finds expression in silty particularly in stony diamictites. The absence of bands extending downglacier from the boulder. primary sedimentary structures suggests that Faceting and even striation of the up-glacier the bands were formed by post-depositional side and the top of the boulder may result (Fig. deformation. Boulton (1976a) listed shear ban­ 9). Boulders have been observed in horizontal ding as a feature of lodgement till, or, if inter­ clusters in the Breidavik Group diamictites, e.g. digitation with stratified sediments occurs, flow in the Fossgil Member (Fig. 10). Such boulder till. Shear banding results when at least two clusters are characteristic of lodgement tills materials are deformed together so that folds (Boulton and Paul 1976). develop and become sheared into bands. Sub­ glacial shearing was discussed by Boulton et al. (1974) who described an upper zone of sheared Flow tillite till within a lodgement till horizon at the margin The diamictites of the Breidavik Group of Breidamerkurjokull in Southeast Iceland. rarely occur in multiple sequences without Presumably, a gradual buildup of lodgement till erosional breaks. The lowest part of the means that every part of it has undergone shear­ Furugerdi Member is an exception. Here a ing. Boulton et al. concluded that a consider­ number of diamictite bodies are interbedded able amount of crushing takes place in the with sorted material ranging from siltstone to shearing zone. This leads to a silt enriched conglomerates above a diamictite which is inter­ matrix. It is noteworthy that some of the silt preted as a lodgement tillite. The stratification bands observed in the Breidavik Group diamict- is often distinct, and the diamictite beds alter- -- ~- o I l~ o I \------Fig. 10. Boulder clusters in Fossgillodgement tillite. 16 nate with sandstone and siltstone lenses of a began to disappear by rapid melting. Hills gra­ limited lateral extent (up to a few tens of dually protruded near the edges as the ice sheet metres). Internal structures are common and became thinner, and thin ice became covered vary from thin, poorly sorted silt bands to dis­ with dirt which had been near the base. tinct folds. The banding and folding of the Goldthwait went on to describe kames: "The upper, stratified part of the Furugerdi Member outstanding deposit of such areas is the glacial diamictite sequence is thought to result from kame. It is an irregular hummocky mound of post-depositional deformation. Recumbent layered sand and gravel, - some coarse, some folds and low angle faults are interpreted as fine. Some layers are dirty (i. e. silty), but most evidence of slumping within soft sediments ori­ layers have silt and clay washed out. Huge ginally deposited in a supraglacial position. boulders may be sprinkled through the deposit Boulton (1968, 1972) has described sedimenta­ like raisins in rice pudding, and not uncom­ tion processes at the snouts of present day monly there are irregular blobs of till" glaciers in Spitzbergen, where ablation of ice (Goldthwait 1959). releases englacial debris which is deposited in a According to Flint (1971), kames are formed fluid state upon the glacier surface. Differential in two principal ways. One type of kame is melting of underlying ice leads to a downslope deposited "in or on the surface of stagnant or flow of the debris. Meltwater streams cause nearly stagnant ice ...". Another type consists local sorting of material and short lived ponds of deltas built outward from ice or inward are common. Sections through flow till formed against ice (kame deltas). According to Flint in this way revealed isoclinal folds. Flow till is kames may grade into kame terraces, collapsed typically deposited on top of lodgement till or masses, ablation drift, and some types of meltout till although other sediments may well eskers. Flint's classification scheme was not intervene. Stratified conglomerates and sand­ based on the location of deposition relative to stones interpreted as ice-contact sediments ice. He suggested supraglacial and englacial were for instance observed between lodgement origin for kames, but discussed "collapsed sedi­ tillite and flow tillite in the H6rgi Member. The ments" separately, these being deposited sup­ Furugerdi Member flow till, however, rests raglacially but becoming deformed as they are directly upon lodgement tillite. The structural let down onto the ground as the ice melts away. and textural properties appear to fit the criteria In Flint's scheme, eskers are typically deposited for distinguishing flow till deposition. in subglacial tunnels. Boulton (1972) distinguished between deposi­ Kame conglomerates tion of ice-contact stratified sediments upon (supraglacial), within (englacial), and beneath Review of kame genesis (subglacial) stagnant and ablating ice, and The research history of kames and ideas based a genetic classification on this position of about their origin were reviewed by Holmes deposition relative to the ice. The supraglacial (1947), who defined kames in the following and englacial models were based on observation way: "A kame is a mound composed chiefly of at the margins of modern glaciers in Spitz­ gravel or sand, whose form has resulted from bergen, and the subglacial one is partly pre­ original deposition modified by any slumping dicted with reference to the englacial one. Boul­ incident to later melting of glacial ice against or ton's classification also included ice-walled upon which the deposit accumulated". deposits as defined in Clayton and Freers Goldthwait (1959) listed two conditions for (1967). Ice-walled deposits resemble supragla­ present day development of areas of ice-contact cial ones but generally retain a flat top. washed deposits: rapid melting and thinning of Karczewski (1971,1974) presented a genetic glacier ice so that ice flow slows down or stops; classification of kames based on structural and and presence of hilly topography to cut off flow. lithological studies of kames in Western Pomer­ Presumably the melting slowed down after the ania, Poland. He put forward a theoretical ice stagnated, and slow melting is generally reconstruction of kame forming conditions and assumed during kame genesis (Embleton and identified four types of kame macrostructures: King 1975). Goldthwait deduced from studies in horizontal, isoclinal, anticlinal, and synclinal. Ohio, U.S.A., that the last ice sheet in that area Original deposition of kame material was envis- 17 aged by Karczewski in supraglacial, englacial, the adjacent mudrocks because of the greater and intraglacial position, in all cases with an resistance of the conglomerates to weathering. original horizontal bedding. Subsequent slum­ The contacts between lodgement tillite and ping and faulting of the margins and irregular the overlying kame complexes are sharp with no melting of ice in the supraglacial case would evidence of an erosional interval preceding the oause the development of anticlinal or synclinal deposition of the gravels (Fig. 11). A continuity forms, while lateral deposition relative to the in sedimentation, if by a different process, is ice would lead to an isoclinal macrostructure. evident. It is an inherent feature of the Ice Age Squeezing up of clay and glaciofluvial material concept that processes associated with an ice may cause an updoming of kames in the sheet and lodgement till deposition will even­ intraglacial case. Karczewski noted, that "boul­ tually be naturally succeeded by processes der clay and ablation sediments" are intimately associated with the melting of that ice sheet. associated with the kame sediments. Consequently it is not surprising to find ice­ Embleton and King (1975) discussed various contact sediments above tillite sheets. hypotheses as to the origin of kames and Although deglaciation processes generate reviewed kame distribution. They noted that kames it does not follow that they will be con­ eskers and kames were formed of stratified served. Even after a modification of their deposits laid down in contact with slow-moving depositional form and structure during the or stagnant ice by glacial meltwater. They dis­ kame genesis, further changes are likely to tinguished between kame terraces formed along occur. Kames are probably short-lived phe­ walley side margins of glaciers, and kames nomena in many cases. Their mound form will deposited at glacier margins in proglaciallakes generally make them accessible to erosion, and or accumulating as lone kames in hollows on or a low conservation potential is therefore pre­ in the decaying ice. dicted. The kame is likely to become modified or destroyed by erosion during ice free periods. This factor may account for the fact that kames are only found in those Breidavik Group Mem­ Distribution of kames in the Breidavik Group bers (Formation in the case of the Horgi unit) Four of the Breidavik Group Formations con­ where there is evidence of submergence below tain spectacular conglomerate bodies, whose sea level soon after the ice retreat phase. thickness is measured in tens of metres. Three Deposition of lacustrine and marine mud tends of the conglomerate horizons are interpreted as to level out the kame topography and increase kame complexes. Stratigraphically they all their conservation probability. A further point belong immediately above lodgement tillites or worth bearing in mind is that not all ice margins a glacially striated substratum. The conglomer­ retreat by stagnation of ice. Kame formation is ates are characterized by an irregular outer hardly expected where glaciers calve into the form and a limited lateral extent. Internal bed­ sea or lakes during retreat. ding structures are highly variable, and deformation structures are common. Grain size and sorting are extremely variable. Many of the Size and shape of the Breidavik Group kames conglomerate bodies are intimately associated The external form of kames is related to at with mudrocks. Ice-contact conditions are sug­ least three factors. In the first instance their gested by their stratigraphical position above form is controlled by channel or local basin lodgement tillites, and by their form, texture form during the original deposition. Secondly, and structure. Their whole appearance brings to the post-depositional removal of lateral and/or the mind a quotation from Flint (1971), when basal support (ice) will lead to a modification of he discussed processes in the immediate prox­ the overall shape of kames. Thirdly, an adjust­ imity of stagnant ice: "In such a place anything ment of shape may take place as kames are can happen, and it often does". A panorama lowered onto the substratum topography. view of one of the kame complexes is exposed As kames are essentially fluvial deposits in the western side of the bay of Breidavik, (Holmes 1947), their depositional form must be where individual kames tower up to 40 m above controlled by the runoff pattern within the stag­ the coastline in vertical cliffs which jut out from nant ice area. This is related to the distribution 18

Fig. 11. Conformable kame conglomerate/lodgement tillite contact in Stapavik. of debris in the ice which affects the ablation moraines), or lodgement till squeezed up into rates, and to crevasses in the ice which are likely crevasses (Hoppe 1953). Any debris ridges on to form natural runoff channels. It is well the surface will reduce ablation of ice if they are known from modern glacier margins that engla­ thick enough, and the meltwater will be chan­ cial dirt tends to be concentrated along planes nelled along elongate depressions, most com­ which are either sub-horizontal or dip up-gla­ monly parallel to the ice margin. This pattern is cier in the snout areas. Release of englacial likely to be interrupted by crevasses in the ice, debris from these planes by melting of ice will which in a stagnant ice may either be controlled lead to an accumulation of supraglacial debris by weaknesses inherited since the ice was active ridges striking parallel to the ice margin (Boul­ or be uncontrolled (Embleton and King 1975). ton 1972). Other debris ridges may consist of In the absence of a three dimensional view of supraglacially transported debris, and these the kame complex and with only a limited would be arranged parallel to ice flow (medial knowledge of the topography beneath the stag- 19 nant dead ice,- as in the case of the Breidavik horizontal no deformation will take place apart Group kames, - it is difficult to establish from that resulting from the melting of adjacent whether the crevasse pattern was controlled or ice. If the kame is lowered onto a sloping sur­ not. In any case, a very indiscriminate runoff face it will be tilted and deformed until equilib­ pattern may develop on the stagnated ice. In rium with the slope has been reached. The addition, all channels are likely to be highly amount of deformation is related to the slope unstable because of the easily penetrated ice, angle and to the shear strength of the kame and frequent avulsions are to be expected. Gen­ material. Internal structures of the kame are erally speaking the depositional form of ice likely to become asymmetrical or f()lded, if not floored kames will be that of elongate channel completely destroyed as a result of this fills. Deposition may stop when the meltwater --deformation. stream is cut off by either an avulsion, a moulin, Field conditions do not allow a detailed three or a decreased rate of melting (seasonal con­ dimensional analysis of the form of the Breida­ trol). More stable channels may develop if the vik Group conglomerate bodies. At best, a two meltwater penetrates through the ice down to dimensional picture can be obtained, but the the substratum. sole is rarely exposed, and the top has often Subsequent to the deposition of kame been modified by erosion. gravels, their form begins to be affected by a slow melting of ice at the margins of and beneath the channels. The ice melts away faster along the flanks than beneath the channel fills Internal structure and genesis of because of the insulation effect of the sedi­ the Breidavfk Group kames ments, and the steep depositional contacts with All the Breidavik Group kames are stratified. the ice in the confined channel spaces will col­ Vertical changes in bedding and lithology are lapse through slumping and faulting at the mar­ commonly often abrupt. Depositional struc­ gins when the lateral support disappears. Due tures vary from massive, poorly sorted con­ to irregularities in sediment thickness along the glomerate beds (sometimes a few metres thick) channels, and possibly also due to to laminated sandstone and siltstone beds. inhomogeneities in the ice below, melting Tabular sets of cross beds occur locally. Graded beneath the kames may not be uniform. There­ bedding is fairly common. The primary struc­ fore, slumping may also take place along the tures are considered consistent with supragla­ axes of channels into hollows created by locally cial fluvial deposition and slumping. Cross bed­ rapid melting. An originally continuous elon­ ded and flat bedded conglomerates and sand­ gate kame may thus disintegrate into a string of stones are interpreted as fluvial deposits, and mounds. Only the former, marginal type of the laminated facies as suspension sediments. deformation is expected, where channels have Very poorly sorted beds and graded beds prob­ penetrated down to the substratum of the ice. ably indicate occasional slumping of supragla­ At the margins of some modern glaciers cial debris into the channels. Some of the (Embleton and King 1975) fluvioglacial mate­ unsorted lenses are interpreted as flow till rial is not only deposited in channels, but also as lobes. outwash fans spreading over relatively flat dead Many of the conglomerate bodies in question ice surfaces. It appears that a complete transi­ display severely deformed bedding. Only the tion from channel deposition to outwash plain elongate kame at Svarthamar has retained the deposition exists in supraglacial position at original bedding intact. The nature of the modern glaciers. The fate of such outwash deformation enables a reconstruction of the sheets is naturally controlled by the melting of genesis of individual kames and kame com­ ice below. If the melting is uniform, no kames plexes. A model explaining common deforma­ will be formed. Minor variations in the thick­ tion structures in kames was presented by Boul­ ness of ice may lead to the development of ton (1972). According to the model, melting of kettle holes and a pitted outwash plain. ice concurrent with supraglacial channel deposi­ Finally, the shape of kames may be affected tion will affect the internal structure of the by topographical features of the substratum resulting kame. A loss of lateral support during (Boulton 1972). If the substratum is flat and deposition will lead to lateral slumping and an 20

anticlinal structure with dips decreasing A Supraglac ial upwards. Subchannel melting during deposition sediment will, on the other hand, produce a syncline, again with dips decreasing upwards. When no appreciable melting accompanies deposition, Ice the resulting kame will be flat bedded, but with secondary dips and faults due to slumping at the 2 kame margins. In all cases, substratum irregu­ larities may cause further deformation and faul­ ting. Exposed kames of the Breidavik Group are most numerous in the Horgi Formation. Several structural varieties present themselves. Fig. 12 shows two kames and a reconstruction of their genesis. Kame A is over 35 m thick (the lower contact is not exposed). Its central part reveals 3 a steep syncline with parallel bedding on the limbs. The axial plane is nearly vertical, which indicates that the kame landed on a flat substra­ tum. The synclinal structure is thought to reflect foundering of the channel bed during deposi­ tion due to simultaneous melting of basal ice. Higher up the bedding is irregularly folded B (compression in the knee of the syncline), and the top section is essentially flat bedded, and no deformation was observed there. The flanks of kame A are characterized by outward dips which are explained as a result of slumping when the lateral support was removed. The Ice kame has a flat top, but this may not be an original feature. Kame B in Fig. 12 has an exposed thickness 2 of 30 m, but its base is not seen. The kame is mound shaped in the coastal outcrop where it is flanked by a smaller kame remnant. The central part of kame B is essentially flat bedded, but C'zgyJ deformation folds occur at a central fault. The central deformation is considered to result from minor irregularities of the substratum. On the western flank there is a marginal fault, beyond 3 which the bedding planes dip outward, presum­ ably because of slumping. On the eastern flank there are also faults, but the bedding is folded. This is explained as abortive slumping against Kame B the juxtaposed kame, the top of which has later been eroded away. There is no evidence of Fig. 12. Hypothetical reconstruction of the genesis basal melting during the deposition of kame B, of two kames of the H6rgi Formation. Kame A was which indicates that the channel was a short deposited on ice (1) and deformed because of basal melting during accumulation (2). Resultant kame (3) lived one or that the sediment was dumped shows synclinal structure. Kame B was deposited on rapidly into a depression in the ice. ice (1) and lowered onto the substratum (2) without A more complicated genesis is proposed for sub-channel melting, producing an essentially flat the evolution of the kame complex in Fig. 13. It bedded kame (3). consists of three separate kame bodies, the two 21

Supraglacial sediments

Ice a.

b.

c.

d.

Fig. 13. Hypothetical reconstruction of the genesis of a kame complex in the Horgi Formation. For explanation see discussion in text.

lower ones have affected the deformation of the lar substratum, which in this case consists of the third one. The inferred genesis of these kames first generation of kames. Detail sketches of may be summed up as follows. Gravel begins to portions of the kame complex in Fig. 13 are accumulate in two closely spaced channels presented in Fig. 14, and a photograph of flanked by debris covered ridges. One of the deformation folds in Fig. 15. channels is fairly short lived (on the left in Fig. Bedrock slope may have played a part in the 13a), but the other one is more stable. Subchan­ deformation of structures in a 20 m high kame nel melting concurrent with fluvial deposition in the Torfh6ll Member exposed in Stapavfk. leads to synclinal deformation. Flow till lobes The bedding is most severely disturbed where from the adjacent ridges interdigitate with the the slope of the tillite bed beneath is steep, and gravel. Eventually the topography between the the deformation decreases and is characterized two channels becomes reversed and a new chan­ by lateral slumping where the tillite flattens out nel is cut into the ice, and partly into the already towards north. deformed older gravels. Flat bedded gravels are deposited in the new channel, and an angular unconformity at the interface between separate channel fills is created (c). Deposition is now Outwash and lacustrine mudrocks halted in the area and slow melting of the and conglomerates adjacent ice begins to affect the depositional Many of the Member units of Breidavfk form. Lateral slumping and faulting takes place. Group contain a vertical sequence where strati­ Deformation is most severe in the central kame, fied ice-contact sediments are succeeded by the bedding planes of which become severely mudrocks. In the absence of ice-contact con­ folded in the process of adjusting to the irregu- glomerates, lodgement tillite beds are often 22

~ ~ ~ ~ - S\~\"\'\ 0 o~"-; L ~D ~CJ 0° \\ ,. "- Ie. --=- __- ~ , . I\~o'~ ~

\\ "--. \1 ~ { '" ~ "c ~/_ ~-7\,-~ 1\(":~~ ,'. ~ ~uL" - ~~\ ~d ~~-= ~~~~---=~ "~o'~, ~ ~ /J ()o<:>o cOD8~"""-- ,,- __ 0 b o

- 0 -:::-- f F 13) . ------14 Detailosketch•es---0 f deformation structures III. a Horgi Formation. kam,comr'" (con",_,. .g. . FIg..Vertrca'1 herg . ht of blow-ups ca 20 m. 23

Fig. 15. Deformation fold in a Horgi Formation kame. Height of exposure 7 m. directly overlain by mudrocks. Sedimentary influx). Erratic pebbles and lenses of diamictite structures in the mudrocks hold clues to the in the siltstone at Fossgil indicate that ice bergs depositional relationship between the mudrocks drifted across the basin and melted there. No and the adjacent sedimentary bodies. fossils are found in this lowest, laminated silt­ The Fossgil Member contains a good example stone facies, and a glacio-lacustrine environ­ of a case where mudrocks overlie lodgement ment is inferred. tillite. Bedding and lamination planes in the A more complicated relationship exists where siltstone are conformable with the surface of the mudrocks overlie or are adjacent to kame com­ tillite, which is irregular. The attitude of the plexes. The mudrocks, which are mostly silt­ bedding is interpreted as evidence of sedimenta­ stones, are generally thickly laminated to thinly tion from suspension in water. The laminae bedded (on a scale proposed by Ingram 1954). indicate variations in the concentration and The bedding may either represent proximal grain size of the suspended material (sediment varves or reflect intermittent input of turbid N +

Fig. 16. Folds and faults in mudrock adjacent to a kame (H6rgi Formation). 25 meltwater on a smaller time scale. Stray pebbles Supraglacial and sand grains are very common, and the a. river general texture of the rocks is very similar to that of mudrocks overlying lodgement tillite. The internal structure of the siltstone units is characterized by deformed bedding on three different scales. Slump structures are common on a small scale involving only convolutions of one or a few laminae. These structures are Ice Till probably synsedimentary, as laminae beneath and above are not affected, and indicate that + + + + + + ++ + + + + + + + the sedimentary basin was not flat. ~Bedrock The second type of deformation manifests itself on a scale of several metres as intense b. Supraglaciallake folding and faulting (Fig. 16). This kind of deformation occurs at steep lateral contacts with kames. The contacts are locally very intri­ cate, and slumping of gravels into muds is indi­ cated. The structures in Fig. 14 are interpreted as a result of a deformation of an unstable lateral contact leading to wall rock crowding within the silt body. The deformation is thus +++++++++++++++ apparently caused by the kames, a strati­ graphically lower unit than the mudrock (low­ c. relief kames are submerged by the very silts that are deformed by the high-relief ones). The ------distribution of the silts in pockets shows that the kames must have had a mound form when mud deposition began. It is considered likely that the original contact of the silts/gravels had a slope approaching the angle of repose for the kame +++++++++++++++ material. A model explaining a postdepositio­ nal increase of the silt/gravel contact is pre­ sented in Fig. 17. Initial deposition of the gravel and sand takes place in a relatively steep sided channel, and the kame sediment is built up to a certain level, which is controlled by the drain­ age conditions in the decaying ice (Fig. 17a). After the deposition of the kame, lateral ice + + + + + + + + melts away, the topography becomes reversed, + + + + + + + and ponds and lakes are formed on the ice between kames (Fig. 17b). Meltwater streams may carry mud into these depressions, which settles out of suspension. The depositional mud/ gravel contact has an initial slope equal to the subaqueous angle of repose for the kame sedi­ ment. The accumulation of such a blanket of sediments composed of mud and gravel upon the ice retards surface melting, but slow subgla­ +++++++++++++++ cial melting continues. At this stage the gravel bodies (kames) may form wedges with ice Fig. 17. A model of kame genesis involving a post­ below and mud above, especially ifthe drainage depositional increase of the dip of mud/gravel con­ deteriorates or the water table of the area is tacts. For explanation see discussion in text. 26 raised by external factors (Fig. 17c). If the ice downthrows away from the silt pocket margins. beneath the sediments is thickest where the Coarser sediments immediately above the silt­ kame gravel wedges out, which would be natu­ stones are also affected by the deformation, ral in the model above, the continued subglacial although decreasingly so, as one goes higher up melting leads to deformation of the sediments in the section. The deformation of the mud­ through a series of gravity faults, which are rocks on this large scale is considered to result more likely to be conserved as planes in the from postdepositional compaction coupled with more cohesive muds (Fig. 17d). The resulting loss of pore water. Rapid deposition and a high fault escarpments involve loss of lateral sup­ original water content of the sediments is infer­ port, and mass movement of soft sediments is red. The kame gravels, having been deposited predicted near the surface. One of the effects of in running water and thereby acquiring a much such gravity faults is an increase in the steepness denser depositional packing than the mudrocks, of the mud/gravel contact and possibly the did not suffer a corresponding compaction, and formation of a graben adjacent to the kame slippage at the contacts has locally taken place. (Fig. 17e). Successive slope failures near the An altogether different contact between the surface may also cause further steepening of mudrocks and the kames is observed adjacent contacts if the kame sediments are affected by to the uppermost regions of the kame bodies, the failures. A third type of deformation where reworking of gravels is indicated by con­ observed in the mudrocks associated with kame glomerate lenses stretching laterally outwards complexes affects entire silt pockets. The large and thinning away from the parent kame scale structure is that of a syncline with bedding bodies. This kind of interfingering relationship planes dipping away from the kames. This is concurrent with an upwards coarsening of the down-warping of the siltstones, which is much mudrocks. In the Horgi Formation the silt­ too steep to represent a depositional slope (Fig. stones change from bedded to massive structure 18), is accompanied by normal faults with where the gravel lenses begin to appear at the

Fig. 18. Steeply dipping bedding planes in intra-kame mudrocks. Height of exposure 25 m. Dark kame remnant to the left. 27 same level as marine fossils. It is inferred from trine environment is inferred. An example of a the presence of siltstone beds between and diamictite lens within a silt facies is present in above ice-contact conglomerates and tillites the Furugerdi Member. The siltstone facies is that ponds and lakes did form during and after generally followed by an upwards coarsening glacier retreat on repeated occasions during the sandstone and conglomerate facies. The sand­ accumulation of the Breidavfk Group rocks. stones are nearly flat bedded in the Horgi Meltwater streams carried suspended glacially Formation, and the conglomerate lenses can derived silt into standing water bodies. generally be traced to kame mounds. Observed Sedimentary processes in active proglacial dips of bedding planes in this facies constitute lakes in Iceland were recently studied by Harris low angle synclines which probably result from (1977, d. also Boulton et a1. (1983), Evensen et compaction. Locally, channels have been cut a1. (1977), and a review of ice-marginal lake into the Horgi Formation sandstone facies and deposits in Embleton and King (1975)). Analy­ have subsequently been refilled with sandstone. ses of glacial lake sediments show that they Lodgement tillite and kame conglomerates of consist largely of silt, but diamicton, gravel and the Torfh611 Member are cut off laterally by an sand lenses are fairly common. Bedding char­ erosional unconformity upon which a cross bed­ acteristics and internal structures may represent ded conglomerate facies has been deposited. varves, or deposition may be dominated by The unconformity defines a broad (ca. 1 km) frequent turbidity currents and slumps from the channel, and the conglomerate above contains adjacent lake slopes into a central basin (Harris upwards fining sequences. The channel strikes 1977). Floating and stranded ice-bergs melt to approximately perpendicular to the direction of yield diamicton lenses within the lake sedimets. ice flow as indicated by grooves and flutes in the Near the ice margin jets of meltwater deposit lodgement tillite at the base of the Torfh611 delta fans which interfinger with the silts. Member. The conglomerate is locally deformed Such features were all present in the mud into folds (Fig. 19), perhaps due to slumping. facies under consideration, and a glacio-lacus- Beds below and above the folded conglomerate

Fig. 19. Folded conglomerate (Torfh611 Member). 28

Fig. 20. Outwash conglomerate west of Stangarhorn. are undeformed. A similar conglomerate facies cial environment where outwash sediments and is present at the base of the Furugerdi Member, channels were subsequently submerged by stan­ but the channel there is narrower (ca. 50 m). A ding water. The sporadic occurrence of tillite thick conglomerate facies with sandstone lenses beneath the Hi:irgi conglomerate west of Stan­ occupies an analogous position at the base of garhorn indicates that the outwash was partly the Hi:irgi Formation at Stangarhorn (Fig. 20. deposited upon stagnant ice. Irregular dips and The conglomerate rests directly on a striated folds are locally observed in these conglomer­ lava flow, and the lodgement tillite observed at ates. Hi:irgi is missing. In all three places the con­ glomerate facies is replaced vertically by a Marine mudrocks and sandstones poorly sorted siltstone facies. These two Many of the fine grained sedimentary rocks sedimentary facies probably formed in a progla- of the Breidavfk Group abound with marine 29 fossils. Molluscs (bivalves and gastropods) are Group. Well rounded pebbles of gabbroic and particularly common, but annelid casts and gneissic composition have been found in this vertebrate bones have also been found. Marine lens. The upper contact, where the conglomer­ fossils have been found in five of the six Breida­ ate is replaced by the fossiliferous sandstone vik Group Formations. A marine environment facies, is gradational. is inferred from the study of lithology, In the Svarthamar Member, it has already sedimentary structures, and fossil assemblages. been noted that the siltstone facies is closely Starting with the finest grained rocks, it was related to a thinly bedded sandstone facies. The already noted in the description of outwash and latter passes laterally into a large scale cross glacio-lacustrine sediments that marine fossils bedded conglomerate facies just east of Fossgil. appear in the sections during silt deposition. Internal bedding planes within the conglomer­ Siltstones with marine fossils directly above gla­ ate dip 330oI15°E. The sandstone facies clearly cio-lacustrine and outwash sediments are pre­ interfingers with this conglomerate, and even­ sent within the Furugerdi Member, the Horgi tually climbs towards southwest over the north­ Formation, and the Fossgil, Svarthamar, and east dipping interfingering contact in the coastal Torfh611 Members. This facies consists of section. Penecontemporaneous deformation poorly sorted siltstone with stray sand grains structures characterize the sandstone facies, and sand lenses, pebbles, and boulders. which grades into siltstone facies laterally as Lamination is notably absent, but sand lenses well as vertically away from the conglomerate. are commonly contorted. Laterally, a gradatio­ East of Breiduvikurlaekur there are marine fos­ nal relationship is observed in the Svarthamar sils in the sandy facies. The conglomerate is Member, where the siltstone facies passes into basally very coarse and there are very large thinly bedded convoluted sandy siltstone facies. boulders at the foot of many of the cross beds. Fossils do not appear to be concentrated in Some of these are derived from a series of thick lenses or horizons in the siltstones, and are tuff layers immediately below the conglomer­ generally unbroken. In Svarthamar two thin ate. Sedimentary pebbles containing marine tuff layers are interbedded in the siltstone fos'sils have also been found. facies. The lower one contains marine fossils. Shoreline sedimentary environments have The undulating attitude of these tuff layers indi­ been studied extensively both in the modern cates a development of considerable topogra­ environment and in ancient sedimentary sequ­ phy within the silts concurrently with their ences (Selley 1970, Pettijohn 1975). The nature deposition. In the Fossgil Member, a thick of shoreline deposits is a reflection of the inter­ sequence of basaltic tuff layers is also interbed­ play between sediment influx and marine ded in a fossiliferous siltstone facies. redistribution. Shoreline environments are The second facies is sandy with abundant (somewhat arbitrarily) divided into deltaic and marine fossils. Individual beds appear to be linear ones, the former being typically sheet shaped. The sandstone facies ranges from regressional with a high rate of sediment influx, fine to coarse grained, and is fairly well sorted. the latter may be either regressional or trans­ Some of the fossils occur in living position, but gressional. Within the deltaic model, three ele­ in other cases they have been transported and ments occur: A delta platform characterized by appear as lenticular shell horizons, mostly with subaerial and subaqueous channel sedimenta­ unbroken shells. Trough cross bedding, tion (gravel, sand, mud, peat), a delta slope, although faint, is fairly common. The Horgi where most of the suspended mud settles out Formation and the Svarthamar and Torfh611 and may slump down to the foot of the delta Members contain this sandstone facies, which slope, and a prodelta, where clays are deposited overlies a siltstone facies everywhere. The con­ from suspension in quiet water. Sedimentary tact is distinct, but only locally in the Svart­ environments associated with clastic linear hamar is it marked by a conglomerate facies, shorelines were listed by Selley (1970). From which is lenticular in shape and consists of well land to sea, in a static shoreline, four major sorted basaltic pebbles with abundant shell frag­ environments can be recognized: Fluviatile, ments and whole shells. Intraformational peb­ coastal plain, lagoonal and tidal flat complex, bles of sedimentary rock are rare, which is barrier island, and offshore marine shelf. Each unusual for conglomerates within the Breidavik one produces distinguishable facies. During 30 regression with high sediment influx,all four conglomerates, in addition to diamictites. One facies build above one another seawards, and of the sandstone beds has been found to contain and upwards coarsening sequence, similar to a plant remains and pollen (Th. Einarsson 1977). prograding delta sequence results. On the other The vertical and lateral changes from shell bear­ hand, a transgression with high sediment influx ing sandstones to nonfossiliferous or plant-bear­ will produce a fining upwards sequence. The ing sandstones and conglomerates seem to actual sequence deposited is a function of both reflect a regressional shoreline environment the sediment availability and of the rate of and a lateral change from marine to fluvial relative sea level changes. coastal plain environment. It is a well established feature of the Late Cainozoic cold periods that drastic changes of sea level took place repeatedly. The regressions Volcanic tuffs and lava flows of and transgressions involved have probably been the Breidavik Group much more rapid than those of more "quiet" Three horizons of basaltic tuff are present in times of geological history. Another feature of the Breidavik Group sequence. The lowest of the cold periods is a high availability of sedi­ these is located above a nonfossiliferous con­ ments. Alternating periods of abrasion and glomerate facies at a high level in the Horgi rapid deposition are borne out by erosional Formation. It is exposed in two outcrops near unconformities and penecontemporaneous Horgi in the coastal section. The tuff bed is up deformation structures in the Breidavik Group to 3 m thick, but it has a lenticular form. Sand sediments. The fining upwards sequences sized basaltic glass is the main constituent, but observed in the lower regions of the Breidavik rounded rock fragments and pebbles indicate, Group cycles are compatible with a trans­ along with the bedding, that the tuff has been gressional shoreline environment. reworked and mixed with fluviatile sediments. A second tuff horizon forms a conspicuous part of the Fossgil Member (Geptner 1973). It is Nonfossiliferous sandstones and conglomerates exposed in the coastal section at and near the The uppermost parts of the Furugerdi Mem­ Fossgil waterfall, and along Trollagil and Foss­ ber, the Horgi Formation, and the Fossgil, gil gullies (Fig. 21). It can be traced southwards Svarthamar, and Torfh6ll Members of the to Ytriklofi, and probably corresponds to a tuff Breidavik Group consist of upwards coarsening layer exposed in Burfell. The tuff is predomi­ seqquences, where marine fossils disappear, nantly made up of silt to sand sized basaltic either vertically or laterally. glass. Characteristically it consists of multiple The contacts between fossiliferous sandstone tuff layers interbedded with a silty sand facies. facies and medium to fine grained cross bedded Load casts and ball and pillow structures are sandstone facies are generally distinct but with very common. The multiple tuff layers reach a no record of a major break. Such contacts were total thickness of some 25 m in Fossgil. The observed in the Furugerdi Member and the attitude of individual tuff layers is affected by Horgi Formation. In the Fossgil, Svarthamar, the topography of the substratum. The tuff and Torfh6ll Members, fossiliferous sandstone layers encroach upon underlying hills and then facies pass laterally (towards south) into coarse submerge them. At the level at which the grained nonfossiliferous sandstones. topography has been levelled out, the tuff The barren sandstone facies is generally fol­ layers dip 2-30 towards NNE. There is no lowed by a conglomerate facies, which is locally detectable break in deposition immediately characterized by upwards fining sequences. In prior to the deposition of the tuffs, and the the Fossgil and Furugerdi Members, a siltstone boundary is gradational. Thin tuff layers occur facies is present, at least locally, beneath the in the underlying siltstone, and siltstone beds conglomerate facies. The barren sandstone alternate with the tuff layers throughout. Hori­ facies is up to at least 20 m thick in the Horgi zon 4 is thicker further inland than in the coastal Formation. It contains carbonized twigs and section, and contains marine fossils there. Near pollen (Schwarzbach and Pflug 1957). In the the coast the top of the tuff layers is marked by sedimentary sequence in Burfell, the predomi­ an erosional unconformity, and the observed nant facies are cross bedded sandstones and thickness is probably a minimum value. Further 31

Fig. 21. 15 m thick series of tuffs in Trollagil. Lenticular conglomerate (channel fill) at top of section. south along Fossgil (at an altitude of ca. 250 m), remarkably uniform. The semiconformable the tuff/siltstone ratio decreases upwards in relation of horizon 5 to the substratum topogra­ vertical sections. Locally, the tuff layers have phy is thought to reflect a distribution of the ash been reworked and contain marine fossils by weak currents. (Tr6llagil, altitude of 160- 170 m). The Breidavik Group contains numerous lava The third tuff horizon corresponds to Bardar­ flows. Most of these are clearly subaerial, being son's horizon 9 (Bardarson 1925) and consists of crystalline throughout. There are four excep­ one to five tuff layers separated by a siltstone tions. A reversely magnetized lava flow at the facies. Locally, it reaches a thickness of 2 m. It top of the H6rgi Formation appears to be is composed of silt sized basaltic glass, and locally affected by rapid cooling, e.g. at Tr6lla­ locally contains abundant marine fossils. A few gil, where a pillow structure was observed. In metres above it there is a thin, discontinuous the coastal section, however, this flow is quite tuff layer of similar composition. regularly jointed. The two lava flows of the The two latter tuff horizons (horizons 5 and Fossgil Member are irregularly jointed and 9) both occupy a position within marine mud­ somewhat brecciated. They have flowed across rocks, which indicates that they were deposited fluviatile sediments at the site of the outcrop, in a subaqueous environment. Rapid, intermit­ which may indicate presence of water and rapid tent influx of the tuff material is indicated by cooling. The lowest part of the Torfh6ll Mem­ load structures and the interbedded silts in both ber lavas (Mana basalts), as exposed in the cases. The variations in sediment supply were coastal section at Voladalstorfa and up to an probably controlled by pulsations in the ash altitude of 120 m, reveals pillowy and brecci­ producing volcano and/or fluctuating water cur­ ated structure. It seems likely that this lava flow rents, tidal or other. The origin of the tuffs has actually entered the sea as a lava delta to cover not been studied thoroughly. It may be tenta­ marine sediments. At an altitude of 120 m in the tively suggested, that the ash was produced by Threngingar area, the fossiliferous sandstone explosive volcanism during shallow water sub­ passes into nonfossiliferous sandstones, and this marine eruptions. The grain size of the tuffs is may reflect the location of the shoreline at that 32 time. The lava flow becomes regularly jointed evolution of environments is the recognition of at the same level. delta sediments, flow tillites, and kame con­ The final exception is the table mountain glomerates. The presence of marine fossils in Burfell. The volcanics at the top of the moun­ sediments is important, particularly when they tain were interpreted as a subglacial volcano by occur in living positions, when it comes to Th. Einarsson et al. (1967). evaluating the wider significance of events recorded in the Tjornes sequence. A manifest demonstration of sea level changes is recorded INTERPRETATION OF in the Breidavik Group rocks and ties in with THE BREIDAVIK GROUP evidence of glaciations to form a picture consis­ A local environment is defined by a set of tent with the ice age concept. physical, chemical, and biological processes It was noted by Th. Einarsson et al. (1967), (Selley 1970, Pettijohn 1975). The net effect of that the lava flows in the Tjornes area generally an environment may be either erosion, originate from eruption sites outside Tjornes nondeposition, or deposition of sedimentary Peninsula. Among exceptions are the table facies. In the Tjornes area, constructive volca­ mountain Burfell, and a lava flow from the nic processes must be added to the sedimentary Grj6thals shield volcano. The average fre­ ones. Local environments are reflected by quency of lava flows in a typical section through geologically significant geomorphic units. The the Icelandic lava pile has been estimated at 1 analysis of these was mainly carried out in the lava flow per 11,150 yr. (McDougall et al. field by means of detailed geological mapping in 1977). Watkins and Walker (1977) concluded order to determine the macroscopic features of that the eastern part of Iceland was built up by sedimentary and volcanic units (Eiriksson virtually continuous volcanism from 13.5 to 1979). 120 sections were measured and analy­ about 2 Ma, producing on average 1 lava flow zed. The combined stratigraphical column was every 16,000 yrs. then analyzed, and a distinct repetitive charac­ The distance between the Tjornes sequence ter in the vertical sense is obvious. &l and the presently active volcanic zone in East­ Palaeoenvironmental reconstructions for the ern Iceland is only a few kilometres. In spite of Breidavik Group rocks are presented below in differences in the tectonic history of the Tjornes six sections with schematic illustrations of area and the area studied by McDougall et al. paleoenvironments within each stratigraphic (1977), it is probable that a similar order of member or formation. Any attempt to decipher availability of volcanic products prevailed dur­ past environments of the Tjornes sequence ing the formation of the Breidavik Group. It must depend on a reliable genetic classification follows that a surface left exposed for a few of the various rock facies types preserved in the thousand years is likely to have been covered by Tjornes sequence. Such a classification for the lava flows. Sediments in the sequence are there­ Breidavik Group rocks is attempted in Table 1. fore unlikely to have suffered much subaerial The table summarizes distinguishing criteria erosion. The probability of a lava flow occur­ and matches facies types with depositing proces­ ring in a given section is related to sever$ll ses. This constitutes the basis of the genetic palaeogeographical factors. Amongst these are classification of the rocks. Sets of processes palaeoslope and distance from active volcanic then add up to an aproximation of centres, both of which have allowed lava flows palaeoenvironments. In addition to rock facies, to reach the Tj6rnes area during the period erosional and angular unconformities, and fos­ under investigation. In addition, land-sea sils, offer important evidence pertinent to the relationships do affect the areal extent of lava approximation. flows. Contact of hot lava with water leads to Some of the features of Table 1 are clearly rapid cooling and the flow is diverted, slowed more relevant than others. Particularly impor­ down or confined. As a result the sea in most tant is the interpretation of some diamictites as cases forms a natural barrier to flowing lava. An lodgement tillites. That conclusion explicitly extensive glacier ice cover will similarly reduce implies that the Peninsula was repeatedly the spread of volcanic products during erup­ covered by glacier ice in the past. Less critical tions and also alter the physical character of the but nevertheless important in determining the solidified rocks. The probability of a lava flow 33

Table 1. Genetic classification of the Breidavik Group rocks

FACIES DISTINGUISHING PRO- GENETIC CLAS- ENVIRON- FEATURES CESSES SIFICATION MENT

Diamictite. Large clasts embedded in a Subgacial ero- Lodgement tillite. Subglacial. (1) matrix offine grained (silty) sion. Till materials. Sheet shape. Stria- lodgement. tions and polshing ofsubstra- Subglacial tum. Flutes. Intimate relation shear. ofpetrological composition to Fluting. local bedrock composition. Silty shear bands.

Diamictite. Large clasts embedded in a Ice melting. Flow tillite. Supraglacial. (2) matrix offine grained (silty) Slumping. materials. Limited lateral extent. Folds and thrust faults.

Conglomerate. Poorly sorted gravel with boul- Fluvial. Ice Kame conglomerate. Supraglacial. (3) ders and sand lenses. Irregular melting. Sag- mounds. Steep contacts intri- ging. Slum- cately associated with (5). Bed- ping. ding deformed into anticlines or irregulary. Interfingering ton- gues of (2).

Conglomerate. Cross bedded gravel and sand Fluvial chan- Outwash con- Proglacial. (4) with fining upwards sequences. nel erosion glomerate. Lenticular shape in cross and section. depostion.

Siltstone. Thickly laminated poorly sorted Suspension Lacustrine mudrock. Glaciolacustrine. (5) silt with stray sand grains, peb- sedimenta- bles and till lenses. Bedding tion. Slum- conformable with substratum, ping. Iceberg locally intricate relationship melting. with (3). Local sand lenses. Channel deposition.

Conglomerate. Large scale cross bedded mode- Fluvial chan- Delta platfonn/fore- Delta platform. (6) rately sorted gravel with sand nels. Floods? set conglomerate. lenses. Wedge shape. Grades into (7) laterally and vertically. Basally abundant angular intra- formational tuff fragments. (continued) 34

Table 1 (continued)

FACIES DISTINGUISHING PRO- GENETIC CLAS- ENVIRON- FEATURES CESSES SIFICATION MENT

Sandy siltstone. Laminated sandy siltstone, Suspension Delta slope Delta slope. (7) locally thinly bedded. Penecon- sedimenta- mudrock. temporaneous deformation tion. Slum- structures. Interfingering con- ping. tact with (6). Wedge shape. Maline fossils.

Siltstone. Massive, moderately sorted silt- Supension Prodelta/lagoon Prodelta (8) stone with erratics. Sheet sedimenta- mudrock. (lagoon). shape, but slump scars affect tion. Iceberg .topography during deposition. melting. rMarine fossils, often in living Slumping. position.

Sandstone. Cross bedded to massive, Waves. Tidal Bar sandstone. Offshore bar. (9) moderately to well sorted sand- currents. stone. Sheet shape. Marine fos- sils, locally in living position, also in shell banks.

Conglomerate. Well sorted gravel with abun- Waves. Beach con- Shoreline beach. (10) dant shells. Lenticular shape. glomerate. Grades upwards into fossil- iferous sandstone (9).

Sandstone and Cross beddeld and flat bedded Fluvial. Alluvial sandstone Alluvial plain. conglomerate. sandstone and conglomerate. and conglomerate. (11) Lag gravels and upwards fining sequences. Sheet or channel shape.

Tuff. Silty-sandy basaltic glass com- Volcanic Glassy tuff. Prodelta (12) position. Sheet shape, conform- eruption. Sus- (lagoon). able with substratum. Load pension structures. sedimenta- tion. Tidal currents.

occurring in a sequence located in the vicinity of It is suggested that lava flows within the active volcanism is thus high in dry, ice free Breidavik Group that do not show water con­ areas where the volcanics enjoy free flow, but tact structures are indicators of subaerial condi­ much lower in one that accumulated during tions. Furthermore, a general statement can be subaqueous and/or subglacial conditions. In the made about the sequence when it is considered, latter two cases the physical character of the that most erosional activity tends to be sub­ rocks will readily suggest contact with water and aerial. As subaerial conditions in the Tj6rnes exclude ambiguity (e. g. pillow structure, area or any area near or within a volcanic zone brecciation, etc.). lead to accumulation of lava flows, subaerial 35 erosion is not expected to have removed large parts of the marine sequence exposed at Table 2. Main facies types of the Furuvik Tjornes to leave significant gaps in the record. Formation The extrusion of lava flows to cover the sedimentary record during ice free periods has 12. Lava flows. in many cases probably contributed towards 11. Diamictite facies. conserving the sediments by providing an 10. Lava flow. armour against subsequent erosion. This fea­ 9. Laminated siltstone facies. ture of the Tjornes sequence was already 8. Coarse grained trough cross bedded sand­ emphasized by Pjetursson (Pjetursson 1905). It stone facies. is a feature shared with many other Icelandic 7. Well sorted conglomerate facies, grades sections of Late Tertiary and Quaternary age, into 8. which are thereby set apart from glacial sequ­ 6. Massive siltstone facies with marine fossils. ences in continental areas, where renewed ero­ 5. Coarse grained wedge shaped sandstones, sion tended to destroy geological evidence occur within 4. about the immediate past. 4. Poorly sorted laminated to massive silt­ stone facies with stray pebbles, deforma­ Palaeoenvironmental reconstruction of tion structures, and lenses of diamictite glacial-interglacial cycles facies. The Furuvfk Formation 3. Moderately sorted cross bedded lenticular conglomerate facies and coarse grained flat The lowest part of the Breidavik Group is bedded lenticular sandstone facies. exposed in the Furuvik Creek, where the Furu­ 2. Diamictite facies. vik Formation begins with an at least 45 m thick 1. Erosional unconformity. sequence of sedimentary rocks, followed by a 3 m thick lava flow, and a 2 m thick sedimentary bed. A series of lava flows then completes the vik Formation is presented below along with Formation. notes on the palaeoenvironmental reconstruc­ Previous research of the Furuvik sections tions: (Bardarson 1925, Askelsson 1941, LindaI1964, 1. Glacial - proglacial. A glacier advances T. Einarsson 1958, Strauch 1963, Th. Einarsson over the Hoskuldsvik zone lava plain. Glacial et al. 1967) generally suffers from insufficiently erosion leaves an erosional unconformity cut detailed mapping and has given rise to con­ across the Hoskuldsvik lavas. The glacier troversy over the history of the Formation. deposits lodgement till upon the unconformity Marine origin was suspected by Bardarson, (facies 2). During retreat, the glacier locally and littoral deposition followed by a questionable repeatedly pushes the lodgement till surface till by Strauch, but Th. Einarsson et al. into ridges (push moraines). Proglacial meltwat­ described the beds in Furuvik as a sequence of er streams cut channels into and locally rework iceberg tillite, conglomerate, sandstone of prob­ the lodgement till, depositing outwash gravels able marine origin, a lava flow, and another and sands (facies 3), which are widely found tillite layer. resting on the till. Facies types and other important features of 2. Glacio-lacustrine. A proglacial lake now the Furuvik Formation as mapped in the pre­ develops at the margin of the retreating glacier. sent study are summarized in Table 2 below, Turbid meltwater jets carry mud into the lake, number 1 being at the base of the Formation. where it is distributed by mixing with the lake A reconstruction of palaeoenvironments water and/or density currents (facies 4). Sandy based on an interpretation of the vertical sequ­ delta fans (facies 5) are built out into the lake ence of Table 2 is presented in Fig. 22. It is mud at rapidly shifting jet mouths. Locally, emphasized that the drawings only show some supraglacially derived till slumps into the lake at steps in the evolution of environments during the glacier snout to form waterlaid flow till the accumulation of the sequence. They are lenses interfingering with the lake mud. Stones schematic, and have not been drawn to scale. and till melt out of ice bergs which calve off the An outline of the geological history of the Furu- glacier snout (ice cliff), these sediments are 36

J. PROGLACIAL. Retreating ice cap. Ice cap Lodgement till Meltwater stream

e ~kA-4 ~A 4 A _- A -.u ;;:: t&"4 - 4- -+\"'- '" ---=p -4

2. GLACIO-LACUSTRINE. Proglacial lake. Turbid water I b Lake mud ~Waterlaid flow till --_[:t~~I.~I '~--~---_DeIta~---:..~-~~ .-~~.-. ~~ ~":il.3; ~ 0_- 02~ ... __ echh* ",·-4- - - -Ad. f/, A-- - - _ -

3. SHORELINE (LAGOON). Transgression. Lagoon mud '------D\-----~~e.9-/~~~L~-~. .. ..:.~. . =: '. :e~: ... -'-" . ~-"., ~..-;.~.-?"it:_ --~

4. SHORELINE (LAGOON - ALLUVIAL COASTAL PLAIN). Regression.

5. ALLUVIAL COASTAL PLAIN. Lava flows across plain

6. GLACIAL.

Lava flows Alluvial plain develops. jit~ti~d~~~~§~~±Ztf ==:-?7:-:-e-=;;-:::---:=.:-=-._-=-.~.F~ ~ ='-;=;-¥~--:~-:----;P--;?-'~-?f' _e_~.A ~k -if i£ &- =-04 1;3: ':¢:A.--£ o::i-'A-i Fig. 22. Palaeoenvironmental reconstruction of the Furuvfk Formation. See discussion in text. 37 found dispersed in the lake mud as dropstones siltstone facies, interpreted as lacustrine sedi­ and diamicton. The glacio-lacustrine environ­ ments, is seen below a conglomerate with sand­ ment evolves into a shoreline environment as stone lenses at the top of the sedimentary sequ­ the area becomes submerged beneath sea level ence. These sedimentary facies are taken to by a eustatic transgression. The only remnants indicate first an intensification of currents, and of life discovered in the sedimentary facies the sedimentary structures and the presence of deposited in the lower part of the sequence are lacustrine sediments to indicate a regression, carbonized twigs of wood found in the delta possibly due to an emergence of the area above fans. These are probably derived from older sea level. sediments (Tjornes beds) and occur as pebbles. 5. Alll1vial coastal/lava plain. The upwards As an alternative, one could possibly argue coarsening trend of the upper part of the sequ­ that the mud, sand, and diamicton lenses were ence di~cussed above is thus thought to reflect a deposited in the sea, and that sedimentation regressional environment, and the development rates were high enough to render the environ­ of an alluvial coastal plain studded with lakes. ment lethal for organisms. However, there is no The sediments are now covered by a lava flow evidence of a redistribution of the sediments by which is seen in both the Furuvfk and Stang­ the sea, and a sheltered fjord model would be arhorn sections. The lava flow is columnar required. There is nothing to suggest landscap­ jointed. .Both in Furuvfk and at Stangarhorn ing of that order at this time, and a glacio­ numerous vertical or nearly vertical up to 20 cm lacustrine environment interpretation is thick sedimentary dykes run up through the favoured. topmost conglomerate and along joints through 3. Shoreline (lagoon). The sediments inter­ the lava flow. Some of the dykes, which are preted so far are all exposed in the western part composed of silt, sand, and granules, are seen of the Furuvfk Creek. To the east of normal to originate in the sediments below. T. Einars­ step faults at Midnef a massive siltstone facies son (1958) mentioned a sandstone dyke at replaces the lacustrine mudrocks to the west of Stangarhorn and Strauch (1966) has described the faults. The massive siltstone is younger than sedimentary dykes in the Breidavfk area. The the lake mudrocks. Although there is no over­ presence of the dykes indicates that the lava lap at the faults, radiometric dating of lava flowed across soft sediments, which were flows below and above excludes ambiguity thereby deformed and squeezed upwards by the (Albertsson 1976). Nevertheless, some suddenly increased load. The lava flow is the sedimentary or other facies may be missing due topmost stratum of the Furugerdi Member. It to these faults. In 1975 I found a cast of a closed completes the first cycle of the Breidavfk bivalve in the massive siltstone facies. It was Group. The cycle started with a glaciation of identified by L. A. Sfmonarson as d. Mya the area followed by deglaciation, marine trans­ truncata. Although only one individual was gression, regression, and finally, development found (by chance), the fact that it was unbroken of an alluvial/lava plain during ice free condi­ and found in a massive, fine grained sediment­ tions. ery facies, is thought to indicate (but not prove) 6. Glacial. The lowest bed of the Midnef a marine environment. Deposition in quiet Member is a diamictite above the Furugerdi water is indicated by fine grain and massive lava. This diamictite (facies 11) is 2 m thick, has structure. A lagoon environment, perhaps a low clast/matrix ratio, and contains striated brackish (hence the apparent paucity of fossils) pebbles. It was interpreted as a tillite bed by is inferred. Th. Einarsson et al. (1967). The stratum has a 4. Shoreline (lagoon - alluvial coastal plain). rather limited lateral extent as far as can be The next sedimentary facies is an upwards judged from the coastal outcrops, being only coarsening conglomerate, starting with sand­ exposed in the eastern half of the Furuvfk stone (facies 7). In the eastern part of Furuvfk Creek. The top of its substratum has clearly where the sedimentary sequence disappears been smoothed and eroded (facies 1), but no below sea level, the conglomerate grades into a glacial striae have been found. The origin of the trough cross bedded sandstone facies (8). At diamictite is somewhat doubtful. It may repre­ Stangarhorn, where the top of this sequence sent a lodgement tillite. In that case, it would crops out again, a nonfossiliferous laminated indicate a second glaciation of the area. Such an 38 interpretation will be assumed in the present The Horgi Formation study. The surface of the top unit of the Midnef 7. Progladal - lava plain. The diamictite Member is erosional. In the coastal section east facies (11) is directly overlain by a series of thin of Stangarhorn a shallow valley, of which only lava flows in Furuvik, and there is no record of the western side is now exposed, has been a transgression after the deposition of the eroded into the Midnef lava plain. The basal diamictite. If the interpretation of the diamic­ sediments of the H6rgi Formation rest on this tite as a lodgement tillite is correct (this is unconformity. assumed in Fig. 22), it is possible that the lava Numerous authors have described the H6rgi flows entered the proglacial area immediately Formation (Pjetursson 1905, Bardarson 1925, after the glacier retreated (corresponding to the Askelsson 1941, Lindal 1964, T. Einarsson glacio-lacustrine environment in the Furugerdi 1958, 1963, Schwarzbach & Pflug 1957, Strauch Member). The lava flows probably represent a 1963, Th. Einarsson et al. 1967, Geptner 1973). very short time span, being of the shield vol­ Pjetursson gave the first description of the cano type (they are probably all derived from Breidavfk sequence and presented two sections. one eruption). This timing is, however, rather Both contain a basal conglomerate unit which speculative. Such an influx of lava flows into the he interpreted as a fluvioglacial and glacial proglacial area, one could hypothesize, would deposit which should be correlated across the have created a topographic high, which might bay. Pjetursson mentioned a tillite bed above explain the lack of a sedimentary record. the conglomerate. The next contribution came Further east along the coastal section towards from Bardarson, who published a profile of the Breidavfk the thin lava flows wedge out and are coastal section in Breidavfk. He assigned the covered by thicker flows with thin sandy and number 1 to the topmost lava flow of the Mid­ gravelly interbeds, indicating an alluvial nef Member at Stangarhorn, and number 15 to environment associated with the lava plain. the lava flow in Voladalstorfa. Later, Strauch According to T. Einarsson (1958) and Th. introduced the notation Hl-H15 for the same Einarsson et al. (1967), all the lavas are strata. Bardarson argued against Pjetursson's normally magnetized. conclusions about a glacial origin of the sedi­ Directional evidence within the Furuvfk ments in Breidavfk, and subsequent workers Formation is scant. A northeast trending expo­ have put forward several possible origins of the sure in Furuvfk reveals a series of sections rocks. through ridges at the surface of lodgement til­ lite. These ridges were interpreted as push moraines, which normally strike perpendicular Table 3. Main facies types of the H6rgi Forma­ to ice flow. The nearly vertical sections make it tion impossible to determine the real strike of these ridges accurately, but it probably lies within a 11. Lava flow. sector between 0-90°, indicating an ice flow 10. Volcanic tuff. direction either towards NW or towards SE. 9. Laminated siltstone facies. Cross beds in the lowest part of facies 7 near 8. Flat bedded sandstone facies with con­ Midnef generally dip towards NNE, wheras glomerate lenses. cross beds in the sandstone at the top of the 7. Sandstone facies with conglomerate lenses Furugerdi Member generally dip towards NE. and marine fossils. Bedding planes in the sandstone dip 60°/11oW. 6. Massive siltstone facies with conglomerate The regional dip here is 45°/8°W. Strauch's lenses and marine fossils. (1963) analysis of the Tj6rnes beds revealed a 5. Poorly sorted laminated siltstone facies palaeoslope and sediment transport towards with deformation structures. north to northwest throughout. The slender 4. Mound to ridge shaped conglomerate palaeodirectional data from the Furuvfk Forma­ facies. tion indicate that the palaeoslope remained 3. Sheet shaped conglomerate facies. similar to that of the Tj6rnes beds, which are 2. Diamictite facies. conformably overlain by the H6skuldsvfk lava 1. Erosional unconformity. zone and the Furuvfk Formation. 39

In the present work, particular attention has meltwater streams (facies 5). The deformation been paid to the relationship between the basal of kames continues, and intricate and fre­ diamictite at H6rgi (it is also sporadically quently collapsed contacts develop between the exposed in the Bay of Breidavfk), the irre­ kame gravels and the lake mud. On the western gularly surfaced conglomerates, and to the side of the valley, meltwater streams locally mudrocks and sandstones. sweep the lodgement till away, and an outwash This problem has already been discussed in plain (facies 3) with lakes (facies 9) develops. connection with the origin of the various types 3. Shoreline (lagoon - alluvial coastal plain). of sedimentary facies. The diamictite facies is A transgression now submerges the area in the interpreted as a lodgement tillite. It rests on a wake of the retreating glacier. The reworking of glacially striated lava flow which belongs to the kames is intensified. Deposition of silt (facies 6) Furuvfk Formation. The conglomerate bodies continues, however, and the conditions appear represent kames, and are juxtaposed by lacus­ to be relatively quiet. A saline lagoon environ­ trine mudrocks, which are followed by marine ment is inferred from the sedimentary facies fossiliferous mudrocks and sandstones. The and marine fossils. main sedimentary facies types and other fea­ 4. Shoreline (bar - alluvial coastalllava tures of the H6rgi Formation are listed in Table plain). The lagoon mud is covered by a sand­ 3. stone (facies 7) with conglomerate lenses and Fig. 23 shows reconstructions of steps in the marine fossils. Reworking of the upper regions evolution of environments during the accumula­ of kames is intensified and the contact with the tion of the rocks of the H6rgi Formation. The marine mud beneath is erosional. The coarse reconstructions are mainly built on the grain, cross beds and fossils of the sandstone are interpretation of the sedimentary facies compatible with a bar environment, initially exposed in the coastal section. Fig. 23 shows a transgressing over the lagoon mud, but later cross section through the western side of a receding again. The bar sediments form a valley carved into the Midnef lava plain. Ex­ wedge shaped body. The lagoon mud, however, posures are also found in the Fossgil and Tr6lla­ is not found again above the bar sands, perhaps gil gullies and in Burfell, which is useful in because the regression was rapid and the allu­ determining the shape and extent of the Forma­ vial plain prograded directly over the bar sands. tion. An outline of the geological history of the Landwards of the shoreline the alluvial coastal H6rgi Formation is presented below. Steps in plain is characterized by fluvial gravels and the palaeoenvironmental evolution refer to Fig. lacustrine sediments. A tuff horizon (facies 10) 23. is conserved in the lacustrine sediments but is 1. Glacial. A glacier advances across the not found elsewhere. It may have been swept Midnef lava plain, initiating valley erosion and away by fluvial erosion and redistributed by the leaving glacial striae and lodgement till (facies sea. A regressional sequence is reflected in the 2) on the erosional unconformity at the base of coastal outcrops in Breidavfk by the disappear­ the H6rgi Formation. As the glacier retreats, ance of marine fossils and deposition of flat­ masses of ice stagnate and supraglacial outwash bedded sandstones with channels and lag sediments (facies 4) accumulate in meltwater gravels (facies 8). A lava now flows across the channels and pools. Lobes of flow till slump plain. It is at present found exposed at H6rgi, in down the snout of the glacier and to and fro on the Fossgil and Tr6llagil gullies, and in Burfell. the dead ice topography. Mud (facies 5) is Glacial grooves on the Stangarhorn lava locally and intermittently deposited adjacent to strike N40oE, and this coincides with the or along with the outwash sediments in pools of approximate trend of the valley at the base of standing water. Melting of ice beneath the the H6rgi Formation, as far as its direction can supraglacial sediments and removal of lateral be deduced from exposures in the coastal cliffs. support leads to sagging, faulting, and deforma­ The strike of the flow lines of the glacier was tion of the sedimentary bodies. therefore northeasterly, at least in the vicinity 2. Glacio-lacustrine. The glacier recedes of the valley, and the palaeoslope probably further back and the buried ice gradually melts remained similar to the one inferred in Furuvfk. away. Kettle holes and depressions in the kame Southwest dipping cross beds were observed in topography form traps for mud carried by the bar sands. The erosional unconformity at +o

1. GLACIAL. Retreating ice cap, locally stagnating. Flow till

Lodgement till

2. GLACIO - LACUSTRINE. Prog/acial outwash

~.e-~a ~A.--:.~-- d~;

3. SHORELINE (LAGOON). Transgression..Reworkmg of kames Ash fa II . Coastal~plain " • Sea level Kames\--'''111ii1~fcollapse mto mud ~ ~--~- i,;Ti-:~Lagoon mud~--~~.b:-;;

Fig. 23. Palaeoenvironmental reconstruction of the H6rgi Formation. See discussion in text. 41 the base of the H6rgi Formation is also an angular one. A gap may exist in the sequence at Table 4. Main facies types of the Fossgil Mem­ that level. ber To summarize, the interpretation of the H6rgi sequence indicates a new cycle of glacia­ 10. Lava flows. tion, deglaciation and transgression. A prograd­ 9. Well sorted conglomerate facies with sand­ ing alluvial plain environment with lava flows stone lanses. during interglacial conditions then completes 8. Thinly bedded siltstone facies with sand­ the cycle. stone lenses. The Fossgil Member 7. Nonfossiliferous flat bedded sandstone facies. The lower unit of the Threngingar Forma­ 6. Coarse sandstone facies with marine fossils. tion, the Fossgil Member, is exposed in the 5. Thickly bedded tuffs with siltstone coastal area near the Fossgil waterfall, in the interbeds and load structures. Tr6llagil and Fossgil gullies, and has been map­ 4. Thinly bedded to massive mudrock facies ped all the way to Burfell Mountain. The tuff with marine fossils. Grades upwards into 5 layers of the Member represent an excellent by an increase in the number and thickness marker key horizon. of tuff beds. Several workers have studied the rocks of the 3. Laminated siltstone facies with stray boul­ Fossgil Member (Bardarson 1925, Lindal 1964, ders and till lenses. T. Einarsson 1958, Strauch 1963, Th. Einarsson 2. Diamictite facies. et al. 1967, and Geptner 1973), and their ideas 1. Erosional unconformity. about the origin of the sediments are contradict­ ory. For ease of reference to earlier literature, the numbers assigned to the Breidavfk horizons are adhered to (Bardarson 1925). The coastal section in the Bay of Breidavfk The present mapping of the Fossgil Member shows that the erosional unconformity (1) forms in Breidavfk and inland on Tj6rnes Peninsula a fairly steep slope dipping towards east. An added considerably to the knowledge of the eastward migration of the valley initiated and extent and nature of these beds. Marine fossils partly refilled during the accumulation of the have been found at two new localities in the H4 H6rgi Formation seems to have taken place. horizon. The position of the normally magne­ The unconformity cuts off the H6rgi Formation tized Fossgil lava flows cannot be fixed with laterally. A reconstruction of the evolution of absolute certainty, but their isolated occurrence the palaeoenvironments of the Fossgil Member within a graben flanked by Fossgil Member is attempted in Fig. 24. The cross sections are strata seems to justify their assignment to the not drawn to scale. top of the Member. In the present interpreta­ 1. Glacial - glacio lacustrine. A renewed tion, H3 represents a lodgement tillite dis­ glaciation of the Tj6rnes area, coupled with a playing banded structure, boulder clusters, stri­ lowering of sea level, continues the valley ero­ ated and faceted boulders, sheet shape, and an sion begun during the H6rgi glaciation. A new irregular surface. H4 begins as a lacustrine sedi­ erosional unconformity (1) is created and subse­ ment with erratic boulders, but grades upwards quently covered by lodgement till (facies 2). into a massive poorly sorted marine siltstone The till is discontinuous and thin on the slope with fossils. It is interrrupted by the multiple exposed in the coastal section. Upon retreat of tuff layers of H5, which have locally been the glacier, an extensive proglacial lake is reworked. Marine sedimentation continued as formed, where laminated to thinly bedded mud the fossiliferous sandstones at the Tr6llagil with erratics (facies 3) is conformably deposited bifurcation were deposited. The section includ­ on the uneven substratum. ing the normally magnetized Fossgil lava flows 2. Shoreline (lagoon). The glacier retreats indicates terrestrial conditions. The main further and sea level rises gradually. The lake sedimentary facies types of the Fossgil Member becomes saline and marine molluscs colonize are presented in Table 4 below, number 1 (an the mud which continues to accumulate (facies erosional unconformity) being at the base. 4). A lagoon environment is inferred from the +­ t--.:l

1. GLACIO- LACUSTRINE. Retreating ice cap. Lake mud Ice cap Lake level Lodgement till

-'~ -~~:. ~---~.:---- _.& -;s: ':c.-

2. SHORELINE (LAGOON). Transgression. Tuffs Sea level Lagoon mud

:~;E~~g;:':"~;~~·····~~~:~;:'"~.4;.~~='" ~~~7~~~~f-~~~~$'~i~i~T~ ~.··(:I~·I~··I '. I'· ~". f 'I'~~ ~~\~II~I':-;~ ~~·~::i·~i.·r0ZEf~q·~~::i·' :;.-t:

Lava flow 4. ALLUVIAL COASTAL PLAIN. t.::.~.~.~~/o~O#J .2S.d.d':CV;-I,~-',)):/\(-i'\..,("(e;QJ[,i)r.),, ~ ll li\\l-'-;~ll~ .'-J/,'-~ •• _ __ ••. 8.. h. __ •. ." " - - , • /'1/ .f.(i( \ r·/,r \-,( / • I ... C

Fig. 24. Palaeoenvironmental reconstruction of the Fossgil Member. See discussion in text. 43 character of these sediments. Volcanic erup­ and a new cycle begins. Soon after the retreat tions (submarine?) now take place in the vicin­ phase of the cycle a transgression takes place ity of the area, and the undulating surface and volcanic activity produces abundant ash topography of the till surface, having persisted carried into the area. During the following throughout the mud deposition, is gradually interglacial the shoreline environment prog­ levelled by tuff beds (facies 5). The lowest tuff rades and lava flows are poured over the allu­ beds are thin (5-10 cm), but gradually the silt vial coastal plain. interbeds become thinner and individual tuff beds reach a thickness of 5 m. The sandy tuffs were clearly deposited rapidly upon the fresh, The Svarthamar Member porous muddy sediments, as the contacts dis­ The stratigraphical relationships and origin of play load casts, flame structures and ball and the sedimentary rocks of the Svarthamar Mem­ pillow structures. Gradually, the silty interbeds ber have been a matter of debate in the litera­ increase in thickness again at the expense of the ture (Pjetursson 1905, Bardarson 1925, Askels­ tuff layers. Lagoon mud continues to accumu­ son 1941, Strauch 1963, T. Einarsson 1958, late. 1963, Geptner 1973). Because of the various 3. Shoreline (bar - alluvial coastal plain). conflicting views on the origin of the rocks of The lagoon mud is followed by a sand facies the Svarthamar Member, and because of the (facies 6). The contact is gradational, but locally conflicting opinions of stratigraphical relation­ there is evidence of reworking, which has pene­ ships within the Breidavfk sequence, all expo­ trated into the tuff beds. An increase in current sures were remapped in 1975 and 1976. Particu­ velocity or a change of water depth is inferred. lar attention was paid to the extent of Bardar­ A transgressing bar environment is reflected by son's (1925) H8 horizon, its physical character, coarse sand with marine fossils, which and the relation to H6 and H7. The irregular encroaches upon the lagoon mud as sea level and discontinuous position of H9 was also stu­ continues to rise. died closely, and evidence of slumping 4. Alluvial coastal/lava plain. The sea level emerged. Glacial striae on the substratum of H6 rise eventually comes to a halt and the situation and the texture of the lowest bed of that horizon becomes reversed. An alluvial coastal plain confirm Geptner's (1973) conclusion about the environment progrades over the bar sands origin of what he called H6a, it represents a (facies 7, 8, and 9). Lake sediments, flat bedded lodgement tillite. It is followed by an upwards sands and channel sequences involving gravels fining sequence interpreted as a delta, H6 and and sands characterize the environment at the H7 corre sponding to the delta platform and H8 top of the Fossgil Member. Two lava flows now to the delta slope. HIO corresponds to the distal reach the area and cover the sediments at Foss­ slope or prodelta mud. The facies types of the gil. Svarthamar Member are listed in Table 5 with The long axes of pebbles in H3 were mea­ other important aspects of the sequence, num­ sured at two localities (in Fossgil and Tr6llagil). ber 1 being at the base of the Member. A strong unimodal distribution is centred The reconstruction of palaeoenvironments around N15°E on a rose diagram over the Foss­ presented in Fig. 25 is based on the facies types gil data (53 measurements). A smaller sample and on field relationships. It is schematic and (20 measurements) from Trbllagil has a unimo­ has not been drawn to scale. dal distribution centred around 0° on a rose 1. Glacial - proglacial.An erosional surface diagram. A 2 m long boulder in the H3 lodge­ with glacial striae (facies 1), lodgement till ment tillite at Fossgil is oriented with the long (facies 2), and kames (facies 3) are products of a axis parallel to N200E. Striae on top of that glacier which advanced over the Tj6rnes area boulder strike N400E. These data are consi­ and receded again, leaving stagnant ice on a dered to show the approximate direction of ice valley floor. Supraglacial outwash sediments flow, which is in agreement with the apparent are preserved as kames at Svarthamar and to trend of the valley. Similar palaeogeographical the east of Svarthamar. conditions to those of the H6rgi Formation are 2. Shoreline (delta). The proglacial environ­ inferred. After the Hbrgi interglacial sea level ment is now submerged by a eustatic rise of sea drops, a new glacier advances across the area, level. Sedimentation rates are high due to an ~ ~

7. PROGLACIAL.

Lodgement till

.?"~=-

_=--..o..~L5:/-_

2. SHORELINE (DELTA). Transgression. Delta platform d Delta slope 5~J~~ ~~~~ ~ _ ';0,- .;'~~-

.~ • 25~

Lagoon mud 3. SHORELINE (LAGOON - DELTA). R k· f k Tuff, deposited upon slumped topography ewor mg 0 ames .

~_ _ • ~o _____ num=uA u __ n ~.,/T :.-;:'~. "" 07.. ~_ .. ~. """ _.. -. . .. . ~ .--'-~~""~~~~. ~.~ k-=~·-o.~~~~:·~·· :~~~~~-",~~.-;~;~~=r:~~~~~ .~

4. SHORELINE (BAR -LAGOON -ALLUVIAL COASTAL PLAIN). Static.

Sea level Slump scar

Fig. 25. Palaeoenvironmental reconstruction of the Svarthamar Member. See discussion in text. 45

The upper regions of kames are reworked con­ siderably at this stage, and the kame topogra­ Table 5. Main facies types of the Svarthamar phy is levelled out by mud deposition. The Member lagoon mud is, however, unstable, and large depressions (scars) form locally, either through 13. Lava flow. slumps or erosion by turbidity currents initiated 12. Nonfossiliferous sandstone facies with con­ in the delta slope. Lag gravels are observed at glomerate lenses. some of these "channels". A tuff layer (facies 8) 11. Coarse sandstone facies with marine fossils. is now deposited on this locally erosional sur­ 10. Conglomerate facies with marine fossils. face of the lagoon mud during submarine condi­ 9. Local erosional unconformity. tions. The tuff layer is colonized by marine 8. Volcanic tuff with marine fossils. fauna. Mud deposition continues unaffected by 7. Local erosional unconformity. the ash fall. Large scale slumping also continues 6. Mudrock facies with marine fossilis and to occur, and the tuff layer (H9) is locally swept erratics. away. The scars created by the slumps (or 5. Thinly bedded to laminated silty sandstone turbidity currents) were yet again filled with facies with deformation structures and marine mud and occasional thin tuff layers. marine fossils. 4. Shoreline (bar - lagoon - alluvial coastal! 4. Cross bedded conglomerate facies. lava plain). A sheet of coarse fossiliferous sand 3. Conglomerate facies. is now deposited above the lagoon mud. A lens 2. Diamictite facies. of gravel (facies 10), which is seen at the base of 1. Erosional unconformity. the sand sheet (facies 11), contains abundant marine shell fragments and complete shells. The gravel is composed of basaltic pebbles with a trace of rhyolitic, plutonic, and meta morphic intensive runoff from the waning glacier. A pebbles. The gravel was deposited on an erosio­ delta sequence is built out from the shoreline nal surface of a soft substratum (9), the contact (facies 4, 5, and 6), but as the rise of the sea is locally intricate and bands of mud stretch level is relatively rapid, the typically regressio­ upwards into the gravel. These facies are inter­ nal delta model is reversed here, leading to an preted as a transgressing bar environment. The upwards fining sequence. Marine fauna colo­ gravel may be partly derived from a protruding nizes distal parts of the delta slope and the kame, and it represents the basal part of the bar prodelta. The delta platform sediments are environment. Vertically, the gravel grades into locally very coarse and contain abundant intra­ fossiliferous sands. Landwards of the shoreline, formational sedimentary pebbles and boulders. an alluvial plain develops. Fluvial channels are Some of these are fossiliferous, and are prob­ cut into the substratum (into H5 along the ably derived from the Tjornes sedimentary zone Trollagil gully), elsewhere nonfossiliferous (Bardarson 1925). The extremely coarse grain sands (facies 12) accumulate, e.g. in the Burfell and steep large scale cross beds, which occur area. A lava flow enters the area and covers the locally in the platform sediments, and the pre­ alluvial plain. sence of intraformational angular boulders may Glacial striae were found beneath lodgement indicate that sheet floods took place during the tillite (on the surface of H5) at the Fossgil development. This may reflect increased valley waterfall. The striae are parallel and strike erosion in the interior, leading to the formation N18°E, which is similar to the direction of ice dammed lakes as the ice cap thinned, and obtained for the Fossgil glacier. The Svartha­ to occasional bursts. mar Member as interpreted here contains evi­ 3. Shoreline (lagoon - delta). The delta dence of advance and retreat of a glacier fol­ environment continues to regrade as sea level lowed by a transgression and a development of rises further. In distal parts marine mud con­ interglacial shoreline en vironments. Lava flows tinues to accumulate rapidly in relatively quiet then enter the area. A valley continues to water, indi cating perhaps a lagoon environ­ migrate eastwards during glacial erosion, but is ment at this time (it is later transgressed by subsequently filled with sediments during late­ fossiliferous sands interpreted as bar sands). glacial and interglacial time. 46

1. GLACIAL. Re treat'Ing Ice, cap,

outwash

2. LAVA PLAIN. Lava flow

3. GLACIAL. Ice cap

4. GLACIO - LACUSTRINE. Lake mud Proglacial outwash ~ ..,

F~g.discussion26: Palaeoenvironmentalin text. reconstruction of the M'ana'FormatIOn. Up to the Dimmidalur Member, See 47

The Mana Formation nized as a tillite by Th. Einarsson et a1. (1967). The basal flute molds, internal shear planes and silt bands, and the frequent striated boulders In earlier stratigraphical nomenclature and support the conclusion that the ed was depo­ classification (e.g. Th. Einarsson et a1. 1967), sited as a lodgement till. The contact between the Breidavik beds were defined to reach up to the beds indexed H13a (the lodgement tillite) reversely magnetized lava flows (HI5) at Vola­ and H13b by Geptner (1973) is not erosional, a dalstorfa. These lava flows are placed within the feature hardly compatible with a glacial burst Mana Formation in the present study, and they event. The internal structure of the conglomer­ constitute the top of the Torfh6ll Member, ate (severe deformation and irregular dip direc­ which then contains Bardarson's (1925) hori­ tions) leads to their interpretation as kames. zons H13 - H15. The conglomerate H13c is interpreted here as a Lindal (1964) traced the lava at Voladalstorfa fluvial deposit (outwash). It is followed by towards south. A more detailed account was lacustrine mud with erratics, and then by cross later presented by T. Einarsson (1958), who bedded sandstones. traced this unit of reversely magnetized lava The three units above (H13c, mudstone, flows (the "Mana basalts") from Voladalstorfa sandstone) belong to the Torfh6ll Member as to Burfell. In Burfell mountain he described a well as the lava flow at the top of the section. group of normally magnetized lava flows above On the northeast side of Tj6rnes (200 m south the Mana basalts, separated from them by a of Skarfafl6s, cf. Eiriksson 1981b), a lodgement conglomerate. T. Einarsson called the two lava tillite is found intercalated between an erosional units "the young basalts of Tj6rnes" and sup­ unconformity and the Mana basalts. This tillite posed that they had originally covered the forms the lowest unit of the Torfh6ll Member, peninsula completely. the unconformity below it probably corres­ Recent mapping (Eiriksson 1981b, Gud­ ponds to the one beneath the conglomerate 13c mundsson and Sigfusson 1982, Eiriksson et a1. in the Torfh6ll section. The lodgement tillite 1982) has led to the definition of 4 member 13a and the kame conglomerate 13b belong to units within the Mana Formation, each contain­ the Stapavik Member, and two lava flows ing a diamictite bed and one or more reversed exposed at Knarrarbrekkutangi form the upper­ polarity lavas. Two of the units, the Stapavik most known units of that Member. and Torfh6ll Members are exposed in the coast­ The reversed polarity Torfh6ll Member lava al sections, but another two, the Dimmidalur unit in Voladalstorfa is correlated with unit and Burfellsa Members crop out in Grasafj6ll GF- 3 in Grasafj6ll (Fig. 27). In that section and Burfell (Fig. 27). there are two additional reversed polarity lava Geptner (1973) described a ground moraine units (GF-5 and GF-7) separated by sedi­ in the northeastern part of the Bay of Breida­ ments. The lower unit, GF-5, can be traced in vik. The bed was numbered H13a by Geptner, the valley sides of Burfellsskardadalur and Dim­ who noted that it cuts across H12 and is overlain midalur and to the southeast slopes of Burfell. by a non-laminated gravel and sand deposit It forms the top of a separate member unit, the with large blocks (HI3b). This was in turn Dimmidalur Member, which belongs to the overlain by a conglomerate (H13c) with a dis­ Mana Formation and is younger than the Tor­ tinct erosional contact at the base. Geptner fh6ll Member. Unit GF-5 rests on a sediment­ found fragments of ground moraine (a term ary horizon consisting of a 0.7 m thick con­ used by him for lodgment till) in H13c where it glomerate, followed by an up to 3 m thick cuts across H13a. He regarded the H13b con­ diamictite, and then by an up to 2 m thick glomerate as a glacial burst deposit, the burst sandstone (unit BS-4, Fig. 27). having been caused by subglacial volcanism. The upper lava unit, GF-7, is separated Geptner's section of the Voladalstorfa coastal from the Dimmidalur lavas by a single up to 2 m cliffs is considerably more accurate than earlier thick diamictite bed, and this couplet forms the versions. His interpretation of the origin of the Burfellsa Member, named after the brook that sediments is, however, different from the one exits Burfellsskardadalur at the foot of section presented below. The lowest part of the H13 GF (Bj6rnsson 1977). The Burfellsa Member horizon in Stapavik had already been recog- forms the top of the Mana Formation. 48

LEGE NO, [iiI] Lava flow ~Pillo'w'lava ~Hyaloclastite

IN!Jm Sill in hyaloc\astite ~Diamictite ~Conglomerate

[]J Sandstone ~Tertiary\avaflo'ois

-Erosional unconformity

Bangastadir Member -

Skeidsoxl Member GRASAFJOLL FORMATION

Midl

Burfellsa - Member

Dimmidalur Member MANA FORMATION

Torfh6ll Member

Kaldakvfsl lavas

Fig. 27. Sections in Grasafj611 and Burfell. 49

ment was still soft and deformable at the time of Table 6. Major facies types of the Mana Forma­ glaciation (facies 1). The flutes strike N2°E, and grooves on an internal plane within the till tion (facies 2) strike N8°W. During retreat the gla­ cier stagnates in places and supraglacial out­ 18. Lava flow. wash sediments accumulate (facies 3). The 17. 2 m thick poorly sorted diamictite with silty supraglacial outwash sediments are lowered matrix and angular to subangular pebbles onto the sloping till surface due to the melting and boulders, up to 0.5 m in diameter. of stagnant ice beneath. In the process, intense 16. Multiple lava flows. folding and faulting takes place within the 15. Up to 2 m thick reddish, medium grained poorly to moderately sorted pebbly sand­ materializing kames. Proglacial meltwater stone with basal lenses of conglomerate. streams rework and erode the kame deposits locally. 14. Very poorly sorted diamictite with angular pebbles and boulders up to 0.5 m in dia­ 2. Lava-plain. Lavas flow across the area during ice free conditions (facies 4). meter. Topmost part reddish with trace of rounded pebbles. 3. Glacial. A glacier covers the area once again and deposits lodgement till (facies 6) 13. 0.7 m thick red-grey conglomerate with upon an erosional surface. rounded pebbles, lower part well sorted, 4. Glacio-lacustrine. The glacier retreats and upper part poorly sorted with pebbly­ a proglacial outwash plane (facies 7) is now sandy matrix. submerged by a proglaciallake, in which poorly 12. Lava flows. sorted mud accumulates (facies 8) along with 11. Nonfossiliferous sandstone and conglomer­ erratics dropped from ice bergs calved off the ate facies. glacier margin. Local deformation of the cross 10. Fine grained trough cross bedded sand­ bedded outwash gravels is taken to indicate a stone facies with marine fossils and erratics. temporary readvance of the glacier front 9. Coarse gravelly sandstone facies with (perhaps as the lake was formed). Possibly, it marine fossils. might have been caused by scouring ice bergs. 8. Pebbly mudrock facies, laminated near 5. Shoreline (lagoon - bar). The appearance base. of marine fauna in the mud facies (facies 8) 7. Cross bedded conglomerate facies with coincides with a marine transgression in the local deformation structures. wake of glacier retreat, and a lagoon environ­ 6. Diamictite facies. ment develops. A transgressing bar environ­ 5. Erosional surface. 4. Lava flows. ment with increased wave action and current velocities then brings cross bedded sands with 3. Conglomerate facies with deformation marine shells across the lagoon mud (facies 8). structures. Reworking of the kames leads to an admixture 2. Diamictite facies. 1. Erosional unconformity. of gravels (facies 9) in the basal part of the bar sands (facies 10) in the vicinity of the kame mounds. 6. Shoreline (alluvial coastal/lava plain). The major sedimentary facies types and other Alluvial sands and gravels (facies 11) are depo­ features of the Mana Formation are presented sited landwards of the shoreline, only to be in Table 6, number 1 being at the base. covered by numerous lava flows. A reconstruction of palaeoenvironments dur­ 7. Lava plain. The glacier has now retreated, ing the accumulation of the Stapavik and Torf­ and alluvial sedimentation and volcanic activity bOll Members is presented in Fig. 26, which is now characterize the environment. schematic and has not been drawn to scale. The Steps 3-7 are recorded in the rocks of the reconstruction of the Dimmidalur and Burfellsa Torfh6ll Member. These rocks crop out in seve­ Members is shown in Fig. 28. ral sections on the east coast of Tjornes, and 1. Glacial. A glacier covers the Tjornes area these sections have recently been analyzed and and deposits lodgement till. Flute molds on the interpreted by Birgisd6ttir (1984). She identi­ sole of the till indicate that the underlying sedi- fied a sedimentary horizon intercalated 50

8.- n. GLACIAL- LAVA PLAIN - GLACIAL - LAVA PLAIN. Dimmida/ur Burfellsa

J. GLACIAL. - Ice cap

2. LAVA PLAIN.

3. GLACIAL. Subglacial volcano Ice cap

Lodgement till

4.-5. LAVA PLAIN- GLACIAL. mountain Ice cap

6.-7. LAVA PLAIN-GLACIAL. Ice cap Bangastadir lava plain

8.- 10. LAVA PLAIN - GLACIAL - ALLUVIAL / LAVA PLAIN. Bakkaa lava plain

~":' ~'.' : ....

)' Ii l ~I Fig. 28. Palaeoenvironmental reconstruction of the Grasafjoll and Husavfk Formations (including the Dimmidalur and Burfellsa Members of the Mana Formations). 51 between the lavas of the Stapavik and Torfh6ll the burrowing molluscs Mya truncata and Members ("Sedimentschicht I") as a lodgement Hiatella arctica are found in living positions in tillite, ice-contact and glaciofluvial deposits and the top of the diamictite, which is covered by a fluvial deposits. sandy conglomerate in which a granite pebble 8. Glacial. Renewed glaciation and deposi­ was found. tion of facies 14. Facies 13 may represent prog­ At Botnsvatn the lava is solid and rests dire­ lacial fluvial sedimentatiop during ice sheet adv­ ctly on a diamictite, which is in turn underlain ance. The ice then melts away and a pebbly by olivine porphyritic lava. The sequence of sandstone facies is deposited on the till plain. exposed strata in Husavik begins with a normal 9. Lava plain. Subaerial volcanism produces polarity olivine porphyritic lava flow followed lava flows (facies 16). by a very thick diamictite with internal shear 10. Glacial. Renewed glaciation. planes in HusavikurhOfdi. This diamictite thins 11. Lava plain. Ice free conditions allow out and disappears towards south because of an lavas to flow across the till plain left by the erosional unconformity. In the southern part of retreat of ice. Husavik the basal lava appears to be overlain by the Grj6thals pillow breccia, but the contact is not exposed. At the top of the section there is The Grasafj611 and Husavfk Formations another diamictite, which should probably be Rocks of the Grasafj6ll Formation are wide­ correlated with a diamictite above the erosional spread on eastern and central Tj6rnes, and have unconformity in the northern part of Husavik. recently been mapped in detail (Eiriksson The uppermost part of this diamictite is loosely 1981b, Gudmundsson and Sigfusson 1982). The cemented, and it probably represents till depo­ work of Lindal (1964), T. Einarsson (1958) and sited by the Weichselian ice cap, which has left Th. Einarsson et al. (1967) clarified the main numerous features on or at the surface of stratigraphic succession, but several new units Tj6rnes, among them glacial striae on the top of have been added. The interpretation of the the Grj6thals lavas. Till, lateglacial marine stratigraphy takes into account rock facies deposits, soil and postglacial lava flows form the exposed along the coast and in Grasafj6ll and uppermost part of the Husavik Formation. Burfell mountains. The Grasafj6ll Formation The main rock facies and other features of contains 3 member units, the Midlaekur Mem­ the Grasafj6ll and Husavik Formations are pre­ ber, the Skeids6xl Member, and the Bangasta­ sented in Table 7, number 1 being at the base. dir Member. A reconstruction of a few steps in the evolu­ The Husavik Formation is the youngest tion of environments during the accumulation formation of the Breidavik Group, and contains of the rocks of the Dimmidalur and Burfellsa two distinct subunits. The lavas of the intergla­ Members of the Mana Formation and those of cial shield volcano Grj6thals represent a key the Grasafj6ll and Husavik Formations is pre­ horizon. At Bakkaa, Reydara and Botnsvatn sented in Fig. 28, and an attempt to interpret the lavas overlie a diamictite, and at the former the record is as follows. two localities they reveal vertical gradation 1. Glacial. Approximately at the time of the from breccia to solid lava, which is also true of geomagnetic polarity reversal from Matuyama exposures south of Husavik and in Aervik. to Brunhes, a glacier covered Tj6rnes Penin­ According to T. Einarsson (1965) and Birgis­ sula, eroded the substratum and deposited a d6ttir (1984) the Grj6thals lava in Aervik rests lodgement till (facies 2). It is now found on sediments containing marine fossils. between reversed and normal polarity lava At the top of the section in H6fdaskard in flows at Engidalsgja. Breidavik there are local remnants of sediment­ 2. Lava plain. When the glacier had retre­ ary horizons that discordantly overlie sediments ated, a lava plain environment developed of the Torfh6ll Member. The sediments were (facies 3). There is no information about sea described by Gudfinnsson et al. (1983), and level during this cycle, which belongs to the they occupy a similar position to that of the Midlaekur Member. Husavik tillite. According to Gudfinnsson et al. 3. Glacial. The lava plain of the Midlaekur the sequence above the unconformity begins Member is now covered by a new glacier. A with a diamictite with shear planes. Fossils of volcanic eruption now takes place. Hyaloclas- 52

volcanics. Upon ice retreat, local lakes and ponds silt up to level a dead ice topography on Table 7. Facies sequence of the Grasafj6ll and the lowlands (facies 9 and 11). Burfell Formations 6. Lava plain. After a retreat of the Banga­ stadir Member glacier, subaerial lava flows 22. Soil with ash layers, and lava flows. cover the tiIVoutwash plain and the topography 21. Sand with marine fossils. is levelled to some extent. The table mountain 20. Nonfossiliferous gravels and sands. Burfell towers over the surroundings. 19. Diamicton with fluted surface. The most conspicuous Bangastadir Member 18. Erosional surface. lavas on the east coast of Tj6rnes are multiple 17. Normal polarity lava flows, basally brecci- flows of highly plagioclase porphyritic basalt, ated in coastal sections. which serve as a useful marker unit. A local 16. Marine fossiliferous siltstone. outcrop of olivine porphyritic basalt is sepa­ 15. Diamictite with grooved internal planes. rated form the plagioclase rich lavas underneath 14. Erosional unconformity. by a sedimentary horizon (mainly sandstone) 13. Multiple flows of normal polarity lavas. north of Hringsbjarg. 12. Up to 4 m thick cross bedded conglomerate 7. Glacial. Tj6rnes is once again covered by with sandstone lenses. a glacier which deposits an exceptionally thick 11. Hyaloclastites, pillow breccias and lava diamictite, which at present juts out as sea cliffs flows of the table mountain Burfel!. in Husaviurh6fdi, Lundey and at the top of the 10. Discontinuous laminated siltstone with Engidalsgja section. A bed of large rounded deformation structures. boulders is intercalated in the basal part of this 9. Up to 5 m thick diamictite with silty matrix, diamictite in the Engidalsgja section. Marine bouldery near base. sediments in Aervik may have been deposited 8. Erosional unconformity. during a transgression in the wake of the 7. Normal polarity lava flow. retreating glacier, and the same is true of the 6. Hyaloclastites with lenses of diamictite. fossiliferous sediments at the top of the section 5. 2.5 m thick platy and banded diamictite in H6fdaskard in Breidavik. with grooved internal planes. 8. Lava plain. After a retreat of the glacier 4. Erosional unconformity. the Grj6tMls shield volcano poured lavas over 3. Normal polarity lava flows. the southern part of the area. The lavas prob­ 2. 5 m thick diamictite. ably entered the sea as they are extremely 1. Erosional unconformity. brecciated at the base. Elsewhere, they covered the till plain of the Bakkaa Member. 9. Glacial. The last glaciation of the Tj6rnes tites (facies 6) are piled up in a cavity which is area sets in and the oldest sediments are till melted into the glacier. The volcanism is deposited by the glacier. This glacier has also intermittent and local lenses of diamictite left glacial striae and grooves at numerous loca­ become intercalated in the volcanic pile, which lities on Tj6rnes Peninsula. Fluted and drumli­ is finally eroded considerably and locally nized till plains are observed in the Kaldakvisl covered by diamictite. area. The strike of these linear features is north­ 4. Lava plain. Upon retreat of the glacier, westerly. subaerial lava flows (facies 7) cover the till 10. Shoreline - alluvial coastal plain. A surface and the topography is levelled to some notch roughly parallel to the present coastline extent. These are the lavas of the Skeids6xl marks an abrupt end of the fluted surface on the Member. Kaldakvisl area till (facies 19). It is tentatively 5. Glacial. The Skeids6xl Member lavas are suggested that a marine transgression at the end now covered by an advancing glacier, deposit­ of the last glaciation reached this lineation. ing diamicton (facies 9). A subglacial eruption Sediments near the top of the costal cliffs near now takes place and the top of Burfell mountain the farm Ytri Tunga contain marine fossils and is piled up. Explosive volcanism continues until are not lithified (Gladenkov 1974). They might the pile exceeds the thickness of the ice, and have been deposited during the same transgres­ subaerial lava covers the fragmental pile of sion. After an isostatic rebound, the present 53 environments evolved with soil formation and sea sediments. According to palaeomagnetic coastal and fluvial processes. Recent lava flows and K/Ar data, the first seven of the lodgement have not reached the northern part of Tj6rnes tillites were deposited between ca. 2 Ma and ca. Peninsula, but are to be found to the southwest, 1.25 Ma. The upper part of the Breidavfk south, and southeast. Group was formed during reduced rate of subsi­ dence and later during uplift on Tjornes, which to-day represents a horst structure. The apparently lower frequency of glacial events CONCLUSIONS may thus not reflect overall changes in the rate The Breidavfk Group documents a remark­ of climatic cyclicity, but instead a reduced con­ ably detailed history of Quaternary glaciations. servation potential of the rocks on Tjornes. The unusual tectonic setting and volcanic activ­ The results presented here need to be sup­ ity are critical conservation factors. The ideas ported by mapping, lithostratigraphical clas­ about the origin of the Breidavfk Group rocks sification and facies analysis of Quaternary sec­ and the interpretation of the lithological cycles tions elsewhere in Iceland, where volcanism has presented in this paper may be summed up as been active. Correlations between different follows: parts of the country may solve questions about 1. Lodgement tillite beds in the sequence the extent of individual glaciations. After this were deposited by ice caps which are assumed task has been completed, correlation with to have covered most of Iceland and the sur­ Quaternary sections elsewhere and a compari­ rounding shelf. son with the deep sea record will probably add 2. Kame conglomerates and glaciolacustrine to our knowledge of the history of glaciations mudrocks were formed during the retreat of ice and climatic changes in the North Atlantic area. caps. 3. Marine transgressions in the wake of retreating ice are evident by the presence of ACKNOWLEDGEMENTS marine sediments with fossils in close associa­ The Science Institute of the University of tion with deglaciation sediments. The fauna is Iceland supported the field work on Tjornes. I arctic in the sediments immediately above gla­ am indebted to the following persons for helpful cial sediments. discussions and advice during the preparation of 4. Marine sediments with interglacial fauna this paper: Professor Thorleifur Einarsson, Dr. cover the tillites and deglaciation sediments in G. S. Boulton, Dr. Leifur A. Sfmonarson, Dr. Breidavfk. These sediments interfinger with Kristinn J. Albertsson, Andres I. Gud­ and are followed in the vertical sense by fluvial mundsson and Helgi Bj6rnsson, cando real. sediments and lava flows indicating regression Andres 1. Gudmundsson assisted with the final and ice free conditions. drafting of the figures. Special thanks are due to Such a sequence of facies (1-4) constitutes Arni Karason and Bjarki Karason and their an ideal Breidavfk Group cycle, and is consi­ families on Hallbjarnarstadir and to Kari Leifs­ dered to have accumulated during a climatic son and his family in Breidavfk for their glacial-interglacial cycle. hospitality and assistance, as well as to the 5. The frequency of the cyclicity of the community of Tjorneshreppur for allowing the Breidavfk Group is similar to that of the oxygen use of the community hall at Hallbjarnarstadir isotope stages defined for the Quaternary deep during field work. 54

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(Received January 26, 1985; accepted April 12, 1985) ACTA NATURALIA ISLANDICA Volume I: 1. T. Einarsson: Origin of the Basic Tuffs of Iceland. 75 pp. 1946. 2. J. Askelsson: A Contribution to the Geology of the KerlingarfjOlI. A Preliminary Report. 15 pp. 1946. 3. S. Steind6rsson: Contributions to the Plant-Geography and Flora of Iceland IV. The Vegetation of fsafjardardjup, North-west Iceland. 32 pp. 1946. 4. I. Davidsson: Notes on the Vegetation of Arsk6gsstr6nd, North Iceland. 20 pp. 1946. 5. S.H. Petursson: Die Herkunft der Milchsiiurelangstiibchen des isliindischen Speisequarks. 8 pp. 1946. 6. 6.B. Bjarnason: The Component Acids of Icelandic Herring Oil. 9 pp. 1946. 7. H. Einarsson: The Post-larval Stage of Sand-eels (Ammodytidae) in Faroe, Iceland and W-Greenland Waters. 75 pp. 1951. 8. H. Lister, R. Jarvis, M. McDonald, I.W. Paterson and R. Walker: S6lheimaj6kull. Report of the Durham University Iceland Expedition 1948. With an Introduction by J. Eyth6rsson. 41 pp. 1953. 9. R. Dearnley: A Contribution of the Geology of Lodmundarfj6rdur. 32 pp. 1954. 10. T. Tryggvason: On the Stratigraphy of the Sog Valley in SW Iceland. 35 pp. 1955.

Volume II: 1. H. Einarsson: On the Post-larval Stages of Ammodytes lancea Cuvier. 7 pp. 1955. 2. S. Th6rarinsson: The Qraefaj6kull Eruption of 1362. 99 pp. 1958. 3. J. Askelsson: Fossiliferous Xenoliths in the M6berg Formation of South Iceland. 30 pp. 1960. 4. S. Steind6rsson: The Ankaramites of Hvammsmuli, Eyjafj6ll, Southern Iceland. 32 pp. 1964. 5. E. Wenk, H. Schwander und H.-R. Wenk: Labradorit von Surtsey. 29 pp. 1965. 6. S.P. Jakobsson. The Grimsnes Lavas, SW-Iceland. 30 pp. 1966. 7. K. Saemundsson: Vulkanismus und Tektonik des Hengill-Gebietes in Siidwest- Island. 105 pp. 1967. 8. K. Nakamura: En Echelon Features of Icelandic Ground Fissures. 15 pp. 1970. 9. G. J6nsson: Fasciation inflorescentielle et fasciation vegetative. 15 pp. 1970. 10. N. Hald, A. Noe-Nygaard & A.K. Pedersen: The Kr6ksfj6rdur Central Volcano in North­ west Iceland. 29 pp. 1971. (continued on outside back cover) ACTA NATURALIA ISLANDICA (continued from inside back cover)

21. L Munda: General features of the benthic algal zonation around the Icelandic coast. 36 pp.1972. 22. T.W. Johnson, Jr.: Aquatic fungi of Iceland: Uniflagellate species. 38 pp. 1973. 23. T.W. Johnson, Jr.: Aquatic fungi of Iceland: Biflagellate species. 40 pp. 1974. 24. A. Ing6lfsson: The feeding habits of Great Black-backed Gulls, Lams marinus, and Glaucous Gulls, L. hyperboreus, in Iceland. 19 pp. 1976. 25. A. Ing6lfsson: Distribution and habitat preferences of some intertidal amphipods in Iceland. 28 pp. 1977. 26. S.P. Jakobsson: Petrology of Recent basalts of the Eastern Volcanic Zone, Iceland. 103 pp.1979. 27. c.r. Morgan: A systematic survey of Tardigrada from Iceland. 25 pp. 1980. 28. E.H. Einarsson, G. Larsen & S. Th6rarinsson: The S6lheimar tephra layer and the Katla eruption of ~ 1357. 24 pp. 1980. 29. J. Eiriksson: Lithostratigraphy of the upper Tjornes sequence, North Iceland: The Breidavik Group. 37 pp. 1981. 30. Bergth6r J6hannsson: A list of Icelandic bryophyte species. 29pp. 1983. 31. J. Eiriksson: Facies analysis of the Breidavik Group sediments on Tjornes, North Iceland. 1985.