proterozoic fluvial styles: response to changes in accommodation space (rivieradal sandstones,...

ELSEVIER Sedimentary Geology 120 (1998) 257–274

Proterozoic fluvial styles: response to changes in accommodationspace (Rivieradal sandstones, eastern North Greenland)

Martin Sønderholm a,*, Henrik Tirsgaard b

a Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmarkb Mærsk Oil and Gas A=S, Esplanaden 50, DK-1263 Copenhagen K, Denmark

Received 9 April 1997; accepted 3 September 1997

Abstract

Fluvial styles recorded by the uppermost part of the Neoproterozoic ‘Rivieradal sandstones’ succession of easternNorth Greenland reflect variations in rate of generated accommodation space and possibly climatic changes. Three faciesassociations, arranged in two genetic sequences, are recognised within the succession. The lower sequence initially recordslittle available accommodation space. A high degree of reworking results in sheet-like, high-energy, bed-load-dominated,braided river deposits lacking recurrent facies patterns. As accommodation space increases upwards through the sequence,reduction in reworking is recorded by the development of fining- and thickening-upward muddy fluvial cycles. Evidenceof desiccation or prolonged periods of drought are absent within the deposits and climate was probably relativelyhumid. Channel deposits in the lower sequence reflect mixed-load, braided fluvial systems with stable channel banks andfloodplains, and the gradient appears to have been low to moderate. These features are generally considered favourable forthe establishment of meandering river systems, but channels, nevertheless, retained an overall braided character and theirdeposits show no evidence of meandering. Despite indications of a climatic setting without significant periods of droughtsediments indicate that large fluctuations in discharge occurred within the mixed-load streams and this is suggested to bethe main cause for the absence of meandering. The swift and rather dramatic response of the fluvial systems to changesin precipitation, probably resulted from rapid runoff rates caused by the absence of vegetation. The upper sequence showsan initial return to shallow, sandy braided river deposition recording little available accommodation space. A subsequentincrease in the rate of generated accommodation space is indicated by the presence of alternating sheet sandstonesand sand-streaked mudstones with abundant desiccation cracks. The sheet sandstones show evidence of high-energy,unconfined ephemeral fluvial flash-flood deposition, while the mudstones are interpreted to represent muddy floodplaindeposits. The change in fluvial style, combined with the widespread evidence of desiccation, suggest an evolution towardsa more semi-arid climate in the upper sequence. This climatic change could account for the reduced clastic input seen inthe overlying marine succession which culminated in carbonate platform deposition. The present study suggests that evenunder conditions considered favourable for the formation of meandering streams, these will rarely occur in Proterozoicdeposits due to the lacking influence of vegetation. Although meandering deposits cannot be ruled out as having formed inpre-vegetational times, the conditions for their formation appear to have been even more restricted than previously realised. 1998 Elsevier Science B.V. All rights reserved.

Keywords: Neoproterozoic; meandering river; shallow braided river; muddy braidplain; ephemeral sheetflood; geneticsequences

Ł Corresponding author. Tel.: C45 3814 2410; Fax: C45 3814 2050; E-mail: [email protected]

0037-0738/98/$ – see front matter 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 3 5 - 9

258 M. Sønderholm, H. Tirsgaard / Sedimentary Geology 120 (1998) 257–274

1. Introduction

During the last thirty years, speculations on howriver characteristics were influenced by the evolutionof vascular land plants during the early Palaeozoichave been put forward (e.g. Schumm, 1968; Cot-ter, 1978; Long, 1978; Sweet, 1988). Prior to thisevolutionary step, which helped to stabilise the finer-grained fluvial plain environments, channel bank sta-bility would have been comparatively low, resultingin a strong dominance of braided river patterns com-parable to those found in present-day arid climates(Schumm, 1968) and non-vegetated mud-deficientenvironments (Smith, 1976). Thus, the developmentof land plants probably resulted in a gradual tran-sition to a more balanced mixture of braided and

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

�

��

��

��

��

��

��

��

�▲

▲

��

��

��

�

��

��

��

��

�

��

��

��

��

��

��

��

��

��

��

��

�

��

�

��

�

��

��

��

��

��

��

��

�

��

�

��

��

��

��

��

��

▲▲

▲

▲▲

�

��

��

��

��

��

��

��

�

��

��

�

Independence Fjord WandelSeaValdemar

GlückstadtLand

Dan

mar

kFj

ord

HagenFjo

rd

Ingolf Fjord

Nioghalvfjerdsfjorden

HovgaardØ

CentrumSø

Mylius-Erichsen Land

��

82°

81°

79°

80°

82°

26° 22° 18° 14°

▲

▲▲

▲

▲

▲

▲

▲▲

▲

HolmLand

��

▲▲

▲

▲

▲

▲▲

▲

▲

Kronprin

sChris

tian

Land

��

��

��

��Lambert

Land

��Rivieradal

Van

dred

alen

▲▲

▲

▲

▲

▲▲

Fault

Studied section

Thrust

��Carboniferous – Tertiary

Wandel Sea Basin

Ice/water

��

Ordovician – Silurian

Franklinian Basin

��Shallow-marine deposits in west, deep-

water clastic sediments in east(mainly Rivieradal sandstones)

��Upper Proterozoic

Platform carbonates and shallow marinesiliciclastics (Hagen Fjord Group)

Lower Proterozoic

Zig-Zag Dal Basalt Formation

��Independence Fjord Group

�Lower Proterozoic and ? Archaean

Crystalline basement complex

22° 18°

50 km

Fig. 1. Location of the ‘Rivieradal sandstones’ basin in eastern North Greenland. The studied fluvial section lies in the northern outcropregion close to the Caledonian nappe front which also includes the lowermost the carbonate deposits of the Hagen Fjord Group.

meandering rivers, with the latter providing a mech-anism for producing more cyclic fluvial successions.This hypothesis was tested and supported by Cot-ter (1978). Long (1978) pointed out that among thelisted examples of presumed Proterozoic meanderingriver deposits most interpretations were based mainlyon the presence of fining-upward cycles. Large-scalelateral accretion surfaces, which are a more diag-nostic — but not unequivocal — feature, were onlyrecorded in a few examples. In a study of Early Pro-terozoic fluvial deposits from Australia, possibly inpart meandering, Sweet (1988) concluded that mean-dering streams should have been stable when slopeand discharge values were below certain critical val-ues, and when a significant proportion of suspendedbed-load existed.

M. Sønderholm, H. Tirsgaard / Sedimentary Geology 120 (1998) 257–274 259

The present study of Neoproterozoic fluvial sed-iments from the ‘Rivieradal sandstones’ of easternNorth Greenland demonstrates that even in fluvialsettings where the rate of formation of accommo-dation space was high, gradients were moderate tolow, and a significant proportion of fine-grained ma-terial was present resulting in the development ofmuddy floodplains, braided streams still formed. Theobservations presented in this paper thus support thecontentions of Schumm (1968), Cotter (1978) Long(1978) and Rust (1978), that even under conditionsotherwise favourable for the formation of meander-ing streams, these rarely occurred in the Proterozoic.

2. Geological setting

Until recently, knowledge on the Neoproterozoicrocks in eastern North Greenland was fragmentary, asmost field work had been carried out mainly by footin the early 1950s (Adams and Cowie, 1953; Frankl,1954, 1955). Apart from a very short reconnaissancemapping program in 1980 by the Geological Survey ofGreenland (Hurst and McKerrow, 1981) these rockswere not studied further until the Survey resumedmapping of this remote region in the mid-1990s.

The ‘Rivieradal sandstones’ consist of an 8–10km thick succession exposed within the Vandredalennappe of the Caledonian fold belt in Kronprins Chris-tian Land, eastern North Greenland (Fig. 1; Henrik-sen, 1995). Together with the overlying Hagen FjordGroup the ‘Rivieradal sandstones’ form part of a ma-jor sedimentary basin fill which was probably relatedto the formation of the Iapetus Ocean in Late Pro-terozoic time (Hurst et al., 1985; Clemmensen andJepsen, 1992).

The major part of the ‘Rivieradal sandstones’ suc-cession is characterised by stacked coarsening- andthickening-upward marine shelf and slope succes-sions, which probably developed in response to tec-tonic activity during formation of the basin. The sedi-ment was derived from uplifted areas along the basinmargin to the west by erosion of Middle Proterozoicfluvial sandstones (Independence Fjord Group; Hurstet al., 1985) and older metamorphic basement rocks.

The sandstone-dominated section discussed in thispaper is found within the uppermost 500 m of the‘Rivieradal sandstones’ and is considered to reflect achange from inner shelf to fluvial deposition (Figs. 2

and 3). The inner shelf deposits underlying the fluvialsediments are represented by a thick succession ofheterolithic sandstones and mudstones. Hummockycross-stratified, coarse-grained sandstone and wave-formed, coarse-grained ripples are present.

The transition from marine deposition to fluvialdeposition is represented by a 35 m thick sand-stone-dominated section overlying the inner shelfdeposits. This interval consists of fine-grained, large-scale cross-bedded sandstones of probable braidedfluvial origin, interbedded with thin, discontinuousup to c. 1 m thick intervals of fine-grained sandstonewith small-scale structures including current lami-nation (occasionally with flaser bedding), climbingripple lamination and parallel lamination (Fig. 3),which most likely constitute either tidal or wave-dominated shallow marine deposits.

The fluvial section is approximately 370 m thickand comprises three facies associations represent-ing different aspects of braided river deposition: asandy braided river association, a muddy braidplainassociation and an ephemeral sheetflood association(Fig. 4). Due to the presence of local ice caps, pre-sent-day outcrops of these fluvial sandstones canonly be followed laterally for approximately 2 km,and their relationships to local conglomeratic units ofprobable fluvial origin occurring in apparently simi-lar stratigraphic positions along the frontal thrust ofthe Vandredalen nappe, are at present unclear.

The ‘Rivieradal sandstones’ succession is over-lain by carbonate platform rocks of the Hagen FjordGroup. The transition from siliciclasticfluvial deposi-tion to deep-water carbonate deposition is representedby an approximately 80 m thick, strongly tectonised,marly, mudstone-dominated unit with 2–4 m thick,laterally extensive, structureless sandstone beds. Theoverall character of this interval resembles the innershelf deposits which underlie the fluvial successionsuggesting marine siliciclastic and marly deposition.

3. Facies associations

3.1. Facies association 1: shallow braided riverassociation

3.1.1. DescriptionThis association occurs as two units within the flu-

vial succession (units 1a and 1b; Figs. 3 and 4). The


Fig. 2. Cliff-section showing transition from marine inner shelf (IS) to fluvial deposition in the uppermost part of the ‘Rivieradalsandstones’. The main part of the studied fluvial section forms the top of the mountain and comprises a shallow braided river association(FA 1) and a more recessive weathering muddy braidplain association (FA 2). The uppermost part of the fluvial section (ephemeralsheetflood association) is exposed in a steep, nearly inaccessible and tectonised section on the back of the mountain. The basal part of thesandstone-dominated succession comprises possible interbedded fluvial and shallow marine deposits (F=M). Carbonate platform deposits(PC) of the Hagen Fjord Group form the top of the exposed succession. Height of cliff approximately 800 m.

units can be followed for approximately 2 km later-ally. They are dominated by amalgamated sandstonesheets, occasionally interbedded with thin mudstonebeds without cyclic facies trends (Figs. 3 and 4).The sandstones are mature, medium-grained quartzarenites and are generally well sorted and rounded.

The sandstone sheets have a tabular to slightlylenticular geometry and can be followed for severalhundred metres laterally (Fig. 5). They are 20–350cm thick and consist of stacked, laterally interdig-itating cosets comprising 2–10 sets; single sets arerare. Sets are characterised by large-scale, semi-pla-nar to tangential cross-bedding; they are 10–50 cmthick, mostly 10–25 cm. Foresets are mostly con-cave upwards and generally dip between 20º and

Fig. 3. Generalised section log of the uppermost part of the ‘Rivieradal sandstones’ succession. A, B and C show position of detailed logsshown in Fig. 4; F=M D interbedded fluvial and possible shallow marine deposits; CP D carbonate platform deposits of the Hagen FjordGroup (HFG). Plotted current directions are from large-scale, semi-planar cross-sets.

30º, but downcurrent transition to parallel beddingis common (Fig. 6) and sigmoidal cross-beddingis occasionally preserved. In rare cases foresetsare mud-draped and display reactivation surfaces.Well-developed trough cross-bedding is only locallypresent. A few, up to 60 cm thick, channel-fill troughcross-beds are also present. Current directions showa strong preference towards the east and southeast(Fig. 3).

Sheet boundaries are sharp, irregular and slightlyerosive. Boundaries are locally underlain by stringersor thin beds of mudstone, generally less than 2 cmthick. Angular and sub-rounded mudstone clasts upto c. 1 cm are common in certain intervals at base ofsheets and along foreset laminae.


��

��

��

��

��

��

��

��

�y��yy�y��yy

��

yyyy

�y��yy

��yy�y��yy�y

��yz{|��

f mm

sandcmud

inne

r sh

elf

F/M

inne

r sh

elf

sand

y br

aide

d riv

ersa

ndy

brai

ded

river

mud

dy b

raid

plai

nep

hem

eral

shee

tfloo

d

3

1b

1a

2

100

200

300

400

500

550

��

��

C

B

C

unit

CP

HF

G��z|

1010

20

15

Limestone

Calcareous mudstone��

Sandstone(with granules/mudclasts)

��Heterolith

Sandy mudstone

��Mudstone

Hummuckycross-stratification

Desiccation cracks

Structureless

Parallel lamination

Wavy lamination

Lenticular bedding

Large-scale planarcross-stratification

Cross bedding

Ripple lamination

Cross lamination

Wave ripples

‘Riv

iera

dal s

ands

tone

s’

n=53


mud f msst

c0

5

10

15

Shallow braided riverassociation

Muddy braidplainassociation

Ephemeral sheetfloodassociation

��

��

��

��

��

m m m

mud f msst

c mud f msst

c0

5

10

15

0

5

10

15

��

yyyyyy��yyyy��

��

yyy��

yyyy

��yy��yy

�

��

yyyyyyyy

�

�y��y

��y��y��yy��

��y

��yy�y��

��

yyyyy��yy��y��y��y��yy�y��yy��y�y�y

��

�

A B C

��

��

��

��

��

��

��

Fig. 4. Sedimentological logs showing comparison of three Late Proterozoic fluvial styles recognised in the ‘Rivieradal sandstones’ ofeastern North Greenland. For legend see Fig. 3.


Fig. 5. Cliff-section showing basal part of shallow braided river association (FA 1; unit 1a). The transition from inner shelf marinedeposits (IS) to fluvial deposits is represented by a 35 m thick unit possibly comprising interbedded fluvial and shallow marine deposits(F=M). Note the sheet-like geometry of individual beds in the shallow braided river association.

3.1.2. InterpretationThe laterally continuous sand sheets dominated

by large-scale semi-planar to tangential cross-bed-ding with common concave-up foresets reflect mi-gration of extensive two- and three-dimensionaldunes in a relatively high-energy environment. Com-bined with the unimodal flow character and the lackof low-energy facies and cyclic facies trends, thissuggests that the bedforms were generated by uni-formly directed currents close to the dune- to upperstage plane-bed transition (cf. Røe, 1987; Dam andAndreasen, 1990). The geometry and internal struc-tures of the sand sheets compare closely with theType B sandstone bodies described in detail by Røeand Hermansen (1993), which reflect migration ofcompound bar-complexes in wide, relatively shallowchannels within a braided stream environment.

Differentiation between sandy tidal channel de-posits lacking significant bipolar palaeocurrent com-

ponents and ephemeral fluvial deposits may be prob-lematic especially in pre-vegetational times (e.g. An-derton, 1976; Leckie and Singh, 1991; Tirsgaard,1993). However, the general lack of sedimentarystructures typical of wave and tidal action, suchas wave ripples, common herringbone cross-stratifi-cation, small cross-bedded sets with opposing cur-rent directions on top of larger sets (ebb and floodcaps), widespread mud-draped reactivation surfacesand undulatory lower set boundaries associated withsuccessive bundles of foresets (Boersma and Ter-windt, 1981), point towards a fluvial rather than atidally influenced marine environment for the majorpart of this association.

Inverse grading and other structures suggestingaeolian (Hunter, 1977) or beach (Clifton, 1969) de-positional environments have not been recorded.

Within the braided river deposits the overall con-sistency of foreset shape, dip azimuth and dip angle


Fig. 6. Detail of shallow braided river association showing typical large-scale semiplanar cross-bedding. The parallel lamination seen inthe upper half of the picture represents downcurrent transitions from tangential cross-bedding (pencil is 13 cm long).

suggest recurring effective floods of similar mag-nitude. Falling-stage and lowstage deposits were,however, originally more common as indicated bythe widespread occurrence of mudstone intraclaststogether with the local appearance of mud-drapedreactivation surfaces. The low incidence of these fea-tures may reflect a high degree of reworking and alow preservation potential. Nevertheless, topographicdifferentiation was probably low as suggested by theuniform lateral and vertical development of the asso-ciation, and thus a shallow, braided river environment(Miall, 1996) with broad channels is envisaged forthis association (Platte-type of Miall, 1977).

3.2. Facies association 2: muddy braidplainassociation

3.2.1. DescriptionThis association consists of fining-upward suc-

cessions (1–6 m thick) comprising a multistoreysandstone unit overlain by heterolithic sandstone andmudstone. These are stacked into an approximately

110 m thick overall fining-upwards unit (Figs. 3and 4) which can be followed laterally for at least2 km.

Typically, the fining-upwards successions com-prise a 50–450 cm thick multistorey scour and fillsandstone unit at the base which can be traced later-ally for at least many tens of metres. However, out-crops do not reveal details of the overall sandstonebody geometries and hence it cannot be determinedwhether these represent ribbon or sheet sandstonebodies (cf. Stear, 1983).

The sandstones are fine- to medium-grainedwith abundant mudflakes throughout and consistof stacked, cross-cutting broad channel-like formswhich can be traced for several tens of metres paral-lel to the flow direction (Fig. 7). Internally, these aremade up of compound cosets (25–200 cm, mostly<50 cm thick) separated by thin (<10 cm, mostly<1 cm), discontinuous mudstone veneers. Each cosetcomprises 2–10 sets. The base is typically erosivewith a relief of up to 40 cm (Fig. 8A). Cosetsare dominated by large-scale, semi-planar, lenticular


Fig. 7. Transition between shallow braided river association (unit 1a) and muddy braidplain association (facies association 2) showingcross-cutting, shallow channels. Fine-grained intervals are thin in the lower part of association 2, but become prominent upwards.

cross-sets and trough cross-sets. These pass upwardsinto small-scale structures such as current-ripple andparallel lamination (Fig. 8B). In the lower part of thesandstone units, large-scale sets are generally 10–25cm thick but may in rare cases reach 50 cm. Foresetsare mostly concave upwards; downcurrent transi-tions to parallel lamination also occur. Mud-drapedforesets and reactivation surfaces are common. Over-

turned cross-bedding is locally present and backflowripples are occasionally preserved in bottomsets. Setthicknesses decrease to 5–15 cm in the upper part ofthe sandstone unit and small-scale structures becomedominant. Mud-drapes between cosets are thickerand laterally more continuous than in the lower part.

The basal sandstone units grade into 50–400 cmthick wavy and lenticular bedded heterolithic units


comprising dark, parallel laminated to structurelessmudstone. These include thin (<15 cm, rarely up to60 cm) tabular and lenticular sandstone beds witha lateral extent exceeding several metres. The sand-stones mostly display a sharp, erosive base and haveflat, small-scale rippled or occasionally wave-rippledtops. The sandstones are fine- to medium-grained,and internally parallel laminated, ripple cross-lam-inated or climbing ripple laminated; thicker bedsare cross-bedded with sets up to 15 cm thick.Graded beds, up to 20 cm thick consisting of fine-to medium-grained current-ripple and parallel lam-inated sandstone topped by a thin mudstone cap,dominate some heterolithic units.

3.2.2. InterpretationThe gradual transition from the underlying

braided river association indicates a close relation-ship between these two associations (Fig. 7). Thedevelopment of fining-upward cycles and the preser-vation of fine-grained deposits indicate increasedrates of generated accommodation space. Althoughcut-banks have not been observed, the lack of ma-jor lateral accretion surfaces and the relatively thin,broad channel-like forms which cut across each othersuggest that deposition of sand occurred within a ma-jor braided fluvial system characterised by migrationof small channels and bars, in part comparable to theDonjek-type of Miall (1977).

The internal architecture of the channel sand-stone bodies indicate more fluctuating current ve-locities than in the previous association; the pres-ence of falling-stage and low-stage deposits repre-sented by sets of current-ripple and parallel lami-nated sand and mud-draped reactivation surfaces andcoset boundaries, all point in this direction (cf. Pi-card and High, 1973; Tunbridge, 1984). The lack ofcoarse-grained and pebbly material probably reflectsmaximum available particle size rather than streampower which was quite high as recorded by the com-mon occurrence of tangential cross-bedding. Thecommon presence of overturned cross-bedding also

Fig. 8. (A) Stacked, cross-cutting channels with a relief of up to 40 cm forming the basal part of the fining-upwards successionstypical of the muddy braidplain association (facies association 2). (B) Channel deposits consisting of compound cosets dominated bylarge-scale semi-planar, lenticular cross-sets passing upwards into current-ripple lamination and parallel lamination separated by a thin,discontinuous mudstone veneer. Ruler is 20 cm long.

suggests relatively strong flow velocities of heavysediment-laden waters. However, upper flow regimeparallel lamination, typical of both channelised andnon-channelised ephemeral floods (e.g. Tunbridge,1981; Stear, 1983, 1985; Abdullatif, 1989), is con-spicuously absent. This suggests that although dis-charge was variable, it was probably continuousrather than catastrophic.

The thicker heterolithic and muddy units mostlikely represent floodplain and overbank depositswhile thinner units may be the result of in-channeldeposition. Mudstones were deposited from suspen-sion fall-out during overbank flooding (or in-chan-nel deposition during low discharge), and includecrevasse splay sediments represented by thin-bed-ded, lenticular sandstones with small-scale struc-tures and graded sandstone sheets (cf. Leeder, 1974).The absence of desiccation features or evidence ofsoil formation indicates that prolonged periods ofdrought did not occur. The dark grey coloration sug-gests a high water table close to or at the depositionalsurface (Walker, 1967).

The muddy braidplain deposits show that al-though vegetation was not present, the fine-grainedmaterial was cohesive enough to stabilise channelbanks, possibly due to a high water table. Thus,large fluctuations in discharge were not accommo-dated through the formation of sheet-floods or wideand poorly confined channels, but through repeatedintra-channel erosion and vertical aggradation, in-cluding formation of overbank sheet splays.

Cyclicity — in the form of stacked, fining-upwardsuccessions — is a not uncommon feature in braidedriver environments and can have several causes, suchas repeated flooding, lateral accretion, channel aggra-dation or avulsion (Miall, 1977, 1996). Although itwas not possible to carry out the detailed field workneeded in order to analyse the architecture of thesandstone bodies, at least three levels of genetic se-quences can be recognised within this association.The lowest level is represented by the mud-drapedcosets of the channel sandstones representing recur-


ring flooding events or pulses within major floods.The second level corresponds to the individual fin-ing-upward successions which probably representchannel-belt aggradation between major river avul-sions — a characteristic feature of distal braidedrivers and alluvial plains (Rust, 1978; Stear, 1983).The third level is recorded by the overall fining up-wards of the association, and is related to longer-termfluctuations in the depositional system.

3.3. Facies association 3: ephemeral sheetfloodassociation

3.3.1. DescriptionThis association is exposed in steep, rather inac-

cessible and tectonised outcrops, and detailed sec-tions have not been measured; description thus relieson general facies descriptions and photographs. Itis characterised by interbedded sheet sandstones andblack, red and green siltstones and mudstones ar-ranged in an overall thickening-upwards succession(Fig. 9). The outcrop comprises three stacked succes-sions with a total thickness of approximately 80 m.Mudstones are dark grey to black in the lower partof the succession, while variegated red and greenmudstones with abundant desiccation features domi-nate the upper part (Fig. 9D). Boundaries are sharpboth to the underlying sandy braided river associa-tion and to the overlying shallow marine mudstonesand sandstones.

The sandstone sheets are 10–50 cm thick andare fine- to medium-grained. They are laterallyvery extensive (>100 m), and generally have asharp, slightly undulating erosive base and a flattop (Fig. 9A, B). Desiccation cracks on top sur-faces are common (Fig. 9D). The basal parts ofsandstone sheets are generally structureless gradingupwards into finer-grained sandstone with parallelto undulating lamination (Fig. 9C). Upper surfacesare mostly capped by siltstone or mudstone bedsfrom <1 cm thick up to 30 cm thick. These oftencontain thin, graded sandstone lenses and beds con-sisting of fine- to very fine-grained, structureless toparallel laminated sand which mostly grade up into astructureless mudstone cap (Fig. 9C).

The internal primary structures of the variegatedmudstones are difficult to discern as most of the tec-tonic deformation is absorbed in these units. When

locally preserved they consist of structureless tofinely laminated red and green siltstone with thin,often graded, fine-grained to very fine-grained sand-stone streaks up to 1 cm thick. Lamination is mostlyparallel but may be irregular and wavy. The varie-gated mudstones show frequent evidence of waterescape and soft sediment deformation such as pipesand rupturing and convolution of laminae (cf. Tun-bridge, 1984). Desiccation cracks are common.

3.3.2. InterpretationStructureless to parallel laminated, thin-bedded

sheet sandstones encased in sand-streaked mudstonesare common sedimentary facies in both storm-influ-enced marine shelf deposits (e.g. Walker and Plint,1992; Arnott, 1993) and ephemeral fluvial deposits(McKee et al., 1967; Tunbridge, 1981; Sneh, 1983;Tunbridge, 1984; Stear, 1985). However, the gen-eral lack of wave-induced structures or structuressuggesting tidal activity, together with the presenceof desiccation cracks suggest a fluvial rather than amarine origin for these deposits.

Multistorey thin-bedded sheet sandstones andmudstones dominated by parallel lamination areregarded as the product of laterally unconfinedephemeral flows, with rapidly falling water levelsat the end of the flood preventing the developmentof rippled tops (Tunbridge, 1981). The abundanceof parallel lamination and paucity of cross-beddingdistinguishes these deposits from deposits of moreperennially flowing meandering and braided streams(Tunbridge, 1984; Miall, 1996). A general low wa-ter table is suggested by the common desiccationfeatures and the red coloration suggesting oxidisingconditions (Walker, 1967).

Each graded component reflects a flash-floodevent followed by a period of drought leading todesiccation of channels and adjacent interfluve ar-eas within the overall thickening- and coarsening-upward successions which record longer-term varia-tions across the flood plain. The latter may simplybe due to intrinsic factors such as flood-lobe progra-dation and migration, but may also be attributableto extrinsic factors such as tectonic rejuvenation ofthe fluvial source area (Miall, 1991), climate-relatedvariations in run-off (cf. Olsen, 1990), or depositionduring a relative base-level fall leading to an increasein slope and sediment load (cf. Miall, 1991).


Fig. 9. Ephemeral sheetflood association (facies association 3). (A) Section (about 30 m thick) showing the general coarsening- andthickening-upward trend of the association. Sandstone sheets are laterally very extensive (>100 m). (B) Interbedded sheet sandstones andmudstones. The sandstones have a sharp, slightly undulating erosive base and a flat top. Pencil (circled) is 13 cm long. (C) The sandstonesheets are structureless in the basal part and grade upwards into finer-grained sandstone with parallel to undulating lamination. Scale is10 cm. (D) Desiccation cracks on top surface of sandstone bed encased in variegated mudstone. Hammer is 30 cm long.


In present-day environments, ephemeral flash-flood deposition dominated by upper flow regimeparallel lamination predominantly occurs in aridand semi-arid climates (e.g. Stear, 1983; Tunbridge,1984; Miall, 1996). The presence of an ephemeralfluvial system may, however, not invariably be usedas an indication of an arid or semi-arid climate inPrecambrian times. The lack of stabilising vegeta-tion would result in a general flashy run-off of riverswhich would respond rapidly to changes in precip-itation and even short dry periods may have led todesiccation of river systems (Schumm, 1968). How-ever, the conspicuous difference in the fluvial stylerecorded by associations 2 and 3 is inferred to reflecta shift from perennial to ephemeral fluvial flow, anda plausible explanation could be the developmenttowards more arid climatic conditions with more in-frequent periods of rainfall. This is further supportedby both the conspicuous lack of desiccation featuresin association 2 and the change in colour of theassociated mudstone deposits in the two associationsfrom grey to red, implying more oxidising conditionsand periodically low water table during formation ofassociation 3 (e.g. Walker, 1967). Although evidenceof pronounced stage fluctuations is also present inthe deposits of association 2, catastrophic upper flowregime deposition is notably absent, indicating thatdischarge variations were considerably smaller thanin association 3.

4. Genetic sequences

Two major genetic sequences can be recognisedwithin the fluvial succession described above. Thelower sequence consists of facies associations 1and 2 and the upper sequence of facies associations 1and 3 (Fig. 2).

4.1. Sequence 1

The lower sequence is underlain by inner shelfdeposits, which form a thick progradational unitcomprising a series of stacked, coarsening-upwardsuccessions recording decreasing rate of added ac-commodation space. This enabled progradation offluvial deposits, and a further slow down in genera-tion of accommodation space resulted in a changeto an aggradational succession characterised by

shallow, braided fluvial deposition (facies associa-tion 1).

During the early stage of deposition the gradi-ent was modest, as suggested by the dominance ofmoderate-energy structures, and the space availablefor sediment storage decreased, enhancing rework-ing of existing sediment (cf. Wright and Marriott,1993; Allen et al., 1996). This is clearly reflected bythe vertical development of the shallow braided riverassociation which is characterised by erosion andlack of repetitive facies patterns suggesting a balancebetween generation of accommodation space, riverdischarge and sedimentation rate.

The gradual transition from shallow, sandybraided river deposition (facies association 1) tomuddy braidplain deposition (facies association 2)reflects renewed generation of accommodation spaceallowing preservation of overbank sediment. Gener-ation of accommodation space may be due to bothreduced sediment influx and base-level rise (Wrightand Marriott, 1993). Increasing aridity may causereduced sediment influx to the river system, butthe presence of perennial river systems suggeststhat relatively humid conditions prevailed through-out the deposition of association 2. Isolated channelswith low interconnectedness separated by thick fine-grained floodplain deposits are considered to developwhen creation of accommodation space is compara-tively fast, while more amalgamated sandbodies arethought to develop if base-level rise is relativelyslow (Wright and Marriott, 1993; Allen et al., 1996).The overall fining-upward character of the muddybraidplain association could thus reflect a gradualretrogradation during an accelerating base-level rise,punctuated by smaller fining-upward cycles repre-senting progradational events related to channel-beltswitching.

4.2. Sequence 2

The return to aggradational braided fluvial depo-sition (facies association 1) characterising the lowerpart of sequence 2 is abrupt and reflects a rejuvena-tion of the fluvial system. The base of the sequencethus probably forms a stratigraphic unconformityrepresenting an alluvial sequence boundary (Allen etal., 1996), possibly formed in response to tectonicactivity along the basin margin. The overall aspects


of this unit strongly resemble those of the braidedriver unit of sequence 1; however, there is a tendencyfor the cross-bedding to be smaller and mudstoneintraclasts to be more abundant, possibly recordingshallower channels and more pronounced stage fluc-tuations. This may reflect a gradual change towardsthe more arid climatic conditions inferred for theupper part of the sequence.

The top of the fluvial succession records an over-all increase in generated accommodation space al-lowing fine-grained floodplain deposits to be pre-served and the development of large-scale cyclicity.This shift is associated with a marked change influvial style from perennial, muddy braidplain toephemeral flash-flood deposition, suggesting an evo-lution towards a more arid climate (e.g. Tunbridge,1981). The continued increase in rate of generatedaccommodation space recorded by the overlying ret-rogradational mudstone-dominated marine succes-sion, may partly be a result of a climatically in-duced reduction of clastic input which eventuallyculminated in a change to carbonate platform depo-sition.

5. Discussion: Proterozoic fluvial architectureand style

The idea that the evolution of vascular land plantsduring the Early Palaeozoic had a profound effect onfluvial style was originally put forward by Schumm(1968). The lack of vegetation would — even inhumid climates — result in high discharge vari-ability, high rates of discharge decline and highsediment yield resulting in bed-load streams (i.e.braided rivers) comparable to those found in present-day arid climates (Schumm, 1968) and non-veg-etated mud-deficient environments (Smith, 1976).This hypothesis was tested and supported by Cot-ter (1978) and Long (1978), but was challenged bySweet (1988) who argued that given adequate lowdischarges and a high proportion of suspended load,meandering rivers would have been stable even inPrecambrian times irrespective of the lack of landvegetation.

In a review of these topics, Miall (1996) con-cluded that the highest stream powers are found inrivers with the most variable discharge. Thus, thesewould be exposed to rapid bank erosion (with conse-

quent high sediment yield) due to the predominanceof channel-forming, high-discharge events. Further-more, Maddock (1969) suggested that streams withbeds and banks consisting of non-cohesive unigranu-lar material would maintain relatively high velocitiesthrough a wide range of discharges, as the cross-sec-tional area of through-flow above a critical streamvelocity would respond quickly to changes in dis-charge. In a comparison between the BrahmaputraRiver and some Proterozoic river systems by Røeand Hermansen (1993) these conditions were used toexplain the apparent difference in stage fluctuations.They suggested that Precambrian rivers, in contrastto the Brahmaputra, show relatively small stage fluc-tuations due to very low bank stability resulting fromthe absence of vegetation and thick overbank muddeposits. This caused rivers to widen considerablyduring increasing discharge (cf. Fuller, 1985).

The sandy braided river association (facies asso-ciation 1) described above resembles the Norwegianexamples described by Røe and Hermansen (1993)as it is mud-deficient (unigranular), and nearly exclu-sively composed of medium- to high-energy faciesrecording relatively small stage fluctuations and dis-playing the same sandstone body geometries. Evenin humid climates high discharge variability is ex-pected due to the absence of vegetation; however,thick stabilising overbank muds were not depositeddue to the high degree of reworking resulting fromlimited accommodation space. Thus, effective flooddischarge in this setting was frequently above thecritical value enabling perennial, high-energy fluvialconditions to persist.

The presence of relatively thick overbank muddeposits in association 2 suggests enhanced rates ofvertical accretion resulting from increasing rates ofgenerated accommodation space. Furthermore, thehigher mud : sand ratio suggests a lower gradientthan for the underlying association. The combina-tion of fluvial style and the absence of desiccationfeatures within the muddy floodplain and channel de-posits point towards perennial flow, probably in a rel-atively humid climate without significant periods ofdrought. This situation would have been favourablefor the development of meandering streams in veg-etated environments, and was also suggested to bea prerequisite for the development of stable mixed-load and suspended-load meandering streams in the


Precambrian (Sweet, 1988). However, these did notdevelop in this setting. The most likely cause isprobably related to the high fluctuations in dischargewhich appear to have prevented the formation ofhighly sinuous channels (cf. Miall, 1996).

The sedimentary structures of the perennial chan-nel deposits of facies association 2 show evidenceof considerable discharge fluctuations, as manifestedby the recurrent shifts between trough cross-bedding,parallel lamination and ripple lamination associatedwith abundant reactivation surfaces and mud-drapedsurfaces. However, in contrast to the channel depositsof association 1, the channels appear not to haveaccommodated discharge fluctuations by wideningof the channels through erosion of channel banks,but through repeated intra-channel erosion and ver-tical aggradation, including formation of overbanksplays The muddy braidplain deposits thus showthat large fluctuations in discharge in Precambriantimes did not necessarily lead to the formation ofwide and poorly confined channels (as seen in fa-cies associations 1 and 3), provided that sufficientfine-grained material was present to stabilise channelbanks. Although the muddy braidplain deposits in-dicate that topographic differentiation could developwithin Proterozoic braided fluvial systems, it seemsthat differentiation was less pronounced than seenin present-day deep braided river systems. Proba-bly only two levels developed, in contrast to thefour recognised in the Donjek River (Williams andRust, 1969; Miall, 1996) in which the upper two arestabilised by vegetation.

As suggested above high fluctuations in dischargein Precambrian deposits can be expected to occurin a much broader range of climatic settings thanis seen today. The deposits of the ‘Rivieradal sand-stones’ suggest that it requires a special combinationof factors for meandering streams to develop in pre-vegetational times; present-day settings favourablefor their development, such as high mud-to-sand ra-tios, high rates of generated accommodation space(to preserve the floodplain deposits), low slopes anda climate with only small fluctuations in precip-itation, do not seem to be sufficient. A suitablecombination of factors may not have occurred verycommonly in Precambrian times and may provideone possible explanation for the rarity of meanderingrivers in pre-vegetational times.

6. Conclusions

(1) Fluvial styles recorded by the uppermost partof the Neoproterozoic ‘Rivieradal sandstones’ suc-cession of eastern North Greenland suggest that evenunder conditions considered favourable for the for-mation of meandering streams, in pre-vegetationaltimes, braided systems developed.

(2) Two genetic sequences reflecting variationsin generated accommodation space and possibly cli-mate have been recognised within the fluvial de-posits.

The lower sequence consists of shallow braidedriver deposits lacking repetitive facies patterns laiddown during a period of slow rate of generatedaccommodation space. Increased rates of generatedaccommodation space resulted in deposition of cycli-cally developed muddy braidplain sediments. Thebraidplain deposits represent a fluvial system witha moderate to low gradient and with relatively con-fined channels and stable channel banks. A humidclimate is inferred for these deposits. All these fea-tures should be favourable for the establishment of ameandering river system; yet the braidplain channelsretain a braided character.

The upper sequence reflects a return to slowrates of generated accommodation space and shal-low braided river deposition. An upwards increasein generated accommodation space is recordedby cyclically developed sheet sandstones encasedin sand-streaked mudstones representing ephemeralflood deposits. It is suggested that the differing flu-vial styles seen in the two periods of increased ratesof generated accommodation space may reflect a de-velopment towards a more arid climate. This couldaccount for the reduced clastic input seen in theoverlying clastic marine succession which eventuallyled to carbonate platform deposition.

Acknowledgements

The field work was carried out with support fromthe Carlsberg Foundation (grant No. 93-0254=10).Mærsk Oil and Gas A=S is gratefully acknowledgedfor having supported H. Tirsgaard’s participationin the field work. Officially appointed reviewersJ.D. Collinson and E.L. Simpson are thanked forconstructive and critical reviews of the manuscript.


A.K. Higgins is thanked for linguistic corrections ofthe final manuscript. This paper is published withthe permission of the Geological Survey of Denmarkand Greenland.

References

Abdullatif, O.M., 1989. Channel-fill and sheet-flood facies se-quences in the ephemeral terminal River Gash, Kassala, Su-dan. Sediment. Geol. 63, 171–184.

Adams, P.J., Cowie, J.W., 1953. A geological reconnaissance ofthe region around the inner part of Danmarks Fjord, North-eastern Greenland. Medd. Grønl. 11 (7), 24 pp.

Allen, G., Lang, S., Musakti, O., Chirinos, A., 1996. Appli-cation of sequence stratigraphy to continental successions:implications for Mesozoic cratonic interior basins of EasternAustralia. Mesozoic geology of the Eastern Australia plateConference, Brisbane, Queensland, Australia. Geol. Soc. Aust.Extend. Abstr. 43, 22–26.

Anderton, R., 1976. Tidal-shelf sedimentation: an example fromthe Scottish Dalradian. Sedimentology 23, 429–458.

Arnott, R.W.C., 1993. Quasi-planar-laminated sandstone beds ofthe Lower Cretaceous Bootlegger Member, north-central Mon-tana: evidence of combined-flow sedimentation. J. Sediment.Petrol. 63, 488–494.

Boersma, J.R., Terwindt, J.H.J., 1981. Neap–spring sequences inintertidal shoal deposits in a mesotidal estuary. Sedimentology28, 151–170.

Clemmensen, L.C., Jepsen, H.F., 1992. Lithostratigraphy and ge-ological setting of Upper Proterozoic shoreline-shelf deposits,Hagen Fjord Group, eastern North Greenland. Rapp. Grønl.Geol. Unders. 157, 27 pp.

Clifton, H.E., 1969. Beach lamination: nature and origin. Mar.Geol. 13, 607–610.

Cotter, E., 1978. The evolution of fluvial style, with specialreference to the central Appalachian Paleozoic. In: Miall, A.D.(Ed.), Fluvial Sedimentology. Mem. Can. Soc. Pet. Geol. 5,361–383.

Dam, G., Andreasen, F., 1990. High-energy ephemeral streamdeltas; an example from the Upper Silurian Holmestrand For-mation of the Oslo Region, Norway. Sediment. Geol. 66, 197–225.

Frankl, E., 1954. Vorlaufige Mitteilung uber die Geologie vonKronprins Christian Land (NE-Gronland). Medd. Grønl. 116(2), 85 pp.

Frankl, E., 1955. Weitere Beitrage zur Geologie von KronprinsChristians Land (NE-Gronland). Medd. Grønl. 103 (7), 35 pp.

Fuller, A., 1985. A contribution to the conceptual modelling ofPre-Devonian fluvial systems. Trans. Geol. Soc. S. Afr. 88,189–194.

Henriksen, N., 1995. Eastern North Greenland 1994, the 1 : 500000 mapping project. Rapp. Grønl. Geol. Unders. 165, 53–58.

Hunter, R.E., 1977. Basic types of stratification in small eoliandunes. Sedimentology 24, 361–387.

Hurst, J.M., McKerrow, W.S., 1981. The Caledonian nappes of

Kronprins Christian Land, eastern North Greenland. Rapp.Grønl. Geol. Unders. 106, 15–19.

Hurst, J.M., Jepsen, H.F., Kalsbeek, F., McKerrow, W.S., Peel,J.S., 1985. The geology of the northern extremity of the EastGreenland Caledonides. In: Gee, D.G., Sturt, B.A. (Eds.), TheCaledonian Orogen — Scandinavia and Related Areas. JohnWiley and Sons, London, pp. 1047–1063.

Leckie, D.A., Singh, C., 1991. Estuarine deposits of the Al-bian Paddy Member (Peace River Formation) and lowermostShaftesbury Formation, Alberta, Canada. J. Sediment. Petrol.61, 825–849.

Leeder, M., 1974. Lower Border Group (Tournaisian) fluvio–deltaic sedimentation and palaeogeography of the Northum-berland Basin. Proc. Yorkshire Geol. Soc. 40, 129–180.

Long, D.G.F., 1978. Proterozoic stream deposits: some problemsof recognition and interpretation of ancient sandy fluvial sys-tems. In: Miall, A.D. (Ed.), Fluvial Sedimentology. Mem. Can.Soc. Pet. Geol. 5, 313–341.

Maddock, T., 1969. The behavior of straight open channels withmovable beds. US Geol. Surv. Prof. Pap. 622A, 70 pp.

McKee, E.D., Crosby, E.J., Berryhill, H.L.J., 1967. Flood de-posits, Bijou Creek, Colorado, June 1965. J. Sediment. Petrol.37, 829–851.

Miall, A.D., 1977. A review of the braided-river depositionalenvironment. Earth Sci. Rev. 13, 1–62.

Miall, A.D., 1991. Stratigraphic sequences and their chronos-tratigraphic correlation. J. Sediment. Petrol. 61, 497–505.

Miall, A.D., 1996. The Geology of Fluvial Deposits: Sedimen-tary Facies, Basin Analysis, and Petroleum Geology. Springer-Verlag, Berlin, 582 pp.

Olsen, H., 1990. Astronomical forcing of meandering river be-haviour: Milankovitch cycles in the Devonian of East Green-land. Palaeogeogr., Palaeoclimatol., Palaeoecol. 79, 99–116.

Picard, M., High, L., 1973. Sedimentary Structures of EphemeralStreams. Developments in Sedimentology 17, Elsevier, Ams-terdam, 223 pp.

Røe, S.-L., 1987. Cross-strata and bedforms of probable transi-tional dune to upper-stage plane-bed origin from a Late Pre-cambrian fluvial sandstone, northern Norway. Sedimentology34, 89–101.

Røe, S.-L., Hermansen, M., 1993. Processes and products oflarge, late Precambrian sandy rivers in northern Norway. In:Marzo, M., Puigdefabregas, C. (Eds.), Alluvial Sedimentation.Spec. Publ. Int. Assoc. Sedimentol. 17, 151–166.

Rust, B., 1978. Depositional models for braided alluvium. In:Miall, A.D. (Ed.), Fluvial Sedimentology. Mem. Can. Soc.Pet. Geol. 5, 605–625.

Schumm, S.A., 1968. Speculations concerning paleohydrauliccontrols of terrestrial sedimentation. Geol. Soc. Am. Bull. 79,1573–1588.

Smith, D.G., 1976. Effect of vegetation on lateral migration ofanastomosed channels of a glacial meltwater river. Geol. Soc.Am. Bull. 87, 209–230.

Sneh, A., 1983. Desert stream sequences in the Sinai Peninsula.J. Sediment. Petrol. 53, 1271–1280.

Stear, W.M., 1983. Morphological characteristics of ephemeralstream channel and overbank splay sandstone bodies in the


Permian Lower Beaufort Group, Karoo Basin, South Africa.Spec. Publ. Int. Assoc. Sedimentol. 6, 405–420.

Stear, W.M., 1985. Comparison of the bedform distribution anddynamics of modern and ancient sandy ephemeral flood de-posits in the southwestern Karoo region, South Africa. Sedi-ment. Geol. 45, 209–230.

Sweet, I.P., 1988. Early Proterozoic stream deposits: braidedor meandering — evidence from central Australia. Sediment.Geol. 58, 277–293.

Tirsgaard, H., 1993. The architecture of Precambrian high energytidal channel deposits: an example from the Lyell Land Group(Eleonore Bay Supergroup), northeast Greenland. Sediment.Geol. 88, 137–152.

Tunbridge, I.P., 1981. Sandy high energy flood sedimentation— some criteria for recognition, with an example from the

Devonian of SW England. Sediment. Geol. 28, 70–95.Tunbridge, I.P., 1984. Facies model for a sandy ephemeral stream

and clay playa complex; the Middle Devonian TrentishoeFormation of North Devon, U.K. Sedimentology 31, 697–715.

Walker, R.G., Plint, A.G., 1992. Wave- and storm-dominatedshallow marine systems. In: Walker, R.G., James, N.P. (Eds.),Facies Models, Response to Sea Level Change. GeologicalAssociation of Canada, Ontario, pp. 219–238.

Walker, T.R., 1967. Formation of red beds in ancient and moderndeserts. Geol. Soc. Am. Bull. 78, 353–368.

Williams, P.F., Rust, B.R., 1969. The sedimentology of a braidedriver. J. Sediment. Petrol. 39, 649–679.

Wright, V.P., Marriott, S.B., 1993. The sequence stratigraphy offluvial depositional systems: the role of floodplain sedimentstorage. Sediment. Geol. 86, 203–210.

proterozoic fluvial styles: response to changes in accommodation space (rivieradal sandstones,...

Documents