J. geol. Soc. London, Vol. 138, 1981, pp. 1-14, 8 figs. Printed in Northern Ireland.

Devonian lake marginenvironments and processes, SE Shetland,Scotland

P. A. Men SUMMARY: The Devonian lake or lakes of SE Shetland were characterized by 4 environ- ments: (1) a marginal environment dominated by the high energies of the and just lakeward of the breaker zone; (2) an additional marginal environment consisting of small, shallow, oxidizing lakes on a low-gradient coastal flood plain over which waters from the main body of the lake fluctuated; (3) an intermediate environment characterized by continuous to intermittent wave agitation, in water depths of probably <5 m; and (4) an offshore environ- ment characterized by a lack of evidence for or wave action, at depths of >S10 m. The marginal environment was influenced by both wave and current processes. A friction- dominated outflow produced sedimentary structures that indicate a south-easterly flowand depositional slope. Outflow competed with shoaling waves which advanced fromE to W onto the lake shoreline. Where outflow was weakest,wave processes dominated. Longshore driftwas an additional componentin the movementof the nearshore waters. The Devonian lakeor lakes of SE Shetland possessed rather shallow shoaling margins.

The Shetland , an 165km N of the natedcarbonate called theExnaboe FishBed. A Scottish mainland, containrelicts of 3 Devonian basins similargroup of facies is shown in Log 2, which of deposition (Mykura 1976). The present-day juxta- describes the sequence from Millbum to Stokki position of the 3 basins has been produced by essen- Geo.Further S in the SumburghHead region the tially lateralmovements along major N-S trending lacustrinesediments are described from the large transcurrent faults. The least deformed sediments are roadside quarry (Log 3) and The Waals (Log 4). present in theeasternmost of these 3 basins and The stratigraphic columns in the northern (Exnaboe outcrop from Rova Head in the N to Sumburgh Head area)and thesouthern (Sumburgh Head)parts of in theS and on severalislands, the largest of which are Dunrossness are shown in Fig. 2. Correlation of indi- Bressay, Noss and Mousa (Fig. 1). The Devonian sedi- vidual sequences exposed on the Scatness , ments of this easternmost basin were deposited in a Sumburgh Head and in the Exnaboe areais hampered variety of environments, such as alluvial fan, fluvial, by the large number of predominantly W-SE trend- aeolian and lacustrine. ing faults but the general relationships indicated on Mykura’s (1976, p. 65) reconstruction of the basins Fig. 2 are probably valid and the total stratigraphic emphasized the limited lateral extent and interdigita- thickness of Devonian sediments in this part of Dun- tion of the lithological facies. It indicated a ‘palaeo- rossness is c. 1125 m. The sedimentarysuccession hill’ of metamorphic strata, separating 2distinct drain- appears to be rather thicker to theN of the study area age systems in the N and S, flowing into a lake situated (Mykura 1976). to the E. Thatthe SE Shetlandbasin contained Devonian lakeshas been known for some time (Finlay 1926), Sedimentary facies and processes basedessentially onits continental position in the Devonian NW European landmass (Geikie 1879) and Figs 3, 4 and 5 represent logsfrom Blo Geo-Swart its vertebrate fauna (summarized in Mykura 1976, pp. Skerry,Millburn GeoStokkiGeo and The Waals, 64-66) of lateGivetian age. Lacustrine sediments respectively. The logs show laminated carbonates or are present as generally thin calcareous intervalsin the calcisiltites sandwichedbetween siltstones andsand- form of symmetrical cycles. These lacustrine intervals stones,providing evidence of wave agitation. The are particularly well exposed in the Dunrossness area large scale grouped cross-stratified sandstones are not of SE Shetland (Fig. l), which is described in this of lacustrine origin. The lacustrine sediments can be study. The results are nevertheless applicable through- divided into the following facies: out the SE Shetland basin. (1) Horizontally stratified fine sandstones.Low angleplanar cross-stratification andripple cross- Location of sections lamination are less common. The fine sandstones con- taininterbedded siltstones andrare thin dark grey 5 sedimentologicallogs from the Dunrossnesscoast mudstoneswith subaqueous shrinkage cracks. Con- are discussed (Fig. 1). Log 1was taken between BloGeo volutelamination, slump and load structures are and Swart Skerry and includes a vertebrate-rich lami- common.

0016-7649/81/0100-0001$02.00. @ 1981 The Geological Society

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 2 P. A. Allen ‘S

SCATNESS PENINSULA

Lacustrine Sediments

FIG. 1. Location map of Dunrossness in SE Shetland with position of sedimentological logs and indication of the main outcrops of lacustrine sediments.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 Devonian lakemargin environments and processes, SE Shetland 3

SUMBURGH action. The fish bedat Exnaboe is speciala develop- SEQUENCE ment of this facies. (C) 200111 The Slithers Facies 1: Horizontally stratified fine -m Large Quarry 3 [- - sandstones This facies comprises, volumetrically,the bulk of the nearshore lacustrine sediments. The fine sandstone is The Waals medium todark grey and ‘flaggy’, withhorizontal parallel laminations and thin (up to 10 cm) very low angleplanar cross-sets andsmall-scale cross-

VerticalI, Scale stratification. The horizontally-stratified and low angle planar cross-stratified sandstonesresemble foreshore to backshore deposits dominated by the breaker zone and zone of and backwash in marine environ- ments (Thompson 1937; McKee 1957). Given a mod- erate fetch for thewind it is reasonable that lacustrine EXNABOE shoreline environments may also be characterized by SEQUENCE Jarlshof well-develooed zones. Wulf (1963). for instance. described a-suite of shoreline features from a lake in Michigan. Where ripple cross-lamination is preserved it is most SCATNESS commonly of the undulatory type. Undulatory lamina- SEQUENCE tion is produced under high intensity oscillatory flows (A) by the accretion of rolling grain ripples (rolling grain

Burgi Ness rippleshave been produced experimentally by Bag- nold (1946) and Sleath (1976) and are further discussed by P. A. Allen (in press)). The required high oscillat-

Brei Geo ory flow intensities areproduced whenwaves considerably in shallowwaters. Harms et al. LocationMap (1975, p. 87) believed that lamination of the undulat- ory type (hummocky cross-stratification) is produced in the lower shoreface to offshore facies by relatively large storm waves of arough sea, at flow velocities greater than those required to form wave ripples. In the lacustrine environment, undulatory lamination is probablyproduced immediately lakeward of the breaker zone in shallow water. The ripplecross-lamination includes features such asopposed unidirectional cross-laminated lenses, bidirectionalbundled lenses, undulatory lamination and draping and offshooting foresets. Top profiles of cosets are of lob amplitude and roughly symmetrical. The cross-lamination is of the undulatory, interwoven and less commonly linsen types (Reineck & Wunder-

Basal lich 1968; de Raaf et al. 1977). unconformity The directionally opposed lenses, bidirectional bun- dled lenses and draping and offshooting laminations FIG. 2. Composite stratigraphic section for Dun- rossness. The totalthickness of sediments is c. suggest that the ripplecross-sets were formed by wave 1125 m. Numbers 1 to 4 refer to positions of oscillation (Boersma 1970; Reineck & Singh 1973, pp. sedimentological logs. 24-28). The occasionalunidirectional tabular cross- setsare probably the only truecurrent rippled de- posits. These tabular cross-sets were themselves mod- (2) Ripplecross-laminated and horizontally- ified by waves, since their top profiles are commonly laminated siltstones withplant remains, subaqueous near-symmetrical. shrinkage cracks and convolute lamination. The sandstone unit above the Exnaboe Fish bed at (3) Laminatedand unlaminated calcisiltites and Swart Skerry (Log 1, Fig. 3) is coarser-grained and is shaly limestones lacking evidence of current or wave exceptional in ‘its great thickness (about 10 m). The

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 4 P. A. Allen 1 l

od

Exnaboe Fish Bed

2m

I

-

7 L L

FIG. 3. Sedimentological log from Blo Geo to SwartSkerry, northern Dunrossness (Log 1). Key to lithologies, sedimentary structures and symbols as in Fig. 5. Numbers 1, 2 and 3 are facies types discussed in text.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 Devonian lake margin environments and processes,SE Shetland 5 l l

J

I t t FIG. 4. Sedimentological log from Millburn Geo to Stokki Geo, northern Dunrossness (Log 2). Key to lithologies, sedimentary structures and symbols as in Fig. 5.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 6 P. A. Allen

TOP

Zm

1

0

BOTTOM L FIG. 5. Sedimentological log at The Waals, Sumburgh Head (Log4).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 Devonianlake marginenvironments and processes, SE Shetland 7

association of flat-beddedsandstones with erosional shorewhere not only stormwaves, but fairweather troughs (sets up to 40 cm thick) due to current scour waves in addition were of influence. An end to wave and fill and occasional high angle planar cross-sets (up activity is marked in most cases by a draping of dark to 20 cm thick) suggests that either these sediments grey mud over the symmetrical profile of the wave- were deposited close to a river outflow, or that they generated coset. There is no evidence of emergence, represent a longshore bar. In the latter case, the low such as desiccation cracks, and all crack-fills are of the angle planar laminations may represent the low slopes non-polygonaloriented subaqueous variety (Picard of the bar facing the basin (McKee & Sterrett 1961) 1966). and the higher angle cross-laminations may have been produced on thesteeper landward-facingside Facies 3: and shaly limestone (Thompson 1937; Hoyt 1962;Davidson-Amott & Calcisiltites Greenwood1974). The troughcross-sets may have Based on their fine and lack of evidence beenproduced by megaripplesat the margin of a of current or wave action, these sediments are inter- longitudinal (Reineck 1963) or by dissection preted as the mostoffshore representatives of the of the bar atlow water stages.The coarse grade of the lacustrineenvironment. The lack of waveagitation sedimentcomprising the troughcross-sets suggests suggeststhat waters were deeper thanstorm wave- that low stage modification was not important. How- base. For the short period waves which characterized ever, the amount of exposure provides too little data the Devonian lake or lakes of Shetland this wave base to substantiate an origin as a longshore bar. was <10m and probablycloser to 5 m.A similar The weaker the wave energy thesmaller the number figure for wave-base is given by Reineck (1971) from of longshore bars (Reineck & Singh 1973, p. 305), and the of Gaeta, Italy. sincethe wave energy in lacustrineenvironments is The preservation of organic matter and the common usually small, the fine to medium sandstones at Swart laminated nature of these sediments suggests deposi- Skerry (Fig. 3) may merely have been deposited close tionbelow a thermocline in ananoxic environment. to a zone of high sediment supply, at a river outflow. The association with unlaminated sediments indicates The presence of small wavelength ripples (microrip- that the thermocline was not a permanent feature. ples, Singh 1969; ormini-ripples, Singh & Wunderlich The presence of more finely laminated sediments in 1978) in this facies suggests the action of very short the upper parts of the fish beds indicates a continual period waves in very shallow water. deepening of thelake during their deposition. The strongly slumped and pseudo-nodular sediments above the fish beds demonstratethat coarser detritus was Facies 2: Ripple cross-laminated and deposited on an easily deformed, soft substrate, and horizontally-laminated siltstones marks the onset of lake shallowing. Donovan (1975) Ripplecross-laminated and horizontally-laminated describedmicro-turbidites in thisposition from the siltstones grade continuously to the fine sandstones of Middle Devonian of Caithness, but there is no such Facies 1, andthis is reflectedin anoverlap of the evidence in SE Shetland. In contrast, the sediments in cross-laminationtypes. Interwoven and linsentypes the lacustrine cycle are symmetrically disposed about (cf. de Raaf et al. 1977) are the most common, with the central calcisiltite/limestone member. subordinate horizontal and undulatory lamination. In generalterms, linsen intervals were probably The sediments of Sumburgh Head produced under conditions of low to moderate wave action. Linsen structure results from starvation of the The lacustrinesediments of theSumburgh Head bed of or -sized sediment necessary for ripple area display significant differences to those of the development. The connected and unconnected lenses Exnaboe area. Facies 1 (beach) sediments are poorly within the mudstone background may represent storm developedand are replaced by a facies of red silt- layersdeposited from suspension but reworked by stonesand very fine sandstones in the largequarry waveagitation in thesame storm. Linsentypes of (Fig. 1). Fig. 6 represents logs from the large roadside cross-lamination areproduced beyond the zone of quarry, Sumburgh Head; this section is interpreted in continuous wave activity, but in waters shallow enough some detail. to bereached by storm waves. The depth towhich this Log 3a (in Fig. 6) is 8.5 m long and is taken from zone corresponds in the Devonian lake of Dunrossness the N end of the larger of the 2 quarries situated to is probably from about 3 m to a maximum of 10 m the E of the road from Grutness to Sumburgh Head based on results of computer simulation of wave con- lighthouse. The strata in this quarry are cutby 2 faults ditions (P. A. Allen, in press). and log 3a records the section in the central block. Forthe interwovenwave rippled sediments it is Log 3b is similarly 8.5 mlong andrecords the reasonable to infercontinuous wave activity, with sedimentary sequence in the southern block. A small successive reworking of sand layers on the lake floor. amount of rotationhas occurred on the2 faults, Thesesediments were produced closer tothe lake causing a slight difference in strike and dip between

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 8 P. A. Allen

::. -a -1 I 1.. .: - j3a/ 13bJ

CENTRAL FAULT BLOCK SOUTHERNFAULT BLOCK

GRAIN SIZE COU)UR

FINE SAND RED

TOP

IIOTTOY FIG.6. Sedimentological logs at the large roadside quarry, Sumburgh Head (Log 3). Key to lithologies, sedimentary structures and symbols as in Fig. 5.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 Devonianlake marginenvironments and processes, SE Shetland 9

the central (010j20E)and the southern block under low aggradation rates, perhaps with a superim- (344/18E). The exact relationship stratigraphically be- posedunidirectional flow (see Harms et al.1975, tween the intervalsdescribed in logs 3aand 3b is p. 55). An end to bedload sedimentation was marked complicated by the intervening fault zone. However, by the fall-out and draping of blue-grey mud over the the intervaldescribed in log 3b is stratigraphically wave-rippled surface. The long wavelength and small higher than that in log 3a. amplitude of this surface is anomalous with respect to typical wave-ripple indices (Reineck & Singh 1973, p. 45;Tanner 1967), but the amount of compaction The central fault block (Log 3a) these sediments have undergonemay be considerable. The lowermostrocks exposed at the large quarry Water depth was evidently shallow enough to allow a (and not shown in Fig. 6)are interlaminated sand- moulding of thesand bed by the wave-generated stonesand mudstones with ptygmatic and bulbous motion of the water. The development of pseudo- crack-fills. The shrinkagecracks are of a sand- nodules and convolutelamination emphasizes the dominated type andwere produced by enlargement water-logged nature and inherent instability of these and deformation of hairline subaqueousshrinkage sediments.Stanley & Surdam (1978,~.568) and cracks under conditions of relatively high aggradation Hubert et al. (1976, p. 1198) also described sandstone of sand. It is possible that some of these structures are pillows and pseudo-nodules from ancientlacustrine due to bioturbation. Donovan & Foster(1972) and sediments. Picard (1966) described similarstructures fromancient Unit 2 comprises grey, wavily laminated and ripple lacustrine rocks. The grey, calcareous nature and pres- cross-laminated very fine and fine sandstones. A de- ence of subaqueous shrinkage cracks suggest that the tailed interpretation of the ripple cross-lamination is environment of deposition of thesesediments was not possible, butthe bidirectional, asymmetrical lacustrine, but the position within the lakeis not clear. character of cross-sets indicates opposing flow direc- The background sediment of dark grey to black mud- tions. The slightly sinuous, roughly symmetrical crest- stone indicates placid conditions in which fine ter- lines of ripples on one bedding surface are not incon- rigenous sediment accumulated from suspension. sistent with awave-generated origin, but details are However, the lake was subject to periodic influxes of lacking in this highly weathered unit. coarser-grainedsandy sediment,perhaps scavenged The overlying sediments (Unit 3) contain 2 different from nearshore zones during storms. types of cross-lamination. Firstly, the grouped cross- The overlying very fine sandstones with thin dark sets contain structures such as directionally opposed grey mudstone beds (Unit 1) contain more diagnostic lenses, scooped basal surfaces and low angle foresets structures. The presence of climbing ripples atthe which are common in wave-generatedripples base indicates high aggradation rates relative to down- (Boersma 1970). The bifurcating crestlines also indi- stream ripple migration. This regime is common where cate wave oscillation (Reineck & Singh 1973, fig. 29, flows are abruptly checked in velocity and are forced p.28). Secondly, the connectedunidirectional lenses to deposit their load. Fluvial run-off, in times of flood, overlying horizontalstratification were produced by into the lake would produce such deceleration. The wave modification of flat-lying sediments. The climbing ripples pass up into horizontal and low angle horizontally-stratified very fine sandstone was pro- planar cross-stratification which was probably pro- duced either by deposition from suspension, perhaps duced by high velocity flows. The low angle cross-set after whirling up of shoreline sediment duringstorms, indicates high velocity flow producing a flat bed on a or by deposition at high flow velocities as a flat bed. slightly inclined depositionalsurface. Parallel- The upwardincrease in red siltstonelaminae and laminated and low angle planar cross-stratified sedi- interbeds in Unit3 heralds the deposition of the ments are characteristic of the breaker zone and zone predominantly redcoloured Unit 4, theuppermost of swash and backwash (Harms et al. 1975; Thompson unit in log 3a (Fig. 6), which is characterized by 1937; McKee 1957)but similar structures may be convoluted, horizontally-stratified and ripple cross- formed in slightly deeper water close to river outfalls laminated fine sandstonesand thin red mudstones. where the lake gradually. The draping of sediment over the symmetrically rip- The shape and internal structure of small scours in pled surface of bundles of unidirectional cross-sets at the parallel-laminated finesandstones indicate the 6.30 m suggests the operation of wave processes. The migration of small, current-generated, sinuous to lin- symmetrical form of the upper surface of the coset has guoid ripples (J. R. L. Allen 1963). The coset (at the a lengthfieightratio of 10, which is thought tobe top of Unit 1) of slightly climbing, small scale ripples typical of wave ripples allowing for a slight amount of indicates more prolonged migration of trains of rip- flattening during compaction (Reineck & Singh 1973, ples. The strongly scooped basal surface with mostly p. 45). The symmetrical form of the upper profile of concordant internal laminations and low angle of the coset at 6.70 m again indicates late stage remould- climb of the basal surfaces suggests that these struc- ing of the sand bed by wave processes. tures may have been produced by wave oscillation The origin of much of the ripple cross-lamination in

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 10 P. A. Allen

Unit 4 is problematical. The unidirectional cross-sets allowingperiodic atmospheric exposure of thelake may have a wave or current-generated origin; New- margin. ton (1968)showed thatmost wave ripples in the nearshorezone are composed of aform-discordant Summary of depositional environments unidirectionalcross-lamination. However, the planar basal surfaces to some of the unidirectional cross-sets In summary, the data available suggest that the lake suggests deposition by unidirectional currents. As cur- was characterized by: rent strength and sediment input waned, wave modifi- 1. A marginal environment dominated by the high cation of current-generatedbedforms took place. energies of the surfzone and just lakeward of the However, the interwovencross-lamination which is breakerzone, in which flat-bedded, very lowangle common in this unit has affinities with wave-generated cross-stratified andundulatory-laminated sandstones ripples (Boersma 1970). were deposited. Locally thick flat-bedded units were The downward projecting structuresof sand into red developed,as at Swart Skerry, but more commonly silty mudstone are crack-fills due either to subaerial thinner units containing wave-ripples are found. desiccation or subaqueous shrinkage. Thesmall scaleof 2. On SumburghHead an additional lake margin many of these features, the high frequency of struc- environmentconsisted of small,shallow oxidizing tures alongbedding planes and the folded shapes lakes on a low-gradient coastal floodplain over which suggest that they were formed subaqueously. Some of the waters from the main body of the lake fluctuated. the larger crack-fills have a polygonal shape in plan At the margin of the main lake direct wave activity and were due to desiccation. and unidirectional flows related to fluvial run-off, out- Towards the top of Log 3a the abundance of red flow dispersal or produced predomin- mudstones withdesiccation cracks and occasional antly small-scale ripples. medium-grained sandstones with granules and rip-up 3. Anintermediate environment characterized by clasts of redmudstone indicate anenvironment of continuous to intermittentwave agitation in which greater extremes of sedimentation. Muds were desic- ripple cross-laminated siltstones were deposited. This cated on a coastal floodplain or lake margin zone corresponded to water depths of c. 3-10 m, but deprived of coarsedetrital sediment supply. That the storm wave-base upper limit of water depth may ‘floods’ occasionally crossed this environment is shown have been shallower for long periods of time. by the thin, erosive bedsof medium-grained sandstone 4. An offshore environment characterized by a lack rich in rip-up clasts. of evidence for current or wave action, and in which fine-grained clastic sediments and primary precipitates of calcium carbonatewere deposited. This environ- The southern fault block (Log 3b) ment existed in water depths of >5 m and possibly Thered sediments in thelower half of Log 3b >l0 m. containevidence of subaerialemergence and drying (desiccationcracks). The origin of the unidirectional Discussion and synthesis ripple cross-lamination in these rocks is unclear, but the planar basal surfaces suggest that the ripples were Many lacustrine basins are filled by a delta-like out- produced byunidirectional currents. However, the growth from an inflowing river, and similar mechan- symmetrical draped laminations and the similarity of isms have been postulated in ancient lake basins (Link thisfacies to Unit 4 in Log 3a indicatesthat wave & Osborne 1978; Stanley & Surdam 1978). Wright & processeswere also operative.The azimuths of the Coleman(1972, 1973)stressed that when the tidal ripple cross-laminations have a large variance. range and incident wave power are negligible or small The grey, calcareous natureof the upper part of log compared to the strength of the river outflow, river- 3b is evidence for a lacustrine environment of deposi- dominated configurations result. Such river-dominated tion. One type of ripple cross-lamination, comprising situations characterize lakes with small tides, particu- grouped sets with undulating and scoop shaped basal larly if fronted by flat offshore slopes which attenuate surfaces was produced by prolonged wave agitation. wavepower. Wright (1977) stated that in river- Theother type of cross-lamination,with connected dominated situations, one of three primary processes lenses of solitary sets may have a wave- or current- will dominate: (1) inertiaand associated turbulent generated origin. Limited sediment supply prevented diffusion, (2) turbulent bed friction, (3) buoyancy. thedevelopment of groupedsets, and the large High outflow velocities, small density contrasts and, amounts of associatedfine-grained sediment suggest in particular,deep water basinward of the river short-livedwave orcurrent activity. The smaller mouths favour the establishment of a fully turbulent crack-filling structures were produced by subaqueous jet due to inertial forces. Inertia dominated outflows shrinkage,supporting a lacustrine origin for these are frequently associated with steep gradient streams sediments. The larger crack-fills may have been due to entering deep, freshwater lakes (homopycnal outflow desiccation, indicating that lake waters were shallow, according to Bates 1953). Agood example of this

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 Devonianlake marginenvironments and processes, SE Shetland 11

wouldbe the Rhine in Lake Constance than a coarsening-up lacustrine-fill (cf. Stanley & Sur- (Forstner et al. 1968).Characteristics of thissystem dam 1978, p. 561). are low spreadingangles (angle between outflow Wright (1977) did not discussin detail the sediments centre line and outflow boundary) and a progressive andstructures formed inhis inertiadominated and but low rate of basinwarddeceleration of thejet. friction dominated outflow types. However, the high Corresponding depositional patterns are a low lateral bed shear stresses experienced near the river mouths, dispersion of sediments from the central core and the in the proximal part of the turbulent jet, should pro- development of a typical Gilbert-type profile extend- ducepredominantly flat-bedded sediments, given a ing a considerable distance into the lake. fine to medium sand bed material. At times of lower Unless there is very significant lateral(longshore) river discharge, these flat-bedded sediments may have re-distribution of sediment, areas between river out- become modified by waves approaching the gradually falls are starved of sediment supply. Nydegger (1976) shoaling lake shoreline. The sediments overlying the showed how the Coriolis force can cause large mass Exnaboe Fish Bed at Swart Skerry may therefore be movements of lake water, and Sturm & Matter (1978) interpreted as the high energy deposits produced by detected the results of such water movements in the turbulent bed friction at a river outflow. Further from lake floor sediments of LakeBrienz, Switzerland. the river outflow, slightly finer-grained sediments con- Forstner et al. (1968) showed from cores in the Rhine tain more evidence of wave processes, and the hori- River delta sediments in Lake Constance that on the zontal laminations of these fine sandstones were pro- sides of the axis of theRhine considerably finer- duced in the surf zone as waves advanced onto small grained sediments are deposited thanin the core zone, . andthere are no along thelake shorelines. The Sumburgh Headarea was characterized by a Weiss (1979) described similar depositional patternsin complexinteraction of channellizedand unchannel- Lake Biel, Switzerland. lized fluvial run-off, direct wave activity, outflow dis- If thewater depths basinward of theoutlet are persal and longshore drift. The grey sediments in the shallow, outflow velocities and bed shear stresses are roadside quarry, Sumburgh Head, were deposited in a high andturbulent bedfriction becomes dominant complex of sub-environments. Some flat-bedded and (Wright1977). Friction, in combination with lateral low angle cross-stratified sediments were deposited at turbulentjet diffusion causes therates of outflow high flow velocities,probably related to a friction- spreadingand deceleration to increase substantially. dominatedoutflow. Decreases in outflow activity al- Gilbert-type, high angle, profiles are not developed. lowed the local development of wave-generated struc- Sand-size sediment would then be available for incor- tures.The red sediments contain current-generated poration in a lake shoreline environment. cosets which were modified by waves and desiccation Friction-dominatedoutflows with shallow lake cracks,indicating that they weredeposited in small shoreline slopes are inferred for the lake or lakes of oxidizing lakes. These lakes possessed marginal mud Shetland. This is based on: and silt flats which were crossed and eroded by land- (1) The presence of horizontally-stratified fine sand- derived floods. stones in almost all logs in lacustrinesequences Van Houten (1964)visualized the existence of mud- throughout SE Shetland, suggesting wide distribution flats and broad floodplains with weak external drain- of river-derived sediment rather than concentration in age as the Triassic Lockatong lake decreased in size, deltas. The lateral changes between Swart Skerry and andthe lake may havebeen broadly similar in SE Shingly Geo are due to the relativeproximity of Swart Shetland.Hubert et al. (1976) also described low Skerry to the axis of the river outflow, or perhaps to gradient floodplains with shallow oxidizing lakesin the the local development of longshore bars. In the Sum- non-marine Hartford Basin in Connecticut and Mas- burgh Head area the facies of horizontally-stratified sachusetts. sandstones is less well developed (Log 4, Fig. 5) sug- gesting that this area was locatedfarther from the Local palaeogeography sediment source. (2) The dominance in the marginal lacustrine sedi- Thedirectional structures in thesediments at the ments by flat-beddingand wave-generated ripples roadsidequarry, Sumburgh Head, provide detailed rather than the complex of structures formed on delta information on local palaeogeography. The major de- wedges (Miiller 1966; Axelsson 1967; Born 1972; positional processes were waves advancing from the E Gustavson et al. 1975). or SE, sediment dispersal or fluvial run-off in a south- (3)The indication, from the thickness and nature of easterly direction and southward longshore drift. Out- lacustrine facies, that water depths were rather shal- flow activity (and the regional palaeoslope to the S) low, promoting turbulent bed friction rather than iner- probably strongly influencedthe direction and strength tial diffusion at river outflows. of longshore drift. (4) The occurrence of lacustrine faciesin symmetri- Forthe entire Dunrossness area, there is a wide cal cycles reflecting lake advance and retreat, rather variance of data from ripple crestline orientations and

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 12 P. A. Allen

shorelines. If subaqueous shrinkagecracks can be used in this way, theyprovide evidence on local palaeogeography independent of the ripple crestlines and cross-laminations. Although the data are few, the average trends of subaqueous shrinkagecracks strongly support the other data. Fig. 8 shows a schematic reconstruction of processes and palaeogeography forthe Dunrossness region of Shetland.

Conclusions

The marginal lacustrine environment in SE Shetland was complex. Such complexity is to be expected where FIG.7. Individual vector means and aggregate rose depositional environments such as fluvial and lacus- diagram for the lacustrine sediments of Dunross- trine interact. Thefollowing conclusions can be made. ness. The marginallacustrine environment was charac- terized by both wave and current processes. Discharge from a friction-dominated outflow produced flat beds internal cross-laminations. The data from Sumburgh and low angle planar and ripple cross-sets indicating a Head indicate that this large variance is due to the south-easterly flow and depositionalslope. Outflow interaction of several processes in a complex deposi- competed with shoaling waves which advanced from E tional environment. Reineck & Singh (1973, p. 305) to W ontothe lake shoreline. Where outflow was noted a wide dispersion of cross-stratification azimuths weakest, wave processes dominated. Longshore drift in the upper shoreface, particularly where there is a in a N-S direction was probably an additional compo- strong longshore current, but there is usually a mode nent in the movement of the nearshore waters. dipping towards the land with another parallel to the Predominantly flat-bedded, low angle planar cross- shore. Fig. 7, which showscross-stratification and stratified and undulatory laminatedsandstones were crestline data from Dunrossness, contains such a pat- deposited close to the lake shoreline in the zone of tern. swash and backwash, surf zone and immediately lake- Themeasurements giving the E-W vector means ward of the breaker zone. The beach-shoreface zones originate from undoubted wave-generated structures, werenarrow compared with those of high energy frequently from unidirectional cross-laminated lenses oceanic . Thicker,coarser-grained sandstones associated with a fine-grained background sediment of accumulated close to river outflows or as longshore silty mudstone (Facies 2). These sediments were depo- bars. More placid conditions existed in locations re- sited near the transition to the offshore zone; the lake moved fromthe mainzones of outflow. Here fine shoreline was consequently oriented roughly N-S. The sands, possibly scavenged from the lake margin during measurements giving the E-W and more commonly storms, were deposited asthin beds in a background of N-S vectormeans originate in general fromboth dark grey muds. wave-generated and wave-modified structures,and Lakeward of the surf zone, or where the surf zone possibly fromsome current-generated sets, in the was absent, wave rippledsiltstones weredeposited morenearshore sediments of the lacustrine plexus. under continuous to intermittentwave activity. At the This isbest seen at The Slithers (Fig. l), Sumburgh lakeward edge of this zone solitary wave ripples were Head.This oblique component was produced by formed during storms. longshore drift and/or outflowdispersal. Similar Beyond storm wave-base, calcareous siltstones and bimodalpatterns in lacustrine sedimentshave been shalesaccumulated under placid conditions in the measured in other parts of the SE Shetland basin. offshore zone. A seasonal thermocline was sometimes Aeolian sediments exposed N of the Exnaboe area developed, allowing the preservation of organic-rich indicate prevailing winds fromthe S, blowing up- laminated sediments. Carbonate was also precipitated valley, whereas the wave-generated ripples in lacus- andproduced primarylaminations in theExnaboe trinesediments suggest an orthogonal advance of Fish Bed. Red sediments were deposited by E- to SE- waves onto the lake shoreline. flowing low velocity currents on a coastal floodplain, Picard (1966) in the Uinta Basin, Utah, and Dono- subject to periodic desiccation. Where lakes were iso- van & Foster (1972) in the Caithness Flagstones, NE latedon the floodplain, waves modified the tops of Scotland, commented on the use of the linear trendof current generated ripples. subaqueous shrinkagecracks in reconstructing palaeo- The symmetrical arrangement of lacustrine facies,

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 Devonian lake margin environments and processes, SE Shetland 13

FIG. 8. Schematic reconstruction of processes and palaeogeography at the Devonian lake margin in Dunrossness.

the widespread occurrence of sandy shoreline zones ACKNOWLEDGMENTS.I thank Dr Peter Friend of the Univer- and lack of delta development suggest that the lake sity of Cambridge and Dr Albert Matter of the University of was characterized by a shoalingmargin, possi- Bernfor their critical reviewing of themanuscript. I also bly built by sedimentdispersal from one or gratefully acknowledge Drs Christine Turner and Fred Peter- friction dominated The Devonian lake in son of the United States Geological Survey in Denver who provided many ideas when this study was in its formative Dunrossnessdeepened in time and predominantly stage. work was cmied out duringthe tenure of a offshore lacustrine Sediments are preserved in the Natural Environment Research Studentshipat the University upper part of the stratigraphic column. of Cambridge.

References

ALLEN, J. R. L.1963. The classification of cross-stratified BAGNOW,R. A. 1946. Motion of wavesin shallow water. units, with notes on their origin. Sedimentology, 2, 93- Interactions of waves and sandy bottoms. Proc. R. Soc. 114. London, A187, 1-15. ALLEN, P. A. (In press). Wave-generated structures in the BA-, C. C. 1953. Rational theoryon delta formation. Bull. Devonianlacustrine sediments of SE Shetland,and Am. Assoc. Petrol. Geol. 37, 2119-62. ancient wave conditions. Sedimentology. BOERSMA,J. R. 1970. Distinguishing Features of Wave-ripple -ON, V. 1967. The Laitaure delta-a study of deltaic Cross-stratification and Morphology. Thesis, PhD, Univ. morphology and processes. Geogr. Annalr, 49A, 1-127. Utrecht (unpubl.).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021 14 P. A. AIYen

BORN,S. M. 1972. Late Quaternary history, deltaic sedimen- MYKURA,W. 1976. British Regional Geology: Orkney and tationand mudlump formation at PyramidLake, Shetland. HMSO, Edinburgh, 149 pp. Nevada.Centre for WaterResources Research, Desert NEWTON,R. S. 1968.Internal structure of wave-formed Research Institute, Univ. of Nevada, Reno, 97 pp. ripple marks in the nearshore zone. Sedimentology, 11, DAVIDSON-ARNO~,R.G. D. & GREENWOOD,B. 1974. 275-92. Bedforms and structures associated with bar topography NYDEGGER, P. 1976. Stromungen in Seen: Untersuchungen in the shallow-water wave environment, Kouchibouguac insito und an nachgebildeten Modellseen. Beitr. geol. , NewBrunswick, Canada. J. sediment.Petrol. 44, Schweiz, Kl. Mitt. 66, 141-177. 698-704. PICARD, M. D.1966. Oriented, linear shrinkage cracks in DERAAF, J. F. M., BOERSMA,J. R. & VANGELDER, A. 1977. GreenRiver Formation (Eocene), Raven Ridge Area, Wave-generated structures and sequences from a shal- Uinta Basin, Utah. J. sediment. Petrol. 36, 1050-7. low marinesuccession, Lower Carboniferous, County REINECK,H. E. 1963.Sedimentgefuge imBereich der Cork, Ireland. Sedimentology, 24, 451-83. sudlichenNordsee. Abh. Senckenb. naturforsch. Ges. NO VAN, R. N. 1975. Devonian lacustrine limestonesat the Frankfurt, 505, 138 pp. margin of the Orcadian basin, Scotland. Q. J. geol. Soc. -1971. Der Kustensand. Nat.Mus. Frankfurt, 101,4540. London, 131, 489-510. -& SINGH,I. B. 1973. Depositional Sedimentary Enuiron- - & FOSTER,R. J. 1972. Subaqueous shrinkage cracks ments. Springer-Verlag, New York, 439 pp. from the Caithness Flagstone Series (Middle Devonian) - & WUNDERLICH,F. 1968. Classification and origin of of NE Scotland. J. sediment. Petrol. 42, 309-17. flaser and lenticular bedding. Sedimentology, 11,99-104. FINLAY,T. M. 1926. The Old Red Sandstone of Shetland. SINGH,I. B.1969. Primary sedimentary structures in Pre- Part 1. South-eastern area. Trans. R. Soc. Edinburgh, 54, cambrian quartzites of Telemark, southern , and 553-72. their environmental significance. Nor. geol. Tiddskr. 49, FORSTNER, U., M-R, G. & REINECK,H. E. 1968. 1-31. Sedimenteund Sedimentgefiige des Rheindeltas im -& WUNDEFUJCH, F. 1978. On the terms wrinkle marks Bodensee.Neues Jahrb. Mineral. Abhandlungen, 109, (Runzelmarken),millimetre ripples, and mini ripples. 33-62. Senckenbergiana marit. 10, 31-7. GEIKIE, A. 1879.On the Old Red Sandstone of western SLEATH,J. F. A. 1976. On rolling grain ripples. J. Hydraulic Europe. Trans. R. Soc. Edinburgh, 28, 345-52. Res. 14, 69-80. GUSTAVSON,G. M,, ASHLEY,G. M. & B~~THROYD,J. C. STANLEY,K. 0.& SURDAM,R. C. 1978. Sedimentation on 1975. Depositional sequences in glaciolacustrine deltas. the frontof Eocene Gilbert-type deltas, Washakie Basin, Spec. Publ. Soc. econ.Paleontol. Mineral. Tulsa, 23, Wyoming. J. sediment. Petrol. 48, 557-73. 264-80. STURM,M. & MATTER, A. 1978. Turbidites and varves in HARMS, J. C., So-, J., SPEAIUNG,D. R. & WALKER, R. Lake Brienz (Switzerland): deposition of clastic detritus G. 1975. Depositional environments as interpreted from by densitycurrents. In: MA=, A. & TUCKER,M. primary sedimentary structures and stratification sequ- (eds.),Modern and AncientLake Sediments, 147-68, ences.Lecture notes, Soc. econ.Paleontol. Mineral., Spec. Publ. Int. Assoc. Sedimentol. 2, 290 pp. Short Course No. 2, Dallas, Texas, 161 pp. TANNER,W. F. 1967.Ripple mark indices and their uses. HOYT,J. H. 1962.High angle beach stratification, Sapelo Sedimentology, 9, 89-104. , Georgia. J. sediment. Petrol, 32, 309-11. THOMPSON, W. 0. 1937. Original structures of beaches, bars HUBERT,J. F., REED, A. A. & CAREY, P. J.1976. and . Bull. geol. Soc. Am. 48, 723-52. Palaeogeography of the East Berlin Formation, Newark VAN HOUTEN,F. B. 1964. Cyclic lacustrine sedimentation, Group, Connecticut Valley.Am. J. Sci. 276, 1183-1207. UpperTriassic Luckatong Formation, central New LINK, M. H. & OSBOS,R. H. 1978. Lacustrine facies in the Jersey and adjacent Pennsylvania. Bull. geol. Sum. Kan- PlioceneRidge Basin Group, Ridge Basin, California. S~S,169, 497-531. In: MATTER, A. & TUCKER,M. (eds.). Modern and WEISS,H. P. 1979. Die Oberflachensedimente des Bielersees. Ancient Lake Sediments, 169-75. Spec. Publ. Int.Assoc. Eclog. geol. Helu. 72, 407-24. Sedimentol., 2, Blackwell Scientific Publ., Oxford. WRIGH'T L. D. 1977. Sediment transport and deposition at MC-, E. D.1957. Primary structures insome recent river mouths: a synthesis. Bull. geol. Soc. Am. 88, 857- sediments. Bull. Am. Assoc. Petrol. Geol. 41, 1704-47. 68. - & STERRETT,T. S. 1961. Laboratory experiments on -& Cow,J. M. 1972. River delta morphology: wave form and structure of longshore bars and beaches. In: climate and the role of the subaqueous profile. Science, PETERSON, J. A. & OSMOND, J. C.(eds.) Geometry of 176, 2824. SandstoneBodies, 13-28. Am. Assoc.Petrol. Geol., -& -1973. Variations in morphology of major river Tulsa. deltas as functions of ocean wave and river discharge MULLER, G. 1966. The new Rhine delta in Lake Constance. regimes. Bull. Am. Assoc. Petrol. Geol. 57, 370-98. In: SHIRLEYM. L.(ed.). Deltasintheir geologic WULF, G. R. 1963.Bars, spits and ripple marks in a framework, 107-125, Houston Geol. Soc. Michigan lake. Bull. Am. Assoc. Petrol.Geol. 47,691-5.

Received 14 March 1980; revised typescript received 10 July 1980. PHILIP A. ALLEN,Geologisches Institut, Universitat Bern, Sahlistrasse 6, CH-3012 Bern, Switzerland.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/1/1/4886901/gsjgs.138.1.0001.pdf by guest on 02 October 2021