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 surf zone 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 current 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 flow and 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 Islands, an archipelago 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 Geo 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 Peninsula, 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 lakes has 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 logs from 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 and sand- 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 sandstones resemble foreshore to backshore deposits dominated by the breaker zone and zone of swash 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 beach 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 shoal 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 a rough 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