Depositional Environments and Palynology of the ...

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Depositional Environments and Palynology of the Devonian South-east Shetland Basin P. A. ALLEN 1 and J. E. A.

MARSHALL 2

^Geologisches Institut, Universitat Bern, Sahlistrasse 6, 3012 Bern, Switzerland 2 Department of Geology, The University, Newcastle upon Tyne, NE1 7RU

SYNOPSIS

The sediments of the SE Shetland basin were deposited in six major depositional environments, (1) the breccias of basement screes, (2) the conglomerates of alluvial fans, (3) the sandstones and conglomerates of braided streams, (4) the fine-grained alluvium of high-sinuosity streams, (5) the clastic and carbonate deposits of lakes and (6) the sandstones of desert dunes. Palynological evidence from the age suggests a stratigraphic position at or near the Gi vetian/Frasnian boundary and certainly younger than the Eday Flags of Orkney. Deposition may have occurred earlier in the northern part of the basin. Kerogen typing suggests the lacustrine environment was less evolved than Orkney and Caithness. The Devonian SE Shetland basin occupied an elongate depression which was oriented NNW—SSE. A major drainage system flowed to the SSE and may have connected this basin to the larger Orcadian basin in the south. Prevailing winds blew up-valley to the north. Small lakes with shallow shoaling margins expanded and contracted in this basin.

INTRODUCTION

T h e Shetland Islands, an archipelago 165 k m north of the Scottish mainland, contain a large assemblage of rock types of metamorphic, igneous and sedimentary origin. T h e Devonian sediments of Shetland were deposited in three distinct basins presently separated b y n o r t h - s o u t h trending major transcurrent faults. T h e Devonian rocks west of the Melby Fault (Fig. 1) contain t w o fish-bearing siltstones which have been correlated with the Achanarras Limestone of Caithness and the Sandwick Fish Bed of O r k n e y (Mykura 1976, p. 52). East of the Melby Fault are the folded Sandness and Walls Formations which M y k u r a (1976, p . 60) estimates to be about 12 600 m thick. T h e sediments of the easternmost basin, exposed to the east of the Walls B o u n d a r y Fault (Fig. 1) are summarized here. A brief history of research into the rocks of Shetland can be found in Mykura (1976, p. 11-14). For SE Shetland M y k u r a visualized t w o depositional systems, separated by a palaeohill of metamorphic strata in the Quarff-Fladdabister region. M y k u r a (1976, p . 62) suggested that this palaeohill separated . . . 'a deep southeastward flowing river valley in the north from a m o r e open river valley and later alluvial plain in the south'. Scott. J. Geol. 17, (4), 257-273, 1981


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30 Km

MELBY / BASIN $$/

Foul a

/

? S.E. SHETLAND BASIN

' §

Fair Isle

FIG. 1. The three Devonian basins of Shetland. After Mykura (1976, p. 56). Mykura also summarizes the palaeontological evidence for the age of the sequence, placing the lower parts equivalent to the Eday Group and John o'Groats Sandstone of Orkney and Caithness with some of the higher beds of Bressay being possibly of Late Devonian age. SEDIMENTOLOGY

The Devonian sediments of SE Shetland were deposited in a variety of environments such as alluvial fan, fluvial, aeolian and lacustrine. The purpose here is to provide a broad picture of the depositional environments and sediment dispersal patterns in SE Shetland.


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The Basal Breccia This coarse-grained breccia is exposed discontinuously at the base of the stratigraphic column in SE Shetland, but is best seen in the Quarff-Fladdabister region (Fig. 2) where it drapes'an underlying hilly topography formed of metamorphic rocks of the Quarff Nappe (Flinn 1967). Many of the coarse (up to 1 m clasts) breccias are chaotically organized and contain a matrix of two size populations, (a) granules and pebbles which grade into the framework, and (b) red siltstone to very fine sandstone which was subsequently infiltrated. Close to the contact with the basement large voids between blocks are locally entirely filled with the red, fine-grained infiltrated matrix. These chaotic breccias represent ancient screes which mantled the hills of pelitic gneiss and granulite. Well bedded, horizontally-stratified, often finer grained breccias are also present, for example in Millburn valley [4390 3310], north of Fladdabister (Fig. 2). The local normal grading of clasts, rare imbricated fabric and lenticular nature of some units indicate that these beds were deposited by water. Evidence of current working increases upwards and the basal breccia is overlain conformably by entirely fluvial sediments, termed the "Brindister Flags" by Finlay (1926), following Peach and Home (1879).

Alluvial Fan Environment There are two areas in SE Shetland containing fine exposures of the alluvial fan facies. The first is the Rova Head area and the nearby coast of Bressay (Fig. 2). The Rova Head area is dominated by thick conglomerates (up to 5 m thick beds), often inverse to normally graded (Allen 1980), and comprising clasts of quartzite and granite, with smaller proportions of psammite, felsite, schist and foliated granodiorite. The quartzite clasts may originate from the Lower Dalradian Scatsta Division (Flinn 1967; Mykura 1976), but the granite clasts probably come from further afield. Large post-Devonian displacements along the Walls boundary and associated faults obscure the provenance of these granitic clasts. Most of the conglomerates were deposited by eastward and northward flowing water as thick gravel sheets of little topographic relief, but broadly analogous to longitudinal bars. Other conglomerates, comparatively of lesser importance, were deposited from highly concentrated clast dispersions. The hydrological environment was one of flash discharges of short duration but of high velocity. Flow was dissipated rapidly, perhaps due to extreme water loss, in a manner similar to that on the terminal fans described by Mukerji (1976) and Williams (1970) and discussed by Friend (1978). To the east the conglomerates of Rova Head interdigitate with fluvial sandstones and lacustrine siltstones and mudstones. They pass to the south and east into a predominantly fluviatile, sandstone-dominated sequence which has been termed the 'Lerwick Sandstone' by Finlay (1926) (Fig. 2).


WIIV

FLAGSTONE FACIES

PEBBLY SANDSTONE FACIES

CONGLOMERATIC FACIES

BASAL BRECCIA

LIMIT OF STEEP BELT

SUMBURGH HEAD

FIG. 2. Location and geological map of the Devonian SE Shetland basin. Based on Mykura (1976, p. 63).


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The second area of the alluvial fan facies is the Scatness Peninsula in the extreme south (Fig. 2). Thin (up to 30 cm thick beds) matrix-supported conglomerates originated as highly concentrated dispersions or perhaps debris flows. A proximal, high-slope environment of deposition for these beds is thought unlikely and an origin by resedimentation of thicker, framework-supported conglomerates in a manner similar to that postulated by Larsen and Steel (1978) seems more probable. The main outcrops of conglomerates between Sheep Geo and Peerie Vati Geo [HU 3885 0880] display thick (up to 1 m thick) framework-supported beds. These beds are locally normally graded, contain imbricated clasts and are sheet-like in geometry, suggesting that they were deposited as longitudinal or sheet-bars (Boothroyd and Ashley 1975). In the downstream direction some of the longitudinal bars evolve into bedforms with avalanche fronts in the form of high-angle planar crossstratified pebbly sandstones, which are commonly developed at relatively low discharges at the margins of transverse bars (Hein and Walker 1977). In other cases, the gravel bodies had riffle margins of low relief and simply wedge out downstream with no avalanche front development. This may be related to higher water and sediment discharge causing downstream rather than vertical accretion of the bar (Hein and Walker 1977). Streams flowed to the south. Associated standstones were deposited in transverse bars or sandwaves (terminology of Harms et al. 1975), as late-stage ripple trains and, locally, as channels filled with megaripples. Braided Stream Environment Much of the Lerwick area and northern Bressay, the area surrounding Gulberwick, the area of Voe, Dunrossness and the Scatness Peninsula (Fig. 2) is occupied by thick pebbly sandstone sequences with only thin conglomerates. This facies appears to be equivalent to the 'Lerwick Sandstone' of Finlay (1926). Little detailed work has been done on this facies in SE Shetland, but on the Scatness Peninsula at Ness of Burgi (Fig. 2) the cross-stratified sandstones represent the lunate-crested bedforms of lowsinuosity stream channels. Horizontally-stratified conglomerates originated as longitudinal bars which locally evolved in time into transverse features with welldeveloped slip-faces producing high-angle cross-stratified pebbly sandstones. There is a scarcity of fine-grained alluvium in this facies. There are fine exposures of the braided stream facies along the southern shore of Trebister Ness, south of Lerwick. The medium to very coarse sandstones contain both grouped trough and planar cross-sets deposited within active channels, with common channel lags one cobble in thickness. The fine-grained alluvium consists of red siltstones and mudstones with desiccation cracks and carbonate nodules, with interbedded cross- and horizontally-stratified sandstones representing the avalanche fronts and tops respectively of small (up to 20 cm high) linear crested bedforms. Thicker accumulations of fine-grained alluvium occasionally fill abandoned channels and contain rootlet horizons.


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High Sinuosity Stream Environment Much of the Brindister area, Sandwick Peninsula and Cunningsburgh Peninsula (Fig. 2) is composed of fine grained fluvial sediments which have been termed the 'Brindister Flags' by Finlay (1926). The Cunningsburgh Peninsula alone contains a 1 km succession of this facies. Interlaminated siltstones and very fine sandstones represents levee-type sediments deposited by vertical accretion at the margin of the channellized zone. Horizontally- and cross-stratified fine sandstones were deposited as overbank sheet-floods and are similar to crevasse-splays. Red mudstones with sandstone lenses were deposited on an alluvial floodplain essentially from suspension. Large-scale, grouped cross-stratified fine to medium sandstones represent substratum sedimentation in the channellized zone as transverse bars, point bars and megaripple channel fills. Streams flowed from the NW to the SE in the Cunningsburgh area.

Lacustrine Environment Lacustrine sediments are present as generally thin calcareous intervals in the form of symmetrical cycles. These lacustrine intervals are particularly well exposed in the Dunrossness area and in the Ness of Sound area south of Lerwick (Fig. 2). The Devonian lake or lakes of SE Shetland were characterized by four sub-environments. (a) Horizontally-stratified, low angle planar cross-stratified and wave-ripple crosslaminated sandstones were deposited in the surf zone of small beaches and just lakeward of the breaker zone. (b) Red siltstones and mudstones were deposited in small, shallow, oxidizing lakes on a low-gradient coastal floodplain over which waters from the main body of the lake fluctuated. (c) Ripple cross-laminated and horizontally-laminated siltstones with subaqueous shrinkage cracks were deposited in an intermediate environment characterized by continuous to intermittent wave agitation in water depths of probably less than 5 m. (d) Laminated and unlaminated calcisiltites and shaley limestones were deposited in an offshore environment characterized by a lack of evidence for current or wave activity, at depths of greater than 5 to 10 m. Hubert et al. (1976) describe similar lacustrine sub-environments from the Triassic Newark Group of New England. River outflows were dominated by frictional processes (Wright 1977) and competed with shoaling waves which advanced from east to west onto the gradually shoaling lake shoreline, and a southward directed longshore current. The contradiction between a southward lake current and northward winds is only apparent since Coriolis effects and inflowing rivers more strongly affect lake circulation patterns than winds (Nydegger 1976, Hamblin and Cormack 1978, Wright and Nydegger 1980). A river inflow situated in the north of this northern-hemisphere lake would

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theoretically cause lake currents to hug the western shore, which is in agreement with palaeocurrent data. Aeolian Environment Large-scale cross-stratified, non-pebbly, aeolian sandstones are found on the Scatness Peninsula near Geo of Newlands, the area north of Exnaboe at Vaakel Crags and, to a lesser extent, south of Levenwick at Gunstie (Fig. 2). The sediments at Geo of Newlands represent essentially transverse aeolian dunes (commonly 1 m high), interbedded with interdune sandstones and siltstones with adhesion ripples (Glennie 1970). The sediments at Vaakel Crags, north of Exnaboe, have already been tentatively ascribed to an aeolian origin (Mykura 1976, p. 64). The sediments are similar to those at Geo of Newlands in the large size of the cross-sets (up to 3 m), lack of pebbles, the well-developed grain size differentiation between adjacent laminae and the characteristic sedimentary deformation concordant with internal cross-strata high on the avalanche face (Bigarella et al. 1969). Bedforms were probably variously transverse and barchanoid. Prevailing winds were from the S or SW.

Palaeocurrent Analysis For a general appreciation of regional palaeocurrent trends, blocks of data have been constructed by aggregation of cross-stratification attitudes from several localities. In this way 13 blocks of data have been constructed from large-scale crossstratification for study of fluvial and aeolian dispersal patterns. An analysis of lacustrine sediment dispersion has been made possible by measurement of predominantly wave-generated ripple cross-lamination and ripple crestlines. The trend of oriented subaqueous shrinkage cracks has also been used. Figure 3 illustrates fluvial and aeolian dispersal patterns. The fluvial sediments are dominated by a N N W to SSE drainage pattern, with NW to SE trends in some areas such as the Cunningsburgh Peninsula. An important exception is the Rova Head area where sediment was dispersed to the north and east in a distinct depositional system. The rose diagrams for aeolian sediments are in sharp contrast to those from fluvial sediments, and indicate a south to north direction of prevailing winds. Such an up-valley direction was undoubtedly influenced by cross basin morphology. Vector means for each locality indicate two trends in the directional data from lacustrine sediments: (1) north-south directed currents due to lake longshore drift and/or river inflows superimposed upon wave activity, and (2) direct wave advance from east to west onto the lake shore. The first trend originates from wave-generated, wave-modified and possible current-generated structures in nearshore sediments, whereas the second trend originates from undoubted wave-generated structures formed near the transition to the offshore zone. Waves were refracted into parallelism with the gradually shoaling lake margin.


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P. A. ALLEN A N D J. E. A. MARSHALL NORTH BRESSAY

A

ROVA HEAD

B

NORTH EAST

C

SOUTH NESS 8 KIRKABISTER

D

VOE OF SOUND

BRESSAY

E

TREBISTER a

F

NESS OF SOUND

BRINDISTER

6

CUNNINGSBURGH PENINSULA

H

SANDWICK PENINSULA

I

VAAKEL CRAGS

J

SOUTH EXNABOE

K

SCATNESS

L

SUMBURGH

M

NESS OF BURGI

HEAD