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Caithness Field Club Bulletin | ||||||||||||||||||||||||
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THE ORKNEY SCIENCE FESTIVAL September 11-14,1993 I was invited to give a paper on the fossil fishes of Orkney at the 0rkney Science Festival and to lay on a display of fossils at the High School in Stromness. I was very impressed with the lecture theatre. Within the Science Festival there was a small symposium on the Framework of the Orcadian Basin, a synopsis of which is given below . GENERAL GEOLOGY OF ORKNEY The Rocks and Age The rocks exposed in Orkney are predominantly flagstones and sandstones deposited in a fresh-water lake. They belong in time to the Devonian (old Red Sandstone) period of earth history (409-363 million years ago). Contemporaneous igneous rocks include basaltic lavas and volcanic vents filled with agglomerate and ash. Younger Permian aged (250 million years ago) intrusive lamprophyre trap dykes are numerous. The oldest rock exposed in Orkney is the Precambriam Basement Complex of metamorphic schists and granitic gneiss formed about 1500 million years ago. These outcrops represent the tops of island hills emerging from the early lake. Lake Orcadie Located in an Equatorial Mid-continental Desert. During the Devonian Period, North America and Northern Europe were joined together forming "Euramerica" one of the three major continental masses on the Devonian globe with Orkney placed south of the equator. Most of Britain formed part of this land mass with mountains in the north-west and the open marine "Devonian Sea" in the south covering south west England. In between the mountains and the sea was a vast desert plain with the lowest topographical levels occupied in a large, shallow, fresh water lake extending from Shetland through Orkney, Caithness, the Moray coast and across to western Norway. This shallow lake is known as "Lake Orcadie". Throughout its existence the lake was fed by numerous rivers flowing from the western mountains carrying sand and mud to fill the lake depression. Cyclic Sedimentation Evaporation in this tropical climate was high and rainfall seasonally variable, resulting in fluctuations in the lake depth and area covered by water, giving rise to extensive lake margin mudflats which were seasonally drowned and dried out. Superimposed on this on this seasonal cycle of wet and dry periods was a much longer climatic periodicity, perhaps on the order of tens of thousands of years, where the lake got steadily deeper and covered even larger areas lapping up to the edge of the western mountains and then rapidly retreating to begin this cycle all over again. These cycles are easily observed in the well exposed coastal sections of the West Mainland where the major cycles are approximately 15-20 meters thick. The cycle starts with shallow water lake margin sediments consisting of sands, silts and light grey muds and ends with a finely laminated dark grey to black muddy flagstone with fossil fish fragments representing periods of greatest water depth in "Lake Orcadie". This cyclicity of lake deposition continued for much of the Middle Devonian until the Upper Middle Devonian and Upper Devonian times when the "Lake Orcadie" waters retreated so far that it was broken up into many small separate lakes in a predominantly desert landscape. This dry period is characterised by sandstone deposits derived from the large braided river channels which continued to flow from the Western Mountains and the sand dunes which migrated across the desert plains. Two intervals of increased rainfall during this period saw the advance of the lake once again, covering over the desert floor but these advances were short-lived and perhaps did not cover the same area as "Lake Orcadie". Talks The structure of the Conference [was] kept fluid so as to accommodate any late changes in talks; the more general topics ... placed in the public sessions, with the specialist subjects discussed in the evening sessions. Field Trips Of particular interest in the field trips [was] the visit to a breached sandstone reservoir where the residual oil is preserved as bitumen showing clearly the relationships on the macro and micro scale of hydrocarbon distribution in such a reservoir. Other locations [included] ... the excellent shore-line exposures of lake sediment evaporate mineralisation fossil fish locations and the aeolian sequences. During the field trips there will be [a visit] to several of the major archaeological sites in the Islands such as Skara Brae and Maeshowe. Exhibition of Fossil Fish Jack Saxon of Thurso [presented] a talk on the fossil fish of the Orcadian Basin. This [was] accompanied by an open exhibition of his fossil fish in the foyer of [Stromness) Academy. OtherActivities The Orkney Science Festival [had] a large programme of talks and exhibitions. Delegates' entertainment [included) an opening ice-breaker for participants reception by the Orkney Islands Council and a "Ceilidh" with local music and whisky. This symposium was very exciting since it brought together people with quite ditterent backgrounds and interests. Dr. John Marshall was concerned with the palynoloqy (or the study of spores) of the Devonian Orcadian Basin. Spores are virtually indestructible and so form useful material for study. Most paleobotanists who visit the Orcadian Basin are struck by the paucity of plant fossils. According to Marshall the palynological studies revealed that a rich flora existed. In the papers by Hillier and Marshall(l) they also discuss the Thermal history and the hydrocarbon generation. They consider that the Orcadian Basin may he considered as a type of continental rift basin. Generally rift basins are characterised by high but variable geothermal gradients and above average levels of organic maturation at relatively shallow depths. They assume that contact metamorphism may be invoked. The regions affected by contact metamorphism are all in close proximity to the Great Glen/Walls Boundary Fault suggesting that this line of weakness may have acted as a conduit for magmas toward the surface. They also suggest that the hydrocarbons of the Beatrice Field may have originated in the Devonian Sediments. Two of the text figures from the paper are of interest to the geology of the basin. Dr. Tim Astin(2) and Dr John Marshall are both concerned with the stratigraphy of the Orcadian Basin and are working on a complete review of the problem. They were surprised to find that I had been counting cyclothems as early as 1965 which is part of the method for determining climate cycles. Dr. Marshall also gave a talk on Devonian sediments in Greenland. I have only seen Greenland from the air and it seemed to me that the poetic expression "Greenland's icy mountains" just about summed it up. Their expedition was, surprisingly, to a relatively snow-free area. Although his work was primarily on palynology he did find a fragment of fossil fish firmly attached to a piece of eruptive rock as though the fish had been baked on during a volcanic "barbecue". I suspected that the fragment was of Groenlandaspis, a very rare fish mentioned elsewhere in the Bulletin. The NIREX boreholes in Caithness were also illustrated in one of the lectures. I would like to have seen more of these cores (and been involved at the time). I would also have liked to have more information on the gamma ray signatures from the boreholes which everybody but me seemed to be familiar with. Tony Hewett gave a very polished performance on the Exploration Well 12/29-2 which gave a complete Devonian sequence of over 5000 feet (1524m) interpreted from a combination of paleo- and lithological indications combined with seismic evidence of sequences and unconformities. An extract from his paper is given below. "The Devonian succession penetrated by I2/29-2 from 6047 ft to 11460 ft has been broken down as follows-
A core was taken at the base of a 232' basement internal of Ordovician age. BASEMENT AGE
DESCRIPTION
FORMATION (1a) Siegennian Fining Upward Sequence The sequence comprises two major fining upward sequences. The lower sequence is 370' in thickness and the upper sequence is 182' thick. The lower sequence has a 140' thick sandstone at its base which consists of clear to pink, fine to very fine, angular quart grains. The sand is well sorted but has increasing amounts of mica and shale upwards, before grading completely into shale. The sand is well cemented by ferroan calcite and dolomite. The sand grains are tightly packed to the extent of being sutured and as a result porosity is very low. The sand grades upwards into claystone through calcareous siltstones. Two types of claystone are present, dark grey to black, soil fissile shales with a high organic content. Both the types of shale have a high calcareous content, and within the shale sequences trace amounts of limestone and dolomite am seen. The interval is interpreted as consisting of two upward-fining sequences. Terrestrial to fluvial-deltaic sands grade upwards into organic and non-organic calcareous shales representing marginal lacustine conditions. Humic materials predominate over algal organic matter suggesting impermanent lacustrine influences. Evidence for Siegennian or latest Geddian age was noted in the upper part of this interval due to the presence of diagnostic palynological forms. (Emphanisporites rotatus and representatives of Retusotriletes and Apiculiretusispora and absence of spores indicating a later age) . (1b) Emsian Lacustrine Shale Sequence This sequence represents a continuation in style of the uppermost Siegennian lacustrine shale unit and is 1158' in thickness. The sequence consists mainly of interbeds of light grey green shales and dark grey shales, which are organically rich with oil shows, together with thin interbedic of limestones and sandstones. Shales are fissile, medium to hard and calcareous with traces of pyrite. Mica is often abundant and a high gamma ray response of around 100 API units is seen throughout the shale sequence. Good oil shows were recorded from the geochemical analysis of cuttings throughout the interval. The sequence becomes more sandy in the uppermost 150' in a gradual coarsening upwards reflected by the gamma ray response. Sands are clear, fine grained and well sorted with a calcareous cement and often exhibit a weak crush cut. Sands often grade from siltstones of similar character. The sequence is interpreted to be of alternating terrestrial and marginal lacustrine deposition. Towards the end of the sequence evidence of increased clastic input is present in the form of increasing amounts of very fine sand. An Emsian age is suggested by diagnostic palynoforms within the lower part of this interval but the upper part is barren. (Larger camerate spores and Ephanisporites annulatus) (2a) Lower Middle Devonian A 1650' sequence of sandstones and conglomerates overlies the Emsian shale sequence with marked unconformity apparent from lithology and on seismic. The sequence displays a moderately constant log response of 50 API units, with slight increases apparent where zones of more conglomeratic material occur, often around 20' thick. The sands and sandy conglomerates are purple red, red brown, pink and grey in colour. Sands are fine to medium grained and consist of angular to subrounded quartz grains poorly cemented by calcite. Conglomerates fine upwards into sand and even siltstone, and contain quartz clasts together with green-black dolerite and mica-chist clasts. The massive coarse clastic sequence is interpreted as a repeated series of fining upwards cycles deposited in a subaerial environment as a continental alluvial fan. An arid environment is suggested by dominant oxidation appearance and lack of biota. A clastic source direction from the south, is further suggested by the presence of basic igneous fragments, which are exposed onshore to the south of the Moray Firth. Mica schists are also present to the south and have derivatives present in the 12/29-2 conglomerates. The interval is palynologically barren. (2b) Massive Sandstone Unit A 700' sandstone unit, with minor conglomerates, overlies the conglomeratic sequence. Log character shows an overall gamma ray increase upwards which appears to relate to poorer sorting and increased amounts of conglomeratic material towards the top. The dominant lithology is sandstone, clear to orange in colour, fine to medium and occasionally coarse grained in size and consisting of angular to rounded, mainly well-sorted quartz grains. The sand is only poorly cemented by calcite and has a log porosity of 23%, even though it occurs at a depth of below 7400'. The sand becomes more poorly sorted upwards with increased amounts of conglomerate clasts of quartzite, gneiss and dolerite. The sequence is interpreted to be a distal fluvial alluvial sheet flood sandstone deposited in an arid subaerial environment. (2c) Upper Conglomerate Unit An upper conglomerate unit 665' in thickness overlies the massive sandstone and displays a markedly high gamma ray response of nearly 100 API units consistently throughout. The conglomerate consists of abundant angular fragments of quartzite ranging in colour from grey-green to dark grey to black dolomite fragments, again suggesting a southerly provenance. The conglomerate has a matrix of sand, siltstone and claystone. The sand is fine-grained and the claystone grades to siltstone and is black, hard, micaceous and partly cemented by calcite. The conglomerate is interpreted as a proximal subaerial alluvial fan. (2d) Upper Shale Unit (mid Eifelian - Earliest Givetian) A 221' thick sequence of silty claystone overlies the upper conglomerate. The interval ranges in colour from red-brown, dark brown and grey-brown shales, which are generally barren, to medium to dark grey shales which have been dated Middle Devonian. The shales are silty, subfissile and very micaceous and are also calcareous. The interval is interpreted to represent a sequence of fine terrestrial basin deposits in an alternating subaerial and lacustrine setting which represents age equivalents of pans of the thick Caithness Flags Laminate sequence seen exposed onshore in NE Scotland. Evidence in particular of Achanarras Fish beds interval or Lower Thurso Flagstone group is considered to be present in the palynology from this interval (by the presence of Ancyrospora, Rhabdosporites and Aurora/spora). (3) Uppermost Devonian Barren Sequence The upper part of the higher gamma ray sequence consists of mixed fine clastic interbeds of white, fine grained sandstone, brown siltstone and orange brown claystone. The fine clastics are very micaceous and calcareous apart from the claystone. The uppermost 170' of the Barren Sequence is a fining-up interval comprising a clean well-sorted sand unit of around 80' thickness which consists of fine to medium grained angular quartz grains well cemented by calcite. Above this clean unit are red-brown claystones and siltstones with purple grey, poorly sorted sandstones containing large amounts of biotite mica and traces of pyrite and progressively increasing amounts of anhydride upwards through the sequence. The interval is interpreted as a combination of alluvial and sheet flood deposits resulting farm intermittent flash floods within an acid continental basin. " The least scientific contribution was from Scottish Natural Heritage. They seemed to be mesmerised by the high-sounding aims set out by the Government when they down-graded it from the Nature Conservancy. I suggested a few practical steps like interpreting the geology using notices: specifically I suggested one at Loch Glencoul showing both the Glencoul Thrust and the Moine Thrust and another at Assynt indicating the double unconformity. They claimed that there was no money for that kind of thing. The entire budget was committed to building a larger hut at Knockan Cliff! It was a disappointing contribution. However the field trip was quite exciting. In addition to the itinerary below, on the Yesnaby leg we saw a medieval boundary dyke, a bronze-age earthwork and, whilst looking at a volcanic vent identical in mineralogy to the diamond-bearing pipes of Kimberley, I also stumbled across a burial cist. GEOLOGY AND LANDSCAPE IN ORKNEY Scottish Natural Heritage Three sites demonstrate some of the many processes which have formed and moulded the islands from around 1200 million years ago to the present. All the localities on the field trip are designated as Sites of Special Scientific Interest (SSIs) because they contain features of national importance. The purpose of this designation is to protect and preserve these features for future generations and to enhance our understanding of Scotland's diverse natural heritage. SITE 1 Yesnaby Cliffs At this locality [are] the oldest rocks in 0rkney - the basement Moine rocks. There is a volcanic vent. ....The rocks of the cliffs .... were deposited as sand dunes and lake sediments during the Devonian Period, some 400 million years ago. Algae grew within these lakes and these are now preserved as unusual banded fossils known as stromatolites. The coastal landscape at Yesnaby has formed since the last ice age and is still actively eroding today. The cliffs, stacks and geos .... are some of the most spectacular in Scotland. SITE 2 Bay of Skaill The Bay of Skaill is a very different coastal environment from Yesnaby, though just a few kilometres away. The soft, sandy sediments around Skara Brae indicate coastal deposition in the past, though erosion has become the dominant process in recent years. [There is] evidence of these fluctuations in the position of the coastline and modern beach processes are forming the sandstones of the future. SITE 3 Cruaday Quarry This is an important SSSI because It contains some of the best preserved fossil fish in Great Britain and so is of considerable value in unravelling the evolutionary development of different fish groups. The range of fossils present also allows the rocks in Orkney to he correlated with those elsewhere in Northern Scotland and has helped establish the position of a great inland lake which covered the region nearly 400 million years ago, when the fish lived. I had never been out with petroleum geologists before. A hard rock geologist cracks open a rock to see fresh structures which he studies with a lens. A palaeontologist looks for bones or shells and only cracks a rock which he needs to collect. The petroleum geologist cracks a rock and sticks it under his nose with the speed of light. He is sniffing for aromatics. Frequently one can see thin horizons in oil-bearing rock with bands of oil "tide marks". Dr. Marshall took us to an Eday Beds locality where he had found undoubted marine fossils; tiny, but there was no doubt about it. We also studied halite pseudomorphs. I had never seen them before, but once seen, these cubic depressions on bedding planes can be seen to be the impressions of crystals. Tim Astin showed me a fish bed where he had measured the oxygen content profile. As expected, in a lacustrine sediment under thermal climate cycling, the highest content was in the middle of the bed. I also asked him for any explanation of the change from alternating dark and light cyclothem for the Wick Beds to a light and light cyclothem for the Thurso Beds. His explanation was that the Wick Beds had been 'cooked' (see the cycle maturation map from the paper by Hillier and Marshall given earlier). I must say that this idea came as something of a surprise. I've seen plenty of contact metamorphisms but I hadn't recognised it in the Wick Beds. All in all I think the papers in the symposium deserved to be published in full. I have been at many an international symposium where the papers have been no more interesting than these. Let us hope that there will be more of the same. Jack Saxon References 1
Hillier, S and Marshall, J.E.A. Organic maturation, thermal history an,
hydrocarbon generation in the 2 Astin, T.R. The Devonian Lacustrine sediments of Orkney, Scotland; implications for climate cyclicity, basin structure and maturation history. J. Geol. Soc. Lond., Vol. 147,1990, pp 141-151. |