n 4 4 4 4 kvolutionary and geological events across the Precambrian- Cambrian boundary M. D. BRASIER

Over the last decade, palaeontologists around the world have been striving to understand one of the most remarkable episodes in the fossil record: the explosive evolution of skeletal fossils around the Precambrian-Cambrian boundary. When examined in the field, the close association of geological and biological events is apparent. A n important succession in this respect is the classic Precambrian and Cambrian area of Chamwood-Nuneaton in the English Midlands. Here it can be seen that the boundary also spans a time of major tectonic and oceanographic changes. Could these have contributed to the 'Cambrian evolutionary explosion'?

T h e remarkable sequence of events that took place on Earth about 700-500 million years ago (Fig. 1) cannot be seen in any single geological succession. There are several places where much of the story is preserved, such as the remote West Anabar region of Arctic Siberia, the Yunnan Province of China, and the foggy penin- sulas of south-eastern Newfoundland. Fortu- nately for those who would like to see the evidence for themselves, the more accessible succession in the English Midlands is yielding new fossil data, proving it to be almost as complete as any other. It contains igneous and tectonic events which are an important part of the story and therefore provides a standard sequence referred to here.

Up to about 700 million years ago, the Pre- cambrian fossil record is sparse, dominated by simple benthic and planktonic algae. Evolution had certainly taken place since the earliest algal and bacterial blooms over 3000 million years ago but was mainly concerned with improvements in cellular organisation and reproduction. The 'prehistory' of animals in this interval is obscured by their poor potential for preserva- tion, but there are rare and poorly documented reports of faecal pellets and burrows up to 1000 million years old. A fauna of rather small, soft bodied and inconspicuous protozoans, platy- helminths, annelid worms and cnidarians (e.g. jellyfish) is thought to have been developing at this time.

From about 700 to 500 million years ago, a series of distinct evolutionary bursts increased the abundance and diversity of the animal fossil record (Fig. 2 ) . When examined in the field, the association of biological and geological events becomes apparent.

Fig. 1. Geological and biological events near the Precambrian-Cambrian boundarv, with reference

Late Precambrian ice age This, the last major Precambrian ice age, affected most continents and reached to low to the seauence in the

END-CHARNIAN REGRESSION

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GEOLOGY TODAY Sept-Ocr 19851141

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The deposits are therefore preserved in tectonic basins, and the platforms were mainly emergent. Late Precambrian tillites are not seen in the English Midlands, though they are developed beneath beds with a Chamia fauna in south- eastern Newfoundland. The Port Askaig tillites of Islay in Scotland are a classic example; here glacial deposits and dolomites occur together, indicating extreme fluctuations of climate. No animal remains have been recorded from this level, although Planolites burrows appear below the tillites in Donegal and faecal trails (Neo- nereites) occur above them on Islay. Planktonic microfossils show that the older Riphean microflora was suffering extinction during this glacial period.

First invertebrate impressions An evolutionary radiation of soft-bodied meta- zoans followed soon after the late Precambrian ice age, during the Vendian Period. The fossils mainly consist of large impressions of cnidarians (cf. jellyfish, colonial hydrozoans) with some flat annelid worms, possible ancestors of the arthro- pods, and other problematica.

The Chamia fauna is rare but almost world- wide, with cosmopolitan elements, such as the spindle-shaped Chamia and disc-shaped Cyc-

Fig. 2. Summary chart of the ranges of invertebrate groups across the Precam brian-Cambrian boundary interval.

Benthic: relating to benthos, the marine life that inhabits the ocean floor.

Planktonic: relating to plankton, aquatic organisms that drift, or swim very weakly.

Fig. 3. Impression of a colony of Charnia-like animals and small Charniodiscus (bottom) ir: the Precambrian Woodhouse Beds of Charnwood. ( x 0.8 approx.)

lomedusa and more restricted elements, like the ?arthropod Pseudovendia , associated with simple ‘worm burrows’ such as Planolites. These can all be seen in the Woodhouse Beds of Charnwood Forest, north-west of Leicester, and were the first indisputably Precambrian animal remains to be described. Some remains await full des- cription (e.g. Fig. 3) and are also known from other horizons in the Charnian. They compare closely both in setting and composition with the Mistaken Point fauna of south-eastern New- foundland.

It is not yet possible to establish evolutionary stages in diversification of the Charnia fauna. Some believe it to be older than the Ediacarian fauna of South Australia; the evidence for this rests largely on a potassium-argon date of 684 k 29 million years obtained from an underlying Charnian intrusive, and on comparison with lower Vendian faunas from the Soviet Baltic. There is no indication that these Charnian and Ediacarian faunas were transformed into the succeeding skeletal fauna of the Cambrian. The preservation of this Precambrian assemblage of large, soft-bodied animals may owe much to the lack of predators and scavengers at that time; and its disappearance from the fossil record about 550 million years ago coincided with the first vigorous exploitation of sediments for food and shelter by burrowing animals, shortly pre- ceding the appearance of fossils with hard skeletons and effective jaws. Both changes also coincided with major tectonic and oceano- graphic events.

Caldecote igneous events Volcanic eruptions were contemporaneous with the Chamia fauna in England and Newfound- land and appear to have reached a climax some time afterwards. Charnian rocks can be traced by geophysical methods beneath Triassic mark

1421GEOLOGY TODAY Sept-Oct 1985

to Nuneaton, with perhaps up to 10 km thick- ness of intervening volcanic or sedimentary rocks. The Caldecote volcanics of Nuneaton (Fig. 4) comprise water-lain tuffs, followed by subaerial crystal tuffs, intruded by basaltic and dioritic dykes dated nearby at about 540 k 58 million years. This rubidium-strontium whole- rock isotopic date provides an important datum point for the Precambrian-Cambrian boundary interval.

The Charnian-Caldecote igneous rocks are the remains of an island arc, active until about 540 million years ago. The Caldecote Igneous Event suggests active plate subduction, indicated by vulcanicity and igneous intrusion of similar age throughout England and Wales and also in south-eastern Newfoundland, New Brunswick, Brittany, the Channel Islands, Spain, Morocco, Israel and Arabia. In England it may be taken as a terminal Precambrian event, punctuating the change from a volcanic to a platform setting. It has been suggested that a Precambrian ‘super- continent’ began to break up at about this time, leading to the expansion of an ‘Iapetus Ocean’ lying between North America, the Baltic and Gondwanaland. The eustatic rise in sea level that soon followed probably relates to this increased production of seafloor.

End-Charnian regression In every region where it can be studied, there is a break in the record of animal ‘body fossils’ between the top of the Chamia or Ediacarian fauna (which suffered widespread extinction) and the first assemblage of skeletal fossils (Fig. 4). Organic remains are scarce in this interval and are largely confined to algal stromatolites, vendotaeniid algae or horizontal trace fossils of Precambrian type, preserved in subsiding basins (e.g. the Kotlin Stage of the Soviet Baltic). A global regression is indicated, particularly at the end of Kotlin times.

Cambrian transgressions and the first diverse animal traces From this point onwards, the geological record is characterised by a series of transgressions that reversed the essentially regressive state of con- tinental shelves, progressively flooding the mar- gins to remote interiors of continents during the Cambrian and Ordovician. The Triassic to Cre- taceous transgressions serve as a parallel and both were apparently linked with the widening of new ocean basins.

It is with the early stages of this Cambrian transgression that the explosion of skeletal fos- sils is associated. Sea level seems to have risen in a series of transgressive-regressive pulses, each of which contained a distinctive fauna. Because of the pattern of the transgression, the first ‘Cambrian’ elements tend to be littoral or shal- low sublittoral, whereas deeper water groups

make their appearance and evolutionary burst later, especially in the Middle Cambrian to Ordovician.

In many regions, the early stages of the trans- gression tend to be found in coastal sandstone facies; but unlike the underlying sandstones, they contain an increasingly abundant and di- verse assemblage of trace fossils (Fig. 5). These are witness to an explosive evolution of animals without mineralised skeletons and to the vigor- ous and increasingly efficient exploitation of nutrient-rich seawater (suspension feeders) and sediment (deposit feeders). An example of such an assemblage can be found above the base of the Hartshill Formation at Nuneaton, where the intense activity of simple Planolites, U-shaped ‘worm’ burrows of Arenicolites and vertical pipes of Monocraterion led to bioturbation of the sedi- ment. The resting traces of arthropods (trilo- bites?) first appear about 240 m above the base of t!ie Hartshill Formation.

On the Siberian platform, the initial trans- gressive phase is mostly represented by littoral dolomites of the Yudoma Formation. But sedimentary depressions preserve ‘windows’ of fossiliferous limestone, where the earliest ske- letal fossils are found. This ‘Manykayan’ fauna comprises one or two species of protoconodont (Protohertzina), tubular Anabarites and hyoliths with simple round cross-sections (e.g. Circo- theca). Their occurrence with annulated pogo- nophoran tubes of Sabellidites compares with the Rovno Beds in the Soviet Baltic.

First diverse small shelly fossil assemblage In many regions, including the classic type region for the Tommotian fauna (Aldan and Lena Rivers of Siberia), the first skeletal faunas are associated with glauconitic and phosphatic sediments at the base of transgressive pulses. At Nuneaton this fauna is found in lag conglomer-

Fig. 4. The unconformable ‘Precambrian-Cambrian’ boundary at Nuneaton. Here are seen the subaerial crystal tuffs of the topmost Caldecote volcanics (Precambrian), weathered into spheroidal masses and covered by basal Hartshill sandstones of the Cambrian transgression. Foreground about 3 m wide.

Tillite: a sedimentary rock formed by the consolidation of glacial till, a mixture of clay, silt, sand, gravel and boulders deposited by a glacier.

Tuff a rock consisting of small fragments of volcanic material that have been welded together by heat or stuck together by percolating solutions.

Stromatolite: a structure produced by the trapping and binding of sediment by micro-organisms, principally blue-green algae.

Littoral: relating to the environment between high and low tide levels.

GEOLOGY TODAY Sept-Oct 19851143

that indicated by the Rispbjerg sandstone of the Baltic.

Fig. 5. Bedding plane covered in meandering traces, from the Lower Cambrian Hartshill sandstones at Nuneaton.

Bioturbation: the churning and stirring of a sediment by organisms.

Lag: relating to a deposit from which the finer grains have been removed by wind or currents leaving behind the coarser grains.

Diachronous: referring to a rock or fossil unit which, though continuous, formed in different areas at different times.

ates and sandstones at the base of the Home Farm Member, near the top of the Hartshill Formation. Its fauna broadly compares with that found in the Sunnagin horizon of the Aldan (Fig. 6): diminutive remains of phosphatic and calcareous tubes (e.g. Hyolithellus, Coleoloides), enigmatic phosphatic scales (e.g. Sunnaginia im- bricata), hyoliths with simple rounded cross- sections (e.g. Turcatheca), and inarticulate brachipods.

In Siberia, the Tommotian fauna also contains unique reefal associations of sponge-like archaeocyathids and algae, which are absent at Nuneaton, but the sequence of small shelly fossils is remarkably similar. The fauna of the Hyolithes limestone still awaits complete descrip- tion but yields an evolving fauna of brachiopods, hyoliths, protoconodonts, problematical tubular fossils, and scale-like sclerites indicative of a lower Tommotian age through to Lower Atdabanian age. Molluscs are represented by minute , cap-shaped ?monoplacophorans (Bemella) and the fauna is dominated by long calcareous tubes of Coleoloides and sponge spi- cules. Both at Nuneaton and the Aldan, there- fore, the fossil record was transformed within a few metres of section.

Develo ing small shelly fossil assemb Y age and the Woodlands regression The next faunal assemblage in the Hyolithes Limestone at Nuneaton contains a distinct and diverse suite of small shelly fossils. Here appear abundant tubular Hyolitha, large mono- placophorans (Randomia), the first sinus-bearing gastropods (Prosinuites) and bivalves (Fordilla) plus new species of brachiopod (Paterina sp. A). Sunnaginia n. sp., and other problematica (Fig. 6 ) . The sculptured protoconodont Rhombocor- niculum insolutum provides an index for inter- national correlation, since it also appears at the base of the Atdabanian Stage in Siberia. The Woodlands sandstone above this assemblage at Nuneaton records a regression, comparable with

First olenellid trilobites The Atdabanian is notable for the widespread appearance of mineralised arthropod skeletons, mostly trilobites and ostracods (Fig. 2 ) . These were probably present earlier, but seem to have evolved the ability to impregnate chitin with calcium salts at a later stage. Many arthropods never adopted this habit, as can be seen in the Middle Cambrian Burgess Shale; and the first appearance of trilobite skeletons was probably diachronous.

At Nuneaton, olenellid trilobite remains appear about 20 m above the Rhombocorniculum insolutum datum, just above the base of the Purley Shales. In Siberia, the first trilobites also appear above the base of the Atdabanian Stage (i.e. Profallotaspis). In muddier sequences, where the plankton may be well preserved, an enrichment of phytoplankton is also recorded near the Tommotian-Atdabanian boundary. Both plankton and shelly benthos seem to have flourished in response to a widespread trans- gression that flooded the ancient landmasses of Scandinavia at this time.

First eodiscid trilobites Towards the end of this stage there also appeared the hrst echinoderm skeletons (in North America) and eodiscid trilobites with a widespread distribution and probable plank- tonic mode of life. An example is the Serrodiscus bellimarginatus assemblage found about 70 m above the base of the Purley Shales of Nuneaton, and in the lower part of the Comley Limestone of Shropshire. There the microfossil index is Rhombocorniculum cancellatum.

First protolenid trilobites and echinoderms This phase in the evolution of the skeletal fauna was a time of extensive reefal development and of diversifying assemblages of trilobites, hyoliths and echinoderms (Botomian stage of Siberia). Broad areas of shallow shelf seas were now available for exploitation as a result of the con- tinuing rise in sea level. This assemblage, with the first protolenid trilobites, can be seen near the top of the Purley Shales and in the upper beds of the Comley.Limestone.

End-Comleyan regression Towards the end of Lower Cambrian time, faunas became more impoverished and there is widespread evidence for emergence of shallow platforms. At Comley in Shropshire, this is well seen at the top of the Comley Limestone. The sandy, phosphatic conditions of the early Tommotian returned, but with a different

144lGEOLOGY TODAY Sept-Oct 1985

Fig. 6. Some small shelly fossils typical of the Precambrian-Cambrian boundary beds in both England and Siberia. (a) Camenella baltica, a phosphatic scale from a ?mollusc-like creature that had a multi-element skeleton. (b) Sunnaginia imbricata, another kind of phosphatic scale. (c) Chancelloria lenaica, a calcareous star-like assemblage of unknown affinity. (d) Coleoloides typicalis, a striated calcareous tube of unknown affinity. (e) A hexactinellid sponge spicule. (f) Torellella lentifomis, a curved phosphatic tube of unknown affinity. (g) and (h) The primitive molluscs Bemella jucutica and Bemella pauper respectively. (i) Prosinuites emarginatus, a possible gastropod viewed from the sinusiate ?anterior end. ( j ) ?Tuojduchithes biconvexus, a hyolith. (k) Rhombocorniculum jnsolutum, a phosphatic protoconodont. (I) l’aterina sp. A , , a primitive phosphatic inarticulate brachiopod. Scale bar is about 200 Fm except for the following: (a) and (d) about 140 pm; (i) about 330 km; ( j ) about 1200 k m ; (1) about 470 Fm.

assemblage of brachiopods and small shelly fos- sils (e.g. Lapworthella nigra) and was followed by local erosion, leaving a gap in the fossil record.

The succeeding Upper Comley Sandstone contains reworked Lower Cambrian fossils at its base; i t bears a different fauna of trilobites, characterised by the ptychopariid Paradoxides and agnostid arthropods. During the interval between these contrasting units, archaeocyathid

reefal assemblages, olenellid trilobites and many of the early problematical organisms seem to have suffered widespread extinction. The fossil record continues with its familiar pattern of evolution and extinction, while trilobites assume their special value for Cambrian biostratigraphy from about this point.

Evolution and geology Seen in this way, it is not difficult to appreciate that the evolutionary bursts and extinctions took place during a time of widespread tectonic, climatic and oceanographic changes. Here we may suggest a tentative hypothetical scenario for this remarkable sequence of events.

Active subduction and island arc growth, represented by the Charnian, may have been connected with the close proximity of continen- tal plates in the late Precambrian. This, and variation in the obliquity of the Earth’s axis of rotation, were possible causes of the late Pre- cambrian ice age, while extremes of climate led to the extinction of Riphean microplankton. Milder post-glacial climates allowed develop- ment of the Charnia fauna but this, too, was partly exterminated during the terminal Precam- brian at a time of regression and the Caldecote igneous event; volcanic ash or dust may have obliterated much of the biota. Rifting, with the formation of new ocean floor and passive mar- gins, resumed near the Precambrian-Cambrian boundary, initiating rising sea levels with ame- liorating climate, and encouraging development of the new skeletal fauna and ichnofauna.

An increase in the ratio of ‘heavy’ sulphur isotopes (the Yudomski event) and increased phosphorus abundance took place in shallow waters at about this time, possibly from volcanic activity and increased upwelling of mineral-rich deep ocean waters. This allowed a bloom in the biomass of phytoplankton, benthic algae and invertebrates. The increased abundance of food encouraged both sessile suspension feeders and vagrant deposit feeders and led to an ecological chain reaction resulting in the evolution of pre- dators and scavengers. Activities of the latter were damaging to the Chamia fauna and Pre- cambrian stromatolite flora but encouraged the development of a diverse new Cambrian fauna with exoskeletons. Initially, many of these skeletons were organic or impregnated with abundant calcium phosphate, but as the phos- phogenic climax passed, the more widely avail- able calcium carbonate was preferred. Over this period, rapid plate movements led to rising sea levels and the expansion of geographically di- verse marine habitats; shallow marine biotas developed and radiated first, with sequential radiations of progressively more offshore com- munities as the transgression proceeded. Early skeletal faunas were relatively cosmopolitan, suggesting proximity of plates, but continental

GEOLOGY TODAY Sept-Oct 19851145

Darcy: the standard unit of permeability, being the passage of 1 cc of fluid of 1 centipoise viscosity flowing in 1 s under a pressure differential of 1 atmosphere through a porous medium having a cross-sectional area of 1 sq cm and a length of 1 cm. (1 millidarcy = 0.001 darcy.)

drift encouraged increasingly provincial differ- ences. Periods of reduced seafloor spreading led to episodic regressions connected with extinc- tions, such as those near the end of the Lower Cambrian.

Suggestions for further reading Brasier, M.D. 1979. The Cambrian radiation

event. In: The Origin of Major Invertebrate Groups (Ed. M.R. House). pp. 103-159. Academic Press.

Brasier, M.D. 1984. Microfossils and small shel- ly fossils from the .Lower Cambrian Hyolithes Limestone at Nuneaton, English Midlands. Geological Magazine, v. 121, pp. 229-253.

Brasier, M.D. & Hewitt, R.A. 1979. Environ- mental setting of fossiliferous rocks from the

uppermost Proterozoic-Lower Cambrian of central England. Palaeogeography , Palaeo- climatology, Palaeoecology, v. 27, pp. 35-57.

Brasier, M.D., Hewitt, R.A. & Brasier, C.J. 1978. On the Late Precambrian-Early Cam- brian Hartshill Formation of Warwickshire. Geological Magazine, v. 115, pp. 21-36.

Cook, P. J. & Shergold, J.H. 1984. Phosphorus, phosphorites and skeletal evolution at the Precambrian-Cambrian boundary. Nature, v.

Glaessner, M.F. 1984. The Dawn ofAnimal Life. 308, pp. 231-236.

Cambridge University Press.

M . D . Brasier is Lecturer in Geology ar the Uni- versity of Hull.

~~

Oil in Chalk JAKE HANCOCK

Chalk is not the sort of rock that you would expect to yield oil. Although it has a high porosity whenfirst formed, the permeability is low. Even the porosity is normally much reduced under overburdens of more than 1000 m. Yet in the North Sea there are special conditions that allow major reservoirs in the Chalk beneath thousands of metres of Tertia y sediments.

Texture of the Chalk If I had been asked 20 years ago - not that I was - on the possibility of the Chalk being a reser- voir-rock for oil under the North Sea, I should have thought in terms of its properties as a water-reservoir. The porosity (the amount of space in the rock not occupied by solids) of the Chalk in southeast England is high; 3547%, plenty of space for water or oil.

To be a useful reservoir the rock must also have a high permeability; that is, the liquid must be able to flow through the rock easily. In the Chalk this depends almost entirely on joints and fractures. In a lump of unfractured chalk the pores that can hold the liquid are very small, measurable in micrometres, and the connections between the pores are even smaller - i.e. the matrix permeability is very low, typically 6- 8 mD (millidarcies). Water can only flow very slowly through such a rock; and oil, with its higher viscosity, would be even slower. On the other hand, the passage offered by fractures may be good; and if those fractures and joints have been enlarged by solution through rainwater, as has quite often happened onshore, the mass permeability may be startingly high: water- velocities of more than 90 m per hour have been recorded.

These properties of the Chalk make it an important natural reservoir for water in Eng- land. Offshore, under the North Sea, the situa- tion is very different. This chalk has never been exposed to rain and there is no reason to expect enlargement by solution along joints and frac- tures. Most of the Chalk is buried beneath 1000 m or more of Tertiary sediments. Whilst this allows the possibility of an overlying im- permeable stratum that could trap oil in the Chalk, it also means an overburden of hundreds of kilograms over each square centimetre. Such pressures tighten fractures, thus reducing the mass permeability, as well as the porosity. At depths of 1500-2000 m the porosities are typi- cally 15-30%; at 2700-3300 m the porosities are generally in the range 2-25%, with per- meabilities no more than 0.5 mD.

Source-rock Deep burial damages the texture of chalk, but burial is necessary to make a source-rock yield its oil (or gas). To generate oil in reasonable quantity from the organic carbon in the source- rock, the rock must be heated to about 90°C and preferably to 120°C. Geothermal gradients under the North Sea range from approximately 22" to 40°C per km. Taking 29" per km as an

146IGEOLOGY TODAY Sept-Oct I985