GCSE Geology

OUGS (SW) Participant Booklet

OUGS (SW)

Geology Fieldwork Weekend Saturday March 21st and Sunday March 22nd 2009

Glamorgan Heritage Coast

A fieldwork weekend based in the area around Ogmore-by- Sea, Glamorgan Heritage Coast, South Wales

G. Neighbour

Contents

Background Information ...... 4 Effective recording of data ...... 5 Rationale for choosing the fieldwork area ...... 9 Introduction to the fieldwork area ...... 9 Comparison with other areas ...... 10 Simplified map of the Field Area ...... 13 Simplified stratigraphic column for the fieldwork area ...... 14 Pant-y-Slade ...... 15 Methyr-mawr Warren ...... 19 The Flats, Ogmore-by-Sea ...... 22 Dunraven Bay ...... 24 Graphic Log Template ...... 27

Background Information

The following books and maps are useful references and contain much useful information:

Hunter, A. (2004). The Geological History of the British Isles. The Open University Press. ISBN: 0335161405

Howe, S., Owen, G., and Sharpe, T. Walking the Rocks – six walks discovering scenery and geology along the Glamorgan coast. Geologists’ Association – South Wales Group. ISBN: 0903222019

Neville George, T. (1970). British Regional Geology – South Wales (3rd Edition). British Geological Survey. ISBN: 0118800841

Owen, T. R. (2006). Geology Explained in South Wales. Fineleaf Editions. ISBN: 0953443765

Toghill, P. (2003). The Geology of Britain – An Introduction. Airlife Books. ISBN: 1840374047

Wilson, D., Davies, J. R., Fletcher, C. J. N., and Smith, M. (1990). Geology of the South Wales Coalfield, Part VI, the country around Bridgend. ISBN: 0118844253

In addition you may find it useful to study the following geological maps (published by the British Geological Survey):

1:50 000 series – Sheet 247 – Swansea – (Solid with Drift Edition)

1:50 000 series – Sheet 262 part 261 – Bridgend (Solid and Drift Edition)

1:50 000 series – Sheet 263 – Cardiff – (Solid Edition)

Effective recording of data

The points given below will not all apply to all areas, but they indicate what to look for in a rock exposure.

Use these ideas to sharpen your observation and to aid your appreciation of the actual geological evidence which can be gathered in a real situation to illustrate theories studied in the classroom.

Describe the position of the exposure you have decided to study a) By occurrence (e.g. quarry, stream section etc). b) By location (You should really include at least a six-figure grid reference also make note of nearby roads, paths, river etc).

Sketch the exposure in your field note book. Show any noticeable features such as bedding planes, faults, folds, joints, intrusions, veins, erosive features like soil creep, potholes, striations etc.

Indicate the scale of your sketch and the thickness of any obvious layers, using metric units. It is also essential to show finer detail in separate enlarged sketches e.g. for fossils that you identify.

Study carefully each rock seen in the exposure.

Try to describe each rock using the following criteria. (Write your observations into your field note book). a) Is it igneous, sedimentary or metamorphic? b) What evidence suggests that it is igneous, sedimentary, or metamorphic? c) Describe the texture of the rock: - • Look at and describe its grain size, the shape of the grains, the relationships of the grains (interlocking, cemented, large embedded in small etc). • Look at the structure of the rock: are the grains randomly distributed, sorted by size, segregated by composition, orientated by shape or uniformly distributed through the rock? d) Describe the composition of the rock. Try to distinguish which mineral or minerals make up the rock and to calculate the approximate proportion of each one in the rock (use %). e) Describe the colour of the rock, noting any red/brown/yellow colouration due to iron staining, yellow sulphur staining, green copper staining etc. • Is the colour of the weathered rock different from a fresh surface? • If so, what is the significance? f) Describe any structural features of the rock: • If it is igneous look for and describe any flow structure, vesicles, amygdales, pyroclastic fragments, xenoliths, joints, veins. Note any variation in grain size such as fine grained margin or margins.

• Sketch any structures found, again indicating the scale of your sketch.

• If it is metamorphic look for and describe any folds, fractured grains, alignment of mineral grains in lines or planes, foliation into bands of similar or different material, distorted pebbles or fossils, flow structure around large crystals. • Sketch any structure found, indicating the scale of your sketch.

• If it is sedimentary look for and describe any sedimentary structures e.g. cross bedding, graded bedding, slump bedding, sole structures (flute casts, load casts), ripple marks, concretions, contorted bedding, jointing. • Sketch any structures found, indicating the scale of your sketch.

• Look carefully for fossils: do not hammer a fresh outcrop any more than absolutely necessary - sift through fallen debris, sketch fossils 'in situ' in the exposure • Identify any fossils by phylum (brachiopod, bivalve) or class (crinoid, ammonite) if at all possible. • Describe the mode of preservation of the fossils, (hard parts, carbonised remains, mineralised remains, mould, cast, tracks, trails etc). • Carefully label any fossils you collect and write down the reference number and rough identification of each one in your field note book. Then you can identify each fossil more accurately when we return to the school.

From all your observations you should now be in a position to classify each rock in the exposure, giving its name and (if applicable) variety, e.g. crinoidal limestone, black shale, green slate etc.

Where possible indicate the probable origin of each rock, e.g. in warm shallow sea, delta swamp, beach etc.

Describe the structure of the exposure. • Study any joints noting how far apart they are, how many different joint directions are present. • Measure the strike and dip of each set of joints. • Types of joint e.g. radial tension joints, sheet joints, columnar joints etc.

Study any folding or tilting of the rocks. • If the rocks are flat-lying or tilted try to measure their strike and dip. • If the rocks are folded measure the strike and dip on either side of the fold axis. • If possible measure the direction of the axial plane of the fold.

Sketch any fold, labelling the axial plane and giving measured dips. • Describe the fold(s) - anticline/syncline, degree of symmetry (symmetrical, asymmetrical, overfold recumbent), shape of hinge area, tightness (interlimb angle), plunging/ non plunging etc.

Study any faults present. • Try to estimate the displacement along the fault plane, especially throw. • If possible try to decide whether the fault is normal, reversed, thrust or tear. • Measure the strike and dip of the fault plane if it is accessible. • Give details of any related phenomena e.g. slickensides, brecciated rock, mineralised veins. • Sketch the field situation.

Study any apparent unconformities present. • Measure the strike and dip of the beds above and below the unconformity. • Sketch this carefully giving the measured dip

Study any cleavage or foliation in the rock and measure its strike and dip. • If the bedding can also be seen, measure its strike and dip and then sketch the relationship between bedding and cleavage or foliation.

Study any mineral veins present. • Describe whether they are related to faults or joints. • If possible measure the strike and dip of each vein. • Identify or collect specimens of the minerals found in each vein, and mention their diagnostic physical properties. • Label any minerals collected and enter their number and description in your field notebook.

Study any igneous intrusions present. • Any evidence of type of intrusion e.g. pluton, dyke, sill. • Are there any chilled margins (finer grained) or baked margins/hardening/spotting within the country rock? • Is there a metamorphic aureole? • If so what is its extent, rock types within it? • Sketch any relevant details.

Other features of the exposure/area. Comment on any other points of the geology not already noted e.g. • Do geological structure and/or rock type influence the relief/topography of the area? (Show this by means of a sketch). • Describe any affects caused by groundwater such as seepage through fractures or pores, formation of springs etc. • Are there any recent weathering (e.g. biological roots) or erosive (e.g. by glacial effects? © G. Neighbour Mar 21, 2009 - Mar 22, 2009 Page 7 of 32 OUGS (SW) Participant Booklet

• Are some beds more easily weathered/eroded than others? • Are there any recent/superficial deposits? Is man making any economic use of the outcrop/area as a result of the geology?

Try to summarise the geological history of the rocks in the exposure from the origin of the oldest rock (as a wind-blown sand for example) through (using e.g. cross cutting relationships) any periods of folding or faulting up to and including present day erosion.

The above information and advice on recording field observations has been adapted from advice provided by Dave Mack of Ashton-under-Lyne Sixth Form College to the Earth Science Teachers Association.

Rationale for choosing the fieldwork area

The rationale for using the Glamorgan Heritage coast for the fieldwork is for many reasons. Firstly unlike the rocks of the Cornubian massif, these have not been significantly affected by a massive mountain building event. The rocks in the Cornwall, Devon and West Somerset have been affected by an event called the Variscan Orogeny (the Variscan mountain building event). This means for instance the sedimentary rocks will have been subject to regional metamorphism. The problem with this is that the structure and the evidence for what has happened in the region are not always clear.

The area around Ogmore-by-Sea has extensive geology ranging in age from the Carboniferous period, through the Triassic and into the Jurassic. With the adjacent sand-dune system, we may also be able to look at more recent (Quaternary) sediments.

Introduction to the fieldwork area

The region is on the whole situated to the south of the South Wales Coalfield. The area to the east forms part of the Glamorgan Heritage Coast. During this weekend you will be given the opportunity to study rocks that represent limestones, sandstones and mudstones of Carboniferous age, the red and green marls of the Triassic and finally the thinly interbedded limestones and shales of the Lower Jurassic.

Comparison with other areas

Within the region, there are many other exposures of Triassic and Jurassic sediments. The coast around Aust Cliff especially and other parts of the Bristol district expose more distal parts of the Triassic-Jurassic sequence. Aust Cliff especially is famed for its exposures of the Rhaetian Bone Bed. This has been the subject of a previous OUGS (SW) field trip.

The Rhaetian Bone Bed, exposed at Aust Cliff, near the first Severn River crossing.

Thalassinoides Isp.

Thalassinoides is interpreted as a combined feeding and dwelling burrow, but has been observed as a boring in some cases. The probable trace-maker was an arthropod.

Indeterminate vertebrate fossil remains exposed in the Rhaetian Bone Bed, at Manor Farm, Aust.

Other exposures, again of a slightly more distal nature can be seen around Watchet in Somerset, where Dr. Eric Robinson led an OUGS (SW) trip in 2007.

Triassic exposures at Helwell Bay, Watchet, Somerset

There are other exposures along the Somerset coast in the Kilve area. There are also exposures of more distal (marine) Jurassic sediments in the Lyme Regis – Charmouth area of the Jurassic coast.

Typical exposures of the Lias sequences, east of Lyme Regis

Simplified map of the Field Area

Beach sediments and alluvium

Jurassic (Lias) ed to use the books and maps

iled locality information. iled locality information. Triassic

of the field area. You will ne

High Tor Limestone (Carboniferous) to confirm the more deta (Scale is approximately 2.5 km of coastal exposure)

Gully Oolite Limestone (Carboniferous) The map shows the basic geology

Simplified stratigraphic column for the fieldwork area

Lithology Description Lower Lias – These rocks are of Hettangian and Lower Sinemurian stages of the Lower Jurassic period. They consist of thinly interbedded limestones and calcareous mudstones. These are sometimes referred to as the ‘Blue Lias’.

These were formed in a shallow epeiric sea. They have been deposited in cycles, with each rhythm showing a rapid deepening, followed by a slow regressive event. Penarth Group – These are sometimes referred to as the Rhaetic. They form the junction between the Triassic and the Jurassic periods. They mainly are made up of a sequence of marine shales.

Deposited in the area as part of a marine marginal facies. Mercia Mudstone Group – These are of Norian age (Triassic), and are a ‘marginal facies’ of conglomerates, breccias and sandstones, that pass laterally into a sequence of siltstones and mudstones.

Formed as part of a storm deposit working to a near shore lacustrine facies. High Tor Limestone – This mainly consists of well bedded, skeletal packstones and wackestones with intercalated thin shales. This is Arundian (Lower Carboniferous) age.

Deposited as part of a barrier complex in a shallow near shore setting Gully Oolite – This is an oolitic limestone of Chadian (Lower Carboniferous) age. It comprises thickly bedded, cross- stratified, pale grey ooid & peloid grainstones.

Deposited as part of a regional development of oolite shoals.

Information summarised from Wilson et al. (1990)

Pant-y-Slade

The Lower Lias succession rests unconformably on the Carboniferous sediments. The Lower Lias rocks here are formed of three distinct members.

The unconformity is irregular. The Lias rocks that immediately overlay these sediments are the Sutton Stone Member. They are thickly bedded, coarsely bioclastic calcarenites and conglomerates. The Sutton Stone Member is at its minimum 10 metres thick and at its thickest 13.5 metres thick.

The Carboniferous limestone exposed here is the same as you will see on The Flats, Ogmore-by-Sea Field Day and the Merthyr-mawr Warren Field Day. What is different about it is that you will see a boulder beach. This it is thought was formed in the Jurassic period and is the result of erosion in the area during the formation of the Lower Jurassic rocks.

The structure that you will see exposed in the cliffs is called the Slade Trough. This trough rises quite quickly from beach level. Corals and oysters encrust the unconformity surface.

Within the Lower Lias rocks of the Sutton Stone Member there are two generally recognised subdivisions. These are separated by an irregular bored surface. The lowermost unit is up to 10 metres thick. It is very conglomeratic toward the base, the pebbles being made up of Carboniferous Limestone. At beach level itself there are course breccias, with irregular Carboniferous Limestone lithoclasts. These are up to 2 metres across. These have been encrusted and bored by fossil marine organisms. They are in a matrix of smaller clasts and shell debris. There is a small calcarenite unit above this and then above that another overlying breccia (unit 3). The clasts are smaller in this unit than the lower breccia unit. These are supported by a coarse bioclastic matrix.

There may be irregular colonial coral masses in the lowermost part of the 3rd unit.

Things to do:

• Measure blocks in the conglomerates and the breccias • Measure if there is a preferred orientation in the blocks e.g. is there any evidence of imbrication? • Construct a graphic log • Produce field sketches • Look for evidence of mineralization • Look for evidence of structures

Fossils along the coastal section

Conglomerates and breccias along the coastal section

Methyr-mawr Warren

This section is different from the others in that for part of it you will be studying recent (Quaternary) sediments.

At this locality you will have the opportunity to study recent (post-last glaciation) and may include estuarine alluvium, marine and storm gravel beach deposits and blown sand.

The marine beach deposits are generally well-sorted sands and gravels which occur below the high water mark. It is important to study these sediments because they will generally consist of examples of the local geology.

The blown sand deposits consist of fine- to medium-grained quartz sands. These are important because within historical times the dune fields have been unstable and this caused the burial of the ancient town of Old Kenfig.

On the return from the Merthyr-mawr Warren locality we will have the opportunity to look at the Carboniferous rocks of the Gully Oolite and the High Tor Limestone. The Gully Oolite was deposited within Oolite shoals that were being formed on a regional basis. The rock is mainly made up of thickly-bedded cross stratified, pale-grey coloured ooid and peloid grainstones. There may be some skeletal grains present. Bioturbation is common throughout the sequence and sometimes allow the mixing of the Gully Oolite with the High Tor Limestone.

The High Tor Limestone was deposited as part of a barrier complex, within an open marine shelf. Sedimentary structures are destroyed for the most part, due to the extensive bioturbation. At its base there is a 3 metre thick crinoidal packstone-grainstone. Above this is about 40 metres of well-bedded skeletal packstones. Finally there is an upper unit that displays evidence of hummocky cross-stratified, skeletal, peloidal packstones and grainstones.

Things to do:

• Construct a graphic log • Produce field sketches • Look for evidence of different minerals within the sands • Take samples from a reasonable number of sample sites (This process is called measuring along a transect). • Look for evidence of structures

Locality of Merthyr Mawr Warren Field locality

The Flats, Ogmore-by-Sea

This locality is also called Sutton Flats and it provides the opportunity to study in three dimensions of a Triassic alluvial fan deposit. These sediments were formed as a result of streams and mudflows from the edges of a Carboniferous Limestone landmass.

There is a large variation between the particle sizes – these may be from clay grade to boulders (these boulders may be up to 3 metres in diameter). The Triassic fan deposits occur in pre-existing channels in the surface of the underlying Carboniferous Limestone.

The conglomerate varies from poorly sorted to well-sorted, and in places there are irregular bedding planes. Trough cross-bedding and imbrication of some of the smaller pebbles may be seen. The cement is made up of calcium-carbonate.

Toward the eastern end of the locality (Bwlch Cae Halen) you should be able to record a fining upward sequence within the conglomerate – you will notice that these are poorly sorted. At about 1 metre from the top of the conglomerate is a breccia (about 25 centimetres thick) with more conglomerates above it.

The sequence has finer-grained sandstones and calcarenites within it. These are typical sediments formed during sheet floods, and are normally found on the more distal parts of an alluvial fan.

This site is an SSSI because the sediments preserved here give an excellent example of facies changes within alluvial fan complexes.

Things to do:

Cement

Breccia

Dunraven Bay

At Dunraven Bay there is the opportunity to further study Lower Lias sediments. These are from higher in the succession than the sediments that were exposed at Pant-y-Slade.

The sediments exposed belong to the Southerndown Member which passes into the overlying Porthkerry Member. The rocks of the Southerndown Member are comprised of lithoclastic and bioclastic sands and gravel-grade conglomerates. The conglomerate contains a lower proportion of limestone lithoclasts and has higher clay content. These occur in thin beds.

In the upper part of the Southerndown Member contains bands of oolitic limestone. The nuclei are formed from calcarenite grains, shell fragments and quartz grains.

The Porthkerry Member is a typical off-shore facies of the Blue Lias. There is alternation between mudstones and argillaceous limestones. These are on centimetre to decimetre scale. As the exposures are studied it should become clear that the cliff changes from a limestone dominated succession to a succession where there is a greater proportion of mudstone.

There are many fossils and some of the beds have a distinctive fossil content. There will be corals or brachiopods. There is a large variation in the fossil content, with some beds having up to 50%, (mainly as bioclastic debris).

Bivalves are especially important, and Gryphaea arcuata (the Devils Toenail) occurs in large numbers at some levels. Ammonites also occur in large numbers and are used for providing relative ages for the formation.

Things to do:

• Construct a graphic log • Record fossils – do not attempt to remove them from beds • Produce field sketches • Look for evidence of mineralization • Look for evidence of structures e.g. faults, folds etc.

Locality of Dunraven Bay Field locality

Graphic Log Template

Location: Formation: Date: Grid Reference:

Clay and Silt F M C Sedimentary Fossils Remarks/Notes Gravel Structures Lithology Colour metres Direction Bed Number Thickness in Palaeocurrent

Figure 1 The Graphic log: You may need to complete the above as part of today’s exercise. Graphic logs come in many different styles and the ones that you will use at Summer school may well not look like the above

Key for a graphic log

Lithology

Siliciclastic sediments Carbonates Others Clay, Pebble Limestone Coal Mudstone Supported Conglomerate Shale Matrix Dolomite Chert Supported Conglomerate Sandstone – Alternating Sandy Volcaniclastic (Undiff.) strata Limestone Sediment sst/shale Siltstone

Sedimentary structures Devise suitable symbols to use for any of the structures you find in your sediments.

Flute Parallel Wave-Ripple Stromatolites Cast Lamination Lamination

Groove Cross Normal Graded Slight Cast Lamination Bedding Bioturbation

Tool Planar Cross- Reversed Intense Marks Bedding Graded Bedding Bioturbation

Load Trough Cross- Imbrication Sharp/Planar Casts Bedding Bed contact

Shrinkage Herring-Bone Slump Sharp/Irregular Cracks Cross-Bedding Structures Bed Contact

Striations/ Low Angle Convolute Gradational Lineations Cross-Bedding Bedding Bed Contact

Symmetrical Flaser Nodules Palaeocurrent: - Ripples Bedding Azimuth

Asymmetrical Lenticular Stylolites Palaeocurrent: - Ripples Bedding Trend

Palaeontology

Fossils - Brachiopods Gastropods Algae (Undiff.)

Fossils – Coral – Graptolites Roots (Broken) Solitary

Coral - Bivalves Crinoids Burrows Compound

Trilobite Fish Reptiles Devise others as required

Typical Carboniferous fossils – Glamorgan Heritage Coast, Ogmore-by-Sea

Colonial corals - Siphonodendron

Colonial Corals - Michelinia

Fossilised algal mat Solitary Coral - Zaphrentites

Typical Jurassic fossils – Glamorgan Heritage Coast, Ogmore-by-Sea

Bivalves - Plagiostoma Bivalves - Pinna

Crinoid debris

Bivalves – Gryphaea arcuata

I hope that you are able to gain as much benefit as I have from this superb section of coastline.

Best wishes,

Gordon Neighbour