An Introduction to Geological Structures and Maps Fifth Edition G. M. Bennison Formerly Senior Lecturer in Geology, University of Birmingham Edward Arnold A . An introduction to Geological Structures and Maps (C.M. Bennison).pdf - Free download as PDF File .pdf) or read online for free. Geological Structures and Maps. A PRACTICAL GUIDE. Third edition Preface. GEOLOGICAL maps represent the expression on the earth's . Introduction.
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An Introduction to Geological Structures and Maps. Authors; (view Front Matter. Pages i-x. PDF · Horizontal and dipping strata. G. M. Bennison. Pages 'An Introduction to Geological Structures & Maps' is a concise text that leads the students in easy stages from the simplest ideas on geological structures right. AN INTRODUCTION TO. GEOLOGICAL. STRUCTURES AND MAPS. Seventh Edition. G.M. Bennison. Chartered Geologist, formerly Senior Lecturer in Geology .
Stress refers to the forces that cause rocks to deform. There are three basic types of stress that deform rocks: tension pulling apart shear twisting or rotating In response to stress, rocks will undergo some form of bending or breaking, or both. The bending or breaking of rock is called deformation or strain. If rocks tend to break, they are said to be brittle.
If a rock breaks, it is said to undergo brittle behavior. If rocks tend to bend without breaking, they are said to be ductile. If a rock bends but is able to return to its original position when the stress is released, it is said to undergo elastic behavior. If a rock bends and stays bent after stress is released, it is said to undergo plastic behavior. A combination of elastic and brittle behavior causes earthquakes. Rocks get bent in an elastic fashion until they reach their limit, then they break in brittle fashion.
The rocks on either side of a break act like rubber bands and snap back into their original shape. The snap is an earthquake and the break along which the rocks slide back to their original shape is a fault.
Earthquakes and faults occur in the shallow crust, where rocks are relatively cold and therefore brittle. In the deep crust and deeper, in the earth's mantle, rocks are very hot and subject to high pressure caused by the weight of the overlying rock. This heat and pressure causes deep crustal and mantle rocks to be ductile. In fact, rocks deep in the continental crust and upper mantle can be so hot and soft that they behave almost like a slow-moving liquid, even though they are actually solid.
They "flow," or bend, or stretch, in a plastic manner, at a geological pace. Now let us look at the specific types of geologic structures - the breaks and bends that deform rock in response to stress.
Folds Ductile rocks behave plastically and commonly become folded in response to stress. Folding can happen in the shallow crust if the stress is slow and steady and gives the rock enough time to gradually bend. If the stress is applied too quickly, rocks in the shallow crust will behave as brittle solids and break. However, deeper in the crust, where the rocks are more ductile, folding happens more readily.
Refer to this table of folds and how they are symbolized on a geologic map. Anticlines and Synclines The most basic types of folds are anticlines and synclines.
Anticlines are "up" folds; synclines are "down" folds. In box diagrams like these, the top of the box is the horizontal surface of the earth, the map view.
The other two visible sides of the box are cross-sections , vertical slices through the crust. The colored layers represent layered geologic formations that were originally horizontal, such as sedimentary beds or lava flows.
Sub-unconformity outcrops An unconformity represents a period of erosion. Tens or hundreds of metres of strata may be removed. How can we examine this phenomenon and deduce what remains of the older strata- Sectionsacross published Geological Survey Maps since they are now covered by post-unconformity strata?
We could, of course, in practice remove all post-unconformity strata with a bulldozer and other earth-moving equipment to reveal the plane of unconformity. We can, by a study of the struc- ture contours, deduce from a map just what we should expect to see if that was possible. What we seek are the 'outcrops' of the older strata on the plane of unconformity.
This plane can be defined by its structure contours. If we now take the structure contours drawn on the geological boundaries of the older set of strata and note where they intersect, not ground con- tours, but the structure contours of the plane of unconformity, we can plot - where the two sets of structure contours intersect at the same height - a number of points which will define the sub-unconformity outcrops.
This exercise can be attempted on Map 8. The topic will be revised on a later page and Maps 10 and 16 give further practical exercises. Assynt Geological Survey special sheet Examine the western partof the map to find the unconformitiesat the base of the Torridonian and the base of the Cambrian.
Drawasection along the north-south grid line 22 to show these unconformities. From your knowledge of the conditions under which the Torridonian and Lower Cambrian were deposited can you explain how and why these unconformities differ? Shrewsbury Map No. How manyof the unconformities indicated in the geological column in the marginsare deducible from the map evidence?
On examining the strata over a wider area it is found that the inclination is not constant and, as a rule, the inclined strata are part of a much greater structure.
For example, the Chalk of the South Downs dips generally southwards towards the Channel - as can be determined by examination of the 1: We know, how- ever, that in the North Downs the Chalk dips to the north passing beneath the London Basin ; the inclined strata of the South and North Downs are really parts of a great structure which arched up the rocks including the Chalk over the Wealden area.
Not all arching of the strata is of this large scale and minor folding of the strata can be seen to occur near the centre of the Brighton sheet. Folding of strata represents a shortening of the earth's crust and results from compressive forces.
In the simplest case the beds on each side of a fold structure, i. In this case a plane bisecting the fold, called the axial plane, is vertical. The fold is called an upright fold whenever the axial plane is Fig.
Where axial planes have a low dip or are nearly horizontal, see Fig. The effect of erosion on folded strata is to pro- duce outcrops such that the succession of beds of one limb is repeated, though of course in the reverse order, in the other limb. In an eroded anti- cline the oldest bed outcrops in the centre of the structure and, as we move outwards, success- ively younger beds are found to outcrop Fig.
In an eroded syncline, conversely, the youngest bed outcrops at the centre of the structure with successively older beds outcropping to either side Fig.
If the beds of one limb of a fold dip more steeply than the beds of the other limb, then the fold is asym- metrical. The differences in dip of the beds of the two limbs will be reflected in the widths of their outcrops, which will be narrower in the case of the limb with the steeper dip Fig.
See also p. Now, the axial plane bisecting the fold is no longer vertical but is inclined and the fold is called an inclined fold. Overfolds If the asymmetry of a fold is so great that both limbs dip in the same direction though with different angles of dip , that is to say the steeply dipping limb of an asymmetrical fold has axial planes. The strata of the limb with the reversed dip, it should be noted, are upside down, i.
Figures 14 to 17 show a progressive decrease in inter-limb angle. Figures 14 to 17 illustrate fold structures pro- duced in response to increasing tectonic stress. Further terminology should be noted.
Where the limbs of a fold dip at only a few degrees it is a gentle fold, with somewhat greater dip Figs. They are open folds with horizontal or near-horizontal axial planes.
The axial planes of a series of such folds will also be approximately parallel over a small area, but over a larger area extending per- haps forty kilometres greater than that portrayed in a problem map they may be seen to form a fan structure.
Theoretically at least two mechanisms are possible: The way in which beds will react to stress depends upon their constituent materials and the level in the crust at which the rocks lie.
Compe- tent rocks such as limestone and sandstone do not readily extend under tension or compress under compressive forces but give way by frac- turing and buckling while incompetent rocks such as shale or clay can be stretched or squeezed. Thus in an alternating sequence of sandstones and shales the sandstones will fracture and buckle while the shales will squeeze into the available spaces.
Concentric folds The beds of each fold are approximately concentric, i. Beds retain their constituent thickness round the curves and there is little thinning or attenuation of beds in the limbs of the folds Fig. Although typically developed in thinly bedded rocks such as the Culm Measures of North Devon most of the problem maps in this book which illustrate folding have straight limbed folds since these provide a simple pattern of equally spaced structure contours on each limb of a fold.
Similar folds The shape of successive bed- ding planes is essentially similar, hence the name Fig. Thinning of the beds takes place in the limbs of the folds and a strong axial plane cleav- age is usually developed.
This type of folding probably occurs when temperatures and pres- sures are high. Two possible directions of strike A structure contour is drawn by joining points at which a geological boundary surface or bedding plane is at the same height. By definition this surface is at the same height along the whole length of that structure contour. Clearly, if we join points X and Y Fig. Thus, if we attempt to draw a structure contour pattern which proves to be incorrect, we should look for the correct direction approximately at right angles to our first attempt.
It should also be noted that an attempt to visualize the structures must be made. For example, in Map 9 the valley sides provide, in essence, a section which sug- gests the synclinal structure, especially if the map is turned upside down and viewed from the north. Map 11 similarly reveals the essential nature of the structures by regarding the northern valley side as a section.
To facilitate this, fold the map at right angles along the line of the valley bottom. Now regard the top half of the map as an approxi- mate geological section. What is the test of whether we have found the correct direction of strike? In these relatively sim- ple maps the structure contours should be paral- lel and equally spaced at least for each limb of a fold structure. Furthermore, calculations of true thicknesses of a bed at different points on the map should give the same value.
Note on Map 10 You should find three areas where Bed E occurs underneath younger beds. However, to confirm that this is correct it is necessary to use the inter- section of the two sets of structure contours on DIE and on the base of X. This method is the only way in which the western extent of Bed E can be defined. Remember see p. The surface on which we are plotting the outcrop of the DIE boundary is the plane of unconformity, defined by the structure contours drawn on the base of Bed X.
You will find it necessary to use intersections of structure contours which are now above ground level: Notes on the Lewes map Here, as elsewhere in the south-east of England, the Chalk of Upper Cretaceous age is a compact rock more resistant to erosion than the softer clays and sands of pre- Chalk age.
There is less information on dip of strata than desirable, although dips are given to the north and south of the town of Lewes. Remember to allow for the vertical exaggeration by multiplying the gradient tangent of given dips by 4.
Where dip information is less than adequate the beds must be fitted to the outcrops at an angle of dip which gives the correct thicknesses of strata shown in the stratigraphic column on the map. Lewes, 1: Notethe general structural trend, close to E-W, and the 'younging' of beds southwards.
Study the relationship of topography to geology. Draw asection along the line of Section 1 south of grid line 11 7. Make the vertical exaggeration x4 the same as the BGS section on this map. Map 10 Indicateon the map the outcrop of the plane ofunconformity.
Inserttheaxial traces of thefolds. Draw asection along the line P-Q to illustratethe geology. Indicateon the mapthe extentof Bed Ebeneath the overlying strata. Strata may also respond to stress compression, extension or shearing by fracturing.
Faults are fractures which displace the rocks. The strata on one side of a fault may be vertically displaced tens, or even hundreds, of metres rela- tive to the strata on the other side. In another type of fault the rocks may have been displaced hori- zontally for a distance of many kilometres. While in nature a fault may consist of a plane surface along which slipping has taken place, it may on the other hand be represented by a zone of brec- ciated composed of angular fragments rock.
For the purposes of mapping problems it can be treated as a plane surface, usually making an angle with the vertical. All structural measurements are made with reference to the horizontal, including the dip of a fault plane. This is a measure of its slope, ct. The term 'hade', for- merly used, is the angle between the fault plane and the vertical and is therefore the complement of the dip.
It causes some confusion but it will be widely encountered in books and on maps and is, down - throw upthrow angleof hade It will, no doubt, gradually drop out of usage. The most common displacement of the strata on either side of a fault is in a vertical sense.
The vertical displacement of any bedding plane is called the throw of the fault. Other directions of displacement are dealt with later in this chapter. Normal and reversed faults If the fault plane is vertical or dips towards the downthrow side of a fault it is called a normal fault Fig. If the fault plane dips in the opposite direction to the downthrow i.
In nature, the dip of a reversed fault is generally lower than that of a normal fault. The outcrop of a normal fault commonly with a dip in the range, but as low as in some examples will usually be much straighter - unless the fault- plane itself is curved see Map Of course, where the area is of low relief, the outcrop of a fault plane, normal or reversed, will be virtually straight.
On some geological maps, such as those pro- duced in Canada and the United States, the direc- tion of dip of fault planes is shown.
The angle of dip of the fault plane may be given in the map description. British Geological Survey maps show the direction of the throw of faults by means of a tick on the downthrow side of the fault out- crop see also Map Any sloping plane, including a fault plane, can be defined by its structure contours.
It is possible on both Map 12 and 14 to construct contours for the fault plane. The method is exactly the same as for constructing structure contours on bedding planes, described in Chapter 1.
From these struc- ture contours the direction of dip of the fault plane can be deduced and then, by reference to the direction of downthrow, it can be deduced whether the fault is normal or reversed. The effects of faulting on outcrops Consider the effects of faulting on the strata: Since this uplift is not as a rule a rapid process and the strata will be eroded away continuously, a fault may not make a topographic feature, although temporarily a fault scarp may be present Fig.
Some faults which bring resistant rocks on the one side into juxtaposition with easily eroded rocks on the other side may be recognized by the presence of a fault line scarp cf. The strata which have been elevated on the upthrow side of a fault naturally tend to be eroded more rapidly than those on the downthrow side. This results in higher younger beds in the strati- I. It follows that we can usually determine the direction of downthrow of a fault, whether normal or reversed. Following the line of a fault across a map, there will be points where a younger bed on one side of the fault is juxtaposed against an older bed on the other side of the fault.
The younger bed will be on the down- throw side of the fault. A fault dislocates and displaces the strata.
The effect of this, in combination with erosion, is to cause discontinuity or displacement in the out- crops of the strata. Classification of faults Faults may be categorized in two ways. We have con- sidered so far normal and reversed faults with a vertical displacement called throw. Movement in these faults was in the direction of dip of the fault plane.
They are called dip-slip faults because the movement - or displacement - was parallel to the direction of dip of the fault plane. On a later page faults with lateral displacement, wrench faults, are described.
Here, displacement is parallel to the strike of the fault plane and they can be described as strike-slip faults. In nature, in some faults the displacement was neither dip-slip nor strike-slip but was oblique.
Naturally, the displacement will have a vertical component throw and a horizontal component lateral displacement. Such a fault may be called an oblique-slip fault Fig. Where the faulting is parallel or nearly so to the direction of dip of strata, the faults are called dip faults. Where the faulting is more or less at right angles to the direction of dip of strata, i.
Examples of both are found on Problem Map Faults which are neither in the dip direction nor the strike direction may be called oblique faults. It is particularly important not to confuse the two schemes of classification discussed above: Test yourself: Figures 26 and 27 show examples of dip-slip faults, the former shows two cases of strike faults, the latter shows a dip fault. Figure 29 is an example of a strike-slip fault. It is, however, also a dip fault. To return to normal dip-slip faults, sequences of outcrops encountered on a traverse may be partly repeated or may be partly suppressed.
Where the fault plane is parallel to the strike of the beds we see either repetition of outcrops Fig. Where the fault plane is parallel to the dip direc- tion of the strata a dip fault , i.
Whatistheamountofthe throw ofthefault? Drawstructurecontourson thefaultplane. Isita normalorareversedfault? Whatisthe thicknessofthesandstone? The outcrops shift progressively down dip as erosion lowers the ground surface. Note the lateral shift of outcrops although the actual displacement is vertical. This must not be confused with lateral movement of the strata see p. Calculation of the throw of a fault Note on Map 12 Construct the structure con- tours on the upper surface of the sandstone bed in the northern part of Map They run north- south and are spaced at Follow the same procedure for the upper surface of the sandstone south of the fault plane.
The m structure contour drawn on the south side of the fault, if produced beyond the fault, is seen to be coincident with the position of the m structure contour on the north side of the fault. The stratum on the south side is therefore m lower relatively. The fault has a downthrow to the south of m. Determine the throw of the fault using the struc- ture contours on the base of the sandstone - on each side of the fault - and check that you obtain the same value.
Map 13 is a slightly simplified version of the western part of the BGS map of Chester, Sheet in the 1: It is here about half scale. The geology of the area is rela- tively simple with strata. The topography is almost flat and drift cover is extensive so that much of the map has been compiled using borehole data. Strata are displaced by a considerable number of faults and they display many important characters of faulting. Note that faults may die out laterally, examples can be seen at F1 and F2.
Of course, all faults die out eventually, unless cut off by another fault, as at C, though major faults may extend for tens or even hundreds of kilometres. Note also that a fault may curve, for example at G. This is not mere curvature of the outcrop of the fault plane due to the effects of topography on a sloping fault plane see Map Most of the faults on Map 13 run approximately north-south, roughly parallel to the general strike of the strata: The displacement is in the direction of dip of the fault planes so they can also be called dip-slip faults.
Two faults are approximately parallel to the direc- tion of dip of the strata, seen at C and D. These are, therefore, dip faults.
They are also probably dip-slip faults. The Chester sheet indicates the direction of downthrow of each fault, as do most published maps, but it has been omitted from most of the faults on Map It can be seen that since the fault plane dips, the intersections with a bedding plane on each side of the fault do not coincide in plan view. Con- sider an economically important bed, for example of coal or ironstone.
In the case of a normal fault there is a zone where a borehole would not pene- trate this bed at all due to the effect of heave see Fig. This is important when calculating eco- nomic reserves, for example in a highly faulted coalfield. In the case of a reversed fault a zone exists where a borehole would penetrate the same bed twice.
Figure 28 shows, in section, how a borehole after pene- trating a coal seam would penetrate the fault plane and, beneath it, the same seam. In calcu- lating reserves it is vital to recognize that it is the same seam which the borehole has encountered or reserve calculations would be wrong by a factor of approximately two. Wrench or tear faults In the case of these faults the strata on either side of the fault plane have been moved laterally rela- tive to each other, i.
In the case of simply dipping strata the outcrops are shifted laterally Fig. Note that the effect on the outcrops is similar to that of anormal dip fault d. In some geological contexts the terminology based on the direction of movement relative to the dip and strike of the fault plane is preferable. Notes on Map 13 The dip of the strata at E is anomalous, the result of a phenomenon called fault-drag.
The effect of faults which are parallel to the dip of the strata is to laterally shift outcrops see p. The extent of this shift depends on two factors, the amount of throw of the fault and the angle of dip of the strata. At C, on Map 13, the geological boundary is displaced very little, but at D the displacement is considerable the shift is almost equal to the width of outcrop of Bed 4. At C and D the dips of the strata are similar so we can conclude that the fault at D has a much larger throw than the fault at C.
It may be assumed that all the faulting here is normal. Map 13 On the map indicate the downthrowdirection ofallfaults where ithasnotbeen shown. Indicateexamples of a agrabenand b stepfaulting.
Drawasection along the nearlyeast-west lineX-Yto illustrate the geology. Pre- and post-unconformity faulting After the deposition of the older set of strata earth movements causing uplift may also give rise to faulting of the strata.
The unconformable series the younger set of beds , not being laid down until a later period, are unaffected by this faulting. Earth movements subsequent to the deposition of the unconformable beds would, if they caused faulting, produce faults which affect both sets of strata. Clearly, it is possible to determine the rela- tive age of afault from inspection of the geological map which will show whether the fault displaces only the older pre-unconformity strata or whether it displaces both sets of strata.
A fault is later in age than the youngest beds it cuts. A fault may also be dated relative to igneous intrusions, a topic dealt with in the last chapter. Structural inliers and outliers The increased complexity of outcrop patterns due to unconformity and faulting greatly increases the potential for the formation of outliers and inliers these terms have been defined on p.
Indicate on Map 16 inliers and outliers which owe their existence to such structural features and sub- sequent erosional isolation. Posthumous faulting Further movement may take place along an exist- ing fault plane. So the displacement of the strata is attributable to two or more geological periods. It follows that an older series of strata may be displaced by an early movement of the fault which did not affect newer rocks since they were laid down subsequently.
The renewed movement along the fault will displace both strata so the older strata will be displaced by a greater amount since they have been displaced twice the throws are added together. The simplest use of isopa- chytes is to show the thickness of cover material overlying a bed of economic importance, such as a coal seam or ironstone. The overlying material - whatever its composition: Its thickness can be determined where the height of the top of an economic bed ironstone, Map 15 is known from its structure contours, and the height of the ground at the same pOint is known from the topographic contours.
Wherever structure contours and topographic contours intersect on the map we can obtain afigure for the thickness of overburden by subtracting the height of the top of the iron- stone from the height of the ground. Lecture 3 introduces you to the art of locating field data, geologic features and their subsequent plotting in a base map. Lecture 4 describes and illustrates to you the practical usage of the compass, clinometer and a hand level and how to use them in determining various attitude data e.
After accumulating sufficient data from the fieldwork session, Lecture 5 will introduce you to the necessary follow-up laboratory investigations of the samples you collected in the field. Among the topics you will cover in this lecture will include the study of rock thin sections using the petrographic microscope; routine mineral identification procedures; and some principle techniques of chemical and X-ray methods of rock and mineral analyses.
The lecture concludes by introducing you on the method used in the plotting of some basic statistical projections such as the rose diagrams and the preparation of rock thin-sections.
Lecture 6 and lecture 7 is a combined and integrated study on aerial photographs. Lecture 6 starts by introducing you on the use of aerial photographs, their nature, the concept of stereoscopy, the architecture of stereoscopes and how they are used to interpret geological features. Lecture 7 goes a notch higher to introduce you on how various geological structures e. Using tone and relief features, this lecture provides an in-depth analysis on how to differentiate lithological units of igneous, metamorphic and sedimentary origin in aerial photographs.