Notes 4 Rock Defoormation and Geological Mapping

PgDip/MSc Energy Programme/Subsurface Rock Deformation & Geological Mapping

The Robert Gordon University 2006 1 campus.rgu.ac.uk

Rock Deformation & Geological Mapping

Review

In this topic the student is introduced to rock deformation andthe construction of geological maps by practical example.

Content

Deformation of the Rock Record Time WarpsAccording to the Law of Original Horizontality rocks are initially depositedas near horizontal layers parallel to the Earths surface. However, due tothe dynamic nature of the crust and the resultant stresses applied to therocks, these layers may become deformed. Folding, tilting or faulting mayoccur depending on, for example, how ductile the rock is. Brittle rockswill tend to fracture, whereas ductile rock will tend to fold. Responses tostress depend upon:

temperature;

depth of rock, eg, shallow rock tends to fracture or fault whereasdeeper rocks tend to be ductile and fold due to confining pressureof surrounding rock;

the nature of the rock, eg, crystalline basement rocks tend to bebrittle, whereas sediments tend to be ductile;

the time the stresses are applied to the rock.

Figure 1 illustrates the types of tectonic forces that act upon rocksleading to deformation of the rock record.

Figure 1. Tectonic Forces Acting on the Rock Record. (FromUNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)



FoldsWhen a rock deforms in a ductile manner under compressive or shearloading it will fold. At very low strain rates even quite shallow rocks maybehave in a ductile manner. Folds can exist on a large scale (mountainbelts) or small scale (individual beds). The folding may be gentle orsevere. The simplest form of this is a monocline, where there is a localsteepening in an otherwise uniformly dipping series of strata. Mostfolding however is more complicated. The axis of a fold is the intersectionof the axial plane (symmetrical division of the fold) with the beds (Figure2). When the limbs of the fold dip at the same angle, the axial plane isvertical and the fold is symmetrical. When the dips of one limb aresteeper than the other, the fold is asymmetrical. The angle of the axialplane to the vertical is known as the plunge of the fold. When the bed onboth limbs of the fold dip in the same direction, the fold is overturned,and if overturned to such an extent that the axial plane is nearlyhorizontal it is termed recumbent (Figure 3). Upfolds, or arches oflayered rocks are called anticlines, and down folds, or troughs are calledsynclines. Note that topographic expression is not necessarily a reflectionof deformation, ie, hills do not necessarily correlate with the top of eitheran anticline, or valleys with the wells of synclines.

Figure 2. Fold Geometry. (From THE DYNAMIC EARTH by B.J. Skinner and S.C.Porter, copyright 2000 John Wiley and Sons. This material is used by permission ofJohn Wiley and Sons, Inc.)

FracturesWhen a rock deforms in a brittle manner, it will fracture. There are twocategories of fractures:

Joints, which are present in almost every outcrop of rock, are formedwhere tectonic forces have cracked the rock but where no appreciablemovement has occurred. This tends to occur where regional stresses areapplied, eg, sediment compression leads to joint formation. If the patternof joints is regular the stress system must have been uniform. Joints canalso form as a result of non-tectonic expansion and contraction of the



rock. If surface layers are removed, for example, the release of containedstresses and expansion leads to joint formation at flaws in the rock. Theformation of joints may speed up the natural weathering process as theyallow passage of air and water into the rock. Weathering of intersectingjoints can cause blocks to break away. Joints may also provide channelsfor magma, leading to parallel swarms of dikes.

Faults occur where rock is displaced either side of or parallel to afracture. Faults are common in mountain belts or where deformation isintense. Faults are assigned different names according to their sense ofmovement, eg, normal, reverse, thrust, dip-slip, strike-slip, oblique-slip,right-lateral, left-lateral etc (Figure 4). Faulting can lead to the formationof structures called horsts and grabens (Figure 5). A graben forms bytensional forces in the crust leading to down dropping of the faultedblock, for example, at mid ocean ridges or rift valleys (East African Rift).Grabens are therefore long narrow valleys bounded by two or moreparallel normal faults. A horst is the opposite. It is a ridge formed byparallel reverse or normal faults.

Figure 3. Fold Geometry (contd). (From UNDERSTANDING EARTH by FrankPress and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used withpermission. Bottom right picture after A Maltman, Geological Maps: An introduction. NewYork: Van Nostrand Reinhold, 1990.)



Figure 4. Example Fault Types. (From UNDERSTANDING EARTH by Frank Pressand Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)



Figure 5. Horst and Graben Formation. (From THE DYNAMIC EARTH by B.J.Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of John Wiley and Sons, Inc.)

Fold and Fault RelationshipsFolds and faults are often combined in a single rock formation. Forexample, faults tend to die out as folds towards the edges of thedeformed region. Folds in turn die out as smaller and smallerdeformations into the surrounding strata. in this manner it can be seenthat continued folding of a rock will tend to lead to faulting asdeformation increases. A fold may overturn, become recumbent and thenfracture forming a thrust fault. In this case the overlying rock in thestratigraphic record may be older than the lower strata (Figure 6).



Figure 6. Large Scale Thrust Fault. (From UNDERSTANDING EARTH by FrankPress and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used withpermission.)

Unconformities Missing TimeIn addition to folding and fracturing, the sequential record ofsedimentation at a particular location may cease or be interrupted atcertain points. This may be because the sediment has accumulated toabove sea level so that no more accumulates, or the sea level may havefallen exposing the sediment to the atmosphere. This break insedimentation is called a hiatus. During this time, the sediment may beuplifted, tilted and eroded, thereby removing more of the record. Ifsubsidence then occurs sedimentation will continue, with layers beingdeposited on the older, truncated, erosional surface (Figure 7). Thiserosional surface represents the break in sedimentation and is known asan unconformity. Unconformities can be classified:

disconformity irregular surface of erosion between parallel strata,which implies cessation of sedimentation but no tilting ordeformation, often only recognisable by widely varying fossilrecords between strata;

angular unconformity angular distinction between older andyounger strata, which implies that the older strata were deformedand eroded before deposition restarted;

nonconformity sedimentary strata overlying igneous ormetamorphic rocks.



Figure 7. Disconformity and Angular Unconformity Formation.(From UNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)

Field Relationships and Geological MappingEvery rock stratum and series of strata can tell us something about thephysical and biological conditions present at some point in the vast timeperiod of geologic time. As can be seen, the sequential record may bevastly distorted or interrupted by tectonic and weathering processes.Figure 8 illustrates how some sedimentary features can indicatestratigraphic sequence and direction. Figure 9 illustrates how gaps in thefossil records can highlight disconformities.



Figure 8. Some Sedimentary Features and Strata. (From THE DYNAMICEARTH by B.J. Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. Thismaterial is used by permission of John Wiley and Sons, Inc.)

Figure 9. Using the Fossil Record. (From THE DYNAMIC EARTH by B.J. Skinnerand S.C. Porter, copyright 2000 John Wiley and Sons. This material is used bypermission of John Wiley and Sons, Inc.)

Observations in the field rarely yield complete information as much of therecord will either be obscured by soils or removed by erosion. However,by recording the geometry of folds, tilts and faults, and throughreconstruction of maps and cross sections, geologists may be able toclosely determine the deformation history and sub-surface geology of thearea concerned (Figure 10).



Figure 10. Geological Map and Derived Cross-section. (FromUNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)

In geologically important areas, oil exploration for example, the surfaceof a particular stratum, such as a sandstone which is favourable to oilaccumulation, is mapped across an area by identifying its depth in aseries of boreholes and a contour map is produced. This information willbe combined with seismic surveys for the construction of a detailedgeological record. The correlation of strata, the structure and themapping of geology are important in petroleum geology for a number ofreasons:

to understand the plate tectonic setting of an area, in order toidentify areas in space and time that are likely to have had thegeological conditions favourable for the generation ofhydrocarbons;

to identify structures which can act to trap hydrocarbons;

to recognise areas which can be correlated with knownhydrocarbon producing regions, in the hope that an analogousgeological situation will lead to identification of new deposits.

The geology of oil and gas reservoirs will be examined in detail in laterTopics.

The Use of MapsA map gives a quantitative representation of the spatial distribution of aparticular feature or set of features. These could be roads, rivers,mountains or continents, for example, depending on scale. The geologistuses 2-dimensional maps to describe geological materials on or near the



Earths surface to represent a 3-dimensional model of the area underexamination.

Topographical maps show a regions elevations and land forms and arefamiliar to most of us in one form or another, whether for navigationaround a city or trekking in the mountains.

Geological maps contain more information than a normal topographicalmap by way of the distribution of rocks and other geological materials atdifferent lithologies and ages over the Earths surface. The geologist isinterested not only in land form but also the pattern of subsurfacestructures, stratigraphic sequences, igneous intrusions andunconformities to name but a few geological features. Certain stages andfeatures are common in the construction of a geological map, and aredemonstrated below.

Deriving subsurface structures and past surface structures removed byerosion can be likened to putting together a 3-dimensional jigsaw withmissing pieces and involves a good deal of intuition with sound geologicalknowledge.

Drawing a Cross-SectionMap 1 is a simple geological map, illustrating the topographical contoursand geological boundaries between different strata. In this case thegeological boundaries are parallel to the topographical contours,indicating the beds are horizontal. This is rare in nature. Most of themaps you meet will show the geological boundaries crossing thetopographical contours. This implies that the layers have been folded ortilted and are dipping at an angle to the horizontal.

To draw a section from Map 1, along the line A-B:

1. draw a base line on graph paper, the exact length A-B;

2. mark off on the baseline points at which the topographicalcontours cross the line of section, and for each point, draw avertical from the base line mark to the appropriate height on avertical scale;

3. a topographic surface can now be constructed by joining all theseintersection points together

Geological details of the section are often lost if the vertical scale used isequivalent to the horizontal scale, eg, if a geological map of scale1:50,000 is used to construct a cross-section, an equivalent vertical scalewould be 1cm = 500m. This vertical scale would be too small to show anydetail, and should therefore be exaggerated, eg 1cm = 200m. Careshould be taken not to over-exaggerate as strata can then appeardistorted.

DipStrata inclined to the horizontal are dipping. Two important concepts arethe direction of dip and the angle of dip. The angle of dip is the maximumangle measured between the strata and the horizontal. The direction ofdip is given as a compass bearing from 0 to 360. For example,



27012 implies a 12 angle of dip to the West. The strike is the direction at

right angles to the dip (Figure 11).

Figure 11. Southerly Dipping Strata in a Quarry.

Structure Contours / Strike LinesContour lines can also be drawn on a geological map for a bedding plane(geological boundary). These are called structure contours or strike lines.They join points of equal height of a bedding plane. On a map, the heightof the geological boundary is known where it crosses a topographiccontour line. A straight line (strike line) can be drawn between points ona geological boundary which are the same height. For simple maps, strikelines are always straight and parallel, and if the dip is constant, they willbe equally spaced.

Calculation of Angle of DipThe angle of dip is calculated from the spacing between the structurecontours. For example, if the distance (measured with a ruler) betweenthe 200m strike line and the 300m strike line for a particular boundary is1.25 cm, and the scale of the map is 2.5 cm = 500m, then a 100mvertical drop of the bed occurs in 1.25 cm = 250m. Hence, the gradient isgiven by 250

100 , or 1 in 2.5. The angle using trigonometry is 2.51 = 22.

Refer to Map 2 for an example.

Recording the angle and direction of dip of a series of strata is importantin constructing a 3-dimensional model. If the surface area underexamination has been eroded flat, some excavation or age analysis of therocks may be required to get things right. Figure 12 demonstrates howthe same surface feature can be resolved to two completely differentmodels, Figure 13 shows how aging as well as dip can infer certainsubsurface structures.



Figure 12. Geometry of Folds Intersecting a Flat Eroded Surface.(From UNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)

Figure 13. Order of Rock Ages in Dome and Basin Formations.(From UNDERSTANDING EARTH by Frank Press and Raymond Siever, 1998, 1994 W.H.Freeman and Company. Used with permission.)

Calculation of the Bed ThicknessRefering to Map 3, the 200m structure contour for the Q-R boundarypasses through the point where the P-Q boundary is at 400m. It followsthat bed Q has a vertical thickness of 200m.

Vertical Thickness and True ThicknessWhen beds are inclined, the vertical thickness (for example, thatpenetrated by a borehole) is greater than the true thickness of the bed,measured perpendicular to the geological boundary. The angle betweenVT (vertical thickness) and T (true thickness) is equal to the angle of dip:

Equation 1 cosineVTTandVTTcosine

The true thickness of a bed is thus the vertical thickness multiplied by thecosine of the angle of dip. Where the dip is low, the cosine is high(approaching 1.0), and VT and T are approximately the same (Figure 14).



Inliers and OutliersAn outcrop of a bed entirely surrounded by outcrops of younger beds iscalled an inlier. An outcrop of beds entirely surrounded by older beds iscalled an outlier. These features are products of erosion.

Figure 14. Relationship Between Vertical Thickness and TrueThickness.

FaultingRefer to lecture notes on stratigraphy and structure for illustrations of thegeometry of faults. The throw of the fault is the vertical displacement ofany bedding plane. The angle of dip of the fault is the angle it makes withthe horizontal, and the angle of hade is its angle with the vertical (Figure15).

Calculation of Fault ThrowStructure contours are drawn for displaced stratum on either side of afault. If, for example, the 100m structure contour for a bed on the westside of the fault coincides with the 500m structure contour for the samebed on the east side of the bed, then the fault has a down throw to theeast of 500m.



Figure 15. Dip, Hade and Throw of a Fault.



Map 1.



Map 2.

Map 2 indicates how the direction of dip is calculated throught thedetermination of strike lines.

Strike lines are drawn through points where the geological boundaryintersects the surface contour at the same height. This must be done forthe same geological boundary, ie, either the top or the bottom of thebed.

This example shows strike lines constructed where the geologicalboundary between beds A and B intersects the 350m and 400m surfacecontours. This indicates the line of dip. Trigonometry can then be used tocalculate the angle of dip.

50m

400m

350m

Measured horizontaldistance

DipAngle

100m

A-B



directionEasterlySouthain26.610050tanAngleDip 1

Map 3.

The continuous black lines are the geological boundaries separating theoutcrops of strata beds P, Q, R, S, T and U. Note that the geologicalboundaries are not parallel to the contour lines, but, in fact, intersectthem. This shows that the beds are dipping.

Some exercises that can be carried out on this map:

1. Draw structure contours for each geological interface;

2. Calculate the direction and angle of dip;

3. Construct a cross-section along line N S, and illustrate on it thedipping beds P to T;

4. Calculate the vertical thickness and true thickness of beds Q and S.

Notes 4 Rock Defoormation and Geological Mapping

Documents

ductile rock

outcrop of rock

fold dip

fold geometry

temperature depth of

ductile manner

reflectionof deformation

shallow rocks maybehave