Lines on the map are drawn to show the boundaries between each of the rock units. The most obvious use of a geological map is to indicate the nature of the near-surface bedrock.
Introduction
In the field, the direction of dip is usually measured with a magnetic compass that includes a device called a clinometer, based on a plumb line or spirit level principle, for measuring dip angles.
Plunge of lines
Strike lines
The direction of the dip is always at right angles to the strike and can therefore be taken from. The map symbol used to represent the dip of bedding usually consists of a bar in the direction of strike with a short dash to the side in the direction of dip (see the list of symbols at the beginning of the book).
Apparent dip
The line on the map that separates the formations has an irregular shape even though the contact between the formations is a planar surface (Fig. 2.9B). Where the beds dip steeply, the contact flow is straighter on the map (Fig. 2.10C, D, E, F).
Representing surfaces on maps
To understand the shapes described by the boundaries of formations on geological maps, it is important to realize that they represent a line (horizontal, dip or curved) through the intersection in three dimensions of two surfaces (Fig. 2.9 B, D). For example, if the ground surface were flat (Fig. 2.9D), the contacts would run as straight lines on the map (Fig. 2.9C).
Properties of contour maps
Drawing vertical cross-sections through topographical and geological surfaces
The apparent dip in the XZ direction is the observed slope of the sandstone bed at true scale (vertical scale = horizontal scale) of a vertical section along the XZ line. The same formula as for dip angle above can be used, except that the "contour spacing" is now the apparent spacing observed along the XZ line.
Three-point problems
To find the angle of incidence, we need to calculate the slope of the line on the surface at right angles to the impact. If it is necessary to find the inclination of the contact, we can use the method of the previous example.
Outcrop patterns of geological surfaces exposed on the ground
Other structure contours for other elevations will be parallel and equally spaced on the map.
Buried and eroded parts of a geological surface The thin coal seam in the previous example only occurs at
Contours of burial depth (isobaths)
V-shaped outcrop patterns
Complete exposure of the thin limestone bed exposed in the northwestern part of the area (Fig. 2.18A). The intersections of the topographic contours with the structural contours of the same height yield points that lie on the outcrop of the thin limestone bed.
Structure contours from outcrop patterns A map showing outcrops of a surface together with
During the mapping, there are a few outcrops where contacts are visible and where dips can be measured, but the rest of the map is based on interpretation. Spacing between contours = contour interval Tangent (dip angle) Since the bed outcrop in the northwest part of the map is at an elevation of 350 meters, the 350 meter structure contour must pass through this point.
Geological surfaces and layers
Stratigraphic thickness
The vertical thickness will be correct on any vertical section, but the true thickness will only be visible on cross sections parallel to the dip direction of the beds. The horizontal thickness is given by the horizontal separation of each pair of structure contours of the same height (one for the base, one for the top).
Isochores and isopachs
It is important to note that the outcrop width of a bed on a map (W in Fig. 2.21) is not equal to the horizontal thickness unless the ground surface is horizontal. Find the vertical thickness, horizontal thickness, true thickness, and dip angle of the sandstone formation from the structure contours in Fig.
Topographic effects and map scale
For each map, determine the direction and dip angle of the geologic contact shown. What is the direction of dip and angle of dip of the Two-foot-nine seam.
Cylindrical and non-cylindrical folding
To demonstrate this, place both hands on a tablecloth and pull them together; shortening the tablecloth results in a number of folds. For example, the current mountain range of the Andes is a fold belt created by the shortening of the rocks of South America since the end of the Cretaceous period.
Basic geometrical features of a fold
For example, the Upper Carboniferous rocks of south-west England and southern Wales show intense folding when compared with rocks of the same age further north in Britain. Zones of concentrated deformation and folding are called fold belts or mountain belts, and these occupy long parallel tracts of the earth's crust.
Terms relating to the orientation of folds
The orientation of the fold hinge and the axial surface are the two most important directional characteristics of a fold. The orientation of the axial surface is described using its dip direction and dip angle.
The tightness of folding
Curvature variation around the fold
Symmetrical and asymmetrical folds
Types of non-cylindrical fold
Layer thickness variation around folds
Accurately measuring the thickness of the layer at many points between the bend points provides data that allows classification of the fold. The latter are called similar folds because the upper and lower surfaces of the bed have the same shape.
Structure contours and folds
The lamination around the limbs also undergoes distortions or strains that lead to a relative thinning of the lamination at some positions in the fold compared to others. Note that the contour lines for one limb intersect those of the other limb (Fig. 3.22C).
Determining the plunge of a fold from structure contours
Since cylindrical folds give equal cross-sections in parallel sections (see Section 3.1), these folds give contour patterns of structure consisting of contours of similar shape and size (Fig. 3.20A). Use chalk to sketch the outlines of the structure on the folded map and draw on the map how these outlines will look from above (Figure 3.20A).
Lines of intersection of two surfaces
Determining the plunge of a fold from the dips of fold limbs
Sections through folded surfaces
For each cut section (Fig. 3.26), carefully note (a) the apparent tightness, (b) the apparent asymmetry, and (c) the apparent curvature of the folds seen in the oblique section. This exercise demonstrates the importance of the orientation of the slice through the fold in determining the fold shape one observes.
The profile of a fold
Horizontal sections through folds
Note the way the fold appears to close on the horizontal and steep parts of the protruding surface.
Construction of true fold profiles
Recognition of folds on maps
Hinge points and axial surface traces
Constructing hinge lines on maps
Determining the nature of folds on maps
Cross-sections through folded areas
Inliers and Outliers
The numbers on the map show the depths in meters (below sea level) to the top of the Wittekalk Limestone. Is it correct to say that the Carboniferous Limestone is thicker in the fold zone?
Fault planes
In the error example, we note that the sequence is not reversed, but repetition of the same sequence occurs. Their attitude relative to the structure of the country rocks allows faults to be classified.
Slip and separation
The dip can sometimes be inferred from the shape of the outcrop over irregular topography. Abeding plane fault runs parallel to the bedding (Fig. 4.5) and is therefore a variety of strike faults.
Separation terms
Dip separation is the bottom shift in the downward direction of the fault (Fig. 4.7). For a given fault, the ratio of heave to throw depends in this way on the dip of the fault plane.
Repetition and omission of strata
Otherwise, this separation could be measured from the map in a direction parallel to the strike of the fault between the structural contours of the same height for the two sides of the displaced surface. Alternatively, the heave can be measured directly from the map (heave = 6 m), as it is the distance between the hanging wall and footwall cut-off lines measured perpendicular to the line of the fault.
Determining the slip of a fault
The direction can be stated as the dip of the line joining the two displaced points. One of these planes is the fault plane itself, the other two are a pair of any .. the above example and Fig. 4.14), older errors, inconsistencies (explained in chapter 5), sheet interferences (chapter 6), etc. .
Components of slip
The strike-slip component is the component of net slip in the direction of strike of the fault plane. The strike-slip component is equal to the length of the HF projected onto the fault line (98 m in Fig. 4.13B).
Classification of faults based on slip
The cross-sectional plane (bottom diagram) coincides with the fault plane. Determine the strike separation, dip separation, dip, rise, and vertical separation of the fault.
Types of unconformity
For example, the nature of sedimentary rocks shows the physical environment in which the sediments were deposited and how this environment changed over time. In a given area, the sedimentary rocks themselves contain only a partial record of geologic time since sedimentation is unlikely to have continued continuously.
Overstep and overlap
The contact between the underlying rocks and the rocks that unconformably overlie them is called an unconformity surface or unconformity plane. A transgression refers to a subconformity relationship where the unconformity surface truncates stratigraphic boundaries (Figure 5.3).
Subcrop maps
On the map (Fig. 5.6A), construct the subcrop of the thin coal seam on the pre-Permian unconformity surface. Use the calculated trend to draw the coal subcrop through point Z on the map (Fig. 5.7B).
The geological usefulness of unconformities Geological map interpretation has two main facets. The first
Shade the regions on the map where the coal seam does not exist at depth. Identify an unconformity on the map and determine the dip of the unconformity surface.
Intrusive igneous rocks
The zone of metamorphosed rocks is called a metamorphic aureole, and is shown on maps as a zone that surrounds or runs parallel to the contacts of the intrusion. For example, a trunk may form an area of upland with topographic contours after the contact of the intrusion (Fig. 6.6B).
Extrusive igneous rocks
The intrusive character of an igneous body can be made less clear by later deformation of the rocks of the area. What do the bodies of igneous rock tell us about relative ages of the rock units in the area.
Foliations
Roof clays, such as those of the Cambrian of North Wales, owe their fissibility to the presence of slate cleavage. From the shape of deformed (contracted) fossils in rifted rocks it can be shown that espalier cleavage planes have formed perpendicular to the direction of greatest shortening.
Axial plane foliations
Through chemical alteration of rocks, the grains that make up the rock can change shape and rotate so that their long dimensions are turned away from the direction of greatest shortening in the rock. Secondary foliation, which is present in fine-grained metamorphic rocks such as shales, is called cleavage.
The relationship of cleavage to bedding
Provided the folds are oriented upward (i.e. the antiforms are anticlines, the synforms are synclines), the overturned beds can be identified by the fact that on this limb the bedding will dip more steeply than the cleavage planes. The cleavage, which shows refraction, runs parallel to the hammer handle in that part of the outcrop (north is on the left of the photo).
