- Email: [email protected]
Horizontal and Inclined Beds
C H A P T E R
2 Horizontal and Inclined Beds Map representation and interpretation of horizontal and inclined beds are discussed in this chapter. Few key points are given below: 1. A solid line on the map represents a plane (Concept 2.1A), and the plane is: horizontal—if the line is parallel to the height contours (i.e., the broken lines); inclined—if the solid line is curved and cuts the height contours; vertical—if on a map of undulating topography, the plane is represented by a straight line that cuts across height contours. 2. On the map, the curvature of the solid line decreases as the bed steepens. A straight solid line indicates a vertical plane for an undulating topography. However, in a planar topography, both inclined and vertical planes appear as straight lines. 3. Concept 2.1B describes the concepts of strike lines and stratum contours associated with an inclined plane. Strike lines are present on the inclined plane and stratum contours are the orthographic/map projection of strike lines on a horizontal surface. 4. Strike lines of a single plane are mutually parallel. Successive strike lines of equal interval (500 m, 400 m, 300 m, etc.) are equidistant from each other. Steeper planes produce more close-spaced strike lines. Concepts 2.1C and 2.1D demonstrate how the outcrop pattern changes in maps for two oppositely dipping beds. Three maps will be discussed in this chapter: Map 2.1 (horizontal beds), Map 2.2 (inclined beds), and Map 2.3 (three point construction of strike lines).
Map Interpretation for Structural Geologists. DOI: http://dx.doi.org/10.1016/B978-0-12-809681-9.00002-4
9
© 2017 Elsevier Inc. All rights reserved.
10
2. Horizontal and Inclined Beds
CONCEPT 2.1A Horizontal and inclined beds as in hillocks and in maps. Schematic diagrams, not to scale. (A) Horizontal beds in a hillock. (C) Inclined beds in a hillock. (E) Vertical beds cropping out in a hillock. B, D and F are the plan-/map- views of A, C and E, respectively.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
HORIZONTAL AND INCLINED BEDS
11
CONCEPT 2.1B Strike lines and stratum contours of a plane dipping towards the reader: towards south. Schematic diagrams, not to scale. (A) Block diagram showing an inclined plane, the strike lines and stratum contours. (B) Representation of the inclined plane in a map.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
12
2. Horizontal and Inclined Beds
CONCEPT 2.1C (A) Exposure of an inclined plane (dipping in a direction opposite to the reader: towards north) in a valley. The valley slopes towards the reader: towards south. Thus, the dip direction of the plane is opposite to the slope of the valley. (B) Intersection between the outcrop of the bed and the height contours. The construction of the stratum contours is to be noted. MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
HORIZONTAL AND INCLINED BEDS
13
CONCEPT 2.1D (A) Exposure of an inclined plane in a valley. The dip direction of the plane and the slope of the valley are in the same direction: towards the reader—towards south. (B) Intersection between the outcrop of the bed and the height contours.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
14
2. Horizontal and Inclined Beds
MAP 2.1
MAP 2.1 Describe topography and structural geology. Draw a suitable cross-section. MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 2.1
15
The map represents an undulating topography with a western valley sloping south, and an eastern spur occurring side-by-side. Here the maximum height is 800 m or more, and the minimum height is 100 m or less. Four litho-units crop out: A, B, C, and D. The bed boundaries do not cross-cut the height contours, but they are sub-parallel. Hence the beds are horizontal. Fig. 2.1 shows a cross-section along PQ. Cross-section 2 (Fig. 2.1): Drawing horizontal beds in cross-section and modification of the topographic outline. Let us discuss this concept using the example of the litho-contact between the units A and B (A/B) in Map 2.1. On the line PQ, the A/B contact is exposed at ~ 550 m height, and this point is marked by “A/B” on the x-axis of the section. As the contact is horizontal, in the vertical cross-section it will be represented by a line parallel to the x-axis. In other words, the contact needs to be shown as a horizontal line. Therefore, ST is drawn at the 550 m elevation to represent the A/B contact. Now, at point A/B, a line perpendicular to the x-axis is drawn, which meets ST at the point T. Point T indicates the point where A/B contact cuts the topography. Hence, the A/B contact (i.e., the ST line) cannot be extended beyond point T. And the topographic outline should also pass through point T. A similar technique is followed to draw litho-contacts in subsequent cross-sections.
●
FIGURE 2.1 Cross-section along “PQ”: refer to Map 2.1.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
16
2. Horizontal and Inclined Beds
MAP 2.2
MAP 2.2 Describe topography and structural geology. Draw a suitable cross-section. Modified after map 4 of Simpson (1961).
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 2.2
17
The map represents an undulating topography with a western valley and eastern spur occurring side-by-side. Here the maximum height is between 800 to 900 m and the minimum height is between 0 and 100 m. Four lithounits crop out: A, B, C, and D. Fig. 2.2A shows strike lines on the map. The attitude of the litho-contacts is 90°/20°→0° (i.e., strike: 90–270°, true dip amount: 20°, true dip direction: 0°/north) (Fig. 2.2B). Cross-section 3: Drawing an inclined bed in cross-section and modification of the topographic outline. Consider the C/D contact. The points where 300 m and 400 m strike lines of the C/D contact intersects PQ is marked on the crosssection as C/D 300 and C/D 400, respectively. Vertically above these two points, the 300 m (for C/D 300) and 400 m (for C/D 400) are marked by black solid circles. The straight line passing through these two black solid circles is the C/D contact on the cross-section. Now, the point where the C/D contact cuts the PQ line is marked by “C/D” on the x-axis of the cross-section. A line perpendicular to the x-axis and through the point “C/D” marks the extent up to which the C/D contact is actually present in this section. Following the similar argument made in “Cross-section 2”, the topographic outline is adjusted. ● Measuring true dip amount of an inclined planar surface from stratum contours: see Fig. 2.2C for detail. ● Measuring bed thickness: The line PQ is drawn perpendicular to the stratum contours (parallel to the true dip direction). A cross-section drawn along such a line represents the true thicknesses of the planar bodies. The orange lines in Fig. 2.2B show the true thickness for beds B, C, and D. According to the scale of the map, the true thicknesses of the beds B, C, and D are 190 m, 360 m, and 200 m, respectively. In Fig. 2.2A, the A/B 600 m stratum contour is superposed on the B/C 400 m stratum contour. This indicates that the upper surface of unit B (i.e., A/B) and the lower surface of unit B (i.e., B/C) are vertically 200 m apart. Hence, the vertical thickness of the unit B is 200 m, as also shown by the green line in Fig. 2.2B. ●
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
18
2. Horizontal and Inclined Beds
FIGURE 2.2A Solution of Map 2.2.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 2.2
FIGURE 2.2B Cross-section along “PQ”.
FIGURE 2.2C Calculation of true dip amount on the map. (A) Map view. (B) Cross-section view.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
19
20
2. Horizontal and Inclined Beds
MAP 2.3
THREE-POINT PROBLEM TO DEDUCE ATTITUDE
MAP 2.3 Measure the attitude of the contact between A and B units by the “three-point method”.
In the previous map the stratum contour was drawn by joining the two points where the litho-contact intersects a particular height contour. However, in Map 2.3 the contact cuts each height contour only once. Here, the stratum contour is to be drawn by the “three-point method”. The solution is given in Fig. 2.3. Consider that the A/B contact is planar. Hence the 600-, 700- and 800-m stratum contours for the A/B contact will be parallel and the distance between successive strike lines will be same. The points where the 800- and 600-m height contours cut the A/B contact is marked by points P and Q, respectively. The A/B 700-m stratum contour will pass through the mid-point of PQ, i.e., R. Now, the A/B 700-m contact also goes through the point of intersection between A/B contact and the 700-m height contour. Hence this point of interaction is joined with R to get the 700-m stratum contour of the A/B contact. Other stratum contours are drawn parallel to it. The attitude of the litho-contact between the units A and B is 153°/14°→243° (Fig. 2.3).
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Three-Point Problem to Deduce Attitude
FIGURE 2.3 Solution for Map 2.3.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
21
This page intentionally left blank
This page intentionally left blank
This page intentionally left blank
2 Horizontal and Inclined Beds Map representation and interpretation of horizontal and inclined beds are discussed in this chapter. Few key points are given below: 1. A solid line on the map represents a plane (Concept 2.1A), and the plane is: horizontal—if the line is parallel to the height contours (i.e., the broken lines); inclined—if the solid line is curved and cuts the height contours; vertical—if on a map of undulating topography, the plane is represented by a straight line that cuts across height contours. 2. On the map, the curvature of the solid line decreases as the bed steepens. A straight solid line indicates a vertical plane for an undulating topography. However, in a planar topography, both inclined and vertical planes appear as straight lines. 3. Concept 2.1B describes the concepts of strike lines and stratum contours associated with an inclined plane. Strike lines are present on the inclined plane and stratum contours are the orthographic/map projection of strike lines on a horizontal surface. 4. Strike lines of a single plane are mutually parallel. Successive strike lines of equal interval (500 m, 400 m, 300 m, etc.) are equidistant from each other. Steeper planes produce more close-spaced strike lines. Concepts 2.1C and 2.1D demonstrate how the outcrop pattern changes in maps for two oppositely dipping beds. Three maps will be discussed in this chapter: Map 2.1 (horizontal beds), Map 2.2 (inclined beds), and Map 2.3 (three point construction of strike lines).
Map Interpretation for Structural Geologists. DOI: http://dx.doi.org/10.1016/B978-0-12-809681-9.00002-4
9
© 2017 Elsevier Inc. All rights reserved.
10
2. Horizontal and Inclined Beds
CONCEPT 2.1A Horizontal and inclined beds as in hillocks and in maps. Schematic diagrams, not to scale. (A) Horizontal beds in a hillock. (C) Inclined beds in a hillock. (E) Vertical beds cropping out in a hillock. B, D and F are the plan-/map- views of A, C and E, respectively.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
HORIZONTAL AND INCLINED BEDS
11
CONCEPT 2.1B Strike lines and stratum contours of a plane dipping towards the reader: towards south. Schematic diagrams, not to scale. (A) Block diagram showing an inclined plane, the strike lines and stratum contours. (B) Representation of the inclined plane in a map.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
12
2. Horizontal and Inclined Beds
CONCEPT 2.1C (A) Exposure of an inclined plane (dipping in a direction opposite to the reader: towards north) in a valley. The valley slopes towards the reader: towards south. Thus, the dip direction of the plane is opposite to the slope of the valley. (B) Intersection between the outcrop of the bed and the height contours. The construction of the stratum contours is to be noted. MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
HORIZONTAL AND INCLINED BEDS
13
CONCEPT 2.1D (A) Exposure of an inclined plane in a valley. The dip direction of the plane and the slope of the valley are in the same direction: towards the reader—towards south. (B) Intersection between the outcrop of the bed and the height contours.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
14
2. Horizontal and Inclined Beds
MAP 2.1
MAP 2.1 Describe topography and structural geology. Draw a suitable cross-section. MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 2.1
15
The map represents an undulating topography with a western valley sloping south, and an eastern spur occurring side-by-side. Here the maximum height is 800 m or more, and the minimum height is 100 m or less. Four litho-units crop out: A, B, C, and D. The bed boundaries do not cross-cut the height contours, but they are sub-parallel. Hence the beds are horizontal. Fig. 2.1 shows a cross-section along PQ. Cross-section 2 (Fig. 2.1): Drawing horizontal beds in cross-section and modification of the topographic outline. Let us discuss this concept using the example of the litho-contact between the units A and B (A/B) in Map 2.1. On the line PQ, the A/B contact is exposed at ~ 550 m height, and this point is marked by “A/B” on the x-axis of the section. As the contact is horizontal, in the vertical cross-section it will be represented by a line parallel to the x-axis. In other words, the contact needs to be shown as a horizontal line. Therefore, ST is drawn at the 550 m elevation to represent the A/B contact. Now, at point A/B, a line perpendicular to the x-axis is drawn, which meets ST at the point T. Point T indicates the point where A/B contact cuts the topography. Hence, the A/B contact (i.e., the ST line) cannot be extended beyond point T. And the topographic outline should also pass through point T. A similar technique is followed to draw litho-contacts in subsequent cross-sections.
●
FIGURE 2.1 Cross-section along “PQ”: refer to Map 2.1.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
16
2. Horizontal and Inclined Beds
MAP 2.2
MAP 2.2 Describe topography and structural geology. Draw a suitable cross-section. Modified after map 4 of Simpson (1961).
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 2.2
17
The map represents an undulating topography with a western valley and eastern spur occurring side-by-side. Here the maximum height is between 800 to 900 m and the minimum height is between 0 and 100 m. Four lithounits crop out: A, B, C, and D. Fig. 2.2A shows strike lines on the map. The attitude of the litho-contacts is 90°/20°→0° (i.e., strike: 90–270°, true dip amount: 20°, true dip direction: 0°/north) (Fig. 2.2B). Cross-section 3: Drawing an inclined bed in cross-section and modification of the topographic outline. Consider the C/D contact. The points where 300 m and 400 m strike lines of the C/D contact intersects PQ is marked on the crosssection as C/D 300 and C/D 400, respectively. Vertically above these two points, the 300 m (for C/D 300) and 400 m (for C/D 400) are marked by black solid circles. The straight line passing through these two black solid circles is the C/D contact on the cross-section. Now, the point where the C/D contact cuts the PQ line is marked by “C/D” on the x-axis of the cross-section. A line perpendicular to the x-axis and through the point “C/D” marks the extent up to which the C/D contact is actually present in this section. Following the similar argument made in “Cross-section 2”, the topographic outline is adjusted. ● Measuring true dip amount of an inclined planar surface from stratum contours: see Fig. 2.2C for detail. ● Measuring bed thickness: The line PQ is drawn perpendicular to the stratum contours (parallel to the true dip direction). A cross-section drawn along such a line represents the true thicknesses of the planar bodies. The orange lines in Fig. 2.2B show the true thickness for beds B, C, and D. According to the scale of the map, the true thicknesses of the beds B, C, and D are 190 m, 360 m, and 200 m, respectively. In Fig. 2.2A, the A/B 600 m stratum contour is superposed on the B/C 400 m stratum contour. This indicates that the upper surface of unit B (i.e., A/B) and the lower surface of unit B (i.e., B/C) are vertically 200 m apart. Hence, the vertical thickness of the unit B is 200 m, as also shown by the green line in Fig. 2.2B. ●
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
18
2. Horizontal and Inclined Beds
FIGURE 2.2A Solution of Map 2.2.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 2.2
FIGURE 2.2B Cross-section along “PQ”.
FIGURE 2.2C Calculation of true dip amount on the map. (A) Map view. (B) Cross-section view.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
19
20
2. Horizontal and Inclined Beds
MAP 2.3
THREE-POINT PROBLEM TO DEDUCE ATTITUDE
MAP 2.3 Measure the attitude of the contact between A and B units by the “three-point method”.
In the previous map the stratum contour was drawn by joining the two points where the litho-contact intersects a particular height contour. However, in Map 2.3 the contact cuts each height contour only once. Here, the stratum contour is to be drawn by the “three-point method”. The solution is given in Fig. 2.3. Consider that the A/B contact is planar. Hence the 600-, 700- and 800-m stratum contours for the A/B contact will be parallel and the distance between successive strike lines will be same. The points where the 800- and 600-m height contours cut the A/B contact is marked by points P and Q, respectively. The A/B 700-m stratum contour will pass through the mid-point of PQ, i.e., R. Now, the A/B 700-m contact also goes through the point of intersection between A/B contact and the 700-m height contour. Hence this point of interaction is joined with R to get the 700-m stratum contour of the A/B contact. Other stratum contours are drawn parallel to it. The attitude of the litho-contact between the units A and B is 153°/14°→243° (Fig. 2.3).
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Three-Point Problem to Deduce Attitude
FIGURE 2.3 Solution for Map 2.3.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
21
This page intentionally left blank
This page intentionally left blank
This page intentionally left blank