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Summary maps
C H A P T E R
6 Summary maps So far, we presented maps with only one structural feature (e.g., unconformity/fold/fault). In this chapter we will discuss maps with more than one such features. The objective is to describe the structural details, to draw suitable cross-sections and decipher time-wise sequence of events.
Map Interpretation for Structural Geologists. DOI: http://dx.doi.org/10.1016/B978-0-12-809681-9.00006-1
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© 2017 Elsevier Inc. All rights reserved.
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6. Summary maps
MAP 6.1
MAP 6.1 Reproduced from ‘maps without contours: 2’ of Bradshaw and Jarman (1969). Permission received by Elsevier.
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Map 6.1
Six litho-units crop out in Map 6.1. Conglomerate, shales, sandy shale, and sandstone constitute a litho-group (Group A). This litho-group is folded and faulted. Group A is underlain by a horizontal basalt layer. The contact between Group A and this layer is an angular unconformity. Group A rocks are intruded by a 64° trending vertical quartz porphyry dyke. At two places alluvium deposits are observed. Details of Group A: Group A rocks are synformally folded. The western limb dips 15° towards east and the eastern limb 15° towards west. It is a symmetric fold. The fold is disrupted by an E trending vertical transverse fault. Here the northern block is downthrown (as understood from the broadening of hinge of the synform). The solution is given in Fig. 6.1. Sequence of events: Deposition of alluvium Basalt layer Intrusion of quartz porphyry dyke Development of angular unconformity Faulting of Group-A Folding of Group-A Deposition of Group-A
(youngest)
(oldest)
FIGURE 6.1 Solution of Map 6.1.
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6. Summary maps
MAP 6.2
MAP 6.2 Reproduced from fig. 56 of Thomas (1966). Permission received by Elsevier.
Seven litho-units, viz. A, B, C, D, E, F, and G crop out in Map 6.2. Units A, B, and C belong to a litho-group (Group 1). Group 1 rocks are folded, faulted, and intruded by dyke D (Fig. 6.2). This sequence is separated from the overlying litho-unit E by an angular unconformity. Units F and G are sills intruded within E. Fault F5 affected the overall rock sequence. The ‘m’ symbol indicates contact metamorphism effects. Details of Group 1: Group 1 rocks are antiformally folded with NE trending axial trace. The northern limb dips towards NW. These folded strata are disrupted by five faults. F1, F2, F3, and F4 disrupted only the Group 1 rocks. Fault F5 is youngest as it displaces/slips the overlying litho-unit E as well. From the broadening or narrowing of the hinge, the upthrown/downthrown blocks can be identified for those four faults. The upthrown block for these faults are: F1—western block, F2—eastern block, F3—western block, F4— western block. Fault F5: This is a map of almost planar topography and the western block of F5 exposes only the top-most litho-unit, i.e., E. Whereas, in the eastern block all the other underlying litho-units expose. Hence, the eastern block is upthrown.
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Map 6.2
Sequence of events (also consult Fig. 6.2): Fault F5
(youngest geological event)
Intrusion of F, G Deposition of E Development of angular unconformity Intrusion of D Faulting F1, F2, F3, F4 Folding of Group 1 Deposition of Group 1
(oldest geological event)
FIGURE 6.2 Solution of “Map 6.2. Alphabets denote different litho-units, which are referred in the structural/geological interpretation.”
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6. Summary maps
MAP 6.3
MAP 6.3 Reproduced from fig. 10.9 of Borradaile (2014). Permission received by Elsevier.
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Map 6.3
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FIGURE 6.3A Solution of Map 6.3.
Five litho-units, A, B, C, D, and E, crop out in Map 6.3 of undulating topography. Litho-units D and E belong to a single group (Group 1). Group 1 rocks are folded. The strike lines of the limbs are parallel. Hence it is a nonplunging fold. The axial trace (trace of the axial plane) trends same as the limbs, i.e., 102°. The attitude of the southerly dipping limb is 102°/27°→192°, and that of the northerly dipping limb is 102°/27°→12°. The interlimb angle of the fold is {180°− (27° + 27°)} = 126°. Hence, it is also a gentle fold. Folded D-E sequence gets truncated against the base of C. The base of C is not parallel with the base of the overlying unit B. Hence there is an angular relation between them. Again both B and C terminate below A. These indicate that the basal planes of the units C, B and A are angular unconformity surfaces. They have been marked as angular unconformity 1, 2, and 3 in Fig. 6.3A.
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6. Summary maps
The attitude of this basal plane of C is 27°/14°→297°. The attitude of the basal plane of B is 58°/12°→328°. The base of A is horizontal (as it is not intersected by any height contours). A cross-section drawn along LR is given in Fig. 6.3B. Sequence of events (Fig. 6.3A): Deposition of A
(youngest)
Angular unconformity 3 Deposition of B Angular unconformity 2 Deposition of C Angular unconformity 1 Folding of D, E Deposition of D, E
(oldest)
FIGURE 6.3B Cross-section drawn for Map 6.3.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.4
MAP 6.4
MAP 6.4 Reproduced from Map no. 5 of Platt (1951). Permission received by Elsevier.
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In Map 6.4 of undulating topography, two groups of rocks crop out: Group-1: litho-units A, F, O, E, and T; and Group 2: litho-units R and S. The Group-1 rocks truncated at the base of Group 2 and the Group 2 rocks are parallel to this plane of truncation. Hence, the base of Group-2 rocks is an angular unconformity. The attitude of Group-1 rocks is 130°/46° → 240°. The attitude of Group 2 rocks as well as the angular unconformity: 95°/19° → 5°. Two vertical faults (F1 and F2) slipped these rocks. F1 affected Group-1 rocks only, whereas F2 affected both the groups. F1 trends 161° and F2 trends 12°. The eastern block is downthrown by F1 and the vertical separation is 50 m (from Fig. 6.4). The western block is downthrown by F2 and the vertical separation is 100 m (from Fig. 6.4), which is also indicated by the superposition of the strike lines ‘A/F 500w’ and ‘A/F 600e’. Sequence of events (Figs. 6.3, 6.4A,B): Faulting F2
(youngest geological event)
Deposition of Group-2 rocks …..Angular unconformity….. Faulting F1 Deposition of Group-1 rocks
(oldest geological event)
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Map 6.4
FIGURE 6.4A Solution of Map 6.4.
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FIGURE 6.4B Cross-section drawn for Map 6.4.
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Map 6.5
MAP 6.5
MAP 6.5 Reproduced from ‘advanced Map 3’ of Bradshaw and Jarman (1969). Permission received by Elsevier.
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6. Summary maps
Seven litho-units belonging to two groups crop out in this map (Map 6.5) of undulating topography. From bottom to top, the lower sequence (Group-1) consists of limestone, mudstone, sandstone and oolite. Group-2 is made up of arkose and sandstone. Group-1 rocks are folded with 104° trending axial plane. The axial plane dips 86° (found from Fig. 6.6). The trend of the northern limb is 149°/23° → 59°, and that of the southern limb is 61°/23° → 151°. The attitude of the fold axis is 17°→195°. The interlimb angle, deduced from stereo-net (not shown here), is 149°. The fold is a gently plunging-symmetric-gentle upright antiform. An acidic igneous rock intruded the Group-1 rocks. From this massive intrusion, three dykes with trends 65°, 77°, 115° emanated but remained within Group-1 rocks. The igneous intrusion and the the dykes created local contact metamorphism (indicated by ‘m’ in the map). Although the host rocks are folded, the dykes are not. Therefore, intrusion postdates folding. The Group-1 rocks along with the intrusion truncates against the base of Group-2. Hence the base of Group-2 is an angular unconformity. The attitude of Group-2 rocks as well as that of the angular unconformity is 15°/13° → 105°. Sequence of events (Figs. 6.5A,B): Deposition of Group-2 rocks
(youngest)
Development of angular unconformity Igneous intrusion in Group-1 rocks Folding of Group-1 rocks Deposition of Group-1 rocks
(oldest)
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.5
FIGURE 6.5A Solution of Map 6.5.
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FIGURE 6.5B Cross-section drawn for Map 6.5.
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Map 6.6
MAP 6.6
MAP 6.6 Reproduced from fig 10.12 of Borradaile (2014). Permission received by Elsevier.
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Litho-units A and B crop out in this map (Map 6.6) of undulating topography. Both of them are intruded by two vertical dykes D1 and D2. Litho-units A and B are folded and faulted. The fault plane is not folded and the strikeline magnitudes on the either side of the fault indicate that the folded sequence is faulted. Hence folding predates faulting. The solution is given in Fig. 6.6A. The trend of axial plane is 15°. The attitude of the west dipping limb is 15°/40° → 285°, and that of the east dipping limb is 15°/40° → 105°. Hence the fold is a symmetric-nonplunging-open fold. The attitude of the fault plane is 74°/63° → 164°. Magnitudes of the strike lines on either side of the fault indicates that the southern block (the hanging wall block) is upthrow. It is a reverse fault. In Fig. 6.6B, EF is the net slip and according to the scale of the map, it is 80 m. The pitch of net slip on the fault plane is 54° from 74°. D1 and D2 trend 166° and 128°, respectively. Dyke D2 is faulted, but not folded. Hence, D2 intruded in between the folding and the faulting events. Dyke D1 is not displaced and it’s pitch on the fault plane (90°) does not match with the pitch of net slip on the fault plane (i.e. 54° from 74°). Hence, Dyke D1 intruded after faulting. A crosssection has been presented in Fig. 6.6C. Sequence of events (Figs. 6.6A,B): Intrusion of dyke D1
(youngest)
Faulting Intrusion of dyke D2 Folding of A and B Deposition of A and B
(oldest)
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Map 6.6
FIGURE 6.6A Solution of Map 6.6.
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FIGURE 6.6B Net slip calculation for Map 6.6 and Fig. 6.6A.
FIGURE 6.6C Cross-section for Map 6.6.
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Map 6.7
MAP 6.7
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
MAP 6.7 Reproduced from Map 3 of Platt (1951). Permission received by Elsevier.
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Six litho-units crop out in Map 6.7. They can be subdivided into the following three groups: Group-1: litho-units L, N, and O; Group-2: litho-units A and E; and Group-3: a 59° trending vertical dyke (D) that intruded Group 1 rocks only. Hence the dyke is younger than Group 1 rocks and older than Group 2 rocks. The pattern of strike lines indicates that the Group-1 rocks are folded. The attitudes of the limbs are 0°/31°→90° and 0°/16°→270°. The Group-1 rocks truncates against the base of Group 2 rocks. Also, Group-2 rocks are parallel to this plane of truncation. Hence an angular unconformity separates Group-1 from Group-2 rocks. The three rock groups are disrupted by a fault. The attitude of the fault plane is 97°/71°→187°. Here the northern block (the footwall block) is upthrown. Hence it is a normal fault. The amount of net slip is 100 m (Fig. 6.7B). The pitch of net slip on fault plane is 66° from 277°. A cross section drawn along XY line is shown in Fig. 6.7C. Sequence of events (6.7A,C): Faulting
(youngest)
Deposition of Group-2 rocks …angular unconformity… Intrusion of dyke Folding of Group-1 rocks Deposition of Group-1 rocks
(oldest)
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FIGURE 6.7A Solution of Map 6.7.
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6. Summary maps
FIGURE 6.7B Calculation of net-slip for Map 6.7. The calculations are done on the fault plane, which trends 97–277 º (orange line). Pitch of dyke on fault plane 66° from 277° (black line, cuts the strike line of the fault plane at “P”). Pitch of the unconformity planes on fault plane is 7° from 97°. The traces of the unconformity planes for southern (blue line) and northern (green line) blocks intersect the strike of the fault plane at Q and R, respectively. Net slip: PS = 1 cm, which is 100 m, according to the map scale. The pitch of net slip (i.e., PS) on fault plane is 66º from 97º.
FIGURE 6.7C Cross-section for Map 6.7.
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Map 6.8
MAP 6.8
MAP 6.8 Reproduced from Map 7 of Platt (1951). Permission received by Elsevier.
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FIGURE 6.8A Solution of Map 6.8.
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Map 6.8
Two groups of rocks crop out in Map 6.8: Group-1: rock units M, N, and E. Group-2: rock units L and T. Besides, recent deposits O and A are also present at places. Group-A rocks folded and faulted. The attitudes of the fold limbs are 0°/64°→90° and 0°/45°→270°. Hence, they are nonplunging asymmetric folds (antiform/synform). The interlimb angle of the folds are 71°. Therefore it is an open fold. The axial traces, that trend N in Fig. 6.8A, have been drawn in the cross-section (Fig. 6.8B) by locating (i) where the axial traces intersect (red dots) the topographic profile along the XY line, and (ii) joining the red dot in the central part of the diagram with the corresponding hinge point. The second axial trace (left to the former in Fig. 6.8B) is drawn parallel to the previous axial trace and passing through its corresponding red dot. Two faults Fault-1 and Fault-2 affected the folded sequence. Both of the faults are vertical and trends 90°. For Fault-1, the southern block (~the central block) is upthrown by 100 m. For Fault 2, the southern block is downthrown by 200 m. The two groups of rocks are separated by an angular unconformity. Its attitude as well as that for Group-2 rocks is 43°/12°→133°. Sequence of events (Fig. 6.8B): Deposition of A and O
(Youngest)
Tilting Deposition of L and T Angular unconformity Folding of Group 1 rocks Deposition of Group 1 rocks (M, N and E)
(Oldest)
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FIGURE 6.8B Cross-section drawn for Map 6.8.
Map 6.9
MAP 6.9
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
MAP 6.9 Reproduced from Map 18 of Platt (1951). Permission received by Elsevier.
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Seven litho-units crop out in this map (Map 6.9) of undulating topography. They can be classified as: Group 1: Silurian units S1, S2, S3; Group 2: Old Red Sandstone (R) and Dolerite (D); Group 3- Trias (T) and Jurassic (J). Based on the pattern of the strike lines, it is seen that the Group 1 units are folded. The trend of the axial planes is 60–240° (Fig. 6.9A). The attitude of NW dipping limb is 60°/76°→330° and that of the SE dipping limb 60°/52°→150°. Folded Group 1 rocks (Silurian) truncate at the base of the unfolded Old Red Sandstones (mainly Devonian). Hence this basal plane (marked green in Fig. 6.9A) indicates a break in deposition and deformation as well. That means, this surface is an angular unconformity (attitude: 59°/11°→329°). The dolerite intrusion appears to be concordant with the host Old Red Sandstone, hence it is a sill (attitude 8°/179°→269°). Group 1 and Group 2 units truncate against the base of Group 3 (marked light blue). Group 3 units are parallel to this base. Hence, the base of Group 3 is an angular unconformity surface (attitude 114°/7°→204°). There are three faults—F1, F2, and F3 present in the map. F1 has affected Group 1 only. F1 is a vertical fault trending 82–262°and the eastern block is upthrown by 200 m. F2 has affected Group 1 and Group 2 units. F2 is a vertical fault trending 174–264° and the western block is upthrown by 300 m. F3 is a normal fault that has affected all three groups of rocks. The trend of F3 is 66°/77°→156°. Here the western block (i.e., the footwall block) is upthrown by 100 m. Hence it is a normal fault. A cross-section is presented in Fig. 6.9B. Sequence of events: Fault 3
(Youngest)
Deposition of Group 2 rocks Development of angular unconformity Fault 2 Angular Unconformity Fault 1 Folding of Group 1 rocks Deposition of Group 1 rocks Intrusion of Dolerite in Old Red Sandstone Deposition of Old Red Sandstone
(Oldest)
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
FIGURE 6.9A Solution of Map 6.9.
FIGURE 6.9B Cross-section drawn for Map 6.9.
Exercises
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EXERCISES For each of the following maps (Maps 6.10–6.14): give geological and structural details, draw a proper crosssections, and find out the sequence of events along with explanations.
MAP 6.10 Reproduced from Map 8 of Simpson (1961). Permission received by Elsevier.
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MAP 6.11 Reproduced from Map 6 of Platt (1951). Permission received by Elsevier.
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MAP 6.12 Reproduced from Map 9 of Platt (1951). Permission received by Elsevier.
MAP 6.13 Reproduced from “ADVANCED MAPS:1” of Bradshaw and Jarman (1969). Permission received by Elsevier.
MAP 6.14 Reproduced from Map 17 of Platt (1951). Permission received by Elsevier.
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6 Summary maps So far, we presented maps with only one structural feature (e.g., unconformity/fold/fault). In this chapter we will discuss maps with more than one such features. The objective is to describe the structural details, to draw suitable cross-sections and decipher time-wise sequence of events.
Map Interpretation for Structural Geologists. DOI: http://dx.doi.org/10.1016/B978-0-12-809681-9.00006-1
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© 2017 Elsevier Inc. All rights reserved.
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6. Summary maps
MAP 6.1
MAP 6.1 Reproduced from ‘maps without contours: 2’ of Bradshaw and Jarman (1969). Permission received by Elsevier.
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Map 6.1
Six litho-units crop out in Map 6.1. Conglomerate, shales, sandy shale, and sandstone constitute a litho-group (Group A). This litho-group is folded and faulted. Group A is underlain by a horizontal basalt layer. The contact between Group A and this layer is an angular unconformity. Group A rocks are intruded by a 64° trending vertical quartz porphyry dyke. At two places alluvium deposits are observed. Details of Group A: Group A rocks are synformally folded. The western limb dips 15° towards east and the eastern limb 15° towards west. It is a symmetric fold. The fold is disrupted by an E trending vertical transverse fault. Here the northern block is downthrown (as understood from the broadening of hinge of the synform). The solution is given in Fig. 6.1. Sequence of events: Deposition of alluvium Basalt layer Intrusion of quartz porphyry dyke Development of angular unconformity Faulting of Group-A Folding of Group-A Deposition of Group-A
(youngest)
(oldest)
FIGURE 6.1 Solution of Map 6.1.
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6. Summary maps
MAP 6.2
MAP 6.2 Reproduced from fig. 56 of Thomas (1966). Permission received by Elsevier.
Seven litho-units, viz. A, B, C, D, E, F, and G crop out in Map 6.2. Units A, B, and C belong to a litho-group (Group 1). Group 1 rocks are folded, faulted, and intruded by dyke D (Fig. 6.2). This sequence is separated from the overlying litho-unit E by an angular unconformity. Units F and G are sills intruded within E. Fault F5 affected the overall rock sequence. The ‘m’ symbol indicates contact metamorphism effects. Details of Group 1: Group 1 rocks are antiformally folded with NE trending axial trace. The northern limb dips towards NW. These folded strata are disrupted by five faults. F1, F2, F3, and F4 disrupted only the Group 1 rocks. Fault F5 is youngest as it displaces/slips the overlying litho-unit E as well. From the broadening or narrowing of the hinge, the upthrown/downthrown blocks can be identified for those four faults. The upthrown block for these faults are: F1—western block, F2—eastern block, F3—western block, F4— western block. Fault F5: This is a map of almost planar topography and the western block of F5 exposes only the top-most litho-unit, i.e., E. Whereas, in the eastern block all the other underlying litho-units expose. Hence, the eastern block is upthrown.
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Map 6.2
Sequence of events (also consult Fig. 6.2): Fault F5
(youngest geological event)
Intrusion of F, G Deposition of E Development of angular unconformity Intrusion of D Faulting F1, F2, F3, F4 Folding of Group 1 Deposition of Group 1
(oldest geological event)
FIGURE 6.2 Solution of “Map 6.2. Alphabets denote different litho-units, which are referred in the structural/geological interpretation.”
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6. Summary maps
MAP 6.3
MAP 6.3 Reproduced from fig. 10.9 of Borradaile (2014). Permission received by Elsevier.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.3
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FIGURE 6.3A Solution of Map 6.3.
Five litho-units, A, B, C, D, and E, crop out in Map 6.3 of undulating topography. Litho-units D and E belong to a single group (Group 1). Group 1 rocks are folded. The strike lines of the limbs are parallel. Hence it is a nonplunging fold. The axial trace (trace of the axial plane) trends same as the limbs, i.e., 102°. The attitude of the southerly dipping limb is 102°/27°→192°, and that of the northerly dipping limb is 102°/27°→12°. The interlimb angle of the fold is {180°− (27° + 27°)} = 126°. Hence, it is also a gentle fold. Folded D-E sequence gets truncated against the base of C. The base of C is not parallel with the base of the overlying unit B. Hence there is an angular relation between them. Again both B and C terminate below A. These indicate that the basal planes of the units C, B and A are angular unconformity surfaces. They have been marked as angular unconformity 1, 2, and 3 in Fig. 6.3A.
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6. Summary maps
The attitude of this basal plane of C is 27°/14°→297°. The attitude of the basal plane of B is 58°/12°→328°. The base of A is horizontal (as it is not intersected by any height contours). A cross-section drawn along LR is given in Fig. 6.3B. Sequence of events (Fig. 6.3A): Deposition of A
(youngest)
Angular unconformity 3 Deposition of B Angular unconformity 2 Deposition of C Angular unconformity 1 Folding of D, E Deposition of D, E
(oldest)
FIGURE 6.3B Cross-section drawn for Map 6.3.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.4
MAP 6.4
MAP 6.4 Reproduced from Map no. 5 of Platt (1951). Permission received by Elsevier.
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6. Summary maps
In Map 6.4 of undulating topography, two groups of rocks crop out: Group-1: litho-units A, F, O, E, and T; and Group 2: litho-units R and S. The Group-1 rocks truncated at the base of Group 2 and the Group 2 rocks are parallel to this plane of truncation. Hence, the base of Group-2 rocks is an angular unconformity. The attitude of Group-1 rocks is 130°/46° → 240°. The attitude of Group 2 rocks as well as the angular unconformity: 95°/19° → 5°. Two vertical faults (F1 and F2) slipped these rocks. F1 affected Group-1 rocks only, whereas F2 affected both the groups. F1 trends 161° and F2 trends 12°. The eastern block is downthrown by F1 and the vertical separation is 50 m (from Fig. 6.4). The western block is downthrown by F2 and the vertical separation is 100 m (from Fig. 6.4), which is also indicated by the superposition of the strike lines ‘A/F 500w’ and ‘A/F 600e’. Sequence of events (Figs. 6.3, 6.4A,B): Faulting F2
(youngest geological event)
Deposition of Group-2 rocks …..Angular unconformity….. Faulting F1 Deposition of Group-1 rocks
(oldest geological event)
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Map 6.4
FIGURE 6.4A Solution of Map 6.4.
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FIGURE 6.4B Cross-section drawn for Map 6.4.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.5
MAP 6.5
MAP 6.5 Reproduced from ‘advanced Map 3’ of Bradshaw and Jarman (1969). Permission received by Elsevier.
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6. Summary maps
Seven litho-units belonging to two groups crop out in this map (Map 6.5) of undulating topography. From bottom to top, the lower sequence (Group-1) consists of limestone, mudstone, sandstone and oolite. Group-2 is made up of arkose and sandstone. Group-1 rocks are folded with 104° trending axial plane. The axial plane dips 86° (found from Fig. 6.6). The trend of the northern limb is 149°/23° → 59°, and that of the southern limb is 61°/23° → 151°. The attitude of the fold axis is 17°→195°. The interlimb angle, deduced from stereo-net (not shown here), is 149°. The fold is a gently plunging-symmetric-gentle upright antiform. An acidic igneous rock intruded the Group-1 rocks. From this massive intrusion, three dykes with trends 65°, 77°, 115° emanated but remained within Group-1 rocks. The igneous intrusion and the the dykes created local contact metamorphism (indicated by ‘m’ in the map). Although the host rocks are folded, the dykes are not. Therefore, intrusion postdates folding. The Group-1 rocks along with the intrusion truncates against the base of Group-2. Hence the base of Group-2 is an angular unconformity. The attitude of Group-2 rocks as well as that of the angular unconformity is 15°/13° → 105°. Sequence of events (Figs. 6.5A,B): Deposition of Group-2 rocks
(youngest)
Development of angular unconformity Igneous intrusion in Group-1 rocks Folding of Group-1 rocks Deposition of Group-1 rocks
(oldest)
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.5
FIGURE 6.5A Solution of Map 6.5.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
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FIGURE 6.5B Cross-section drawn for Map 6.5.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.6
MAP 6.6
MAP 6.6 Reproduced from fig 10.12 of Borradaile (2014). Permission received by Elsevier.
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6. Summary maps
Litho-units A and B crop out in this map (Map 6.6) of undulating topography. Both of them are intruded by two vertical dykes D1 and D2. Litho-units A and B are folded and faulted. The fault plane is not folded and the strikeline magnitudes on the either side of the fault indicate that the folded sequence is faulted. Hence folding predates faulting. The solution is given in Fig. 6.6A. The trend of axial plane is 15°. The attitude of the west dipping limb is 15°/40° → 285°, and that of the east dipping limb is 15°/40° → 105°. Hence the fold is a symmetric-nonplunging-open fold. The attitude of the fault plane is 74°/63° → 164°. Magnitudes of the strike lines on either side of the fault indicates that the southern block (the hanging wall block) is upthrow. It is a reverse fault. In Fig. 6.6B, EF is the net slip and according to the scale of the map, it is 80 m. The pitch of net slip on the fault plane is 54° from 74°. D1 and D2 trend 166° and 128°, respectively. Dyke D2 is faulted, but not folded. Hence, D2 intruded in between the folding and the faulting events. Dyke D1 is not displaced and it’s pitch on the fault plane (90°) does not match with the pitch of net slip on the fault plane (i.e. 54° from 74°). Hence, Dyke D1 intruded after faulting. A crosssection has been presented in Fig. 6.6C. Sequence of events (Figs. 6.6A,B): Intrusion of dyke D1
(youngest)
Faulting Intrusion of dyke D2 Folding of A and B Deposition of A and B
(oldest)
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
Map 6.6
FIGURE 6.6A Solution of Map 6.6.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
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6. Summary maps
FIGURE 6.6B Net slip calculation for Map 6.6 and Fig. 6.6A.
FIGURE 6.6C Cross-section for Map 6.6.
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Map 6.7
MAP 6.7
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
MAP 6.7 Reproduced from Map 3 of Platt (1951). Permission received by Elsevier.
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6. Summary maps
Six litho-units crop out in Map 6.7. They can be subdivided into the following three groups: Group-1: litho-units L, N, and O; Group-2: litho-units A and E; and Group-3: a 59° trending vertical dyke (D) that intruded Group 1 rocks only. Hence the dyke is younger than Group 1 rocks and older than Group 2 rocks. The pattern of strike lines indicates that the Group-1 rocks are folded. The attitudes of the limbs are 0°/31°→90° and 0°/16°→270°. The Group-1 rocks truncates against the base of Group 2 rocks. Also, Group-2 rocks are parallel to this plane of truncation. Hence an angular unconformity separates Group-1 from Group-2 rocks. The three rock groups are disrupted by a fault. The attitude of the fault plane is 97°/71°→187°. Here the northern block (the footwall block) is upthrown. Hence it is a normal fault. The amount of net slip is 100 m (Fig. 6.7B). The pitch of net slip on fault plane is 66° from 277°. A cross section drawn along XY line is shown in Fig. 6.7C. Sequence of events (6.7A,C): Faulting
(youngest)
Deposition of Group-2 rocks …angular unconformity… Intrusion of dyke Folding of Group-1 rocks Deposition of Group-1 rocks
(oldest)
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
FIGURE 6.7A Solution of Map 6.7.
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6. Summary maps
FIGURE 6.7B Calculation of net-slip for Map 6.7. The calculations are done on the fault plane, which trends 97–277 º (orange line). Pitch of dyke on fault plane 66° from 277° (black line, cuts the strike line of the fault plane at “P”). Pitch of the unconformity planes on fault plane is 7° from 97°. The traces of the unconformity planes for southern (blue line) and northern (green line) blocks intersect the strike of the fault plane at Q and R, respectively. Net slip: PS = 1 cm, which is 100 m, according to the map scale. The pitch of net slip (i.e., PS) on fault plane is 66º from 97º.
FIGURE 6.7C Cross-section for Map 6.7.
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Map 6.8
MAP 6.8
MAP 6.8 Reproduced from Map 7 of Platt (1951). Permission received by Elsevier.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
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6. Summary maps
FIGURE 6.8A Solution of Map 6.8.
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
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Map 6.8
Two groups of rocks crop out in Map 6.8: Group-1: rock units M, N, and E. Group-2: rock units L and T. Besides, recent deposits O and A are also present at places. Group-A rocks folded and faulted. The attitudes of the fold limbs are 0°/64°→90° and 0°/45°→270°. Hence, they are nonplunging asymmetric folds (antiform/synform). The interlimb angle of the folds are 71°. Therefore it is an open fold. The axial traces, that trend N in Fig. 6.8A, have been drawn in the cross-section (Fig. 6.8B) by locating (i) where the axial traces intersect (red dots) the topographic profile along the XY line, and (ii) joining the red dot in the central part of the diagram with the corresponding hinge point. The second axial trace (left to the former in Fig. 6.8B) is drawn parallel to the previous axial trace and passing through its corresponding red dot. Two faults Fault-1 and Fault-2 affected the folded sequence. Both of the faults are vertical and trends 90°. For Fault-1, the southern block (~the central block) is upthrown by 100 m. For Fault 2, the southern block is downthrown by 200 m. The two groups of rocks are separated by an angular unconformity. Its attitude as well as that for Group-2 rocks is 43°/12°→133°. Sequence of events (Fig. 6.8B): Deposition of A and O
(Youngest)
Tilting Deposition of L and T Angular unconformity Folding of Group 1 rocks Deposition of Group 1 rocks (M, N and E)
(Oldest)
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
FIGURE 6.8B Cross-section drawn for Map 6.8.
Map 6.9
MAP 6.9
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
MAP 6.9 Reproduced from Map 18 of Platt (1951). Permission received by Elsevier.
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6. Summary maps
Seven litho-units crop out in this map (Map 6.9) of undulating topography. They can be classified as: Group 1: Silurian units S1, S2, S3; Group 2: Old Red Sandstone (R) and Dolerite (D); Group 3- Trias (T) and Jurassic (J). Based on the pattern of the strike lines, it is seen that the Group 1 units are folded. The trend of the axial planes is 60–240° (Fig. 6.9A). The attitude of NW dipping limb is 60°/76°→330° and that of the SE dipping limb 60°/52°→150°. Folded Group 1 rocks (Silurian) truncate at the base of the unfolded Old Red Sandstones (mainly Devonian). Hence this basal plane (marked green in Fig. 6.9A) indicates a break in deposition and deformation as well. That means, this surface is an angular unconformity (attitude: 59°/11°→329°). The dolerite intrusion appears to be concordant with the host Old Red Sandstone, hence it is a sill (attitude 8°/179°→269°). Group 1 and Group 2 units truncate against the base of Group 3 (marked light blue). Group 3 units are parallel to this base. Hence, the base of Group 3 is an angular unconformity surface (attitude 114°/7°→204°). There are three faults—F1, F2, and F3 present in the map. F1 has affected Group 1 only. F1 is a vertical fault trending 82–262°and the eastern block is upthrown by 200 m. F2 has affected Group 1 and Group 2 units. F2 is a vertical fault trending 174–264° and the western block is upthrown by 300 m. F3 is a normal fault that has affected all three groups of rocks. The trend of F3 is 66°/77°→156°. Here the western block (i.e., the footwall block) is upthrown by 100 m. Hence it is a normal fault. A cross-section is presented in Fig. 6.9B. Sequence of events: Fault 3
(Youngest)
Deposition of Group 2 rocks Development of angular unconformity Fault 2 Angular Unconformity Fault 1 Folding of Group 1 rocks Deposition of Group 1 rocks Intrusion of Dolerite in Old Red Sandstone Deposition of Old Red Sandstone
(Oldest)
MAP INTERPRETATION FOR STRUCTURAL GEOLOGISTS
FIGURE 6.9A Solution of Map 6.9.
FIGURE 6.9B Cross-section drawn for Map 6.9.
Exercises
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EXERCISES For each of the following maps (Maps 6.10–6.14): give geological and structural details, draw a proper crosssections, and find out the sequence of events along with explanations.
MAP 6.10 Reproduced from Map 8 of Simpson (1961). Permission received by Elsevier.
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6. Summary maps
MAP 6.11 Reproduced from Map 6 of Platt (1951). Permission received by Elsevier.
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MAP 6.12 Reproduced from Map 9 of Platt (1951). Permission received by Elsevier.
MAP 6.13 Reproduced from “ADVANCED MAPS:1” of Bradshaw and Jarman (1969). Permission received by Elsevier.
MAP 6.14 Reproduced from Map 17 of Platt (1951). Permission received by Elsevier.
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