MASTER`S THESIS MASTER`S THESIS

Transcripción

Faculty of Science and Technology
MASTER’S THESIS
Study program/Specialization:
Spring, 2013
Petroleum Geosciences Engineering
Open
Writer:
Dora Luz Marín Restrepo
Faculty supervisor: Nestor Cardozo
External supervisor(s): Camilo Montes, Universidad de Los Andes (Bogota-Colombia)
Title of thesis:
Structural analysis of the Tabaco anticline, Cerrejón mine, Northern Colombia (South
America)
Credits (ECTS): 30
Keywords:
Strain, curvature, restoration, strike slip
fault, throw, 3D model
Pages: 71
Stavanger, June 13, 2013
i
Copyright
by
2013
ii
Structural analysis of the Tabaco anticline, Cerrejón mine, Northern
Colombia (South America)
by
Master Thesis
Presented to the Faculty of Science and Technology
The University of Stavanger
The University of Stavanger
06-2013
iii
Acknowledgements
I would like to thank my supervisors Nestor Cardozo and Camilo Montes for their help with
preparation and processing of the data, as well as for their valuable comments, edits and
constructive discussions. Thanks to Dave Quinn at Badleys for his fast and effective answers
about TrapTester, which were crucial to accomplish my goals. I would also like to thank Chris
Townsend for help in the construction of the 3D model, and Lisa Bingham for help with ArcGis.
Finally, thanks to Andreas Habel for IT support. The computers programs TrapTester (Badleys),
3DMove (Midland Valley), Petrel (Schlumberger), Matlab (Mathworks), and OSXStereonet
(Cardozo and Allmendinger, 2013) were used in this thesis.
iv
Abstract
Structural analysis of the Tabaco anticline, Cerrejón mine, Northern Colombia (South
America)
University of Stavanger, 2013
Supervisor: Nestor Cardozo Ph.D.
External Supervisor: Camilo Montes Ph.D.
The Tabaco anticline is located in the Cesar-Ranchería basin of northern Colombia, South
America, close to the transpressional collision between the Caribbean and South American
plates. The anticline is bounded by the Cerrejón thrust to the east, the right lateral strike slip Oca
fault to the north, and the left lateral strike slip Ranchería fault to the south. The anticline is
asymmetric and verges to the SE, with a NW limb dipping in average 26°W, a SE limb dipping
41°E, and a fold axis 217°/7° (trend and plunge). The fold’s vergence is opposite to that of the
Cerrejón thrust. In this thesis, I make a 3D structural model of the Tabaco anticline using a high
resolution dataset from the Cerrejón open coal mine in an area of about 10 km2. This 3D model
contains 17 coal seams and 67 faults. The thesis is divided in three main parts: Construction of a
3D structural model, fault displacement analysis, and restoration of the anticline. Four different
patterns in the contours of fault throw were observed: 1) Low throw in the middle of the fault
and high throw in the areas around, 2) Highest throw at one corner of the fault and not in the
center, 3) Highest throw in the middle of the fault, and 4) In conjugated faults, highest value of
throw at the intersection of the two fault planes. Most of the faults show pattern 3. Patterns 1 and
2 are mostly due to lack of sampling of the entire fault surfaces. Faults show a consistent pattern
of slip in the area. 3D restoration of the anticline using a flexural slip technique suggests a total
v
shortening of 18%. Fault-related strain (from the analysis of fault displacement) and fold-related
strain (from the restoration of the coal seams) are related, with the highest values of fold-related
strain associated with a fault in the core of the anticline, and the faults located in the SW
anticlinal limb. Fold-related strains are also high in the SE, steeply dipping anticlinal limb. The
results of this study show that the anticline was affected by uplift of the Santa Marta massif,
Perijá range deformation, and strike-slip movement of the Oca , Samán and Ranchería faults.
vi
Table of Contents
List of Tables................................................................................................................................viii
List of Figures................................................................................................................................ix
1 Introduction.................................................................................................................................11
2 Geological setting.......................................................................................................................15
2.1 Regional tectonic setting..............................................................................................15
2.2 Cenozoic stratigraphy of the Cesar-Ranchería Basin..................................................18
2.3 The Tabaco Anticline and the Cerrejón mine data.......................................................19
3 Methods.......................................................................................................................................26
3.1 Construction of 3D structural model............................................................................26
3.1.1 Coal seams construction...............................................................................26
3.1.2 Fault network construction..........................................................................28
3.2 Fault displacement analysis.........................................................................................29
3.3 Restoration of the anticline..........................................................................................30
3.4 Curvature......................................................................................................................32
4. Results........................................................................................................................................33
4.1 3D structure of the Tabaco anticline............................................................................33
4.1.1 Fault geometry....................................................................................................33
4.1.2 Curvature...........................................................................................................39
4.2 Fault displacement analysis............................................................................................43
4.2.1 Fault displacement patterns................................................................................43
4.2.2 Fault array summation and strain.......................................................................45
4.3 Restoration......................................................................................................................54
4.3.1 Strain maps.........................................................................................................57
5. Discussion..................................................................................................................................59
5.1 Summary of the main events affecting the Tabaco anticline in a regional context........63
References......................................................................................................................................66
vii
List of Tables
Table 1. Summary of main faults in the area.................................................................................17
viii
List of Figures
Figure 1. Location of the study area in Northern Colombia (South America)..............................12
Figure 2. Geologic map of the northern Cesar-Ranchería Basin in the area of the Tabaco
anticline..............................................................................................................................13
Figure 3. Schematic illustration of an isolated fault.....................................................................14
Figure 4. Generalized Cenozoic stratigraphy of the northern Cesar-Ranchería Basin.................16
Figure 5. Systematic dissection of the Tabaco Anticline in ten horizontal mining levels.............22
Figure 6. Map showing the location of measured bedding data (red lines) in the Tabaco
anticline..............................................................................................................................23
Figure 7. Lower hemisphere stereographic projection of poles to bedding in the Tabaco
anticline..............................................................................................................................24
Figure 8. Lower hemisphere stereographic projection of poles to bedding from reconstructed
coal seams surfaces in the Tabaco anticline.......................................................................24
Figure 9. Down-plunge projection of the Tabaco anticline...........................................................25
Figure 10. Steps in the construction of the coal seam surfaces.....................................................27
Figure 11. Steps in the construction of a fault plane.....................................................................29
Figure 12. a) Map showing the location of the cross sections A-A’, B-B’ and C-C’ b) Three cross
section of the Tabaco anticline after the reconstruction of upper coal seams with parallel
folding................................................................................................................................32
Figure 13. Geologic curvature classification................................................................................33
Figure 14. Faults in the 3D model and their occurrence in each of the coal seams......................39
Figure 15. Distribution of maximum curvature (kmax), minimum curvature (kmin), and geologic
curvature with kt = 0..........................................................................................................42
Figure 16. Fault throw patterns observed in the 3D model...........................................................44
Figure 17. Distribution of fault patterns in coal seam 130............................................................45
Figure 18. Left side: Maps displaying the faults affecting each coal seam, contoured by their
throw attribute. Right side: Plots of individual and cumulative fault throw, and cumulative
ix
fault related strain vs. distance. The initial and the last position of the sampling line are
indicated in the map view. e-o contain two groups of plots, one corresponding to NW-SE
striking faults, and another to NE-SW striking faults..........................................................54
Figure 19. Restoration of the Tabaco anticline using the flexural-slip technique.........................56
Figure 20. Maps of maximum principal elongation (e1)..............................................................58
Figure
21.
Stearn and Friedman (1972) model showing the fractures set associated with
folding...................................................................................................................................60
Figure 22. Individual and aggregate fault throw, and fault-related strain vs. distance for coal
seam 123 after the re-interpretation of fault 4......................................................................61
Figure 23. Summary of the main events affecting the Tabaco anticline.......................................66
x
1. Introduction
3D structural models allow the integration of scattered 2D and 3D data (e.g. field mapping, 2D
and 3D seismic, wells) into a common framework, where the data must complement each other
and give rise to an internally consistent model. There are beautiful examples of 3D models,
particularly in areas where the data have very high resolution, large coverage, and are measured
on 2D slices of various orientations (e.g. Yorkshire coal mines data in the UK). Integration of
these data in 3D has given us tremendous insight into the geometry, displacement fields and
growth of faults, particularly in extensional settings (e.g. Rippon 1985; Walsh and Watterson,
1987, 1988; Huggins et al, 1995). Restoration of 3D structural models is a great tool to better
understand the spatial and temporal evolution of geological structures, and in the subsurface
where often only seismic and sparse well data are available, it is the most relevant tool to predict
subseismic features such as fractures. 3D structural models are thus key to represent and
characterize reservoirs. They are the basic framework of hydrocarbon flow models.
Inconsistencies in 3D structural models (e.g. incorrect positioning of the fault network and layer
juxtapositions) have higher impact on fluid flow models than the exact calibration of fluid flow
model parameters (Fisher and Jolley, 2007).
In this thesis, I make a 3D structural model of the Tabaco anticline, using a high resolution
dataset from the Cerrejón open coal mine in La Guajira department, northern Colombia, South
America (Figure 1). The dataset consists of differential GPS measurements of coals seams and
fault traces on 10 horizontal slices (i.e. mining levels) in an area of about 10 km2. These coal
seams delineate the geometry of the anticline in 3D. Additionally, this dataset offers an unique
opportunity to understand faulting and folding in a transpressional setting, in an area bounded by
regional strike-slip faults to the north and south, and a thrust to the east. This study is a
continuation of previous research by Montes et al. (in prep.) who acquired and processed the
GPS data, measured kinematic indicators in the area, and constructed a pseudo-3D model of the
anticline.
11
79°00`W
12°00`N
19.69
78°00`W
±
77°00`W
76°00`W
75°00`W
74°00`W
73°00`W
72°00`W
Caribbean Plate
12°00`N
lt
d
me
Be
r
efo
D
an
11°00`N
Cartagena
10.87
sin
C
ba
e
uth
So
CR
rn
11°00`N
Santa Marta
be
b
ari
15.28
10°00`N
Perijá
10°00`N
Maracaibo
Lake
6.87
Panamá
9°00`N
9°00`N
16.53
22.06
5.25
3.16
South American
Plate
29.17
8°00`N
4.92
30.48
8°00`N
9.53
7°00`N
7°00`N
0 1530 60 90 120
Km
79°00`W
78°00`W
77°00`W
76°00`W
75°00`W
74°00`W
Figure 1. Location of the study area in Northern Colombia (South America). The red rectangle
Tuesday,
13
is June
the11,study
area (Figure 2). Black lines are faults. CR = Cesar-Ranchería Basin. Black arrows
are GPS velocity vectors relative to stable South America (1991, 1994, 1996, and 1998 CASA
campaigns; Trenkamp et al., 2002). Numbers are velocity vectors in mm/yr.
The Tabaco anticline is located in the Cesar-Ranchería Basin of Northern Colombia, South
America, close to the transpressional collision between the Caribbean and South American plates
(Figure 1). The anticline is an asymmetric fold plunging to the southwest (Ruiz, 2006; Palencia,
2007). The asymmetry of the fold is defined by steeply dipping strata (average of 41°E) on its
southeastern flank, and shallowly dipping strata (average of 26°W) on its northwestern flank
(Montes et al., in prep.). The Tabaco anticline is bounded to the southeast by the northwestverging, Cerrejón thrust (which is also the boundary between the Cesar-Ranchería Basin and the
Perijá range), to the north by the right-lateral Oca fault, to the south by the left-lateral Ranchería
fault, and to the west by the 5700 m high Santa Marta massif (Figure 2). The trend of the
anticline (N20°E) is oblique to the Oca and Ranchería strike-slip faults, and its vergence is
12
opposite to that of the Cerrejón thrust (Montes et al., 2010). The Tabaco anticline is important for
the geology of the area because it records the deformation of the strike-slip and thrust faults, and
the uplift of the Santa Marta massif and Perijá range.
72°50'W
72°40'W
72°30'W
Tep: Palomino Fm
Tet: Tabaco Fm
Tpc: Cerrejon Fm
Tpm: Manantial Fm
Ku: Cretaceus undiff.
Kc: Cogollo Gr.
±
Oca fault
11°10'N
Tabaco
anticline
11°10'N
ta
ar
a M sif
t
n s
Sa ma
13
26
Tet
14
Samán
fault
Tep
16
Ku
41
Tpc
n
ejó
rrr lt
e
u
C fa
Tpm
ría fault
Ranche
e
Tep
Tpc
ng
Tet
á
rij
ra
Pe
11°0'N
Kc
0
1
2
4
6
11°0'N
8
Km
72°50'W
72°40'W
72°30'W
Figure 2. Geologic map of the northern Cesar-Ranchería Basin in the area of the Tabaco
anticline. Modified from Montes et al. (2010). The red rectangle shows the area of the Cerrejón
open coal mine where the GPS data were collected.
This thesis is subdivided in three main topics: Construction of a 3D structural model, fault
Thursday, April 25, 13
displacement analysis, and restoration of the anticline. The 3D model of the anticline was
constructed using the traces of coal seams and faults on horizontal mining levels, giving a total
of 17 coal seams and 67 faults. The faults were divided into four structural domains as suggested
by Palencia (2007). The 3D model was used to calculate the displacement field on faults. An
important concept is that the faults should show a reasonable variation in displacement, with zero
displacement at the fault tipline and maximum displacement at the center of the fault surface
(Kim and Sanderson, 2005; Figure 3).
13
Tip
Point
Tip
Point
Hanging wall
Dmax
Footwall
Height (H)
Displacement (D)
Dmax
Length (L)
Figure 3. Schematic illustration of an ideal, isolated fault. Displacement is maximum at the
center of the fault and decreases outwards to be zero at the fault tipline (modified from Fossen,
2012).
Using TrapTester (Badleys), the throw was calculated on each fault. Four different patterns in the
contours of fault throw were found: 1) Low throw in the middle of the fault and high throw in the
areas around, 2) Highest throw at one corner of the fault and not in the center, 3) the most
common pattern, highest throw in the middle of the fault, and 4) In conjugated faults, highest
throw at the intersection of the two fault planes.
Pattern 1 can be explained by the fault linkage model (Peacock and Sanderson, 1991; Cartwright
et al., 1995), in which the faults grow as isolated faults at early stages and then link to produce
larger faults. In this case, the area of low throw is indicating the zone of fault linkage. A possible
explanation for pattern 2 is that the fault is actually larger, but there are not enough data to
14
resolve the complete fault plane. Pattern 3 is the consistent pattern of fault displacement of
Figure 3. In pattern 4, the two conjugate faults add to a larger displacement.
Profiles of fault displacement versus distance were also constructed to visualize the 3D
distribution of fault displacement in the anticline, and to make inferences about fault-related
strain (i.e. the gradient of fault displacement) in the area. The faults with highest value of throw
are fault 59 (an E-W fault located in the core of the anticline), and fault 45 (the Samán fault). The
faults that show the highest values of strain are located in the SW limb of the anticline and have a
strike NE-SW.
Finally a restoration of the Tabaco anticline was performed using a flexural-slip technique. This
technique preserves volume in 3-D, line length in the unfolding direction, and orthogonal bed
thickness (Griffiths et al., 2002). The restoration shows that the total shortening of the Tabaco
anticline is 18%. The shortening of coal seam 130 is 6%, 115 is 2%, 105 is 7%, and 100 is 3%.
The maximum fault-related strain for coal seam 130 is 5%, 115 is 3%, 105 is 10 % and 100 is
2.5%. The results of this study show that the anticline was affected by the uplift of the Santa
Marta massif and Perijá range, and the strike slip movements of the Oca, Samán and Ranchería
faults.
2 Geological setting
2.1 Regional tectonic setting
The Tabaco anticline is located in the Cesar-Ranchería basin, northern Colombia, South America,
close to the transpressional collision between the Caribbean and South American plates (Figure
1). The northern part of the Cesar-Ranchería basin is defined by a southeast-dipping monocline
that shows structural continuity with the Santa Marta massif to the west (Figure 2). The
monocline is bounded to the north by the right-lateral Oca fault and to the east by the northwestvergent Cerrejón thrust (Montes et al., 2010) which limits the Perijá range (Kellogg, 1984;
Figure 2). Faults and folds in the footwall of the Cerrejón thrust are only affecting Cenozoic
15
rocks (Figure 4), these structures include the left-lateral Ranchería fault and the Tabaco anticline
(Montes et al., 2010). Table 1 shows a summary of the main faults in the area. Sánchez and
Mann (2012) describe 3 major periods of shortening for the Cesar-Ranchería Basin that include:
an event in the Paleocene- early Eocene, an Oligocene- early Miocene event, and a last period in
the Pliocene-Pleistocene.
Legend
S175
Sandstone
Age
Formation
Lithology
Limestone
Ma
Siltstone
Palmito
55
Early
Eocene
S170
S160
Claystone
S155
59
Upper Cerrejón Fm
Tabaco
S150
Coal
S145
S135
S130
3
S125
Cerrejón
Middle to Late Paleocene
57
Late Paleocene
to Early Eocene
Shale
S123
S120
S115
61
Manantial
Early Paleocene
S110
S106
S105
S102
S100
S95
S90
20 m
FigureMay
4. 21,
Generalized
Tuesday,
2013
Cenozoic stratigraphy of the northern Cesar-Ranchería Basin and detail of
the Upper Cerrejón Formation where the coal seams (S) analyzed in this study are located.
Identification numbers on the coal seams are the same as those of the 3D model (modify from
Bayona et al., 2011).
16
Table 1. Summary of main faults in the area
Name of Fault
Oca
Cerrejón
Ranchería
Type
Displacement
Right lateral
strike slip
Feo-Codecido (1972): 15 to 20 km
Tchanz et al. (1974): 65 km
Montes et al. (2010): 75-100 Km
Kellogg (1984): 90 km
Pindell et al. (1998): 100 km
Age
Middle to Late Miocene
(Konn in Shagam, 1984
Reverse
Total throw between 16-26 km
(Kellogg and Bonini, 1982)
Early Eocene and Late
Oligocene (Montes et
al., 2010)
Left lateral
strike slip
5 km (Sánchez, 2008; Montes et al.,
2010)
No information
available
The western boundary of the area is the Santa Marta massif (Figure 2), an isolated, triangular
basement block with a maximum elevation of 5700 meters above sea level. Cardona et al. (2008)
estimated exhumation rates for the Santa Marta massif of 0.7 km/Ma between 65-48 Ma, 0.16
km/Ma until the Late Oligocene, and 0.33 km/Ma in the middle-late Miocene.
The eastern limit of the Cesar-Ranchería basin is the northwest-vergent Cerrejón thrust with an
average dip of 9-12° towards the SE in the surface (Montes et al., 2010). This fault is the western
boundary of the Perijá range (Figure 2). The Perijá range consists of Mesozoic and Palaeozoic
igneous and sedimentary rocks with a maximum elevation of 3650 meters above sea level
(Kellogg, 1984). Four deformation phases have been described in the area starting in the Early
Eocene (53 Ma) and Middle Eocene (45 Ma), intensifying during the Late Oligocene with thrust
sheet emplacement and unroofing of 3–4 km (Kellogg, 1984), and ending between the Late
Miocene to recent. Four post-Jurassic, thrust detachment levels have been proposed in the Sierra
de Perijá: at the base and top of the Upper Cretaceous shales of the Colón Formation, at the
shales of the Guaimaros Member of the Cretaceous Apón Formation, at the shale and sandstone
with high mica content of the Lisure Formation; and an intrabasement detachment level (Duerto
et al., 2006).
17
2.2 Cenozoic stratigraphy of the Cesar-Ranchería Basin
A Late Cretaceous to Eocene sedimentary succession is preserved in the area (Figure 4). The
Early Paleocene Manantial Formation is composed of glauconitic shales and sandy limestones
(Bayona et al., 2011). The upper Manantial Formation include calcareous sandstone and
biomicrite beds, followed by a thick succession of dark-coloured mudstone and siltstone beds
with plant remains and signs of bioturbation. Towards the top, calcareous and fossiliferous
sandstone beds interbedded with laminated mudstone and siltstone beds and local conglomerates
occur. Bayona et al. (2011) described a change in thickness of this unit eastward from 600 m at
the west of the basin, to 180 m in a well close to the Ranchería fault.
The Cerrejón Formation is a 1 km thick coal-bearing unit that consists of very fine to fine
grained argillaceous sandstones, dark colored sandy siltstones and interbedded mudstones, shales
and coal seams (Bayona et al., 2011). Jaramillo et al. (2007) established an age of Middle-to-Late
Paleocene for this formation. The Cerrejón Formation is a deltaic sequence that were deposited
in less than 2 My (Bayona et al., 2007; Jaramillo et al., 2007). Almost all the coal seams that
were modeled in this thesis are located in the upper part of the Cerrejón Formation (S 100-175,
Figure 4), only coal seams 90 and 95 are located in the lower part of the Formation. Provenance
analyses in the Paleocene Manantial and Cerrejón Formations in the northernmost part of the
Cesar-Ranchería valley indicate that these Formations were supplied from the Santa Marta
massif (Cardona et al., 2010; Bayona et al., 2007), indicating uplift of the massif from the
Paleocene.
The Late Paleocene-Early Eocene Tabaco Formation is a 75 m thick unit that includes variedly
colored, massive mudstone beds interbedded with cross-bedded conglomeratic sandstone beds
(Bayona et al., 2011; Cardona et al., 2010). Montes et al. (2010) interpret the Tabaco Formation
as syntectonic strata based on thickness changes and field relations with the Cerrejón Formation.
They suggested that mild deformation took place in the Cesar-Ranchería basin during the
accumulation of this Formation. Provenance analyses in these syntectonic strata indicate a source
of the Santa Marta massif (Cardona et al., 2010), but also point to tectonic activity of the Perijá
18
range (Bayona et al., 2007). Finally, the Palmito Formation (Early Eocene) is composed of lightcolored, massive mudstones (Bayona et al., 2011). Beck (1921) estimated a thickness of 60 m for
this Formation.
2.3 The Tabaco Anticline and the Cerrejón mine data
The Tabaco anticline is an asymmetric fold plunging to the southwest (Ruiz, 2006; Palencia,
2007). The asymmetry of the fold is defined by steeply dipping strata (average of 41°E) on its
southeastern flank, and shallowly dipping strata (average of 26°W) on its northwestern flank
(Montes et al., in prep.). The anticline affects Cenozoic rocks, including the Upper Cerrejón
Formation where the coal seams analyzed in this study are (Figure 4).
I use in this thesis a dataset collected in the Cerrejón open coal mine (red square in Figure 2) by
several geologists working in the mine. The data are the result of routine in-pit mapping of the
intersection of dipping coal seams and horizontal mining levels (Montes et al., in prep.). Each
coal seam intersection was followed with differential GPS. Attributes such as the name of the
seam, elevation, roof and floor lithologies, dip angle, and apparent thickness along the dip
direction were recorded. Structural features such as faults, minor folds, bedding, kinematic
indicators (minor folds and slickensides) were also recorded. All this information was stored in a
GIS database (Montes et al., in prep.). The coal seams and fault traces in the GIS project were
cleaned and interpreted (Montes et al., in prep.). This “clean” database is the initial information
for this thesis.
The GIS project contains the traces of 19 coal seams in 10 horizontal mining levels, and more
than 1000 fault traces (i.e. fault intersections with the mining levels). The stratigraphic position
of the coal seams is shown in Figure 4. The faults are classified by their level of confidence as
observed, interpolated or inferred. Figure 5 shows the 10 mining levels. Of the 19 mapped coal
seams, 17 were used in the construction of the 3D model. Coal seams 90 and 175 (Figure 4) were
not used because there are not enough data to reconstruct them. A limitation of the dataset is that
19
as the layers become younger, the data coverage becomes less in the core of the anticline.
Younger coal seams are only mapped in the limbs of the anticline.
100
72°34'30"W
72°34'0"W
72°33'30"W
72°35'0"W
72°34'30"W
72°34'0"W
95
10
5
106
123
13
5
120
13
5
15
5
120
14
5
11
5
11°7'30"N
12
5
11
5
10 95
2
10 1
2 00
10
2
100
100
12
0
11
5
10
2
105
100 102
110
12
5
13
0
5
11
11
0
5
13
0
16
14
5
115
100
105
120
130
11
0
125
115
125
123
5
13
5
14
5
15
145
0
13
0
12
3 25
12 1
14
5
135
135
130
145
150
13
5
135 130
155
10
0
15
5
13
5
15
0
11
5
14
5
11°7'0"N11°7'0"N
11
5
95
102
10
5
15
0
115
135 130
13
0
135
125 120
12
3
13
115
0 12
3
13 123
5
110
145
10
5
106
11
5
10
0
11
0
130
0
13
11
0
102
110
110
95
100
10
102
5
10
0
95
10
2
10
5
10
0
100
95
115
95
145
120
130
135
14
5
12
0
11
5
5
15
5
15
145
105
135
0
11
105
0
13
123 25
1
160 160
150
135
106
123 123
15
5
0
16
5
10
6
10
110
5
14
155
5
12
0
10
110
0
12
5
13
170
155
13
5
2
10
6
10
3
12
135
5
15
0
16
11°7'0"N
5
15
0
17
0 100 200
72°34'30"W
123
90 0
10
102
115
150
6
10
160
123 25
1
130
150
115
5
15
72°35'0"W
145
0
11
5
14
0
12
11°7'0"N
Wednesday,
June 12, 13
155
11°7'30"N11°7'30"N
152
5
11
130
145
0
17
0
15 152
125
102
130
130
150
2
10
11°8'0"N
DAFS
DFSS
120
150
90
72°33'30"W
DFTAB
DFLSE
152
110
115
5
12
135
150
2
15
12
3
13
0
±
600
m
Level 30
11°8'0"N11°8'0"N
120
130
125
5
15
5
15
105
115
72°35'0"W
11°7'30"N
110
10
2
72°34'0"W
Level 20
11
5
102
13
5
72°34'30"W
DAFS
DFSS
16
0
13
0
11
5
12
5
72°35'0"W
DFTAB
DFLSE
15
0
15
5
15
0
12
3
13
0
72°34'0"W
400
110
!
72°34'30"W
±
115 11
3
12
5
10
6
95
102
105
106
11
0
0 100 200
11
0
!
!!
! !
!
600
m
106
! !
!
150
72°35'0"W
11°8'0"N
10
100
10 5
6
105
102
115
110
105
5 12
11
0
0
110
10
5
13 130 12
115
3 12
5
115
115
3
130 120
130
145
14
130
5 135
10
5
95
10
2
10
5
16
0
11°7'0"N
5
0
15 15
14
5
400
5
15
0
15
11°7'0"N
11°7'0"N
0 100 200
5
14
0
15
135
3
12
5
0
15
5
14
11°7'30"N
15
0
! !
5
12
15 1
5 50 1
45
! !
0 3
13 1
5
14
123
10
0
135
12
0
110
12
5
125
115
13
5
!
! !
3
12
14
5
10
0
!
!
5
!
12
0
12
115
11
5
13
0
10
5
115
!
125
14
5
!
!
12
3 12
3
14
5
13 123
0
115
115
!
120
14
5
!
!
!
12
0
115
110
10
0
10
95
110
90
95
10
2
!
!
!
!
!
10
10 5
6
115
!
135
!
125
!
!
!
125 125
123
120
0
13
2
15
130
5
13
0
13
5
14
! !
12
3
10
6
10
0
2
! !
135
130
!
135
!
15
5
!
10 105
6
!
! !
0
12
5
11
5
13
0
12
5
12
10
2
10
5
!
!
! !
!
150
130 135
5
15
6
10
0
11
11°8'0"N
115
120
115
12
13 5 123
5
120
130
130 12 120
5
145
145
!
!
!
!
! !
12
3
13
5
3
12
5
12
0
15
0
11
13 12
0
14 0
12
5
115
10
10
10
2
5
6
10
110
51
11 106
05 10
0
2
10
115
110
5
11
0
115
120
115
115
12
5
135 13
0 130
!
!
115
12
3
125
0
12
0
13 45
1
105 2
10
150
115
3
12
145
170
106
0
11
150
5
14
135
145
45
145 1
0
15
5
12
110
130
5
12 130
0
12
0
11
115
13
115
135
3
12
35
51
2
10
160
123
0
13
105
15
5
5
12
3
12
6
10
11°7'30"N
11°7'30"N
155
105
3
12 3
12
150
11°7'0"N
! !
150
2
15
152
! !
! !
155
!
0
16
! !
12 123
5
DFSS
!
!
!
5
15
! !
130
DAFS
!
145
!
!
3
12
12
5
!
!
!
106
110
5
15
95
0
10
!
105
! !
110
14
5
130
12
3
5
11
! !
!
12
5
5
13
DFTAB
DFLSE
115
!
105
0
12
150
130
!
±
72°33'30"W
115
0
10
!
11°8'0"N
11°8'0"N
72°34'0"W
Level 10
5
10
DFSS
11°7'30"N
12
0
110
115
DFTAB
DFLSE
DAFS
11°8'0"N
10
5
10
2
100
Level 0
72°34'30"W
110
72°33'30"W
72°35'0"W
150
±
72°34'0"W
135
130
72°34'30"W
!
72°35'0"W
72°34'0"W
400
600
m
0 100 200
72°35'0"W
72°34'30"W
400
600
m
72°34'0"W
20
0
17
12
0
100
10
6
13
5 130 12
5 123
15
0
110
16
011
11
0
5 1
15
10
2
11
5
12
3
12
0
16
0
105
17
5
16
0
15
0
10
2
10
5
110
11
0
105
16
0
5
10
6
0
12
12
0
11
0
3 15
17 51
11
12
3
0
17
5
14
0
13
0
12
0
17
5
14
11°7'0"N
5
12 35
1
17
0
5
13
16
0
17
3
5
13 5
14
0
16
0 100 200
400
600
m
17
0
3
17
72°34'0"W
0
16
11°7'0"N
0
17
170
175
72°35'0"W
11°7'30"N
5
14
17
5
11
0
12
3
105
15
5
105
0
12
5
12 35
1
5
14
0
15 155
160
0
12
11°7'30"N
5
14
100
10
12
6
115
1
14 30 125 0
5
11
120
0
1
14 30 13 1
5
0 23
17
15 14 1
12
0
0 5 35
31 1
13 20 73
16
15
0
0
0
12 175
5
16
13
0
12
5
15
3
0
14
5
13
5
105
105
0
13
90
100
102
5
11
0
16
5
11
3
12
11°8'0"N
0
13
3
3
12 105 1
5
13 055
114
0
13
5
13
115
10
2
100
100
10
2
130
5
12
5
11
15
5
3
12
10
5
90
72°33'30"W
90
0 100 200
600
m
600
m
12 102
10
0
6
105
10
2
10
12
6
115
1
14 30 125 0
5
11
120
90
0
130
13 1
14
5
0 23
11 15 114
1
0 0 055 13 02 123 1
5
13 20
110 1
16
0
50
0
10
12
115
5
5
16
13
0
12
5
15
3
0
14
5
13
5
105
105
115
10
6
17
0
130
145
16
0
13
5
15
0
16
0
17
0
72°34'0"W
11
5
100
110
12
11
11
3 12
10
0
5 1
3
6
15
13
5 130 12
59 123
5
95
11
10 10 102 10 1
12
11
13
5
10 10 102 10
6 5 13 0 12
5
5
5
17
6
5
0 10
050
10 5
5
10
5
0
12
5
1012
115
102
110
5 123
115
110
5 123
10 105
12
10 105
6
12
115
13
6
0
11
1
0
0
15
16
5
30
110
11510
0
0
14
0
1
5
45
13 125 120
13 125 120
5
5
135
12
135
0
125 123 1
135
125 123 1 120
30
135
30
130 135
130 135
160
14
160
14
150 5
150 5
155
155
160
160
17
17
0
0
10
2
10
2
102
105
100
10
2
5
11
0
11
115
115
130
155
400
0
16
72°34'30"W
152
0 100 200
5
10
0
12
145
0
13
106
170
155
155
11°7'0"N11°7'0"N
102
10
6
110
72°35'0"W
72°34'0"W
123
105
100
105
12
12 5
3
102
12
12 5
3
135
135
135
130
135
15
5
135
170
170
115
0
11
0
13
5
13
0 15
17 1
12
3
175 12
5
145
152
160
17
0
15
5
16
0
15
0
123
15
5
16
0
120
135
123
150
160
145
130
15
0
16
0
155
160
155
160
10
2
10
5
17
0
11
5
102
11
0
10
5
102
106
110
17
5
160
5
12
5
14
0
15 155
130
3
17
600
m
5
17
17
5
102
135
123
150
160
145
130
120
14
5
115
14
5
15
0
10
5
10
5
10
2
10
110
11
6
5
10
10
2
115
2
12 120
13
115
12 120
5
15
5
5
1
10 0
12
11
90
1
0
0
90
13
10
12
13 100 201
5 1
0
0
2950 95
5 1
16
10
30
95 95
10
3100
6
0
10
6
21
1
13
2
0 12 20
301012 120
110
102
110
16
5 12
25 1
0
3
23
15
1
1
20
12
0 14
50 1
11
5 135 1
45 13
16
0
115
16 5
0
5 13
30
120
0
12
0 1 120
17
5
25
0
123 170
14
123
14
5
5
135
130 135
135
130 135
145
145
102
102
11
0
115
115
135
130
12
5
15
0
106
110
115
120
130
10
6
120
15
5
16
0
123
115
106
105
95
110
102 100
106
105
110
115
120
14
5
14
5
10
5
11
5
11
0
11
5
102
12
3 12
0
13
0
10
2
10
6
10
2
10
2
11
5
12
3 12
0
13
0
10
6
95
10
6
11
0
11
5
102
100125
102
10 1
10 0 02
2
102
102
102
0
10
2
130 10
10
5
110
15
0
115
123
125
123
10 1
10
2 00
6
15 105
0
14
5
14
5
102
102
100
10
2
10
2
10
2
10
5
10
5
105
11
01
10
125
123
130
130
125
105
11
01
10
110
120
115
123
120
120
145
17
0
400
3
12
130
110
115
12
3
12
5
0
12
106
130
95
95
106
5
10
6
10
106
130
110
11°7'30"N11°7'30"N
152
0 100 200
400
11°8'0"N
105
105
115
5
11
0
12
130
DFSS
115
5
14
5
15
0 100 200
5
10
6
10
155
0
13
600
m
11°7'0"N
72°33'30"W
102
5
14
400
72°34'0"W
5
10
11°7'0"N11°7'0"N
0 100 200
3
17
106
5
13
5
17
0
17
Level 70
DFTAB
DFLSE
DAFS
5
12
3
12
11°7'0"N
5
14
72°34'0"W
DFSS
11°7'30"N
72°34'0"W
0
17
106
3
12
120
5
5
0 12 13
12
0
16
5
13
72°34'0"W
130
6
10
0
11
0
16
5
13
3
17
3
12
5
13
5
17
0
12
145
0
17
5
17
0
17
0
13
DFTAB
72°34'30"W
DFLSE
72°34'30"W
DAFS
5
14
0
15
125
5
15
145
123
130
130
130
120
130
130
125
135
130
135
145
130
130
130
135
105
106
120 123
123
5
15
100
106
120
3
12
0
13
5
115
17
0
12
105
106
6
10
120
3
12 5
12
123
5
14
0
11606 0
17
11°7'30"N
0
11
5
14
5
10
6 10
10 1
5
13
11°8'0"N
3
12
5
13
Level 70
170
155
115
150
72°34'30"W
120 123
160
5
14
0
15
0
17
170
170
123
72°34'30"W
5
15
5
14
3
12
5
15
0
16
106
130
155
3
122
120 10
5
15
145
0
10
2
10
0
16
3
12
0 25
12 1 5
13
5
14
0
13
5
13 145
5
14
5
13
5
10
0
13
105
102
2 35
1
3 10
12
595
2
0
12 1 5
13
5
14
5
10
5
17
5
12
125
152
120
3
12
130
5
14
0
15
2
15
11°7'30"N11°7'30"N
95
110
0
3
17
17
0
72°34'30"W
0
16
150
12
170
145
3
12
3
12
135
5
15
145
160
115
115
123
155
72°35'0"W
102
72°34'30"W
Wednesday, June 12, 13
0
10
5
5
0 12 13 5
12
10
105
105
115
6
10
110
155
5
11
170
95
125
152
160
160
0
16
155
11°8'0"N11°8'0"N
5
10
90
5
15
152
150
152
0
10
105
0
16
135
5
14
5
14
72°33'30"W
72°35'0"W
95
115
0
12
±
±
72°34'0"W
0
10
6
10
110
115
5
15
170
11°7'0"N
600
m
72°35'0"W
11°8'0"N11°8'0"N
0
10
6
10
0
11
123
150
15
5
16
0
135
135
72°34'0"W
105
150
160
11°7'0"N
400
95
102
125
5
15
0 100 200
95
DFTAB
DFLSE
DAFS
11°7'30"N
155
72°34'0"W
2
10
115
0
15
152
72°35'0"W
11°7'0"N11°7'0"N
72°33'30"W
72°35'0"W
90
5
11
13
0
0
13
5
13 145
5
14
130
Level 60
DFSS
2
15
170
155
DFTAB
72°34'30"W
DFLSE
DAFS72°34'30"W
DFSS
600
m
72°34'0"W
Level 60
160
400
0
15
0
15 55
1
120
0
11
5
10
102
102
6 10 95
10 1
6
10
0
10
2
10
105
6
10
110
145
0 100 200
0
15
5
15
0
15
5
15
150
150
152
0
13
155
120
5
14
72°34'30"W
15
5
16
0
160
0
12
145
130
135
13
0
130
170
150
5
12
0
13
5 145
13
155
125
120 123
5
14
152
152
0
12
3 25
12 1
6
10
11°7'0"N11°7'0"N
5
14
5
13
115
72°34'30"W
0
13
5
12
0 0
11 11
160
11°7'30"N
11°7'30"N
11°7'30"N
11°7'30"N
5
10
72°33'30"W
95
170
0
15 55
1
105
0
13
155
0
3
12 12
5
14
5
10 6
10
5
13
72°34'0"W
Level 50
DFTAB
DFLSE
DAFS
DFSS
160
120
145
5
13
0
15
2
10
5
15
5
15
150
145
11°7'30"N
72°35'0"W
2
105
14
0
15
0
15
0
13
5
12
5
10
72°33'30"W
11°8'0"N
5
14
102
5
13
11°8'0"N
11°8'0"N
170
0
12
145
150 150
170
11°8'0"N
105
5
11
135
125
150
130
120 123
125
5
14
150
155
72°35'0"W
95
5
10
0
13
0
12
3 25
12 1
5
12
0
13
5 145
13
130
135
145
13
0
±
±
DFTAB
DFLSE
72°34'30"W
DAFS
DFSS
120
3
12 5
12
0
3
12 12
0
13
152
152
170
13
0
145
5
10 6
10
95
0 0
11 11
5
10
115
2
10
2
10
102
110
130
72°35'0"W
110
13
12 0
3
13
5
±
±
72°34'0"W
Level 50
6
10
145
150 150
155
11°7'0"N
13
5
125
102
100
102
10 1
10 0 02
2
102
5
11
135
125
130
150
152
5
10
12
3
10
2
102
125
145
10 1
2 00
10
5
95
DFTAB
DFLSE
5
10
115
95
11°7'30"N
72°35'0"W
11°8'0"N
72°33'30"W
72°35'0"W
102 100
72°34'0"W
Level 40
DAFS
DFSS
72°34'30"W
11°8'0"N
11°8'0"N
DFSS
11°7'30"N
11°7'0"N
72°35'0"W
72°33'30"W
72°35'0"W
100
11°8'0"N
DFTAB
DFLSE
72°34'30"W
DAFS
120
72°35'0"W
±
±
72°34'0"W
Level 40
135
11°8'0"N
72°34'30"W
145
72°35'0"W
72°34'30"W
400
600
m
72°34'0"W
21
72°35'0"W
11°8'0"N
±
72°34'30"W
72°35'0"W
72°33'30"W
72°34'0"W
Level 80
DFTAB
DFLSE
DAFS
±
11°8'0"N11°8'0"N
72°34'30"W
72°34'0"W
72°33'30"W
Level 90
DFTAB
DFLSE
DAFS
11°8'0"N
DFSS
10
5
11
0
115
DFSS
11
0
10
5
110
12
0
130 1
25
15 1
0 45 1
45
12
0
16
17 0
0
5
17
5
15
0
17
17
3
16
0
0
15
0
17
3
17
5
17
17
3
5
15
17
0
5
14
5
14
17
3
12
0
12
3
115
17
5
13
5
106
15
0
125
5
130 13
17
3
5
15
105
13
0
1
13 20
0 12
3
106
10
5
115
12
3
125
12
135 130 3
135
11
5
3
12
0
16
11°7'0"N
11°7'0"N
0
16
11°7'0"N
11°7'30"N
5
0 12
12
5
12
5
13
0
0 25 13 45
12 1 35 1
1
5
11
16
0
0
16
5
11
0
12
17
0
5
15
6
10
5
11
115
0
17
16
0
17
0
5
13
145
12
3
11
0
3
0
12
120
13 45
1
15
2
5
12
0
15
0
16
13
5
115
3
0 12
12
0
13
30
0
12 30 1 145
1
0
13
5
10
06
106 1
13
5
10
6
105
10
6
105
105
115
5
11
11°7'30"N11°7'30"N
106
6
10
6
10
110
11°7'30"N
11
5
10
6
5
10
105
11°7'0"N
3
17
5
17
5
17
3
17
173
170
0 100 200
175
72°35'0"W
72°34'30"W
72°34'0"W
400
17
3
600
m
17
0
175
72°35'0"W
72°34'30"W
0 100 200
400
600
m
72°34'0"W
Figure 5. Coal seams (black lines with identification numbers) and fault traces (colored lines) in
ten horizontal levels in the Cerrejón mine. Number on level is its elevation a.s.l. in meters. Faults
are colored according to the structural domains defined by Palencia (2007). DFTAB : Tabaco
fault domain, DFLSE: Southern limb domain, DAFS: Samán antithetic fault domain, DFSS:
Samán fault domain.
488 bedding measurements were taken by geologists in the Cerrejón mine (Montes et al., in
prep.). The bedding information is located in all mining levels in both, the core of the anticline to
the north of the modeled coal seams, and in the limbs of the anticline where there are GPS data
(Figure 6). A cylindrical best fit to the data suggests that the anticline is roughly cylindrical, with
a fold axis (trend and plunge) 217/7 (Figure 7). Strike and dip data from the reconstructed coal
seam 3D surfaces (best-fit plane routine of Fernandez, 2005 in 3DMove) suggest also a roughly
cylindrical fold with a fold axis 204/5 (Figure 8).
22
72°34'30"W
72°34'0"W
o
±
11°9'0"N
o
ooooo o oo
o oo
oooooooo
oo
o
oo
oo
o
oooo
o
o
o
oo
o
o
o
o
o
o
oo ooo
o
oo
o oo
o
ooo ooooo o
37
oo
o
o
o
ooo o oo
o o ooooo o o oo
o
o
oo
oo oooooo o o o
o
oo
oo o
oo ooo
o
o
o
o
oo
ooo o
ooooooooooo
oo
oooo o
o
ooo
oo
22
65
oooooooo
oo o
o
55
11°8'30"N
11
34
24
o
15
o
22
6
11°8'30"N
72°33'30"W
11
17
50
25
34
110
12
3
130
115
5
11
11°8'0"N
12
0
38
10
2 1
10
00
5
110
13
5 12
1
10
5 12 115
10
5 1 02
6
3
115
05 10
11
106 1 2
0
05
1
1
1
10
12
5
5
110
115
115 120 115 115
150 13
0 130
135 130
11°8'0"N
10
2 10
0
10
5
o
ooo oo
oo ooooo ooo
ooo oo oo
o ooooooo
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oo
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21
5
10
20
71
25
5
12
120
123
123
12
3
120
12
5
12
0
13 5
14
5
14
10
0
110
115
95
0
12
23 5
31
13
115
10
6
14
5
11
5
3
12 35
1
5
12
0
13
5
14
0
15
5
15
28
0
200 400
54
72°35'0"W
72°34'30"W
11°7'0"N
0
15
o
23
90
10
5
115
115
115
3
12
0
11
28
110
125
130 135
130
135
5
10
11°7'30"N
13
0
130
95
0
10
6
10
0
12
125
3
12
31
3
12
135 45
1
145 50
1
135
12
5
13
0
130 130
152
5
15
0
16
150
10
2
95
125
145
5
15
110
135
15
5
14
oo oo ooo
5
13
14
5
11°7'0"N
o
0
10
105
0
12
11°7'30"N
115
130
150
o
o
o
May 21, 2013
72°35'0"W
11°9'0"N
72°34'0"W
800
m
72°33'30"W
Figure 6. Map showing the location of bedding data (red strike and dip symbols) in the Tabaco
anticline. Bedding data was measured in all mining levels. The black lines are the coal seams and
faults traces in the lowest mining level 0. Numbers on coal seams traces are the coal seams ids.
(data from Montes et al., in prep.).
23
Equal Area
Lower Hemisphere
Kamb Contours
C.I. = 2.0 Sigma
N = 488
Figure 7. Lower hemisphere stereographic projection of poles to bedding in the Tabaco anticline.
A cylindrical best fit to the data (red great circle and eigenvectors) suggests a fold axis of 217/7.
Equal Area
Lower Hemisphere
Kamb Contours
C.I. = 2.0 Sigma
N = 20947
Figure 8. Lower hemisphere stereographic projection of poles to bedding from the reconstructed
coal seams 3D surfaces. A cylindrical best fit to the data (red great circle and eigenvectors)
Thursday,
April 25,
suggests a fold
axis
of13 204/5.
24
A down-plunge projection of the coal seams along the fold axis 217/7 was performed
(Allmendinger et al., 2012). Since the anticline is not perfectly cylindrical, not all points on a
coal seam surface fall along the same line in the projection (Figure 9). However the down-plunge
projection is still useful to visualize the geometry of the anticline. The projection shows that the
anticline is asymmetric, with a SE vergence, and a rounded hinge. Lower points in the projection
suggesting a west dipping, overturned forelimb, should not be taken into account. They result
from projecting the entire data to the profile plane. Disharmonic folds, for instance in beds
100-105 and 150-155 towards the east, were observed in the anticline (cross section C-C in
Figure 12). Small thickness changes are also observed between the coal seams.
400
200
100
50
0
m
up m
0
2500
−200
a
−400
2000
2500
1500
2000
−600
1500
1000
1000
500
north m
−800
500
0
0
b
east m
−1000
−800
−600
−400
−200
0
200
400
600
800
m
Figure 9. Down-plunge projection of the Tabaco anticline. a) 3D view of the anticline. b) Downplunge projection of the anticline. Notice that since the fold axis plunges south (Figure 7), the
cross section in b is looking to the south (east is to the left).
c
Based on the dataset above, detailed field mapping, and kinematic indicators; Palencia (2007)
divides the anticline into four structural domains (Figure 5). 1) The Tabaco fault domain
Friday, April 26, 13
(DFTAB), characterized by a main SE dipping reverse fault (Tabaco fault) and a series of SE
dipping, right-stepping in echelon reverse faults, the average dip of these faults is around 54°. 2)
The Samán fault domain (DFSS) (left-lateral strike slip fault), characterized by short faults with
high dip angles and striking NNW-SSE. 3) The Samán fault antithetic domain (DAFS),
characterized by short segment faults with an average dip of 55° S-SW and NE with almost no
25
associated folding. And 4) The southeast limb domain (DFLSE), characterized by short segment
faults with an average dip of 60° NE and SW. Additionally, the kinematic indicators (in the core
of the anticline) suggest a tectonic transport direction to the northwest (between 310° and 330°)
consistent with shortening perpendicular to the strike of the Cerrejón thrust, but in a direction
opposite to the vergence of the Tabaco anticline.
3 Methods
3.1 Construction of 3D structural model
To describe and analyze the geometry of the anticline, a 3D structural model was built. The 3D
model was important since it was a way to integrate the data on the different mining levels.
When the coal seams and the fault planes were reconstructed, and the relationship between these
two were checked, it was possible to evaluate the consistency of the model (whether the model
was geologically reasonable or not). The 3D model was made in two steps: 1. Coal seams
construction, and 2. Fault network construction.
3.1.1 Coal seams construction
Since the initial GIS project was the result of cleaning and interpretation of the original GPS data
(Figure 10a, Montes et al., in prep.), the first step in the construction of the 3D model was the
resampling of the coal seams and fault traces to increase the number of vertices on them. This
was done by setting vertices every 2 meters along the traces, using a resampling tool in ArcGIS.
This process did not change the shape of the traces, it just increased the number of vertices on
them (Figure 10b).
26
N
N
120 m
120 m
a
N
b
N
120 m
c
120 m
d
Figure 10. Steps in the construction of the coal seam surfaces, in this case coal seam 100. a)
b) Resultant points (light blue) after resampling the original data (dark blue). c)
Resultant points (green) after gridding. d) Final coal seam surface with contours. In c and d, the
blue lines are the resampled data in b. Notice that the modeled surface closely follows the data.
Original
points.
Friday,
April 26,
13
Second, each coal seam was interpolated in Matlab using a grid fitting routine called gridfit
(D’Errico, 2005). Gridfit fits a surface to scattered or regular 3D data points. It allows the
existence of point replicates in x and y but with different elevation. Gridfit also allows building a
gridded surface directly from the data, rather than interpolating a linear approximation to a
surface from a Delaunay triangulation. A low smoothness was used in this process to closely
follow the traces. Only interpolated areas with control data were exported. The purpose of this
step was to increase the number and regularity of data points in the coal seam, while closely
following the original data (Figure 10c-d). Steps 1 and 2 were performed to guarantee that the
27
coal seams were continuous enough. Near a fault scarp for instance, the coal seam must have
enough resolution and continuity to create lines of intersection (cutoff lines) with the fault.
3.1.2 Fault network construction
The first step was to quality control and edit the fault traces, to make sure they follow the offsets
displayed by the traces of the coal seams. The second and most crucial step was the correlation
of the fault traces on the different mining levels (Figure 11a). This undoubtably was the step with
highest uncertainty and where it was possible to introduce interpretation errors. The traces were
visually correlated level by level in Petrel (Figure 11a). Some of the challenges I faced were:
Fault traces were mapped by different geologists with perhaps different connotations as to what a
fault trace is. Where fault traces of similar strike were very close in a mining level, it was
difficult to identify the correct trace to correlate. Another challenge was that in some areas one
trace in one level looked as it could be correlated with another in a different level, but the strike
of the traces were different, giving an irregular fault plane. The question here was whether to
include or not the fault trace.
Each group of correlated fault traces were triangulated into a fault plane (Figure 11b). Some of
the fault planes were very irregular (Figure 11b). To make a more geologically reasonable plane,
an interpolation of the fault traces with a small degree of smoothing was made using gridfit
(D’Errico, 2005). The result was a more regular fault plane that still followed the fault traces
(Figure 11c). Finally a double check of the fault surfaces was made with the original fault traces
and with the coal seams.
28
a
80 m
b
80 m
80 m
d
80 m
c
e
80 m
Figure 11. Steps in the construction of fault planes. a) Correlation of fault traces on different
mining levels. b) Triangulation of fault traces to obtain a fault plane. c) Gridding of fault plane to
obtain a more geologically reasonable structure. d) Computation of fault-coal seams
intersections. e) Estimation of the throw attribute in the fault planes.
3.2 Fault displacement analysis
Fault displacement analysis was used in this thesis with three main objectives: 1) To check
inconsistencies in the interpretation, 2) to understand the behavior of the fault network in the
anticline in terms of displacement, and 3) to look at the fault displacement gradients or faultrelated strains.
The first step was to calculate in TrapTester the faults-coal seams intersections. This process
generated a series of polygons that represent the hanging wall and footwall cutoffs (Figure 11d).
The input for this step were the fault planes and the coal seams of the 3D structural model. I only
worked in areas with control data, so in the case of the younger coal seams, fault-horizons
intersections were not computed in the core of the anticline (Figure 12).
29
Some of the faults-horizons intersections polygons show anomalies (for example abrupt spikes).
After checking the consistency of the modeled coal seams and fault planes, and if these
irregularities persist, a minor edition of the faults-horizons intersection polygons was performed.
Then the throw attribute was computed on the fault planes (Figure 11e). The throw on the fault
surface is derived from the cutoffs generated in the fault-horizons intersection step. With the 3D
model and the throw information, fault statistics were calculated, including fault orientation
plots, fault displacement profiles, and fault array summation and strain. This was done using the
fault statistics tool of TrapTester.
3.3 Restoration of the anticline
To restore the anticline, the coal seams data and the fault surfaces were imported into 3DMove.
In the case of the coal seams, I reconstructed the part of the anticline without data. This applies
mostly to the younger coal seams without data in the core of the anticline (Figure 12). To avoid
errors in the reconstruction of the surfaces, for instance crossing of surfaces in the core of the
anticline, the area without control data was reconstructed from the older surface assuming
parallel folding or constant stratigraphic thickness across the structure (Figure 12). This
assumption is not completely correct because as we can see in the cross sections in the area with
control data (black dots), the beds are not completely parallel (Figure 12). However, this is a
reasonable approach to represent the core of the anticline in younger levels.
After reconstructing the coals seams, a kinematic restoration of the anticline was performed
using a 3D flexural-slip restoration technique in 3DMove. This technique utilizes a slip method
that preserves volume in 3D, line length in a given unfolding direction, and orthogonal bed
thickness (Griffiths et al., 2002). The technique only deals with the coal seams and does not take
into account the slip on the fault surfaces (that would be very hard to do with so many fault
surfaces). My aim is to compare the kinematics and strain of this “unfolding” restoration with
those obtained in the fault displacement analysis of the previous section.
30
72°34'30"W
±
11°8'0"N
72°34'0"W
72°33'30"W
C
11°8'0"N
B
A
11°7'30"N
11°7'30"N
C´
B´
11°7'0"N
11°7'0"N
A´ 0
72°35'0"W
72°34'30"W
A
72°34'0"W
100 200
400
600
m
72°33'30"W
a
A’
E
W
100m
B
B’
W
E
100m
100
105
Friday,106
April 26, 13
C
C’
W
E
130
135
145
110
115
150
155
120
123
160
170
control
31
100m
100
105
106
110
C’
W
E
130
135
145
115
150
155
120
160
123
170
control
data
125
C
100m
Figure 12. a) Map showing the location of cross sections A-A’, B-B’ and C-C’. These cross
sections are shown after the reconstruction of the younger coal seams with parallel folding.
Black dots in the sections show the areas with data. Sections have no vertical exaggeration.
Monday, June 10, 13
Numbers on legend refer to the coal seams IDs.
3.4 Curvature
The curvature of the coal seams was calculated using differential geometry algorithms by Mynatt
et al, (2007). The Matlab codes for the calculations were taken from Pollard and Fletcher (2005,
their chapter 3). The principal curvatures, kmax and kmin, were calculated, as well as the
geological curvature. The geological curvature is based on the gaussian curvature (kG = kmin *
kmax) and the mean curvature (kM = (kmin+ kmax)/2) (Mynatt et al., 2007). The geologic curvature
defines the shape and orientation of points on the folded surface. The curvature threshold
specifies an absolute curvature value below which calculated principal curvatures are set to zero,
thereby allowing the classification of ‘‘idealized’’ shapes (Bergbauer, 2002). Figure 13 shows the
geologic curvature classification scheme for points on a surface as a function of the Mean and
Gaussian curvatures (Mynatt et al, 2007).
32
I. Mynatt et al. / Journal of Structural Geology 29 (2007) 1256e1266
1259
Fig. 2. Geologic curvature classification. The geologic curvature of a point on a surface can be determined from the Gaussian (kG ) and mean kM curvatures at the
Figure 13. Geologic curvature classification. The geologic curvature of a point on a surface can
point. The color code is used throughout the paper. Modified from Roberts (2001) and Bergbauer (2002).
be determined from the gaussian (kG) and mean kM curvatures. The color code is used in the
By curvature
specifying kcalculation
parabola
defined as
(Mynatt
et can
al., be
2007).
t , sections of the
having zero curvature for subsequent analyses or calculations.
For kt ¼ 0.1 m"1, all locations on the parabola with jkj <
0:1 m"1 (R > 8.8 m) are assigned a curvature of zero (light
4. Results
4.1 3D structure of the Tabaco anticline
4.1.1 Fault geometry:
Figure 14 shows the faults in the 3D structural
interpreted in 17 coal seams. Most of the faults
grey) and treated mathematically as linear. This isolates the
sections of the parabola with greater curvature (dark grey and
black) and continues to consider them parabolically curved.
Making a greater approximation and setting kt ¼ 0.5 m"1 assigns sections of the parabola with jkj < 0:5m"1 (R > 1.9 m)
curvature values of zero. With this approximation all light
and dark grey sections are treated as linear. For a surface, this
process can be used to define points as synformal, antiformal
and planar.
In order to approximate geologic surfaces as perfect saddles
(Fig. 2), a similar algorithm is applied. Recall that saddles
have principal curvatures kmin and kmax of opposite signs,
and a perfect saddle would have kmin ¼ "kmax , or kM ¼ 0.
The principal curvature values will never be of equal magnitude for
butlabeled
may be close
to warrant
model.
A geologic
total ofsurfaces,
67 faults
1 toenough
67, were
this approximation. Idealized perfect saddles can therefore be
kmax j falls
below kt.
points where
jkmin þ
arespecified
reverse,at except
fault the
45 sum
(Samán
fault),
which
is as a left lateral strike-slip fault (Palencia, 2007).
Only faults
than
one coal
3. Description
of displacing
an anticlinalmore
surface
using
differential geometry
seam are present in the 3D model. Rose diagrams of
fault orientations are also included in Figure
14.
Fig. 3. Application of the curvature threshold kt to the parabola y ¼ x2 . Light
grey sections of the curve are treated as linear when kt ¼ 0:1 m"1 and dark
grey are treated as linear when kt ¼ 0:5 m"1 . In both cases the black section
retains its curvature values.
3.1. Description of field area and creation of
surface model
Sheep Mountain anticline is a doubly plunging asymmetric
33
fold located near Greybull, Wyoming (Forster et al., 1996;
The four fault domains mapped by Palencia (2007) in the field, were identified in the 3D model
(Figure 14). The four domains are: 1) The Tabaco fault domain (DFTAB) which is present in the
anticline’s S-SW limb. In the 3D model, the faults belonging to this domain are long (300-400 m
along strike) reverse faults with a strike NE-SW (except for fault 59 that has an E-W strike). 2)
The domain of the SE limb (DFLSE). The faults belonging to this domain are mainly short (100
to 150 m along strike), except for fault 1 (340 m along strike). Their strike is NW-SE and
approximately E-W to the south of the anticline. Between coal seams 120 and 145, some of the
faults show a conjugated pattern. 3) The antithetic Samán fault domain (DAFS). The faults
belonging to this domain are very short segments (60 m along strike) close to the Samán fault.
These faults get longer as their distance to the Samán fault increases (for instance fault 63 is 340
m along strike, Figure 14 coal seam 123). The strike of the faults in this domain is NW-SE. 4)
The Samán fault domain (DFSS) which consists of the Samán fault and a couple of very short
faults (50 m along strike). In the original database (Figure 5) there were more fault traces
belonging to this domain. However, these faults offset just one coal seam and therefore were not
included in the 3D model.
Coal seam 95 is affected by 4 main faults with a main orientation NW-SE (Figure 14). These
faults are located in the eastern limb of the anticline, and belong to the fault domains DFLSE,
DFTAB, and DAFS. Coal seam 100 is affected by 7 main faults with a main orientation NW-SE
(Figure 14). These faults are mainly located in the eastern limb of the anticline. In addition, fault
59 crosses almost the whole seam in an E-W direction (Figure 14). The faults affecting this coal
seam belong to the domains DFLSE, DFTAB, and DAFS.
Coal seam 102 is affected by 6 main faults with a main orientation NW-SE (Figure 14). Almost
all the faults that offset this coal seam are the same as those of the lower coal seam 100, except
fault 60 which has a different strike NE-SW, and it is located in the SW part of the anticline.
Additionally fault 45 (the Samán fault) was identified in this coal seam. The faults affecting this
coal seam belong to the domains DFLSE, DFTAB, DAFS and DFSS. Coal seam 105 is affected
34
by 7 main faults with two main orientations, NW-SE which are located in the NE limb of the
anticline, and NE-SW which are located to the south of the anticline. The faults in the south with
a strike NE-SW belong to the domain DFTAB, and those striking NW-SE belong to the domains
DFLSE, DAFS and DFSS.
Coal seam 106 is affected by 6 main faults with orientations similar to those in the lower seam
105. The NW-SE striking faults are located in the NE limb of the anticline, and the NE-SW
striking faults are located to the south of the anticline. In this coal seam (and the seams above),
there are no data from the core of the anticline, so there is no control for the faults affecting this
part of the anticline. Therefore it is not possible to know the behavior of faults such as fault 59 in
younger coal seams. The faults in the south with a strike NE-SW belong to the domain DFTAB,
and those in the NE limb with a strike NW-SE belong to to the domains DFLSE, DAFS and
DFSS.
Coal seams 110 and 115 are affected by 15 and 16 main faults respectively, with strike NW-SE in
the NE limb, NE-SW in the south, and a new fault, fault 1, striking E-W. In these coal seams, the
number of faults with strike NE-SW increases. The faults in the south with strike NE-SW belong
to the domain DFTAB and the faults in the NE with strike NW-SE belong to to the domains
DFLSE, DAFS and DFSS. Fault 1 (E-W) also belongs to the domain DFLSE.
Coal seam 120 is affected by 22 faults with strike NW-SE in the NE limb, NE-SW in the south,
and fault 1, striking E-W. In this coal seam, a conjugated pattern of the faults in the eastern limb
was recognized. This conjugated pattern include two orientation of thrust faults and it was very
clear in coal seams 120 to 145. The faults that show the conjugated pattern belong to the DFLSE
domain. The other 3 domains were also recognized here.
Coal seams 123, 125, 130, 135 and 145 are affected by 25, 29, 29, 28 and 25 faults respectively,
with strike NW-SE in the NE, NE-SW in the SW, and E-W in the SE. One characteristic of these
coal seams is that they have a series of E-W faults in the southern part of the anticline.
35
Additionally the conjugated pattern in the east limb is also present. Both the E-W faults and the
conjugated pattern belong to the DFLSE domain. The other 3 domains were also recognized
here.
In coal seam 150 the number of faults start to decrease. This coal seam is affected by 16 faults
with two main strikes, NW-SE and NE-SW. Faults striking E-W have almost disappeared. In this
coal seam the conjugated pattern was not observed. This is the last coal seam where the 4 faults
domains were observed. Coal seam 155 is affected by 12 faults with two strikes, NW-SE and
NE-SW. In this coal seam, there was not a lot of information available in the north part of the
anticline, so only 2 domains were recognized, DFLSE and DFTAB.
Coal seam 160 is affected by 8 faults with a main strike NW-SE. In this coal seam there are only
faults in the eastern limb that belong to the DFLSE, DAFS and DFSS domains. The last modeled
coal seam, 170, is affected by 4 faults with a main strike NW-SE in the SE. The faults affecting
this coal seam belong to the DFLSE domain.
95
100
N
N
DAFS
DAFS
DFLSE
DFLSE
DFTAB
DFTAB
59
1:10000
1:10000
N=4
N=7
102
105
N
N
DAFS
DFSS
DFSS
45
45
DAFS
DFTAB
DFLSE
DFTAB
59
DFLSE
59
1:10000
1:10000
36
102
105
N
N
DAFS
DFSS
DFSS
45
45
DAFS
DFTAB
59
DFLSE
DFTAB
DFLSE
59
1:10000
1:10000
N=7
N=6
106
110
N
N
106
110
DFSS
DAFS
45
N
DFTAB
DAFS
DFSS
N
45
63
DFSS
DAFS
DAFS
63
DFSS
45
DFLSE
DFTAB
DFLSE
DFLSE
DFTAB
37
1
N=7
1:10000
37
DFTAB
63
37
1:10000
45
DFLSE
63
1:10000
1
37
N=15
1:10000
N=7
N=15
115
120
115
120
45
N
N
45
DFSS
N
45
N
45
DAFS
DAFS
DFSS
63
DAFS
63
44
DAFS
44
44
37
37
37
DFLSE
37
DFLSE
1
4
DFTAB
DFTAB
1:10000
25
4
DFTAB
1
DFSS
44
63
63
DFTAB
DFSS
1
N=16
DFLSE
DFLSE
2
2
1:10000
25
1
1:10000
1:10000
N=16
N=22
N=22
Wednesday,
Tuesday, June
11, 13 June 12, 13
37
123
125
N
45
123
DFSS
63
63
DFSS
DAFS
37
63
63
DFLSE
37
1
2
DFTAB
25
4
25
4
25
DFTAB
37
DFLSE
25
4
DFLSE
1
1
1:10000
2
3744
44
1
2
44
44
DAFS
4
45
DAFS
DFSS
N
45
DFTAB
DFSS
125
DAFS
N
45
N
DFLSE
2
DFTAB
1:10000
N=25
N=29
1:10000
1:10000
N=25
130
N=29
135
N
N
130
135
45
N
DFSS
45
DAFS
45
N
DAFS
4
25
DFTAB
37
DFTAB
DFTAB
37
25
DFLSE
1
4 2
25
DFLSE
2
DFTAB
44
4
25
37
63
44
37
1
44
DFSS
DAFS
DFLSE
1
24
63
44
DFSS
DAFS
63
DFSS
45
63
1
2
DFLSE
17
171:10000
1:10000
N=29
N=28
1:10000
1:10000
N=29
N=28
Tuesday, June 11, 13
145
150
N
N
45
45
DFSS
DAFS
DFSS
44
DAFS
37
DFTAB
DFTAB
DFLSE
DFLSE
2
17
17
1:10000
1:10000
N=25
N=16
155
160
N
45
DFSS
N
DAFS
DFTAB
DFLSE
38
DFLSE
17
17
1:10000
1:10000
17
17
1:10000
1:10000
N=25
N=16
155
160
N
45
DFSS
N
DAFS
DFTAB
DFLSE
DFLSE
17
17
1:10000
1:10000
N=12
N=8
170
N
DFLSE
1:10000
N=4
Figure 14. Faults in the 3D model and their occurrence in each of the coal seams. Faults are
labeled with number 1 to 67. Numbers on top of the figures refers to the coal seam (95 is the
lowest coal seam). Rose diagrams show the orientation of the faults in each coal seam. N is the
number of faults. DFTAB : Tabaco fault domain, DFLSE: Southern limb domain, DAFS: Samán
fault antithetic domain, and DFSS: Samán fault domain.
4.1.2 Curvature:
Curvature was calculated in the older, more complete stratigraphic seams (95, 100, 102, 105,
Wednesday,
22, 2013
106). Curvature
wasMay
also
calculated in coal seam 130, which has the highest amount of faults.
Figure 15 shows the maximum curvature (kmax, left side), minimum curvature (kmin, middle side)
and geologic curvature (right side).
39
kmax highlights the anticline’s hinge and minor folds hinges. The anticline’s hinge is clearly
visible in coal seam 100 suggesting a fold axis trending 210° (approximately the same as in
Figures 7 and 8). In the other coal seams, the highest values of kmax indicate minor hinges. kmin
mainly highlights synclines but also erosional features (drainage pattern). The principal
curvatures clearly show that the fold is no cylindrical. A folded surface is cylindrical when the
principal curvature parallel to the hinge line is zero (Mynatt et al., 2007).
By using the concept of geologic curvature and implementing a curvature threshold (kt), it is
possible to examine how far a coal seam departs from the cylindrical geometry (Mynatt et al.,
2007). Figure 15 (right side) shows the geologic curvature calculated across the surface with kt =
0. The color code of Figure 13 is used in the coal seams. The geologic curvature calculated
across the surface with kt= 0 shows that no principal curvatures are approximated as zero, so no
cylindrical or planar points appear. As it can be seen, there are no orange, yellow or light blue
colors representing cylindrical synforms, planes or antiforms (Figures 13 and 15 right side).
Areas on the surface are identified as domes, basins, and antiformal and synformal saddles. The
domes (blue) and the antiformal saddle (green) coincide with the anticlinal hinge, and the basin
(dark red) and synformal saddles (dark yellow) coincide with the synclines position or valleys
(Figure 15).
In coal seam 130 there is a clear difference between the size of the basins (dark red) and
synformal saddles (dark yellow), in both limbs. In the western limb the basins are bigger than in
those in the eastern limb (Figure 15 right side). For this coal seam the eastern limb of the
anticline is affected by more faults that the western limb (Figure 14).
40
Kmax
Kmax
0.3
0.3
Kmin
Kmin
95
95
0.3
0.3
Kt=0
Kt=0
2
2
N
N
0.2
0.2
0.2
N
N
0.2
0.1
0.1
0
0.1
0.1
0
100 m
100 m
Kmax
Kmax
N
N
0
0
0
0
-0.1
-0.1
-0.1
-0.1
-2
100 m
100 m
-0.2
-0.2
0.08
Kmin
Kmin
100 m
100 m
-0.2
-0.2
100
100
0.08
0.08
-2
Kt=0
Kt=0
-4
-4
2
0.08
2
N
N
N
N
0.04
0.04
N
N
0.04
0.04
0
0
0
0
-0.04
-0.04
-0.04
-0.04
-0.08
-0.08
0
0
-2
-2
-0.08
-0.08
100 m
100 m
100 m
100 m
100 m
100 m
-4
-4
102
Kmax
Kt=0
Kmin
2
0.1
0.1
N
Friday, May 24, 2013
100 m
N
N
0
0
-0.1
-0.1
100 m
-0.2
0
-2
100 m
-0.2
-4
105
Kt=0
Kmin
Kmax
2
0.1
N
0
0.1
N
-0.2
N
-0.1
-0.1
100 m
0
100 m
-0.2
0
-2
41
100 m
-4
100 m
100 m
-0.2
100 m
-0.2
-4
105
Kt=0
Kmin
Kmax
2
0.1
0.1
0
N
0
N
-0.1
-0.1
100 m
0
N
-0.2
100 m
-0.2
-2
100 m
-4
106
Kmax
Kt=0
Kmin
0.1
0.1
2
0
0
0
N
N
N
-2
-0.1
-0.1
100 m
100 m
100 m
-4
130
Kmax
0.1
Kmin
0.1
Kt=0
125
2
Kmax
Kmin
0.1
N
0.1
N
-0
Kt=0
N
0
-0
-2
N
200 m
-0.1
N
0
N
0
200 m
-0.1
200
-4
Figure 15. Distribution of maximum curvature (kmax), minimum curvature (kmin) and geologic
200 m
200105,
m
curvature with kt = 0. Curvature was-0.1calculated for coal seam 95, 100,
106 and 130.
-0.1
42
Saturday, May 25, 2013
200 m
4.2 Fault displacement analysis
4.2.1 Fault displacement patterns
The throw was calculated for all faults in the 3D model. Figure 16 shows four different fault
throw patterns: 1) Low throw in the middle of the fault and high throw in the areas around
(Figure 16a), 2) Highest throw at one corner of the fault and not in the center (Figure 16b), 3) the
most common pattern, highest throw in the middle of the fault (Figure 16c), and 4) In conjugated
faults, the highest value of throw is located at the intersection of the two faults (Figure 16d).
Figure 16a shows a fault plane with low throw in the middle of the fault and high throw in the
areas around. This pattern was observed in two faults of the model. Three possible explanations
for this pattern are: 1) the area of low throw suggests a tip point which indicates that the mapped
fault is actually 2 faults, so the fault plane should be split. 2) The linkage fault model (Peacock
and Sanderson, 1991; Cartwright et al., 1995), in which the faults grew as isolated faults at early
stages and linked to produce additional fault length. In this case the area of low throw is the
region of fault linkage. 3) Outer areas of high displacement result from the intersection of two
faults with different orientations that together add to a larger displacement (Kim and Sanderson,
2005). This third option is discarded because there are no other fault crossing the fault plane.
However, it is difficult to know which of the two first options is generating this fault throw
pattern.
Figure 16b shows the highest throw value in a corner of the fault and not in the center as we
would expect (Figure 3). Two possible explanations for this pattern are: 1) the fault could
actually be larger, but there are not enough data to cover the complete fault plane. 2) there is a
problem in the interpretation of the faults.
Figure 16c shows two fault planes with the area of high throw in the middle of the fault. This is
the most common pattern in the faults of the 3D model. Although this is not exactly the ideal
43
pattern of Figure 4, it is a consistent pattern of fault displacement. Figure 16d shows a pair of
conjugated faults, where the highest value of throw is at the intersection of the two fault planes.
The two faults with different orientations add to a larger displacement.
60m
a
60m
b
30
30m
c
d
Figure 16. Fault throw patterns observed in the 3D model. The blue-purple color indicates the
highest values of throw a) fault plane with low throw in the middle of the fault b) fault plane
with
highest
throw at the corner of the fault c) fault planes with the highest throw in the middle
Friday,
May 3,
2013
of the fault d) fault throw in conjugated faults.
Figure 17 shows the distribution of the four displacement patterns in coal seam 130. This coal
seam has the highest number of faults, so the distribution of the patterns here reflects very well
the behavior in the anticline. Figure 17 shows that pattern 3 (P3) is the most common pattern,
followed by pattern 2 (P2). Pattern 1 (P1) was observed in two faults in the anticline, and pattern
4 (P4) in conjugated faults.
44
N
P2
P3
P3
P3
P3
P3
P2
P3
P3
P3
P3
P1
P1
P3
P3
P4
P3
P3
P3
100 m
Figure 17. Distribution of fault displacement patterns (P1 to P4) in coal seam 130.
4.2.2 Fault array summation and strain
A series of fault displacement versus distance profiles were made for each coal seam in the
anticline. Figures 18a-q contain maps showing the faults affecting each coal seam contoured by
their throw attribute (left side), and plots of individual displacement, fault array displacement
summation, and strain. The throw was measured along sample lines oriented perpendicular to the
average strike of the faults. Sample line spacing (in the horizontal) was 10 m. The location of the
sample grid was adjusted for each coal seam according to the area where the faults were located.
That allows sampling and visualization of specific details in each case. The initial and the last
position of the sampling line are shown in the map views of Figure 18.
45
Figures 18e-o for coal seams 106 to 155 show faults with two main strikes, NW-SE and NE-SW.
For this reason two different plots of fault array summation and calculated strain (labeled 1 and
2) were made. The first one is for the faults with strike NW-SE (in the map view the area
surrounded by the square 1). The second one is for the faults with strike NE-SW (in the map
view the area surrounded by the square 2). In both cases the sample line spacing was 10 m.
The fault displacement versus distance profiles (Figure 18, middle and right side) show the
vertical offsets (throws), the fault array summation, and the strain for the 67 interpreted faults in
the 17 coal seams of the 3D model. The fault that shows a highest apparent throw is fault 59,
which shows a throw of 21 m in coal seam 100, 27 m in coal seam 102, and 35 m in coal seam
105 (Figures 18b, c, and d). It is not possible to know the behavior of fault 59 in younger coal
seams, due to lack of data in the core of the anticline.
Fault 45 (Samán fault) shows also an important amount of displacement (25 to 26 m in coal
seams 105 and 110, Figures 18d and f). However, there is some uncertainty in the calculation of
throw of fault 45 due to lack of data in some coal seams. Other faults that show an important
amount of displacement are fault 63 in coal seam 115 (19 m, Figure 18g), fault 17 in coal seam
155 (18 m, Figure 18o), fault 25 in coal seam 130 (18 m, Figure 18k), fault 44 in coal seam 120
and 123 (17 m, Figures 18h and i), fault 37 in coal seam 120 (17 m, Figure 18h), fault 4 in coal
seams 120 and 123 (17 m, Figures 18h and i), and fault 2 in coal seam 130 (16 m, Figure 18k).
On each coal seam, the aggregate vertical component of displacement (dashed line) was also
calculated. For this analysis it is important to mention that the younger coal seams did not have
any data in the core of the anticline. This can result in underestimation of cumulative throw in
these younger seams. The coal seam with the highest value of cumulative throw (50 m) is 105,
followed by coal seams 130 (44 m), and 125 (43 m) (Figures 18d, 18k and 18j). Coal seam 105 is
affected by a limited number of faults (7 in total), so the high cumulative throw in this coal seam
is due mainly to faults 59 and 45 (Figure 18d). In contrast, coal seams 125 and 130 are the ones
affected by more faults (29 faults in both coal seams), so in these two cases the highest value of
46
the total throw results from the contribution of small throw faults (Figures 18j and 18k). In
younger coal seams than 130, the total throw starts to decrease gradually up-section. Coal seam
135 shows a maximum cumulative throw of 29 m, 145 a maximum of 27 m, 150 a maximum of
15 m, 155 a maximum of 21 m, 160 a maximum of 12 m, and 170 a maximum of 15 m (Figures
18l to 18q).
In some of the coal seams, it is easy to correlate the spikes in the cumulative throw curve with
the different fault domains of Palencia (2007). Between coal seams 120 to 145 (Figures 18h and
m), it is possible to identify 3 clear spikes. The first one corresponds to the cumulative throw
generated by the conjugated faults of the DFLSE domain. The second spike is generated by the
DFLSE domain (the no conjugated faults), but in some cases there are also faults of the DAFS
domain. The third spike is generated by the faults that belong to the DAFS domain. The
magnitude of these spikes changes through the coal seams. In coal seams 120 and 123 the
highest spike is the third one, which includes the DAFS domain, with a throw of 30 m (Figures
18h and 18i). However, in coal seams 125 to 135, the highest spike is the first one, which
belongs to the conjugated faults of the DFLSE domain, with the highest value of throw of 45 m
(Figures 18j and 18k).
The spike generated by the domain DFSS is more difficult to see due to the lack of data in the
coal seams in the most northern part of the anticline. However this pattern is clearly indicated in
coal seam 125 (Figure 18j). The pattern of the DFTAB is mainly shown on the right side of
Figure 18. However, in half of the cases the cumulative throw curve does not resemble a
continuos curve due to the faults being widely spaced. In general, it is not possible to observe
that the displacement in the anticline gradually decreases towards one specific direction, but it
seems to decrease up-section younger coal seams than 130 as indicated before.
Figure 18 also shows the fault-related strain (i.e. horizontal elongation calculated from aggregate
heave of all faults divided by original length of horizon) for each coal seam over the 3D model.
The fault-strain in the modeled coal seams of the anticline varies between 0.0015 in coal seam
47
150, and 0.2 in coal seam 123 (Figures 18n and 18i). Only coal seam 123 shows strain values
higher than 0.1, while coal seam 105 has values of 0.1 (Figures 18i and 18d). Freeman et al.
(2010) based on a large number of published data for strike dimension and maximum
displacement for faults, established that 0.1 represents a realistic upper limit of the longitudinal
strain when it is measured in the displacement direction of a fault. The strain value 0.2 identified
in coal seam 123 (Figure 18i, right side) is higher than the upper limit suggested by Freeman et
al. (2010). This value of 0.2 is generated by fault 4 (Figure 18i, right side). Additionally to the
high value of fault-related strain, fault 4 has the second fault throw pattern: fault plane with
highest throw at the corner of the fault (Figure 16), which is not a consistent pattern of fault
displacement. For those two reasons it was very plausible that there was a problem in the
interpretation of the fault 4. So it was necessary to check the interpretation of fault 4 again. The
re-interpretation of fault 4 will be shown in the discussion chapter.
In Figures 18e-o, the plots for faults with strike NW-SE (middle) show lower strain values than
those for faults with strike NE-SW (right). The only exceptions are coal seams 115 and 155
(Figures 18g and 18o). The right side plots show the faults that belong mainly to the DFTAB
domain, while the middle plots show the faults that belong to the other 3 fault domains.
Strain
95
N
0.01
0.005
SE
NW
47
DFTAB and DAFS
6
Apparent throw (m)
DFTAB
40
50
25
e
in
gl
in r
pl mbe
m
sa nu
40
47
50
4
2
0
4
200m
8
12
16
Sampling line number
20
a
48
Strain
100
N
0.02
0.01
47
SE
NW
40
42
47
48
50
58
59
DFTAB and DAFS
40
25
Apparent throw (m)
50
25
59
e
in
gl
in r
pl mbe
m
sa nu
20
15
10
0
5
4
8
12
16
20
200m
b
102
Strain
0.04
N
0.02
47
SE
40
NW
35
59
DFTAB and DAFS
40
47
59
60
Apparent throw (m)
30
60
25
e
in
gl
in r
pl mbe
m
sa nu
25
20
15
10
0
5
5
Monday, May 27, 13
10
15
20
25
200m
c
105
N
Strain
0.1
51
47
0.05
45
SE
40
50
25
Apparent throw (m)
59
60
e
in
gl
in r
pl mbe
m
sa nu
40
NW
40
45
47
51
59
60
DFSS and DFTAB
DFTAB
DAFS
30
20
10
0
5
200m
10
15
20
25
d
49
1
106
2
47
0.01
0.005
0
1
63
0.02
SE
12
Apparent throw (m)
10
0
3
NW
NE
38
47
63
38
60
0.04
Strain
e
in
gl
in r
pl be
m um
a
s n
SW
3
60
7
e
in
gl
in r
pl be
m m
sa nu
2
Apparent throw (m)
20
N
Strain
0.015
DFLSE and DAFS
DAFS
8
6
DFTAB
6
5
4
4
25
4
2
2
4
2
6
8
10 12 14
16
18
5
10
15
20
25
200m
e
110
1
2
47
45
0.02
30
25
Apparent throw (m)
37
60
Monday, May 27, 13
67
20
NE
SW
DFTAB and DFLSE
12
DFLSE and DAFS
15
10
1
60
67
10
8
6
4
0
1
NW
DAFS and DFSS
37
38
41
45
47
63
63
38
0.04
0.02
SE
0
1
41
Strain
e
in
gl
in r
pl mbe
m
sa nu
0.04
Apparent throw (m)
N
Strain
0.06
35
5
2
e
in
gl
in r
pl be
m m
sa nu
2
5
10
35
15
20
25
5
30
10
15
20
25
30
200m
f
1
115
2
45
63
44
1
SE
NW
Mainly DAFS
Apparent throw (m)
67
0
1
0.02
0.01
37
43
44
45
53
63
30
60
55
0.02
0
43
37
57
Strain
e
in
gl
in r
pl mbe
m
sa nu
0.03
0.04
25
DFSS
20
15
NE
SW
1
55
57
60
67
9
Apparent throw (m)
N
Strain
0.06
42
7
DFTAB and DFLSE
5
3
10
2
e
in
gl
in r
pl be
m m
sa nu
1
5
5
10
60
25
15
20
30
35
40
5
15
35
25
45
55
200m
g
50
1
120
2
0.06
e
in
gl
in r
pl mbe
m
sa nu
40
Strain
N
Strain
0.03
0.02
0.04
0.01
0.02
45
1
SE
0
NW
30
37
32
25
28
4
Apparent throw (m)
35
5
39
1
0
2
25
DAFS
DFLSE
conjugated
NE
25
26
28
34
35
37
39
43
44
49
63
64
DFLSE
43
20
15
10
SW
1
2
4
5
16
DFTAB and DFLSE
Apparent throw (m)
44
63
12
8
2
e
in
gl
in r
pl be
m m
sa nu
4
5
65
5
25
10
15
20
10
20
30
40
50
60
200m
h
N
e
in
gl
in r
pl mbe
m
sa nu
40
0.015
0.2
Strain
123
2
Strain
1
0.01
0.1
0.005
44
37
SE
NW
43
39
30
DAFS
32 35
Apparent throw (m)
27
64
4
28 25
1
2
25
DFLSE
conjugated
20
NE
1
25
27
28
32
35
39
43
44
49
63
64
DFLSE
15
SW
2
4
16
DFTAB
0
Apparent throw (m)
0
1
63
12
8
10
4
e
in
gl
in r
pl be
m m
sa nu
Monday, May 27, 13
5
2
65
4
16
12
8
10
20
30
200m
i
1
125
2
44
63
49
43
0.01
0
SE
1
37
40
39
13
1
Apparent throw (m)
27 33 35
64
32
4
5
28 25
20
2
62
Strain
e
in
gl
in r
pl mbe
m
sa nu
32
DFLSE
conjugated
0.02
NW
DFLSE
and
DAFS
30
20
0.04
DAFS
1
20
25
26
27
28
29
31
32
33
35
37
39
43
44
49
63
64
DFSS
NE
SW
2
4
5
13
21
22
62
20
DFTAB and DFLSE
Apparent throw (m)
N
Strain
0.06
0.02
16
12
8
0
22
21
2
52
e
in
gl
in r
pl be
m m
sa nu
10
4
20
10
30
10
20
30
40
50
200m
j
51
1
0.02
Strain
e
in
gl
in r
pl be
m um
a
s n
30
N
2
Strain
130
0.01
0.04
0.02
44 49
5
SE
43
39
35
32
33
Apparent throw (m)
27
64
4
13
40
28 25
2
20
62
DFLSE
conjugated
30
NE
SW
20
35
37
39
43
44
46
63
64
20
0
DAFS
22
21
NW
20
25
26
27
28
29
31
32
33
DFLSE
and
DAFS
Apparent throw (m)
37
0
1
63
10
2
4
5
13
21
22
23
DFTAB and DFLSE
16
12
8
4
e
in
gl
in r
pl be
m m
sa nu
2
52
4
8
12
16
20
10
20
30
40
200m
k
1
e
in
gl
in r
pl mbe
m
sa nu
0.02
Strain
25
N
Strain
135
2
0.01
0.04
0.02
49
44
0
46
43
37
1
SE
NW
39
5
25
4
28 25
1
2
Apparent throw (m)
35
27 33
64
32
20
17
23
0
21
DFLSE
conjugated
20
DFLSE
and
DAFS
15
10
1
20
25
26
27
28
29
31
32
NE
33
35
37
39
43
44
46
63
64
2
4
5
21
22
23
10
DAFS
22
48
e
in
gl
in r
pl be
m m
sa nu
2
Monday, May 27, 13
SW
DFTAB and DFLSE
Apparent throw (m)
63
5
8
6
4
2
4
8
12
16
20
20
30
10
40
200m
l
28
39
0.01
0.005
46
43
1
0
SE
NW
DFLSE
and
DAFS
25
5
Apparent throw (m)
27 31 33
64
32
26
2
20
23
0.01
0.005
17
22
17
20
26
27
31
32
33
39
43
44
46
64
20
DFLSE
conjugated
15
10
NE
12
10
Apparent throw (m)
44
e
in
gl
in r
pl mbe
m
sa nu
Strain
N
2
Strain
1
145
SW
2
5
22
23
DFTAB and DFLSE
8
6
4
0
2
35
e
in
gl
in r
pl be
m m
sa nu
Monday, May 27, 13
DAFS
5
2
5
10
15
20
25
10
20
30
200m
m
52
1
Strain
e
in
gl
in r
pl mbe
m
sa nu
28
N
45
2
0.03
0.002
Strain
150
0.001
0.02
0.01
46
0
7
1
SE
NW
14
10
61
23
Apparent throw (m)
27
17
NE
17
27
33
39
45
46
33
12
10
8
0
2
31
e
in
gl
in r
pl be
m m
sa nu
6
SW
7
10
23
61
4
Apparent throw (m)
39
DFTAB
3
2
4
1
2
5
10
15
20
25
5
10
15
20
25
30
200m
n
1
N
0
e
in
gl
in r
pl mbe
m
u
sa n
0.01
SE
2
14
17
16
16
DFLSE
14
12
10
8
SW
DFTAB
8
Apparent throw (m)
31
1
0
NE
16
17
18
23
0.005
NW
20
Apparent throw (m)
e
in
gl
in r
pl be
m m
sa nu
7
0.02
Strain
Strain
155
2
6
14
23
4
6
4
2
2
Monday, May 27, 13
1
2
3
4
5
5
6
10
15
20
25
30
200m
o
160
N
e
in
gl
in r
pl mbe
m
sa nu
Strain
0.006
15
0.004
0.002
SE
0
NW
12
16
17
19
24
30
36
DFLSE
Apparent throw (m)
10
36
30
8
6
4
19
17
2
Monday, May 27, 13
24
2
16
200m
4
6
8
10
p
53
Strain
170
N
15
30
0.01
0.005
e
in
gl
in r
pl mbe
m
sa nu
SE
NW
0
18
Apparent throw (m)
14
24
16
16
18
24
30
DFLSE
12
10
8
6
4
2
2
200m
4
6
8
q
Figure 18. Left side contains maps displaying the faults affecting each coal seam contoured by
their throw attribute. Right side shows plots of individual and aggregate fault throw, and fault
related strain vs. distance. The initial and the last position of the sampling line are indicated in
the map view. e-o contain two groups of plots, 1 and 2, corresponding to NW-SE striking faults
(1) and NE-SW striking faults (2). DFTAB : Tabaco fault domain, DFLSE: Southern limb
domain, DAFS: Samán fault antithetic domain, DFSS: Samán fault domain.
4.3 Restoration
Monday, May
13
Figure 19 shows
the27,results
of the restoration of the anticline using the flexural-slip technique. At
each restoration stage, the uppermost stratigraphic surface was unfolded to a datum (300 m)
while the underlying surfaces were carried as passive objects. The underlying layers were
sequentially unfolded as the restoration proceeded. Since the older beds have more control data,
the restoration was performed only in this part. The coal seams that were included in the
restoration are 130, 115, 105 and 100. Figure 19 shows the incremental restoration of 3 cross
sections through the anticline. When the restoration was done, the strain was captured in the
restored beds (Figure 19a). Strain maps from the unfolding will be explained in the next section.
The coal seams were restored incrementally through four unfolding stages. The total shortening
of the anticline is 18%. The shortening of the restoration of coal seam 130 is 6%, of coal seam
54
115 is 2%, of coal seam 105 is 7%, and of coal seam 100 is 3% (Figure 19). Shortening was
measured across the surface in the maximum shortening direction (127°) normal to the fold axis.
When coal seam 130 was restored, the main fold disappears almost completely. This suggests
that the fold was formed after the deposition of the the Cerrejón Formation. However after the
restoration of coal seam 130, it is still possible to observe some minor folds in coal seams 105
and 100 (Figure 19b left side). The restoration shows that: 1) flexural slip was not the only
deformation mechanism that operated in the anticline. Flexural slip is an important mechanism in
systems consisting of mechanically strong layers and beds of easy slip such as thinly bedded
sandstone-shale sequences (Lisle, 1999). In the older coal seams (95-105) this interbedding
between strong layers and incompetent beds is not observed (Figure 4), and for that reason the
slip between those older beds may not easily happen. 2) Small scale folding occurs in coal seams
100 and 105 located in a sedimentary package surrounded by shale, and claystone (Figure 4).
This material is very ductile and can flow relatively easy. Despite minor folds in coal seams 100,
105 and 115, after the restoration of coal seam 130, the regional dip is towards the west (Figure
19b right side). The same regional dip is observed after the restoration of coal seams 115 and 105
(Figure 19b left side).
N
A
B
C
A´
B’
100 m
C’
a
55
Figure 19. Restoration of the Tabaco anticline using the flexural-slip technique. The cross-
b
56
200m
200m
200m
200m
200m
NW
A
Restoration of 100
Restoration of 105
Restoration of 115
100
100
105
105
100
105
100
100 105
Restoration of 130
Present Day
115
115
115
130
130
SE
A´
200m
200m
200m
200m
NW
B
Restoration of 105
Restoration of 115
Restoration of 130
Present Day
105
105
105
105
115
115
115
130
SE
130
B´
NW
C
200m
200m
200m
Restoration of 115
Restoration of 130
Present Day
115
115
115
130
130
SE
C´
sections have no vertical exaggeration. a) plan view of the horizon 130 with the position of the
cross sections. The colors indicate maximum principal elongation e1 (which is explain in the next
chapter) b) Left side: Restoration stages in cross section A-A’ showing the present-day geometry,
the restoration of coal seam 130, restoration of coal seam 115, restoration of coal seam 105 and
restoration of coal seam 100. Note that in the present day section some small change of
stratigraphic thickness across the fold can be observed. In the middle of the figure: Restoration
stages in cross section B-B’ showing the present-day geometry, restoration of coal seam 130,
restoration of coal seam 115 and restoration of coal seam 105. Right side: Restoration stages in
cross section C-C’ showing the present-day geometry, the restoration of coal seam 130 and
restoration of coal seam 115.
4.3.1 Strain maps
When the restoration was done the strain was captured in the model beds between restoration
steps. The principal strain orientations were calculated using Move (Midland Valley). It is
calculated using the coordinate position from the strained coal seams relative to the position of
unstrained coal seams. The xyz position is given for each principal strain axes e1, e2 and e3.
Figure 20 shows the values of maximum principal elongation e1 for coal seams 130, 115, 105
and 100, which was captured between restoration steps. After the restoration of coal seam 130
the main fold disappears almost completely (Figure 19b), and for that reason the strain captured
in coal seams 130 and 115 (Figures 20a and b) is mainly associated with the bigger scale of
folding, while the strain captured in coal seams 105 and 100 (Figures 20c-d) is mainly associated
with small scale folding. A common characteristic in coal seams 130 and 115 is an area of high
strain values located in the SE limb (Figures 20a and b), which has the steeply dipping strata.
The shallow, W-dipping limb shows relative lower values of strain in coal seams 130 and 115
(Figures 20a and b). Additionally coal seams 130 and 115 show a localized area of high strain in
the SW limb, where faults striking NE-SW are located (arrows in Figures 20a-b).
Coal seams 100 and 105 show an area of high strain with orientation approximately E-W (arrows
in Figures 20c and d). Those areas of high strain in coal seams 100 and 105 may correspond with
the minor folds, which remain after the restoration of coal seam 130 (Figure 19b). Coal seam 105
shows higher values of e1 that the others coal seams. When the restoration was done, it was also
the coal seam with the highest values of shortening (7%). The cross-section A-A’ in figure 19b
57
shows that coal seam 105 is the one with more amount of minor folds, indicating a higher
shortening compared with the others coal seams.
115
115
130 130
N
0.12
0.12
0.120.12
N
N N
100 m
100 m
a
0.080.08
0.08
0.08
0.040.04
0.04
0.04
100 m
100 m
a
0
105
0
100
100
0.19
0.19
105
b
b
0
0
0.10
0.10
N
N
N
N
0.13
0.13
0.07
0.07
0.06
0.03
0.06
c
100 m
100 m
c
0
0
0.03
100 m
100 m
d
d
0
0
Figure 20. Maps of maximum principal elongation e1. a) coal seam 130 b) coal seam 115 c)coal
seam 105 and d) coal seam 100. The strain was captured between restoration steps.
58
5. Discussion
The 3D model shows a total of 67 faults distributed in 17 coal seams. The amount of faults
affecting the older coal seams are lower (in coal seams 95, 100, 102, and 105 the number of
faults are 4, 7, 6, and 7 respectively) that in intermedium coal seams (in coal seams 125 and 130
there are 29 faults). After coal seam 130, the number of faults decrease. The lower amount of
faults in the lower coal seams can be the result of the incompetence of the rocks surrounding
these coal seams (Figure 4). Furthermore, it seems like the faults are splitting upsection in the
anticline, with the lower beds having few fault branches while the intermedium beds have more
fault branches (Figures 14 and 18). Since the lithology of the upper coal seams is almost the
same as that of the intermedium coal seams, the number of faults in these coal seams (e.g. 160
and 170) can be the same as in the intermedium seams. However, because there are less data in
the upper beds, it is not possible to test this hypothesis.
In the 3D model, it was also possible to observe a series of reverse faults with an E-W direction
at the south nose of the plunging anticline, between coal seams 115 and 145 (Figure 14). A
possible origin for these faults is a N-S compressional event proposed by Kellogg (1984) in the
Sierra de Perijá. This event, which is post-middle Eocene, generated stylolites, folds and reverse
faults with an E-W direction. Kellogg (1984) suggested that this event may be caused by oblique
movement on the Oca fault system.
A conjugated pattern was described in the eastern limb of the anticline, between coal seams 120
to 145. Conjugated fractures are common in folded strata (Stearns, 1968; Cooper et al., 2006).
These conjugates are formed systematically with respect to the fold axis and bedding (Stearns,
1968; Bergbauer and Pollard, 2004) (Figure 21). The conjugated faults described in the Tabaco
anticline may have been generated when the strata of the anticline were folded.
59
the maximum (s1) and minimum (s3)
stresses are inferred to lie within the b
plane. The inferred directions of maxim
minimum principal stresses are differe
each fracture set. The lower diagram i
view of the fracture sets with conjugat
fractures shown as dashed lines and e
fractures as solid lines. The two fractur
delineated by shading in the lower dia
with upper-diagram fractures colored
middle-diagram fractures colored blac
jugate fractures are not shown on the
of the plan-view diagram to emphasize
tion in the extension fractures. The two f
sets are reprinted from Stearns and Fried
(1972) with permission from AAPG.
Figure 21 Stearn and Friedman (1972) model, showing the fractures set associated with folding
(Cooper et al., 2006).
Four different fault patterns in the contours of fault throw were observed (Figures 16 and 17): 1)
Low throw in the middle of the fault and high throw in the areas around, 2) highest throw at one
corner of the fault and not in the center, 3) the most common pattern, highest throw in the middle
of the fault, and 4) in conjugated faults the highest value of throw is located at the intersection of
Basin of central Australia. Their fold-related fracture-
plunging, breached anticline has dips in exces
the two fault planes.
Those
throw
were
not sets
only
a vary
useful technique
for checking
on the forelimb
and up tothe
23j on the backlim
trajectory
map
showspatterns
extension
fracture
that
anticline strikes roughly north-northwest an
in orientation with respect to the fold. These fractures
quality of the interpretation
and construction of the 3D model, but they
also help to understand
by several normal-oblique, northeast-striking
perpendicular faults. Fractures strike roughly pa
the hinge throughout the fold. Fractures striking
perpendicular to the fold hinge are composed
Fractures in Folds Associated with Strata Overlying
basic types: (1) those with trace lengths extend
Deep-Seated Thrusts
The fault plane with an area of low throw in the middle of the fault can indicate: 1) that the area
eral meters and (2) those that terminate at i
tions
hinge-parallel
fractures.
DeSitter
(1956)
documented
normal
faults
parallel
to
of low throw is a tip point, which suggest two instead of one fault, 2)
thewith
linkage
fault model
Three joint sets have been described within
fold hinges in several anticlines (Kettleman Hills, Cali(Peacock and Sanderson,
1991; oil
Cartwright
et al.,
et al., 2000),
in which
the atfaults
Formation
sandstones
Oil Mountain, approx
fornia; Quitman
field, Texas;
Sand1995;
Draw Kim
oil field,
30 mi (48 km) west of Casper, Wyoming (H
Wyoming; La Paz oil field, Maracaibo district, Venegrew isolated at zuela)
early and
stages
and then linked to produce a larger fault,
and 3) areas of high
et al., 1998, 2000). Oil Mountain is a northwestattributed them to tension in the upper arc
doubly
plunging,
breached anticline. It is uniqu
of a doubly plunging
anticline.
faults
displacement due or
to crest
the intersection
of two faults
withNormal
different
orientation
(Kim
and Sanderson,
category because it is interpreted to have thru
roughly perpendicular to the fold hinges at the same
2005). After a careful
review
of the interpretation,
options
and 3 were
so forthat
theare related to deep
in thediscarded,
Mesozoic section
locations
were attributed
to tension resulting
from13-D
ment thrusts. A fracture set parallel to the fo
closure (DeSitter, 1956). Engelder et al. (1997) docuTabaco anticline, the
mostsimilar
likelyfaults
alternative
is theanticline
linkageinfault
model. at Oil Mountain is interpreted as a preexisting
mented
at Elk Basin
the Bigset due to its presence in Frontier Formation pa
horn Basin of Wyoming. This basement-cored, doubly
correspond to the fracture sets described by Stearns
and and
Friedman
(1972) (Figure
3). faults.
fault growth patterns
the interaction
between
The second pattern is a fault plane with the area of highest throw value in a corner of the fault Cooper et al.
and not in the center. This suggests: 1) that the fault plane is not complete because there were not
enough data to cover it. 2) In the case of fault 4, there is a problem in the interpretation because it
is not a consistent pattern of fault displacement. Additionally, the fault-related strain value for
this fault was 0.2, higher than the upper limit of 0.1 suggested by Freeman et al. (2010). When
60
the faults-horizons intersections polygons were re-checked in the 3D model an anomalous high
value in the displacement of this fault was observed, so a re-interpretation of fault 4 was done.
The new values of throw and fault-related strain for fault 4 in coal seam 123 can be seen in
figure 22. The highest value of fault-related strain is 0.09 in this case, which is consistent with
the upper limit suggested by Freeman et al. (2010). With this result it is clear that the fault throw
Strain
pattern and fault-related strain values were a very useful tool to validity the fault interpretation.
0.1
SE
NW
2
4
Apparent throw (m)
8
6
4
2
5
10
15
20
25
30
Figure 22 Individual and aggregate fault throw, and fault-related strain vs. distance for coal seam
123 after the re-interpretation of fault 4.
The third pattern with high throw values in the middle of the fault shows a gradual variation in
throw along strike, which is the expected behavior of a single fault. The last pattern occurs in
conjugated faults. In these cases, the highest value of throw is located at the intersection of the
two fault planes.
61
The fault domain that have the highest values of strain is the DFTAB, showing strain values
(fault-related strain) between 0.5% for coal seam 155, to 9% in coal seam 123 (Figure 18i and
18o). The strike of the faults in this domain is mainly NE-SW, similar to the trend of the Perijá
mountains. This can indicate a propagation of the Perijá deformation towards the west. However
there is one fault in the DFTAB domain that does not have the NE-SW trend. Fault 59 in the
northern part of the DFTAB domain and in lower coal seams, strikes almost E-W (Figure 14,
coal seam 100-105). Two interpretations are possible for the strike of this fault. The first
interpretation is that this fault was generated in the N-S compressional event described above.
The other option is that this part of the anticline is affected by left lateral movement of the
Samán fault, which rotated fault 59 to its current position. This second interpretation is supported
by the shape of the axis of the anticline, which bends close to the area of the Samán fault (Figure
2).
For coal seam 130 the curvature values are showing a difference between the size of the basins
(dark red) and synformal saddles (dark yellow), in both limbs. In the western limb the basins are
bigger than those in the eastern limb (Figure 15). The eastern limb has more faults affecting this
area of the coal seam (Figure 14), so this situation does not allow the development of big basins
in the eastern limb. While the faults affecting the western limb are less but with a higher
displacement, making that the size of the basins and synformal saddles bigger in that part of the
anticline.
The Cerrejón formation is a pre-folding formation, because when the upper coal seam 130 was
restored, the anticline almost vanished. The total shortening of the Tabaco anticline calculated
with the flexural slip technique is 18%. The shortening in the restoration of coal seam 130 is 6%,
of coal seam 115 is 2%, of coal seam 105 is 7%, and of coal seam 100 is 3%. In the restoration
process, a regional dip towards the west was identified. This shows that the depocenter of the
basin at the onset of folding was to the west of the Tabaco anticline .
62
When the shortening calculated with flexural slip is compared with the fault-related strain, it
does not show significant difference (in most cases only 1%). The maximum fault-related strain
for coal seam 130 is 5% (Figure 18 k), for coal seam 115 is 3% (Figure 18 g), for coal seam 105
is 10 % (Figure 18 d), and for coal seam 100 is 2.5% (Figure 158b). Fault-related strain, can be
slightly different that the fold-related strain calculated with flexural slip, because the gradual
accumulation of regional strain leads to fault slip; therefore the fault-related strain is related to
local perturbations superimposed on the regional strain (Dee et al., 2007).
High strain values (e1, Figure 20a and b) are located in the SE limb associated with the steeply
dipping strata in coal seams 130 and 115. Coal seams 100 and 105 show an area of high strain
with orientation E-W (arrows in Figure 20c and d). The strain captured in coal seams 130 is
mainly associated with the bigger scale of folding, while the strain captured in coal seams 105
and 100 (Figures 20c-d) is mainly associated with small scale folding. The high values of strain
in coal seams 130 and 115 shows that the SE limb with steeply dipping strata accommodated the
highest amount of strain in the anticline.
5. 1 Summary of the main events affecting the Tabaco anticline in a regional context
In the Middle-to-Late Paleocene the deposition of the Cerrejón Formation started in the area
(Figure 23a). The depocenter was towards the west of the future position of the Tabaco anticline.
Even though some small thickness changes are identified in the Cerrejón Formation, it is a prefolding unit. By that time, the Santa Marta massif had some episodes of uplift (Cardona et al.,
2010).
In the Late Paleocene-Early Eocene, the deposition of the Tabaco Formation started in the basin.
Montes et al. (2010) interpreted this Formation as syntectonic. If so, the Tabaco anticline was
probably formed during this time. The Santa Marta massif continued its uplift (Cardona et al.,
2010). Bayona et al. (2007) and Kellogg (1984) suggested that the tectonic activity of the Perijá
63
range started in the Early-Middle Eocene. The vergence of the faults in the western part of the
Perijá range is towards the west, however the vergence of the Tabaco anticline is towards the
east. One possible explanation can be that the Tabaco anticline was formed by a backthrust of the
west vergent thrust belt of the Perijá range (but there is not evidence of this backthrust). However
in the western limb of the Tabaco anticline, a series of thrust (belonging to the DFTAB domain)
with the same strike and the same vergence as the Perijá thrust belt are observed. So an
alternative interpretation for the formation of the anticline is that it was generated as a
detachment fold, trigged by the uplift of the Santa Marta massif and using the incompetent
Cerrejón formation as a detachment (Figure 23b). In addition, the reverse faults in the western
limb of the anticline could be generated after the anticline was formed and were related with the
propagation of the Perijá mountain front towards the west. This second alternative can explain
the eastern vergence of the anticline and the reverse faults in its western limb (Figure 23c).
However with the information analyzed in this thesis, there is not evidence that the faults of the
western limb are posterior to the main fold event.
The conjugated faults were generated when the beds of the Cerrejón Formation were folded
(Figure 23b). This conjugated pattern include two orientation of thrust faults and can be
reflecting locally developed of bending or buckling (Stearns, 1968 and Bergbauer and Pollard,
2004), so it is related with the folding of the beds.
The Samán fault may be formed in an event posterior to the formation of the Tabaco anticline
and the generation of the faults of the DFTAB domain. This left lateral strike-slip fault generated
a series of small antithetic faults close to its area of influence. Additionally, it bent the axis of the
anticline and some the faults of the DFTAB domain (Figures 2 and 23d). Another event posterior
to the formation of the Tabaco anticline was the generation of the E-W faults generated by a
north-south compression event (Figure 23d).
64
Proto Santa
Marta massif
Proto Santa
Marta massif
Proto Santa
Marta massif
Pro
Ma
Proto Santa
Marta massif
Pr
M
Proto Santa
Marta massif
a
Tabaco
anticline
Proto Santa
Marta massif
Tet
TpcTabaco
anticline
Ku
Proto Santa
Marta massif
Tet
Tabaco
anticline
Tpc
Ku
b
Tet
Tpc
Proto Santa
Marta massif
Ku
Tabaco
anticline
Perijá
range
Proto Santa
Marta massif
Tabaco
anticline
Perijá
range
c
Proto Santa
Marta massif
Tabaco
anticline
Perijá
range
65
rijá
nge
Oca
fault
Proto Santa
Marta massif
Samán
fault
Perijá
range
Tabaco
anticline
d
Figure 23 Summary of the main events affecting the Tabaco anticline. a) Middle-to-Late
Paleocene deposition of the Cerrejón Formation. b) Late Paleocene-Early Eocene generation of
the Tabaco anticline as a detachment fold, Ku: Cretaceous undiff., Tpc: Cerrejón Formation, Tet:
Tabaco Formation. c) Reverse faults in the western limb of the anticline are generated and are
related with the propagation of the Perijá mountain front towards the west. Perijá range is
interpreted here as thin-skin due to the structural cross section from Montes et al. (2010) and
Kellogg (1984) d) The Samán strike-slip fault generated antithetic faults in the anticline and
bends it. Faults striking E-W are also generated.
References
Allmendinger, R.W., Cardozo, N., Fisher, D., 2012. Structural Geology Algorithms. Cambridge
University press. 289 p.
Bayona, G., Lamus, F., Cardona, A., Jaramillo, C., Montes, C., and Tchegliakova, N., 2007.
Procesos orogénicos del Paleoceno para la Cuenca de Ranchería (Guajira, Colombia) definidos
por análisis de procedencia. Geol. Colomb., 32, p 21-46.
Bayona, G., Montes, C., Cardona, A., Jaramillo, C., Ojeda, G., Valencia, V., Ayala-Calvo, C.,
2011. Intraplate subsidence and basin filling adjacent to an oceanic arc-continent collision: a case
from the southern Caribbean-South America plate margin. Basin research, 23, p 403-422.
66
Beck, E., 1921. Geology and Oil Resources of Colombia, South America, Economic Geology,
16, p 463-465.
Bergbauer, S., 2002. The use of curvature for the analysis of folding and fracturing with
application to the Emigrant Gap Anticline, Wyoming. PhD thesis, Stanford University.
Bergbauer, S., and Pollard, 2004. A new conceptual fold-fracture model including prefolding
joints, based on the Emigrant Gap anticline, Wyoming. GSA Bulletin, 116, p 294-307
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