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Fall 2016
Periclinal folding in Ozark plateau: A record of local karst collapse or regional tectonic forces?
Chao Liu
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PERICLINAL FOLDING IN OZARK PLATEAU:
A RECORD OF LOCAL KARST COLLAPSE OR REGIONAL TECTONIC
FORCES?
by
CHAO LIU
A THESIS
Presented to the Faculty of the Graduate School of the
MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY
In Partial Fulfillment of the Requirements for the Degree
MASTER OF SCIENCE IN GEOLOGY AND GEOPHYSICS
2016
Approved by
John P. Hogan, Advisor Andreas Eckert Wan Yang
ii
iii
ABSTRACT
The Ordovician Rubidoux Formation of the Ozark Plateau, Missouri is typically identified as mechanically competent sub-horizontal beds of medium grained sandstone.
North of Licking, MO, the Rubidoux Formation was deformed into a series of dome-and- basin shaped buckle folds, exposed in road cuts over a distance of ~10 km. Such folds are highly unusual within the Ozark Plateau and their origin remains controversial.
Three major hypotheses have been proposed based on field observations and stereographic analysis. Given the pervasive distribution of karst collapse throughout Missouri, one hypothesis to explain these folds is that the Roubidoux Formation sagged into collapsed caverns. A second hypothesis is that these folds are the result of compression induced in a possible transpression zone between two left-lateral strike-slip faults. A combination of the two processes is also possible.
Field measurements of 37 fold structures including more than 300 recordings of bedding attitudes and stereographic projections confirm that these Roubidoux folds are periclinal folds. They elongate along N59E to S59W, with the maximum shortening occurring along N60W to S60E. The uniformity of the fold orientations over this large distance is inconsistent with an origin by sag folding related to karst collapse. While the orientation of the shortening direction is inconsistent with the general trend of midcontinent deformation structures, two possible left-lateral strike-slip faults mapped to the north and south of the folding zone may have locally reoriented the stress field. This is supported by a simple 2D finite element model indicating that transpressional deformation in a restraining jog results in localized shortening direction consistent with the distribution field data.
It is concluded that a regional tectonic event induced localized buckle folding whereby local karst effects may have contributed in forming the initial perturbation. iv
ACKNOWLEDGEMENTS
I would like to thank Dr. Hogan for his guidance and advising. His sparking suggestions and encouragements are critical for this project. I would like to thank Dr. Eckert for his guidance on numerical models, which strong support the possibility of the hypothesis proposed in this thesis. I would like to thank Dr. Yang for giving advice on field work during his field trip to Roubidoux Formation. Also, I would like to thank those colleagues who offered assistance of field work.
I would like to acknowledge the financial support from a Geological Society of
America Research Grant and University of Missouri University of Science and Technology
Radcliffe endowment.
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TABLE OF CONTENTS
Page
ABSTRACT ...... iii
ACKNOWLEDGMENTS ...... iv
LIST OF ILLUSTRATIONS ...... vii
LIST OF TABLES ...... ix
SECTION
1. INTRODUCTION ...... 1
2. GEOGLOGIC SETTINGS ...... 5
2.1 TECTONICS ...... 5
2.2 STRATIGRAPHY ...... 7
3. RESULTS ...... 12
3.1 FIELD WORK STRATEGY ...... 12
3.2 FIELD WORK ...... 14
3.2.1 Site One --- Maximum Deformation Intensity ...... 14
3.2.2 Site Two --- Medium Intensity of Deformation ...... 25
3.2.3 Site Three --- Minimal Deformation Intensity ...... 32
3.2.4 Site Four --- End of Folding Zone ...... 40
3.3 FIELD WORK SUMMARY ...... 41
4. DISCUSSIONS ...... 43
4.1 KARST COLLAPSE INVOLVED DEFORMATION ...... 43
4.2 REGINAL TECTONICS INDUCED BUCKLE FOLDING ...... 45
4.2.1 Multi-Event Origin Buckle Folding ...... 46
4.2.2 Single-Event Origin Buckle Folding...... 47 vi
4.3 THE COMBINATION OF HYPOTHESES ...... 48
4.4 2D FINITE ELEMENT ANALYSIS ...... 48
4.5 UNSOLVED QUESTIONS AND FUTURE WORKS ...... 50
5. CONCLUSIONS ...... 51
APPENDIX ...... 52
BIBLIOGRAPHY ...... 55
VITA ...... 60
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LIST OF ILLUSTRATIONS
Figure Page
1.1 Geologic map of study area. (After Middendorf, 2003) ...... 3
2.1 Principal midcontinent fault and fold zones. (Marshak and Timothy, 1996) ...... 5
2.2 Distribution of rocks at different ages in Missouri. (Thompson, 1995) ...... 7
2.3 Stratigraphic column of Canadian Series, Ordovician. (Thompson, 1995) ...... 8
2.4 Stratigraphic column of Roubidoux Formation. (Thompson, 1991) ...... 10
3.1 Map view of site one with important features labeled...... 15
3.2 Sketch of folds information in site one...... 17
3.3 Stereographic plot of site one...... 18
3.4 Fold #1_W (Upper) and fold #2_W (Lower) on west side of site one...... 19
3.5 Fold #1_E (Lower) on east side of site one...... 19
3.6 Fold #2_E (Upper) on east side of site one...... 20
3.7 Fold #4_W (Upper) on west side of site one...... 20
3.8 Fold #5_W (Upper) on west side of site one...... 21
3.9 Fold #4_E (Upper) and fold #5_E (Lower) on east side of site one...... 23
3.10 Fold #6_E (Upper) on east side of site one...... 23
3.11 Fold #6_W (Upper) on west side of site one...... 24
3.12 Fold #7_E (Lower) and fold #8_E (Upper) on east side of site one...... 24
3.13 Fold #9_E (Upper) on east side of site one...... 25
3.14 Map view of site two with important features labeled...... 26
3.15 Sketch of folds information in site two with rose diagram...... 27
3.16 Fold #1 on west side of site two...... 28
3.17 Fold #1 on east side of site two...... 28
viii
3.18 Fold #2 on east side of site two...... 29
3.19 Fold #2 on west side of site two...... 29
3.20 Gap area on west side of site two...... 30
3.21 Fold #3 on west side of site two...... 31
3.22 Fold #3 on east side of site two...... 31
3.23 Fold #4 on west side of site two...... 32
3.24 Fold #4 on east side of site two...... 32
3.25 Map view of site three with important features labeled...... 33
3.26 Sketch of folds information in site three with rose diagram...... 34
3.27 Fold #1 on west side of site three...... 35
3.28 Fold #2 on west side of site three...... 36
3.29 Fold #3 on west side of site three...... 36
3.30 Fold #4 on west side of site three...... 37
3.31 Fold #5 on west side of site three...... 37
3.32 Fold #1 on east side of site three...... 38
3.33 Fold #2 (Upper) and fold #3 (Lower) on east side of site three...... 38
3.34 Fold #6 on west side of site three...... 39
3.35 Fold #8 on west side of site three...... 39
3.36 Fold #5 on east side of site three...... 40
3.37 Map view of site four with important features labeled...... 41
4.1 Karst collapse involved deformations...... 44
4.2 Rose diagram of hinge line trend distributions...... 46
4.3 Geologic map of study area. (Modified Middendorf, 2003) ...... 48
4.4 2D finite element simulation by reproducing the study area...... 49
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LIST OF TABLES
Table Page
3.1 Data table of folds information of site one ...... 16
3.2 Data table of folds information of site two ...... 27
3.3 Data table of folds information of site three ...... 34
1
1. INTRODUCTION
An unusual zone of folding of Ordovician Roubidoux Formation is exposed in road cuts along highway 63 near Licking Missouri. The presence of these folds is enigmatic as the midcontinent region is characterized as a relatively stable craton, and within this area the
Rubidoux Formation is typically undeformed and crops out as flat lying strata with low to sub-horizontal dips (Unklesbay and Vineyard, 1992; Thompson, 1995).
The focus of this thesis is to constrain the origin of this zone of folding of Roubidoux
Formation within the Ozark Plateau. . The study area includes the road cuts of Highway U.S.
63, for a distance of about 10 kilometers, in the southernmost part of Phelps County, MO to about 2.5 kilometers north of Licking, Texas County, MO (Figure 1.1). In this specific area, sandstones of the Lower Ordovician Roubidoux Formation are deformed into a series of folds, inconsecutively, exposed along road cuts. The variation in fold geometry is recognized in terms of dipping directions, symmetricity, fold amplitude, and fold wavelength. The different intensity of deformation between inconsecutive outcrops, as recorded by fold shape, shows deceasing trends from center towards both the southern and northern ends of the zone of folding. The road cuts provide the only opportunity to examine these folds because surrounding areas are either heavily coverage by vegetation or owned by private farms. The questions of how and when these deformation event happened remains unanswered.
Three major hypotheses are proposed to explain the origin of these folds. Given the considerable distribution of karst caves throughout Missouri, one hypothesis is that Rubidoux
Formation sagged into collapsed caverns (i.e., sink-holes) developed in the underlying
Gasconade Dolomite. In this model, the Gasconade dolomite was dissolved by naturally slightly acidic precipitation and removed by groundwater, creating void space for the overlying Roubidoux sandstones to sag into the sink (Spencer, 2011)., Bertz (1960) proposed 2 filled–sink structures to interpret how this type of folds formed in Missouri. Folds are recognized as important structures accompanied with karst collapse (Hack, 1965; Hubbard,
1983; Orndorff and Goggin, 1994; Doctor et al., 2008). Many detailed examples can be found in Missouri (Bertz, 1960; Spencer, 2011) and other states (Doctor et al., 2008). A second hypothesis is that these folds record buckling as a result of tectonically induced horizontal shortening as the result of far-field forces related to plate motions. Fold and fault zones have been reported in the Midcontinent region (Nelson, 1991; Park and Jaroszewski,
1994; Nelson and Marshak, 1996). Several different scenarios have been discussed based on field mapping analysis. Possibilities of single and multiple folding events have been taken into consideration. Realistic scenarios are those that are consistent with maximum principal stress derived from field data with surrounding structures from published resources
(Middendorf, 2003; Cox, 2009). Finally, a combination of the two processes is also possible, where tectonic deformation is dominating the folding zone and subsequent karst collapse modifies the form of these folds
In order to test which model is more feasible, a field mapping analysis has been conducted. This includes tracing folds and plotting measured data on stereographs to construct their geometry and fold architectures. This approach included the following steps:
1) Stereographic analysis of fold hinge lines. When the trend and plunge of all measured hinge lines for folds are plotted on one stereograph, karst collapse induced folding will lead to a more random distribution of hinge lines with respect to each collapse. This reflects the immediate local control of the sink hole on the form of the fold. In contrast, a more systematic pattern for the orientation of fold hinges is expected for tectonic buckle folding. The orientation of tectonic folds will reflect control by the orientation of the maximum principle stress directions at the time of folding. 3
2) Spatial relationship determination. By checking the attitude of an individual fold, a conceptual map can be constructed to indicate the spatial relationship to the neighboring folds within a more local structural domain within the entire zone of folding. Instead plotting all folds in one stereograph, each fold will be plotted separately with information hinge line and axial plane along fold cut. In this case, a 2D map view display can be derived from the field data. Such maps display fold orientation information so that fold pattern can be revealed.
Figure 1.1 Geologic map of study area. (After Middendorf, 2003)
3) Whole picture trend analysis. Since the folding zone is comprised of inconsecutive groups of outcrops, separated by covered zones where the Roubidoux does not crop out, it is necessary to examine whole folding trend. Fold amplitude and wavelength are used to 4 determine the intensity of deformation, where high degree of deformation will correspond to folds that exhibit higher amplitudes and tighter wavelengths.
4) If the folds form by tectonic processes, then the shortening directions and possible force sources able to predict. Comparison with published maps with orientations of structures will be used to make comparisons. If fold orientations are consistent with other regional structures, this will support a tectonic origin for these folds. Otherwise, unmapped structures will be proposed.
5) 2D finite element approach. Numerical simulation will be used to test the possibility of proposed hypotheses to produce these fold patterns. 2D finite element model is able to provide the distributions of maximum principle stress, which will be used to compare with the results from field data.
The results of this thesis will be used to characterize and interpret the origin of
Roubidoux folds.
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2. GEOLOGIC SETTINGS
2.1 TECTONICS
The midcontinent region in North America is considered as a stable area, but different structural domains have been recognized. In the Ozark Plateau, two dominant sets of structural trends can be observed (Figure 2.1), which are north-northeast trends related to reactivation of the Late Proterozoic Central North American rift system; and west-northwest trends related to reactivation of Early/Middle Proterozoic basement faults (Snyder, 1968;
McCracken, 1971; Kisvarsanyi, 1984; Yarger, 1989; Marshak and Timothy, 1996). Different interpretations of those structural trends have been reported. Kisvarsanyi (1984) proposed that northwest tending Proterozoic faults may related to the Central North American rift system and Mississippi Valley rift system, but Marshak and Paulsen (1996) suggest that this
Figure 2.1 Principal midcontinent fault and fold zones. (Marshak and Timothy, 1996) 6 fault set is related to a Proterozoic rift graben system. Alternatively, Sims and Peterman
(1986), Sims et al. (1987), Bowring et al. (1988) relate these structures to a suture zone formed during the Early Proterozoic Central Plains Orogen.
During the Appalachian–Ouachita orogeny, thrusting along the edge of the North
American craton margin acts as driving force to reactivate structures of foreland uplifts and basins in the midcontinent region (Quinlan and Beaumont, 1984; and Arbenz, 1989).
Comprehensive reviews on Appalachian–Ouachita orogeny are provided by Hatcher et al.
(1989); Keller and Robert (1999). In the Ozark Plateau, this compression mainly reactivated basement northwest-trend structures, and overlying sedimentary rocks were deformed by thrust faults (Flint, 1926), gravity faults (McCracken, 1971; Nelson and Lumm, 1985), reverse faults (Nelson and Lumm, 1985), and strike-slip faults (Snyder and Gerdemann,
1968; Clendenin et al., 1989; Schultz et al., 1992; Clendenin et al., 1993; Harrison and
Schultz, 1994).
In the order from earliest to oldest, four regional episodes of Pennsylvanian deformation in the midcontinent region have been reported by Cox (2009). Event one is represented by northwest trending jointing and normal faulting, indicating the approach of the Ouachita terrane and its initial collision with the North American continental margin.
Event two shows a symmetric change, from north-northwest in the south to northeast in the north, in extensional deformation. This period of extension has been related to minor northeast lateral escape of lithospheric blocks away from the northwest-moving Ouachita terrane. Event three is dominated by contractional and strike-slip deformation with a strong east-northeast regional trend. These structures are consistent with northeast–southwest shortening during the late Paleozoic Ancestral Rockies deformation in the southern Great
Plains and Rocky Mountains. Event four has a general northwest trend of contractional and
7 strike-slip deformation, consistent with late stage northwest convergence along the southeastern margin of North America during the Alleghanian Orogeney.
2.2 STRATIGRAPHY
Most exposed rocks in Missouri are at age of Paleozoic, where Cambrian,
Ordovician, Mississippian and Pennsylvanian rocks are most observed in term of distribution and total thickness. Figure 2.2A shows the distribution of rocks at different ages. Rocks at
Cambrian and Ordovician ages are exposed mostly on southern part of Missouri.
Pennsylvanian rocks occupied northern and northwestern part of Missouri. In between,
Mississippian rocks crop out at northeastern and southwestern part of Missouri. And a few
Silurian and Devonian rocks are present in the southeastern, northeastern and Central
Missouri. Accumulated thickness of Paleozoic rocks in Missouri is approximately 10,000ft, while, not a single outcrop has even half of this thickness (Thompson, 1995).
A B
Figure 2.2 Distribution of rocks at different ages in Missouri. (Thompson, 1995)
Ordovician system, which is originally descripted by Lapworth (1879), is divided into four parts by Ross et al. (1982). From oldest to youngest, they are Canadian Series (called
Ibexian by Ross et al), Whiterockian Series, Mohawkian Series and Cincinnatian Series. 8
Before that, Missouri Ordovician system has been divided into three series (Martin et al.,
1961) and five series (Kay, 1960). Ordovician Rocks, exposed over one third of Missouri, where most of them are present around the flanks of the St. Francois Mountains, with accumulated thickness of approximately 3800 feet.
From oldest to youngest, the formations of Canadian series in Missouri are
Gasconade Dolomite, Roubidoux Formation, Jefferson City Dolomite, Cotter Dolomite,
Powell Dolomite and Smithville Dolomite (Figure 2.3). Generally, rocks of Canadian Series are mainly arenaceous and cherty dolomite and sandstone. They exposed in a large area south of the Missouri River (Figure 2.2B), extended from Mississippi River to Cedar County. There is a major unconformity at the top of Canadian series. The base is generally conformable with upper Cambrian, but locally unconformities are noticed in some area.
Figure 2.3 Stratigraphic column of Canadian Series, Ordovician. (Thompson, 1995) 9
Roubidoux Formation, which is first described by Nason (1892), consists of cherty dolomite, sandy dolomite, dolomitic sandstone, and sandstone. The thickness of Roubidoux
Formation ranges from 100 to 300 ft., where thickest parts are exposed in the southwestern and southeastern Ozarks and thinnest in northeastern Missouri (Thompson, 1991). Fossils are not pervasive in Roubidoux Formation, but some Mollusks can be found locally in chert. In many places, the primary structures are well preserved and observed. They are mud cracks, ripple marks and cross beddings.
The sandstone in Roubidoux Formation is composed with fine to medium grained, subrounded, poor to well sorted quartz sand. They have grey or brown on their weathered surface, while, light yellow tan or rend in their fresh surface. In central Missouri, Roubidoux
Formation is mainly quartzose sandstone but in other area, most rocks are cherty dolomite, so that it is difficult to distinguish from the underlying Gasconade Dolomite. The dolomite in
Roubidoux Formation is fine to medium crystalline, light gray to brown, thin to thick bedded.
Some beds contain brown to gray oolitic sandy chert (Thompson, 1991).
Type section of Roubidoux Formation is designated as The Roubidoux Creek Section in the Texas County, central Missouri (Heller, 1954). And The Roubidoux Creek Section is the only complete section as Heller stated. It has approximately 150 ft. and 44 members have been described. Another reference section is described by Muilenberg and Beveridge (1954) on the east side of highway 17, at the bridge over Jacks Fork, southeastern Texas County, south-central Missouri. In this place, over 100 ft of Roubidoux is exposed (Figure 2.4). It contains 27 members, where dolomite and sandy dolomite are prominent in this section with several sandstone beds.
Gasconade Dolomite Formation is underlying the Roubidoux. The average thickness of Gasconade is about 300ft in south-central Missouri. The well-log data shows a maximum thickness of 700ft in that area. Gasconade Formation is mainly composed of light brown to 10
Figure 2.4 Stratigraphic column of Roubidoux Formation. (Thompson, 1991)
gray, cherty dolomite. At the lowermost part, it contains a sandstone unit called Gunter
Sandstone Member. Separated by amount and type of Chert, Gasconade Formation is divided into “lower” and “upper” Gasconade. The “Lower Gasconade” is fine to medium crystalline, and thin to medium bedded dolomite with varying amounts of cherts. While, the “Upper
Gasconade” is a nearly chert-free, massive, fine to medium crystalline dolomite. Fossils can be found in this formation, which are Mollusca, stromatolite, gastropods, brachiopods, and trilobites (Stinchcomb, 1986). Karst collapse are very common when dolomite are dissolved. 11
Instead of forming karst caves, the filled-sink structures are also the result of erosion (Bertz,
1960; Spencer, 2011), where dolomite were dissolved slowly over time to create spaces for overlying beds to sink slowly into the structures.
Jefferson City Dolomite is carbonate formation overlying the Roubidoux. The thickness of Jefferson City Dolomite ranges from 100ft to 350ft and its average thickness is
200ft. this formation is mainly composed of light brown to brown, fine to medium crystalline dolomite and argillaceous dolomite. Locally, shale, conglomerate and orthoquartzite can be found. In the study area of this thesis Jefferson City Dolomite is eroded completely.
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3. RESULTS
3.1 FIELD WORK STRATEGY
Field description and structural measurement of the folded Roubidoux Formation has concentrated on several road cuts as most of the surrounding area is farmland. Rocks in folding zone are mainly cherty sandstones and sandstones. Two distinct sandstone layers can be recognized. One is on top of the other, in this thesis, the upper one is referred as “Upper
Roubidoux Sand” and the lower one is “Lower Roubidoux Sand”. They are interbedded with a thin clay layer at the thickness of about 50 cm. When both sandstones cropped out at the same location, they had very similar geometry. This incompetent inter-bed has been highly eroded on its surface. Upper Roubidoux sandstone is a light reddish to reddish, medium grain, well sorted, rounded, massive clean sandstone. The Lower Roubidoux sandstone is light in color with minor reddish portion, medium grain, well sorted, and rounded, massive sandstone. Lower Roubidoux sandstone is silicified, resulting higher hardness. Lots of fractures and joints are observed in every location and in individual layers. Some joints penetrated the entire beds, some are contained within a single layers. As another type of primary structures, ripple marks are observed in the whole study area.
Within this folding zone, road cuts provide the only accessibility to examine the folded rocks. Five accessible road-cut exposures have been chosen to conduct field analysis across the length of the folding zone .Four of the road cuts exhibit several folds and an additional site is chosen to represent undeformed Roubidoux sandstone for comparison. The folding zone extends over ten kilometers along highway U.S. Route 63 from the Phelps &
Texas county boundary to just outside the city of Licking. It’s been noticed that the rocks at north end have higher hardness and more resistant to erosion.
13
These folds appear as several consecutive groups of exposures in the form of road cuts. The major rocks in folding zone are sandstone and cherty sandstone of the Roubidoux
Formation.
Within the folding zone, highway U.S. 63 follows a nearly perfect in north-south route. Google Earth mapping shows that the heading of this segment of highway is 179.96 degrees and there is no major elevation change. The trend of route 63 is oblique with strikes of the axial planes of the folds. Instead of being able to follow the true strike of folds or perpendicular cross-sections, field work and fold analysis is confined along the N-S direction of the road cuts. The north-south trending highway in this area is an important reference to reconstruct fold architecture.
The intensity of deformation decreases from north to south. Undeformed rocks quickly reached maximum degree of folding in the north and gradually turned back into flat layers to the south. In the whole folding zone, there are nine groups of folds exposed over a distance of ten kilometers. Roubidoux sandstone crops out as horizontal, or nearly horizontal layers at both ends and folded rocks were only observed in between and in road cut exposures.
About 1000 meters south of the northern most end of the folding zone, where rocks are horizontal, sandstone layers are gently dipping in the second road cut, but not shortened enough to have formed anticline-syncline pairs. On the west side, sandstone layers are limited in exposed and slightly folded. The entire east side repents a long wavelength anticline which has some subtle minor wavy undulations on its southern limb.
Fold measurements start on the spot 1500 meters south from the northern end, and cover three groups of folds in a row (labeled as site one, two and three in the order from north to south). The first group of folds is one of the most important spot in the folding zone, where maximum deformation was achieved. The greatest fold amplitude and steepest dips are 14 observed is this place. Here the sandstone layers were deformed into prominent anticlines and synclines. By the reference of north-south trending road segment, the deformation center of whole folding zone is biased heavily to north in this specific viewing aspect (Fig 1.1). Three measured sites, covering the distance about 2000 meters record a decrease in the intensity of folding from maximum amplitude and shorter wavelengths to a relative low amplitudes and longer wavelengths. Detailed mapping results are used to reconstruct the fold layout and 3D architecture. The back-and-forth variations of fold axial planes and hinge lines can be easily noticed from both field and stereographic analysis. The periclinal geometry has recognized where the two shortening directions follow two regional trends in those folding area: northeast-southwest and northwest-southeast.
There are three more unmapped groups of exposures between south end and the third measured site. Because of very low dips folds south of this area were not investigated. The decreasing intensity of folding is clearly visible travelling south along highway 63. Similarly, to the north, the last outcrop where folds are still recognizable exhibit low dips and form large-wavelength, low amplitude, anticline over hundreds meters.
3.2 FIELD WORK
Near the boundary between Phelps and Texas Counties, the first field site has been chosen to represent maximum deformation intensity. Site one is next to Hutchason Cemetery at the intersection of Highway 63 and Co Rd 3974, which is approximately seven miles North of Licking city, Texas County, Missouri (Figure 3.1).
3.2.1 Site One --- Maximum Deformation Intensity. The Roubidoux Formation is highly deformed into folds with high amplitudes and short wavelengths. The fold amplitudes range from 6 to 8 meters approximately. The actual wavelength is not directly measurable because road cut has an angle with fold strike. But larger number and tighter arrangement of 15 folds are present in this site. This place occupies about 300 meters in distance along road and there are fifteen folds present on both side of road in total. Interestingly, the orientation of fold hinge lines (both plunge and trends) of these folds alternates from one fold to the next.
These folds are preserved with both symmetric and asymmetric geometries along the road cuts, where dramatic changes in dip on the limbs of folds can be observed. The difference between two limbs can be as much as 31o where one has 45o dip angle, while other has only 14o. The limb transitions of asymmetric folds are typically very sharp, especially for synclines. However, there is no visible pattern of the alterations of fold symmetricity.
Referencing north-south trending road, two changes are notices in this place: non- systematic changes of symmetricities and back-and-forth variations of fold axial plane and hinge line directions.
Figure 3.1 Map view of site one with important features labeled.
16
Both Upper and Lower Roubidoux Sands are present in this spot. However, most of the road cut is comprised of Upper Roubidoux sandstone. Only the top part, mainly the hinge zone, of Lower Roubidoux Sand are observed when they cropped out in form of anticlines.
There is no Lower Roubidoux sandstone observed in synclines within the folding zone. Some folds, with unexpected orientations, may be considered as related to karst collapse, and will perturb the construction of structural architecture in some degree.
There are fifteen folds been measured in site one and four of them are from Lower
Roubidoux Sands (Figure 3.4, 3.5, 3.9, and 3.12). One outlier is noticed because of its anomalous fold shape (Figure 3.7), and its adjacent folds were influenced by this fold.
From north to south, the folds are numbered and plotted in separate stereograms.
Table 3.1 shows data of all measured fold in site one. For example, “W1_U” in Table 3.1 stands for the first fold of west side of road, and it is from Upper Roubidoux sandstone. The table includes the information of fold hinge lines, axial surfaces, fold type, and symmetricity.
Based on the field information a sketch is built to reconstruct the spatial relationship between those folds (Figure 3.2).
Table 3.1 Data table of folds information of site one.
In Figure 3.2, fold orientation is plotted with respect to their location. Two parallel straight north-south thin lines represent road boundaries. Thicker segments represent the fold 17 exposures along road: light orange stands for Upper Roubidoux anticlines; light blue stands for Upper Roubidoux synclines; light red stands for Lower Roubidoux anticlines. On top of those segments, the fold names and their attitude including hinge lines and axial surfaces are plotted.
Figure 3.2 Sketch of folds information in site one.
Periclinal forms have been recognized from sketch. It refers to the alternative patterns of basin-and-dome forms, where each basin or dome has two nearly orthogonal shortening 18 directions to form a four-way-closure structure. In this case, each fold exposed along road cut represent a vertical cut in north-south direction on certain part of one pericline, and the trend of hinge line or strike of axial plane indicates one shortening direction of folding system.
Figure 3.3 shows the stereoplot of hinge line in site one and its related rose diagram. A cluster in NWW trend is noted with some scatters around in this quadrant. At the same time, some other scatters are plotted in NE quadrant.
Figure 3.3 Stereographic plot of site one.
The variations of hinge line directions of these folds are one of the main observations and evidences for periclines. On the west side of road, the stacked Upper and Lower
Roubidoux sandstones W#1, W#2 have very similar hinge lines with 7.6 degree difference trending to southeast: they are 150.4o and 142.8o respectively (Figure 3.4). However on the other side of road, the Lower Roubidoux sandstone E#1 shows the opposite trend in direction of NWW (Figure 3.5), resulting in a basin form in between.
On the southern end of the basin form discussed above, folds W#3 and E#2 (Figure 3.4 and
3.6) define a NE-SW trend in form of anticlines, which indicates a dome form in the road area. The more southwards trend of fold W#3 than fold E#2 may be the result of possible 19 karst collapse in the area of fold W#4 (Figure 3.7). It is likely that this dome form elongates in NE-SW direction indicating shortening in NWW-SEE direction.
Figure 3.4 Fold #1_W (Upper) and fold #2_W (Lower) on west side of site one.
Figure 3.5 Fold #1_E (Lower) on east side of site one. 20
Figure 3.6 Fold #2_E (Upper) on east side of site one.
Figure 3.7 Fold #4_W (Upper) on west side of site one.
A bowl shaped basin represents an outlier that contrasts with the more systematic orientation of the other folds. On west side of road, a syncline, W#4, with distinct appearance stands out among those folds (Figure 3.7). The steep limbs give it a geometry in bowl shape, 21 and it is likely to be the result of enhancement of karst collapse. From sketch figure, it can be noticed that its adjacent neighbors, two adjacent anticlines (W#3 and #5) are influenced by the proximity to this fold compared with fold attitudes on the opposite side of road (Figure
3.8). The trend of this syncline is 294.5o in azimuth and there is no surprise that a syncline is trending to nearly same direction on the other side of road with its hinge located on the reverse extension line of hinge line of the bowl shape syncline. It strongly indicates that they may belong to the same basin form, extending from one side of the road to the other.
Figure 3.8 Fold #5_W (Upper) on west side of site one.
However, even the following fold W#5 and fold E#4 are anticlines, their different attitudes exclude the possibility that they belong to the same dome. In this case, two different dome forms are plotted at their location with respect to the shortening direction.
Alternatively, they could be a tortured dome form, where west side is influenced by karst collapse.
22
Similar with first basin form discussed above, three anticlines W#5, E#4, and E#5
(Figure 3.8 and 3.9) dipped toward one point to define another basin form. In Figure 3.8, the sharp transition from southern limb of anticline W#5 to next anticline indicates a local small fault. The syncline E#6 (Figure 3.10), indicates the presence of another basin form. However the existence of a pair of stacked anticlines E#4 and E#5 (Figure 3.9) separates the two basins. So anticlines W#5, E#4, E#5, and a syncline together form a basin-dome-basin pattern from northwest to southeast in this small area.
However, even though anticline W#6 and another pair of Upper and Lower
Roubidoux anticlines E#7 and E#8 (Figure 3.11 and 3.12) have a general west trend, the differences in size, location, and bedding thickness indicate two different dome forms. The wavelength of folds E#7 and E#8 are much shorter than fold W#6. Fold W#6 has thicker beddings in general.
At the southernmost end of this site, a southeast trending syncline E#9 probably indicates another basin area (Figure 3.13). At this position the fold amplitude decreases a lot, reflected by the dipping angle of fold limbs.
Along the road, which is oblique to the overall shortening direction, alternating basin and dome forms are recognized. This observation strongly supports the reconstructed pericline model. Giving the consideration that the width of road is shorter than the observed fold wavelength exposed in the road cut and some basin areas are restricted to the road area, the general shape of periclines is elongated in NNE-SSW direction and the shortening in
NWW-SEE is larger than in the other direction.
Along the road, which is oblique to the overall shortening direction, alternating basin and dome forms are recognized. This observation strongly supports the reconstructed pericline model. Giving the consideration that the width of road is shorter than the observed fold wavelength exposed in the road cut and some basin areas are restricted to the road area, 23 the general shape of periclines is elongated in NNE-SSW direction and the shortening in
NWW-SEE is larger than in the other direction.
Figure 3.9 Fold #4_E (Upper) and fold #5_E (Lower) on east side of site one.
Figure 3.10 Fold #6_E (Upper) on east side of site one.
Embracing the pericline model, the non-systematic symmetry can be explain as both the differences between local rock properties, and the result of oblique cuts through arbitrary 24 directions in the periclines: if the cut planes are parallel to symmetric or through the symmetric center, the folds exposed on road cuts are symmetric; if not so, asymmetric outcomes will be expected.
Figure 3.11 Fold #6_W (Upper) on west side of site one.
Figure 3.12 Fold #7_E (Lower) and fold #8_E (Upper) on east side of site one. 25
Figure 3.13 Fold #9_E (Upper) on east side of site one.
Another unexpected result is the attitudes between the Upper and Lower Roubidoux
Sands are slightly different. The thin clay inter-bed may allow slip to occur between the upper and lower folds. This suggests that lithostatic load is insufficent to constrain the folds to develop as one system.
3.2.2 Site Two --- Medium Intensity of Deformation. The next group of folds is about 850 meters south of site one and two hundred meters north of the intersection of highway 63 and highway CC on the east side (Figure 3.14).
Folds in this site are less deformed than those in site one as recorded by a decrease in fold amplitudes and increase in fold wavelength. These folds are 4 to 6 meters high above surface. Amplitudes range from 2 to 4 meters accompanied by the increase of wavelength varying from 40 meters to 65 meters.
Similar to site one, both asymmetric and symmetric fold shapes are observed. Two trends in the orientations of hinge lines are clearly recognized: NWW-SEE and NE-SW
(Figure 3.15) Only Upper Sands crop out in this exposure.
26
Figure 3.14 Map view of site two with important features labeled.
On east side of road in this site, approximately sixty meters of vegetation cover stands out in contrast to the continuous exposure of folding on west side (Figure 3.20). This gap is about 65 meters long and present as topographic low. On both ends of this covered interval, rocks dip into the ground at gentle dips. Taking into consideration fold geometry and trends this anomaly may be related to karst collapse where Roubidoux Formation sagged into underlying dissolved Gasconade Dolomite Formation.
Separated by road, eight anticlines have been measured. All of them exposed only the
Upper Roubidoux Sands. Compared with site one, the intensity of deformation declined significantly, reflected by the decrease in fold amplitude and increase in wavelength. In the order north to south, these folds are numbered and plotted in stereographs separately. Table
3.2 shows detailed information. Following the same method, a sketch is built for site two to help reconstruct the fold layout (Figure 3.15). Similar with site one, the variation in the 27 orientation of fold axial surfaces and their hinge lines are mapped out and systematic orientations of the folds with respect to one another is visible (Figure 3.15). The axial surfaces dip from 0.8 to 11.4 degrees.
Table 3.2 Data table of folds information of site two.
Figure 3.15 Sketch of folds information in site two with rose diagram.
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As the starting points, folds W#1 and E#1 show as low amplitudes (Figure 3.16,
3.17). Folds W#2 and E#2 (Figure 3.18, 3.19) together indicate a dome form across the road.
There is a 62.2 degrees difference between the two shortening directions. They have similar bedding thickness, wavelength, amplitude, and location, which strongly link them together as one structure.
Figure 3.16 Fold #1 on west side of site two.
Figure 3.17 Fold #1 on east side of site two. 29
Figure 3.18 Fold #2 on east side of site two.
Figure 3.19 Fold #2 on west side of site two.
The green line on the west side of sketch represents the vegetation covered gap
(Figure 3.20). There is no field data for this area, and it is believed to be caused by local karst effect. However, this area is bounded by anticlines along either side, so alternatively a structural basin form can be inferred here as well. The shortening direction would be similar to the adjacent anticlines E#2 and E#3. At the same location on the west side, and a basin 30 form is inferred by two anticlines W#2 and W#3 (Figure 3.19, 3.21), trending toward one point.
Figure 3.20 Gap area on west side of site two.
The combination of folds W#3 and E#3 (Figure 3.21, 3.22) define a structural dome form in the road area, because their hinge line trends are pointing in opposite directions:
287.9 degrees and 116.5 degrees. Similarly, anticlines W#4 and E#4 (Figure 3.23, 3.24) indicate a structural basin form in the intervening road area by trending towards each other.
And the smaller size and less deformed shape of these two anticlines also indicate that another structure or more were remove when the road was built.
Two more structural dome forms are inferred on both side of the road at the back of anticlines W#4 and E#4, and their shortening direction would be the same as these two anticlines. In the sketch, even the dome one the west side of W#4 looks the extension of dome defined by W#3 and E#3. The existence of anticlines W#3 and W#4 indicates that two domes are separated. 31
Figure 3.21 Fold #3 on west side of site two.
Figure 3.22 Fold #3 on east side of site two.
Although fewer folds are exposed in this site, clear periclinal patterns are identified.
Along the road, an alternating basin-and-dome pattern is present, and the presence of other structural basins and domes can be inferred. 32
Figure 3.23 Fold #4 on west side of site two.
Figure 3.24 Fold #4 on east side of site two.
3.2.3 Site Three --- Minimal Deformation Intensity. One thousand and twenty meters south from site two, another group of folds is exposed in a road cut. This site is located at about four hundred and fifty meters south from the intersection of highway 63 and highway CC on the West side (Figure 3.25). 33
Figure 3.25 Map view of site three with important features labeled.
As expected, lower amplitudes and longer wavelength folds are observed. Some folds are partially exposed and the intervening area between the folds is covered by vegetation – this region is represented by green segments in sketch (Figure 3.26). There is a small segment of Lower Roubidoux sandstone exposed on the east side of road, but the overlying Upper
Roubidoux sandstone has been eroded away (Figure 3.33). Here the Upper Roubidoux sands is exposed along the limbs of the anticline and the Lower Roubidoux sandstone crops out only in the core of the anticline. On the west side of the road, a bowl shape syncline is well exposed among folds in this site (Figure 3.30). This structural basin may fall into the category of having been influenced by local karst collapse.
Separated by the road, site three has five folds on its east side and nine folds on the west. One of them is formed by the Lower Roubidoux sandstone (Figure 3.33). One definite outlier with regards to the other fold forms is an anomalous bowl shaped structural basin 34
(Figure 3.30). In order from north to south, these folds are numbered and plotted in stereographs separately, and information regarding these folds is listed in Table 3.3.
Table 3.3 Data table of folds information of site three.
Figure 3.26 Sketch of folds information in site three with rose diagram.
35
A schematic sketches of these folds is presented to illustrate the spatial distribution of the folds (Figure 3.26). Two general fold orientation trends are recognized in site three:
NWW-SEE and NE-SW reflecting a “basin-and-dome” pattern. At the northern end of this site, folds are exposed on the west side of the road. Two anticlines W#1 and W#2 (Figure
3.27, 3.28) crop out and the intervening area between the folds is covered by vegetation and soil. Because these folds are partially exposed, their wavelengths are likely longer than what is presently seen in the field. The hinge lines of these folds trend in different directions: 314 degrees and 208 degrees in azimuth. These trends are consistent with the two general shortening directions of this folding system. Two dome forms are indicated at the location of these two anticlines and two possible basin forms are inferred at their plunging directions.
Figure 3.27 Fold #1 on west side of site three.
A bowl shape synclinal basin W#4 is similar to a structural basin present in site one
(Figure 3.30). This syncline is considered to have been affected by local karst collapse. The attitudes of the adjacent anticlines W#3 and W#5 are also affected to some degree by this sagging when compared with folds E#1, E#2 and E#3 (Figure 3.29, 3.31, 3.32, 3.33) on east side of the road at this location. 36
Figure 3.28 Fold #2 on west side of site three.
The three folds on the east side trend more E-W and are only partially exposed. The hinge area of fold E#1 was eroded, but its two limbs can be still be seen. One small segment of Lower Roubidoux sandstone is exposed in the core of the anticline. At the locations of each fold, both basin and dome forms appear to be present. Among these folds E#1, W#4 and
E#3 have hinge lines with similar orientations. The existence of a basin form at location of
W#4 can also be inferred by the trend and plunge of the hinge line for fold E#1.
Figure 3.29 Fold #3 on west side of site three. 37
Figure 3.30 Fold #4 on west side of site three.
The Upper Roubidoux sandstone forms an anticline W#6 that has a near parallel axis with fold E#3 but in opposite direction (Figure 3.33, 3.34): they are 254.6 degrees and 077 degrees respectively. Giving the consideration of their position and the fact that the Lower
Roubidoux sandstone anticline had Upper Roubidoux sandstone stacked on it with similar attitude (Upper one was eroded away), there could be one dome form that extended from fold
W#6 to fold E#3 cross the road.
Figure 3.31 Fold #5 on west side of site three. 38
Another possibility is that fold E#3 and fold W#6 represent two different dome forms to keep the size of periclinal patterns symmetrical.
Figure 3.32 Fold #1 on east side of site three.
Figure 3.33 Fold #2 (Upper) and fold #3 (Lower) on east side of site three.
Synclines W#7 and E#4 are at similar position along the road, however, differences in their attitudes and their sizes indicate that are two independent structural basin forms. 39
Figure 3.34 Fold #6 on west side of site three.
South of them, two anticlines W#8 and E#5 (Figure 3.35, 3.36) have parallel but opposite trend directions and very likely, they belong to one dome form, consistent with their similar appearance and position.
Figure 3.35 Fold #8 on west side of site three.
40
If they compose one dome form, the east side may be irregular due to the existence of syncline E#4. Another structural basin form is indicated by syncline W#9 at the southernmost part of this site. Its hinge line is parallel to E#5 and W#8 in direction of 224 degrees.
Figure 3.36 Fold #5 on east side of site three.
3.2.4 Site Four --- End of Folding Zone. There are three more unmapped groups of folds exposed between site three and site four. However, because of the very low dip of the fold limbs these folds were not studied in detail. The shallowing of the dips continues south towards Licking, Missouri, consistent with a decrease in deformation intensity. Some road cuts expose a single anticline over a distance of a couple hundred of meters or more. Finally, sandstones of the Roubidoux Formation croup out as horizontal layers. Site four is located about 10 kilometers from starting point of folding zone, and around 2400 meters north of
Licking City, MO (Figure. 3.37). This site is the first group of rocks that do not exhibit evidence of folding. Limited sandstones beds are exposed 1.5 meter in height above the ground. Here the sandstone are Upper Roubidoux sandstone, with slightly less reddish components. 41
Figure 3.37 Map view of site four with important features labeled.
As stated above, all four chosen folding sites have been examined in detail.
Throughout the entire folding zone, fractures and joints are abundant. These competent sandstone layers failed brittlely as a result of the applied stress which also forced them to fold.
Throughout the entire zone of folding, adjacent folds exhibit differences in orientation and spatial distribution that are consistent with a structural basin-and–dome pattern.
3.3 FIELD WORK SUMMARY
From north to south, the overall trend over the entire folding zone can be described in the following major points: 42
1) Folding is first recognized as small low amplitude long wavelength buckle folds.
Folding quickly reaches maximum intensity of deformation in a relative short distance, then exhibits a gradual decrease in intensity as reflected by a general decrease in fold amplitude and increase in fold wave length in the fold groups. Finally, undeformed, subhorizontal sandstones crop out in road cuts just outside of Licking Missouri. In terms of deformation level, this folding zone has a bias with maximum fold intensity area on north.
2) Both significant asymmetric and symmetric geometries of folds are common in this folding zone. The location of asymmetric folds vs symmetric folds crop out in road cut does not appear to be systematic. The oblique relationship between the strike of fold axial planes and the orientation of the road may affect discerning true fold geometries.
3) There is considerable variation in the orientation of fold axial surfaces and hinge lines but they are not chaotic or random. The reconstructed folding pattern is consistent with these folds being periclines and does not necessitate multiple folding events to create a basin and dome pattern.
4) Locally there are outliers that are inconsistent with the overall pattern created by the folds in general. The orientations of theses outlier folds are likely modified by karst collapse.
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4. DISCUSSIONS
Based on field observation and stereographic data analysis, several hypotheses have been proposed to interpret the origin and geometric patterns of these folds. They can be divided into three major categories: they are local karst collapse, regional tectonics, and the combination of the two.
4.1 KARST COLLAPSE INVOLVED DEFORMATION
Karst caves are extremely pervasive through Missouri State. Around more than 6,300 caves are recorded, and the number keeps growing each year (Spencer, 2011). Karst induced folding is common in Missouri. Examples occur within the Ozark Plateau, mainly in carbonate formations. Some features in study area may be related to karst effects, such as unexpected bowl-shape synclines (Figure. 4.1A) and gap areas.
One hypothesis states that Rubidoux Formation sagged into collapsed caves developed in the underlying Gasconade Dolomite (Figure. 4.1B) after they were deposited.
The Gasconade dolomite was dissolved by naturally slightly acidic precipitation and removed by groundwater, creating void space for overlying Roubidoux Sandstones to sink. Spencer,
2011, has listed a lot of filled sink features around Missouri:
1) Wea Shale syncline at the west end of the ramp from Raytown Road to westbound
US50/I-470.
2) Chouteau Group Limestone anticline in a road cut along the westbound lanes of
US50, 0.5 mile west of Cooper County Route A near Otterville.
3) Jefferson City Dolomite. a) Along the westbound lanes of US50 at Cole County
Route D. b) At the third cut east of the MO 28 South junction, about 0.4 mile west of the
44
Gasconade River. c) Along the northbound lane of US 63 halfway between Maries County
Route A and MO 28 North.
4) Cedar valley Limestone along the westbound lanes of US 50 west of the Moreau
River.
Figure 4.1 Karst collapse involved deformations.
5) Pennsylvanian limestones in a road cut about 2 miles east of Route Y and 2 miles west of Route C, Franklin County.
Another karst related possibility is that those Roubidoux layers are deposited on a wavy, eroded surface of Gasconade Dolomite after karst collapse. Similar geological structures also can be found in Missouri (Spencer, 2011). 45
1) The St. Peter Sandstone along I-44 between mile markers 254.9 and 255.2.
2) Unconformity between the Proterozoic Grassy Mountain Ignimbrite and the
Cambrian Lamotte Sandstone in a road cut along the eastbound lane of MO 72, about 1.6 miles west of US 67.
Folding related to localized karsting is expected to produce a high degree of scatter in the orientation of folds. Figure. 4.1C shows the contour plot of hinge lines which indicates several prominent orientation for the folds but not a random distribution. From rose diagram
(Figure 4.2) and statistical analysis, two trends are recognized. The major one is 330 degrees in azimuth and the minor one is 059 degrees in azimuth. There is a 119 degrees difference between two shortening directions. This distribution of fold orientations is inconsistent with an origin of the majority of the folds within the folding zone being related to karstng, although karsting may have had a subsequent effect on the shape and orientation of some of these folds.
4.2 REGINAL TECTONICS INDUCED BUCKLE FOLDING
Compared to local karst collapse effects, reginal tectonics has higher possibility to be the dominant factor of these folds. More than three hundred pieces of data was collected in study area, containing information of thirty seven folds. From field observation and stereographic analysis, several critical points are noted:
1) The variation of dip angles of fold limbs strongly denies the possibility of cylindrical folding. Instead, groups of periclinal folds have been recognized. Those periclines have the maximum principle shortening in NWW-SEE trend (Figure 4.2), resulting dome- and-basin patterns elongating in perpendicular direction.
2) Highway U.S.63 is nearly N-S in study area, where folds are exposed. So, there is an oblique angle between where folds are exposed and the axis of periclinal folds. Therefore, 46 different orientations of folds can be explained as a result of different cut angles through pericline style folds.
3) With an overall decrease of deformation intensity from the center towards both ends of the folding zone, it is noticed that intensity drops more quickly towards north than south.
Different scenarios have been taken into consideration to explain the field observations and stereographic analysis of those folds.
Figure 4.2 Rose diagram of hinge line trend distributions.
4.2.1 Multi-Event Origin Buckle Folding. Groups of periclines, with dome-and- basin patterns, are commonly interpreted as requiring multiple folding events to produce an 47 interference pattern. Lots of research has been done to interpret periclines as the result of multiple (at least two) directional shortening. They are mainly in orthogonal directions
(Marinin & Sim, 2015), and some are in non-orthogonal directions (Treagus & Treagus,
1981). Therefore, a multi-event origin buckle folding model has been proposed. Giving the fact of the layout of periclinal folds, this area may be the results of at least two major deformations with shortenings in orthogonal or sub orthogonal directions. Pericline folds elongate in the direction perpendicular to maximum shortening direction. In this case, different strain rates in minor shortening direction are required with response to the asymmetric distribution of deformation intensity. However, this traditional model is flawed by requiring at least two orthogonal folding events, as there is no evidence to support the sources of forces in the two orthogonal directions, based upon geologic map for this area.
4.2.2 Single-Event Origin Buckle Folding. In this thesis, a new simplified hypothesis is proposed to explain those folds. It is single event origin buckle folding as the dominant factor. Instead of requiring multiple stresses, basin-and-dome periclines can be achieved by unidirectional stress, referred as single-event origin (Ramsay and Huber, 1983;
Ramsay and Huber, 1987; Ghosh et al., 1995; Schmalholz, 2008). Bordering the study area are two nearly parallel NWW trending (Figure 4.3). If these two faults exhibited left lateral strike-slip motion they would have a created a transpressional zone that provided a compressive stress regime leading to the formation of these folds. This unidirectional shortening will lead to a minor shortening in its orthogonal direction, while field data shows a
119 degrees difference instead of ideal 90 degrees. The reasons could be that 1) the heterogeneous rock properties and local karst effects result; 2) different strikes of the two faults. They have slight different strikes that north one is 294 degrees and south one is 311 degrees in azimuth; and 3) different strain rates of two faults. Although, tectonic theory explains overall folding event, the importance of local karst effects cannot be negligible. 48
Figure 4.3 Geologic map of study area. (Modified Middendorf, 2003)
4.3 THE COMBINATION OF HYPOTHESES
Local karst effects may be the provider of initial perturbations, which strongly affect the development of folds. Also initial perturbation is necessary prerequisite to form a fold.
On the other hand, karst effects may also change local structures later on. In this thesis, the most possible hypothesis is that the combination of local karst effects and reginal tectonics. It can be concluded as a regional tectonic event induced localized buckle folding whereby local karst effects may have contributed in forming the initial perturbation.
4.4 2D FINITE ELEMENT ANALYSIS
A simple 2D finite element model has been run to test the possibility of transpression model formed in a restraining jog. The main purpose of this numerical simulation is to examine whether the maximum principle stress direction provided by two left-lateral strike slip faults matches with field data (Figure 4.4). 49
Figure 4.4 2D finite element simulation by reproducing the study area.
By reproducing the localities of two faults at original scale, a displacement of 5 mm per year is assigned to both faults. Detailed model setup is stated in appendix. Figure 4.4 shows the results of model, displaying distributions of maximum principal stress. Two oblique black solid lines represent the locations of faults and vertical straight line represents the study area as road cut. Red arrows indicate maximum principal stress produced in transpression zone in a NW trend across road cut. Overall, this NW trend is consistent with field data as N60W trend. This 2D finite element simulation strengthens the possibility of transpression zone hypothesis.
50
4.5 UNSOLVED QUESTIONS AND FUTURE WORK
There are several questions that remain unsolved, which will be, and can be main directions of future work. Paleomagnetism fold test, seismic investigation, and fracture analysis are considered as possibilities for future research.
Constraining the timing of deformation, when these folds and faults formed are critical to understanding the significance of the overall geologic events. When did those faults create transpression zone? Did the fault movements match general midcontinent trend at that time? If so, what geologic event was responsible for them? In order to answer these questions, a paleomagnetism fold test will be conducted. It will not be concluded in this thesis, but it is the one of the most important step in the future. So far field core samples have been collected, waiting for laboratory examination.
Seismic investigation may also be helpful to confirm the subsurface distribution of folds or the presence of karst related features. How far do those folds extend? Are Gasconade rocks underlying Roubidoux Formation deformed? Do they exhibit a similar pattern? Or are some Gasconade dolomites dissolved? Due to the limited exposure of folds, the Gasconade
Formation does not crop out in this area in the field. In this case, seismic interpretations may be able to provide answers.
Fracture analysis on those folds is potentially a new aspect of field work. How does the field work match numerical modeling on periclines? How do pre-existing fractures and joints affect the distributions of fracturing during folding?
51
5. CONCLUSIONS
Based on field work and data analysis, a new hypothesis has been proposed to interpret Roubidoux folds based upon their geometry and orientation as documented by my field work and stereographic analysis. The following statements can be concluded:
1) A structural dome-and-basin pattern has been identified. This pattern is consistent with the Roubidoux sandstones folds having a periclinal form. They elongate in N59E to
S59W with maximum shortening occurs in N60W to S60E. The maximum principal stress direction is inconsistent with major trend of midcontinent fold and fault zone. This folding zone is the result of localized tectonic deformation.
2) A single folding event hypothesis is proposed to interpret the origin of these folds.
It states that two possible left lateral strike slip faults bound the folds may be the dominant tectonic force by generating a transpression zone, while, local karst effects provide initial perturbations and subsequently may modify the form and orientation of some of the folds.
3) A 2D finite element model supports the single folding event hypothesis by resulting the consistent maximum principal stress with field data.
4) This thesis provides one possibility to explain the origin of Roubidoux folds in the midcontinent from a new approach of recognizing these folds as periclines. Additionally, the results of this thesis can be applied as a field example for the study of the development of basin-and-dome patterns as the result of a single event of buckle folding to form pericline folds.
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APPENDIX
2D FINITE ELEMENT MODEL SETUP
A.1 GENERATE GEOMETRY
In order to proof the possibility of transpression zone model, a 2D finite element analysis has been conducted. A detailed setup of model geometry and parameters assignment will be explained. The software HyperMesh of Altair HyperWorkTM 12.0 is selected to generate model geometry.
The dimension of this model is translated from a geologic map on ration 1:1 (Figure
A.1). In order to clearly assign displacement on strike slip faults, six components have been classified. Node sets have been created to set up boundary conditions and apply movements on the faults.
Figure A.1 Geometry sketch and mesh in HyperMeshTM 53
A.2 CODING AND RESULT
During the coding, rock property and parameters are assigned to geometry.
Homogeneous properties are given, where Young’s module is 10 to the power of 11 Pa; passion’s ratio is 0.25. And this model is independent of temperature. Gravity is not considered because it’s a two-dimension model. Boundaries of this model are assigned as rollers. Lateral displacements of 5 mm per year along two strike slip faults are set.
The result can be viewed by AbaqusViewer of AbaqusTM. Due to the different sign convention of software, in order to check the distribution of maximum principal stress, instead, minimum principal stress has been selected.
In Figure A.2, Red arrows represent the orientations and distributions of maximum principal stress. One thing is noticed that the length of each arrow has not physical meanings in this display. Red arrows do not reflect the magnitudes of stresses.
Figure A.2 2D finite element model result of transpression zone. 54
The purpose of this numerical modelling is to examine the possible maximum principal stress distributions produced by two left lateral strike slip faults, and then, to provide evidence to poof the possibility of transpression zone hypothesis. 55
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VITA
Chao Liu was born in Karamay, Xinjiang, China. In September 2009, he received his offer from China University of Petroleum (Huadong), and finished his first three grades in
Geology Major. In August 2012, He went to Missouri University of Science and Technology,
Rolla, Missouri, USA as a transfer student by “2+2 transfer program”. In May 2014, he received his B.S. degrees in Geology and Geophysics from both China University of
Petroleum (Huadong) and Missouri University of Science and Technology. In 2014, August, he was enrolled as a master student in Missouri University of Science and Technology. In
2016, August he was enrolled as a doctoral student in in Missouri University of Science and
Technology.
Starting from summer in 2012, he has been a teaching assistant for geologic field trips, structural geology and remote sensing technology.
His studies were focused on polygonal faults in Egypt and folding zone in Ozark
Plateau. In 2015, he received 1,500 dollars founding from Geological Society of America
(GSA) for study on folds in Ozark Plateau. In December 2016, he received his M.S. degree in
Geology and Geophysics from Missouri University of Science and Technology.