HARDING, MATTHEW RYAN, M.S. MAY 2017 GEOLOGY
A GEOPHYSICAL STUDY OF UPPER SILURIAN SALINA GROUP IN NORTHEASTERN
PENNSYLVANIA (135 pp)
Thesis Advisor: Chris Rowan
The Upper Silurian Salina Group in Pennsylvania’s Appalachian basin consists of 2,000+ feet of salt, which have been a significant influence on the tectonic & structural development of the Appalachian Mountains during the late Paleozoic. Understanding how halokinesis and décollement thrusting within the Salina Group has contributed to the present‐day structure of the
Appalachian Basin is of great importance due the organic‐rich shale plays (Marcellus and Utica) currently being explored and developed within this region. Given that most of the seismic data collected from this region was before the advent of high-resolution 3D seismic surveys, a more detailed investigation of structures associated with the Salina Group was warranted.
Based on preliminary examination of seismic data from North‐Central Pennsylvania, I propose that the reactivation of basement faults during the Allegheny orogeny influenced halokinesis in the overlying Salina, and therefore acted as a control on the development of the salt cored anticlines and associated faulting in the overlying units. This was tested by detailed mapping of structures in the basement and in the Paleozoic sequence above and below the Salina, noting any spatial associations between the structures. Isochronopach maps and profiles were created and analyzed, to constrain the deformation of the Salina Group. Several faults were found within three primary stratigraphic sections of the seismic volume. Those are the above Salina Group (AB1 and AB2), Salina Group (S1, S2, and S3), and basement (B1, B2, B3, B4, B5, and B6). Along with T1, sub-divided into three stratigraphic sections; faulting ~50ms TWTT above top of basement; monoclinal folding and faulting within the Trenton-Black River Group; and faulting between ~1.5-1.8s TWTT. Further a pop-down structure is formed by salt evacuation between S1 and S2 is found from Inline 300 to Inline 90, being most developed at Inline 95 and 90.
Structures found within the 3D volume suggests indirect basement control on Salina development. Along with the development of the pop-down structure of the S1/S2 complex further offer evidence of basement influence. Results appear to strengthen the model put forward by Mount (2014) however result maybe only applicable this specific region. The main conclusion of the study are as follows; there is a spatial correlation between the location and development of faulting within and above the Salina Group and a structural high formed by
Neoproterozoic basement faults. With the intersection of B1 and B6 associated with the SW ends of S1 and S2, where above the S1 and S2 propagate highest upwards, and where AB2 initiates above where S1 offsets Tully Limestone above this B1/B6 intersection. This suggests that the location of Salina Group faulting is the resultant influence of Grenville basement topography caused by Neoproterozoic rifting. There is also evidence of diffuse deformation below Salina décollement in the same location T1, whose features are most common below the S1/S2 pop- down and above the B1/B6 intersection. Where the changes in overburden thickness due to basement structures acted as foci for later Salina deformation during the Alleghanian orogeny.
A GEOPHYSICAL STUDY OF UPPER SILURIAN SALINA GROUP IN NORTHEASTERN PENNSYLVANIA.
A thesis submitted To Kent State University in partial Fulfillment of the requirements for the Degree of Master of Science
by
Matthew Ryan Harding
May 2017
© Copyright
All rights reserved
Except for previously published materials
Thesis written by
Matthew Ryan Harding
B.S., Indiana University of Pennsylvania, 2012
M.S., Kent State University, 2017
Approved by
Dr. Chris Rowan, Assistant Professor (NTT), Ph.D., Geology, Masters Advisor
Dr. Daniel Holm, Chair, Ph.D., Department of Geology
Dr. James Blank, Dean, Ph.D. College of Arts and Sciences
TABLE OF CONTENTS
TABLE OF CONTENTS v
LIST OF FIGURES vii
PREFACE xi
ACKNOWLEDGEMENTS xii
1. INTRODUCTION 1
1.1WHAT IS ROCK SALT? 1
1.2 HALOKINESIS 1
1.3 THE APPALACHIAN MOUNTAIN CHAIN: A BRIEF OVERVIEW 3
1.4 STRATIGRAPHY AND TECTONIC HISTORY OF PENNSYLVANIA 3
1.5 STUDY AREA 5
1.6 DETAILED STRATIGRAPHY OF THE SALINA GROUP 6
1.7 ROLE OF SALINA GROUP IN APPALACHIAN BASIN STRUCTURAL
DEVELOPMENT IN PENNSYLVANIA 14
1.8 AIMS 17
2. METHODOLOGY 20
2.1 SEISMIC VOLUME 20
2.2 SALINA THICKNESS MAPPING 23
3. SEISMIC RESULTS 29
3.1 REGIONAL SEISMIC VIEW 31
3.2 STRUCTURES WITHIN THE SALINA GROUP 52
3.3 SALINA THICKNESS 76
3.4 STRUCTURES ABOVE THE SALINA GROUP 83
v
3.5 BASEMENT STRUCTURES 90
3.6 DEFORMATION BETWEEN BASEMENT AND SALINA 100
4. DISCUSSION & CONCLUSIONS 103
4.1 SUMMARY OF FAULTS 103
4.2 SPATIAL CORRELATION BETWEEN BASEMENT FAULTING AND
ALLEGHANY DEFORMATION. 104
4.3 MECHANISM LINKING BASEMENT STRUCTURES TO DEFORMATION
ABOVE THE SALINA GROUP. 107
4.4 OTHER VIEWS ON SALINA DEVELOPMENT 109
4.5 COMPARISON WITH OBSERVED AND MODELLED BASEMENT SALT
INTERACTIONS 110
4.6 PROPOSED TECTONIC HISTORY OF STUDY AREA 110
4.7 SUMMARY OF RELATIONSHIP OF STRUCTURES FOUND 113
4.8 CONCLUSION 113
6. WORKS CITED 115
7 APPENDIX A: SALINA CONTOUR SOURCE CODE 120
vi
LIST OF FIGURES
FIGURE 1: PHYSIOGRAPHIC PROVINCES OF PENNSYLVANIA 7
FIGURE 2: SCHEMATIC CROSS SECTION OF PENNSYLVANIA 8
FIGURE 3: SCHEMATIC CROSS-SECTION OF PENNSYLVANIA’S APPALACHIAN
PLATEAU. 9
FIGURE 4: GENERALIZED STRATGRAPHIC COLUMN, NE PENNSYLVANIA 10
FIGURE 5: LOCATION OF SALINA SALT BASINS 11
FIGURE 6: ISOPACH MAP OF THE SALINA GROUP 12
FIGURE 7: GENERALIZED STRATIGRAPHIC COLUMN OF UPPER SILURIAN ROCKS
OF NORTH CENTERAL PENNSYLVANIA 13
FIGURE 8: HYPOTHESIZED STRCUTURAL LINKAGE OF SALINA GROUP AND
BASEMENT 19
FIGURE 9: SCHEMATIC SHOWING THE FULL EXTENT OF 3D SEISMIC DATA 24
FIGURE 10: SYNTHETIC WELL TIE PROCESS 25
FIGURE 11: THE TYPICAL SEISMIC REFLECTORS OF THE PICKED HORIZONS 26
FIGURE 12: A SEISMIC CROSS-SECTION 27
FIGURE 13: EXAMPLES OF ERROR OF AUTOPICK HORZIONS 28
FIGURE 14: LOCATION OF SELECT INLINES AND CROSSLINES 30
FIGURE 15A: INLINE 300 [UN-INTERPRETED] 32
FIGURE 15B: INLINE 300[INTERPRETED] 33
FIGURE 16A: INLINE 220 [UN-INTERPRETED] 34
FIGURE 16B: INLINE 220[INTERPRETED] 35
FIGURE 17A: INLINE 180 [UN-INTERPRETED] 36
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FIGURE 17B: INLINE 180 [INTERPRETED] 37
FIGURE 18A: INLINE 115 [UN-INTERPRETED] 38
FIGURE 18B: INLINE 115 [INTERPRETED] 39
FIGURE 19A: INLINE 95 [UN-INTERPRETED] 40
FIGURE 19B: INLINE 95 [INTERPRETED] 41
FIGURE 20A: INLINE 90 [UN-INTERPRETED] 42
FIGURE 20B: INLINE 90 [INTERPRETED] 43
FIGURE 21A: INLINE 60 [UN-INTERPRETED] 44
FIGURE 21B: INLINE 60 [INTERPRETED] 45
FIGURE 22A: CROSSLINE 150 [UN-INTERPRETED] 46
FIGURE 22B: CROSSLINE 150 [INTERPRETED] 47
FIGURE 23A: CROSSLINE 180 [UN-INTERPRETED] 48
FIGURE 23B: CROSSLINE 180 [INTERPRETED] 49
FIGURE 24A: CROSSLINE 250 [UN-INTERPRETED] 50
FIGURE 24B: CROSSLINE 250 [INTERPRETED] 51
FIGURE 25A: INLINE 300 ZOOMED IN [UN-INTERPRETED] 54
FIGURE 25B: INLINE 300 ZOOMED IN [INTERPRETED] 55
FIGURE 26A: INLINE 220 ZOOMED IN [UN-INTERPRETED] 56
FIGURE 26B: INLINE 220 ZOOMED IN [INTERPRETED] 57
FIGURE 27A: INLINE 180 ZOOMED IN [UN-INTERPRETED] 58
FIGURE 27B: INLINE 180 ZOOMED IN [INTERPRETED] 59
FIGURE 28A: INLINE 115 ZOOMED IN [UN-INTERPRETED] 60
FIGURE 28B: INLINE 115 ZOOMED-IN [INTERPRETED] 61
viii
FIGURE 29A: INLINE 95 ZOOMED-IN [UN-INTERPRETED] 62
FIGURE 29B: INLINE 95 ZOOMED-IN [NTERPRETED] 63
FIGURE 30A: INLINE 95 ZOOMED-IN (VERTICAL) [UN-INTERPRETED] 64
FIGURE 30B: INLINE 95 ZOOMED-IN (VERTICAL) [INTERPRETED] 65
FIGURE 31A: INLINE 90 ZOOMED-IN [UN-INTERPRETED] 66
FIGURE 31B: INLINE 90 ZOOMED-IN [INTERPRETED] 67
FIGURE 32A: INLINE 90 ZOOMED-IN (VERTICAL) [UN-INTERPRETED] 68
FIGURE 32B: INLINE 90 ZOOMED-IN (VERTICAL) [INTERPRETED] 69
FIGURE 33A: INLINE 60 ZOOMED-IN [UN-INTERPRETED] 70
FIGURE 33B: INLINE 60 ZOOMED-IN [INTERPRETED] 71
FIGURE 34A: INLINE 60 ZOOMED-IN (VERTICAL) [UN-INTERPRETED] 72
FIGURE 34B: INLINE 60 ZOOMED-IN (VERTICAL) [INTERPRETED] 73
FIGURE 35: Z-PLANE 1100MS TWTT 74
FIGURE 36: Z-PLANE SCHEMATIC OF S1, S2, AND S3 AT 1100MS TWTT 75
FIGURE 37: SALINA THICKNESS TO BOTTOM ONE 78
FIGURE 38: THICKNESS PROFILE OF VARIOUS INLINES FOR THE SALINA
GROUP 79
FIGURE 39: THICKNESS PROFILE OF VARIOUS CROSSLINES FOR THE SALINA
GROUP 80
FIGURE 40A: SALINA POP-DOWN AT INLINES 300, 220, AND 180 81
FIGURE 40B: SALINA POP-DOWN AT INLINES 115, 95, AND 90 82
FIGURE 41A: INLINE 220 AB1 ZOOMED IN [UN-INTERPRETED] 84
FIGURE 41B: INLINE 220 AB1 ZOOMED IN [INTERPRETED] 85
ix
FIGURE 42A: INLINE 180 AB1 ZOOMED IN [UN-INTERPRETED] 86
FIGURE 42B: INLINE 180 AB1 ZOOMED IN [INTERPRETED] 87
FIGURE 43:Z-PLANE VIEW AT 625MS TWTTOF AB1 AND AB2 88
FIGURE 44: SCHEMATIC OF AB1 AND AB2 AT ~600MS TWTT 89
FIGURE 45A: CROSSLINE 180 ZOOMED IN [UN-INTERPRETED] 92
FIGURE 45B: CROSSLINE 180 ZOOMED IN [INTERPRETED] 93
FIGURE 46A: CROSSLINE 150 ZOOMED IN [UN-INTERPRETED] 94
FIGURE 46B: CROSSLINE 150 ZOOMED IN [INTERPRETED] 95
FIGURE 47A: INLINE 180 ZOOMED-IN [UN-INTERPRETED] 96
FIGURE 47B: INLINE 180 ZOOMED-IN [INTERPRETED] 97
FIGURE 48: Z-PLANE AT 2502MS TWTT 98
FIGURE 49: SCHEMATIC OF B1-B6 AND S1/S2 99
FIGURE 50: SCHEMATIC OF T1 AND T2 WITH ALL OTHER FAULTS 102
FIGURE 51: BASEMENT RIFT COMPLEX AT 2502 TWTT 105
FIGURE 52: SCHEMATIC OF PROPOSED TECTONIC HISTORY OF NORTH CENTRAL
PENNSYLVANIA. 115
x
PREFACE
To those that read this thesis and those that flip through to see the pretty pictures, I recommend to look at those first, please be kind to remember that this project came into being by two ancient but recurring questions. The first is a bane of professors, teaching assistants and parents everywhere, the unassailable and incontrovertible question of “why?” The second as equally profound or profane (depending on the ever-changing context) that has ever driven the fields of science ever upwards into that always ephemeral boundary of the realm of known and unknown knowledge, which is so inglorious known in the common parlance of “Um, that looks funny”. So it was during a summer internship looking at the end of an 8 hour day of seismic interpretation, I came across a section of seismic that seemed off and utter the latter first and the first last. And after many strained eyes and acetaminophen relived headaches I am able to present to you the complete thesis. So dear reader, look at the pictures and ask why and how, as you continue on the ever changing journey I leave you with two quotes.
“Ad astra per aspera”
-Unknown
“If you're going through hell, keep going.”
- Winston Churchill
xi
ACKNOWLEDGEMENTS
I first would like to thank Talisman Energy for allowing me to use their seismic volume for my thesis, and for the internship that ultimately led to this thesis. I would like to thank the following groups for their financial support of my thesis; the Department of Geology at Kent State,
Sigma Gamma Epsilon, the Graduate Student Senate. I wish to thank my thesis committee members Dr. Hacker and Dr. Holm for their invaluable advice and support. I would also like to thank Dr. Chris Rowan, my advisor, for the countless hours of advice, support and troubleshooting.
Further, I would like to thank Darren Reilly for helping me rebuild the analogue model over the course of two sleepless nights. I would like to thank my parents who both nurtured my love of science and geology, and supported me during whatever I endeavored.
Finally, I wish to dedicate this work to my grandfather, who sadly passed away before I finished my master’s degree.
xii
1. Introduction
1.1 What is Rock Salt?
Evaporite deposits colloquially referred to as salt (and referred to as such hereafter) are typically made up of halite, gypsum and anhydrite (Hudec and Jackson, 2007). There are multiple properties of salt that make it geologically unique. It is mechanically weak, and because it is relatively incompressible it has a lower density than other rock types, particularly after compaction (Hudec and Jackson, 2007; Warren, 2006). Salt is found throughout the world in many restricted marine basins that allow evaporative concentration and chemical precipitation of dissolved minerals (Hudec and Jackson, 2007; Hudec et al., 2011). The Salina Group that is the focus of this study was deposited into a similar restricted marine environment (Schultz, 1999).
1.2 Halokinesis
Halokinesis or salt tectonics is a consequence of salt’s low strength, which causes it to deform under low tectonic stress, and its incompressibility, which means it is much less dense at depth than surrounding rock units. These properties allow salt layers to migrate both laterally and vertically through denser overlying sediments. Where evaporate units are present, halokinesis can be a very strong influence on the regional deformation of a tectonic system (i.e. orogenies or rifting). Salt is normally the most easily deformed rock type within a given system (Hudec and
Jackson, 2007) and often acts as a lubricating layer (Warren, 2006). Compressional tectonics in regions with salt bearing layers, which are often caused by overburden thickening (i.e. orogenic events), produces thin-skinned deformation regimes, i.e. structural decoupling via the salt layers
(Warren, 2006).
1 In addition to décollement thrusting: salt pillows, salt rollers and salt-cored anticlines result from lateral and vertical migration of salt are common (Hudec and Jackson, 2007). The commonly create a deformed areas that have more folding structures than faulting. Salt décollements are inter-salt slippage features, which allow for the transmission of tectonic energy along weak planes within the salt unit (Hudec and Jackson, 2007). Salt pillows are areas of thickened salt with a surrounding zone of thinned salt, the scale of which can vary from several hundreds of meters in both length and width. Salt rollers are more elongated and have a cross- sectional shape similar to an asymmetric tidal ripple, with thinned salt or even no salt occurring between individual rollers. Larger salt anticlines, where substantial thickening is typically associated with an internal thrust fault, can be on the order of tens to hundreds of kilometers in length (Hudec and Jackson, 2007; Hudec et al., 2011; Warren, 2006).
It is known that basement faulting can influence salt tectonics (Hudec and Jackson, 2007;
Koyi and Petersen, 1993; Rubinat et al., 2013). Some of the best examples of this phenomenon are observed in the extensional regime of the Danish North Sea, where Koyi and Petersen (1993) have shown that many of the salt diapirs in the region are above basement faults. Modelling by
Koyi and Petersen (1993) suggests that faulting weakens the overburden, allowing the diapirs to more easily intrude and form over the basement faults. However in the Danish North Sea the salt tectonics have shown that faulting and diapirism appear to be coevolutional, and that active extension rifts, as they build, weaken the overburden which assists active salt flow (Koyi and
Petersen, 1993). Another example is the Bicorb-Quesa Diapir in southeastern Spain, where the diapir experienced first compressional forces and then an extensional regime, above pre-existing extensional basement faults, and underwent reactivational growth. They state that the pre- existing basement fault by overburden removal, provided a favorable location for diapir
2 formation (Rubinat et al., 2013). Rubinat et al, (2013) show that the diapirs can be developed if the basement fault's dip direction and the direction of propagation are in the right geometric relationship. They found that the right geometric relationship for diapirs to develop in the
Bicorb-Quesa Diapir was when the salt extended over the basement fault with a salt detachment below the zone of basement influenced overburden removal (Rubinat et al., 2013), that diapir growth and flow driven by differential overburden pressures appears to be common in extensional salt tectonic regimes.
1.3 The Appalachian Mountain Chain: A brief overview
The Paleozoic Appalachian Mountains can be traced from Newfoundland, Canada to
Georgia in the United States along the eastern edge of North America (Clark, 2001; Rast, 1989), and represents the terminal collision, known as the Alleghanian orogeny, between
Gondwanaland and Laurentia (Hibbard and Karabinos, 2013). The Appalachian Mountains themselves can be divided into northern and southern halves, and with several embayments and promontories (zones of basement rocks that either recede or proceed from the average location of basement), with the division between the two halves being the New York promontory (Hibbard and Karabinos, 2013). The southern Appalachian Mountains can be divided structurally into the western Appalachian Plateau (basin), the Valley and Ridge Province, Blue Ridges Province and the Piedmont (Figures 1, 2 and 3). Within the Appalachian plateau, an extensive Silurian salt unit known as the Salina Group is present within the states of New York, Pennsylvania, Ohio, and West Virginia (Figures 4 and 5) (Schultz, 1999).
1.4 Stratigraphy and Tectonic History of Pennsylvania
The geology of Pennsylvania records several major tectonic events (Figures 1 and 2).
Crystalline basement of Grenville age (1.0-1.2 Ga) (Kulander and Ryder, 2005), is overlain by a
3 sequence of Paleozoic sandstones, limestones, and as well as regionally extensive, laterally persistent shale formations (Figure 4). With the rifting of the Iapetus Ocean basin at 600-550 Ma, a carbonate bank developed on the passive margin during the Cambrian and Ordovician (Schultz,
1999). Following the initiation of subduction on the margin during the Ordovician, a foreland basin formed inland of a volcanic arc. This was a restricted marine basin that had only limited connection to the Iapetus Ocean, allowing vast quantities of halite and anhydrites to chemically precipitate, forming the Salina Group. In the middle Devonian (~375-325 Ma), clastic sedimentary input associated with the Acadian orogeny, began to dominate deposition. The sequence was then deformed by the Pennsylvanian-Permian (~320-275 Ma) Alleghany orogeny that marked the collision of Gondwana and complete closure of the Iapetus Ocean (Schultz,
1999). Structures developed during the Alleghanian orogeny are regionally extensive and include a number of large regional salt-cored anticlines in the relatively undeformed Appalachian basin of Western Pennsylvania (Schultz, 1999).
The Appalachians in Pennsylvania can be divided into a number of domains with characteristic geology and structural style (Figures 1, 2, and 3). Moving from east to west, the
Piedmont domain is the metamorphic core of the ancient Appalachian Mountains, overlain with
Jurassic and Triassic age deposits associated with the rifting of the proto-Atlantic (Schultz,
1999). The Appalachian Mountains of Pennsylvania are formed by the Valley and Ridge
Province, a curved fold and thrust belt where the entire Paleozoic cover sequence has been intensely faulted and folded into short-wavelength anticlines and synclines. Within the Valley and Ridge Province, numerous plunging and non-plunging anticlines and synclines trend northeast along the Pennsylvania – Maryland border, shifting to an east-northeast trend just west of the Pennsylvania – New Jersey border. Blind thrust faults are typically are found within the
4 cores of anticlines in the Valley and Ridge Province propagating upwards from the detachment faults that typically originate within Cambrian units. The general shortening direction recorded in the fold and thrust belt of the Valley and Ridge Province is southeast to northwest (Schultz,
1999; Wise, 2004). The transition from the Valley and Ridge Province to the Appalachian
Plateau, across the Appalachian Structural Deformation Front, (Figures 1 and 2), is controlled in part by the Salina Group’s deposition, where within the Valley and Ridge Province Silurian aged rocks are primarily clastic, i.e. shales, versus on the Appalachian Plateau, where the Salina
Group is present (Frey, 1973; Gwinn, 1964; Schultz, 1999).
On the Appalachian Plateau, the amount of deformation is quite low and is concentrated within thick Salina Group evaporites, which act as a regional décollement. The dominant structures are broad, long-wavelength folds formed by migration of the Salina Group salt into the core of salt anticlines(Figure 3) (Kulander and Ryder, 2005; Schultz, 1999). These anticlines are typically asymmetric with a steeper southeast facing fold limb, and a gentler northwest fold limb
(Schultz, 1999). Faults within the plateau have varying degrees of steepness; faults within the basement are typically high angle, within the Salina Group and above, faults range from high angle to shallow low angle faults (Schultz, 1999). Faults and lineaments related to
Neoproterozoic rifting, are detectable in seismic surveys of the deep basement subsurface
(Kulander and Ryder, 2005; Schultz, 1999). One such structure within the basement is the southwest – northeast trending Rome trough (Figure 3) (Schultz, 1999). Kulander and Ryder
(2005), show that the Rome trough’s boundary faults have been reactivated as thrusts likely during the Alleghanian orogeny (Kulander and Ryder, 2005). This thesis studies the possible control that these Precambrian basement structures have had on the structural development of
Alleghanian thin-skinned thrusting in the Appalachian Plateau.
5 1.5 Study Area
The study area covers an area located on the Appalachian Plateau in northeastern and north-central Pennsylvania (Figures 5, 6). In this area, the subsurface thickness of the Salina
Group is reported to vary from ~2,000 feet (Fergusson and Prather, 1968) to greater than 2,500 feet (Rickard, 1969) (Figure 7).
1.6 Detailed Stratigraphy of the Salina Group
The Salina Group is an Upper Silurian package of carbonates and evaporitic rocks found within the Appalachian and Michigan basins (Figure 5) (Fergusson and Prather, 1968). The
Salina Group is bracketed by carbonate units, overlying the Lockport Dolomite or McKenzie
Formation (Figure 4), depending on location, and overlain by the Bass Island Dolomite or the
Keyser Formation. It is divided into several sub-units, classified by the letters A through H, which represent different salt and shale/carbonate lithologies (Figure 7) (Fergusson and Prather,
1968; Schultz, 1999). The H sub-unit of the Salina Group is not within the study area, and typically found only in Ohio (Oinonen, 1965).
6 7
Figure 1: The different physiographic provinces of Pennsylvania. The red line represents the location of Figures 2 and 3. After (Sevon, 2000)
8
Figure 2: A schematic cross-section of Pennsylvania across the three principal physiographic provinces. After (Evans, 1989; Kulander
and Ryder, 2005; Mount, 2014; Schultz, 1999).
Salina Group Salt-cored Anticline 9 Basement Faulting
Figure 3: A schematic cross-section of Pennsylvania’s Appalachian Plateau. After (Bureau of Topographic and Geologic Survey,
2007; Kulander and Ryder, 2005; Mount, 2014; Schultz, 1999) Tully Limestone
Marcellus Shale
Trenton/ Black River
Figure 4: Generalized stratigraphic column for north-central Pennsylvania, showing the Salina
Group and surrounding rock units correlated to major regional tectonic events and structures.
After Carter, (2007).
10
Figure 5: Approximate distribution of the Salina Group within the Michigan and Appalachian
Basins. After Fergusson and Prather, (1968).
11
Figure 6: Isopach map for the Salina Group in the Appalachian basin, showing the spatial relationship with the Allegheny deformation front. The approximate location of the study area is highlighted. Modeled after Rickard, (1969).
12
Figure 7: Generalized stratigraphic column for the Silurian rocks found within Pennsylvania, showing subunits A-G of the Salina Group. With the approximate amount of shale (brown), dolomite/limestone (orange), and salt (grey) in subunits A-G after Carter, (2007).
13 Unit A is a sandy pale green/light gray shale with some interbeds of anhydrite and gypsum (Fergusson and Prather, 1968); however, in the western part of the basin (near the
Pennsylvania/Ohio border) the unit appears as a mixture of dolomitic shales and brown dolomites and is known as the Greenfield Formation (Oinonen, 1965). Unit B consists of interbedded anhydrite, shales and dolomites, with two major salt horizons. Unit C is made up of green-gray shales and argillaceous dolomite (Fergusson and Prather, 1968). In the western part of the basin, a thin layer of orange shaley salt is locally present within Unit C (Oinonen, 1965).
Unit D is primarily composed of salt with minor interbedded shale beds. Unit E is primarily composed of dolomite (Fergusson and Prather, 1968), with some thinner layers of interbedded shales and thin ribbons of salt (Oinonen, 1965). Unit F is the thickest salt bed in the Salina
Group and is divided into six distinct salt-bearing subunits (F1 to F6). These subunits are not always laterally continuous across the basin, but subunits F1, F2 and F3 are found throughout the basin and are on average 150 feet thick. Unit G is primarily made up calcareous dolomite, with local thin interbeds of anhydrite (Fergusson and Prather, 1968).
1.7 Role of Salina Group in Appalachian Basin Structural Development in Pennsylvania.
The Pennsylvania Geological Survey conducted the first major study of the stratigraphy and physical properties of the Salina Group within Pennsylvania (Fergusson and Prather, 1968).
L.V. Rickard produced several large-scale maps of the Salina Group’s unit thickness as well as a cross section across the Appalachian depositional basin (Rickard, 1969). Fergusson and Prather
(1968) concluded that the shallow subsurface faults developed throughout the Appalachian
Plateau during the Alleghany orogeny likely did not continue below the basal unit (Unit A) of the
Salina Group. Ge et al. (1997) demonstrated that progradation into a restricted sedimentary basin would create large diapirs and salt sheets. Although clastics deposited on top of the Salina in the
14 Appalachian Basin might be expected to produce similar structures, previous seismic work done by Kulander and Ryder (2005) show that the Salina Group has instead formed large-scale salt- cored anticlines, suggesting other controls on salt flow.
Previous interpretations of seismic data from the Appalachian Plateau (Frey, 1973) have argued the Salina Group acts as a regional décollement, and that folding and faulting above the
Salina Group have no structural relationship with underlying structures. Reverse ramp faults above the décollement are typically associated with thickened sections of the Salina salt produced by lateral migration (Frey, 1973). Kulander and Ryder (2005) further state that the
Salina Group deforms internally, with individual salt horizons acting as individual weak décollements. These small weak décollements can only be inferred by flexure of the seismic reflectors rather than the offset traditionally seen in faulted seismic sections (Frey, 1973;
Kulander and Ryder, 2005)
Mount (2014) describes the general structural style of north-central Pennsylvania’s
Appalachian Plateau. In this region the Salina group is over 2,500ft thick (Rickard, 1969), with the halite-dominated subunits greater than 500ft/125m thick. Mount (2014) identifies three different structural domains, all occurring in within their study area, within the Appalachian
Plateau’s fold belt. Domain 1 is dominated by folds that are both low amplitude (30-150m) and short wavelength (1.5-2km). Domain 2 contains large salt-cored anticlines, with large amplitudes
(1km+) and wavelength (13-16km), with long gently dipping northwestern fold limbs, and steeper southeastern faces. Domain 3 contains large synclines, with large wavelengths (13-16km.
Here Mount (2014) argue that the detachment within the Salina Group is non-planar and they can link depressions/rises to synclines/anticlines, which developed prior to halokinesis. In addition,
15 Mount (2014) does not directly state a cause for the above, but suggest it may be depositional/primary or it may be secondary and developed early in deformation.
Gillespie et al., (2015) offers an alternative explanation for structures above the Salina
Group structures within the décollement sheet of the Appalachian plateau. Here they argue that high angle kink bands developed due to deformation being accommodated on multiple décollement surfaces, with the internal Salina Group decollement as the lowest. The location of the salt-cored anticlines are related to where kink bands interact (i.e. pop-down structures)
Gillespie et al., 2015). Gillespie et al. (2015) further show that the Salina structures are the result of a two stage process: 1) originally formed on the axial core of a salt pillow, then 2) further compression the high angle kink bands develop with the original structure off-flank of the new axis. Gillespie et al. (2015), confine Salina deformation to internal Salina décollements and above Salina detachments from the Tully Limestone and Marcellus Shale, and from Alleghanian orogenic stresses. Therefore Gillespie et al., (2015), argue that the interactions between kink bands also controlled halokinesis. However, they caution that this same deformation may not occur to the same extent in the rest of the Appalachian plateau (Gillespie et al., 2015).
Although these studies propose different controls on the flow of the Salina Group into the core of the anticlines, they both follow Frey (1973) in restricting Alleghanian structural development to the units above the Salina Group. In at least one case however, faulting in the
Pre-Cambrian basement has been demonstrated to influence later deformation, particularly flow in the Salina. In the Lackawanna Synclinorium (Figure 3) a structural trough over 100km in length contains inferred basement thrust faults in some of the seismic lines that are interpreted to be reactivated normal faults (Harrison, et al.2004). The Lackawanna Synclinorium, Harrison et al. (2004) state, was developed in part by salt collapse (due to flow or dissolution) appears to
16 occur in conjunction with reactivated basement faults. Existence of a thick salt deposit within the
Salina Group in the first place may also be due to localized subsidence associated with these faults.
In summary, Mount (2014) sees the Salina Group flow and fold formation as controlled by decollement topography, which could be either a pre-existing feature or created by early deformation. Gillespie et al. (2015) see Salina Group deformation effectively as a response to kink bands forming between various detachment levels, of which the Salina Group is the lowest.
The style of folding and halokinesis is essentially controlled by factors internal to the Salina in the former cases and within the Salina overburden in the latter. The sub-Salina units and basement play no role in either case. Whereas, Harrison et al. (2004) demonstrate that pre- existing basement structures have likely influenced salt removal in the Lackawanna
Synclinorium.
1.8 Goals of Studfy:
The study area and surrounding region has seen renewed interest due to the Marcellus and Utica shale-gas plays in northeastern Pennsylvania. Most of the seismic data collected from this region was before the advent of high-resolution 3D seismic surveys (PA DCNR, 2007).
Given the current state of seismic processing and imaging as compared to the state of seismic capabilities of the 1970’s, a more detailed investigation of structures associated with the Salina
Group is warranted. Using a 3D seismic dataset donated by Talisman energy, I have investigated the potential linkage between structural development of the Salina and overlying units and pre- existing underlying basement faults.
Based on preliminary examination of seismic data from north-central Pennsylvania, it is proposed that reactivation of basement faults during the Allegheny orogeny influenced
17 halokinesis in the overlying Salina, and therefore acted as a control on the development of the salt-cored anticlines and associated faulting in the overlying units (Figure 8). This is tested by detailed seismic mapping of structures in the basement and in the Paleozoic sequence above and below the Salina especially noting any spatial associations between the structures. To further constrain the deformation of the Salina Group, isochronopach maps and profiles were created and analyzed. This work has shown a possible interaction between basement rift-transform faults and the overlaying Salina Group decollement faults, through a zone of disruption occurs between the basal Salina and the top of the Grenville Basement.
18
Figure 8: Showing the 3 differing views of Salina Group deformation development (After
Gillespie, et al., 2015; Mount, 2014; Harrison, et al., 2004)
19 2. Methodology
2.1 Seismic Volume
The 3D seismic volume, provided in SEG- Y (Barry et al., 1975) format by Talisman
Energy (now part of Repsol) was collected prior to 2011, and has been time migrated (Kirchhoff
Pre-stack Time Migration method) but not depth converted. Seismic migration is a method of seismic reflection survey data processing that creates a 2D or 3D useable image of the data. Time migration is a form of seismic migration, that uses time coordinates, and has benefits of being less computationally expensive than depth migration, and can be used in areas with moderate velocity fields and less complex geologic conditions. Although depth migration is more expensive (resource-wise), it provides more information, including “true” depth of structures, easier checks on horizons, and better imaging of areas with larger lateral velocity variation, such as extensional salt diapirs. The main limitations of time-migration are lower reliability in sub-salt features in regions with larger lateral variability within the salt and lack of true depths and dips of features within the volumes (Yilmaz et al., 2001). The Kirchhoff pre-stack time migration, an industry standard method, is a ray-base method that migrates each ray individually and separately (Etgen et al., 2009; Yilmaz et al., 2001). The volume studied here has not been depth converted primarily because of the lack of required well log information to produce a reliable velocity model (Yilmaz et al., 2001). The volume has 356 NW-SE oriented inlines, 439 NE-SW oriented crosslines, and 4998 Z-planes (Figure 9), and was analyzed using the seismic processing software OpendTect 4.6.0.
20 Major reflectors within the volume were delineated by correlating reflection data to a - synthetic seismic profile (Figure 10) created from well logs of sonic, density and formation top depths in the study area. A schematic syntheic is plotted in Figure 11, along with selected horizon tops of the Tully Limestone, Marcellus Shale, Salina Group (top and bottom), Trenton-
Black River Group and the Grenville Basement (Figures 11 and 12). The Tully Limestone horizon is characterized by a strong positive reflector wavelet. The Marcellus Shale is typified by a positive reflector wavelet about two-thirds the strength of the Tully reflector. The Salina
Group top has a strong negative reflector wavelet, which is often a doublet. The basal Salina has a mid-range negative reflector wavelet. The tops of the Trenton-Black River Group and the
Grenville Basement also produce strong negative wavelet.
The synthetic wavelets for each reflector were used to identify these horizons on 15 lines where the selected horizons were particularly well expressed (Figure 12). For example, the top of the Tully Limestone is predicted to produce a strong positive reflection, which can be matched to a strong positive reflector at the correct approximate depth in the seismic volume. The manually selected points on these initial 15 lines then became seed points for tracing each reflector across the 3D seismic volume using OpendTect's automated horizon tracking tools. The relative difference option was selected, which traces the seed reflector across the seismic volume until a break is found that exceeds a predetermined step offset. The relative difference method uses the amplitude of the last point tracked compared to the next possible point, continuing until the amplitude of the next point exceeds the seed step percentage. The steps are the maximum percentage difference allowed between the seed point wavelet and the next possible tracking point. The seed step percentages that were used are 1, 5, 10, and 15 steps, or percentage difference allowed, from the seed point wavelet. The horizon tracking first uses differences of ≤
21 1% (1 step) around the seed points, then progress to ≤ 5% (5 steps), then to the ≤ 10% (10 steps) and finally ≤ 15% (15 steps). All steps were used in the generation of the horizons but they were weighted differently. In general; 1 step and 5 step percentages are considered to have good to very good reliability; 10 step percentages have moderate reliability; and 15 step percentages are considered to have poor reliability (Schlumberger, 2010; db Earth Sciences, 2015).
As a focus of this study both the top Salina and the basal Salina horizons were picked.
The large shale layer (Subunit C), within the Salina Group was not visible within the Salina
Group layer seismic signals. The basement horizon was the most difficult to resolve due to the attenuation of the seismic signal by overlying reflectors, particularly the Salina Group. Based on the interpretation of seismic profiles in surrounding regions by Kulander and Ryder (2005) and by Schultz (1999), the basement top is marked by a basal strong negative reflector below which no coherent, laterally extensive continuous reflectors are imaged.
Following horizon tracing, an initial survey of the volume was then performed by stepping through the crosslines and inlines at 10 line intervals, and through the Z-plane every
100 lines. Major features, such as obvious discontinuities and other reflector disruptions, were noted for further investigation and tracing at smaller line intervals, to fully characterize the structures in the volume and their spatial and structural relationships.
The selected inlines and crosslines, as well as zoomed-in sections of these lines, were exported from Opendtect as PDF files. These files were then imported into Adobe Illustrator as the base layer and cropped to the required dimensions. Interpreted faults and horizons were then retraced onto new layers over the seismic line and annotated. Direct export of the lines produced errors in the picking of the horizons, particularly in the region near the southern edge of the volume (south of crossline 150) for the Salina Group. With manual tracing of the horizons, false
22 horizon picks were removed and more accurate interpretation of the horizons was accomplished, along with the more control on horizon visibility in exported images at closer zoomed-in views
(examples of this can be seen in Figure 13). Retracing also allowed for easier control of finer scale fault editing as well as an additional check on horizon data.
2.2 Salina Thickness Mapping
To better understand the variations of Salina Group TWTT (Two Way Travel Time) thickness within the volume, the TWTT depths of the constructed 3D horizons for the Salina
Group Top and Basal Salina horizons were used to construct isochronopach maps. These maps were constructed by exporting the horizon xyz data from OpendTect and processing the resulting data files in Python (see appendix A for source code). Cross-sections of selected crosslines and inlines provide additional 2D visualizations of Salina thickness changes.
23 N
Figure 9: Schematic showing the full extent of 3D seismic data, looking from map view.
24
Figure 10: A simplified representation of the process by which a synthetic well log is created.
25
Figure 11: A schematic representation of the synthetic seismic profile, illustrating the predicted characteristics of the picked horizons. Dashed lines represent gaps in the profile.
26
Figure 12: A seismic cross-sections (Top: un-interpreted; bottom: interpreted) showing the traced horizons for the Tully Limestone (blue), Marcellus Shale (Tan), Salina Group Top (Yellow),
Basal Salina (Lemon), Black River Group (Red) and the Grenville Basement (Green) horizons.
27 A
B 28
Figure 13: Two examples of errors caused by auto-tracking horizon picking within the Top Salina and Basal Salina Horizons. (A)
shows an upward jump in the tracing, ignoring the pop-down located here. (B) shows the skipping over of the horizon tracing where
thrusting has duplicated the Top Salina reflector. 3. Seismic Results
The location of selected lines analyzed within the volume are shown in Figure 14. Major horizons and structures are identified and described on raw and uninterpreted full inlines
(Figures 15 - 21), which are oriented from the NW to the SE. Additionally structures are identified on three raw and uninterpreted full crosslines oriented SW to NE (Figures 22 – 24).
Additional zoomed in images of full Inlines and Crosslines, full Z-planes, and associated interpreted figures can be found in Figures 25 to 50. Many of the major structures (faults and folds) found within the volume trend NE-SW and are concentrated in the southern part of the volume. Minor structures (such as small-scale faults) are found within the basement horizon.
Faults and folds developed above the Salina décollement, between the basement and the Salina
Group, and within the basement itself have been identified by offset markers and bending of reflectors, are described in more detail in the following sections. Potential spatial and geometric relationships between these features are discussed in Chapter 4.
29
Figure 14: Approximate extent of seismic volume showing location of Inlines (NW to SE) shown in Figures 15 – 21, and crosslines (SW to NE) shown in Figures 22-24.
30 3.1 Regional Seismic View
In the SE area of the study volume, a salt pillow is indicated by significant thickening of
Salina TWTT from the NW to the SE, from ~50ms TWTT to ~350ms TWTT, along Inlines 300,
220 and 180, (Figures 15, 16, and 17) and from ~50ms TWTT to ~250ms TWTT along Inlines
115, 95, 90 and 60 (Figures 18 – 21) (Section 3.3). A salt pillow is an oblong upwelling with consistent strata above it (Hudec and Jackson, 2007). Within the study volume the Salina in
Crosslines maintains a relatively uniform thickness, varying less than 50ms TWTT, from SW to
NE. To the east of roughly Inline 95 deformation above the Salina décollement consists of thrust faults rising from two main detachments, located within the Salina itself and just above the Tully
Limestone. Major faults, i.e. those that disrupt basement cover, trend across the volume.
Additionally small-scale faults that do not disrupt the cover are found within the basement (e.g.
Figure 15), across the study area volume.
31 SE NW 32
Figure 15a: Full un-interpreted profile view of Inline 300.
SE NW
33 Fig 25
TWTT
Figure 15b: Full interpreted profile view of Inline 300. Black box shows extent of Figure 25, which provides a more detailed view of
faults disrupting the Salina Group (S1, S2). SE NW 34
Figure 16a: Full un-interpreted profile view of Inline 220
SE NW Fig 26 Fig 41 35
Figure 16b: Full interpreted profile view of Inline 220, showing the location of faulting within the Salina Group (S1, S2), a fault
developed above the Tully Limestone (AB1), zone of disruption between the basement and Salina Group (T1), and a basement fault
(B5). See also Figure 26. 36
Figure 17a: Full un-interpreted profile view of Inline 180 Fig 27 Fig 42 37
Fig 47
Figure 17b: Full interpreted profile view of Inline 180, showing the location of faulting within the Salina Group (S1, S2), a fault
developed above the Tully Limestone (AB1), a zone of distruption between the basement and Salina Group (T1), and the basment
fault (B2). See also Figure 27. 38
Figure 18a: Full interpreted profile view of Inline 115. Fig 28 39
Figure 18b: Full interpreted profile view of Inline 115, showing the location of faulting within the Salina Group (S1, S2), a fault
developed within the Tully Limestone (AB2), a zone of disruption between the basement and Salina Group (T1), and a small basement
fault (B6). See also Figure 28. SE NW 40
Figure 19a: Full un-interpreted profile view of Inline 95.
SE NW Fig 29 41
Fig 30
Figure 19b: Full interpreted profile view of Inline 95, showing the Salina Group faults S1 and S2, the above Tully Limestone fault
AB2, and AB4, and a zone of disruption between the basement and Salina Group (T1), and the Basement Fault B6. See also Figure 30. SE NW 42
Figure 20a: Full un-interpreted profile view of Inline 90. SE NW Fig 31 43
Fig 32
Figure 20b: Full interpreted profile view of Inline 90, showing the faulting within the Salina Group (S1, S2), the above Tully
Limestone fault AB2, a zone of disruption between the basement and Salina Group (T1), and Basement Fault B6. See also Figure 32. SE NW 44
Figure 21a: Full un-interpreted profile view of Inline 60. SE NW
Fig 33 45
Fig 34
Figure 21b: Full interpreted profile view of Inline 60, showing the above Tully Limestone fault AB2, the Salina related fault S3, and a
zone of disruption between the basement and Salina Group (T1), and basement fault (B6). SW NE 46
Figure 22a: Full un-interpreted profile view of Crossline 150.
SW NE 47
Fig 46
Figure 22b: Full interpreted profile view of Crossline 150, showing basement Fault B1. SW NE 48
Figure 23a: Full un-interpreted profile view of Crossline 180.
SW NE 49
Fig 45
B4
Figure 23b: Full interpreted profile view of Crossline 180 showing basement Faults; B1, B4, and B5.
SW NE 50
Figure 24a: Full un-interpreted profile view of Crossline 250.
SW NE 51
Figure 24b: Full interpreted profile view of Crossline 250, showing faults developed in the basement (B3) and between the basement
and the Salina Group (T2).
3.2 Structures within the Salina Group
In the southern part of the volume, two NE-SW trending faults, S1 and S2 (Figures 25 –
34), disrupt the Salina Group and horizons immediately above it. These structures extend across the volume between crosslines 70-120 and 900-1300ms TWT (Figure 35), and appear to be the primary structures accommodating shortening above basement east of Inline 100 (Figures 35 and
36). Upward displacement of reflectors between the top of the Salina Group and the Tully
Limestone on the hanging wall of both faults (e.g. Figure 25b); indicate that they are thrust faults, with a down-thrown block between them. The basal Salina is not displaced, suggesting that the faults are propagating upwards from within the Salina decollement.
S1 can be identified across 259 Inlines from Inline 333 to Inline 74 and has an along- strike length of ~29, 900ft / 9,133m. It dips to the SE, with an approximate down-dip fault plane length of ~1,667ft / 508m with an average vertical extent of ~355ms. S1’s up-dip extent increases from the NE to SW across the volume from 2200ft/ 671m on Inline 300 (Figure 25) to
2150ft / 655m on Inline 90 (Figures 31, 32). In the region of maximum up-dip extent on Inlines
90-115, S1 offsets the Tully Limestone reflector (Figures 28-32), but it terminates below the
Tully Limestone reflector on inlines 180, 220 and 300 (Figures 25 -27). S1 is a thrust fault with offset determined by the upward throw of the hanging wall (SE) over the footwall (NW). S1’s offset, as determined by both the Top Salina and the Tully Limestone reflectors remains uniform at ~100ms between Inline 300 and 115. The offset of S1 of the Top Salina decreases sharply to
~65ms between Inlines 115 and 95, and increases slight to ~80ms at Inline 90. Within the Tully
Limestone there is an apparent increase in the amount of offset from ~20-25ms from Inlines 15 to Inline 95 to ~50ms at Inline 90.
52 S2, the NW member of the conjugate set, can be identified from Inline 321 to Inline 72, across 247 Inlines and has an along strike length of ~28,500ft / 8, 691m. It dips to the NW, with an approximate down-dip fault plane length of 1,668ft / 508m from its highest terminus at Inline
95, just slightly offsetting the Tully Limestone (Figure 30) to its lower terminus above the basal
Salina reflector and an average vertical extent of ~190ms. S2’s up-dip length increases to the NE
(Figures 26) across the volume from 2150ft/ 655m on Inline 300 (Figure 25) to 1250ft / 380m on
Inline 90 (Figures 31, 32). S2 is typically terminates below the Tully Limestone reflector, with little lateral variation (Figures 25 – 34), but slightly offsets the Tully Limestone in Inline 95
(Figure 30). S2’s offset of the Top Salina is more variable across the volume than S1 the offset ranges from ~50-90ms between Inlines 300 and 220, increases to ~150ms between Inlines 220 and 115. Then it decreases again to ~50ms from Inlines 115 to 90.
West of Inline 100 both S1 and S2 start to diminish in their down-dip extent (Figure 36) and ultimately terminate. However, a further, minor thrust fault S3 (Figures 32 and 33) disrupts horizons above the top of the Salina. This fault was identified across 22 Inlines, from Inline 45 to
Inline 67 and has an along strike length of ~1,700ft / 521m. S3 dips steeply to the NW, with an approximate down dip fault plane length of ~520ft / 160m from its highest terminus below the
Tully Limestone to its lower terminus just below the Top Salina reflector, and a vertical extent of
~110ms. S3’s up-dip extent increases from the NE to SW across the volume, from 435ft / 132m at Inline 60 (Figure 27) to 490ft / 150m at Inline 45. The upward motion of the Top Salina on the
NW hanging wall indicates thrust displacement over the SE footwall, with an offset of ~50ms.
S1 and S2 terminate and are replaced by S3 between Inlines 95 and 60, with the transition occurring over a ~ 5-line interval between Inlines 73-71 and68.
53 SE NW 54
Figure 25a: Full un-interpreted zoomed in view of Inline 300.
SE NW 55
Figure 25b: Full interpreted zoomed in view of Inline 300, showing Salina faults S1 and S2. SE NW 56
Figure 26a: Full un-interpreted zoomed in view of Inline 220.
SE NW 57
Figure 26b: Full interpreted zoomed in view of Inline 220, showing Salina faults S1 and S2, the disrupted horizon between the
basement and the base of the Salina (T1) and basement fault B5. SE NW
Figure 27a: Full un-interpreted zoomed in view of Inline 180.
58
Figure 27b: Full interpreted zoomed in view of Inline 180, showing Salina faults S1 and S2, and disrupted horizons between the Salina and Trenton-Black River Groups (T1).
59
Figure 28a: Full zoomed-in un-interpreted view of Inline 115.
60
Figure 28b: Full zoomed-in interpreted view of Inline 115, Salina faults S1/S2, disrupted horizons between the basement and Trenton-Black River Groups (T1), and showing basement fault B6.
61 SE NW 62
TWTT
Figure 29a: Zoomed in un-interpreted view of Inline 95.
SE NW 63
Figure 29b: Zoomed in interpreted view of Inline 95; showing the above Tully Limestone faults AB2 and AB4, and Salina faults S1
and S2.
Figure 30a: Zoomed in un-interpreted view of Inline 95.
64
Figure 30b: Zoomed in interpreted view of Inline 95, showing the above Tully Limestone fault
AB2, Salina faults S1 and S2, the T1 deformation zone with monoclinal folding of the Black
River, and basement normal fault B6.
65 SE NW 66
TWTT
Figure 31a: Zoomed in un-interpreted view of Inline 90.
SE NW 67
TWTT
Figure 31b: Zoomed in interpreted view of Inline 90, showing the above Tully Limestone fault AB2, Salina faults S1 and S2.
Figure 32a: Zoomed in un-interpreted view of Inline 90.
68
Figure 32b: Zoomed in interpreted view of Inline 90, showing the above Tully Limestone fault
AB2, Salina faults S1 and S2 , the T1 deformation zone with monoclinal folding of the Black
River, and basement normal fault B6.
69
SE NW 70
TWTT
Figure 33a: Zoomed in un-interpreted view of Inline 60.
SE NW 71
Figure 33b: Zoomed in interpreted view of Inline 60, showing the above Tully Limestone fault AB2 and Salina fault S3.
Figure 34a: Zoomed in un-interpreted view of Inline 60.
72
Figure 34b: Zoomed in interpreted view of Inline 60; showing the above Tully Limestone fault
AB2, Salina fault S3, and basement normal fault B6. The T1 deformation zone consists of monoclinal folding of the Black River Group Reflector and a series of smaller thrust faults.
73
74
Figure 35: Z-plane at 1100ms TWTT. The locations of S1, S2 and S3 are shown (See Figure 14, for line locations).
Figure 36: Horizontal traces of Salina Group Faults at ~1100ms TWTT. Inline 100 is marked for reference. The perpendicular marks on S1 and S2 show the relative down-dip extent of these faults.
75 3.3 Salina Thickness
To better understand and constrain the internal deformation of the Salina Group, the traced surfaces of the Salina Group top and bottom horizons were used to contour TWTT thickness in Z-plane view. The results of the contouring are shown in (Figure 37) and depicted in select profile views (Figures 38 and 39). As discussed in Section 3.1, the Salina Group gradually thickens to the SE from an average TWTT thickness of 55-65ms in the NW section of the volume to 170 -300ms in the SE (Figure. 37), with the maximum thickness changes of the Salina occurring at oblique angles to the Inlines and Crosslines (Figure 37) and the greatest thickness of salt being observed on the SE side of the volume (Figures 38 and 39). On Crossline profiles, the
Salina Group starts to thicken at roughly crossline 250 (Figure 38), reaching a maximum thickness of >250ms TWTT at the SE end of both Inlines 325 and 225, ~180 – 200ms TWTT on the SE ends of Inlines 125 and 25.
In map view Salina group thickness variations (Figure 37) reveal the pop-down structure associated with faults S1 and S2 in the thinned area at Inline 100 / Crossline 100, The thickness
(e.g. thinning of the Salina between S1 and S2) of the pop-down structure changes across the volume, as also shown in Figure 40. Further west, the thickness changes little, decreasing to about 180ms TWTT in the far SW corner of the study volume (Inline 25) (Figure 38). The thickness of the Salina Group (Table 1) shows a larger reduction of the Salina inside versus outside the pop-down structure around Inlines 115-90 than Inlines 300-220. An increase in the reduction occurs between Inlines 220 and 180, with the greatest occurring in Inlines 95 and 90, a
55% reduction between inside versus outside the pop-down (Figures 29-32).
76 Salina Thickness Around Salina Pop-down
Thickness Outside Thickness Inside Reduction of Pop-down Pop-down Thickness 300 275 195 29%
220 270 190 30% 180 205 105 49% 115 190 95 50%
Volume Inlines Volume 95 195 85 56% 90 200 90 55%
Table 1: The thickness of the Salina Group inside the pop-down versus outside the pop-down, with the calculated reduction of thickness across S1/S2.
77 Salina Thickness (Salina Top to Basal Salina) 78
Figure 37: Contoured thickness of the Salina Group in the study area. The red outline is the approximate extent of the study area. The
discontinuity in the NW of the volume that parallels ~Inline 175 is an artifact produced by the automatic horizon-tracing tool. SE NW
Crosslines
Crosslines
Crosslines
Crosslines
Figure 38: The thickness profile of various Inlines for the Salina Group.
79 SW NE Crossline 400
Inlines Crossline 275
Inlines
Crossline 150
Inlines Crossline 50
Inlines
Figure 39: The thickness profile of various Crosslines for the Salina Group.
80
Figure 40a: Detail of the Salina pop-down structure from eastern Inline 300 to western Inline
180, showing thinning of the Salina Group between S1 and S2.
81
Figure 40b: Detail of Salina pop-down structure from Inline 115 to Inline 90.
82 3.4 Structures above the Salina Group
Two major faults were revealed by disrupted reflectors in the units above the Tully
Limestone, a prominent reflector that lies about 300ms TWTT above the top of the Salina Group.
Fault AB1 is found in the NW of the volume (Figures. 41, 42, 43, and 45) from Inline 278 to
Inline 150, giving it an along strike length of ~14,900ft/ 4,541m (129 inlines), striking to the NE.
It dips moderately to the SE, with an approximate down-dip fault plane length of ~7700ft/
2,346m. AB1’s up-dip extent increases from the NE to SW across the volume from 5000ft/
1525m on Inline 220 (Figures 16 and41) to 7800ft / 2380m on Inline 180 (Figure 42). Near
Inline 180, a kink of the Tully Limestone can be seen at the base of AB1. The upward throw of the hanging wall reflectors (SE) over the footwall (NW) indicates a thrust fault: offset, is relatively uniform at ~40ms on Inline 220 (Figure 41) and Inline 180 (Figure 42).
Fault AB2 is a shallow to moderately SE dipping fault found in the SW corner of the volume (Figures 29, 33, 34, 43, and 44).The fault can be identified from Inline 110 to Inline 40, giving it an along strike length of ~8,000ft/ 2,438m (72 inlines), striking NE. The approximate down-dip fault plane length is ~7,500ft/ 2,286m. AB2’s up-dip extent increases from the NE to
SW across the volume from 6800ft/ 2075m on Inline 95 (Figure 28) to 8500ft / 2592m on Inline
60 (Figures 31 and 32). The upward throw of the hanging wall reflectors (SE) over the footwall
(NW) (Figures 29 - 34) indicates a thrust fault: offset is relatively uniform at ~45ms between
Inline 95 and Inline 90, decreases to ~40ms on Inline 60. AB2 is nearly directly above Salina faults S1, S2, and S3 (Figure 29 – 34, 44). AB2’s mapped extent also overlaps with the interval between Inlines 74-110 where Salina faults S1/S2 reach their maximum up-dip extent (Figure
44) and have propagated up through the Tully Limestone (Figure 40d-f). However, AB2 continues SW past the termination of S1 and S2, while increasing its up-dip extent (Figure 44).
83 84
Figure 41a: Zoomed in un-interpreted view of Inline 220.
SE NW 85
Figure 41b: Zoomed in interpreted view of Inline 220, showing the above Tully Limestone fault AB1.
SE NW
TWTT 86 1 0.5 0 0.5 1
Figure 42a: Zoomed in un-interpreted view of Inline 180.
0 SE NW
TWTT 87 1 0.5 0.5 1
Figure 42b: Zoomed in interpreted view of Inline 180, showing the above Tully Limestone fault AB1. 88
Figure 43: Z Plane view at 625 TWTT, showing faults AB2 and AB1.
Figure 44: Spatial relationships of faults within and above the Salina Group. Traces of faults S1 and S2 at ~1100ms TWTT (Figures 25-32), and AB1 and AB2 (Figures 29-34, 41, 42) at
~600ms TWTT, are marked with perpendicular lines on the faults showing their up-dip extent.
89 3.5 Basement Structures
In the study area the Precambrian Grenville basement occurs at depths greater than 5,000m
/16,400ft below surface (Schultz, 1999), which corresponds to ~2.5s TWTT in the study volume.
Six faults with significant length and/or vertical displacement have been identified in the basement.
Numerous additional minor faults that offset the basement, but have limited lateral extent, are marked on Figures 15-23 but not described further.
Fault B1 (Figures 45 and 46) was identified in the SW corner of the volume. This feature strikes NW, between crosslines 219 and 95 (124 crosslines), giving it an approximate along strike length of 6000-8,000ft / 1800-2400m. B1 dips to SW with an approximate down-dip fault plane length of ~2600ft / 810m. B1’s up-dip extent decreases from the NW to SE across the volume from 2900ft / 885m on Crossline 200 to 1992ft / 608m on Crossline 180 (Figure 45) to
1800ft / 550m at Crossline 150 (Figure 46). From Crossline 150 the up dip extent of B1 increases to 3000ft / 915m on Crossline 100 (Figure 48). Basement is offset within the SW side being up- thrown; however, the fault cannot be traced into the basement. Reflectors 50-100ms above the basement are also offset: here thrust motion is evident. Therefore, either the disrupted horizons are syn-rift and thrust or disrupted horizons are signs of reactivation and the original sense of motion unclear.
Fault B2 (Figure 17 and 47) was identified from Inline 200 to Inline 160 in the NE area of the volume. The strike of the fault is SW to NE and it dips to the SE. It has an along strike length of ~5,400ft / 1633m (40 inlines), and an approximate down-dip fault plane length of
~1,200ft / 375m. B2’s up-dip extent decreases from 1580ft / 482m on Inline 195 to 1370ft /
418m on Inline 180 (Figure 47), and decreases more to 1168ft / 356m on Inline 165. B2 is a normal fault as shown by the downward throw of basement reflector in the hanging wall (SE)
90 with respect to the footwall (NW). B2’s offset is uniform across its lateral extent, roughly
~10ms at Inline 180 (Figure 47), a look at ~50ms above the basement appears to be disrupted above B1, B2, and B6. This consistency might indicate syn-rift deposition and displacement.
Fault B6 (Figures 28, 30, 32, and 34), can identified across 50 Inlines from Inline 125 to
Inline 40 and has an along strike length of 8500ft / 2,600m, striking NE. B6 dips to the SE, with an average down-dip fault plane length of ~1,000ft / 300m. B6’s up-dip extent increases from the
NE to SW across the volume from 516ft/ 158m on Inline 115 (Figure 28) to 1100ft/ 335m on
Inline 60 (Figure 34). B6 appears to continue west of Inline 40, but cannot be reliably traced due to edge effect. Fault B6 is a normal fault (Figures 28, 30, 32, and 34), based on the downward throw of the hanging wall (SE) below the footwall (NW). B6’s offset of the basement reflectors is ~150ms between Inline 125 and 40, and increases to ~200ms at Inline 60.
A series of basement faults B3, B4, and B5, all trending NE/N, offset only the basement reflector making it hard to infer their sense of motion. B3 (Figures 24 and 48) the SW basement reflector is down-thrown. B3 is identified from Inline 179 to Inline 124 and has an along strike length of ~7,400ft (55 inlines). There is little variation in offset (~15ms on Crossline 250
(Figure 24b). Basement Fault B4 (Figures 23 and 45) was identified from Inline 205 to Inline
175, and has a SW down-thrown side. B4 has an along strike length of ~3,300ft / 1,005m (30 inlines), strikes NE and offsets the basement reflector by ~10ms at Crossline 180 (Figure 45). B5
(Figures 16, 23, 26, and 45) was identified from Inline 251 to Inline 201, and has an along strike length of 6600ft / 2,011m (50 inlines), striking NE. B5 cuts obliquely across the volume and appears on both Inlines and Crossline with its west ( SW: Crossline 180 / NW: Inline 220) side down-thrown. The dip is unable to be determined. The offset of B5 is relatively uniform and is
~15ms at Crossline 180 (Figure 45).
91 SW NE 92
Figure 45a: Full zoomed-in un-interpreted view of Crossline 180.
SW NE 93
Figure 45b: Full zoomed-in un-interpreted view of Crossline 180, showing basement faults B1, B4 and B5. SW NE 94
Figure 46a: Full zoomed-in un-interpreted view of Crossline 150. SW NE 95
Figure 46b: Full zoomed-in un-interpreted view of Crossline 150, showing basement fault B1.
TWTT 96
Figure 47a: Full Un-interpreted zoomed in view of Inline 180.
97
Figure 47b: Full interpreted zoomed in view of Inline 180, showing the basement fault B2 98
Figure 48: Z-plane at 2502ms TWTT, showing the locations of basement faults B1, B2, B3, B4, B5, and B6.
Figure 49: Geospatial relationship of basement fault, with Salina-deforming faults S1 and S2, higher in the volume stratigraphically.
99 3.6 Deformation between Basement and Salina
In most of the volume, little deformation is observed. However, some of the structures are observed in the SE of the volume beneath fault S1 and S2 (T1) (Figures 26-28, 30, 32, 34,
50). These structures are disruptions (kinking, monoclinal folding and/or faulted offsets) of strong reflectors between Salina and basement that appear to be geographically associated but are not linked into a single through-going structure. These structures, collectively noted as T1, predominantly trend from the SW to the NE, predominantly dipping to the SE, with a smaller number dipping to the NW. (Figures 27, 28, 30, 32 and 34). T1 can be divided into 3 groups of structures, based on the stratigraphic level at which they occurred at. The first group is thrust faulting below the Trenton-Black River Group, particularly in the ~50ms above basement near basement fault B6, between Inlines 115 and 90 (Figures 28, 30, and 32 (Blue, Figure 50)). The second group is the monoclinal folding and thrust faulting developed within the Trenton-Black
River Group at 2-2.1s TWTT, between Inlines 115 and 60 (Figures 28, 30, 32, and 34 (Purple,
Figure 50)). Finally the third group is the thrust faulting above the Trenton-Black River Group at
~1.5-1.8s TWTT, observed between Inlines 220 and 180 (Figures 26 and 27) and also Inlines 95 and 90 (Figure 30 and 32 (Pink, Figure 50))
The monoclinal folding has the largest folding at Inlines 95 and 90, below the most disrupted sections of S1, S2, and AB2, and above the basement B6 and likely the intersection zone of B1 and B6. Similarly the below Trenton-Black River Group T1 structures are found in the same locations of the monoclinal T1.
Elsewhere in the volume, T2 disrupts horizons at ~1.8-1.9s TWTT above the Trenton-
Black River Group reflector in the NE of the volume (Figure 24). T2 is identified across 70 inline from Inline 200 to Inline 130 and has an along strike length of ~7,700ft / 2,346m. This
100 fault has an approximate down-dip fault plane length of ~680ft / 207m from terminus to terminus. The up-dip extent of T2 appear uniform across its lateral extent, and at Crossline 250
(Figure 24) is 1,600ft/ 488m, and appears to be a thrust fault with the SW hanging wall reflectors to the up-thrown NE reflector at ~1800ms (Figure 24). T2 has a footwall offset of ~15ms at
Crossline 250 (Figure 24). In contrast to the T1 structures, T2 appears to have no geographic relationships with any other faults within the study area.
101
Figure 50: The three parts of the T1 deformation system shown in map view, representing TWTT slices from above basement to below basal Salina. T1-Purple represents the monoclinal folding of T1. T1-Blue represents structures found below the Trenton-Black River Horizon. T1-Pink represents structures found above the Trenton-Black River Horizon. The dashed lines represent the possible continuation of these sub-structures.
102 4. Discussion & Conclusions
In this seismic volume, most structures found above the basement horizon strike from the northeast to the southwest. This trend parallels the Allegheny structural deformation front, which for the study area has a mean strike of 60o±10oNE (Schultz, 1999; Wise, 2004). The faults both in and above the Salina Group strike in this direction. S1 and S2 are major thrust faults with S1 dipping southeast which is the general direction of the other faults within the study area.
4.1 Summary of Faults
The basement as noted in Section 3.5 is disrupted by numerous faults. Six of these faults cross several dozen seismic lines (Figure 49). Two major NE-SW trending faults with normal displacement (B3, B6) are linked by a NW-SE trending fault (B1) (Figure 49). The other faults have an indeterminate motion (B2, B4, & B5). At the eastern terminus of fault B6, the southeastern terminus of fault B1 is found in close proximity to each other. At the northwestern terminus of fault B1, the southeastern terminus of fault B3 is found nearby. The basement faults
B2, B3, B4, and B5 form a fan-like array-changing trend from NE-SW (B2) to a NNE-SSW (B5)
(Figure 49).
Several lineaments (as discussed by Schultz) share the SW-SE trend of B1 in the area of the study region. Many of these faults, within the study area, are likely related to the rifting of the proto-Iapetus ocean during the late Precambrian and earliest Cambrian (Figure 52 A & B)
(Schultz, 1999). NE-SW trending normal fault and SE-NW trending lineaments are commonly observed in the Pre-Cambrian basement in the region, and are related to Iapetus rifting (Schultz,
103 1999). B3 and B6 can therefore be interpreted as rift boundary faults linked by a transform fault
(B1).
The principal structures accommodating deformation within the Salina and above, as described in Sections 3.2 and 3.4 are faults with NE-SW trends that parallel the Allegheny structural deformation front (Schultz, 1999; Wise, 2004). Within the Salina group S1 and S2, form a pop-down structure. S1 and S2 are present in the southeastern portion of the volume from
Inline 70, to Inline 300, and likely continuing off volume.
4.2 Spatial Correlation between Basement Faulting and Alleghany Deformation.
The major structures found within the volume above the basement appear to be the Salina
Faults of S1/S2 complex and the above Salina faults of AB1 and AB2. There is a clear geographical relationship between at a minimum, three of these faults and the basement structures discussed above (Figure 51). The SW end of S1 and S2 is associated with the intersection of B1 and B6. In the region these structures coincide, S1 and S2 both propagate furthest upwards in the volume. Here they disrupt the Tully Limestone (Section 3.2). This is also the region where the thickness of the Salina within the pop-down structures has been the obviously reduced by salt flow (Figure 40). AB2, which appears to overlap with S1 and S2, and initiates directly above where S1 displaces the Tully Limestone reflector in the region of overlap
(Figures 29, 30, 31), is confined to the basement high formed by B1 and B6. Although the link between AB2 and any basement structures is less direct (Figure 17), the step-back to the NW of
AB1 relative to AB2 occurs in the same region that B6 step-back NW to B2 (Figure 51).
The basement is relatively higher to the southwest of B1 (i.e. the B6 footwall), visible on
Crossline 180 (Figure 23), forming a possible rift promontory. This rift promontory likely acted as an indirect control of the basement changing the stress field, and therefore allowing the
104 change of the above Tully deformation to Salina deformation, through the deformation zone of
T1. There is a change in the strength and style of deformation the east of B1/B6 versus west of
B1/B6. East of this, zone the S1 and S2 complex is the only expression in the Salina and above.
West of B1/B6 structures expressed are just S3 in the Salina and the only above deformation becomes the Tully limestone fault AB2. Further, AB2 begins to develop above S1/S2, along with
S1 and S2 reaching their maximum up-dip fault plane length and their most up section offsets.
Additionally the most intense folding of the Black River Group, which is relatively uniform and undisturbed across the volume below S1 and S2 occurs at the Inlines between Inlines 60 and 115
(i.e. the area above B1/B6).
All of this indicates that the intersection point between B1 and B6, likely became the focal point (seed point) for the development of S1 and S2 during the salt flow of the Salina. By changing the stress field above this promontory to allow differences in development of the Salina and above structures. As in the area where these basement faults meet there was more intense faulting of the Salina group to S1 and S2, along with the stronger expression of the pop down structure. Additionally a change in expression of the structures can be seen changing east to west across the volume.
105 106
A
Figure 51: The inferred Basement Rift Complex at 2515 TWTT. The left image shows the location of the basement faults B1, B3, B4,
B5 and B6. The right image shows the inferred basement relationships of the faults. A is the promontory that is created by the B1, B3
and B6 faults. B is inferred to be possible connected to A. When looking at other structures within the volume, e.g. above Tully limestone fault
AB1, there appears to be no relationship between this fault in the basement faults nearby (B2,
B3, B4, and B5). Other minor faults within the volume (i.e. T2) also appear randomly distributed with respect to the basement, so that no clear relationship is perceived between them.
Additionally the thickening of the Salina group salt to the southeast appears not to be related to these basement faults either. So that these basement faults appear not to have exerted any influence on the development of the Salina group and its salt movement during the terminal
Alleghanian orogeny.
4.3 Mechanism Linking Basement Structures to Deformation above the Salina Group.
The overlap between Pre-Cambrian basement structures and the thrust faults found above
Salina decollement during the Alleghenian Orogeny suggest that the basement structures may have influenced later structural development despite decoupling associated with the Salina
Group. While there are no clear through going structures, there is however evidence of post-
Neoproterozoic deformation beneath the Salina Group decollement, which has been collectively interpreted as T1 (Section 3.6). This is shown primarily by disruptions of horizons above the basement faults B1, B2, and B6, possibly indicating reactivation (Figures 17, 28-34, and 45-46).
Furthermore, monoclinal faulting and thrusting within the Trenton-Black River Group, a prominent set of reflectors beneath the Salina Group (Figures 28, 30, 32 and 34) show similar disruptions.
Most of the deformation features of T1 occurs below the pop-down structure formed by the Salina faults S1 and S2, which suggests that they may be linked to its formation. T1 features also seem to be particularly common in the region where B1 and B6 intersect, and the above
Salina deformation seems the most intense (see previous sections). These observations strongly
107 suggest that the location of Salina Group faulting, formed during the Allegheny orogeny, is influenced by the Grenville basement topography generated during Neoproterozoic rifting. The evidence is consistent with the following: S1 and S2 initializing in the SW if the volume, close to the intersection of the basement faults B1 and B6. AB2 also initializing also in the region when
S1 propagated upwards through the Tully Limestone.
The possible mechanisms are discussed below. Reactivation of the basement faults as thrust during the Allegheny orogeny and their progression up through the sub-Salina Paleozoic sequence has directly stressed the base of the Salina Group. This requires that T1 structures are just the most visible evidence of more continuous faulting, visible only where they disrupt strong reflectors. Previous studies of basement influence on salt flow in deforming regions have highlighted the role of changing overburden thickness across a fault on the vertical stress field, which then causes salt flow to concentrate near basement structures. This part of the volume is in the footwall of B6 and the basement is raised relative to the B6 hanging wall to the SE and the region NW of B1 (Crossline 180, (Figure 23)). Although these studies (i.e. Dooley, et al. (2005)) have largely focused on the flow of salt during orogenic extension, it is possible that a similar mechanism could lead to salt deformation associated with the step to thinner overburden on this basement high. In this case, T1 would reflect a secondary response to a changing stress field across the basement step, which would explain its rather diffuse nature.
The most logical scenario is one where, as suggested above, T1 reflects a response to a changing stress field across the basement step rather than a through-going structure. This would explain the diffuse offsets between the basement and the Salina, in rather competent seismic units, such as across the Trenton Black River Group (Figures 28, 30, 32, and 34) where T1 is expressed at this horizon as monoclinal folding, above the basement step. Additionally the stress
108 field change explains the rather quick change from the S1/S2 complex to the AB2 fault as the primary expression of Salina and above deformation (Figures 25-32). Furthermore, this also explains the lack of large-scale volume-sized (51+ Line length) structures elsewhere within the volume, and the concentration of structures above the basement-step.
4.4 Other Views on Salina Development
The more regional studies of the Appalachian plateau deformation by Mount (2014) and
Gillespie, et al. (2015) both have identified structures similar to the S1/S2 pop-down structure identified within the study area volume. The small wavelength Domain 1 structures of Mount
(2014) and the thrust from the Salina décollement associated with lower angle thrusting above the Tully Limestone (Figure 7; Mount, 2014), which is very similar to the relationship between
S1/S2 and AB2 observed in Figures 29 and 30. Of several similar structures by Gillespie, et al.
(2015) (e.g. their Figures 11, 12, and 14), the most direct equivalent to S1/S2 is a pop-down on the gently dipping north flank of a salt pillow (Figure 17; Gillespie, et al., 2015), which they propose represents the crest of a symmetric buckle fold, which has then been modified by later deformation. Neither of these studies consider the possibility of underlying basement control on
Salina Group deformation, but apparent influence of the basement faulting on the development of very similar structures in this study indicates that more examples of basement control may be found elsewhere on the Appalachian plateau.
These authors also propose different controlling mechanisms on Salina deformation
(Section 1.7). Mount (2014) suggests that topography of the Salina décollement was the main control. One possible consequence of the mechanisms for basement influence on Salina deformation discussed in Section 4.3 is the disruption of the basal Salina Group. The regional interpretation of Mount (2014) is therefore more obviously compatible with basement control
109 then the interpretation of Gillespie, et al. (2015), suggest Salina deformation is controlled by interaction between the Salina décollement and numerous overlying detachments. However, without clear mapping of basement structures beneath Salina deformation across the study areas of Mount (2014) and Gillespie, et al. (2015), this question remains open.
4.5 Comparison with Observed and Modelled Basement Salt Interactions.
Associations between salt flow and basement structures have been observed in both extensional, i.e. the North Sea (Koyi and Peterson, 1993) and compressional, i.e. the Zagros
Mountains of Iran (Bahroudi and Talbot, 2003) settings. In the case of the Zagros Basin, reactivation of basement faults appears to be a strong control on the basin’s structural evolution despite the presence of the Hormuz Salt (Bahroudi and Talbot, 2003).
Analogue modeling using a synthetic polymer as a salt analogue can provide insight into the possible mechanisms of these interactions. Modelling of salt flow in extensional regimes by
Vendeville, et al. (1995) indicates that if the layer of salt is thick enough, it flows in response to induced stresses, which effectively decouples the basement from the overburden. However, if the salt is sufficiently thinned, or the viscosity or the strain rates are too high to allow salt flow to dissipate the stress, then force-folding of the overburden above the basement step can occur.
More explicitly three-dimensional modelling by Dooley, et al. (2005), emphasizes the important of corner points where basement fault sets intersect in generating diapirism. The intersection between B1 and B6 that appears to be the focus of deformation in the overlying units is a direct equivalent of the outboard corner points where diapirism was focused in these models.
However, it should be noted that none of these localities or models is a direct equivalent of the situation in the Salina Group. Where the Salina Group is a salt layer that is separated from
110 the basement by a significant thickness of sedimentary cover, as well as the deformation of interest occurring in a much later deformation event.
4.6 Proposed Tectonic History of Study Area
The proposed history for the study area is that there was reactivation of the B1 and B6
(Figure 29) fault with a thrust component during the Alleghanian Orogeny during formation of the
T1 and influencing the development of S1, S2, and AB2.
The proposed tectonic and structurally history of the region is as follows (Figure 52). The rifting of the proto-Iapetus ocean created a series of normal and transform faults across the region, with B1 being a transform fault linking major normal rift faults B2 and B6 (Figure 52: Box A).
The carbonate passive margin of the Cambrian-Ordovician time was deposited over the rifted basement (Figure 52: Box B), foreland basin subsidence in the Taconic and Acadian orogenies led to deposition of the Salina Group in the late Silurian (Figure 52: Box C), and post Salina (Figure
52: Box D) sedimentation. During the Alleghanian orogeny, some basement faults were reactivated as thrust faults (Figure 52). The B1 and B6 faults provided a focus for the deformation to begin on and thus deformation system T1 developed (Figure 52: Box E). As time progressed,
T1 began to transmit stress to the base of the Salina Group causing flow and development of the
Salina faults S1 and S2;as well as the above Salina faults, where S1 and S2 breached the Tully
Limestone (Figure 52: Box F).
111
Figure 52: The proposed tectonic history of the study area and by extension north central
Pennsylvania. Boxes A-D are at regional cross-section scale. Boxes E-F are at study area scale.
112 4.7 Summary of Relationship of Structures Found
The relationship within the seismic volumes can be divided spatially into non-related faults and a complex of closely spatially related faults. The close relationship of the northern terminus of B6 and the southern terminus of B1 around Inline 115, in which B6 is a normal fault and B1 is a thrust fault. The northern terminus of B1 ends near the southern terminus of B3. The terminus junction of B1 and B6, lie below the Salina faults of S1 and S2, as well as the T1 deformations, in addition to the above Tully fault of AB2, around Inline 100. When viewing the inlines of the Salina pop-down and Salina Faults, S1 & S2, the change in the deformation of S1 and be seen as a possible influence from B1 and B6, where the Tully Limestone is offset in Inline 115 to where S1 terminates. Basement fault B6 underlies to the east of Inline 100, the above Tully Limestone fault of AB2 and the Salina fault of S3. Above Tully fault AB1 is bracketed by the basement faults B2 and B3. With the Basement fault B3 under T2 around crossline 250. Basement faults B4 and B5 have no other structures found above them. Basement fault B6 lies between AB4 and S3, but its northeastern terminus appears close to the southeastern terminus of B1.
4.8 Conclusion
Structures found within the basement and below Salina Group, suggest that there is some indirect basement control on structures developed both within the Salina Group and above it. The basement rift complex appears to be underlying the change in both the Salina pop-down and the differences in deformation styles west and east of the B1- B6 interaction zone. Additionally the structures found at the S1 S2 complex, appears to strengthen the model put forward by Mount
(2014), where the basement indirectly influence the Salina Group ’s topography through T1.
Further, the change in the Salina thickness differing to such a degree across B1, especially in the pop down structure of the S1-S2 complex, further stresses that the pre-existing basement has
113 indirectly influence the Salina development. As discussed in Section 4.6, the proposed tectonic history relies on reactivation as well as passive stress changes to become foci for development of faults within the Salina and above units. It is to be noted that much like Mount’s (2014) work that this may only be relative to the study area and nowhere else within the Appalachian basin.
Further research to provide definitive answers would require analog models of different basement conditions as well as analysis of other 3D seismic surveys.
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119 Appendix A: Source Code for Salina Contour Program. import os import numpy as np import matplotlib.pyplot as plt import pandas as pd os.chdir("/Users/crowan/Dropbox/Research/Matthew Harding/Salina contours/")
Salina_top=pd.read_table('Salina-Top.txt',header=0) Salina_bottom1=pd.read_table('Salina-Bottom1.txt',header=0) Salina_bottom2=pd.read_table('Salina-Bottom2.txt',header=0) def contour_layer(x,y,z,title='Contour Plot'): xi = np.linspace(min(x), max(x),max(x)/2) yi = np.linspace(min(y), max(y),max(y)/2) zi = griddata(x, y, z, xi, yi, interp='linear') fig, ax1 = plt.subplots(figsize=(6, 5), dpi=150, facecolor='w', edgecolor='k') ax1.contour(xi, yi, zi, 15, linewidths=0.5, colors='k') CS = ax1.contourf(xi, yi, zi, 15, cmap=plt.get_cmap('YlGnBu'), vmax=max(z), vmin=min(z)) plt.xlabel('Inline') plt.ylabel('Crossline') plt.title(title) cb=plt.colorbar(CS) # draw colorbar cb.ax.invert_yaxis() return CS def inline_profile(value): title='Profile through Inline '+`value` filter=Salina_top[Salina_top.Inline==value] plt.plot(filter.Xline,filter.Z,'gold') filter=Salina_bottom1[Salina_bottom1.Inline==100] plt.plot(filter.Xline,filter.Z,'lightblue') filter=Salina_bottom2[Salina_bottom2.Inline==100] plt.plot(filter.Xline,filter.Z,'blue') plt.ylabel('TWTT (ms)', fontsize=8) plt.title(title, fontsize=8) plt.ylim([950,1550]) plt.tick_params(axis='both', which='major', labelsize=7) plt.ylim(plt.ylim()[::-1]) def xline_profile(value): title='Profile through Xline '+`value` filter=Salina_top[Salina_top.Xline==value]
120 plt.plot(filter.Inline,filter.Z,'gold') filter=Salina_bottom1[Salina_bottom1.Xline==100] plt.plot(filter.Inline,filter.Z,'lightblue') filter=Salina_bottom2[Salina_bottom2.Xline==100] plt.plot(filter.Inline,filter.Z,'blue') plt.ylabel('TWTT (ms)', fontsize=8) plt.title(title, fontsize=8) plt.ylim([950,1450]) plt.tick_params(axis='both', which='major', labelsize=7) plt.ylim(plt.ylim()[::-1])
def inline_thickness(value): title='Salina Thickness, Inline '+`value` filter1=Salina_top[Salina_top.Inline==value] filter2=Salina_bottom1[Salina_bottom1.Inline==value] filter3=Salina_bottom2[Salina_bottom1.Inline==value] thickness1=filter3.Z-filter1.Z thickness2=filter3.Z-filter2.Z plt.plot(filter1.Xline,thickness1,'gold') plt.plot(filter1.Xline,thickness2,'lightblue') plt.ylabel('TWTT (ms)', fontsize=8) plt.title(title, fontsize=8) plt.tick_params(axis='both', which='major', labelsize=7) plt.ylim([0,500]) def xline_thickness(value): title='Salina Thickness, Xline '+`value` filter1=Salina_top[Salina_top.Xline==value] filter2=Salina_bottom1[Salina_bottom1.Xline==value] filter3=Salina_bottom2[Salina_bottom1.Xline==value] thickness1=filter3.Z-filter1.Z thickness2=filter3.Z-filter2.Z plt.plot(filter1.Inline,thickness1,'gold') plt.plot(filter1.Inline,thickness2,'lightblue') plt.ylabel('TWTT (ms)', fontsize=8) plt.title(title, fontsize=8) plt.tick_params(axis='both', which='major', labelsize=7) plt.ylim([0,500])
#contour plots contour_layer(Salina_top.Inline,Salina_top.Xline,Salina_top.Z,'Depth to Salina Top (TWTT ms)') contour_layer(Salina_bottom1.Inline,Salina_bottom1.Xline,Salina_bottom1.Z,'Depth to Salina Bottom 1 (TWTT ms)') contour_layer(Salina_bottom2.Inline,Salina_bottom2.Xline,Salina_bottom2.Z,'Depth to Salina Bottom 2 (TWTT ms)') contour_layer(Salina_bottom2.Inline,Salina_bottom2.Xline,Salina_bottom2.Z-Salina_top.Z,'Salina Thickness to Bottom 2 (TWTT ms)') contour_layer(Salina_bottom2.Inline,Salina_bottom2.Xline,Salina_bottom1.Z-Salina_top.Z,'Salina Thickness to Bottom 1 (TWTT ms)')
121 contour_layer(Salina_bottom2.Inline,Salina_bottom2.Xline,Salina_bottom2.Z-Salina_bottom1.Z,'Salina Bottom1 to Bottom 2 (TWTT ms)')
# Inline - depth plots plt.figure(figsize=(5, 5), dpi=150, facecolor='w', edgecolor='k') plt.subplot(4, 1, 1) inline_profile(25) plt.subplot(4, 1, 2) inline_profile(125) plt.subplot(4, 1, 3) inline_profile(225) plt.subplot(4, 1, 4) inline_profile(325) plt.tight_layout()
# Xline - depth plots plt.figure(figsize=(5, 5), dpi=150, facecolor='w', edgecolor='k') plt.subplot(4, 1, 1) xline_profile(400) plt.subplot(4, 1, 2) xline_profile(275) plt.subplot(4, 1, 3) xline_profile(150) plt.subplot(4, 1, 4) xline_profile(50) plt.tight_layout()
# Inline - thickness plots plt.figure(figsize=(5, 5), dpi=150, facecolor='w', edgecolor='k') plt.subplot(4, 1, 1) inline_thickness(25) plt.subplot(4, 1, 2) inline_thickness(125) plt.subplot(4, 1, 3) inline_thickness(225) plt.subplot(4, 1, 4) inline_thickness(325) plt.tight_layout()
# Xline - thickness plots plt.figure(figsize=(5, 5), dpi=150, facecolor='w', edgecolor='k') plt.subplot(4, 1, 1) xline_thickness(400) plt.subplot(4, 1, 2) xline_thickness(275) plt.subplot(4, 1, 3) xline_thickness(150) plt.subplot(4, 1, 4) xline_thickness(50) plt.tight_layout()
122 #interactive surface plot - not quite working right in terms of z axis from mayavi.mlab import * def surface_layer(x,y,z,title='Contour Plot'): xi = np.linspace(min(x), max(x),max(x)/2) yi = np.linspace(min(y), max(y),max(y)/2) X,Y = np.meshgrid(xi,yi) zi = griddata(x, y, z, xi, yi, interp='linear') s=surf(xi,yi,zi) return s surface_layer(Salina_bottom2.Inline,Salina_bottom2.Xline,Salina_bottom2.Z-Salina_top.Z,'Salina Thickness to Bottom 2 (TWTT ms)')
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