EVOLUTION AND SUBSIDENCE MECHANISMS OF THE NORTHERN
CORDILLERAN FORELAND BASIN DURING THE MIDDLE CRETACEOUS
by
Yongtai Yang
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Department of Geology
In the University of Toronto
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While these forms may be included Bien que ces formulaires in the document page count, aient inclus dans la pagination, their removal does not represent il n'y aura aucun contenu manquant. any loss of content from the thesis. Canada EVOLUTION AND SUBSIDENCE MECHANISMS OF THE NORTHERN
CORDILLERAN FORELAND BASIN DURING THE MIDDLE CRETACEOUS
A Doctor of Philosophy thesis, 2008
by
Yongtai Yang
Department of Geology
University of Toronto
ABSTRACT
Mainly based on detailed allostratigraphic and sedimentologic study of the Early
Cenomanian Fish Scales Formation and the mid-late Cenomanian Belle Fourche Formation in southern Alberta, this study reconstructs the evolution and explores the subsidence mechanisms of the Cordilleran foreland basin during the mid-Cretaceous.
The middle Cretaceous succession consists of tectonically-driven cycles containing strata deposited in an underfilled basin during the orogenic loading and early unloading periods, and strata deposited in an overfilled basin during the late orogenic unloading period.
High-frequency tectonism (106-year time scale) and sediment distribution in the basin are the main causes of the tectonic cyclicity.
The positions of the proximal foredeep, forebulge and backbulge depozones from the Late
Albian to Early Coniacian time are defined in northern Cordilleran foreland basin. During the
Late Albian-Middle Cenomanian, the forebulge extended northwest through the northwest corner of Wyoming and curved northeastward through the present-day location of Calgary. At the end of the mid-Cenomanian, the forebulge retreated westward within Alberta, reflecting the change of convergence vectors along the western margin of North America during the mid-
n Cretaceous. The mid-late Cenomanian forebulge zones controlled the deposition of the upper and lower Belle Fourche sandstones in southern Alberta, southwestern Saskatchewan and northern Montana. This study suggests that the forebulge setting needs to be emphasized as an important regional control in any basin-scale model for offshore sandstones in the Cordilleran foreland basin.
During the late orogenic unloading period, a peripheral sag was formed in front of the uplifted orogenic belt and with a depocenter located in the forebulge zone of the preceding orogenic loading period and early unloading period. The Fish Scales and Second White Specks formations, formed during marine transgressions in the latest Albian-earliest Cenomanian and the Early Turonian, are interpreted to have been deposited in wide shallow interior seaways during late orogenic unloading periods. This study calls into question the general significance of units such as the Mowry Shale and the Greenhorn Formation as indicators of eustatic high-stands of sea level, and suggests regional tectonism as an alternative or additional control on basin evolution.
iii ACKNOWLEDGMENTS
I am grateful to my supervisor Professor Andrew Miall, for providing me a great opportunity to study at the U. of T., professional guidance and advice, and financial support.
His course "Seminars in Basin Analysis" shown me how to do basin analysis professionally, and is and will be an asset for my professional career. I am also grateful to my Ph.D. committee members, Sandy Cruden, Nick Eyles, Russell Pyaklywec, and Uli Wormian in the
Geology Department for their effort to keep me on track.
I thank the external appraiser Peter DeCelles and reviewers of our papers: Octavian
Catuneanu, Brian Currie, Paul Heller, Terry Jordan, Brendan Murphy, James Schmitt, Glen S.
Stockmal, Brian Turner, and Tim White, for their critical review and much help in improving this thesis.
I thank Guy Plint, Bogdan Varban, Aditya Tyagi and other members in Basin Analysis
Group, University of Western Ontario, for their many good suggestions and much help in well- log selection and core measurement.
I would like to thank my wife Meng and my mother-in-law Xiuying for taking care of my daughter Victoria, so I could concentrate on this Ph.D. thesis.
IV TABLE OF CONTENTS
CHAPTER 1. INTRODUCTION 1
CHAPTER 2. EVOLUTION OF THE NORTHERN CORDILLERAN FORELAND BASIN
DURING THE MIDDLE CRETACEOUS 8
ABSTRACT
INTRODUCTION
STRATIGRAPHY
ALLOSTRATIGRAPHY
COLORADO GROUP IN ALBERTA AND MONTANA
Upper Albian
Lower Cenomanian
Middle-Upper Cenomanian
RECONSTRUCTION OF THE MID-CRETACEOUS FORELAND BASIN
Late Albian
Early Cenomanian
Middle Cenomanian
Late Cenomanian
DISCUSSION
CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES
CHAPTER 3. MIGRATION AND STRATIGRAPHIC FILL OF A FORELAND BASIN
SYSTEM: MID-LATE CENOMANIAN BELLE FOURCHE FORMATION IN THE
NORTHERN CORDILLERAN FORELAND BASIN 59
v ABSTRACT
INTRODUCTION
ALLOSTRATIGRAPHY
SEDIMENTOLOGY
A QUALITATIVE MODEL
DISCUSSION
CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES
CHAPTER 4. MARINE TRANSGRESSIONS IN THE MID-CRETACEOUS OF THE
CORDILLERAN FORELAND BASIN RE-INTERPRETED AS OROGENIC
UNLOADING DEPOSITS 93
ABSTRACT
INTRODUCTION
ALLOSTRATIGRAPHY
Typical fades successions
Regional distribution of allomembers
BASIN ANALYSIS
DISCUSSION
CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES
CHAPTER 5. TECTONIC CYCLES OF FORELAND BASINS 133
ABSTRACT
vi INTRODUCTION
STRATIGRAPHIC CYCLE DRIVEN BY OROGENIC LOAD AND SEDIMENT LOAD
TECTONIC CYCLES IN THE NORTHERN CORDILLERAN FORELAND BASIN
CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES
vn LIST OF FIGURES
Figure 1.1. The two-phase stratigraphic model for foreland basins, (A) orogenic loading period;
(B) orogenic unloading period.
Figure 2.1. Location of study area. (A) Thickness of the Late Albian to Santonian Colorado
Group in the northern Cordilleran foreland basin. (B) Location of 1480 well logs in
southern Alberta and 35 well logs in northwestern Montana.
Figure 2.2. Stratigraphic nomenclature and microfaunal zones of the Late Albian to Turonian
strata in the northern Cordilleran foreland basin.
Figure 2.3. The distribution of the time equivalent Belle Fourche Formation in southern
Alberta and western Montana.
Figure 2.4. Allostratigraphy of the Belle Fourche Formation in Well 6-4-17-13 W4.
Figure 2.5. A west-east cross section of the Belle Fourche Formation in southern Alberta.
Figure 2.6. A south-north cross section of the Belle Fourche Formation in southern Alberta.
Figure 2.7. Isopach maps of five allomembers of the Belle Fourche Formation in southern
Alberta.
Figure 2.8. Cross Section of the Cenomanian strata in the northern Cordilleran foreland basin.
Figure 2.9. (A) The Late Albian foreland basin system in southern Canada. (B) The latest
Albian foreland basin system in southern Canada. (C) The Late Albian foreland basin
system in northern US.
Figure 2.10. The reconstructed Middle Cenomanian foreland basin system in western North
America.
Figure 2.11. The reconstructed Late Cenomanian foreland basin system in western North
America.
viii Figure 3.1. (A) Restored Middle and Late Cenomanian foreland basin systems in western
North America (Chapter 2), mainly showing the forebulge zones. (B) Isopach map of the
Late Cenomanian Upper Belle Fourche Formation in southern Alberta (Chapter 2),
showing location of 1480 well logs in southern Alberta and 35 well logs in northwestern
Montana.
Figure 3.2. Stratigraphic nomenclature of the Early Cenomanian -Early Coniacian strata in the
northern Cordilleran foreland basin.
Figure 3.3. Allostratigraphy of the Belle Fourche Formation in Well 9-13-15-12W4.
Figure 3.4 A cross section of the Belle Fourche Formation in southern Alberta.
Figure 3.5 A cross section of the Belle Fourche Formation in southern Alberta.
Figure 3.6 A cross section of the Belle Fourche Formation in southern Alberta.
Figure 3.7. Isopach maps of allomembers B-E2 of the Upper Belle Fourche Formation in
southern Alberta.
Figure 3.8. Core photograph in Well 9-13-15-12W4 (596.46-601.29 m).
Figure 3.9. Sand distribution of the Upper Belle Fourche Sandstone in Allomember E2.
Figure 3.10. A qualitative model showing migration and stratigraphic fill of a foreland basin
system during the orogenic loading period (A), early orogenic unloading period (B) and
last period of early orogenic unloading period (C).
Figure 3.11. Allostratigraphy of the Cardium Formation, Alberta Basin, modified from Plint et
al. (1986) and Walker and Plint (1992).
Figure 3.12. The reconstructed Late Turonian-Early Coniacian foreland basin system in
western North America.
ix Figure 4.1. Location of study area, (A) showing the latest Albian-earliest Cenomanian Mowry
Seaway and Early Turonian Greenhorn Seaway (Williams and Stelck, 1975); (B) showing
location of 1480 well logs used in the study.
Figure 4.2. Stratigraphic nomenclature of the Late Albian-Early Turonian strata in the
Cordilleran foreland basin.
Figure 4.3. The distribution of the time equivalent Fish Scales Formation in southern Alberta.
Figure 4.4. Allostratigraphy and typical facies successions of the Fish Scales Formation.
Figure 4.5. Core photographs. (A) Photograph in well 16-24-8-23W4. (B) Photograph of
Allomember II in well 1-11-3-23W4. (C) Photograph in well 16-24-13-26W4. (D)
Photography in well 6-4-17-13.
Figure 4.6. (A-D) Cross sections in southern Alberta. (E) Enlarged lithologic columns of core
in A-D.
Figure 4.7. (A) Isopach map of Allomember I of the Fish Scales Formation in southern
Alberta. (B) Interpreted paleography during the deposition of Shingle 3 of Allomember I of
the Fish Scales Formation in southern Alberta.
Figure 4.8. Isopach map of Allomember II of the Fish Scales Formation in southern Alberta.
Figure 4.9. (A) Restored Late Albian-Late Cenomanian foreland basin systems in the northern
Cordilleran foreland basin (Chapter 2). (B) Isopach map of the Middle Cenomanian Lower
Belle Fourche Formation in southern Alberta. (C) Isopach map of the Late Cenomanian
Upper Belle Fourche Formation in southern Alberta.
Figure 4.10. Foreland basin model showing stratigraphic response of the Fish Scales
Formation during the orogenic unloading period in the Early Cenomanian.
Figure 4.11. The distribution of the time equivalent Second White Specks Formation in
southern Alberta.
x Figure 4.12. Photograph of the Second White Specks Formation in well 6-4-17-13 (631.70-
635.61 m).
Figure 5.1. A qualitative model showing migration and stratigraphic fill of a foreland basin
during the orogenic loading period (A), early orogenic unloading period (B) and late
orogenic unloading period (C).
Figure 5.2. Chronostratigraphic chart of the late Albian-early Coniacian strata in the northern
Cordilleran foreland basin.
Figure 5.3. Restored late Albian-early Coniacian foreland basin systems in western North
America.
Figure 5.4. Cross section constructed from 12 well-logs in southern Alberta.
xi CHAPTER 1
INTRODUCTION
STATEMENT OF CO-AUTHORSHIP
Chapter 2. Evolution of the northern Cordilleran foreland basin
during the middle Cretaceous
Authorship: Yongtai Yang, Andrew Miall
In press in: Geological Society of America Bulletin
Contribution of authors:
Yongtai Yang selected 1515 well-logs and did well-log correlation; measured 61 cores; wrote text and drew figures.
Andrew Miall had a thorough review, gave suggestions and helped to edit the text.
Chapter 3. Migration and stratigraphic fill of a foreland basin system: mid-late
Cenomanian Belle Fourche Formation in the northern Cordilleran foreland basin
Submitted to a journal
Authorship: Yongtai Yang, Andrew Miall
Contribution of authors:
Yongtai Yang selected 1515 well-logs and did well-log correlation; measured 61 cores; wrote text and drew figures.
Andrew Miall had a thorough review, gave suggestions and helped to edit the text.
Chapter 4. Marine transgressions in the mid-Cretaceous of the Cordilleran
foreland basin re-interpreted as orogenic unloading deposits
Authorship: Yongtai Yang, Andrew Miall
In press in: Bulletin of Canadian Petroleum Geology
Contribution of authors:
1 Yongtai Yang selected 1500 well-logs and did well-log correlation; measured 60 cores; wrote text and drew figures.
Andrew Miall had a thorough review, gave suggestions and helped to edit the text.
Chapter 5. Tectonic cycles of foreland basins
Authorship: Yongtai Yang, Andrew Miall
Submitted to a journal
Contribution of authors:
Yongtai Yang wrote text and drew figures.
Andrew Miall had a thorough review, gave suggestions and helped to edit the text.
TWO-PHASE STRATIGRAPHIC MODEL FOR FORELAND BASINS
The two-phase stratigraphic model has been proposed for the stratigraphic fill in foreland basins, suggesting a rapid basin subsidence in the synorogenic phase succeeded by flexural rebound of the thrust belt and proximal part of the foreland basin in the postorogenic phase
(Heller et al., 1988) (Fig. 1.1). The strata produced during the orogenic loading (synorogenic) period show a typical foreland basin system as defined by DeCelles and Giles (1996), with foredeep, forebulge and back-bulge depozones, but those produced during an orogenic unloading (postorogenic) period show a peripheral sag in front of the uplifted orogenic belt and proximal part of foredeep zone of the preceding orogenic loading period (Jordan and Flemings,
1991; Beaumont et al., 1993; Catuneanu et al, 1998).
SCIENTIFIC PROBLEMS AND THE MAIN OBJECTIVES OF THIS RESEARCH
Evolution of the northern Cordilleran foreland basin in the mid-Cretaceous
In southern Alberta and northwestern Montana, the Late Albian-Santonian Colorado Group consists primarily of relatively thin marine shale, and does not show the typical isopach pattern of a foreland basin with a thick foredeep deposit. Structural geologists suggested that the Late
2 Cretaceous-Paleocene convergence resulted in intense northeast-verging folding and thrusting
in the Rocky Mountain fold and thrust belt (Price and Sears, 2000). Therefore, it is suspected
that a substantial volume of foreland basin strata was uplifted and cannibalized in the present
Rocky Mountains.
Based on detailed allostratigraphic study of the mid-late Cenomanian Belle Fourche
Formation in Chapters 2 and 3 and the Early Cenomanian Fish Scales Formation in Chapter
4, this study reconstructed the mid-Cretaceous Cordilleran foreland basin in western North
America. Chapter 2 and Chapter 3 define the position of the proximal foredeep, forebulge
and backbulge depozones in the northern Cordilleran foreland basin in the Late Albian-
Cenomanian time and Late Turonian-Early Coniacian time, respectively. The foredeep depozone and part of the forebulge depozone in the present Rocky Mountains in southern
Canada and northern United States have been uplifted and cannibalized by post-depositional thrusting and shortening during the Late Cretaceous-Paleocene. Chapter 4 suggests that the
Fish Scales Formation, and the Turonian mid-upper Kaskapau Formation and the lower part of the Cardium Formation were deposited during late orogenic unloading periods. Therefore, the marine shale of the Colorado Group in southern Alberta and northwestern Montana consists mainly of strata deposited in back-bulge zones during orogenic loading and early orogenic unloading periods and strata deposited in peripheral sags during late orogenic unloading periods.
Main control of the stratigraphic fill in the Cordilleran foreland basin
It is commonly a challenge to separate the possible effects of eustatic sea-level change from those of relative sea-level change generated by tectonism. Jordan and Fleming (1991) suggested that a cycling between tectonic activity and quiescence can produce stratigraphic features in a foreland basin similar to those generated by eustasy, including the subaerial
3 erosional surface, transgression and regression. Detailed documentation of stratigraphic architecture and isopach patterns, based on a tightly defined allostratigraphy, are the key to unraveling the tectonic and eustatic history of the basins (Ryer, 1993; Jordan, 1995).
Detailed allostratigraphic study of the mid-Cretaceous in the northern Cordilleran foreland basin show that it is composed of tectonically-driven cycles containing strata deposited in an underfilled basin during the orogenic loading and early unloading periods and strata deposited in an overfilled basin during the late orogenic unloading period. Chapter 2 proposes that the latest Albian Westgate Formation and the Cenomanian Lower and Upper Belle Fourche
Formation were deposited in an underfilled condition during orogenic loading and early unloading periods. Chapter 3 proposes a qualitative model for the migration and stratigraphic fill of a foreland basin system period and suggests that the upper part of the Cardium
Formation was deposited in an underfilled basin during the Late Turonian-Early Coniacian.
Chapter 4 suggests that the Fish Scales Formation, and the mid-upper Kaskapau Formation and the lower part of the Cardium Formation were deposited in overfilled conditions during late orogenic unloading periods. A qualitative model is proposed in Chapter 5 for the migration and stratigraphic fill of a foreland basin during a tectonic cycle, and three orders of tectonic cycles are defined, reflecting the short- to long-term history of tectonic episodicity and changes in contractional stress patterns during the evolution of the orogen. Therefore, it is clearly indicated that the tectonism is the main control for the stratigraphic fill in foreland basins and calls into question the general significance of units such as the Mo wry Shale and the
Greenhorn Formation as indicators of eustatic high-stands of sea level.
Forming mechanisms of mud-enclosed sandy sediments in the Cordilleran foreland basin
A number of sandstones enclosed in offshore marine mudstone have been identified in the
Cretaceous Cordilleran foreland basin. Various mechanisms have been proposed for the
4 deposition of these sandstones and recently geologists generally interpreted them as prograding shorefaces deposited during eustatic lowstands of sea-level (Plint et al., 1986).
This project studied the Barons Sandstone and the Belle Fourche Sandstone in southern
Alberta to explore the forming mechanism of mud-enclosed sandy sediments in the Cordilleran foreland basin. In Chapter 3, the Belle Fourche Sandstone is interpreted to have been transported by storms from the foredeep zone to the backbulge zone and deposited along the flank of the uplifting forebulge zone. The so-called lowstand shoreface conglomerates in the upper part of the Cardium Formation are interpreted to be forebulge controlled deposits. In
Chapter 4, the Barons Sandstone, interbedded within the mudstone of the Fish Scales
Formation, is interpreted as a shoreface, distributary channel and barrier island sandstone deposited during shoreline regression in an orogenic unloading period.
REFERENCES
Beaumont, C, Quinlan, G., and Stockmal, G.S., 1993, The evolution of the western interior basin:
causes, consequences and unsolved problems, in Caldwell W.G. and Kauffman, E.G., eds.,
Evolution of the Western Interior Basin: Geological Association of Canada Special Paper, 39, p.
97-117.
Catuneanu, O., Hancox, PJ. and Rubidge, B.S., 1998, Reciprocal flexural behaviour and contrasting
stratigraphies: a new basin development model for the Karoo retroarc foreland system, South Africa:
Basin Research, v. 10, p. 417-439.
DeCelles, P.G., and Giles, K.A., 1996, Foreland basin system: Basin Research, v. 8, p. 105-123.
Heller, P.L., Angevine, C.L., Winslow, N.S., and Paola, C, 1988, Two-phase model of foreland basin
sequences: Geology, v. 16, p. 501-504.
Jordan, T.E., 1995, Retroarc Foreland and Related Basins, in Busby, C. and Ingersoll, R., eds.,
Tectonics of Sedimentary Basins: Blackwell Scientific Publications, p. 331-362.
5 Jordan, T.E., and Flemings, P.B., 1991, Large-scale stratigraphic architecture, eustatic variation, and
unsteady tectonism: A theoretical evaluation: Journal of Geophysical Research, v. 96. p. 6681-6699.
Plint, A. G., Walker, R. G., and Bergman, K. M., 1986, Cardium Formation 6: Stratigraphic framework
of the Cardium in subsurface: Bulletin of Canadian Petroleum Geology, v. 34, p. 213 - 225.
Price, R.A., and Sears, J.W., 2000, A preliminary palinspastic map of the Mesoproterozoic Belt-Purcell
Supergroup, Canada and USA: implications for the tectonic setting and structural evolution of the
Purcell anticlinorium and Sullivan deposit, in Lydon, J.W., et al., eds., The Sullivan Deposit and its
Geological Environment: Geological Association of Canada, Mineral Deposits Division, p. 61-81.
Ryer, T. A., 1993, Speculations on the origins of mid-Cretaceous clastic wedges, central Rocky
Mountain region, United States, in Caldwell, W. G. E. and Kauffman, E. G. eds., Evolution of the
Western Interior Basin: Geological Association of Canada Special Paper 39, p. 189-198.
6 A: Orogenic loading period
Foredeep Forebulge Backbulge •'•*' . -: \ -= -Z •: _ - Datum p^Qrogenic belt™ "
c ,* S* * ;• ••*• '-i •'>* "•
B: Orogenic unloading period
Peripheral sag -Datum
Figure 1.1. The two-phase stratigraphic model for foreland basins, showing surface profiles in
orogenic loading period (A) and orogenic unloading period (B) (modified from Heller et al.,
1988; Jordan and Flemings, 1991; Beaumont et al., 1993; DeCelles and Giles, 1996;
Catuneanu et al., 1998). CHAPTER 2
EVOLUTION OF THE NORTHERN CORDILLERAN FORELAND BASIN DURING
THE MIDDLE CRETACEOUS
Yongtai Yang, Andrew D. Miall
In press in the Geological Society of America Bulletin
ABSTRACT
A detailed study of the Middle-Late Cenomanian Belle Fourche Formation in southern
Alberta helps to elucidate the evolution of the Cordilleran foreland basin during the middle
Cretaceous. By combining isopach data from the study area with previously published research on middle Cretaceous stratigraphy in adjacent areas of northern Alberta and Montana, it is possible to define the position of the proximal foredeep, forebulge and backbulge depozones from the Late Albian to Cenomanian time.
The foredeep was probably located to the west of the present fold and thrust belt in western Montana from the late Albian to middle Cenomanian, with the forebulge extending northwest through the northwest corner of Wyoming and curving northeastward through the present-day location of Calgary. Thick Dunvegan deposits (middle Cenomanian) of northwestern Alberta are interpreted as foredeep deposits. Palinspastic reconstruction of the southern Rocky Mountains of Alberta is consistent with a location of the foredeep within the area of the present fold-thrust belt.
At the end of the middle Cenomanian the forebulge retreated westward within Alberta, defining a nearly straight northwest-southeast trend through the present position of the
Foothills and Front Ranges of Alberta and northwestern Montana. The change in trend of the foreland basin at the end of the middle Cenomanian may reflect the change of convergence vectors along the western margin of North America during the middle Cretaceous.
8 INTRODUCTION
Quantitative flexural models have demonstrated that the subsidence in the Cretaceous
Cordilleran foreland basin was mainly driven by the lithospheric loading of Cordilleran thrust sheets (Beaumont, 1981; Jordan, 1981). Flexural subsidence and sedimentary infilling in the basin can be correlated with thrusting events. For example, the thick deltaic and coastline deposits of the Late Albian-Santonian Colorado Group in northwestern Alberta (Leckie and
Smith, 1992; Bhattacharya, 1994; Leckie et al., 1994), and in western Montana (Schwartz,
1982; Schwartz and DeCelles, 1988; Dyman et al., 1997) reflect intense flexural subsidence of the foreland basin during the middle Cretaceous (Figs. 2.1 A, 2.2). However, in southern
Alberta and northwestern Montana, the Colorado Group consists primarily of marine shale with an almost uniform thickness of 500-700 m, and does not show the typical isopach pattern of a foreland basin with a thick foredeep deposit (Fig. 2.1 A). At times of widespread shale deposition through northern Montana and southern Alberta, the area may not have experienced much flexural subsidence, with sedimentary accommodation reflecting eustatic sea-level change and regional downflexing of the North American craton (e.g. McMechan and
Thompson, 1993; Pang and Nummedal, 1995).
A foreland basin system comprises four distinct zones, the wedge-top, foredeep, forebulge and backbulge depozones (DeCelles and Giles, 1996). The position, orientation and tectonic evolution of these components are dependent on the orientation and rate of convergence of the supracrustal load. A synthesis of stratigraphic and structural data for the US portion of the
Cordilleran foreland basin enabled DeCelles (2004) to reconstruct the position of the foredeep and forebulge at several times during the Late Jurassic to Eocene evolution of that basin.
Isopach patterns outline the changing positions of the foredeep and forebulge of the foreland basin in response to the changes in orientation of crustal stress. Preservation of a stratigraphic
9 record over the forebulge may require that accommodation was available as a result of high eustatic sea levels or subsidence due to dynamic topography, or both. This paper attempts to make a comparable reconstruction of the Montana-Alberta portion of the basin, based on a synthesis of stratigraphic data.
The reconstructed displacement history between western North America and adjacent oceanic plates shows that along the western margin of North America, convergence was in a more-or-less east-west direction at a velocity of about 100 km/m.y. during the Late Jurassic-
Early Cretaceous and in a northeast-southwest direction at a rate of about 200 km/m.y. during the Late Cretaceous-Eocene (Engebretson et al., 1985). This change is reflected in the structural evolution of the Front Range fold-thrust belt. The formation of several northwest- dipping reverse faults in the southern Canadian Cordillera has been attributed to the Late
Jurassic-Early Cretaceous phase of convergence (Price and Sears, 2000) (Fig. 2.1 A). The Late
Cretaceous-Paleocene convergence resulted in intense northeast-verging folding and thrusting in the Rocky Mountain fold and thrust belt, and horizontal shortening of more than 250 km across the belt near latitude 53°N (Price 1994; Price and Sears, 2000) (Fig. 2.1 A).
The middle-late Cenomanian Belle Fourche Formation of southern Alberta has been chosen as the main focus for this paper, in order to explore the evolution and subsidence mechanisms of the northern Cordilleran foreland basin during the middle Cretaceous (Fig. 2.2). We investigated an area of about 60,000 km in southern Alberta, using data from 61 cores and
1515 wells including 35 wells in northwestern Montana (Fig. 2.IB) in order to establish a detailed allostratigraphy for this unit. Based on the more subtle stratigraphic features of the forebulge and backbulge zones, where strata are well preserved, this study focuses on deducing the position of the foredeep zone and the evolutionary history of the northern Cordilleran foreland basin during the middle Cretaceous. It is suggested that the foredeep zone of the
10 foreland basin would have been located within the area of the Front Ranges of the Rocky
Mountains during the middle Cretaceous, but is now mostly missing due to post-depositional erosion and cannibalization. This paper provides an example for studies in other foreland basins in which facies patterns and the geometries of strata may be used to deduce the subsidence patterns and basin evolution through time.
STRATIGRAPHY
A stratigraphic framework for the undifferentiated shale of the Colorado Group in the subsurface of southern Alberta and southern Saskatchewan includes, in ascending order, the
Westgate, Fish Scales, Belle Fourche and Second White Specks Formations (Bloch et al., 1993;
SchroderAdams et al., 1996) (Fig. 2.2). The Belle Fourche Formation consists of shale and siltstone coarsening upward to fine-grained sandstone near the top and in general conformably overlies the Fish Scales Formation (Bloch et al., 1993). The Belle Fourche Formation is subdivided into lower and upper parts (Ridgley et al., 2001; Pedersen, 2004; Tyagi et al., 2007).
The Lower Belle Fourche Formation contains Verneuilinoides perplexus foraminiferal zone and Conlinoceras tarrantense- Conlinoceras gilberti ammonite zones, and is of Middle
Cenomanian age (Ridgley et al., 2001; Tyagi et al., 2007) (Fig. 2.2). The Upper Belle Fourche
Formation contains Spiroplectammina ammovitrea foraminiferal zone which equates approximately to the Late Cenomanian Dunveganoceras pondi- Dunveganoceras conditum ammonite zones, and is of Late Cenomanian age (Ridgley et al., 2001; Tyagi et al., 2007) (Fig.
2.2). On the basis of our detailed well-log correlation, an isopach map of the Belle Fourche
Formation in southern Alberta has been constructed (Fig. 2.3). In general, it shows a markedly northwest trending prismatic geometry, thinning from 50-70 m in a central thick zone southwestwards to less than 30 m and northeastwards to less than 40 m.
ALLOSTRATIGRAPHY
11 An allostratigraphic unit is defined and identified as a mappable body of rocks bounded by discontinuities (NACSN, 1983). These bounding discontinuities can include unconformities, disconformities, omission surfaces, discontinuity surfaces and flooding surfaces (Bhattacharya and Walker, 1991). Allostratigraphic methods have been used for many subdivision of the
Cretaceous rocks of western Canada, and major flooding surfaces correlatable over hundreds of kilometres are usually selected as representing chronostratigraphically significant bounding discontinuities (e.g. Bhattacharya and Walker, 1991; Plint, 2000).
This study also uses widespread flooding surfaces as the boundaries of allomembers and subdivides the Belle Fourche Formation into five allomembers, A-E (Figs. 2.4, 2.5A, 2.6A).
Our Allomember A and Allomembers B-E correspond to the Lower and Upper Belle Fourche
Formation named by Tyagi et al. (2007), respectively (Fig. 2.2). These allomembers coarsen upward from dark shale to bioturbated or laminated shaly siltstone. In southeastern Alberta and southwestern Saskatchewan, there are hummocky cross stratified or parallel laminated fine sandstones at the top of Allomember A and Allomember E, which are important reservoirs for biogenic gas.
Allomember A is mainly composed of shale with thin shaly siltstone and fine sandstone at the top (Figs. 2.4, 2.5A, 2.6A). It forms a thinning wedge, decreasing from 20 m at the
Alberta/Saskatchewan border westwards to 0-8 m in southwestern Alberta (Fig. 2.7A). Its isopach lines trend north in southeastern Alberta but north-northwest in southwestern Alberta.
Well-log correlation in southwestern Saskatchewan shows that the Lower Belle Fourche
Formation (Allomember A), with a thickness of 20-45 m, extends beyond central southern
Saskatchewan (Pedersen, 2004) (Fig. 2.8).
Allomembers B and C mainly consist of shale coarsening upwards into shaly siltstone (Figs.
2.4, 2.5A, 2.6A). They have similar isopach patterns, with a northwest-trending thick zone near
12 the southern Alberta Foothills thinning northeastwards to several meters at the
Alberta/Saskatchewan border (Fig. 2.7B, C). The thickest zones show a southwestward tapering tendency towards the southwestern corner of the study area.
Allomember D mainly consists of shale coarsening upwards into shaly siltstone (Figs. 2.4,
2.5A, 2.6A). It shows a markedly northwest-trending prismatic geometry, thinning from 16-20 m in the central portion of the study area southwestwards to 0 m and northeastwards to less than 2 m (Fig. 2.7D).
Allomember E is mainly composed of shale and shaly siltstone with fine sandstone at the top (Figs. 2.4, 2.5A, 2.6A). Its thick zone trends northwest in the northeastern part of the study area with a tapering tendency northeastwards, and thins southwestwards to a northwest trending thin zone with thickness of 0-4 m in southwestern Alberta (Fig. 2.7E). Note that
Allomember E gradually thickens southwestward to 8 m in the southwestern corner of the study area.
Compared with isopach maps of the other four allomembers of the Belle Fourche
Formation, that of Allomember A has a notably different pattern (Fig. 2.7). The isopach lines of Allomember A trend mainly north in southeastern Alberta and show a north-northwest trending thin zone about 150-200 km wide in southwestern Alberta. However, it can be seen that the thin zones and thick zones shift progressively northeastwards from Allomember B to E
(Figs. 2.5, 2.6, 2.7). Younger strata of the Upper Belle Fourche Formation onlap underlying
Allomember A northeastwards and the whole Upper Belle Fourche Formation thins northeastwards. The gradually-thinning Upper Belle Fourche Formation dies out in central southern Saskatchewan (Pedersen, 2004) (Fig. 2.8). Therefore, we disagree with the suggestion by Ridgley et al. (2001) that the thinning of the Upper Belle Fourche Formation is because of a
13 widespread unconformity between the Belle Fourche Formation and the Second White Specks
Formation in southern Alberta, southern Saskatchewan and northern Montana.
COLORADO GROUP IN ALBERTA AND MONTANA
In order to develop a regional picture of the evolution of the Belle Fourche Formation we discuss here the correlation of this unit with older and younger strata in Montana and elsewhere in Alberta. In northeastern British Columbia and northwestern Alberta, the Late Albian-
Santonian Colorado Group has a thickness of more than 1150 m and its isopach lines intersect the Rocky Mountain thrust belt at a high angle (Leckie and Smith, 1992) (Figs. 2.1 A, 2.2). In western Montana the Colorado Group has a maximum thickness of 2,500 m and it thins eastwards markedly to 411 m in the Wolf Creek area (Schmidt, 1978; Schwartz and DeCelles,
1988) (Fig. 2.1A).
Upper Albian
The Late Albian Joli Fou and Viking Formations thin from 160 m in northeastern British
Columbia to 20-40 m in central Alberta and then thicken to 60-160 m in southern Alberta and southern Saskatchewan (Reinson et al., 1994) (Figs. 2.2, 2.9A). The latest Albian Shaftesbury
Formation has a thickness of 500 m in northeastern British Columbia and its equivalent
Westgate Formation is 0-20 m thick in central Alberta and 140 m in southwestern
Saskatchewan (Leckie et al., 1994) (Figs. 2.2, 2.9B). Well-log cross sections in southern
Alberta show thinning and pinch-out of the Westgate Formation westwards to the Alberta
Foothills (Fig. 2.5A). Using outcrop and subsurface sections, Glaister (1959) correlated the
Early Cretaceous strata in central and southern Alberta and northwestern Montana, and Lang and McGugan (1988) correlated the Late Albian-Early Turonian strata in southern Alberta and northwestern Montana (Fig. 2.2). These studies show a remarkable feature of the isopach lines of the Late Albian strata curving from a northeast trend in southern Alberta to a northwest
14 trend in northwestern Montana. Lang and McGugan (1988) suggested that the present north- northwest trending Sweetgrass Arch in northwestern Montana may have been tectonically active during the middle Cretaceous because of the rapid thinning of the Vaughn Member of the Blackleaf Formation from more than 210 m in northwestern Montana northeastwards to 0 m in central northern Montana (Fig. 2.2). A series of north-northwest trending intraforeland uplifts developed in southwestern Montana during the Early Cretaceous-early Late Cretaceous and were interpreted as possible manifestations of migratory forebulge uplift (Schwartz, 1982,
Schwartz and DeCelles, 1988) (Figs. 2.9C, 2.10).
Lower Cenomanian
The Early Cenomanian Fish Scales Formation is a regional stratigraphic marker in the
Western Interior of Canada and is mainly composed of silty mudstone with abundant fish remains and bentonite beds (Figs. 2.2, 2.8). This formation was correlated to the upper part of the Bootlegger Formation and Mowry Formation in northern Montana where it mostly consists of interbedded and interlaminated sandstone, siltstone and sandy shale with abundant fish scales (Glaister, 1959; Cobban et al., 1976; Lang and McGugan, 1988) (Fig. 2.2).
Middle-Upper Cenomanian
Detailed well-log correlation shows that the Dunvegan Formation and the lower part of the
Kaskapau Formation in northeastern British Columbia and northwestern Alberta are correlative to the Belle Fourche Formation in central and southern Alberta (Tyagi et al., 2007) (Figs. 2.2,
2.8). The Dunvegan Formation contains Verneuilinoides perplexus foraminiferal zone and can be correlated with the Lower Belle Fourche Formation (Allomember A). It is a large deltaic complex comprising ten coarsening-upward allomembers, showing a southeastward thinning tendency (Bhattacharya and Walker, 1991; Bhattacharya, 1994; Plint, 2000; Tyagi et al., 2007)
(Fig. 2.10). The lower Kaskapau Formation shallow marine mudstones consist of A-X, Doe
15 Creek, Pouce Coupe and Unit I units and can be correlated to the Upper Belle Fourche
Formation (Allomember B-E) (Tyagi et al., 2007) (Figs. 2.2, 2.8). The Dunveganoceras ammonite zone, indicative of a Late Cenomanian age, was found in the Pouce Coupe and the
Unit I units in northwestern Alberta (Warren and Stelck, 1940; Varban and Plint, 2005). The
A-X unit exhibits a wedge shape, thinning from about 50 m in northeastern British Columbia to about 10 m in central northern Alberta (Plint, 2000). An isopach map of the Doe Creek unit also shows a wedge shape, thinning from 115 m in northeastern British Columbia to less than 5 m in central northern Alberta (Kreitner and Plint, 2006). The Pouce Coupe unit with a thickness of 0-100 m occurs mainly in northeastern British Columbia and dies out at the British
Columbia/Alberta border (Kreitner and Plint, 2006) (Figs. 2.8, 2.11). Unit I shows a wedge shape, thinning from about 110 m in northeastern British Columbia to about 10 m in northwestern Alberta (Varban and Plint, 2005). The Upper Belle Fourche Formation in central
Alberta thins eastwards from about 70-80 m near the foothills to 4-6 m near the
Alberta/Saskatchewan border (Tyagi et al, 2007) (Figs. 2.8, 2.11).
In central and southern Alberta Foothills there is an unconformity between the Sunkay
Member of the Blackstone Formation and the Blairmore Group (Stott, 1963, 1984) (Fig. 2.2).
The Sunkay Member composed of marine shale interbedded with siltstone and very fine sandstone shows a northward thickening tendency, with a minimum thickness of 2-5 m on the
Castle River, thickening to 62 m on the Ghost River (Stott, 1963, 1984; Leckie et al., 2000)
(Fig. 2.3). The lower part of the Sunkay Member, containing the Verneuilinoides perplexus foraminiferal zone (Caldwell et al., 1978; Tyagi et al., 2007), and the upper part, containing the
Dunveganoceras ammonite zone (Stott, 1963), are of Middle and Late Cenomanian age, respectively (Fig. 2.2). Tyagi et al. (2007) correlated the Sunkay Member and the subsurface
Belle Fourche Formation in central and southern Alberta. Leckie et al. (2000) attributed the
16 Late Albian-Middle Cenomanian unconformity and thickness change of the Sunkay Member in the Alberta Foothills to a paleohigh created mainly by the volcaniclastic sediments of the
Crowsnest Formation during the Late Albian (Fig. 2.2). However, the Crowsnest Formation has a very restricted distribution area near three source areas or pipes for the volcanic detritus in southern Alberta Foothills (Pearce, 1970; Leckie and Burden, 2001).
In northwestern Montana, the Late Cenomanian Dunveganoceras albertense and
Calycoceras canitaurinum ammonite zones have been found in the Floweree Member of the
Marias River Formation (Cobban et al., 1976) (Fig. 2.2). This unit is mainly composed of marine shale interbedded with shaly siltstone and generally shows a southwestward thinning tendency (Fig. 2.3). In the subsurface, it has a thickness of 42-54 m in central northern
Montana (Ridgley et al., 2001), and 16-29 m in northwestern Montana. It is 3-19.4 m thick in the exposed sections near Great Falls, 11.6 m thick in Summit Creek and 9 m in Sun River
Canyon (Cobban et al., 1976). Merewether and Cobban (1986) suggested that a north- northwest-trending uplift developed near Great Falls during the 93-94 Ma period (Fig. 2.3).
Upper Cenomanian-Santonian strata, more than 2000 m thick, are exposed near Drummond,
Montana, and consist of interbedded sandy limestone and sandstone deposited in shallow brackish-water (Wallace et al., 1990) (Fig. 2.3). In the Wolf Creek area, Montana, the Middle-
Late Cenomanian Floweree Member of the Marias River Shale, with a thickness of 18 m, consists of marine shale, sandstone, bentonite and conglomerate and rests disconformably on the Blackleaf Formation (Schmidt, 1978) (Fig. 2.3). Wallace et al. (1990) suggested that there was a barrier in the present location of the Lewis and Clark line during the Late Cretaceous, which separated the sedimentation in different foreland basins north and south of the line (Figs.
2.3, 2.11). The foreland basin to the south of the Lewis and Clark line was located in southwestern Montana.
17 Thick Middle Cenomanian-Early Santonian Frontier Formation is exposed in southwestern
Montana, with a maximum thickness of 2135 m in the Lima Peaks (Dyman et al., 1997) (Figs.
2.2, 2.3, 2.8). U-Pb data for porcellanitic samples from the top of the underlying Blackleaf
Formation indicate that the Frontier Formation was deposited since 95 Ma (Dyman et al.,
1997). Palynologic data show that the lower coarse-clastic unit of the formation is of
Cenomanian age (Dyman et al., 1989, 1997). Therefore, the Lower Frontier Formation was deposited during the Middle-Late Cenomanian, coeval with the Belle Fourche Formation in other areas in northern Great Plains, United States (McGookey, 1972) and in southern Alberta and southern Saskatchewan. This unit shows a northeastward thinning tendency, from 457 m in the Lima Peaks to 102m, 100m and 76m respectively in the Madison Range, northern
Snowcrest Range and eastern Pioneer Mountains (Dyman et al., 1988, 1989; Tysdal, 1991; and
Dyman and Tysdal, 1998) (Figs. 2.3, 2.8). Merewether and Cobban (1986) proposed that the
Middle Cenomanian Acanthoceras ammonite zone is represented by a north-northwest- trending unconformity in southwestern Montana and northwestern Wyoming (Fig. 2.3). The
Lower Frontier Formation in the Lima Peaks area was deposited in a fluvial environment and is mainly composed of nonmarine siltstone, mudstone, sandstone and conglomerate with a strong volcaniclastic component (Dyman et al., 1988, 1989, 1997) (Fig. 2.8). It mainly consists of interbedded mudstone, silty mudstone, sandstone and tuff in the eastern Pioneer Mountains and
Madison Range and was deposited in a broad deltaic environment (Tysdal, 1991; Dyman and
Tysdal, 1998) (Fig. 2.8). Data from fluvial and delta plain sandstones and conglomerates in the
Lower Frontier Formation suggest a north-to-south paleocurrent direction (Dyman et al., 1988).
Because no data support a further subdivision of the Lower Frontier Formation into the Middle and Upper Cenomanian parts, the consistent lithologic and sedimentary characteristics of the
18 unit and its northeastward thinning reflect continuous subsidence and infilling in southwestern
Montana under similar tectonic background during the middle-late Cenomanian.
RECONSTRUCTION OF THE MIDDLE CRETACEOUS FORELAND BASIN
Based on our detailed stratigraphic study for the Belle Fourche Formation in southern
Alberta, combined with tectonic studies in the Cordilleran fold and thrust belt in southern
Canada and the northern United States, especially by Price and Sears (2000), and stratigraphic studies for the equivalent Belle Fourche Formation and older and younger strata in Alberta and
Montana mentioned above, we have restored the evolution of the northern Cordilleran foreland basin for Late Albian-Cenomanian time (Figs. 2.9-2.11).
The Albian-Santonian Colorado Group consists of strata deposited during orogenic loading
(synorogenic or thrusting) periods alternating with strata deposited during orogenic unloading
(postorogenic or tectonic quiescence) periods (Yang and Miall, Chapter 4), in accordance with a two-phase stratigraphic model for foreland basins (Heller et al. 1988; Jordan and Flemings
1991; Beaumont et al., 1993; Catuneanu et al., 1998). The strata produced during the orogenic loading period show a typical foreland basin system as defined by DeCelles and Giles (1996), with wedge-top, foredeep, forebulge and back-bulge depozones. During an orogenic unloading period a peripheral sag is developed in front of the uplifted orogenic belt and the proximal part of the foredeep zone of the preceding orogenic loading period, with a depocenter located over the forebulge zone of the preceding orogenic loading period.
Due to intense post-depositional thrusting and cannibalization in the Rocky Mountains, the precise locations of the western margin of the foreland basin cannot be determined.
Late Albian
Because we have not carried out detailed stratigraphic study for the Upper Albian strata, orogenic loading and unloading deposits for these intervals can not be recognized. Mainly
19 based on published stratigraphic data, according to the definition of the foreland basin system
(DeCelles and Giles, 1996), we present a simple analysis of basin evolution during the Late
Albian here (Fig. 2.9). This reconstruction indicates that the proximal margin of the foredeep may have been located much further to the west than present outcrop patterns would suggest.
The positioning of the foredeep and forebulge is consistent with the significant northwest- southeast shortening in southern Canada during the Early Cretaceous (Price and Sears, 2000; major faults shown in Fig. 2.1 A).
The stratigraphic pattern of the equivalent Joli Fou and Viking Formations indicates that a north-northeast trending foreland basin system was developed in western Canada during the
Late Albian, with a foredeep in northeastern British Columbia and northwestern Alberta, forebulge in central Alberta and backbulge in southern Alberta and Saskatchewan (Fig. 2.9A).
The strata of the Cadotte and Harmon Members in northwestern Alberta consist mainly of marine shale onlapping the forebulge zone southeastwards, and the equivalent Joli Fou
Formation in southern Alberta consists of marine shale onlapping the forebulge zone northwestward (Leckie et al., 1994; Reinson et al., 1994) (Fig. 2.2), probably reflecting deposition during an orogenic loading period. The Paddy Member in northwestern Alberta and the equivalent Viking Formation in southern Alberta are mainly composed of regressive fluvial, coastal plain and coastline deposits (Leckie et al., 1994; Reinson et al., 1994) (Fig. 2.2), probably reflecting uplifting of the proximal foreland basin and cratonward shifting of the depocenter during an orogenic unloading period. The Westgate Formation shale, deposited in the backbulge zone in southern Alberta and southern Saskatchewan, thinning to zero westwards onto the forebulge (Fig. 2.5A), and together with the Shaftebury Formation in northwestern Alberta, deposited in the foredeep zone, reflects a pattern typical of a foreland basin system developed during orogenic loading (Fig. 2.9B). Because the proximal parts of the
20 foredeep depozones have been uplifted and cannibalized, mainly distal marine shale is found in the Albian formations in northeastern British Columbia and northwestern Alberta (Leckie et al.,
1994;Reinsonetal., 1994).
The curving of the Upper Albian from a northeast trend in southern Alberta to a northwest trend in northwestern Montana (Glaister, 1959; Lang and McGugan, 1988) reflects the change of the trend of the Late Albian foreland basin around the international border (Fig. 2.9). It has a north-northwest trend in the northern United States, with a foredeep depozone in eastern Idaho and westernmost Montana, a forebulge depozone in western Montana and back-bulge depozone in eastern Montana (Fig. 2.9C). In the foredeep region in southwestern Montana, the
Blackleaf Formation is 490-1170 m thick and consists mainly of fluvial deposits (Schwartz,
1982). In northern Montana, the rapid thinning of the Vaughn Member of the Blackleaf
Formation northeastwards (Lang and McGugan, 1988) reflects the onlap of the foredeep deposits on the forebulge depozone. The forebulge inherited the Early Cretaceous forebulge developed in southwestern Montana (DeCelles, 2004) and the north-northwest trending intraforeland uplifts reflect the continuous migration of the foreland basin during the Late
Albian (Schwartz, 1982; Schwartz and DeCelles, 1988). In the back-bulge depozone in eastern
Montana, shoreface sandstone and marine shale were deposited with a thickness of about 200-
300 m (McGookey, 1972; Schwartz, 1982; DeCelles, 2004) (Fig. 2.9C).
Early Cenomanian
According to the two-phase stratigraphic model for foreland basins (Heller et al. 1988;
Jordan and Flemings 1991; Beaumont et al., 1993; Catuneanu et al., 1998), we interpret that the Early Cenomanian Fish Scales Formation was deposited during an orogenic unloading period (Yang and Miall, Chapter 4). The Westgate Formation below and the Lower Belle
Fourche Formation (Allomember A) above the Fish Scale Formation in southern Alberta,
21 together with their equivalent strata in northwestern Alberta, exhibit typical isopach patterns of foreland basin systems (Figs. 2.5, 2.6, 2.9B, 2.10). However, the Fish Scales Formation has a relatively constant thickness of 10-20 m (Fig. 2.8). The relationship among the Westgate, Fish
Scales, and Lower Belle Fourche Formations suggest that strata were deposited alternately during orogenic loading periods and unloading periods.
Middle Cenomanian
The reconstructed Middle Cenomanian foreland basin system is drawn on the background of the palinspastic map of the Rocky Mountains in southern Canada and the northern United
States (Price and Sears, 2000) (Fig. 2.10). It trends north-northeast in southern Canada, and changes to a north-northwest trend in the northern United States.
The deltaic Dunvegan Formation was deposited in the foredeep depozone in northeastern
British Columbia and northwestern Alberta, and progressively onlapped the forebulge depozone southeastward (Fig. 2.10). Based on the estimated shortening of about 100 km in the
Rocky Mountain fold and thrust belt and Foothills near 56° N (McMechan and Thompson,
1989), and the distribution of middle Cenomanian strata in the subsurface and outcrop sections
(Plint, 2000), a foredeep zone with a width of about 500-600 km is estimated in northwestern
Alberta. The isopach lines of the Dunvegan Formation are oriented at a high angle to the
Rocky Mountain fold and thrust belt (Fig. 2.10). We therefore suggest that the Middle
Cenomanian foredeep zone can be extended to southeastern British Columbia. The northeastward thinning, northeastward facies change from fluvial to deltaic, and northward paleocurrent direction of the middle-late Cenomanian Lower Frontier Formation in southwestern Montana (Figs. 2.8, 2.10), coupled with the distribution of the thick Colorado
Group in western Montana (Fig. 2.1 A), suggest that a foredeep zone was developed in western
Montana and eastern Idaho during the Middle Cenomanian. Due to the northeast-southwest
22 shortening of the Rocky Mountains during the Late Cretaceous-Paleocene time (Price and
Sears, 2000; Sears, 2001; Ross et al., 2005), the foredeep deposits in southeastern British
Columbia, eastern Idaho and northwestern Montana were uplifted and cannibalized.
A forebulge about 250 km wide was developed in central Alberta and southwestern Alberta
(Fig. 2.10). Well-log correlation shows that the Lower Belle Fourche Formation is 10-18 m thick above the forebulge in central Alberta (Tyagi et al., 2007) and is 0-8 m in southwestern
Alberta and northwestern Montana. Submarine erosion above the forebulge formed a local erosional surface between the Belle Fourche Formation and the underlying Fish Scale
Formation in southwestern Alberta (Yang and Miall, Chapter 4). We also postulate that the
Middle Cenomanian north-northwest-trending unconformity in southwestern Montana and northwestern Wyoming described by Merewether and Cobban (1986) represents a forebulge, just as White et al. (2002) suggested for the Cenomanian-Turonian unconformities in Colorado documented by Merewether and Cobban (1986). This interpretation is in accordance with the formation of intraforeland uplifts in southwestern Montana during the Early-early Late
Cretaceous (Schwartz, 1982; Schwartz and DeCelles, 1988) (Fig. 2.10). The locations of forebulges show that the foreland basin system propagated progressively east-southeastwards in Alberta from the Late Albian to the Middle Cenomanian (Figs. 2.9,2.10).
The Lower Belle Fourche Formation (Allomember A) in southeastern Alberta and its equivalent strata in eastern central Alberta (Tyagi et al., 2007), Saskatchewan (Ridgley et al.,
2001; Pedersen, 2004) and northeastern Montana (Ridgley et al., 2001) were deposited in the extensive back-bulge depozone (Figs. 2.8, 2.10). According to the cross section in southern
Saskatchewan constructed by Pedersen (2004) and our isopach map of Allomember A, a width of about 300 km is estimated from the axis of the backbulge to the crestline of the forebulge
(Fig. 2.10).
23 Late Cenomanian
A palinspastic reconstruction of the Rocky Mountains in southern Alberta and northern
Montana indicates significant shortening in a northeast-southwest direction during the Late
Cretaceous-Paleocene (Price and Sears, 2000) (Fig. 2.11). The reconstruction of the Late
Cenomanian foreland basin system is consistent with the palinspastic reconstruction and the following stratigraphic data.
All the features of the Lower Frontier Formation in southwestern Montana, northeastward thinning, northeastward facies change from fluvial to deltaic, and northward paleocurrent direction, reflect proximal foredeep deposition, onlapping the forebulge depozone northeastward, in accordance with the proposed rapidly subsiding foreland basin to the south of the Lewis and Clark line during the Late Cretaceous (Wallace et al., 1990) (Figs. 2.8, 2.11).
The restored Colorado Group in western Montana (Fig. 2.1 A) also indicates that a foredeep developed in western Montana during the Late Cenomanian. The foredeep depozone extended northwestward to southeastern British Columbia. However, continuous northeast-verging thrusting and shortening of the Rocky Mountains resulted in cannibalization of the Late
Cenomanian foredeep depozone in southeastern British Columbia and the northern United
States (Price and Sears, 2000; Sears, 2001, Ross et al., 2005). The marine shale interbedded with shoreline and delta front sandstone of the Lower Kaskapau Formation in northeastern
British Columbia, with a thickness of about 350 m, represents deposition in the foredeep depozone (Plint, 2000; Varban and Plint, 2005; Kreitner and Plint, 2006) (Fig. 2.8). Based on the estimated shortening of about 100 km in the Rocky Mountain fold and thrust belt and
Foothills near 56° N (McMechan and Thompson, 1989), and the distribution of Upper
Cenomanian in the subsurface and outcrop sections (Plint, 2000; Kreitner and Plint, 2006), a foredeep zone with a width of about 200-250 km is estimated in northwestern Alberta.
24 Based mainly on the thicknesses of the middle-late Cenomanian strata in western Montana, the north-northwest trending Late Cenomanian uplift in northwestern Montana (Merewether and Cobban, 1986), and the isopach map of the Upper Belle Fourche Formation (Allomembers
B-E), a north-northwest trending forebulge about 150 km wide is restored in western Montana and southeastern British Columbia (Fig. 2.11). Although we use the total thickness of the
Upper Belle Fourche Formation to define this forebulge, it is clearly indicated that the formation of the forebulge resulted from propagation of the foreland basin system cratonwards, as suggested by DeCelles and Giles (1996) (Figs. 2.5, 2.6, 2.7). The equivalent Belle Fourche
Formation in northwestern Montana and southern Alberta Foothills cannot be subdivided into the Middle and Upper Cenomanian parts, but based on its thickness in these areas the Upper
Cenomanian strata are 0-10 m above the crestline of the forebulge (Figs. 2.3, 2.11). The forebulge in southeastern British Columbia and northwestern Montana has been uplifted and cannibalized (Price and Sears, 2000; Sears, 2001; Ross et al., 2005). Kreitner and Plint (2006) mentioned that there is a hinge zone at the border between northeastern British Columbia and northwestern Alberta, and the Pouce Coupe unit is mainly distributed west of it (Figs. 2.8,
2.11). The cross-sections in Kreitner and Plint (2006) also show that the sandstone in the Doe
Creek unit is mainly distributed west of the hinge zone. Several allomembers of the overlying
Unit I of the Lower Kaskapau Formation are absent west of the hinge zone but appear abruptly east of it (Varban and Plint, 2005). Kreitner and Plint (2006) attributed the hinge zone to control of the Precambrian basement. However, Ross and Eaton (1999) suggested that there is little direct relationship between the structures of the Precambrian basement and presence and orientation of faults in the sedimentary section in the Alberta basin. In addition, in an earlier study by Plint et al. (1993), it was suggested that at the B.C.-Alberta border there was a
25 forebulge zone which controlled the deposition of the Doe Creek and Pouce Coupe sandstones.
Therefore, we interpret the hinge zone as a migratory forebulge (Fig. 2.11).
The Late Cenomanian back-bulge depozone trends north-northwest, with deposition of the
Upper Belle Fourche Formation in central Alberta (Tyagi et al., 2007) and Allomembers B-E of the Belle Fourche Formation in southern Alberta (Fig. 2.11). In southern Alberta,
Allomembers B-E shows a key isopach pattern of back-bulge accumulation, with regional closure around a central thick zone (DeCelles and Giles, 1996) (Figs. 2.7B-E, 2.11). A width of about 200 km is estimated from the axis of the backbulge to the crestline of the forebulge. The depocenters of the back-bulge depozones propagated progressively cratonwards from
Allomember B to E, thinning and onlapping the eastern margin of the Western Interior Seaway
(Figs. 2.5-2.8). The backbulge strata thin to zero in central southern Saskatchewan (Figs. 2.8,
2.11). Based on the northwest trending tendency of the back-bulge depozone in southern
Alberta, the Upper Belle Fourche Formation with thicknesses of 60-82 m near the central
Alberta Foothills (Tyagi et al., 2007) represents deposition in the axis of the back-bulge, and part of the back-bulge deposition near the forebulge side has been uplifted and cannibalized
(Fig. 2.11). In northwestern Alberta, the Lower Kaskapau Formation east of the forebulge shows a similar pattern and thickness as the Upper Belle Fourche Formation in central Alberta
(Tyagi et al., 2007) and southern Alberta, thinning east-northeastward from about 80-100 m in northwestern Alberta to about 10 m in central northern Alberta (Plint, 2000; Varban and Plint,
2005; Kreitner and Plint, 2006) (Fig. 2.8).
DISCUSSION
It has been argued here that the Cordilleran foreland basin was developed with a north- northeast trend in southern Canada and a north-northwest trend in the northern United States during the Late Albian-Middle Cenomanian (Figs. 2.9, 2.10). During the Late Cenomanian a
26 north-northwest trending foreland basin was developed that extended through southern Canada and the northern United States (Fig. 2.11). The foredeep depozone and part of the forebulge depozone in the present Rocky Mountains in southern Canada and the northern United States were uplifted and cannibalized by post-depositional thrusting and shortening during the Late
Cretaceous-Paleocene. The unconformity between the Sunkay Member and the Blairmore
Group and the northward thickening of the Sunkay Member in the southern and central Alberta
Foothills (Stott, 1963, 1984; Leckie et al., 2000) is related to uplifting of the forebulge zones during the Late Albian-Late Cenomanian (Figs. 2.2,2.3).
Four major northwest-dipping thrust faults in southern Canadian Cordillera, Moyie-Dibble
Creek, St. Mary, Hall Lake and Mount Forster faults, change southwards into west-dipping or southwest-dipping thrust faults in the northern United State (Price and Sears, 2000) (Figs. 2.1 A,
2.10). A series of west-northwest dipping reverse faults have been documented in southeastern
Yukon Territory in the Omineca Belt west of the Canadian Rockies (McMechan and
Thompson, 1993; Price, 1994). Geochemical and geochronological studies of granitic plutons that intruded these thrust faults near the international border and in southeastern Yukon
Territory show that the latest movement on the faults is at about 94 Ma (Baadsgaard et al.,
1961; Archibald et al., 1984; Hoy and van der Heyden, 1988; Price and Sears, 2000; Coulson et al., 2002). By the end of the Middle Cenomanian (about 94 Ma) thrusting and folding west of the Moyie fault had ended (Price and Sears, 2000), approximately consistent with the end of a regional metamorphism and deformation event along the Salmon River suture zone in western Idaho before about 93 m.y. (Lund and Snee, 1988). Therefore, we suggest that the episode of crustal loading and shortening represented by the activity of these faults resulted in the formation of the foreland basin with a north-northeast trend in southern Canada and a north-northwest trend in the northern United States during the Late Albian-Middle
27 Cenomanian. Our interpreted Late Albian-Middle Cenomanian foreland basin in the northern
Cordilleran foreland basin, together with the Early Cretaceous north-northeast trending
foreland basin in the central Cordilleran foreland basin (Currie, 2002), reflects the east-west convergence vector at the North America plate margin during the Late Jurassic-Early
Cretaceous (Engebretson et al., 1985). Because the palinspastic reconstruction by Price and
Sears (2000) was mainly focused on the Late Cretaceous and Paleocene northeast-southwest shortening in the Rocky Mountains, the palinspastic locations of the Moyie-Dibble Creek, St.
Mary, Hall Lake and Mount Forster faults may not reflect their original locations before 94 Ma.
We suggest that the western margin of the Late Albian-Middle Cenomanian foreland basin might have been located east of original locations of these faults before the end of the Middle
Cenomanian.
The northeast-verging thrusting and folding in the Rocky Mountains in southern Canada began after the end of the Middle Cenomanian (about 94 Ma), and is related to the northeast- southwest convergence at the North America margin during the Late Cretaceous-Eocene
(Engebretson et al., 1985). This is consistent with the formation of a north-northwest trending foreland basin in western North America during the Late Cenomanian (Fig. 2.11). We suggest the western margin of the Late Cenomanian foreland basin might have been located northeast of the palinspastic locations of the westernmost northwest trending thrust faults (Purcell,
Redwall, and Lussier) in southern Canada (Fig. 2.11).
We attribute the westward retreat of the forebulges within Alberta at the end of the
Middle Cenomanian not only to the change of thrusting directions of the Cordilleran thrust belt as mentioned above, but also to the increase of orogenic load due to more intense thrusting of the Cordilleran thrust belt since the late Cenomanian (Figs. 2.10, 2.11). The more intense thrusting was caused by higher convergence velocities of about 200 km/m.y. along the western
28 margin of North America during the Late Cretaceous-Eocene compared to that of about 100 km/m.y. during the Late Jurassic-Early Cretaceous (Engebretson et al., 1985). The bigger load as a result of more intense thrusting of the Cordilleran thrust belt resulted in the formation of a foreland basin with a shorter wavelength in the Late Cenomanian than that formed in the
Middle Cenomanian, in accordance with the predictions of numerical modeling (Flemings and
Jordan, 1989; Sinclair et al., 1991). Therefore, the Middle Cenomanian foreland basin system has wider foredeep, forebulge and backbulge depozones. Study of the Karoo foreland basin of
South Africa also shows that the change of orogenic loads resulted in the orogenward migration of the foreland basin system during the Triassic-Middle Jurassic (Catuneanu et al.,
1998).
CONCLUSIONS
Using widely-distributed flooding surfaces as the boundaries of allomembers, the Middle-
Late Cenomanian Belle Fourche Formation is subdivided into five allomembers, A-E in southern Alberta. The Belle Fourche Formation in southern Alberta is correlative to the
Dunvegan Formation and the Lower Kaskapau Formation in northeastern British Columbia and northwestern Alberta, the Sunkay Member of the Blackstone Formation in the central and southern Alberta Foothills, the Floweree Member of the Marias River Formation in northwestern Montana, and the Lower Frontier Formation in southwestern Montana.
The reconstructed Late Albian-Middle Cenomanian foreland basin trends north-northeast in southern Canada, and changes to a north-northwest trend in the northern United States. The formation of the basin was driven by thrusting of northwest-dipping thrust faults in Cordillera in Canada and west-dipping or southwest-dipping thrust faults in Cordillera in the northern
United States. The reconstructed Late Cenomanian foreland basin trends north-northwest in the northern Cordilleran foreland basin. The formation of the basin was driven by northeast-
29 verging thrusting and folding in the Rocky Mountains since the Late Cenomanian. The foredeep zones and part of the forebulge zones of the middle Cretaceous foreland basin in the present Rocky Mountains in southern Canada and the northern United States were uplifted and cannibalized by post-depositional thrusting and shortening during the Late Cretaceous-
Paleocene. The change in trend of the foreland basins at the end of the middle Cenomanian may reflect the change of convergence vectors along the western margin of North America during the middle Cretaceous.
ACKNOWLEDGEMENTS
We thank Guy Plint, Bogdan Varban, Aditya Tyagi and other members in Basin Analysis
Group, University of Western Ontario, for their many good suggestions and much help in well- log selection and core measurement. We also thank Jennie Ridgley in USGS for her review of
Yang's Ph.D. proposal. Feedback from Peter DeCelles and Paul Heller helped to improve earlier version of the manuscript. Critical reviews by Brian Currie, Brendan Murphy, James
Schmitt and Tim White assisted greatly in improving the manuscript.
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38 Figure 2.1. Location of study area. (A) Isopach map of the Late Albian to Santonian Colorado
Group in Alberta and western Saskatchewan (Leckie and Smith, 1992), restored isopach map of the Colorado Group in western Montana (Schwartz and DeCelles, 1988) and the thickness of the Colorado Group in the Wolf Creek (tp. 15N, rge. 4W) in western Montana (Schmidt, 1978).
Cretaceous granitic plutons and major thrust faults of the Cordilleran orogenic belt are shown
(Price and Sears, 2000), MF: Mount Forester fault, HL: Hall Lake fault, SM: St. Mary fault,
MD: Moyie-Dibble Creek fault. (B) Location of 1480 well logs in southern Alberta and 35 well logs in northwestern Montana used to define the allostratigraphy of the Belle Fourche
Formation. Location of well logs used to construct cross sections in Figures 2.5, 2.6 is shown.
39 B
112W 110W
Fig. 2.6
•T.'.:a6-4-iM3yy4 •;/'>.'.":•£.' •'
4^ O
Rio R5 MontanaR'
——Thrust fault
SS) Cretaceous granitic plutons
^ Contour line of the Colorado Group Figure 2.2. Stratigraphic nomenclature and microfaunal zones of the Late Albian to Turonian strata in the northern Cordilleran foreland basin. Stage boundaries and ages are from Kauffman et al. (1993). Foraminiferal zones are after Caldwell et al. (1978); Tyagi et al. (2007).
Ammonite zones are after Obradovich (1993). Nomenclature for column 1 is after Leckie et al.
(1994); Plint (2000); Varban and Plint (2005). 2 after Glaister (1959); Stott (1963). 3 after
Bloch et al. (1993); SchroderAdams et al. (1996); Tyagi et al. (2007). 4 after Cobban et al.
(1976); Lang and McGugan (1988). 5 after Schwartz (1982); Dyman and Tysdal (1998). 6 after
McGookey(1972).
41 1 2 3 4 5 6 Foraminiferal Ammonite Stage zones zones Northwest plains & Southern Alberta Southern Alberta & Northwestern Southwestern American Northern Foothills Foothills Southern Saskatchewan Montana Montana Great Plains
Pseudocla vulina U Opabin Mbr Ferdig Mbr ^mt^ Carlile Fm (part) Carlile Fm (part) sp. JNeocardiocerasjuddii Haven Mbr (Part) Unit ll-V Turanian Hedbergetla IBurmceras cfydense Second White jEuomphaloceras septemseriatun M Greenhorn Fm loetterfei Vimy Mbr Specks Fm Cone Mbr 93.4 I Vascoceras diartianum Dutweganoceras conditum Unit I Spiroplectammina Dunveganoceras a/berfense (part ) Dunveganoceras problimaticum L Pouce Coupe Upper Belle (Part ) L ammovltrea Lower Calycoceras canHaurinum - Kaskapa u F m Doe Creek Fourche Fm
Floweree Frontie r F m Fm .
Dunveqanoceras pondi Maria s Rive r F m Frontier Belle Fourche Fm 94.7 A-X Blackston e F m Sunkay Mbr Mbr Vemeuilinoides Plestacanthoceras wyomingense Lower Belle M perplexus Acanitioceras amphhclum Dunvegan Fm Bell e Fourch Fourche Fm to 96 Acanthoceras beBense Cenomania n Acanthoceras mukhonense ^ E Barren Acanthoceras granerosense Fish Scale Zone ^* v Fish Scales Fm \ S 97.2 Conljnoceras tarrantense - S; 1 Conlinoceras gjlberti >r o Grou p (par t Mowry Fm Miliammina Shaftesbury Fm orowsnesi *-- Westgate Fm manitobensis Volcanics s Interval A-D Late Bow Island/ Paddy Mbr Mill Albian Haplophragmoides Peace Viking Fm 'faft Hill Mbr Muddy Sandstone Blacklea f F m gigas Creek Blacklea f F m
River Fm Colora d Cadotte Mbr v (j>> Fm Flood ^ Joli Fou Fm Skull Creek Shale 103.4 Harmon Mbr Mbr Figure 2.3. The distribution of the time equivalent Belle Fourche Formation in southern
Alberta and western Montana. The thicknesses of the Belle Fourche Formation in outcrops in southern Alberta Foothills are from Stott (1963) and Leckie et al. (2000), with GR: Ghost
River (sec. 4, tp. 27, rge. 7, W5), SR: Sheep River (tp. 19, rge. 5, W5), HR: Highwood River
(sec. 28, tp. 18, rge. 3, W5), BC: Bruin Creek (50°00.919', 114°24.926'), CR: Castle River (sec.
11, tp. 6, rge. 4, W5). The thicknesses of the Floweree Member of the Marias River Formation in outcrops in northwestern Montana are from Cobban et al. (1976), with SC: Summit Creek
(tp. 30N, rge. 13W), SRC: Sun River Canyon (tp. 22N, rge. 8W), VA: Vaughn (sec. 6, tp. 21N, rge. IE), FL: Flowere (sec. 16, tp. 23N, rge. 6E); in the Well 1 Schwartz and Well 1 Johnnye in northern Montana are from Ridgley et al. (2001); and in the Wolf Creek (tp. 15N, rge. 4W) in western central Montana is from Schmidt (1978). The Middle and Late Cenomanian crestlines of uplift is from Merewether and Cobban (1986). The thicknesses of the Lower Frontier
Formation in outcrops in southwestern Montana: Eastern Pioneer Mountains (sec. 28, tp. 4S, rge. 9W) is from Dyman and Tysdal (1998); Madison Range (sec. 7, tp. 9S, rge. 4E) from
Tysdal (1991); Northern Snowcrest Range (sec. 18, tp. 9S, rge. 3W) from Dyman et al. (1988); and Lima Peaks (sec. 18, tp. 15S, rge. 8W) from Dyman et al. (1989).
43 f 114W 112W 110WI Calgary 51N-
\ tasiem Pionee• r Mnt. 76
Snowcrest Range • |» a in?
Lima Peaks 100 km >• •'—457--'' Thrust fault • 62 (m) Thickness of equivalent Belle O 20 (m) Thickness of equivalent Belle CX F°Urche Fm''" °UtCr°P v FourcheFm.inwell *0J Contour line of Belle Fourche \7T Formation (m) #K Crestline of uplift
44 Second White Specks
MFS
MFS
MFS
MFS
MFS Major Flooding Surface x x x| Bentonite —< Hummocky cross stratification Shale $ Slightly bioturbated Shaly siltstone 1 Siltstone % Mediumly bioturbated J Sandstone § Oyster
Figure 2.4. Allostratigraphy of the Belle Fourche Formation in the Well 6-4-17-13W4. See
Figure 2. IB for location. The subdivision of the formation into five allomembers was based on the recognition of major flooding surfaces (MFS) from well logs and core logs. The arrows show shallowing upwards of relative sea level.
45 Figure 2.5. (A) A west-east cross section of the Belle Fourche Formation in southern Alberta, using Gamma ray log in the left and Resistivity log in the right for every well. See Figure 2.IB for location. (B) Basin evolution interpretation of five allomembers according to the foreland basin system concept (DeCelles and Giles, 1996).
46 w 10-7-12-28W4 8-17-12-26W4 8-KM2-24W4 6-13-12-22W4 8-25-12. 20W4 8-13-12-18W4 6-12-12-16W4 11-11.12-14W4 8-15-12-12W4 11. 33-12-10W4 14+13-8W4 16-11-12-6W4 9-24-11-4W4 4-18-12-1W4 E Second j i) t% n •> s i-) White Specks
- ml
4^ -J
-•• -« :J; »VS ••&•• Ji !*'• <4 •&a ?
\<$0 Wimwr .«# Figure 2.6. (A) A south-north cross section of the Belle Fourche Formation in southern
Alberta, using Gamma ray log in the left and Resistivity log in the right for every well. See
Figure 2.IB for location. (B) Basin evolution interpretation of five allomembers according to the foreland basin system concept (DeCelles and Giles, 1996).
48 S N 10-23-1-16W4 12-12-3-16W4 16-25-4-15W4 6-22-6-14W4 14-26-MSW4 10-15-9-15W4 6-31-10-15W4 6-12-12-16W4 14-10-14-16W4 2-22-15-15W4 6-26-17-15W4 12-11-19-16W4 7-13-20-15W4 6-24-21-16W4
-1^ E linym MM :fST"T _Wr• :*• ^^^g^ fO* W a
7. .- t --. ' LAV •__>• .•_*• — • _ _ _- • - • •«. X *i .,. * ^. .£*. --c s it- *&, t ". .. . v-V vj£..dhh«VhJSl.t: Atf A -. * .kj c 111 ••"' *&*• Wi ^lli *•' • ' :>vVli*v* *. *;5» Li*'-^••'»'• r-:4».-'-Tj9ftf'-'dH! !.'**.-*"i V A: Allomember A D: Allomember D 0 50 km
^ V *• .ie - **» 7 ^ -•""-" 'U \ 0 ^ ~~V - ^ 1* } tf- - ;. - " - • i r^>_ *«" - i "
• -,. -'•!.- "\, .- •£Ji V c - g*4. "^ < . %gt'.i>i st '. ".(ft. VT "^—
rJ( —-, \ -^Vr-V^ v< -,'."' 'vi R3D R25 R20 R15 R10 lli R
B: Allomember B 0 50 km l E: Allomember E i ' . f^X/V Wc^ I if. -1."- HT:r\ «" '••'* '• -Vs.- i ; V s •"rV\ ' •:^S '--/' ^3 -7, r~~\s •' .-^ r 'e- '• : 14-. 1 ,14- '•"<.- . -;- ^
u
RIO R25 R20 R1S RIO R5 R R15 R10 R5 R'
C: Allomember C
Figure 2.7. Isopach maps of five allomembers of the Belle Fourche Formation in southern
Alberta.
50 Figure 2.8. Cross Section of the Cenomanian strata in the northern Cordilleran foreland basin.
1 is from Plint (2000), Varban and Plint (2005) and Kreitner and Plint (2006); 2 and 3 are from
Tyagi et al. (2007); 6 and 7 are from Pedersen (2004); 8 is from Ridgley et al. (2001); 9 is from
Cobban et al. (1976); 10 is from Tysdal (1991) and 11 is from Dyman et al. (1988).
51 Z9
- Dunvegan Formation Lower Kaskapau Formation 8?
JLMl_AJiiKjw\vrvy^^
>/*N i^
Upper Belle Fourche Fm ^-v^AvvV'V'*^ 3 NJ
y co
y *.
N S
v
100 m
in
3 ^ Figure 2.9. (A) The Late Albian foreland basin system in southern Canada. Isopach map of the
Joli Fou and Viking Formations is from Reinson et al. (1994). (B) The latest Albian foreland basin system in southern Canada. Isopach map of the Westgate Formation in southern Alberta and southern Saskatchewan and the Shaftesbury Formation in northwestern Alberta and northeastern British Columbia, is from Leckie et al. (1994). (C) The Late Albian foreland basin system in the northern US. Isopach map of the Upper Albian in Montana is from DeCelles
(2004), and intraforeland uplifts in southwestern Montana is from Schwartz and DeCelles
(1988).
53 A: Joli Fou + Viking Formations B: J/Vestqate Formation (Shaftesbury)
C: Blackleaf Formation
H Boundary ofdepozones of foreland basin system
Schematic foredeep region
Intraforeland uplift (Schwartz &DeCelles, 1988)
Schematic Crestline of forebulge \ Figure 2.10. The reconstructed Middle Cenomanian foreland basin system in western North
America. Palinspastically restored thrust faults are from Price and Sears (2000). The isopach map of Allomember A of the Belle Fourche Formation is drawn in southern Alberta. The isopach map of the Dunvegan Formation in northwestern Alberta is from Bhattacharya (1994).
The thicknesses of the Lower Belle Fourche Formation in central Alberta are measured from well-logs in Tyagi et al. (2007), and in southwestern Saskatchewan from well-logs in Pedersen
(2004). The thicknesses of the Lower Belle Fourche Formation in the Well 1 Schwartz and the
Well 1 Johnnye in northern Montana are from Ridgley et al. (2001). Crestline of uplift and intraforeland uplifts in southwestern Montana are after Merewether and Cobban (1986) and
Schwartz and DeCelles (1988), respectively. Although the equivalent Belle Fourche Formation in outcrops of western Montana cannot be subdivided into the Middle and Upper Cenomanian parts, we mark the outcrops with thickness of the whole formation for correlation convenience.
55 56 Figure 2.11. The reconstructed Late Cenomanian foreland basin system in western North
America. Palinspastically restored thrust faults are from Price and Sears (2000). The isopach map of Allomembers B-E of the Belle Fourche Formation is drawn in southern Alberta. The isopach map of the Pouce Coupe unit in northeastern British Columbia is from Kreitner and
Plint (2006). The thicknesses of the Upper Belle Fourche Formation in central Alberta are measured from well-logs in Tyagi et al. (2007), and in southwestern Saskatchewan from well- logs in Pedersen (2004). The thicknesses of the Upper Belle Fourche Formation in the Well 1
Schwartz and the Well 1 Johnnye in northern Montana are from Ridgley et al. (2001). Crestline of uplift in northwestern Montana is after Merewether and Cobban (1986). The Late
Cretaceous foredeep region in southwestern Montana is after Wallace et al. (1990). Although the equivalent Belle Fourche Formation in outcrops of western Montana cannot be subdivided into the Middle and Upper Cenomanian parts, we mark the outcrops with thickness of the whole formation for correlation convenience.
57 120W 118W 116W 114W 112W Palinspastic thrust fault ' (Price & Sears, 2000) \ I)) » X """^ *«• Boundary of depozone MilW// \ * of foreland basin system Schematic foredeep region •w'-V'; \ Late Cretaceous foredeep V'I- \ a region (Wallace etal., 1990) o v Crestline of uplift k (Merewether & Cobban, 1986) 1 A < 1 ™76 22 (m) Thickness of Upper Belle \ 69 0 Fourche Formation in well • O 11 W Thickness of equivalent Belle 65 Fourche Formation In outcrop -53N \ -V\ \ o "V 7 110W 108W o° 6 1 en 057 \ •> ;S 75 7o 1 £ V 011 • £? '.. • 9 012 1 S 6a>11 ' 3 ;. V\ ',78 13° 11 I3 \ ^ 0 15o 09 " •\-. ", ' 66 • *. '. \ 0 6 \ 0 ^ ! x V \\\° 34 | '"'•> '••• 4.' * 78 • ', 68 6160 58 53 « ° 026 ° °l •A \ ••oc''i2000oi»0 ° 027 -51N .. -. -.. '. '71 '. • 024 |
—Calgary
»/ • v"*1 V *-a "*, • ' vV- •• '•V <% • v. •• v. v--.: -i % •*'/ . \ \«-/-« K
-49N ' \ • X -•" i ' B. C. S=- J52 49 4 8 ^O DO 0 4j« § % a 8 8 o °° '•. V.asT."!ta,;r ••h\i.Jfftt 026 Montana V 026 0 i. 41 \ 18oi1 i r&\ "18 N o OK V 27 \ 1 \ <& \ \ •l9.4\ N •"•• -47N \ \ •-.'•. •.''.'• X N
I . 1 N 1 ' / : \\ N N : / .' V * / • * A • \i » * N ; \ . Y_ 100 km : x r \ • - ' U 1C2 • 1 Wyoming \ •J . \ :\^:
58 CHAPTER 3
MIGRATION AND STRATIGRAPHIC FILL OF A FORELAND BASIN SYSTEM:
MID-LATE CENOMANIAN BELLE FOURCHE FORMATION IN THE NORTHERN
CORDILLERAN FORELAND BASIN
Yongtai Yang, Andrew D. Miall
In review by a journal
ABSTRACT
Mainly based on detailed allostratigraphic and sedimentologic study of the Middle-Late
Cenomanian Belle Fourche Formation in southern Alberta, combined with other stratigraphic studies in the Cordilleran foreland basin and numerical models for foreland basins, a qualitative model for the migration and stratigraphic fill of a foreland basin system is proposed.
During the orogenic loading period, flexural subsidence is created in the region proximal to the mountain belt due to thrusting loading. The foredeep zone receives relatively limited sediment supply from the mountain system and the basin is in an underfilled condition. During the early orogenic unloading period, cessation of thrusting shifts accommodation space from the region proximal to the thrust belt gradually to the distal foredeep zone, leading to rapid regression of the shoreline on the proximal basin. Cratonward shift of subsidence center of sediment load in the foredeep zone result in the cratonward migration of the forebulge and backbulge zones. At the last period of the early orogenic unloading, the whole foreland basin shows a relatively flat topography and forebulge zone is uplifted locally. During the late orogenic unloading period, the foreland basin enters into an overfilled condition.
Some units of conglomerate and sandstone, which have previously been interpreted as the product of a eustatic lowstand of sea-level are interpreted here as deposits generated by erosional reworking of underlying strata during episodes of forebulge uplifting. Other fine
59 sandstone deposits were transported from the foredeep zone by storms on a relatively flat seafloor and deposited in the backbulge zone during the last period of orogenic unloading.
INTRODUCTION
Although foreland basins have been studied for many years, there are still problems in making detailed correlation between orogenic tectonics and basin stratigraphic record, and in deciphering the main controls of basin strata. For example, (1) A direct correlation between orogenesis and basin styles has been generally accepted, i.e., orogenic loading and unloading periods correlating with underfilled basin (foreland basin system) and overfilled basin
(peripheral sag), respectively (Heller et al., 1986; Beaumont et al., 1993; Catuneanu et al., 1998;
Yang and Miall, Chapter 4). However, Jordan and Flemings (1991) suggest that there still is a migratory forebulge zone during the tectonic quiescence period. (2) Numerical experiments and geological data suggest that the cratonward migration of a foreland basin results in a progressive onlap of the foredeep strata onto the forebulge-related unconformity (e.g. Flemings and Jordan, 1989; Jordan and Flemings, 1991; Sinclair et al., 1991; Crampton and Allen,1995;
Plint, 2000; Plint et al, 2001; White et al., 2002). However, few practical stratigraphic studies demonstrate the migrating processes of the forebulge zone and backbulge zone. (3) A number of sandstones enclosed in offshore marine mudstone have been identified in the Cretaceous
Cordilleran foreland basin, and recently geologists generally interpret them as prograding shorefaces deposited during eustatic lowstands of sea-level (Plint et al., 1986). However, it is worthy to suspect the principal control of eustatic sea level in a tectonic active foreland basin.
In earlier study (Yang and Miall, Chapter 2), it is shown that the Middle Cenomanian
Lower Belle Fourche Formation and the Late Cenomanian Upper Belle Fourche Formation could be interpreted in terms of the expected syntectonic subsidence pattern of a foreland basin system (DeCelles and Giles, 1996), respectively (Figs. 3.1, 3.2). High isopach values close to
60 the Rocky-Mountain fold-thrust belt may be interpreted as representing sedimentation in the foredeep zone, a linear trend of low values indicates the presence of the forebulge zone, and beyond this to the east or southeast are located the modestly thicker trends associated with a backbulge zone. Therefore, the Middle and Upper Belle Fourche Formation in southern
Alberta provide an opportunity for an in-depth study of the mechanisms of the migration and stratigraphic fill of the forebulge and backbulge depozones.
In this paper, we present detailed stratigraphic data of the Belle Fourche Formation within an area of about 60,000 km2 in southern Alberta, using data from 60 cores and 1515 wells including 35 wells in northwestern Montana (Fig. 3.1). Based on the more subtle stratigraphic features of the forebulge and backbulge zones, combined with other studies on the stratigraphic fill in foredeep zones, a qualitative model for the migration and stratigraphic fill of a foreland basin system is proposed.
ALLOSTRATIGRAPHY
In Yang and Miall (Chapter 2), using widespread flooding surfaces as the boundaries of allomembers, we subdivided the Belle Fourche Formation in southern Alberta into five allomembers, A-E (Figs. 3.2, 3.3). Allomember A and allomembers B-E can be correlated with the Middle Cenomanian Lower and Late Cenomanian Upper Belle Fourche formations in central Alberta, respectively (Tyagi et al., 2007). The Dunvegan Formation in northwestern
Alberta was deposited in the foredeep zone of the Middle Cenomanian foreland basin system
(Plint, 2000; Plint et al., 2001) and the Lower Belle Fourche Formation in southern Alberta in the forebulge and backbulge zones (Yang and Miall, Chapter 2) (Figs. 3.1 A, 3.2). The Lower
Kaskapau Formation in northeastern British Columbia was deposited in the foredeep zone of the Late Cenomanian foreland basin system (Plint, 2000; Varban and Plint, 2005; Kreitner and
Plint, 2006) and the Upper Belle Fourche Formation in southern Alberta in the forebulge and
61 backbulge zones (Yang and Miall, Chapter 2) (Figs. 3.1, 3.2). The isopach map of the Upper
Belle Fourche Formation in southern Alberta shows a typical pattern of the backbulge zone, with a northwest trending central thick zone (Fig. 3.IB).
In this study, in order to clearly illustrate the cratonward propagating processes of the
Late Cenomanian foreland basin system, we also used widespread flooding surfaces and subdivided allomembers D and E into Di and D2, and Ei and E2, respectively (Figs. 3.3-3.6).
Based on detailed well-log correlation, isopach maps of allomembers Di, D2, Ei and E2 have been constructed (Fig. 3.7). Allostratigraphic study of the Upper Belle Formation shows several important stratigraphic features.
The axes of backbulge zones of Allomembers B and C have the same location in southwestern Alberta (Figs. 3.4-3.7). From Allomember Di to E2, crestlines of forebulge zones gradually migrated northeastwards. From Allomember C to E2, axes of backbulge zones also migrated northeastwards. Areas around the crestlines of uplifting forebulge zones received relatively thin sediments or experienced submarine erosion. The areas of previously uplifted forebulge zones to the southwest of the uplifting forebulge zone also received relatively thin sediments or even experienced submarine erosion. With the northeastward migration of the uplifting forebulge zone, a northwest-trending thin zone representing uplifted and uplifting forebulge zones in the southwestern part of the study area was gradually widened. The continuous migration of the forebulge zones resulted in the gradual uplifting of the previous backbulge zone. The total Upper Belle Fourche Formation thickens northeastwards from the most southwestern part of the study area (Fig. 3. IB). The Belle Fourche Formation is only 2-5 m thick in Castle River in the southern Alberta Foothills (Stott, 1963).
During the deposition of Allomember E2, the uplifting forebulge zone was locally active in the northwestern part of the study area (Figs. 3.4, 3.7F). The allomember can be traced
62 through the southern part of the study area (Figs. 3.5, 3.6, 3.7F). Moreover, Allomember E2 has thickness difference of 6 m between the crestline of the forebulge and the axis of the backbulge, showing a relatively flat pattern (Figs. 3.4-3.6, 3.7F). However, other allomembers have larger thickness difference of 8-14 m and show a wedge shape from the axes of backbulge zones to the crestlines of forebulge zones (Figs. 3.4-3.6, 3.7C-E).
SEDIMENTOLOGY
The Belle Fourche Formation mainly consists of shale and siltstone in southern Alberta
(Fig. 3). At the top of the Lower and Upper Belle Fourche formations there are very fine and fine sandstones several meters thick, that were named Lower and Upper Belle Fourche sandstones, respectively (Ridgley et al., 2001) (Figs. 3.2-3.6). These sandstones in southeastern
Alberta and southwestern Saskatchewan and the equivalent Phillips Sandstone in northern
Montana are very important shallow gas reservoirs (Ridgley et al., 2001) (Figs. 3.1 A).
The Upper Belle Fourche Formation has a coarsening-upward section composed of several smaller allomembers which coarsen upwards from shale, to bioturbated shaly siltstone and fine sandstone (Fig. 3.3). The Upper Belle Fourche Sandstone is contained in Allomember E2 and is composed of siltstone and fine sandstone interbedded with shale (Figs. 3.3-3.5, 3.8). Fine sandstone has hummocky cross stratification and is in sharp contact with underlying shale. The very fine sandstone and siltstone about 1 meter thick just below the bentonite of the Second
White Specks Formation is slightly to moderately bioturbated, with fossils of Planolites, and
Teichichnus (Pemberton et al., 1992). We interpret that the sandstone was deposited in a storm- dominated inner offshore environment. The sand map shows that the Upper Belle Fourche
Sandstone has a northwest trend and is distributed mainly to the east of the forebulge zone of
Allomember E2 (Fig. 3.9).
A QUALITATIVE MODEL
63 Numerical models of foreland basins show that the stratigraphic fill and geometry of foreland basins are mainly related to several factors: thrust advance, lithospheric stiffness, erosion and deposition (Flemings and Jordan, 1989; Sinclair et al., 1991).
However, the allostratigraphic data of the Belle Fourche Formation suggest that the progradation of the orogenic wedge can not be the main cause of the cratonward migration of the forebulge zone of more than 200 km during the late Cenomanian - a period of about 1.3 million years (Kauffman et al., 1993) (Figs. 3.2, 3.7). The total horizontal shortening of the
Rocky Mountain fold and thrust belt is only about 250 km in southern Canada during the Late
Cretaceous-Paleocene convergence (Price and Sears, 2000). Kuznir and Karner (1985) propose that old lithosphere (> 100 Ma) will undergo no observable viscoelastic relaxation. Therefore, we can assume that the lithosphere in southern Alberta is elastic and had a constant lithospheric flexural rigidity during the Late Cenomanian. In addition, the viscoelastic flexural model supports an orogenward migrating forebulge zone (Beaumont et al., 1993), out of accord with the stratigraphic data of the Upper Belle Formation, as described here (Fig. 3.7).
Based on our detailed allostratigraphic and sedimentologic data of the Belle Fourche
Formation in the forebulge and backbulge zones in southern Alberta, and detailed allostratigraphic and sedimentologic data of the middle Cenomanian Dunvegan Formation in the foredeep zone in northwestern Alberta (Plint, 2000; Plint et al., 2001) (Fig. 3.2), we propose a qualitative model for the migration and stratigraphic fill of the northern Cordilleran foreland basin during the late Cenomanian (Fig. 3.10). It is indicated that the sediment flux from the mountain belt and sediment transport in the basin has a critical influence for the high frequency change of the basin geometry, as suggested by numerical modeling (Flemings and
Jordan, 1989; Sinclair et al, 1991; Pelletier, 2007).
64 When intense thrusting begins, newly emplaced thrust sheets result in an advance of the deformation front and an increase of surface elevation in the thrust belt. The flexural response to the orogenic load shifts the accommodation space from the distal basin to the region proximal to the thrust belt (Heller et al., 1988; Jordan and Flemings, 1991) (Fig. 3.10A).
Isolated piggyback basins in the wedge-top depozone are formed (DeCelles and Giles, 1996), providing relatively small volume of sediments to the foredeep depozone. Because the flexural subsidence driven by orogenic load overwhelms sediment supply, the foreland basin is underfilled (Jordan, 1995), showing a typical style of a foreland basin system (DeCelles and
Giles, 1996). In the proximal foredeep zone, relatively small deltaic and shoreface sandbodies are deposited, and in the distal foredeep zone, marine shale is mainly deposited (Plint et al.,
2001). The forebulge zone experiences submarine erosion. If the underlying strata in the forebulge zone contain sandy and conglomeratic sediments, they will be reworked and transported by storms both to the foredeep and backbulge sides. In the distal backbulge zone, small-scale deltaic and shoreface sandbodies might be formed. Allomembers B and C of the
Upper Belle Fourche may be representatives of deposition during the thrusting period. There is little migration of the axes of the backbulge zones from Allomember B to C (Fig. 3.7), indicating that the foreland basin had a consistent flexural subsidence and similar geometry during thrusting and folding in the orogenic belt.
After thrusting ceases, the foreland basin enters into the early orogenic unloading period.
The basin is still underfilled and has obvious forebulge zone (Fig. 3.10B-C). The available accommodation space shifts from the region proximal to the thrust belt to that space left over from previous loading period: the bathymetric low filled with water (Jordan and Flemings,
1991). As the subsidence center of sediment loading shifts cratonward in the foredeep zone, the forebulge zone and the backbulge zone also migrate cratonward (Figs. 3.7, 3.10B-C). With
65 continuous mountain-belt erosion, relatively extensive drainage systems are formed in the wedge-top depozone, gradually increasing sediment supply to the foredeep depozone. Owing to rapid regression of shoreline on the proximal basin (Jordan and Flemings, 1991), extensively distributed prograding delta is formed in the foredeep zone and sediments gradually onlap the forebulge zone of previous loading period (Plint et al., 2000; Plint et al., 2001). Reworked muddy, sandy and conglomeratic sediments of the underlying strata in the uplifted and newly uplifting forebulge zones are transported by storms to the distal foredeep zone and backbulge zone. Migration of the forebulge zone and subsequent deposition of sandy sediments in the backbulge zone produce coarsening-upward sections. Due to the gradual decrease of sandy sediments moving towards the distal backbulge zone, there are no readily recognized coarsening-upward sections there (Figs. 3.4-3.6). Continuous cratonward migration of the new forebulge results in gradual erosion and reworking of the previously deposited backbulge strata.
The allomembers D], D2, Ei and E2 of the Upper Belle Fourche Formation are representatives of deposition during the early orogenic unloading period.
At the last period of the early orogenic unloading, due to continuous sedimentation, little accommodation space is left in the foredeep zone and the whole foreland basin shows a relatively flat topography (Fig. 3.10C). The deltaic facies in the foredeep zone prograde onto the previously uplifted forebulge zones (Plint, 2000; Plint et al., 2001). Forebulge zone is uplifted locally. Silts and fine sands are transported by storms from the foredeep zone to the backbulge zone and deposited along the flank of the uplifting forebulge zone. Allomember E2 of the Upper Belle Fourche Formation is a representative of deposition during this period. The upper Belle Fourche Sandstone was transported from the foredeep zone by storms and deposited along the flank of the uplifting forebulge zone (Figs. 3.8, 3.9).
66 During the late orogenic unloading period, plentiful sediment supply from mountain belt and efficient transport of sediment across the basin lead to the overfilled condition, and accommodation space shifts from the foredeep zone to the forebulge area of the previous orogenic loading period and early unloading period (Jordan and Flemings, 1991; Jordan, 1995;
Yang and Miall, Chapter 4).
Note that the focus of this study is the migration and stratigraphic fill of the foreland basin during an orogenic loading and unloading cycle, just several million years. Over a longer time period, the progradation of the orogenic wedge results in the cratonward migration of forebulge zones of different tectonically-driven cycles, as suggested by Crampton and Allen (1995). The southeastward migration of forebulge zones from the Late Albian to Late Cenomanian probably reflects the southeastward progradation of the orogenic wedge (Yang and Miall,
Chapter 2). The horizontal shortening of the Sevier belt of about 220 km in western United
States from the Late Jurassic through the Cretaceous resulted in the eastward migration of forebulge zones (DeCelles and Coogan 2006).
DISCUSSION
A number of sandstones enclosed in offshore marine mudstone, some of which are important oil/gas reservoirs, have been identified in the Cretaceous Cordilleran foreland basin.
In general, there are two prominent features for these shelf sandstones, containing storm- formed sedimentary structures (e.g. hummocky cross-stratification) and parallel to the strike of the thrust belts of the Rocky Mountains. Various mechanisms have been proposed for the deposition of these sandstones, including deposition in the lee of a seaward break in slope
(Swagor et al., 1976; La Fon, 1981), temporal and spatial velocity variations of along-shelf geostrophic storm flows over a slight topographic irregularity (Swift and Rice, 1984; Gaynor and Swift, 1988), concentration over a tectonic paleo-high (Rice, 1984; Tillman and Martinsen,
67 1984), shoaling and reworking over a forebulge (Tankard, 1986; White et al., 2002), lowstand shoreface (Plint et al., 1986; Plint, 1988; Walker and Plint, 1992), basement control (Hart and
Plint, 1993; Donaldson et al., 1998).
Based on detailed allostratigraphic and sedimentologic data of the Belle Fourche Formation in southern Alberta, our proposed qualitative model provides a new interpretation for the offshore sandstone (Fig. 3.10). During the orogenic loading period and early orogenic unloading period, reworked sandy and conglomeratic sediments eroded from the underlying strata in the forebulge zone can be transported by storms and deposited in the distal foredeep zone and backbulge zone. During the last period of the early orogenic unloading, the seafloor shows a relatively flat topography, and inner offshore environments develop around the rising forebulge. Sandy sediments are transported by storms from the foredeep zone and deposited on the flank of the forebulge in the backbulge zone. We suggest that this model can be used to explain most of the offshore sandstones in the Cretaceous Cordilleran foreland basin. Next we will use this model to interpret the marine mudstone enclosed sandstone and conglomerate in the Late Turonian-Early Coniacian Cardium Formation in Alberta.
Plint et al. (1986) recognized seven basin-wide erosional surfaces related to sea level fall in the Cardium Formation and interpreted sharp-based conglomeratic sediments to be lowstand shoreface deposits. Criticism has been raised for their argument of frequent basin-wide sea level changes (Rine et al., 1987; Hayes and Smith, 1987), but this model has become a commonly accepted interpretation for offshore sandstones (Walker and Plint, 1992; Johnson and Baldwin, 1996). According to Plint et al. (1986), the most important conglomeratic reservoirs in the Cardium Formation are located on the erosional surface of E5 and below the flooding surface of T5 (Fig. 3.11). They are storm-formed conglomeratic bodies and trend
68 northwest, parallel to the strike of the thrust belts in the Rocky Mountains (Krause and Nelson,
1984, 1991) (Fig. 3.12).
The lower part of the Cardium Formation (below T4) progrades and thins eastwards (Plint et al., 1986), showing similar stratigraphic pattern to the mid-upper Kaskapau Formation
(Varban and Plint, 2008b) (Figs. 3.2, 3.11). It has been interpreted that the mid-upper
Kaskapau Formation and the lower part of the Cardium Formation were deposited during the late orogenic unloading period (Yang and Miall, Chapter 4). Sandstones and conglomerates in the lower part of the formation are prograding shoreface deposits.
We suggest that the lower part (below T4) and upper part (above T4) of the Cardium
Formation were deposited in different basin systems. After the deposition of the lower part of the Cardium Formation, renewed intensive thrusting in the orogen shifted the accommodation space back to the proximal zone, and a northwest-trending foreland basin system developed in western North America during the deposition of the upper part of the Cardium Formation in the
Late Turonian-Early Coniacian (Figs. 3.11, 3.12). This foreland basin has similar trend to the late Cenomanian foreland basin (Yang and Miall, Chapter 2), and both of them were driven by northeast-verging thrusting and folding in the Rock Mountains since the Late Cretaceous (Price and Sears, 2001). Wallace et al. (1990) suggested that there was a foredeep zone in southwestern Montana during the Late Cretaceous (Fig. 3.12). We propose that the foredeep zone in southwestern Montana can be extended north-northwestward to northwestern Montana and southern Canada. A series of north-northwest trending uplifts developed during the Late
Turonian-Early Coniacian time have been documented in southwestern Montana and northwestern Wyoming (Merewether and Cobban, 1986). Towards the northwest they intersect the structural trends of the Rocky Mountains in northwestern Montana. It is suggested that these uplifts are forebulges, the trend of which would have been continued to northwestern
69 Montana and the southern Alberta Foothills. The isopach map of the E5-E7 interval of the
Cardium Formation in northwestern Alberta (Hart and Plint, 1993) shows that the forebulge zone went through the British Columbia/Alberta border (Fig. 3.12). Therefore, we interpret that the upper part of the Cardium Formation was deposited in the backbulge zone during the orogenic loading period and early orogenic unloading period. The forebulge zone was located within the present area of the Alberta Foothills and experienced submarine or subaerial erosion
(Figs. 3.11, 3.12). The non-marine deposit of the lower part of the Cardium Formation was uplifted in the forebulge zone and provided sandy and conglomeratic sediments which were transported by storms to the backbulge zone. Therefore, the so-call lowstand shoreface deposit of the conglomerates below the flooding surface T5 can be interpreted to be forebulge controlled sandbodies. Detailed allostratigraphic study for the upper part of the Cardium
Formation in central Alberta is needed to recognize the location of the uplifting forebulge zones in different time and to determine the relationship between the uplifting forebulge and the conglomerates, as reported here for the Upper Belle Fourche Formation in southern Alberta.
CONCLUSIONS
A qualitative model for the migration and stratigraphic fill of a foreland basin system during the orogenic loading period and early orogenic unloading period is proposed. This model is mainly based on detailed allostratigraphic and sedimentologic study of the Middle-
Late Cenomanian Belle Fourche Formation in southern Alberta, combined with other stratigraphic studies in the Cordilleran foreland basin and numerical models for foreland basins.
The foreland basin is underfilled and shows a typical foreland basin system during the orogenic loading period and early orogenic unloading period. During the late orogenic unloading period, the foreland basin is overfilled and the previous forebulge is covered by strata from the thrust belt. The region proximal to the orogenic belt experiences intense flexural
70 subsidence driven by the orogenic load during the orogenic loading period. During the early orogenic unloading period, the forebulge zone migrates cratonward rapidly due to rapid shift of the subsidence center of sediment loading in the foredeep zone. It is suggested that the forebulge setting needs to be emphasized as an important regional control in any basin-scale model for offshore sandstones in the Cordilleran foreland basin.
ACKNOWLEDGEMENTS
We are grateful to Steven Davis, Peter DeCelles, Nick Eyles, Paul McCarthy, Russell
Pysklywec, Pierre-Yves F. Robin and Ulrich G. Wortmann for many helpful comments on the manuscript.
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76 Fig. 3.1. (A) Restored Middle and Late Cenomanian foreland basin systems in western North
America (Yang and Miall, Chapter 2), mainly showing the forebulge zones. The foredeep zones are located to the west of the forebulge zones, and backbulge zones are to the east of the forebulge zones. Location of shallow gas production from the mid-upper Cemomanian Belle
Fourche Formation in Alberta, Saskatchewan and Montana is shown (gray) (Ridgley et al.,
2001). (B) Isopach map of the Late Cenomanian Upper Belle Fourche Formation in southern
Alberta (after Yang and Miall, Chapter 2), showing location of 1480 well logs in southern
Alberta and 35 well logs in northwestern Montana used to define the allostratigraphy of the
Belle Fourche Formation. Location of well logs used to construct cross sections in Figures 3.4,
3.5, 3.6 is shown. The thickness of the equivalent Belle Foruche Formation in Castle River is from Stott (1963).
77 1* 1 IS : v •..• -•.• • -v.- v.--.: v. ••>•. ••/• #•i2
* -'. I" " \ • •
• •.•... • v." .* • *T" • 5>. ! • • ••.-••IJ.^.'.>-/ • •. • "-. ^S»/"T ° "•'.'••• '"••¥1 •V-'' N-' '• '• • • •'• *--Y 'J "2 m J* • -S X •• N, d "V • • ^ J-* H " V» • • • • v* • • ,' . • i
.••.•>..• K<^/.- S ^ • V?'.'- -'d^ f
"A . _ .A».J« . • . »• . J.I > 5 • ,<•• •**- "tiff* ^==^g£*Sl!L : ^J*^"" n^ *j#\
78 Conglomerate
Fig. 3.2. Chronostratigraphic chart of the Early Cenomanian-Early Coniacian strata in the northern Cordilleran foreland basin, showing stratigraphic nomenclature, lithology and basin style (underfilled and overfilled). Stage boundaries and ages are from Kauffman et al. (1993).
Nomenclature and lithologic data in northeastern British Columbia and northwestern Alberta are based on Plint et al. (1986); Plint (2000); Varban and Plint (2005); Kreitner and Plint
(2006). Nomenclature and lithologic data in southern Alberta are based on Bloch et al. (1993);
Nielsen et al. (2003); Tyagi et al. (2007); Yang and Miall (Chapter2, Chapter 4).
79 Gamma Resistivity Ray Second White Specks
MFS Major Flooding Surface Bentonite —' Hummocky cross stratification teot-x-:-?] Carcareous shale Shale ? Slightly bioturbated IQJ Shaly siltstone 55 Mediumly bioturbated ~H Siltstone $3; Highly bioturbated • • I Fine sandstone §• Oyster
Fig. 3.3. Allostratigraphy of the Belle Fourche Formation in the Well 9-13-15-12W4 with location in Figure IB. The subdivision of the formation into allomembers A-E2 was based on the recognition of major flooding surfaces (MFS) from well logs and core logs. The arrows show shallowing upwards of relative sea level.
80 Fig. 3.4 (A) A cross section of the Belle Fourche Formation in southern Alberta, using gamma ray log in the left and resistivity log in the right for every well. See Figure 3.IB for location. (B)
Basin evolution interpretation of allomembers B-E2 according to the foreland basin system concept (DeCelles and Giles, 1996).
81 3-25-10-29W4 16-1-11-28W4 4-7-12-26W4 10-16-13-25W4 16-14-14-24W4 6-14-15-23W4 11-24-16-22W4 5-12-17-21W4 11-1-18-19W4 8-19-19-17W4 14-32-19-16W4 7-13-20-15W4 6-26-20-14W4 11-4-21-12W4
Second L <
Basin evolution interpretation of allomembers B-E2 according to the foreland basin system concept (DeCelles and Giles, 1996).
83 13-33-3-26W4 6-1M-24W4 7-2-5-22W4 8-21-6-21W4 6-4-7-19W4 16-20-8-18W4 2-32-9-17W4 4-22-11-15W4 15-35-13-13W4 9-13-15-12W4 8-11-16-10W4 6-35-17-9W4 10-24.19-7W4 10-21-21-5W4 Second ^ ^ <~ ~-* *\ ( S > ( 'is ; f VI I r \ ^=-Sf\ i \ \ White
oo Fig. 3.6 (A) A cross section of the Belle Fourche Formation in southern Alberta, using gamma ray log in the left and resistivity log in the right for every well. See Figure 3.IB for location. (B)
Basin evolution interpretation of allomembers B-E2 according to the foreland basin system concept (DeCelles and Giles, 1996).
85 1 Blackfeet 1-23 CBAU 13-21 Parker 1 Moles 1-34 Federal 33-6 State 10-28-1-13W4 6-24-2-12W4 8-12-3-10W4 6-22-5-9W4 16-9-6-7W4 9-30-7-5W4 6-22-8-3W4 10-11-10-1W4 13-31N-7W 23-33N-6W 21-34N-5W 29-35N-4W 34-36N-3W Second White Specks sra
oo A: Allomember B D: Allomember D2
t*fr: -
/10v\
^ .- >„ .» . . • >
f J.W\
^ 1
B: Allomember C E: Allomember E1
R30 R25 R20 R15 R10 R5 R1
Vs. Crestline of forebulge ' • •, Axis of backbulge
Fig. 3.7. Isopach maps of allomembers B-E2 of the Upper Belle Fourche Formation in southern
Alberta, showing crestlines of uplifting forebulge zone and axes of backbulge zone. A and B are after Yang and Miall (Chapter 2).
87 Fig. 3.8. Core photograph in Well 9-13-15-12W4 (596.46-601.29 m) with interval place in
Figure 3.3. Scale is 10 cm. B: bentonite bed, H: Hummocky cross stratification, and SWS: base of the Second White Specks Formation. The Upper Belle Fourche Sandstone is composed of hummocky cross stratified and bioturbated siltstone and fine sandstone interbedded with shale.
88 50 km
R25 R20 R15 R10 R5
Fig. 3.9. Sand distribution of the Upper Belle Fourche Sandstone in Allomember E2, mainly based on core logs and well logs, showing location of shallow gas production (stars) (Ridgley et al., 2001). See location in Figure 3.7F.
89 A. Orogenic loading period
Sea level
Backbulge
Foredeep
B. Early orogenic unloading period
Inactive Sea level Thrust Belt Backbulge
Foredeep
C. Last period of early orogenic unloading
Inactive a level Thrust Forebulge Backbulge
Foredeep
Shale Sandstone Erosional surface
Fig. 3.10. A qualitative model showing migration and stratigraphic fill of a foreland basin system during the orogenic loading period (A), early orogenic unloading period (B) and last period of early orogenic unloading period (C). The interpretation of the figure is in the text.
90 Fig. 3.11. Allostratigraphy of the Cardium Formation, Alberta Basin, modified from Plint et al.
(1986) and Walker and Plint (1992). Based on the similar stratigraphic pattern of the lower part of the Cardium Formation (below T4) and the mid-upper Kaskapau Formation (Varban and
Plint, 2008b), we interpreted they were deposited during a Turonian orogenic unloading period
(Yang and Miall, Chapter 4). Sandstones and conglomerates in the lower part of the formation can be interpreted as prograding shoreface deposits. During the Late Turonian and Early
Coniacian, a foreland basin system was formed and the upper part of the Cardium Formation
(above T4) was deposited in the backbulge zone. The non-marine deposit of the lower part of the Cardium Formation in the forebulge zone provided sandy and conglomeratic sediments which were transported by storms to the backbulge zone, forming the conglomeratic reservoirs below the flooding surface T5.
91 Fig. 3.12. The reconstructed Late Turonian-Early Coniacian foreland basin system in western
North America, showing the isopach map of E5-E7 interval of the Cardium Formation in northwestern Alberta (Hart and Plint, 1993), oil/gas fields in the Cardium Formation (Krause et al., 1994), the Late Cretaceous foredeep region in southwestern Montana (Wallace et al., 1990) and crestlines of the Late Turonian-Early Coniacian uplifts in southwestern Montana and northwestern Wyoming (Merewether and Cobban, 1986). The northwest trending forebulge zone in the Alberta Foothills provided an important regional control for the formation of the storm-formed northwest trending conglomeratic bodies of the Cardium Formation.
92 CHAPTER 4
MARINE TRANSGRESSIONS IN THE MID-CRETACEOUS OF THE
CORDILLERAN FORELAND BASIN RE-INTERPRETED
AS OROGENIC UNLOADING DEPOSITS
Yongtai Yang, Andrew D. Miall
In press in the Bulletin of Canadian Petroleum Geology
ABSTRACT
A detailed stratigraphic and sedimentologic study of the Early Cenomanian Fish Scales
Formation and the Barons Sandstone in southern Alberta, based on detailed core measurement and well log correlation, was undertaken in order to assess the principal controlling factors on the mid-Cretaceous stratigraphic fill of the Cordilleran foreland basin. Fine-grained units in the basin, such as the Fish Scales Formation (Mowry equivalent) and the Early Turonian Second
White Specks Formation (Greenhorn equivalent), have traditionally been attributed to deposition during episodes of high eustatic sea levels. However, facies, allostratigraphy, and isopach patterns of the latest Albian Westgate, Fish Scales, mid-late Cenomanian Belle
Fourche and Second White Specks formations in southern Alberta suggest that strata were deposited alternatively during orogenic loading periods and unloading periods. We interpreted the Fish Scales Formation, a deposit formed during the marine transgression of the Mowry
Seaway in the latest Albian-earliest Cenomanian, to have been deposited in a wide shallow interior seaway during an orogenic unloading period. The Barons Sandstone, interbedded with the mudstone of the Fish Scales Formation, is interpreted as a regressive shoreface, distributary channel and barrier island sandstone deposited during the unloading period. We also tentatively interpreted the Second White Specks Formation, a regional stratigraphic marker deposited during the so-called peak eustatic sea level in the Early Turonian, to have been deposited in a
93 wide shallow interior seaway during a period of regional orogenic unloading. This study questions the significance of the Mowry Shale and Greenhorn Formation as indicators of eustatic highstands of sea level, and suggests regional tectonism as the primary control on basin evolution.
INTRODUCTION
In the Cordilleran foreland basin, Cretaceous strata have been subdivided into transgressive-regressive cycles generated during repeated cycles of eustatic sea-level change
(Williams and Stelck, 1975; Kauffman, 1977; Caldwell, 1984; Weimer, 1986). Several widespread fine-grained clastic and carbonate units, such as the Mowry Formation and the Fish
Scales Formation of Late Albian-Early Cenomanian age, the Greenhorn Formation and Second
White Specks Formation of Early Turonian age, and the First White Specks Shale of Santonian age, have long been interpreted as the product of high eustatic sea levels. In an earlier study
(Yang and Miall, Chapter 2), we presented detailed isopach maps for several intervals of the
Albian to Cenomanian age and argued that these isopachs could be interpreted in terms of the expected syntectonic subsidence pattern of a foreland basin, as described by DeCelles and
Giles (1996). High isopach values close to the Rocky-Mountain fold-thrust belt could be interpreted as representing sedimentation in the foredeep zone, a linear trend of low values indicated the presence of the forebulge zone, and beyond this to the east or southeast are located the modestly thicker trends associated with a backbulge zone. We also suggested that the foredeep zone in southeastern British Columbia and northwestern Montana has been uplifted and cannibalized. Thus, the overriding tectonic control on the large-scale stratigraphy of the basin raises the question as to the significance of eustatic cycles of sea-level change during the Cretaceous. In this paper, we examine one of these units, the Fish Scales Formation, and further discuss the Second White Specks Formation, in order to explore the principal
94 factors controlling the stratigraphic fill in the Cordilleran foreland basin during the mid-
Cretaceous.
The Mowry Seaway was developed during marine transgression in the Late Albian-Early
Cenomanian (McGookey, 1972; Williams and Stelck, 1975) (Figs. 4.1 A, 4.2). The Fish Scales
Formation (also called the Fish Scale Marker Bed and Fish Scale Zone) is a regional stratigraphic marker in the Western Interior of Canada and has been interpreted to be a condensed section deposited during the Mowry transgression (Leckie et al., 1992). Based on detailed well-log correlation, we constructed an isopach map of the formation in southern
Alberta showing eastward thickening from 6-8 m near the Foothills to 20 m in southeastern
Alberta (Fig. 4.3). The formation mainly consists of silty mudstone interbedded with bentonite beds, with abundant fish remains. In the subsurface of southwestern Alberta, a series of isolated pods of sandstone and conglomerate named Barons Sandstone (also called the Fish
Scales Sandstone) is interbedded with the mudstone of the Fish Scales Formation and is an important oil and gas reservoir (Leckie et al., 1994) (Fig. 4.2). On the basis of several cores,
Putnam and Oliver (1983) suggested that the Barons Sandstone was deposited by storm activities. Leckie et al. (2000) proposed that the patchily distributed conglomerate and sandstone of the Barons Sandstone in the southern Foothills (Fig. 4.3) was deposited in a wave- dominated shelf environment, related to a eustatic sea-level rise overprinted by a relative sea- level fall caused by uplift of the paleohigh of the Crowsnest Formation. Plint et al. (1992) correlated some Cretaceous sandstones of Alberta with eustatic sea-level lows appearing on the eustatic sea-level curve of Haq et al. (1987). In this model, the Barons Sandstone was correlated to a third-order eustatic sea-level fall. However, it has been suggested that the Exxon global cycle chart mainly represents an amalgam of regional and local tectonic events (Miall,
95 1992; Miall and Miall, 2001), and Miall (1994) suggested that the Cretaceous sandstones in
Alberta are syntectonic in origin.
We investigated an area of about 60,000 km in southern Alberta, using data from 60 cores and 1480 wells (Fig. 4.1). On the basis of detailed core measurement and well log correlation, a depositional model was established for the Fish Scales Formation. It is proposed that the Fish Scales Formation is not a marine transgressive deposit but was developed during an orogenic unloading period between two orogenic loading episodes during the latest Albian and the Middle Cenomanian, respectively. We also suggest that the Second White Specks
Formation was also deposited during the Early Turanian orogenic unloading episode.
ALLOSTRATIGRAPHY
An allostratigraphic unit is defined and identified as a mappable body of rocks bounded by discontinuities (NACSN, 1983). These bounding discontinuities can include unconformities, disconformities, omission surfaces, discontinuity surfaces and flooding surfaces (Bhattacharya and Walker, 1991). This study uses widespread flooding surfaces as the boundaries of allomembers and subdivides the Fish Scales Formation into two allomembers, I and II (Figs. 4.4, 4.5, 4.6). Allomember I is subdivided into three regressive shingles (Fig. 4.6) capped by regressive surfaces of erosion (c.f., Bhattacharya and Walker, 1991). Allomember II is subdivided into three parts on the basis of two bentonites markers (Fig. 4.6D).
Typical facies successions
A succession typical of wave-dominated shorefaces, as described by Walker and Plint
(1992), is present in Allomember I of well 16-24-8-23W4 (Figs. 4.4A, 4.5A). It displays a relatively smooth coarsening-upward pattern with a smooth funnel shape in the gamma ray and resistivity log response. This succession begins with wave rippled silty mudstone and mudstones interbedded with wave rippled very fine sandstone, and grades upward into
96 hummocky cross stratified fine sandstone interbedded with mud laminae, which in turn are overlain by 2 m of planar cross bedded medium-coarse sandstone. It is slightly to moderately bioturbated and contains trace fossils of Planolites, Thalassinoides, Teichichnus and
Terebellina (Pemberton et al., 1992). A relatively complete succession was also found in
Allomember I of wells 8-9-8-22W4 (Figs. 4.6B, 4.6E, 4.7B) and 14-33-8-23W4 (Fig. 4.7B) in cores. Based on well logs, we found wells with this succession form a northwest trending linear body with a thickness of about 12 m (Fig. 4.7A). This succession is interpreted as the deposit of a prograding, wave-dominated shoreface, with two regressive surfaces of erosion indicating a seaward shift of the shoreface (Figs. 4.4A, 4.6, 4.7B). As the shoreface continued to prograde, more sands were moved by waves and storms into offshore environments. The planar cross bedded medium-coarse sandstone at the top of the succession was deposited in a beach environment.
A typical distributary channel fill succession (Bhattacharya and Walker, 1992) is present in Allomember II of well 1-11-3-23W4 (Figs. 4.4B, 4.5B). It is underlain by a regressive surface of erosion (RSE), and has a fining-upward pattern with a bell shape in the gamma ray and resistivity log response. The hummocky cross-stratified, very fine sandstone is sharply overlain by trough cross-bedded conglomerate. Grain size of clasts ranges from 1 to 5 mm and chert pebbles are common. Upward it grades into dark mudstone interbedded with conglomerate and fine sandstone layers. The mudstone is very friable due to its high carbon content. This succession was also found in Allomember II of wells 9-11-1-21W4 (Figs. 4.6C,
4.6E, 4.8) and 6-17-1-20W4 (Fig. 4.8) in cores. We interpret that the seaward shift of the coastline resulted in the formation of the erosional surface between the shoreface fine sandstone below and the conglomerate above. The cross-bedded conglomerate was deposited
97 in a distributary channel on a delta plain. The dark mudstone and interbedded conglomerate and sandstone reflect an interdistributary bay environment.
A typical lagoon and barrier island succession (Reinson, 1992) is present in Allomember
II of well 16-24-13-26W4 (Figs. 4.4C, 4.5C). It consists of very friable black shale interbedded with planar cross bedded coarse sandstone. The gamma ray log shows a serrated feature. This succession was also found in Allomember II of wells 6-4-13-24W4, 11-21-11-24W4 and other wells in cores, together defining a northwest trending linear belt (Figs. 4.6A, 4.8). It is indicated that the succession becomes muddier southwestwards from wells 16-24-13-26W4 and 6-4-13-24W4 to well 11-21-11-24W4 (Figs. 4.6A, 4.8). In well 11-21-11-24W4, black shale dominates the succession and coarse sandstone several centimeters thick was only found at the top of the allomember. The cross bedded coarse sandstone is interpreted as a washover deposit carried by storm surges and deposited on the landward side of the barrier, and the black shale is interpreted as a back barrier lagoonal deposit. The coalescing washover fans formed an elongate northwest trending body, thinning southwestward towards the lagoon (Figs. 4.6A,
4.8).
A typical offshore succession is present in allomembers, I and II of well 6-4-17-13 (Figs.
4.4D, 4.5D). It is characterized by millimeter- to centimeter-scale graded siltstone-mudstone beds. Wave ripples occasionally occur in siltstone. The succession is slightly bioturbated and trace fossils of Planolites, and Teichichnus occur (Pemberton et al., 1992). This succession was also found in allomembers, I and II of well 8-17-18-18W4 in core (Fig. 4.6A, 4.6E). The interbedded mudstone and siltstone succession is interpreted as an offshore deposit with the siltstone beds, probably deposited from waning, storm-generated flows (Walker and Plint,
1992).
Regional distribution of allomembers
98 Allomember I of the Fish Scales Formation shows a roughly northwest trending lens shape with a maximum thickness of about 12 m (Fig. 4.7A). It thins to about 4-6 m northeastward and southwestward. It contains three sandier-upward shingles, separated by regressive surfaces of erosion (Figs. 4.4A, 4.6). These shingles offlap northeastward beneath the major flooding surface between allomembers I and II. They dip northeastward and become thinner and finer, recording northeastward progradation of shorefaces (Fig. 4.6). However, only in Shingle 3 have proximal beach and shoreface sandstones been preserved. Allomember I has a northwest trending thick belt of 8-12 m in the southwestern part of the study area because of the preserved coastline deposits in Shingle 3 (Figs. 4.7).
Allomember II has a completely different isopach pattern, thickening eastwards from 4-6 m near the Foothills to 12-14 m in southeastern corner of Alberta (Fig. 4.8). According to
Reinson (1992), barrier deposits parallel the strandline. Here, the barrier island sandstone has a northwest trend (Fig. 4.8), suggesting a northwest trending coastline similar to that developed in Allomember I (Fig. 4.7). To the northwest, two bentonite markers in Allomember II are truncated beneath the base of the Belle Fourche Formation (Fig. 4.6D). Yang and Miall
(Chapter 2) suggested that the forebulge depozone of the Middle Cenomanian foreland basin went through the western part of the study area (Figs. 4.6, 4.9). It is interpreted that uplifting of the forebulge during the Middle Cenomanian resulted in local erosion of Allomember II, removing much of the proximal part of the facies successions.
BASIN ANALYSIS
The two-phase depositional model has been proposed for the stratigraphic fill in foreland basins, suggesting a rapid basin subsidence in the synorogenic phase succeeded by flexural rebound of the thrust belt and proximal part of the foreland basin in the postorogenic phase
(Heller et al., 1988). The strata produced during the orogenic loading (synorogenic) period
99 show a typical foreland basin system as defined by DeCelles and Giles (1996), with foredeep, forebulge and back-bulge depozones, but those produced during an orogenic unloading
(postorogenic) period show a peripheral sag in front of the uplifted orogenic belt and proximal part of foredeep zone of the preceding orogenic loading period (Jordan and Flemings, 1991;
Beaumont et al., 1993; Catuneanu et al., 1998). Isopach patterns of the Westgate, Fish Scale and Belle Fourche formations, as reported here and in Yang and Miall (Chapter 2), are consistent with the two-phase development of a foreland basin. The following details suggest that the Fish Scales Formation was deposited during an orogenic unloading period in the Early
Cenomanian. Note that the orogenic loading and unloading terms here refer to changes in the positions and/or magnitudes of supra-lithospheric tectonic loads, i.e., loading related to thrusting and shortening in the orogen.
1) The Westgate Formation below and the Lower Belle Fourche Formation above the
Fish Scale Formation in southern Alberta, together with their equivalent strata in northwestern
Alberta, show typical foreland basin systems, with foredeep, forebulge and backbulge depozones (Yang and Miall, Chapter 2) (Figs. 4.6, 4.9). The Westgate Formation thins westwards and dies out near the southern Foothills where a northeast trending forebulge developed during the latest Albian. The Lower Belle Fourche Formation overlying the Fish
Scale Formation shows gradual thinning of the deposition from the backbulge zone to the uplifting forebulge zone during the Middle Cenomanian (Figs. 4.6, 4.9B). The relationship among the Westgate, Fish Scales, and Lower Belle Fourche Formation suggests that strata were deposited alternatively during orogenic loading periods and unloading periods (Heller et al. 1988; Jordan and Flemings 1991).
2) Due to uplifting of the forebulge during the Middle Cenomanian, Allomember II was eroded in the southwestern part of the study area (Fig. 4.8). However, the relatively well
100 preserved Allomember I shows a lens shape with the depocenter located close to the forebulge region of the latest Albian (Figs. 4.7A; 4.9A). Allomember I shows a regressive shoreline pattern (Fig. 4.6). In addition, thin Barons Sandstone is sporadically distributed in the southern
Foothills (Leckie et al., 2000) (Fig. 4.3). All these stratigraphic features are consistent with the flexural model of Jordan and Flemings (1991) that during the orogenie unloading period the depocenter migrates rapidly from the region proximal to the thrust belt to the forebulge zone of the preceding orogenic loading period.
We suggest that a wide interior seaway, shallower than storm-wave base, developed during the deposition of the Fish Scales Formation. This interpretation is in accordance with lithologic, paleontologic and geochemical features of the formation. The widespread, slightly bioturbated offshore succession around the basin (Figs. 4.4D, 4.7), and the richness of fish debris and barrenness of foraminifera (Bloch et al., 1993) together suggest a shallow marine environment above storm-wave base. Because there was no topographic expression of the forebulge between the foredeep and backbulge zones during the Early Cenomanian orogenic unloading period, volcanic ash and organic matter could be transported extensively throughout the seaway. Total organic carbon values in the Fish Scales Formation, are higher than those in the Westgate and Belle Fourche formations, with an average 3.2 wt% and up to maximum of 8 wt% (Bloch et al., 1993). Bentonite beds are abundant (Figs. 4.4,4.5D) and there are more than
15 bentonite beds in the formation in some wells.
We reconstruct the depositional history of the Fish Scales Formation as follows (Fig.
4.10). After the deposition of the Westgate Formation and its equivalent strata in the foreland basin system during the latest Albian (Fig. 4.9A), the thrust belt and proximal foreland basin began to isostatically rebound and flexurally uplift during an episode of orogenic unloading and the depocenter of the basin rapidly migrated to where the forebulge zone had been during
101 the latest Albian (Fig. 4.10A). During the deposition of Allomember I of the Fish Scales
Formation, a series of regressive shoreface deposits were developed (Fig. 4.1 OB). At the same time, the proximal part of the basin underwent erosion and bypass (Fig. 4.IOC). Part of
Allomember I was also eroded due to the continuous regression of shorelines and thus coastline deposits are only found in Shingle 3. Then, a marine transgression occurred, which can be attributed to internal deformation in the Cordillera during the orogenic unloading period
(Beaumont et al., 1993) (Fig. 4.10D). The depocenter of the basin was still located around the forebulge zone of the latest Albian during the deposition of Allomember II, and a series of regressive deltaic and barrier island deposits were developed (Fig. 4.10E). At the same time, part of Allomember II was also eroded due to the continuous regression of shorelines (Fig.
4.1 OF). At the beginning of the Middle Cenomanian, the emplacement of new thrust sheets in the Cordillera resulted in renewed fiexural loading, and the depocenter of the basin shifted back to the proximal zone (Figs. 4.9, 4.10G). Deposition of the facies assigned to the Fish
Scales Formation ended. A new foreland basin system was developed and the Dunvegan
Formation and the Lower Belle Fourche Formation was deposited in the foredeep zone and backbulge zone, respectively. Part of Allomember II around the new forebulge zone experienced submarine erosion during the early stage of the orogenic loading period.
DISCUSSION
The Second White Specks Formation in the Western Interior of Canada, equivalent to the
Greenhorn Formation in the U.S. part (McGookey, 1972), is a regional stratigraphic marker throughout the Cordilleran foreland basin. It mainly consists of gray, calcareous, white- speckled, coccolith-bearing fissile mudstone and silty mudstone and is commonly interpreted as a deposit of the maximum marine transgression of the Western Interior Seaway at the Early
Turanian for the Cretaceous (Williams and Stelck, 1975; Kauffman, 1977; Caldwell, 1984;
102 Weimer, 1986) (Fig. 4.1 A). Partly based on the mid-upper Cretaceous strata in the foreland basin, Haq et al. (1987) proposed a global eustatic sea-level curve showing that sea-level reached peak highstand during the Early Turonian, the highest during the past 250 million years.
However, we tentatively interpret the Second White Specks Formation to have been deposited during a period of the regional orogenie unloading. The complete orogenic unloading deposit includes the mid-upper Kaskapau Formation and the lower part of the Cardium
Formation (below the T4 surface, Plint et al., 1986) and the Second White Specks Formation is its lower part (Fig. 4.2).
The main evidence for this interpretation is: 1) A north-northwest trending foreland basin system developed in the northern Cordilleran foreland basin during the Late Cenomanian orogenic loading period (Yang and Miall, Chapter 2) (Figs. 4.6, 4.9). The foredeep zone and main part of the forebulge zone in southeastern British Columbia and northwestern Montana has been uplifted and cannibalized. The Upper Belle Fourche Formation below the Second
White Specks Formation in southern Alberta reflects continuous northeastward migration of the depocenter of the backbulge depozone (Fig. 4.6). 2) The mid-upper Kaskapau Formation and the lower part of the Cardium Formation (below the T4 surface, Plint et al., 1986) in northwestern Alberta have similar stratigraphic pattern and show long-term progradation of shoreface deposits and advance of the lap-out limits towards northeastern Alberta (Varban and
Plint, 2008b). The mid-upper Kaskapau Formation in northeastern British Columbia is about
500-600 m thick and comprises several coarsening-upward units with coastline and fluvial deposits at the top, and thins to marine shale of about 50 m thick in northwestern Alberta
(Varban and Plint, 2005) (Fig. 4.2). The lower part of the Cardium Formation also progrades westwards from on-marine deposits to marine shale (Plint et al., 1986; Hart and Plint, 1993).
103 The depocenter of the mid-upper Kaskapau Formation and the lower part of the Cardium
Formation is located at the British Columbia/Alberta border near the Foothills where a forebulge zone went through during the Late Cenomanian (Yang and Miall, Chapter 2) (Fig.
4.9A). In addition, due to the uplift and cannibalization of the proximal foredeep zone, the Late
Cenomanian Lower Kaskapau Formation below the mid-upper Kaskapau Formation in northeastern British Columbia is mudstone-dominated (Kreitner and Plint, 2006) (Fig. 4.2). In southern Alberta, the Second White Specks Formation thickens from generally less than 5 m in southeastern Alberta to about 60-70 m in southwestern Alberta, up to 100 m in southern
Foothills (Stott, 1963) (Fig. 4.11). The depocenter of the Second White Specks Formation is located at the forebulge region which developed mainly in the Rocky Mountains in southeastern British Columbia and southwestern Alberta during the Late Cenomanian (Yang and Miall, Chapter 2) (Fig. 4.9). 3) A north-northwest trending foreland basin system developed in the northern Cordilleran foreland basin and the forebulge zone was located at the
Rocky Mountains and the Alberta Foothills during the Late Turonian-Early Coniacian (Yang and Miall, Chapter 3). The upper part of the Cardium Formation (above T4 surface, Plint et al.,
1986) was mainly deposited in the backbulge zone of the foreland basin. Therefore, the relationship among the Upper Belle Fourche Formation, the mid-upper Kaskapau Formation and the lower part of the Cardium Formation, and the upper part of the Cardium Formation reflect alternation of strata deposited during orogenic loading periods with strata deposited during orogenic unloading periods (Heller et al. 1988; Jordan and Flemings 1991) (Fig. 4.2).
All the stratigraphic features of the mid-upper Kaskapau Formation (including the Second
White Specks Formation) reflect a rapid subsidence in the forebulge zone of the preceding orogenic loading period, in accordance with the flexural model of the orogenic unloading period (Jordan and Flemings, 1991).
104 Because the calcareous mudstone of the Second White Specks Formation commonly contains wave rippled fine-grained sandstones rich in shell and fish debris in northwestern
Alberta, Varban and Plint (2005; 2008a) suggested that it was deposited as a sediment-starved condensed section in a low gradient ramp with water depths of a few tens of meters above storm-wave base. Based on core data in southern Alberta, we found the calcareous mudstone of this formation contains abundant silt and very fine grained particles and is not pure shale (Fig.
4.12). Therefore, we also suggest a wide shallow interior seaway was developed during the deposition of the Second White Specks Formation. Volcanic ash was extensively transported on the relatively flat seafloor during the deposition of the formation, thus leading to abundant bentonite beds (Fig. 4.12). The western coastline of the Turanian seaway was located at the forebulge region of the Late Cenomanian foreland basin system. The eastern coastline of the
Turanian seaway was in westernmost Ontario in southern Canada (Williams and Stelck, 1975), further east than the eastern margin of the Late Cenomanian seaway, which was located in central Saskatchewan (Yang and Miall, Chapter 2). In the U.S. part of the foreland basin, both the western and eastern coastlines of the Turanian Greenhorn Seaway were also located much further east than those of the seaway developed during the deposition of the Belle Fourche
Formation in the Cenomanian (McGookey, 1972). However, we can not determine transgression and regression of the eustatic sea-level from these seaways developed in the
Cenomanian and the Turanian, because they were developed in different basin systems.
Compared with the Fish Scales Formation, we believe the Second White Specks Formation represents an orogenic unloading event with more regional influence and longer period, based on the thickness of its equivalent strata and its extensive distribution.
Varban and Plint (2008b) attributed the long-term advance of the lap-out limits of the mid- upper Kaskapau Formation towards northeastern Alberta to renewed in-phase thrusting or
105 sediment loading and excluded the contribution of dynamic topography (Mitrovica et al., 1989) for the <1 million year timescale onlap-offlap cycles. Dynamic topography operates over a millions to tens of millions of years scale. It cannot explain high frequency cycles, and is, in any case, a regional process that affects the entire Alberta Basin area in much the same way
(vertical movements and very broad regional tilts). Numerical modeling suggests that a sufficient sediment flux from the thrust belt coupled with efficient mass transport can produce a deep long-wavelength basin (Flemings and Jordan, 1989). We suggest that during the
Turonian orogenic unloading period, with the continuous flexural rebound of the thrust belt and proximal part of the basin, the lithospheric flexure due to sediment loading resulted in a rapidly deposited peripheral sag in the forebulge zone of the Late Cenomanian loading period.
The more distal parts of the peripheral sag became involved, and basin strata continuously onlapped the craton (Varban and Plint, 2008b).
There are several possible reasons why the fine-grained deposits, such as the Fish Scales and Second White Specks formations, have been attributed to eustatic sea-level rise instead of flexural unloading. 1) Because of the separation by the forebulge, strata deposited during orogenic loading periods in the foredeep and equivalent strata in the back-bulge may be of different facies and not readily correlated. 2) Except for coarse deposits around the forebulge region of the preceding orogenic loading period, the orogenic unloading strata mainly consist of mudstone, silty mudstone and calcareous mudstone which can be traced much farther towards the craton, easily resulting in an assumed interpretation of marine transgression. 3)
Continued thrusting of thrust belts destroyed the original foreland basin, hindering reconstructing of basin evolution. For example, due to the intensive northeast-southwest shortening of the Rocky Mountains during the Late Cretaceous-Paleocene time (Price and
Sears, 2000), foredeep and forebulge deposits of the mid-late Cenomanian orogenic loading
106 periods and coastline coarse deposits of the Early Turonian orogenic unloading period in southeastern British Columbia and southern Alberta Foothills have mainly been uplifted and cannibalized.
CONCLUSIONS
(1) Four typical facies successions are recognized in the Fish Scales Formation: wave- dominated shoreface succession, distributary channel fill succession, lagoon and barrier island succession and offshore succession.
(2) Using widely-distributed flooding surfaces as the boundaries of allomembers, the Early
Cenomanian Fish Scales Formation in southern Alberta is subdivided into two allomembers, I and II. Allomember I shows a roughly northwest trending lens shape and Allomember II thickens eastwards from the Foothills to southeastern Alberta. Allomember I shows a pattern of northeastward progradation of the coastline. Part of Allomember II experienced submarine erosion due to uplifting of the forebulge in the Middle Cenomanian.
(3) Based on stratigraphic and lithologic features of the Fish Scales Formation and strata underlying and overlying it, the formation is interpreted to have been deposited in a wide shallow interior seaway above storm-wave base during the Early Cenomanian orogenic unloading period. During the deposition of the Fish Scales Formation, the Barons Sandstone was deposited as shoreface deposits in Allomember I and as distributary channel and barrier island deposits in Allomember II.
(4) Based on stratigraphic and lithologic features of the Second White Specks Formation and strata underlying and overlying it, we also tentatively interpret it to have been deposited in a wide shallow interior seaway above storm-wave base during the Early Turonian of a regional orogenic unloading. The complete orogenic unloading deposit includes the mid-upper
Kaskapau Formation and the lower part of the Cardium Formation. The rebound of the
107 orogenic belt and proximal foreland basin resulted in the continuous eastward advance of the eastern coastline of the seaway to westernmost Ontario in southern Canada.
ACKNOWLEDGEMENTS
We thank Guy Plint, Bogdan Varban, Aditya Tyagi and other members in Basin Analysis
Group, University of Western Ontario, for their many good suggestions and much help in well- log selection and core measurement. We are grateful to reviewers, Terry Jordan and Brian
Turner, for their critical review, and to editors, Octavian Catuneanu and Glen S. Stockmal, for their excellent editing work.
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Environment. Lydon, J.W., Hoy, T., Slack, J.F. and Knapp, M.E. (eds.). Geological Association of
Canada, Mineral Deposits Division, p. 61-81.
Putnam, P.E. and Oliver, T.A. 1983. Barons Sandstone, southern Alberta. In: Sedimentology of selected
Mesozoic clastic sequences. McLean, J. R. and Reinson, G.E. (eds.). Canada Society of Petroleum
Geologists, p. 95-99.
Reinson, G.E. 1992. Transgressive barrier-island and estuarine systems. In: Facies models: response to
sea level change. Walker R.G. and James, N.P. (eds.). Geological Association of Canada,
Newfoundland, p. 179-194.
Stott, D.F. 1963. The Cretaceous Alberta Group and equivalent rocks, Rocky Mountain Foothills,
Alberta. Geological Survey of Canada, Memoir 317, 306 p.
Tyagi, A., Plint, A.G. and McNeil, D.H. 2007. Correlation of physical surfaces, bentonites and biozones
in the Cretaceous Colorado Group from the Alberta Foothills to south-west Saskatchewan, and a revision of the Belle Fourche/Second White Specks formational boundary. Canadian Journal of Earth
Sciences, v. 44, p. 871-888.
Varban, B.L. and Plint, A.G. 2005. Allostratigraphy of the Kaskapau Formation (Cenomanian-
Turonian) in subsurface and outcrop: NE British Columbia and NW Alberta, Western Canada
Foreland Basin. Bulletin of Canadian Petroleum Geology, v. 53, p. 357-389.
Varban, B.L. and Plint, A.G. 2008a, Palaeoenvironments, palaeogeography, and physiography of a
large, shallow, muddy ramp: Late Cenomanian-Turonian Kaskapau Formation, Western Canada
foreland basin. Sedimentology, v. 55, p. 201-233.
Ill Varban, B.L. and Plint, A.G. 2008b. Sequence stacking patterns in the Western Canada foredeep:
influence of tectonics, sediment loading and eustasy on deposition of the Upper Cretaceous
Kaskapau and Cardium Formations. Sedimentology, v. 55, p. 395-421.
Walker, R. G., and Plint, A. G. 1992. Wave- and storm-dominated shallow marine systems. In: Facies
models: response to sea level change. Walker R.G. and James, N.P. (eds.). Geological Association of
Canada, Newfoundland, p. 219-238.
Weimer, R.J. 1986. Relationship of unconformities, tectonics, and sea level change in the Cretaceous of
the Western Interior, United States. In: Paleotectonics and sedimentation in the Rocky Mountain
region, United States. Peterson, J.A. (eds.). American Association of Petroleum Geologists. Memoir
41, p. 397-422.
Williams, G.D. and Stelck, C.R. 1975. Speculations on the Cretaceous paleogeography of North
America. In: The Cretaceous System in the Western Interior of North America. Caldwell W.G.E.
(eds.). Geological Association of Canada, Special Paper 13, p. 1-29.
Yang, Y. and Miall, A. D. Chapter 2 (in press). Evolution of the northern Cordilleran foreland basin
during the mid-Cretaceous. Geological Society of America Bulletin.
Yang, Y. and Miall, A. D. Chapter 3 (in review). Migration and stratigraphic fill of a foreland basin
system: mid-late Cenomanian Belle Fourche Formation in the northern Cordilleran foreland basin.
112 Figure 4.1. Location of study area, (A) showing the latest Albian-earliest Cenomanian Mowry
Seaway and Early Turanian Greenhorn Seaway (Williams and Stelck, 1975); (B) showing location of 1480 well logs used to define the allostratigraphy of the Fish Scales and Second
White Specks formations, and location of the well logs used to construct the cross sections in
Figure 4.6.
113 B
': Boreal-Sea 100 km
Study area-ipl
Latest Albian-earlies' Cenomanian Mowry
"15 Fig. 4.6d R10 R5 MontanaP' Stage Northwestern Alberta Plains & Foothills Coniacian Cardium Fm.
Turanian Unit ll-V
93.4 Unit I Pouce Coupe Doe Creek A-X Dunvegan Fm.
Fish Scale Zone
Shaftesbury Fm.
Albian Peace River Fm. (Part)
Figure 4.2. Stratigraphic nomenclature of the Late Albian-Early Coniacian strata in the
Cordilleran foreland basin. Stage boundaries and ages are from Kauffman et al. (1993).
Nomenclature for column 1 is after Leckie et al. (1994); Plint (2000); Varban and Plint (2005).
2 after Glaister (1959); Stott (1963). 3 after Bloch et al. (1993); Nielsen et al. (2003); Tyagi et al. (2007). 4 after McGookey (1972). 5 after Williams and Stelck (1975).
115 Ghost River 114W 112W 110W 4m 0 50 100 km • i i
11 Kl • ^alaary w 1 PI •
Sheep River TZO 1 \ O•m
\ Highwood River - • • 7m ... -. :•-•• • 5-*i.«.•.»; .:•.„•... T15
»Bruin Creek \ • .' Om . . . ™*. ' 4i'i» '^'t^MMMBjSvIj !*V ... J. .*_- - -. ^»n»M*>. ft" *f <* .... * •*.. * ""• -»T« •*>- M. "• •*?&•• *S V ..
T10 \ ".Castle River 1 '2 ,v $*••/: fjspu'jg-*?-^-^'^ -y-'^KSv \ Om
"' • J 4s£?g^-^ '* jyJKSB 49N ?..L._ "^f. -^^. *??rrraKHI R30 R25 " R20 " "'""'•"" Rf5 R10 '* " R5 R1
Figure 4.3. The distribution of the time equivalent Fish Scales Formation in southern Alberta.
The thicknesses of the Barons Sandstone in outcrop sections in southern Alberta Foothills are from Leckie et al. (2000).
116 A: 16-24-8-23W4 C: 16-24-13-26W4
MFS
MFS
BFS Westgate •BFS Westgate
B: 1-11-3-23W4 D: 64-17-13W4
RSE MFS MFS
Westgati
BFS Base of Fish Scales Formation loSixSxSoj Shale •-• Fish remains \y Trough cross bedding Major flooding surface i 1 Sandstone with -^ Ripples 5 Slightly bioturbated RSE Regressive surface of erosion 1 : ' mud laminae GR Gamma Ray 1 • ' • ' -1 Sandstone —^ Hummocky cross-stratification <% Moderately bioturbated Rt Resistivity | : '. '.] Conglomerate xxx Bentonite ^ Planar cross-bedding | Photograph interval in Figure 4.5
Figure 4.4. Allostratigraphy and typical facies successions of the Fish Scales Formation with well location in Figure 4.IB.
117 Figure 4.5. Core photographs, see interval places in Figure 4.4. Scale is 10 cm. M: major flooding surface, B: bentonite bed and R: regressive surface of erosion. (A) Photograph in well
16-24-8-23W4. Hummocky cross stratified fine sandstone interbedded with mud laminae is overlain by planar cross bedded medium-coarse sandstone in Allomember I, showing a wave- dominated shoreface succession. The top of Allomember I is indicated by a major flooding surface and is overlain by wave rippled silty mudstone and mudy sandstone in Allomember II.
(B) Photograph of Allomember II in well 1-11-3-23W4. Hummocky cross-stratified, very fine sandstone is sharply overlain by trough cross-bedded conglomerate which grades upward into dark mudstone interbedded with conglomerate and fine sandstone layers. These conglomerate and dark mudstone are interpreted to have been deposited in a distributary channel and an interdistributary bay. (C) Photograph in well 16-24-13-26W4. The major flooding surface (M) is marked by a sharp contact between the wave rippled and bioturbated muddy sandstone in
Allomember I and very friable black shale interbedded with planar cross bedded coarse sandstone in Allomember II. The cross bedded coarse sandstone is interpreted as a washover deposit carried by storm surges and deposited on the landward side of the barrier, and the black shale is interpreted as a back barrier lagoonal deposit. (D) Photograph in well 6-4-17-13. The
Fish Scales Formation is characterized by millimeter- to centimeter-scale graded siltstone- mudstone beds interpreted as offshore deposits. Note the abundant bentonite beds (B) in the formation.
118 A: 16-24-8-23W4 (1131.5-1126m) C: 16-24-13-26W4(1711.48-1707.86m)
B: 1-11-3-23W4(1406.25-1402.39m) D: 6-4-17-13W4(697.05-699.78m)
119 Figure 4.6. (A-D) Cross sections in southern Alberta, showing the Westgate, Fish Scales, Belle
Fourche and Second White Specks (SWS) formations. Gamma ray log is shown on the left, the resistivity log on the right, and the lithologic column of core on the middle (if available). See location in Figure 4.IB. The Westgate Formation thins westwards and dies out near the southern Foothills. Allomember I of the Fish Scales Formation shows a regressive shoreline pattern. The top of Allomember II is truncated in southwestern Alberta. The Upper Belle
Fourche Formation reflects continuous northeastward migration of the depocenter of the backbulge depozone. The Second White Specks Formation shows a westward thinning tendency. (E) Enlarged lithologic columns of cores in A-D.
120 SW NE 6-8-8-27W4 2-22-9-25W4 11-21-11-24W4 6-4-13-24W4 16-24-13-26W4 16-35-14-24W4 16-9-16-22W4 6-23-17-20W4 8-17-18-18W4 16-24-20-18W4 6-24-21-16W4
Major flooding surface Bentonite marker f,ii;:!ii:iiii:i: l Shale Sandstone r» i i -u XXX Regressive surface of erosion Bentonite I Conglomerate l.i'.i' i'l mud laminae CO
122 cS.W NE
7-2-1-21W4 9-11-1-21W4 10-14-2-18W4 12-12-3-16W4 16-25-4-15W4 14-36-5-14W4 15-10-7-12W4 7-9-9-10W4 14-19-9-7W4 2-22-11-5W4 10-8-13-3W4 2-27-14-1W4 10r1-11-30W4 3-25-10-29W4 16-23-10-28W4 15-18-10-26W4 2-22-9-25W4 3-11-9-24W4 16-24-8-23W4 8-9-8-22W4 10-29-7-21W4 2-24-6-21W4 12-16-6-19W4 7-16-5-17W4 12-12-3-16W4 10-28-1-13W4
I t
to 4^
\7 2-22-9-25W4 11-21-11-24W4 6-4-13-24W4
8-17-18-18W4 6-16-6-22W4 8-9-8-22W4
7-2-1-21W4 9-11-1-21W4 14-36-5-14W4
125 ;/>• 1fc1fr24-a-23W4 :V(Q^iB«M-22W4:; Q "^
R30 R25 R20 R15 RIO R5~ • Well with beach and shoreface sandstone O Well with shoreface sandstone
Figure 4.7. (A) Isopach map of Allomember I of the Fish Scales Formation in southern
Alberta. (B) Interpreted paleography during the deposition of Shingle 3 of Allomember I of the
Fish Scales Formation in southern Alberta, showing wells with beach and shoreface sandstone in cores.
126 0 50 km
R30 R25 R20 R15 R10 R5 R1
D Well with distributary channel conglomerate A Well with lagoon mudstone interbedded with barrier island sandstone "^-^ Schematic location of lagoon environment, based on core and well log data
Figure 4.8. Isopach map of Allomember II of the Fish Scales Formation in southern Alberta, showing wells with deltaic and barrier island sandstone in cores and schematic location of the lagoon.
127 Figure 4.9. (A) Restored Late Albian-Late Cenomanian foreland basin systems in western
North America (Yang and Miall, Chapter 2), mainly showing the forebulge zones. The foredeep zones are located to the west of the forebulge zones, and backbulge zones are to the east of the forebulge zones. (B) Isopach map of the Middle Cenomanian Lower Belle Fourche
Formation in southern Alberta, showing the boundary of the forebulge and backbulge zones.
(C) Isopach map of the Late Cenomanian Upper Belle Fourche Formation in southern Alberta, showing the boundary of the forebulge and backbulge zones.
128 Figure 4.10. Foreland basin model showing stratigraphic response of the Fish Scales Formation during the orogenic unloading period in the Early Cenomanian. The Belle Fourche Formation
(BFF), Fish Scales Formation (FSF) and Westgate Formation (WF) and schematic location of wells in Figure 4.4 are shown in G. The interpretation of the figure is in the text.
129 A: Rebound of the proximal basin due to unloading Sea level
Forebulge
B: Deposition of regressive shoreface deposits
Sea level
C: Erosion and bypass in the proximal basin Sea level
D: Marine transgression Sea level
E: Deposition of regressive deltaic and barrier island deposits Sea level
F: Erosion and bypass in the proximal basin
G: Shift of depooenter to the proximal basin due to renewed loading Sea level Forebulge Backbulge Foredeep
I I 16-24-8-23W4 16-24-13-26w4 6-4-17-13W4
Shale Siltymudstone Sandstone Erosional surface Flooding surface Overlaid erosional and flooding surface
130 Ghost River 114W 112W 110W 87m 0 50 100 km
51N- ialgary
Sheep River
100m
\Highwood River
80m
49N R5 Rl
Figure 4.11. The distribution of the time equivalent Second White Specks Formation in southern Alberta. The thicknesses of the Vimy Member in outcrop sections in southern Alberta
Foothills are from Scott (1963).
131 SF yy\ •»• *"$£ 1 ' ,;1- ,f: w ' i !l k._.^ *" - • m !tt ' 7 A 'K \ ~ "J
&
-. A '
Figure 4.12. Photograph of the Second White Specks Formation in well 6-4-17-13 (631.70-
635.61 m), with interval place in Figure 4.6B. SWS is the base of the formation. The calcareous silty mudstone is interpreted to have been deposited in a shallow marine environment. Note the abundant bentonite beds in the formation.
132 CHAPTER 5 TECTONIC CYCLES OF FORELAND BASINS
Yongtai Yang, Andrew D. Miall
In review by a journal
ABSTRACT
Allostratigraphic and sedimentologic analysis of the middle Cretaceous in Alberta, combined with previously published research on stratigraphy and basin modeling in the
Cordilleran foreland basin, provides the basis for the definition of three orders of tectonic cycles, reflecting the short- to long-term history of tectonic episodicity and changes in contractional stress patterns during the evolution of the orogen.
High-frequency tectonism (106-year time scale) and sediment distribution in the basin are the main causes of what we term tertiary tectonic cyclicity. During the orogenic loading period, flexural subsidence is created in the region proximal to the mountain belt due to thrusting and the foreland basin is in an underfilled condition with a prominent forebulge zone. During the early orogenic unloading period, the basin is still in an underfilled condition, and cratonward shift of subsidence center of sediment loading in the foredeep zone results in the cratonward migration of the forebulge and backbulge zones. During the late orogenic unloading period, plentiful sediment supply and efficient sediment transport lead to an overfilled condition in the basin, and a peripheral sag with a lens-shaped isopach pattern is formed in front of the uplifted proximal foredeep zone of the previous loading and early unloading periods.
The six clastic cycles in Western Canada are related to the docking of accretionary terranes on the western edge of North America and constitute secondary tectonic cycles. The mid-
Mesozoic-early-Cenozoic Zuni Sequence deposited during the long-term basin-forming period of the Cordilleran foreland basin is defined as the primary tectonic cycle.
133 Keywords: orogenesis, foreland basin, allostratigraphy, tectonic cycle.
INTRODUCTION
A foreland basin is a depression that develops because of the lithospheric flexure created by crustal thickening associated with the evolution of a mountain belt. Therefore, the strati graphic record in a foreland basin contains valuable information of the tectonic history of the bounding mountain belt. The Sevier orogenic wedge exhibits several short-term (<10 Ma) cycles of behavior in terms of deformation, erosion, and sedimentation (DeCelles and Mitra,
1995). Numerical modeling shows a cycling between tectonic activity and quiescence in orogenic belt can produce stratigraphic features in a foreland basin similar to those generated by eustasy, including the subaerial erosional surface, transgression and regression (Jordan and
Fleming, 1991). However, few practical studies in foreland basins have demonstrated the short- term stratigraphic cycles driven by the cyclic behavior of mountain belts. In addition, we are still not very clear about the relationship between orogenesis and basin styles, i.e., orogenic loading and unloading, and underfilled basin (with forebulge zone) and overfilled basin
(without forebulge zone) (Jordan, 1995). It is usually proposed that the forebulge zone is created during an orogenic loading period and basin shows a peripheral sag during an unloading period (e.g. Beaumont et al., 1993; Yang and Miall, Chapter 4). However, Jordan and Flemings (1991) suggest that the forebulge zone still develops and migrates cratonward during the tectonic quiescence period.
Based on our earlier detailed stratigraphic studies of the middle Cretaceous in the northern Cordilleran foreland basin (Yang and Miall, Chapters 2, 3, 4), this paper proposes a qualitative model for migration and stratigraphic fill in a foreland basin driven by a short term tectonic cycle in the orogenic wedge (Fig. 5.1). A more clear correlation is also established between tectonic evolution of the orogenic belt and foreland basin geometry. Although our
134 following discussion focuses on shallow marine settings, the principles established here also apply to nonmarine foreland basins.
STRATIGRAPHIC CYCLE DRIVEN BY OROGENIC LOAD AND SEDIMENT LOAD
Foreland basin geometry and stratigraphy is mainly related to several factors: thrust load, erosion and deposition, and lithospheric stiffness (Flemings and Jordan, 1989; Sinclair et al.,
1991). For a given area with relatively constant lithospheric flexural rigidity during a relatively short time period (several million years), foreland basin strata largely depend on the orogenic loading due to thin-skinned thrusting, and sediment supply from the mountain belt and sediment distribution in the basin.
During the orogenic loading period, flexural subsidence is created in the region proximal to the mountain belt due to thrusting (Jordan and Flemings, 1991; Yang and Miall, Chapter 3)
(Fig. 5.1A). The proximal foredeep zone receives relatively limited sediment supply from the mountain system, forming relatively small deltaic and shoreface sandbodies (Plint et al., 2001), and the basin is in an underfilled condition with a prominent forebulge zone (Jordan, 1995).
The eroded sediments from the underlying strata in the forebulge zone are transported to the distal foredeep region and the backbulge zone.
During the early orogenic unloading period, cessation of thrusting shifts accommodation space from the region proximal to the thrust belt gradually to the distal foredeep zone, leading to regression of the shoreline on the proximal basin (Jordan and Flemings, 1991; Plint et al.,
2001; Yang and Miall, Chapter 3) (Fig. 5.IB). Cratonward shift of subsidence center of sediment loading in the foredeep zone result in the cratonward migration of the forebulge and backbulge zones. Therefore, the basin with a migratory forebulge is still in an underfilled condition. The continuous migration of new forebulge results in formation of a wide erosional surface and reworking of the previously deposited backbulge strata. At the last period of the
135 early orogenic unloading, the whole foreland basin shows a relatively flat topography and forebulge zone is uplifted locally. Silts and fine sands are transported by storms from the foredeep zone to the backbulge zone proximal to the uplifting forebulge.
During the late orogenic unloading period, plentiful sediment supply from the mountain belt and efficient transport of sediment across the basin lead to an overfilled condition (Jordan,
1995), and the accommodation space shifts from the distal foredeep zone to the forebulge area of the previous orogenic loading period and early unloading period, resulting in a regression of the shoreline and subaerial erosion of the proximal basin (Jordan and Flemings, 1991; Yang and Miall, Chapter 4) (Fig. 5.1C). A peripheral sag is formed with lens shaped isopach pattern.
During a relatively long orogenic unloading period, continuous subsidence of the sag driven by sediment loading can lead to a coastline deposit with a thickness of several hundreds meters
(Varban and Plint, 2005, 2008; Yang and Miall, Chapter 4). With the continuous rebound of the thrust belt and proximal basin, the subsidence centre gradually migrate cratonward and strata in the distal seaway continuously onlap the craton (Varban and Plint, 2008).
TECTONIC CYCLES IN THE NORTHERN CORDILLERAN FORELAND BASIN
Tectonic cyclicity in the northern Cordilleran foreland basin occurred over three overlapping time scales. On the shortest time scale, the middle Cretaceous succession consists of tectonically-driven cycles containing strata deposited in an underfilled basin during the orogenic loading and early unloading periods and strata deposited in an overfilled basin during the late orogenic unloading period (Fig. 5.2). The orogenic loading and unloading cycles lasted for several million years, and we term these "tertiary tectonic cycles" which may correspond with shorter-term (<10 m.y.) cyclic behavior of the orogenic wedge (DeCelles and Mitra,
1995). In practice, it is much easier to use isopach maps and cross sections to differentiate the underfilled foreland basin which has forebulge zone and the overfilled foreland basin which
136 shows lens shape than to find the boundary between the orogenic loading deposit and unloading deposit. Therefore, we define the middle Cretaceous as intervals deposited in the underfilled condition and overfilled condition (Fig. 5.2).
The marine shale of the late Albian Cadotte and Harmon Members in northwestern Alberta and that of the equivalent Joli Fou Formation in southern Alberta onlap southeastward and northwestward on to the northeast-trending forebulge zone, respectively (Leckie et al., 1994;
Reinson et al., 1994) (Figs. 5.2, 5.3), probably reflecting deposition in an underfilled condition.
There is not a prominent forebulge zone in the late Albian Paddy Member in northwestern
Alberta and the equivalent Viking Formation in southern Alberta which are mainly composed of regressive fluvial, coastal plain and coastline deposits (Leckie et al., 1994; Reinson et al.,
1994) (Fig. 5.2), probably reflecting deposition in an overfilled condition.
The latest Albian Westgate Formation shale, deposited in the backbulge zone in southern
Alberta, thinning to zero westwards on to the northeast-trending forebulge, and the equivalent
Shaftebury Formation in northwestern Alberta, deposited in the foredeep zone, reflect deposition in an underfilled condition (Yang and Miall, Chapter 2) (Figs. 5.2, 5.3, 5.4). The early Cenomanian Fish Scales Formation consists of silty mudstone interbedded with regressive shoreface, distributary channel and barrier island sandstones, with depocenter located above the latest Albian forebulge zone, representing deposition in an overfilled condition during the late orogenic unloading period (Yang and Miall, Chapter 4).
The Lower Belle Fourche Formation shale in the backbulge depozone in southern Alberta
(Yang and Miall, Chapter 2) and the deltaic Dunvegan Formation in the foredeep depozone in northwestern Alberta (Plint, 2000; Plint et al., 2001), onlapping southeastward and northwestward the northeast-trending forebulge zone, respectively, were deposited in an underfilled condition during the middle Cenomanian (Figs. 5.2, 5.3, 5.4). Before the basin
137 entered into a late orogenic unloading period, the late Cenomanian orogenic loading period started and therefore this tectonic cycle does not have a corresponding deposit in an overfilled condition.
During the late Cenomanian, the forebulge retreated westward within Alberta and has a northwest trend, probably related to the change of convergence vectors along the western margin of North America (Yang and Miall, Chapter 2) (Figs. 5.2, 5.3, 5.4). The Lower
Kaskapau Formation in northeastern British Columbia and the Upper Belle Fourche Formation in southern Alberta represent foredeep and backbulge deposits, respectively. The mid-upper
Kaskapau Formation and the lower part of the Cardium Formation in northwestern Alberta show long-term progradation of shoreface deposits and advance of the lap-out limits towards northeastern Alberta (Varban and Plint, 2008), with depocenter located above the late
Cenomanian forebulge zone, reflecting deposition in an overfilled condition during a late orogenic unloading period (Yang and Miall, Chapter 4) (Figs. 5.2, 5.3, 5.4). A sufficient sediment flux from the thrust belt, coupled with efficient mass transport (Flemings and Jordan,
1989), produces a wide shallow interior seaway during the Turonian late unloading period
(Varban and Plint, 2005; Yang and Miall, Chapter 4), with the eastern coastline located as far east as westernmost Ontario (Williams and Stelck, 1975).
During the late Turonian-early Coniacian, a northwest-trending forebulge developed in the foreland basin (Yang and Miall, Chapter 3) (Figs. 5.2, 5.3, 5.4). The equivalent strata of the upper part of the Cardium Formation in Alberta represent deposition mainly in a backbulge depozone, thinning westwards on to the forebulge. Detailed stratigraphic study is needed to find the corresponding late unloading deposit in an overfilled condition.
The Western Canada foreland basin contains six clastic wedges bounded by widespread unconformities or by significant and rapid changes in sedimentary facies, correlating to the
138 docking of accretionary terranes on the western edge of North America (Stockmal et al., 1992).
We suggest that each of these clastic wedges can be defined as a "secondary tectonic cycle" consisting of several tertiary tectonic cycles (Fig. 5.2). One secondary tectonic scale represents deposition during an orogenic episode lasting tens of million of years. During this episode, thrusting in the orogen has a similar direction, and orogenic loads in different tertiary tectonic cycles do not exhibit prominent differences. Therefore, the forebulge zones developed during the orogenic loading and early unloading periods of different tertiary tectonic cycles migrate systematically cratonwards with the progradation of the orogenic wedge (Crampton and Allen,
1995). The southeastward migration of forebulge zones in Alberta from the late Albian to late
Cenomanian reflects the southeastward progradation of the orogenic wedge during a secondary tectonic cycle (Yang and Miall, Chapter 2) (Fig. 5.3). Different secondary tectonic cycles reflect different thrusting events in the orogen with prominent difference in thrusting directions or thrusting intensity. The change of thrusting directions and the increase of orogenic load in the Cordilleran thrust belt since the late Cenomanian resulted in the westward retreat of the forebulges within Alberta at the end of the Middle Cenomanian (Yang and Miall, Chapter 2), thus defining two different secondary tectonic cycles (Figs. 5.2, 5.3). The "primary tectonic cycle" represents deposition during the whole developing period of the foreland basin. The
Cordilleran foreland basin was formed during Late Jurassic to Eocene, driven by the
Cordilleran orogenesis in response to subduction of the Pacific plate beneath the North
American continental plate (e.g. Monger and Price, 1979). The mid-Mesozoic-early-Cenozoic
Zuni Sequence (Sloss, 1963) can be defined as the primary tectonic cycle (Fig. 5.2).
CONCLUSIONS
The geometries of strata and facies patterns are used to deduce the subsidence patterns and basin evolution in the northern Cordilleran foreland basin during the middle Cretaceous. The
139 middle Cretaceous is subdivided into several-million-year tectonic cycles driven by orogenic loading in the Cordillera and sediment loading in the basin. A qualitative model is proposed for the stratigraphic fill and migration of foreland basins during a tectonic cycle including orogenic loading, early unloading and late unloading periods. The basin flexural subsidence is mainly controlled by the orogenic load and sediment load, respectively, during the orogenic loading period and unloading period. The basin is in an underfilled condition during the orogenic loading and early unloading periods, and in an overfilled condition during the late orogenic unloading period.
ACKNOWLEDGEMENTS
We are grateful to Peter DeCelles, Nick Eyles, Pierre-Yves F. Robin and Ulrich G. Wortmann for many helpful comments on the manuscript.
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143 Fig. 5.1. A qualitative model showing migration and stratigraphic fill of a foreland basin during the orogenic loading period (A), early orogenic unloading period (B) and late orogenic unloading period (C) (modified from Yang and Miall, Chapter 3, Chapter 4). The interpretation of the figure is in the text. Note that the flexural amplitude is exaggerated.
144 Fig. 5.2. Chronostratigraphic chart of the late Albian-early Coniacian strata in the northern
Cordilleran foreland basin, showing stratigraphic nomenclature and tectonic cycles. Stage boundaries and ages are from Kauffman et al. (1993). Nomenclature in northwestern Alberta is based on Leckie et al. (1994); Reinson et al. (1994); Plint (2000); Varban and Plint (2005).
Nomenclature in southern Alberta is based on Bloch et al. (1993); Leckie et al. (1994);
Nielsen et al. (2003); Tyagi et al. (2007). Tectonic cycle analysis is based on Yang and Miall
(Chapters 2-4); Stockmal et al. (1992); Sloss (1962).
145 Fig. 5.3. Restored late Albian-early Coniacian foreland basin systems in western North
America (Yang and Miall, Chapters 2, 3), mainly showing the forebulge zones and forebulge crestline. The foredeep zones are located to the west of the forebulge zones, and backbulge zones are to the east of the forebulge zones. The numbers of orogenic loading periods are in fig.
5.2.
146 SW NE
Fig. 5.4. Cross section constructed from 12 well-logs in southern Alberta, showing thicknesses of the strata deposited in an underfilled condition and in an overfilled condition during the late Albian-early Coniacian. See location in fig. 5.3. The numbers of depositing periods are in fig. 5.2. The construction is based on detailed well-log correlation of Nielsen (2003) and Yang and Miall (Chapter 2, 4).
147