Tectonic Subsidence Analysis of the Williston Basin a Thesis Submitted
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Tectonic Subsidence Analysis of the Williston Basin A thesis Submitted to the faculty of Graduate Studies in Partial Fulfillment of the Requirements for the degree of Master of Science in Geology Department of Geological Sciences University of Saskatchewan by John R. Raid 1991 The University of Saskatchewan claims copyright in conjunction with the author. Use shall not be made of the material contained herein without proper acknowledgement. In presenting this thesis in partial fulfillment of the requirements for a Postgraduate degree from the University of Saskatchewan, I agree that the Libraries of this University may make it freely available for inspection. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the professor or professors who supervised the thesis work recorded herein, or, in their absence, by the Head of the Department or the Dean of the College in which the thesis work was done. It is understood that due recognition will be given to the author of this thesis and to the University of Saskatchewan in any use of the material in this thesis. Copying or publication or any other use of the thesis for financial gain without approval by the University of Saskatchewan and the author's written permission is prohibited. Requests for permission to copy or to make other use of material in this thesis in whole or in part should be addressed to: Head of the Department of Geological Sciences, University of Saskatchewan Saskatoon, Saskatchewan S7N OWO i To my parents ii Abstract The Williston Basin is one of a number of North American intracratonic basins whose subsidence is poorly understood. The Basin is circular in shape and contains nearly 5 kIn of preserved sedimentary rocks at its center; ranging from Cambrian to Tertiary in age. The Basin's subsidence is investigated quantitatively using the backstripping method in order to gain a better understanding of its origin. Data from 24 oil exploration wells located in central and northwest portions of the basin were correlated and used in the study. A computer program called UNPAK was developed to perform the backstripping analysis. Tectonic subsidence curves were calculated to evaluate episodic subsidence models and extrapolated across unconformities to evaluate continuous subsidence models. Extrapolated tectonic subsidence curves near the center of the basin display a relatively constant subsidence through the Paleozoic, at a rate of slightly less than 1 em ka- 1, and then decrease to the present-day. Improbable behavior of subsidence curves was used to constrain the magnitude of the eustatic models used in the backstripping calculations. A Cambro-Ordovician first-order rise of between 100 and 160 m, and a Cretaceous first-order rise of approximately 150 m above present-day sea level are consistent with the stratigraphic record in the Basin. No bathymetric corrections were used in the backstripping calculations, however, results from this study indicate water depth during deposition of the Bakken Formation was probably in the range of 50 to 100m. Calculated point-loads necessary to account for the deflection of the Basin (using an elastic plate model) ranged from 2.5 x 1018 N to 2.9 x 1018 N, for an episodic model. If a continuous subsidence model was invoked, then a load of between 5.3 x 1018 Nand 6.5 x 1018 N is required to account for the deflection of the Basin. iii Of the previously proposed models to account for the basin's subsidence, continuous subsidence models were favored. A phase-change model, involving progressive metamorphic reactions of gabbro to eclogite, seemed the most likely of these models. Evidence in favor of the continuous subsidence models includes: an excellent fit of the extrapolated tectonic subsidence data to a smooth curve (implying a single subsidence mechanism); an axis of sedimentation which shifted in a circular fashion around a relatively stationary center of deposition; very close correspondence between unconformities in the basin and falls in second-order relative changes in sea level from published curves; and evidence for uplift and erosion of the surrounding cratonic arches in between depositional cycles. Future acceptance of a continuous subsidence model to explain the Williston Basin's subsidence will ultimately depend upon the outcome of the debate surrounding the existence and origin of short-term eustatic changes. iv Acknowledgements The author is very grateful to Dr. E. G. Nisbet who initiated and supervised this study and provided assistance throughout the preparation of the thesis. Dr. W. G. E. Caldwell provided guidance and support in the initial phase of the study. Dr. C. M. R. Fowler provided guidance and input through the course of the study and helped me to understand numerous geophysical concepts. Dr. M. J. Reeves provided invaluable computer programming assis~nce, along with his time and sense of humor while helping to develop the UNPAK computer program. Drs. H. E. Hendry and Z. Hajnal are thanked for their supervision. Esso Resources Canada Ltd. provided digital well logs, access to proprietary lithology analysis software and use of their reprographics department in the preparation of the thesis. A number of the figures in the thesis, along with the cross section in the back pocket were drafted by Esso's drafting department. The author is also very grateful to a number of Esso employees: Dr. Gary Holmes, who provided a great deal of advice and encouragement in the latter stage of the study and critically read the initial draft of the thesis; Terry Buchanan, who provided assistance with the SOLAR lithological analysis; and Dr. Henry Posamentier, who explained sea-level change concepts that he and his co-workers developed at Exxon Production Research Co. The Saskatchewan Department of Energy and Mines provided a computer printout of well data along with access to well logs, copYing facilities and drill cuttings. Dr. Jim Lorsong (then an independent consulta~t) provided valuable advice and perspective during my data-gathering trips to Regina. Support for this study was through an operating grant to Dr. E. G. Nisbet from the Natural Sciences and Engineering Research Council of Canada and scholarships from the Petroleum Society of the Saskatchewan Section of the elM and a graduate v summer scholarship from the College of Graduate Studies and Research of the University of Saskatchewan. The author is also grateful to (Aunt) Jean Connor who provided Sunday brunches and warm hospitality through the course of my stay in Saskatoon. vi Table of Contents Abstract.............................................................................................................. iii Acknowledgements..... v List of Tables x List of Figures................................. xi 1. Introduction........................ 1 1.1 Regional and Tectonic Setting of the Williston Basin.. 1 1.2 Objectives....................................................................................... 4 1.3 Previous Work.... 5 1.3.1 Deep Crustal Seismic Surveys.................. 5 1.3.2 Basement Under the Williston Basin 6 1.3.3 Heat Flow in The Williston Basin 7 1.3.4 Review of Basin-Forming Mechanisms......................... 9 1.3.5 Review of Subsidence and Load Analysis.. 13 2. Geological History.... 19 2.1 The Middle Cambrian-Lower Ordovician Sauk Sequence 19 2.2 The Middle Ordovician-Upper Silurian Tippecanoe Sequence 23 2.3 The Middle Devonian-Upper Mississippian Kaskaskia Sequence....... 24 2.4 The Lower Pennsylvanian-Lower Jurassic Absaroka Sequence 29 2.5 The Middle Jurassic-Paleocene Zuni Sequence and Post- Paleocence Tejas Sequence. 31 3. Sedimentary Basin Subsidence Analysis: Theoretical Background 37 3.1 Controls on Basin Subsidence 37 3.2 Tectonic Subsidence Analysis 38 3.2.1 Development of the Backstripping Method 38 3.2.2 Burial and Thermal History 44 3.3 Eustatic Changes 44 3.3.1 Measuring Eustasy 44 3.3.2 Rates of Change of Sea Level 47 3.4 Compaction of Sediments 51 3.4.1 Measuring Sediment Compaction - Porosity/Depth Relationship. 51 3.4.2 Siliciclastics 55 3.4.3 Carbonates 57 3.4.4 Evaporites 61 V11 4. Method for Calculating Tectonic Subsidence: UNPAK Computer Program............. 64 4.1 Introouction............... 64 4.2 Input 65 4.3 Description of Calculations. 66 4.4 Output 70 s. Data 73 5.1 Data Acquisition 73 5.2 Formations Data File 76 5.2.1 Formation Depths 76 5.2.2 Age of Stratigraphic Units ~ 80 5.2.3 Eustatic Changes 81 5.2.4 Paleobathymetry, Flexural Parameter and Temperatures 83 5.2.5 Lithology Determination Using SOLAR Program 85 5.3 Lithology Data File 89 5.3.1 Density and Porosity-Depth Values 89 5.3.2 Thermal Parameters..................... 93 6. Tectonic Subsidence Analysis Results and Discussion 95 6.1 Backstripping Diagrams... 95 6.2 Consideration of Unconformities - Extrapolated Tectonic Subsidence Curves 105 6.3 Tabulation of Results and Discussion of Model Input Variables 108 6.4 Discussion of Tectonic Subsidence and Extrapolated Tectonic Subsidence Curves 116 7. Interpretation of Tectonic Subsidence Analysis: Eustasy, Load and Possible Basin-Forming Mechanisms 123 7.1 Eustasy and Paleobathymetry in the Williston Basin 123 7.1.1 The Cambro-Ordovician First-Order Eustatic Rise 123 7.1.2 The Cretaceous First-Order Eustatic Rise 127 7.1.3 Second-Order (short term) Eustatic Changes 128 7.1.4 Paleobathymetry of the Bakken Formation 130 7.2 Consideration of the Basin-Forming Load 130 7.3 Discussion of Possible Basin-Forming Mechanisms 136 7.3.1 Continuous Subsidence