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Stratigraphy and Sedimentation of the Upper Cretaceous Montana Group University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1986 Stratigraphy and sedimentation of the Upper Cretaceous Montana group of northwestern Montana with detailed bedform analysis of Two Medicine Formation complexly cross-bedded sandstone Jeffery E. Larson The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Larson, Jeffery E., "Stratigraphy and sedimentation of the Upper Cretaceous Montana group of northwestern Montana with detailed bedform analysis of Two Medicine Formation complexly cross- bedded sandstone" (1986). Graduate Student Theses, Dissertations, & Professional Papers. 7568. https://scholarworks.umt.edu/etd/7568 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. COPYRIGHT ACT OF 1976 This is an unpublished manuscript in which copyright sub­ s i s t s . Any further reprinting of its contents must be approved BY THE AUTHOR. Mansfield Library University of Hontana Date : ^ ^ stratigraphy and Sedimentation of the Upper Cretaceous Montana Group of Northwestern Montana, with Detailed Bedform Analysis of Two Medicine Formation Complexly Cross-bedded Sandstone By Jeffery E. Larson B.A., St. Cloud State University, 1983 presented In partial fulfillment of the requirements for the degree of Master of Science UNIVERSITY OF MONTANA 1986 Approved by: ^ Chairman, Board of Examiners an, Graduate Scnabl Date / UMI Number: EP38369 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Oisswlation Publishing UMI EP38369 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Larson, Jeffery E., M.S., Geology Stratigraphy and Sedimentation of the Upper Cretaceous Montana Group of Northwestern Montana, with Detailed Bedform Analysis of Two Medicine Formation Complexly Cross-bedded Sandstone Director: Johnnie N. Moore The Upper Cretaceous (Campanian) Montana Group comprises a 750 meter thick regressive clastic wedge of transitional marine Telegraph Creek, nearshore Virgelle, and dominantly terrestrial Two Medicine Formations. The Virgelle Formation was deposited along a prograding beach in microtidal conditions with oblique bar and ebb-tidal channel preserved, and longshore sediment transport from south to north. The overlying lignitic shale and sandstone of the Two Medicine Formation was deposited on a marshy coastal plain with distributary channels, crevasse splays, and two transgressive sheet sandstones preserved. The complexly cross-bedded sandstone in the lower 100 meters of the Two Medicine Formation was deposited as a coastal eolian dune system that was then transgressed. The large bedforms in the lower part were wavy-crested and migrated unidirectionally to the southwest. Individual bedforms were less than 10 meters high and largely truncated. Superimposed straight-crested and lunate bedforms had a large bedform normal migration direction. The upper part was deposited by straight-crested and lunate bedforms with no superimposed bedforms and bipolar current directions. Bedform size has decreased vertically, resulting from a decrease in sediment transport or velocity. A shorter relaxation time for upper bedforms may be responsible for preserved secondary azimuth trends. il TABLE OF CONTENTS Abstract................................................. il Table of Contents........................................ ill List of Figures.......................................... iv Acknowledgements..........................................v INTRODUCTION............................................. p.1 CHAPTER ONE: STRATIGRAPHY AND SEDIMENTATION OF THE TELEGRAPH CREEK FORMATION AT SECTION ONE ............ p.8 Descriptions of Stratigraphie Units ................... p.8 Interpretations of Stratigraphie Units................ p. 10 CHAPTER TWO: STRATIGRAPHY AND SEDIMENTATION OF THE VIGELLE FORMATION AT SECTION ONE . p. 12 Descriptions of Stratigraphie Units . p.12 Interpretations of Stratigraphie Units. p. 13 Summary of Interpretations........... p.25 CHAPTER THREE: STRATIGRAPHY AND SEDIMENTATION OF THE TWO MEDICINE FORMATION AT SECTION ONE ............. p.26 Descriptions of Stratigraphie Units ........... p.26 Interpretations of Stratigraphie Units......... p.29 Summary of Two Medicine Formation Interpretations p.41 CHAPTER FOUR: STRATIGRAPHY AND SEDIMENTATION OF THE MONTANA GROUP AT SECTION TWO................... p.44 Summary of Interpretations for Section Two. p.50 CHAPTER FIVE: BEDFORM ANALYSIS OF COMPLEXLY CROSS-BEDDED SANDSTONE IN THE TWO MEDICINE FORMATION . p.51 Descriptions of Stratigraphie Unit . p.54 Descriptions of Stratigraphie Unit 3b . p.60 Descriptions of Stratigraphie Unit 3c . p.64 Interpretations of Stratigraphie Unit 3& p.66 Interpretations of Stratigraphie Unit 3b p.80 Summary of interpretations of 3& and 3b p.85 Interpretations of Stratigraphie Unit 3c p.86 iii REFERENCES................................................p.89 APPENDIX................................................. p.98 One: Legend............................................ p. 98 Two:Sketches of several sedimentary structures in strat. unit three.................................... p.99 PLATES................................................. p. 100 One:Section one outcrops and sedimentary structures • • p.101 Two:Strat* unit 3a sedimentary structures and cross-beds.......................................... p. 102 Three:Sedimentary structures and cross-beds in strat* unit 3a...................................... p. 103 Four :Sedimentary structures and cross-beds in strat. unit 3&...................................... p. 104 Five:Cross-bed associations in strat. unit 3 b ........ p. 105 Six:Sedimentary structures in strat* unit 3b and outcrop picture of 3 c ...........................p. 106 iv List of Figures Figure 1: Location map of Two Medicine Formation. p.2 Figure 2: Correlation chart of Upper Cretaceous rocks of Montana ............... P-3 Figure 3: Location map of study area................. p.5 Figure 4: Location map of stratigraphie columns measured p.6 Figure 5: Telegraph Creek Formation at section one. p.9 Figure 6: Virgelle Formation at section one ......... p. 13 Figure 7: Typical Virgelle prograding beach sequence. p. 15 Figure 8: Modern beach profile: subenvironments p. 16 Figure 9: Two Medicine Formation at section one p.27 Figure 10 Major Facies of lower deltaic plain. p.28 Figure 11 Montana Group at section two .... p.44 Figure 12 Two Medicine Formation at section two p.46 Figure 13 Correlation of sections ......... p.49 Figure 14 Complexly cross-bedded unit....... p.52 Figure 15 Outcrop map of complexly cross-bedded sandstone p.53 Figure 16 Rose diagrams at stratigraphie unit 3a p.55 Figure 17 Rose diagrams of superimposed bedforms p.57 Figure 18 Rose diagrams of stratigraphie unit 3b p.59 Figure 19 Rose diagrams of stratigraphie unit 3b p.61 Figure 20 Rose diagrams of stratigraphie units 3b and 3c p.63 Figure 21 Rose diagrams of stratigraphie unit 3c . p.65 Figure 22 Distribution of stratification types on dune p.69 Figure 23 Contrasting climbing translatent strata. p.71 Figure 24 Wavy-crested bedforms..................... p.73 Figure 25 Superimposed dune, and scour pit associations p.75 Figure 26 Lee-eddles on wavy-crested bedform ....... p.76 Figure 27 Reactivation surface formation ........... p.79 Figure 28 Straight-crested and lunate dunes......... p.82 Figure 29 Transgressive sequence at section two. p.87 Figure 30 Depositional environments on modern barrier island p.88 ACKNOWLEDGEMENTS The research for this thesis was largely funded by grants from the American Association of Petroleum Geologists, and Sigma Xi Scientific Research Society, to which I am very grateful. This thesis was largely improved from discussions and encouragement from Dr. Johnnie N. Moore, to whom I am greatly indebted. I would also like to thank Dr. Donald Winston for valuable insights into sedimentological problems. Dr. David Rubin for discussions on bedform relationships, and William Cobban for help identifying pelecypods. Thanks also to Dr. Potts for his interest in my project. Also a special thanks to my parents for moral, and periodic financial support. Vi INTRODUCTION The Upper Cretaceous (Early Campanian) Two Medicine Formation (Flg.1) comprises the dominantly terrestrial part of a 750 meter thick clastic wedge of Montana Group rocks (Fig.2) that were deposited as an overall regressive sequence In, and adjacent to, an extensive epicontinental sea that covered much of Montana during the Cretaceous Period (Vlele and Harris, 1965). The Montana Group In western Montana consists of the Telegraph Creek, Virgelle, and Two Medicine
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