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Geological Survey Standards. CONTENTS UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Depositional Environment of the Middle Proterozoic Spokane Formation-Empire Formation Transition Zone, West-Central Montana By James W. Whipple Open-File Report 80-1232 1980 This report is preliminary and has not been edited or reviewed for conformity with U.S. Geological Survey standards. CONTENTS Page Introduction*........................................................... 1 Acknowledgments.................................................... A Purpose and scope.................................................. A Previous studies................................................... A Geologic setting................................................... 5 Stratigraphic subdivision.......................................... 6 Primary sedimentary structures.......................................... 7 Bedding and lamination............................................. 8 Graded lamination............................................. 8 Planar lamination............................................. 11 Rhythmic sand and mud bedding................................. 1A Wavy lamination............................................... 1A Ripple bedding and lamination................................. 17 Algal lamination and loferites................................ 19 Bedding-plane marks and imprints................................... 19 Subaerial shrinkage cracks.................................... 21 Subaqueous shrinkage cracks................................... 21 Raindrop impressions.......................................... 25 Gas expulsion pits............................................ 25 Salt crystal casts............................................ 28 Wrinkle marks................................................. 28 Mudchip breccias.............................................. 30 Target marks.................................................. 30 Flow structures............................................... 32 Ripple marks.................................................. 32 Deformational structures........................................... 35 Load structures............................................... 35 Ball-and-pillow structures.................................... 35 Small-scale diapiric structures............................... 38 Fluid escape structures............................................ Al Fluidization channels......................................... Al Birdseye structures........................................... Al Contorted structures............................................... 45 Lithofacies........................................**................... A5 Lithofacies A...................................................... A5 Lithofacies B...................................................... A5 Lithofacies C...................................................... A7 Lithofacies D...................................................... A7 Correlation............................................................. A7 Depositional environment................................................ 52 Lithofacies interpretation......................................... 52 Lithofacies A................................................. 52 Lithofacies B................................................. 52 Lithofacies C................................................. 53 Lithofacies D................................................. 53 Depositional model................................................. 5A Paleotidal range................................................... 56 References.............................................................. 58 Page Appendices.............................................................. 66 Appendix A, Terminology............................................ 67 Appendix B, Stratigraphic sections*................................ 67 McCabe Creek section.......................................... 67 Copper Creek section.......................................... 80 Diamond drill holes........................................... 96 ii ILLUSTRATIONS Page Figure 1. Relative position of Belt basin.............................. 2 2. Present stratigraphic terminology and correlation in the Belt basin............................................. 3 3 * Graded lamination in red beds................................ 9 4. Thin section of graded lamination in green beds.............. 10 5. Graded lamination (wavy) in green beds....................... 9 6. Planar lamination............................................ 12 7. Thin section of planar lamination............................ 13 8. Rhythmic sand-to-mud bedding with well-preserved flasers... 15 9* Lenticular bedding and lamination............................ 15 10. Typical, wavy, even parallel lamination of green beds........ 16 11. Ripple lamination in green calcareous siltite................ 18 12. Sinusoidal ripple lamination in red, subfeldspathic arenite.................................................... 18 13. Cryptalgal lamination in grayish-green loferites............. 20 14. A large oncolite preserved in lower Empire Formation......... 20 15* Subaerial shrinkage cracks................................... 22 16. Compacted subaerial shrinkage cracks in profile.............. 22 17 Linear subaqueous shrinkage cracks........................... 23 18. Birdsfeet subaqueous shrinkage cracks........................ 23 19* Subaqueous shrinkage cracks in profile....................... 24 20. Elliptical raindrop impressions.............................. 26 21. Circular raindrop impressions................................ 26 22. Gas expulsion pits on bedding plane surface of green calcareous siltite......................................... 27 23. Closeup of gas expulsion pits showing marginal, concentric cracks.......................................... 27 24. Probable salt crystal casts.................................. 29 25. Wrinkle marks on surface of red argillite.................... 29 26 Mudchip breccia ............................................. 31 27* A typical target mark........................................ 31 28. Oriented flow structure...................................... 33 29* Closeup of flow structure ................................... 33 30. Truncated, asymmetrical ripple marks in arenite ............. 34 31. Load structure of subfeldspathic arenite deforming interlaminated argillite................................... 36 32. Closer view of load structure................................ 37 33. Ball-and-pillow structure at McCabe Creek.................... 39 34. Typical ball-and-pillow structure at Copper Creek............ 39 35. Diapiric structures of dark-red argillite penetrating an arenite bed............................................. 40 36. Fluidization channel in slabbed green core from Blacktail Mountain......................................... 42 37. Fluidization channel in red beds at Copper Creek............. 43 38. Birdseye structure in green calcareous siltite............... 44 39* Contorted structure in red argillites........................ 46 40. Anticlinal contorted structures with fluidization channels occupying axes.................................... 46 41. Correlation of red and green lithofacies within the transition zone............................................ 48 iii Page 42. Correlation of lithofacies between McCabe Creek and Copper Creek....*...*.*.................................... 49 43. Distribution and percentage of rock types in the McCabe Creek section....................................... 50 44. Distribution and percentage of rock types in the Copper Creek section....................................... 51 45. Peritidal depositional model for the Spokane Formation- Empire Formation transition zone........................... 55 46. A view of the McCabe Creek exposures......................... 68 47.. Geologic map of the McCabe Creek area........................ 69 48. Copper Creek exposures....................................... 81 49. Geologic map of the Copper Creek area........................ 82 iv INTRODUCTION During late Precambrian time, fine-grained siliciclastic and carbonate sediments were deposited in and around an epicratonic basin presently located over portions of Washington, Idaho, Montana, and southern British Columbia and Alberta, Canada (fig. 1). The basin was relatively shallow and tectonically stable over a long period of time, and accumulated a package of rocks approximately 20 km thick known as the Belt Supergroup (Harrison and others, 1974; Harrison, 1972). This basin of deposition has been termed the Belt basin. Over the last few years, Harrison (1972) and others (Mudge and others, in press) have correlated the stratigraphy in the basin. This monumental task has provided subsequent workers a firm base for studying more detailed aspects of basin sedimentation. Figure 2 represents present-day interpretation of the stratigraphy in the Belt basin. Traditionally, the contact between the Spokane Formation and the Empire Formation is placed at the boundary where the dominant color of the section changes from red to green. This has led to some inconsistency in mapping, correlation, and interpretation of these formations; however, the change in dominant color has been recognized basin wide and thus remains a primary criterion used for distinguishing these formations. A distinct transition from red beds of the Spokane Formation to green beds of the conformably overlying Empire Formation is marked by a zone of alternating red-
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