UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY The Purcell Lava Glacier National Park, Montana by Robert G. McGimsey 1 Open-File Report 85-0543 1985 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editoral standards and stratigraphic nomenclature Denver, Colorado CONTENTS Page LIST OF FIGURES.......................................................... iii LIST OF TABLES........................................................... vii ABSTRACT................................................................. 1 ACKNOWLEDGMENTS.......................................................... 2 CHAPTER I. INTRODUCTION..................................................... 3 Purpose of Investigation....................................... 3 Location and Geologic Setting.................................. 3 Previous Work.................................................. 4 Field and Laboratory Methods................................... 5 II. DIVISION AND DESCRIPTION OF FACIES............................... 6 Introduction................................................... 6 Pillow Lava Facies............................................. 6 Two-Dimensional Pillow Exposures............................. 7 Three-Dimensional Pillow Exposures........................... 9 Isolated- and Broken-Pillow Breccia.......................... 9 Coalesced Pillows and the Upper Contact...................... 10 Hyaloclastite Breccia........................................ 10 Intercalated Sediment........................................ 11 Pahoehoe Facies................................................ 11 Morphology................................................... 11 Internal Structures.......................................... 13 Interbedded Pillow Interval.................................. 14 Intercalated Sediments....................................... 15 Vent Facies.................................................... 15 Hypabyssal Diabase Sill........................................ 16 III. PETROGRAPHY AND GEOCHEMISTRY..................................... 17 Petrography.................................................... 17 Introduction and General Petrography......................... 17 Plagioclase................................................ 17 Pyroxene................................................... 17 Olivine.................................................... 17 Iron-Titanium Oxide Minerals............................... 17 Copper-Iron Sulfide Minerals............................... 18 Pillow Lava.................................................. 18 Hyaloclastite Breccia........................................ 18 Flow Units................................................... 18 Vent Facies.................................................. 18 Diabase Sill................................................. 19 Amygdules.................................................... 19 Oxide Mineralogy............................................. 19 General Oxide Mineralogy................................... 19 Pillow Lava................................................ 19 Flow Units................................................. 20 Vent Facies................................................ 20 Hypabyssal Diabase Sill.................................... 20 Geochemistry................................................... 20 Introduction................................................. 20 Determination of Petrogenetic Rock Type...................... 21 Effects of Diagenetic Alteration on Geochemistry............. 22 Immobile Elements................................ 23 Mobile Elements............................................ 23 Geochemical Relation of Purcell Lava to the Middle Belt Carbonate Sill and Dike.................... 24 IV. DISTRIBUTION, CORRELATION AND INTERPRETATION OF FACIES........................................................ 25 Vent Facies.................................................... 27 Diabase Sill................................................... 28 Broken-Pillow Breccia and Isolated-Pillow Breccia.............. 29 Pahoehoe Flow Units............................................ 30 Environment of Emplacement..................................... 32 Age of the Purcell Lava and its Relationship to Intrusives..... 33 V. CONCLUSIONS...................................................... 35 REFERENCES............................................................... 37 APPENDICES............................................................... 44 A. SNOWSLIP FORMATION............................................... 45 B. MEASURED SECTIONS OF PURCELL LAVA IN GLACIER NATIONAL PARK............................................ 48 C. CHEMICAL ANALYSES OF MIDDLE BELT CARBONATE SILL.................. 68 ii FIGURES Figure 1. Location of Glacier National Park and present Belt terrane......... 69 2. Physiographic map of Glacier National Park with location of study sites..................................................... 70 3. Correlation chart for the Belt and Purcell Supergroups............. 71 4. Correlation diagram of past and present stratigraphic terminology within Glacier National Park........................... 72 5. Cross-sectional sketches of the general strati- graphic relationship of Purcell Lava facies........................ 73 6. a: Outwardly protruding pillows................................... 74 b: Distinctive hummocky outcrop produced by outwardly protruding pillows............................................. 75 7. Matrix (hyaloclastite) supported pillows........................... 75 8. Ruptured undeformed sediment slabs displaced during emplacement of pillow lava......................................... 76 9. Photographs of basal contact of pillow lava with Snowslip strata showing soft-sediment deformation.................. 77 10. Relatively undeformed sediment slabs displaced during emplacement of pillow lava......................................... 78 11. Hyaloclastite tuff beds interlayered with Snowslip beds at basal contact with pillow lava, Kootenai Peak section.............. 79 12. Large, massive zones in the pillowed facies at Kootenai Peak....... 80 13. Cross section of a pillow showing thick, concentrically layered, hyaloclastite breccia..................................... 81 14. a: Subspherical vesicles concentrically arranged about the perimeter of a pillow.......................................... 82 b: Pipe vesicles concentrically arranged about the perimeter of a pillow.......................................... 82 15. Cross-section of a fractured pillow illustrating the probable origin of hyaloclastite breccia....................... 83 16. Cross-sectional view of interconnected pillows..................... 84 17. a: Tightly-packed pillows with little interstitial hyaloclastite breccia.......................................... 84 b: Elongate, closely-packed, tubular pillows...................... 85 18. Bulbous and cylindrical pillows.................................... 86 19. Elongate, sinuous pillows.......................................... 87 20. a: "Neck and bulb" shape of pillows which were pinched and swelled during formation.......................... 90 b: Elongate, sinuous pillow that abruptly changed direction during budding....................................... 90 21. Isolated-pillow breccia............................................ 91 22. Broken-pillow breccia.............................................. 93 23. a: Coalesced pillow zone near top of the pillowed facies.......... 93 b: Coalesced pillows at the transition from pillowed facies to pahoehoe facies...................................... 94 24. Ropy texture of pahoehoe toes at the transition from the pillowed facies to pahoehoe facies............................. 95 25. Cross-sectional views of the transition from the upper pillowed facies to the pahoehoe facies............................. 96 26. Interpillow hyaloclastite breccia.................................. 97 27. Erosionally truncated flow units forming a stair-stepped slope..... 98 28. a: Tabular flow unit in the pahoehoe facies....................... 98 b: Lenticular flow units in the pahoehoe facies................... 99 c: Fan-shaped lava tongues in pahoehoe facies..................... 100 iii 29. Three flow units illustrating some of the features used to discern upper and lower contacts........................... 101 30. Thin, pale-olive flow tops characteristic of many flow units....... 102 31. Ropy textures of pahoehoe flow units............................... 103 32. Bleached and oxidized weathering zone at the top of an uppermost flow unit............................................. 105 33. Typical flow units of the Purcell Lava............................. 106 34. Thinning and pinching out of flow units............................ 107 35. Thin lobes of scaly pahoehoe having smooth, billowy surfaces and protruding toes....................................... 108 36. Top of a flow unit having adjacent surface pahoehoe ropes that are convex in opposite directions....................... 109 37. Cross-sectional view of massive pahoehoe with tumulus.............. 109 38. Thin, tightly overlapping flow units of scaly pahoehoe............. 110 39. Amygdaloidal pipe vesicles at the base of a flow unit.............. 110 40. Sketch showing how some flow units were emplaced from opposite directions based on the inclination of pipe vesicles............... Ill 41. Layers of coalesced vesicles in flow units......................... Ill 42.