Geological Survey Professional Paper 754—C The Portage Lake Volcanics (Middle Keweenawan) on Isle Royale, Michigan

Cover

Abstract

Introduction

Historical background
Scope of study
Acknowledgments

The Portage Lake Volcanics on the Keweenaw Peninsula

The Portage Lake Volcanics on Isle Royale

General description
Volcanic rocks
     Classification and nomenclature
     Composition
     Stratigraphic units
          Porphyrite flows
               Scoville Point, Middle Point, and Tobin Harbor Flows
               Huginnin and Grace Island Flows
          Trap flows
               Edwards Island and Long Island Flows
               Minong Flow
               Amygdaloid Island Flow
          Ophite flows
               Greenstone Flow
               Hill Point Flow
               Washington Island Flow
Sedimentary rocks
Pyroclastic rocks
Stratigraphic summary
Stratigraphic thickness

Correlations

Correlation between Isle Royale and the Keweenaw Peninsula
Relation to other Keweenawan volcanic rocks

Structure

Origin of the Portage Lake Volcanics

References cited

FIGURE

1. Sketch map showing generalized distribution of Keweenawan rocks in the Lake Superior region

2. Chart showing nomenclature for middle and upper Keweenawan rocks of northern Michigan and adjacent parts of Wisconsin

3. Longitudinal stratigraphic section of the Portage Lake Volcanics on the Keweenaw Peninsula

4. Photographs showing characteristic textures of volcanic rocks on Isle Royale

5. Photograph showing coarse ophite with knobby weathered surface

6. Chart showing nomenclature for named volcanic flows within the Portage Lake Volcanics on Isle Royale

7. Plot showing tholeiitic affinities of analyzed specimens of rocks from the Portage Lake Volcanics on Isle Royale

8. Plot illustrating the predominantly basaltic nature of analyzed specimens of rocks from the Portage Lake Volcanics on Isle Royale

9. Shaded-relief map of Isle Royale National Park

10. Schematic columnar section of the Portage Lake Volcanics on Isle Royale

11. Simplified map of bedrock geology of Isle Royale

12—16. Photographs showing:

12. Porphyrite from Huginnin Flow

13. Beach cobble of porphyrite from Grace Island Flow

14. Columnar jointing in Edwards Island Flow

15. Specimen of Amygdaloid Island Flow with characteristic agate amygdules

16. Specimen of ophite from Washington Island Flow

17. Columnar section of upper part of Portage Lake Volcanics showing distribution of sedimentary units in the Chippewa Harbor area

18—21. Photographs showing:

18. Scour-and-fill structure in pebbly sandstone near the west end of Chippewa Harbor

19. Breccia occurring above the Amygdaloid Island Flow

20. Tuff-breccia overlying the Greenstone Flow

21. Agates typical of those occurring in the tuff-breccia overlying the Greenstone Flow

22. Geologic section of Isle Royale in the vicinity of Lake Desor and Houghton Ridge

23. Longitudinal stratigraphic section showing variations in thickness of the Portage Lake Volcanics on Isle Royale

24. Stratigraphic diagram illustrating correlation of sedimentary horizons in the Portage Lake Volcanics between Isle Royale and the Keweenaw Peninsula

TABLES

1. Analyses of volcanic rocks of the Portage Lake Volcanics, Isle Royale

2. Correlation of clastic units in the Portage Lake Volcanics between the Keweenaw Peninsula and Isle Royale

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UNITED STATES DEPARTMENT OF THE INTERIOR
ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY
V. E. McKelvey, Director

Library of Congres catalog-card No. 73-600161.

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