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By Thomas J. Holecek Prepared in Cooperation with the George

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By Thomas J. Holecek Prepared in Cooperation with the George UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY THE RELATIONSHIP OF MORPHOLOGY, STRUCTURE AND LITHOLOGY TO THE EMPLACEMENT OF THE HOT CREEK RHYOLITE FLOW, LONG VALLEY, CALIFORNIA By Thomas J. Holecek f-. U. S. Geological Survey OPEN FILE REPORT 79 ~ This report Is preliminary a^d haa not fcoe~s edited or reviewed for conformity xvith Geo!o.gical Survey standards or nomenclature. Prepared in cooperation with the George Washington University Reston, Virginia December 1978 The relationship of morphology, structure and lithology to the emplacement of the Hot Creek Rhyolite flow Long Valley, California By Thomas J. Holecek B.S. June, 1975, George Washington University A Thesis submitted to The Faculty of The Graduate School of Arts and Sciences of The George Washington University in partial satisfaction of the requirements for the degree of Master of Science September 30, 1978 CONTENTS PAGE INTRODUCTION.............................................. 1 PREVIOUS STUDIES.................................... 1 FIELD DATA......................................... 3 ACKNOWLEDGEMENTS. ......... .^*. .»».»............,.. 3 MORPHOLOGY AND GENERAL GEOLOGY.* .....>...........»......... .4-11 ERUPTIVE AND POST-ERUPTIVE HISTORY. ............. * *»..».12-13 MINERALOG*rANB CHEMISTRY ................. ..............14-15 LITHOLOGY AND 20NATION...................................16-27 SOUTHWEST CLIFFS SECTION a........................ 18 HOT CREEK GORGE SECTION b......................... 23 COMPOSITE FLOW TOP SECTION C...................... 24 SUMMARY............................................ 24 PERLITIC ALTERATION...............................26-27 STRUCTURE................................................ 28-41 FLOW RIDGES........................................ 28 FOLIATION.......................................... 28 LINEATION.......................................... 28 FLOWFOLDS.......................................... 28 JOINTS AND FRACTURES..............................28,37 INTERPRETATION OF STRUCTURES...................... 37,40 STRUCTURAL PATTERNS............................... 40-41 EMPLACEMENT MECHANICS (CONCLUSION).......................42-47 REFERENCES...............................................48-49 LIST OF FIGURES, TABLES AND PLATES FIGURE PAGE 1. GENERALIZED GEOLOGIC MAP OF LONG VALLEY............. 2 2. AIRPHOTOS OF THE HOT CREEK FLOW..................... 5 3. N-S AND E-W PROFILES OF HOT CREEK FLOW. ............. 6 4. FOLDED LAKE SEDIMENTS............................... 7 5. TABULAR DELTAIC SEDIMENTS OVERLYING FOLDED SEDIMENTS 8 6. PHOTOGRAPH OF SUBAERIAL PART OF THE FLOW SHOWING TOPOGRAPHY AND VEGETATION........................... 9 7. TYPICAL KNOBBY OUTCROP-SUBLACUSTRINE PART OF FLOW... 10 8. ZONATION MODEL...................................... 17 9. A. THREE SECTIONS THROUGH THE FLOW.................. 19 B. SYNTHESIZED CROSS SECTIONS SHOWING INFERRED ZONATION......................................... 19 10. LITHOLOGIC MAP (MAP OF ZONES)....................... 20 11. PHOTOGRAPH OF FUSED BASAL BRECCIA................... 21 12. PHOTOGRAPH OF SPHERULITIC OBSIDIAN IN CONTACT WITH BANDED FELSITE................................. 22 13. PHOTOGRAPH OF CEMENTED BASAL BRECCIA................ 25 14. MAP OF FLOW RIDGES.................................. 29 15. MAP OF FOLIATION.................................... 30 16: MAP OF LINEATION................................ .... 31 17. MAP OF FLOW FOLDS................................... 32 18. PHOTOGRAPH OF FOLIATION AND TENSION GASHES.......... 33 19. PHOTOGRAPH OF LINEATION AND HAIRLINE FRACTURES...... 34 19. A. PHOTOGRAPH OF HAND SAMPLE SHOWING LINEATION...... 35 20. PHOTOGRAPH OF FLOW FOLDS............................ 36 21. PHOTOGRAPH OF WELL-DEVELOPED FOLIATION AND FRACTURES 38 (continued) PAGE 22. PHOTOGRAPH OF CONJUGATE JOINTS..............~...... 39 23. A. INFERRED N-S STRUCTURAL CROSS SECTION (A-A'K... 43 B. PHOTOGRAPH OF RAMP STRUCTURE.................... 43 24. SCHEMATIC DIAGRAM SHOWING EMPLACEMENT HISTORY...... 44 TABLE I CHEMICAL ANALYSIS, NORM AND MODAL ANALYSIS 5 PLATE I LITHOLOGIC MAP............................BACK PLATE II STRUCTURE MAP. ....... .................. .POCKET INTRODUCTION The Hot Creek rhyolite flow/ located 13 km east of Mammoth Lakes/ California, is one of 12 Pleistocene rhyolite flows that erupted in the moat of Long Valley caldera following collapse of the caldera (Bailey and others, 1976.) It erupted 280,000 years ago within Pleistocene Long Valley lake and was in large part sub- lacustrine (Fig.l). The general purpose of this study is to describe the morph­ ology and internal structure of the flow and to infer from these its emplacement and cooling histories. As about two-thirds of the body was emplaced sub-aqueously, the effect of the lake water on the flow is an important aspect of study. PREVIOUS STUDIES The earliest description of rhyolite flow features, in Eng­ lish, was included in a paper by Iddings (1888). He described and illustrated various textures in the rhyolite of Obsidian Cliff at Yellowstone Park, Wyoming. Iddings also presented an explanation of the process by which foliation developed. A classic study of domes by Williams (1932) and investiga­ tions of the Mono Craters in California by Putnam (1938) and Loney (1968) provided basic information on the structure and lith- ology of rhyolite extrusions which were useful in the present study. Christiansen and Lipman (1966) described the emplacement and cooling history of Comb Peak Rhyolite flow in Nevada. Fund­ amental concepts drawn from this paper were used as a basis for study and interpretation of the structure and lithology in Hot Creek flow. A similar study by Haeffner (1975, 1976) of the Shoshone volcanics. Death Valley, California, also provided use- Figure 1. Generalized geologic map of Long Valley, California (modified from Bailey, 1974) showing relations of Hot Creek flow to other features of Long Valley caldera. INYO DOME HOLOCENE I RHYOLITES. 37* W- . \ OUTLINE OF LONG VALLEY QUARTZ LATHES' CALDERA FLOOR ; - MOAT RHYOLITES PLEISTOCENE LONG VALLEY LAKE SHORELINE (7200') ro fill background information. FIELD DATA The field work upon which this study is based was carried out during the summers of 1975 and 1976 and forms part of a broader study of the Long Valley caldera. The topographic map upon which the field data was plotted, was prepared at a scale of 1:7900 from U.S. Forest Service air photos using a Kearn PG-2 plotter. The contours are generally accurate to within +10 ft. Due to lack of adequate control points, contours at the southern tip of the flow are only accurate to within + 40 ft. ACKNOWLEDGEMENTS John Lewis and Roy Bailey were co-advisors of this thesis project. I wish to thank John Lewis of the George Washington University for his encouragement during revision of the first and final drafts of the thesis. I also wish to thank Roy Bailey of the U.S. Geological Survey who suggested this study and gave me initial assistance in the field. He contributed many hours of dis­ cussion as well as thoughtful guidance and encouragement. He played a major role in the final stages of this project especially as an editor. Bob Hackman of the U.S. Geological Survey helped prepare the topographic base map. I also wish to thank Joanna Kalber for typing part of the final manuscript and Cynde Sears for helping prepare the lithologic map (Plate I) and proof reading. MORPHOLOGY AND GEOLOGY The Hot Creek flow is roughly elliptical in plan (Fig. 2) and thins from south to north (Fig. 3). Its southern perimeter is bounded by steep slopes and vertical cliffs whereas its northern perimeter is topographically more subdued and partly buried by lake sediments. The presence of the Pleistocene lake is indicated by 1) the surrounding deltaic lake deposits, 2)the gravel-covered lake terraces at various locations around the caldera, and 3) the distribution of ice-rafted granitic and metamorphic float below 7250' on the surface of Hot Creek flow. The lake was fed by a stream flowing in the vicinity of Hot Creek as shown by alluvial gravel deposits south and west of the flow. The upper surface of the flow exhibits two distinctly diff­ erent morphologies. (Fig. 2 and Fig. 3a and 3b). The southern third of the flow (Fig. 3c) has greater local topographic relief than the northern two-thirds (Fig. 3d) which has more subdued, rounded topography. The boundary between these two parts of the flow is very irregular and generally coincides with the 7200* to 7250* ft. topographic contours, which in turn coincide with the post-eruptive lake level (see Bailey, 1976). Thus, the boundary is the former lake shoreline. Figures 3a and 3b show in profile, the gradual transition from subaerial to sublacustrine. The southern, subaerial surface (Fig. 2 and Fig. 6) is com­ posed of nearly equally-spaced, asymmetrical, arcuate ridges as much as 200 m long, that grade into randomly spaced, linear to arcuate ridges of diverse size and shape, separated by narrow swales. The sublacustrine part of the flow surface is character­ ized by relatively small, linear ridges and rounded knobs which rise above broader, rounded hills and extensive flat areas (for Figure 2. Stereo-pair of airphotos showing the flow outline, and morphologic differences above and below the paleoshoreline. Figure 3. N-S and E-W profiles of Hot Creek flow showing difference between subaerial and sublacustrine morphology. approximate s lake level c- VIEWED FROM czjaWafusf/tmr 1 contour SOUTH subaericr/ VIEWED FROM WEST o> Figure 4. Tuffaceous conglomeritic
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