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Fabric Criteria for Distinguishing Pseudo Ripple Marks from Ripple Marks

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Fabric Criteria for Distinguishing Pseudo Ripple Marks from Ripple Marks BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA FABRIC CRITERIA FOR DISTINGUISHING PSEUDO RIPPLE MARKS FROM RIPPLE MARKS BY EARL INGERSON CONTENTS Page Abstract...................................................................................................................................... 558 Introduction............................................................................................................................... 558 Ripple-marked sandstone and quartzite.............................................................................. 559 Moenkopi sandstone....................................................................................................... 559 Minnelusa sandstone...................................................................................................... 560 Baraboo quartzite............................................................................................................. 561 Pseudo ripple marks................................................................................................................. 562 Drag folds in Baraboo quartzite................................................................................... 562 Drag folds in Archean of the Grand Canyon............................................................ 563 Weisner quartzite............................................................................................................ 565 Fluted vein quartz.......................................................................................................... 567 Conclusions................................................................................................................................ 569 Works to which reference is made....................................................................................... 569 ILLUSTRATIONS Figure Page 1. 300 c-axes of quartz grains from Moenkopi sandstone............................................. 559 2. 200 normals of muscovite flakes from Moenkopi sandstone.................................... 559 3. 200 c-axes of quartz grains from surface of ripple mark in Minnelusa sandstone. 560 4. 400 c-axes of quartz grains from Minnelusa sandstone............................................. 560 5. 300 quartz axes from the specimen of Baraboo quartzite shown in Figure 1 of Plate 1 ............................................................................................................................ 561 6. Poles to the cleavage planes of 250 muscovite flakes from specimen of drag- folded Baraboo quartzite shown in Figure 2 of Plate 1 ...................................... 562 7. 200 quartz axes from the same specimen as Figure 6 .............................................. 562 8. Poles of 96 biotite flakes from surface of crenulations of Archean schist speci­ men of Figure 3 of Plate 1......................................................................................... 563 9. Poles of 100 biotite flakes from part of Archean specimen between crenulations and quartzite layer...................................................................................................... 564 10. Poles of 100 biotite flakes from quartzite layer of Archean specimen of Figure 3 of Plate 1 ....................................................................................................................... 564 11. 200 quartz axes from schist layer of Archean specimen.......................................... 565 12. 200 quartz axes from quartzite layer of Archean specimen................................... 565 13. Poles of 100 muscovite flakes from specimen of Weisner quartzite shown in Figures 1 and 2 of Plate 2 ......................................................................................... 566 14. 200 quartz axes from specimen of Weisner quartzite shown in Figures 1 and 2 of Plate 2 ....................................................................................................................... 566 15. Axes of 200 large quartz grains from fluted vein quartz, Alabama M ine............ 567 16. Axes of 150 large quartz grains from fluted vein quartz, Alabama M ine............ 567 (557) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/4/557/3431008/BUL51_4-0557.pdf by guest on 26 September 2021 558 EABL INGEES0N— DISTINGUISHING EIPPLE MAEKS Figure Page 17. Axes of 200 small quartz grains from veinlet in fluted quartz 568 18. Normals to 200 sericite flakes from veinlet in fluted quartz.. 568 Plate Facing page 1. Ripple mark and pseudo ripple marks......................................... ......... 564 2. Pseudo ripple marks......................................................................... ......... 565 ABSTRACT Statistical grain orientation studies of two unmetamorphosed ripple-marked sand­ stones, a ripple-marked quartzite, and four pseudo ripple marks show that a dis­ tinction between ripple marks and pseudo ripple marks can be made from fabric characteristics even when field data are equivocal. Some of these pseudo ripple marks had been previously interpreted as ripple marks. Quartz diagrams were prepared for all the rocks, and mica diagrams for those that contained mica. Without exception, the axes of the pseudo ripple marks are important fabric directions, both for mica and quartz. The ripple-mark axes are also principal fabric directions for mica, but there are important and easily recog­ nizable differences between the arrangement in the ripple marks and pseudo ripple marks. Quartz orientation in the ripple-marked sandstones may or may not be significant, but it is radically different from that found in the pseudo ripple marks. The origin of each pseudo ripple mark is discussed briefly. INTRODUCTION In the study of folded and metamorphosed sedimentary rocks there are many structural elements that are important in solving complicated relationships and deducing the geologic history of a region. It is well known that secondary structures may closely resemble primary sedi­ mentary features—so closely, in fact, that well-trained geologists may be unable to agree from the field evidence upon the origin of a given feature. Any microscopic criteria or fabric criteria, therefore, that will serve to distinguish between such features, or that can locate elements (such as fold axes) when they cannot be determined in the field, should prove of value in structural interpretations. Perhaps the commonest example is the problem of distinguishing with certainty between bedding and secondary s-planes in metamorphosed sedimentary rocks. Less frequently encountered, but no less important in certain interpretations, are other distinctions: between cross-bedding and s-folds, between concretions and fossils, between elongate fillings of Joint intersections and fossils, between ripple marks and drag folds of similar size, and between color-streaking in sedimentary rocks and secondary lineations in metamorphic rocks. This paper has to do with the distinction between ripple marks and features of metamorphic rocks that can be confused with them—that is, “pseudo ripple marks.” The following groups of specimens have been studied: (1) Unmetamorphosed ripple-marked sandstones, (2) ripple- marked quartzite, (3) pseudo ripple marks in quartzite and schist, and Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/4/557/3431008/BUL51_4-0557.pdf by guest on 26 September 2021 INTRODUCTION 559 (4) fluted vein quartz. Grain orientation diagrams were prepared for each of the specimens, quartz diagrams for all of them, and mica diagrams for those that contained mica. RIPPLE-MARKED SANDSTONE AND QUARTZITE (1) Moenkopi sandstone.—Two specimens of undoubted ripple marks in sandstones that have not been subJected to regional metamorphism F igure 1.—300 c-axes of quartz grains F igure 2.—ZOO normals of muscovite from Moenkopi sandstone flakes from Moenkopi sandstone St = bedding plane. R a = ripple-mark Small sketch in center represents entire area axis. Numbers are percentages of all quartz of thin section. (X %)• axes measured that fall on one per cent of area of proJection sphere. were studied. The first of these is from 100 feet below the top of the Moenkopi formation in Davis Canyon, southeastern Utah, furnished by A. A. Baker, U. S. Geological Survey. The quartz diagram from this specimen (Fig. 1) shows essentially random orientation, since the areas of maximum concentration of the quartz axes show only 3 per cent of the axes falling on 1 per cent of the area of the proJection sphere, and the areas showing minimum concentrations (blank spaces in the dia­ gram) are small in extent. There is a faint suggestion that the c-axes of the quartz tend to lie parallel to the ripple axis rather than in any other position. This orientation would be expected if the grains tend to be elongate parallel to the c-axis and if the elongate grains are rolled along with their long axes normal to the direction of motion as they are deposited. (See Wayland (1939) for a discussion of elongation and optical orientation of quartz grains in sandstone.) Of the 300 grains measured, 100 were on the ripple-mark surface and the other 200 were measured along lines normal to the bedding in two separate partial diagrams. All three partial diagrams were very similar. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/4/557/3431008/BUL51_4-0557.pdf by guest on 26 September 2021 560 EARL INGERSON— DISTINGUISHING RIPPLE MARKS In contrast to quartz, the muscovite in the Moenkopi
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