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Directional-Current Structures from the Prealpine Flysch, Switzerland

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Directional-Current Structures from the Prealpine Flysch, Switzerland BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 66. PP. 1361-1384. 16 FIGS.. 2 PLS. NOVEMBER 1966 DIRECTIONAL-CURRENT STRUCTURES FROM THE PREALPINE FLYSCH, SWITZERLAND BY JOHN C. CROWELL ABSTRACT Gurnigel sandstone beds in the Paleocene-Eocene Flysch of the External Prealps, Switzerland, contain primary structures which display common orientation through many feet of strata. The linearity of some of these directional-current structures is easily measured in the field. The bottom of beds display groove-cast and load-cast primary- current lineations; within a layer are found clast lineations subdivided into grain and charcoal-fragment lineations, and parting lineations consisting of streaks on bedding- planes. Flute casts, torose load casts, current stratification, convolute bedding, and ripple marks also occur. These, and other factors, suggest that the sediments were laid down in a relatively sheltered basin by turbidity currents. In three sections of the Gurnigel Flysch 401 field measurements of the orientation of current structures show that during the Paleocene and Eocene a source for sediment lay not far to the northwest. This information, with published data from clast types and the distribution and facies of other Paleocene and Eocene units, implies that a land- mass or island probably lay within the northwestern part of the Ultrahelvetic sedimenta- tion region within which the Gurnigel Flysch accumulated, and near the border of the Helvetic region. The Oligocene and Pliocene culminations of the Alpine orogeny carried the Gurnigel Flysch far to the northwest within Ultrahelvetic nappes. CONTENTS TEXT Figure Page 5. Stereonet plot of cross-stratification poles Page in current stratification 1361 Introduction 1352 6. Stereonet plot of cross-stratification Description of sections studied 1352 poles 1362 Directional-current structures 1357 7. Stereonet plot of cross-stratification Lineation 1357 poles 1363 Current structures on bottom of beds 1358 8. Current-rose diagram, La Mortivue sec- Substratal lineation 1358 tion 1 1365 Flute casts 1359 9. Current-rose diagram, La Mortivue sec- Torose load casts 1360 tion II 1366 Current structures inside of beds 1360 10. Current-rose diagram, La Mortivue sec- Internal lineation 1360 tion III 1367 Current stratification 1361 11. Current-rose diagram, Schwarzenbiihl sec- Convolute bedding 1364 tion 1368 Ripple marks on top of beds 1364 12. Current-rose diagram, Falli-Holli sec- Field measurements of directional-current tion 1369 structures 1364 13. Scheme of tectonic units in the Prealps. 1371 Site and manner of deposition 1366 14. Diagrammatic section showing possible Tectonics of External Prealps 1371 distribution of land, sea, and regions of Conclusions 1383 sedimentation during the Paleocene References cited 1383 and Eocene 1372 15. Tectonic cross section through the Ro- mande Prealps 1372 ILLUSTRATIONS Plate Facing Page Figure Page 1. Gurnigel sandstone and current lineations.. 1358 1. Sketch map of External Zone of Romande 2. Current structures 1359 Prealps, Switzerland 1353 2. Sketch maps showing locations of observa- tion stations in the La Mortivue sec- TABLES tion 1353 Table Page 3. Sketch maps showing locations of observa- 1. Current structures from Prealpme Flysch. 1357 tion stations in the Schwarzenbiihl and 2. Number of directional observations of each FaUi-Holli sections 1354 type 1369 4. Geologic cross sections through External 3. Field measurements of directional-current Prealps 1355 structures 1374 1351 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/11/1351/3426832/i0016-7606-66-11-1351.pdf by guest on 24 September 2021 1352 J. C. CROWELL—DIRECTIONAL-CURRENT STRUCTURES INTRODUCTION Kuenen (1952; 1953a; 1953b), Kuenen and Carozzi (1953), and Kopstein (1954) have Sandstone layers in the Flysch of the External pointed out the ease with which information on Prealps, Switzerland, display numerous small- current direction and sense can be obtained by scale sedimentary structures, such as current noting the orientation of small-scale sedi- bedding, groove casts, and load casts, which mentary structures. The work of Kuenen has were formed when the strata were deposited. given new insight into the way that certain Inasmuch as these structures show a remarkable sequences were laid down by turbidity cur- degree of common orientation through many rents, the significance of which had been largely feet of strata, they may be useful in determining overlooked by geologists and oceanographers. the nature and direction of the currents re- Our understanding has come about through sponsible for them. They are therefore de- studies of sedimentation processes in the labora- scribed and classified here, particularly from the tory (Kuenen and Migliorini, 1950), through viewpoint of the field geologist interested in the recognition of sand layers in relatively deep obtaining rapidly some information on the slope basins of present-day oceans (Shepard, 1951; of the sea floor when the sediments were laid Erickson et al., 1951; 1952; Ewing, 1953, etc.), down, and the direction to the source of sedi- and through the study of sedimentary rocks in ment supply. Data gathered within the External the field (Natland and Kuenen, 1951; Kuenen, Prealps and other facts have forwarded under- 1953a, 1953b; Kopstein, 1954). standing of the paleogeography during deposi- The writer is very grateful for the Guggen- tion; an effort has been made to fit these data heim Fellowship and the Fulbright Award into present concepts of the geologic history and which have made his Alpine studies possible tectonic emplacement of the Prealps. while on sabbatical leave from the University Previous workers have also employed sedi- of California, Los Angeles. He also thanks mentary structures to determine current Professor Bruno Sander, University of Inns- direction and sense in the area of deposition. In bruck, Austria, for the use of space and equip- 1843, Hall (p. 234) used groove and flute casts ment in his Institute where the laboratory and as indicators of current directions in the office work were undertaken. Appreciation is Devonian of New York, which has been studied extended to Professor Jean Tercier, University more recently from this viewpoint by Clarke of Fribourg, Switzerland, for critical reading of (1918) and Rich (1951, p. 12). Cross-stratifica- the manuscript and for illuminating discussions tion was used as an indicator of current sense in on the geology of the Prealps. The writer is also 1858 by Sorby (1908, p. 197), and has since indebted to Edward L. Winterer and Ph. H. been studied by Knight (1929), Brinkman Kuenen for comments on the manuscript and to (1933), Shotten (1937), Jungst (1938), Reiche Betty Bruner Crowell for the drafting. (1938), Bausch van Bertsbergh (1940), McKee (1940), Wilson et al. (1953), Potter and Olson DESCRIPTION OF SECTIONS STUDIED (1954), and others, as have ripple marks by Hyde (1911), Bucher (1919), Kay (1937; 1945), The three sections of the Paleocene-Eocene Bausch van Bertsbergh (1940), and King Flysch studied for this paper are exposed in the (1948). Oriented fossils, such as graptolites, northeastern arc of the external Prealps be- cephalopods, and fusulinids, were used by tween Lake Geneva and the A are River in Ruedemann (1897), Kay (1945), and King western Switzerland (Fig. 1). They lie near the (1948). Many of these papers contain maps with northwestern limit of the Ultrahelvetic Nappes, current arrows. In Germany, Richter (1935), and just above the complex of southeast-dipping Hantzschel (1935), Kraus (1935), Kieslinger thrusts which marks the base of the giant (1937), Riicklin (1938), and Bausch van Berts- Prealpine klippe. Large-scale sketch maps of bergh (1940) investigated many of the kinds of the three sections are shown in Figures 2 and 3 structures described in this paper, and other and the local geology on Figure 4. workers have also contributed to an under- The Flysch rocks of the External Prealps, standing of these structures (Fuchs, 1877; 1895; which reach a thickness of about 3000 m, con- Fugger, 1899; Shrock, 1948). sist of two main stratigraphic parts with an Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/11/1351/3426832/i0016-7606-66-11-1351.pdf by guest on 24 September 2021 DESCRIPTION OF SECTIONS STUDIED 1353 EXPLANATION s-ac SWITZERLAND Kilofnlton SHOWING AREA TO LEFT After "Corfe oeoloaigue genera/ de to Suisse", Feuitle S, 1946 6 Feuille 6, 1942. FIGURE 1.—SKETCH MAP OF EXTERNAL ZONE OF ROMANDE PREALPS, SWITZERLAND Location of the three sections studied, and the general current sense as inferred from primary sedimentary structures, are shown LA MORTIVUE SECTION Dams for erosion control with number painted on each LOCATION MAP From Voulrui (359) & Cholel St.-Denis (455) sheets jAtlos topogrophiaue d< la Suisse. FIGURE 2.—SKETCH MAPS SHOWING LOCATIONS OF OBSERVATION STATIONS IN THE LA MORTTVTJE SECTION interfingering gradation between: the Wild- of marly shale with interbedded sandstone, flysch below and the Gurnigel sandstone above breccia, conglomerate, and limestone, but is (Tercier, 1928, p. 3, 65). The Wildflysch, of best known for its exotic blocks and thrust variable character and thickness, is composed slices in the shale. The Gurnigel sandstone, Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/11/1351/3426832/i0016-7606-66-11-1351.pdf by guest on 24 September 2021 1354 J. C. CROWELL—DIRECTIONAL-CURRENT STRUCTURES with which we are concerned
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