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WILLIAM LEE STOKES Department of Geological and Geophysical Sciences, University of Utah, Salt Lake City, Utah

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WILLIAM LEE STOKES Department of Geological and Geophysical Sciences, University of Utah, Salt Lake City, Utah WILLIAM LEE STOKES Department of Geological and Geophysical Sciences, University of Utah, Salt Lake City, Utah Primary Current Lineation or Parting Lineation? Abstract: The terms "primary current lineation" and "parting lineation" have both been ap- plied to the same depositional feature of sedimentary rocks. It is agreed that the origin is a stream- lining effect on loose sand grains parallel with current direction. A superficially similar effect is seen on parting planes of certain metamorphic rocks and is due to the intersection of s-surfaces. This lineation is at right angles to the tectonic movement. It is suggested that the term "primary current lineation" be retained for the sedimentary depositional feature to avoid confusion with the lineation imposed by movement within metamorphic rocks. Introduction would convey the concept Cloos had in mind. Stokes (1947) independently proposed the term Geologic nomenclature is a mixture of terms "primary current lineation" from examples with genetic and non-genetic connotations. In seen in fluvial sandstones of the Colorado general, if the origin of a feature is clear, a Plateau. Later, Crowell (1958) introduced the genetic term should prove useful; if the origin term "parting lineation" in connection with is not clear a non-genetic or purely descriptive studies in the early Tertiary Flysch of the term is better. The field of sedimentary struc- External Prealps, Switzerland. tures furnishes examples of both usages and also Stokes defined primary current lineation as not a few ambiguous terms. For example, the "a streamlining or streaming effect of sand genetic term "oscillation ripple mark" is com- particles in relatively loose and poorly defined monly applied to the same feature as the non- windrow-like ridges parallel with the current genetic "symmetrical ripple mark." Recurring direction" (Stokes, 1947, p. 152). Crowell's discussions of ripple marks reflect the problems description defines parting lineation as "faint that arise when origins and concepts are not streaks parallel to flow. These streaks are clear. very small warps, creases, or faint shallow grooves in the bedding plane, and seem to have Discussion been formed during sedimentation . ." This note is intended to present arguments (Crowell, 1955, p. 1361). Crowell noted the in favor of the genetic term "primary current tendency for rocks to break or "part" along lineation" as opposed to the non-genetic term surfaces showing lineation, hence the descrip- "parting lineation." The historical background tive term based on a mechanical property. He is pertinent. Lineation of particles in flowing also noted that parting lineation is "similar" water was evidently produced experimentally to current lineation, seeming to imply that two by Sorby as early as 1848 (1908, p. 188); he distinguishable features might in fact exist so called the effect "graining in the line of cur- that two names are justified. Observations by rent." However, no experimentation appears many geologists would seem to establish beyond to have produced successive thin laminations doubt that parting lineation and current linea- with lineated surfaces that are without question tion are the same. duplicates of natural occurrences. The feature In the very large and ever-growing family was recognized in the field as a primary direc- of lineations and tectonites there is a type of tional indicator by Hans Cloos and described lineation which needs to be clearly distin- by him in 1938. His very good photograph of a guished from that just described. Lineation specimen bears the title "primaren Parallel- which superficially resembles current lineation gefiiges auf Schichtflachen von Sandstein" is found in metamorphic rocks and is termed (Cloos, 1938, Fig. 2, p. 358). I assume that a c-lineation. As defined by Turner and Weiss translation such as "primary parallel structures (1963, p. 101-103), c-lineation is produced by or textures of bedding planes in sandstone" intersecting s-surfaces (s-surfaces being pene- Geological Society of America Bulletin, v. 79, p. 1419-1420, October 1968 1419 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/10/1419/3428041/i0016-7606-79-10-1419.pdf by guest on 03 October 2021 1420 W. L. STOKES—LINEATION TERMS trative planar structures such as slaty cleavage). lineated rocks and tectonites have made a dis- Extensive masses of quartz-mica schist and tinction that is applicable here. They describe mica-quartz schist displaying c-lineation have primary tectonites as those with fabric com- been seen by the writer in the east-central ponents created in response to movement in an Snake Range, Nevada, and the Raft River enclosing medium; sedimentary rocks with Range, northwestern Utah; the feature is prob- primary directional structures are included ably present also at other localities. These rocks here. Secondary tectonites are defined as those are widely used as building stones and trim in in which components and fabric elements have the Western states chiefly because they are responded directly to deformational forces; easily split (or parted) into thin slabs showing dynamically metamorphosed rocks are in- attractive, grained, lined, or shadowy surfaces. cluded here. Rocks showing current lineation and rocks Plate 1 illustrates lineated fluvial sandstone showing c-lineation both show an obvious from Mesozoic rocks of the Colorado Plateau tendency to split or part. In the metamorphic together with lineated quartz-mica schist from rock planes of weakness are caused by a con- the Raft River Range, northwestern Utah. centration of platy or micaceous minerals. In These are randomly selected specimens typical fluvial sandstone, in comparison, the parting of large masses of rock. The illustration shows planes are apparently due to original difference better than words why, in the opinion of the in cohesiveness between and within the layers writer, the term "parting lineation" is unsatis- or laminations. At the time of formation of factory. It makes no distinction between two lineated sand there is apparently a tendency types of lineation that are deceptively similar for the grains of one layer to attain a certain in appearance but entirely different in origin degree of effective cohesiveness before the next and meaning. In contrast, the term "primary layer is deposited. In both types of rock being current lineation" designates a specific type of discussed the distances between planes of maxi- lineation and leaves no doubt as to origin and mum weakness range from one-eighth to one geologic context. Inasmuch as no formal rules inch, but such planes are unequal in distribu- of priority have yet been applied in the descrip- tion. tion of sedimentary structures, it would seem Lineation in fluvial sandstones is parallel best to use the term primary current lineation with the direction of flow of the transporting as originally proposed. The term c-lineation ap- medium. The direction of movement within pears to be entirely suitable for designating the metamorphic rocks under discussion is con- lineation of the type found in metamorphic sidered to have been transverse to the lineation. rocks such as those here described. Knopf and Ingerson (1938) in their study of References Cited Cloos, Hans, 1938, Primare Richturgen in Sedimenten der rheinischen Geosyncline: Geol. Rundschau, v. 29, p. 357-367. Crowell, J. C., 1955, Directional-current structures from the Pre-alpine Flysch, Switzerland: Geol. Soc. America Bull., v. 66, p. 1361-1384. Knopf, E. B., and Ingerson, E., 1938, Structural petrology: Geol. Soc. America Mem. 6, 270 p. Sorby, H. C., 1908, On the application of quantitative methods to the study of the structure and history of rocks: Geol. Soc. London Quart. Jour., v. 64, p. 171-233. Stokes, W. L., 1947, Primary lineation in fluvial sandstones: A criterion of current direction: Jour. Geology, v. 55, p. 52-53. Turner, F. J., and Weiss, L. E., 1963, Structural analysis of metamorphic tectonites: New York, McGraw- Hill, 545 p. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 4, 1968 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/10/1419/3428041/i0016-7606-79-10-1419.pdf by guest on 03 October 2021.
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