Abstract

Tabular sets of planar cross-stratification are characteristic structures produced by transverse bars in the sand-bed Platte River in Nebraska. Certain sedimentologic characteristics of the Platte bars provide potentially useful information for interpreting the origin of planar cross-strata in ancient deposits. Alternating coarse and fine-graded foreset laminations result from continuous avalanching at bar margins of sediment previously sorted by small scale bed forms on the bar surface. Surface dune transport yields thicker and more distinct coarse-fine foreset laminae than ripples. Foreset angles of inclination tend to decrease with increasing flow regime. Several structures are produced at active bar margins swept by side currents, including ripples and dunes perpendicular to the bar slip face, foreset spurs, and straight-crested ripples on the slip face. Each of these structures produce small scale cross-stratification oriented at high angles to the associated planar cross-bed. Large differences between current and foreset dip azimuths frequently occur because of the irregular and lobate shapes of transverse bars. Only 30.5% of the bar foreset directions correspond to within 5 degrees of the currents that formed them. Foreset dip azimuths in a straight 2.1-km reach of the lower Platte are widely dispersed, although their vector mean accurately identifies the direction of the main channel complex. Planar cross-bed orientations computed for the entire Platte plus the Nebraska portion of the South Platte yielded a variance of 6,129 arid a vector magnitude of 35.6%, indicating dispersions greater than commonly assumed for braided streams. Possibilities for distinguishing planar cross-stratification formed by braided stream transverse bars from similar stratification types produced in other environments are discussed.

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