Problems in Interpretation of Cross-Stratification in a Borehole
Terry D. Taylor, Rudy L. Slingerland
The proper use of cross-bedding produced by the migration of large-scale ripples in estimating paleoflow requires an understanding of the geometry produced by the migration of three-dimensional ripples.
Three-dimensional sinuous and lunate crested ripple forms are simulated using a numerical geometric model. A ripple train with a constant or random phase angle and planar or tangential avalanche faces is migrated over many times steps to form ripple sets. A borehole is then cut through a closet for which the bed dips and azimuths of the cross-strata are recorded. As the simulated borehole is opened, the curviplanar surface of the infilling beds and their erosive bottoms are shown on a two-dimensional surface. A plot of the deviation of the measured dip direction from the true dip direction to the number of beds measured shows the number of beds that must be measured to give an accurate estimate of the true flow direction.
Cases were studied in which the phase angle and the three-dimensionality of the crestline were varied. Ripple crests with a constant phase angle show the range of bed dip directions to increase as the crest-line passes from a straight to a lunate form. For a constant phase angle and sinuous crestline, less than 20 bed dip directions must be measured to predict the true flow direction. Ripple crests with a random phase angle and increasing three-dimensionality show a dramatic increase in the number of beds that must be measured to estimate the true flow direction. For random phase angles and sinuous crestlines, at least 40 bed dip directions must be measured to predict the true flow direction.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.