Tabular (planar) cross-beds consist of cross-bedded units that are large in horizontal extent relative to set thickness and that have essentially planar bounding surfaces. Tabular cross-bedding is formed mainly by migration of large-scale, straight-crested ripples and dunes. Significance -Formation -Environments -Referencestrough cross bedding formation. Shiguai Formation, Inner Mongolia, China. Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments. Planar Bedding. Beach and Longshore Bar Cross-bedding.
The inititial formation of ripples is not well understood, but probably has something to do with burst and sweep processes which disrupt the smooth surface (sweeps) and deposit a small pile of grains as a result of decelleration (bursts). Note the trough cross-bedding on the cut that is perpendicular to flow (i.e. the v-w cut) This figure shows flow along the bed across a set of sinuous-crested bedforms. CROSS-BEDDING is a feature that occurs at various scales, and is observed in conglomerates and sandstones. Diagram illustrating the formation of a graded bed (turbidite). Similar large scale trough cross-bedding occurs in the Paleocene Fort Union Formation of Wyoming. Based on the grain size of the Newcastle Sandstone, the minimum current velocity necessary to erode the troughs was about 20 cm/sec; deposition of the cross-beds probably occurred in current velocities of less than 3 cm/sec.
See: trough cross stratification and other examples at: dune cross stratification See ripple cross lamination on Mars: martian ripples. The basic ideas of dune and ripple formation are the same. Trough cross-stratification is generated by the migration of linguoid ripples or dunes. Sand-sized grains comprising the planar cross-bedding found in a Late Wisconsinan outwash deposit near Caledon, Ontario, show an obliquely downslope preferred o. This asymmetric mode of grain fabric was formed during deposition of a laterally accreted outwash bar.
Dawn’s Sed Strat Lecture Notes: Cross Stratification
Current ripples are small bedforms formed by the effects of boundary layer separation on a bed of sand. Trough cross-bedding formed by the migration of sinuous subaqueous dunes typically has asymptotic bottom contacts and an undulating lower boundary.