Cross-bedding

Abstract: The South Saskatchewan River has a long term average discharge of 275 m3/sec, with flood peaks in the range of 1500 to 3800 m3/sec. South of Saskatoon, the four major types of geomorphological elements recognised are channels, slipface-bounded bars, sand flats and vegetated islands and floodplains. Major channels are 3-5 m deep, up to 200 m wide, and flow around sand flats which are 50-2000 m long, and around vegetated islands up to 1 km long. At areas of flow expansion, long straight-crested cross-channel bars form. During falling stage, a small part of the crest of the cross-channel bar may become emergent, and act as a nucleus for downstream and lateral growth of a new sand flat. The dominant channel bedforms are dunes, which deposit trough cross bedding. Cross-channel bars deposit large sets of planar tabular cross bedding. Sand flats that grow from a nucleus on a cross-channel bar are mostly composed of smaller planar tabular sets, with some parallel lamination, trough cross-bedding, and ripple cross-lamination. A typical facies sequence related to sand flat growth would consist of in-channel trough cross-bedding, overlain by a large (1-2 m) planar tabular set (cross-channel bar), overlain in turn by a complex association mostly of small planar tabular cross-beds, trough cross-beds and ripple cross-lamination. By contrast, a second stratigraphic sequence can be proposed, related only to channel aggradation. It would consist dominantly of trough cross-beds, decreasing in scale upward, and possible interrupted by isolated sets of planar tabular cross-bedding if a cross-channel bar formed, but failed to grow into a sand flat. During final filling of the channel, ripple cross-lamination and thin clay layers may be deposited. In the S. Saskatchewan, these sequences are a minimum of 5 m thick, and are overlain by 0.5-1 m of silty and muddy vertical accretion deposits.

Abstract: Dunes that are morphologically of linear type, many of which are probably of longitudinal type in a morphodynamic sense, are common in modern deserts, but their deposits are rarely identified in aeolian sandstones One reason for non-recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long-term sand-transport direction, thereby depositing cross-strata that have unimodal cross-bed dip directions and consequently resemble deposits of transverse dunes Dune-parallel components of sand transport can be recognized in ancient aeolian sands by examining compound crossbedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along-crest direction over the large dunes

Abstract: The distinction of many ancient eolian from aqueous cross bedded sandstones remains controversial. Although several diagnostic small-scale features have been recognized recently, little attention has been given to the genesis of contorted cross bedding, which is common in some formations. Types of deformation affecting cross bedded sands include sandflow-deformed, parabolically overturned and complexly contorted, and brecciated and/or faulted. Sandflows are due to oversteepening of lee faces of both subaerial and subaqueous dunes. Brecciated sands form on subaerial dunes due to gravity failure of lee faces moistened with capillary water as by coastal fog or rain; faulting can form in that same situation but also in brittle layers deformed at depth. Paraboli overturning generally forms at a subaqueous depositional interface due to current shear and scour of a partially liquefied sand bed, but it can form by drag folding along a buried shear surface as well. More intensely contorted cross laminae also can form either at the depositional interface or after burial due to liquefaction of saturated sand. Excess pore pressure and liquefaction required for parabolic overturning and complex contortion must have developed under water-saturated conditions. Data from modern' cross bedded sands show that porosities of 40 to 50 percent are common, especially where sandflow layers are abundant. Experimental evidence indicates that the angle of internal friction drops sharply when porosity exceeds 43 percent. Such porous sands are therefore very susceptible to liquefaction by collapse of grain packing due either to mechanical loading by deposition, storm waves, floods, or seismicity. When loosely-packed sands fail, increase of pore pressure results in a rapid liquefaction chain reaction. As excess pore pressure dissipates, a liquefied zone "freezes," consolidation proceeds from the margins inwa d. Greatest deformation should occur near the center, which remains liquefied longest. The most extreme condition is an almost complete loss of stratification due to irregular motions of grains during liquefaction. The Weber and Navajo sandstones of Utah display exceptional examples of most types of deformation. Most of the cross bedded sandstones of both formations display features characteristic of eolian deposition yet also display contorted zones indicative of failure in a saturated condition. Gradational or faulted upper boundaries of most of these contorted zones indicate that most deformation occurred after burial and below a water table. Deformation was triggered by liquefaction most likely due to seismicity or rapid fluctuations of water table. Because deformations modify porosity and permeability, their presence must affect sandstones as potential water and petroleum reservoirs.