Genetic Significance of Deformed Cross Bedding--With Examples from the Navajo and Weber Sandstones of Utah

Abstract

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.