35 Sediments, Clastic: New Insights on Old Problems (Posters)

Sunday, 18 October 2009
9:00 AM - 6:00 PM
Hall A

SOFT SEDIMENT DEFORMATION FEATURES IN THE JURASSIC NAVAJO SANDSTONE OF WHITE POCKET, VERMILION CLIFFS NATIONAL MONUMENT

Paper number: 35-3
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The White Pocket area of the Vermilion Cliffs National Monument exhibits well-preserved facies of soft sediment deformation in the upper Jurassic Navajo Sandstone, exposed in an area ~1.6 km2 close to the Utah-Arizona border. The types of deformation can be categorized into three facies: 1) breccia blocks, 2) contorted bedding, and 3) massive facies.

The breccia blocks are angular to subangular clasts (cm- to m-scale) of eolian crossbedded and laminated sandstone in a massive to contorted crossbed sandstone host. This breccia facies is generally the stratigraphically lowest deformation facies. It is 0-10 m thick, and has an abrupt and sharp contact with underlying, undeformed crossbedded sandstone. Larger blocks are commonly contorted or misshapen. Block orientation appears to be random, with no apparent directionality yet determined.

Microfault offsets (few centimeters) of the contorted eolian laminae indicate brittle deformation. Plastic deformation ranges from wavy or ribbon-like folds that are gently curving, to bedding that is completely folded back (recumbent) on itself.

Massive sandstone typically caps the other deformed facies and exhibits a bleached white color. The exposed surface weathers into a distinctive polygonal pattern, with larger first-order polygons (meter scale diameter), and second-order (inset) smaller polygons.

Although some soft-sediment deformation features in the literature are attributed to water table fluctuations, the origin of the deformed features suggest a large triggering event, perhaps from bolide impacts, earthquakes, or loading from burial. The deformation took place after burial while sediment was slightly lithified, where deformation and liquefaction is constrained within major bounding surfaces.