Tag Archives: structure

Geopic of the week: Deformation bands

Def Bands (18)b

The features that crisscross the surface of the sandstone pictured above are deformation bands.  They are micro faults that form where the bedrock is under strain.  They typically develop near larger faults and the orientation of the bands is determined by the orientation of the stresses acting on the rock.

The grains along deformation bands have been crushed, rotated, and reorganized.  The resultant bands are harder and less permeable than the rock they formed in, which causes them to stand in relief when the rock weathers.

Geologists look for deformation bands as indicators that a fault may be nearby.  This photo was taken in Van Buren County, Arkansas on the downthrown block of a normal fault.

For more views of deformation bands click here

Geopic of the week: well exposed fault

DSCN0743

The picture above shows a normal fault exposed in an old railroad grade just north of Greers Ferry lake in the Ozark Mountains of central Arkansas.  Two different lithologies are juxtaposed against one another: siltstone on the left and sandstone on the right.   Exposures such as this one are relatively rare in the Ozarks where most of the bedrock is covered by loose sediment and vegetation.

This fault is extensive and roughly parallels the northern portion of Greers Ferry lake.  The fault gives the northern portion of the lake its linear shape (see below).

Greers Ferry lake

Geopic of the week: Recumbent folds

City Quarry Fox Pass6

Tight recumbent folds in a fresh exposure of Bigfork Chert, Ouachita Mountains Arkansas.  Thin chert beds divided by siliceous shale beds are the basic lithology of this Ordovician age (540 to 490 Ma) formation.  Complicated structures like those pictured below are characteristic of the rocks that form the Ouachita Mountains.  The complex folds and faults resulted from plate tectonic forces that compressed the rock and caused many of the structures to rotate and overturn.

For more views of this and similar folds click here