Tag Archives: Soft Sediment Deformation

Geo-pic of the week: Syn-depositional Faulting

Fault with lables

 

Pictured above is one of many faults, closely spaced together, in an outcrop of the Atoka Formation, near Lake Fort Smith, Arkansas.  The fault pictured extends from the upper right to the lower left and is highlighted.  This type of faulting is called syn-depositional faulting, meaning it occurred at about the same time the rock was being deposited.  It results in disturbed-looking outcrops like this one.

Around 300 million years ago, plate tectonic forces were deforming the Ouachita Mountains in south central Arkansas. Those forces also caused faulting in the southern Ozark Plateaus, as the sediment that composes this rock outcrop was being deposited.  The freshly deposited sediment wasn’t fully consolidated when the faulting took place and the rock surrounding the fault got contorted by the stress. 

Some of the deformed features of the outcrop are labeled above.  The Zone of Soft-Sediment Deformation is the area surrounding the fault where the rock has been deformed by shearing: there is no recognizable bedding in that zone.  The soft clay-rich Deformed Shale was squeezed plastically between the fault blocks in that soft sediment deformation zone.  The bedding orientations surrounding the deformation zone (indicated by magenta lines) vary greatly, because the soft bedrock was broken and heaved around by the fault.

GeoPic of the Week: Soft Sediment Deformation

clip_image002

Soft sediment deformation in silty shale of the Imo Formation near the Middle Fork of the Little Red River near Shirley, Arkansas. Features like that pictured here result from sediment being exposed to pressure prior to being completely lithified. Many factors can contribute to soft-sediment deformation including uneven settling, slumping, and escape during compaction of water trapped in the sediment.