Tag Archives: jackfork formation

Geo-pic of the week: Slickensides

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The grooved surface pictured above is a slickenside.  Slickensides indicate the relative direction of movement between fault blocks (hanging wall moved up, down, laterally, etc..).  

Slickensides form when fault blocks move against each other.  The natural irregularities on each scratches grooves into the other.  The grooves are parallel to movement;  for instance in this example, movement was either to the right or the left.  To tell whether it was right or left, you can rub your hand along the slickensides.  They feel smooth in the direction the fault moved and rough in the opposite direction – it’s like petting a dog from tail to head.  Slickensides are a valuable tool because determining fault movement can be a challenge when there are no easily-recognized beds that can be correlated across the fault to show the sense of offset.

The shale above was photographed in Big Rock Quarry, North Little Rock, AR.  It’s a part of the Jackfork Formation (Pennsylvanian).

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Geo-pic of the week: Flute casts

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Fig. A – Many flute casts that truncate one another.  Black arrow points downstream.

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Fig. B – Flute casts with characteristic round heads pointing in upstream direction.  Black arrow points downstream.

 

Pictured above are beds of sandstone displaying flute casts.  Flute casts are common in channel environments (for instance river channels) where water is carrying sediment and debris (rocks, shells, sticks etc..). 

As debris is carried along, it randomly grazes the mud in the channel bottom, scouring divots.  The divots are typically deeper and narrower in the upstream direction, with a round head pointing upstream.  Flute casts form subsequently when sand in-fills these divots and later becomes a rock, preserving casts of the divots on the bottom of the sand bed. 

Flow direction indicators, such as flute casts, are one of many clues geologists use to reconstruct the history of the earth.

Geopic of the week: Reverse fault or normal fault?

Fault for geopic

Depicted above is a fault in the Jackfork Formation.  The fault is exposed in a quarry in southern Arkansas in the Ouachita Mountains.  Along the plane that separates the two blocks is about a one meter (3 ft) zone of highly fractured rock called fault gouge that formed due to the friction between the blocks as they moved.  The block on the left has moved upward relative to the one on the right; I could determine that because I was able to trace beds from one block to the other and see the sense of offset between them.

The beds on the left block are bent upward near the fault plane (center of photo); this implies that they have moved down relative to the block on the right – the opposite of what I said in the last paragraph.   Most likely this fault has experienced multiple episodes of movement, first thrusting upward due to compressional stress, and later sliding back down as the compression relaxed.  This kind of complex structure is typical of the greatly deformed rocks of the Ouachita Mountains.

To see more views of this fault click here

Geopic of the week: injection feature

 

Sandstone injection feature hollywood quarry

Pictured above is what geologists term an injection feature or sand dike.  It formed when sand was violently forced upward into overlying clay before the sediment was cemented to form rock.  In environments where sediment is accumulating very quickly, water can get trapped and buried in a sand body;  as more sediment is deposited on top of the sand, the pressure causes the sand body to compress.  When water erupts upward to relieve the pressure, it carries sand with it which fills the fissure created by the escaping water. 

Geologists look for clues like injection features when trying to unravel the mystery of what conditions were like when a rock was deposited.  This particular rock is part of the Jackfork Formation which is exposed at the surface around Little Rock Arkansas and surrounding areas; it was deposited when the area was at the bottom of a deep ocean basin more than 300 million years ago.  Ink pen is for scale.