Tag Archives: shale

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.

Geopic of the week: Turbidite deposits in Baumgartner Quarry

The Baumgartner quarry is located a little South of Kirby Arkansas along highway 27 in the Ouachita Mountains.  It exposes approximately 160 m. (590 ft.) of the upper Jackfork Formation which consists of interbedded sandstone and shale deposited  about 300 million years ago when the area that is now the Ouachita Mountains was a deep ocean basin.  Deep oceanic deposits such as these are the kind petroleum geologists explore for oil.  Because these deposits are exposed at the surface in southern Arkansas, geo-scientists from all over come here to study our rocks and gain a greater understanding of the deeply buried rocks they look for oil in in places like the Gulf of Mexico.  This is a view looking west along strike.

DSCN0777