Just wanted to let you know that the Statemap 2014-15 field mapping project has resulted in the publication of three new geologic maps. These are the Parma, Prim, and Greers Ferry quadrangles. Reduced images are posted below. These should be available as .pdfs on our website in the near future. I’ll keep you posted!
Greers Ferry Quadrangle
Also, I would like to thank the many people who helped with data collection in the field this year, without whom this project would have been impossible.
Andy Haner Danny Rains
Angela Chandler Stefanie Domrois
Doug Hanson Ty Johnson
Now it’s off to the Brownsville quad for next year!
This is a picture of sandstone and shale of the Maumelle chaotic zone that outcrops along highway 10 west of North Little Rock, Arkansas. The Maumelle chaotic zone is part of the Jackfork Formation which forms the bedrock around much of the Little Rock area. The chaotic zone is called that because of the disarray the rock is in there. In the example above, broken blocks of sandstone are interspersed with disorganized shale beds that have been rolled, squashed and otherwise deformed (rock hammer at center is for scale). The rocks weren’t deposited this way but were originally organized into horizontal beds on a deep-water ocean slope. Before they could be hardened into solid rock, the slope failed and the beds were transported down hill in a massive submarine landslide.
Note: Other interpretations for this zone have been proffered. The author of this blog prefers the above interpretation.
For more views of the Maumelle chaotic zone click here
This is a “sandstone pipe” in the Witt Springs Formation of north central Arkansas. At first glance, it seems that someone has managed to insert a pipe into the outcrop (either that, or someone had fairly sophisticated plumbing 320 million years ago). Actually, these naturally occurring features result from iron minerals precipitating out of ground water as it moves through rock. When minerals precipitate from a solution, they do so in concentric bands known as liesegang bands. They were named forRaphael Liesegang: the chemist that first produced them in the laboratory. Once the band of minerals has formed, it makes that part of the rock harder, and, as the rock erodes, the iron-fortified band stands out in relief. Typically liesegang bands form in organic shapes like the ones that surround the pipe above. When they form a cylindrical band, however, they look almost identical to iron pipes.
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.
Pictured above is about all that remains exposed of the rock outcrop that is the namesake of the capitol of Arkansas, La Petite Roche or The Little Rock. The outcrop was given its name by early french explorers to the area who first arrived in 1722. Though unimpressive in stature, it is notable because it is the first exposure of solid rock that one sees when navigating up the Mississippi and Arkansas rivers, starting at the Gulf of Mexico.
The geologic explanation for this is simple; The land between New Orleans and Little Rock Arkansas along that route is in the Mississippi River Valley, where the bedrock is buried beneath river-lain and wind-blown sediment. The city of Little Rock straddles the boundary between the lowlands of the Mississippi River Valley and uplands of the Ouachita Mountains, which stretch west well into Oklahoma. Therefore this little sandstone and siltstone outcrop marks that physiographic boundary.
If you boated all the way from New Orleans to Little Rock without seeing a single rock you would probably be impressed by it too.