Tag Archives: depositional environments

Geopic of the week: The Maumelle Chaotic Zone

Chaotic zone

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

Geopic of the week: Bioturbation

Knabioturbation

Pictured above is clay and sand of the Nacatoch Formation of southern Arkansas.  The clay beds, which stand slightly in relief, have been churned, and the sand has numerous cylindrical structures of various sizes.  These are hallmarks of bioturbation, or reworking of sediment by living things.  The cylindrical structures are the preserved casts of roots.  The disrupted clay beds are evidence of the burrowing of mud-loving critters.

Bioturbation structures are commonly preserved in rock and offer glimpses into the environment where sediment was deposited.  Specifically, it tells us that this sediment was near the surface in a relatively calm near-shore marine environment long enough after deposition for living organisms to move in and set up shop.  Clues like these are one of many tools geologists use to piece together the early history of the earth from the rock record.

For more views of bioturbation click here

Geopic of the week: Oolitic Limestone

oolitic pitkin

This is a close-up picture of a hand-sized specimen of oolitic limestone.  It’s called “oolitic” because it’s composed chiefly of ooliths which are the round, sand-sized grains that make up the majority of the rock.  An Oolith is a grain of marine sediment formed by repeated precipitation of minerals from sea water around a nucleus; the nucleus is typically a tiny fossil fragment or speck of sand.  They form in very shallow marine shoals where waves are agitating the grains on the sea floor causing them to tumble around.  As they tumble they accrete concentric mineral layers (usually calcium carbonate but sometimes other minerals) around them and grow larger.  Once formed, ooliths can be transported by currents in the same way as sand grains, accumulate in various marine environments and form rocks.