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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.
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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.
This is a picture of a sedimentary rock called novaculite collected in the Ouachita Mountains of central Arkansas. On the surface are thin coatings of a manganese oxide mineral in shapes reminiscent of trees. When a mineral forms in these tree-like patterns, it is called a dendrite (named for the impulse-transmitting nerve component of a similar shape). Manganese oxide minerals commonly form in this peculiar morphology.
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Here is a photograph of Pitkin Limestone from the Ozark Mountains near Fox, Arkansas. This exposure displays a classic orthogonal joint set. The joints are the easy-to-see fractures that divide the bedrock into square blocks. Orthogonal means the joints formed at roughly 90 degree angles to each other, hence the resultant square blocks.
Joints are common features in sedimentary and crystalline bedrock, and they form in a variety of patterns in response to the stresses the rock has been subjected to. Essentially, bedrock is being compressed, and the joints form to relieve that pressure. The squeezing and resultant fracturing result from natural processes such as burial, erosion, and plate tectonics.
Joints are important because they convey information about stress-fields that have acted on the rocks in the past. They can also be useful for understanding the flow of fluids through a petroleum reservoir or aquifer when trying to maximize production from an oil or water well.
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