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).
Pictured above is one of many faults, closely spaced together, in an outcrop of the Atoka Formation, near Lake Fort Smith, Arkansas. The fault pictured extends from the upper right to the lower left and is highlighted. This type of faulting is called syn-depositional faulting, meaning it occurred at about the same time the rock was being deposited. It results in disturbed-looking outcrops like this one.
Around 300 million years ago, plate tectonic forces were deforming the Ouachita Mountains in south central Arkansas. Those forces also caused faulting in the southern Ozark Plateaus, as the sediment that composes this rock outcrop was being deposited. The freshly deposited sediment wasn’t fully consolidated when the faulting took place and the rock surrounding the fault got contorted by the stress.
Some of the deformed features of the outcrop are labeled above. The Zone of Soft-Sediment Deformation is the area surrounding the fault where the rock has been deformed by shearing: there is no recognizable bedding in that zone. The soft clay-rich Deformed Shale was squeezed plastically between the fault blocks in that soft sediment deformation zone. The bedding orientations surrounding the deformation zone (indicated by magenta lines) vary greatly, because the soft bedrock was broken and heaved around by the fault.
Pictured above is a mineralized vug (approximately 3 inches long) in chert. A vug is a void or open space in a rock. Many vugs are filled with minerals after water that is saturated with a certain mineral flows through the rock. This mineralization can happen in multiple stages. The vug above was initially filled with silica-rich fluid therefore quartz precipitated out of solution and lined the walls of the vug. Afterwards calcite precipitated, as is evident from the larger crystal on the interior left of the vug.
This vug is present in a section of ornamentally banded chert. Chert is a sedimentary rock made up of microcrystalline quartz. It can be a variety of colors or banded and quite beautiful. The chert above is Devonian age (416-359 million years ago) from northwest Arkansas.
Accessory minerals are minerals found in igneous rocks that are not used for the classification or naming of the rock. These minerals may be commonly present in a type of rock, but the absence of the mineral would not change the general classification geologists give to the rock.
The two accessory minerals in the center of the picture above are greenish-black needles of aegirine (AY-jur-EEN) and orangish-pink analcime (uh-NAL-seem) crystals. These minerals are frequently found together in igneous intrusions of syenite like the one present at Granite Mountain, where this sample was collected.
Accessory minerals give important clues to geologists when trying to determine details about how a rock formed and how it changed over time. They can make up a substantial portion or a fairly insignificant portion of a rock. Some accessory minerals make up a sufficient portion of the rock to be included as a modifier in the name, such as “biotite syenite”. Adding such a modifier gives geologists quick and useful information about how this rock differs from standard syenite.
Continuing with our previous theme “Sharkansas”, this week’s geo-pic is on Arkansas corals. Of course, corals don’t live in Arkansas today, but from about 480 million years ago, up until roughly 40 million years ago, coral would have been a fairly common sight in the natural state.
The picture above is of a tabulate coral: a now-extinct variety of colonial coral. Each hexagonal corallite chamber housed a simple, individual animal, called a polyp, that could protrude and retract to filter food from the water. The chambers in this fossil are in-filled with the mineral calcite, but that occurred after the coral died and was incorporated into the rock. It was photographed in the Ozark Plateaus, in the Prairie Grove Member of the Hale Formation.
Other varieties of coral are found in the rocks of Arkansas. For more views of Arkansas corals click here
You wouldn’t know it to look around now, but Arkansas, at times in the distant past, was teaming with sharks (and other marine fish). Indeed, Arkansas was in part or wholly covered by ocean many times in the past. One such time was 250 million years ago, during the Carboniferous Period. The fossilized Cladodus tooth pictured above belonged to a primitive shark that had sharp teeth with multiple points of varying size that it used to gig fish before gulping them down. The long point at the middle of the tooth is broken off and displaced to the right in this picture.
This particular specimen was found near West Fork, Arkansas. It was collected from the Prairie Grove Member of the Hale Formation, a limey sandstone. However, shark teeth can be found locally, throughout other parts of the state, in marine rock layers spanning hundreds of millions of years.