Like the White Cliffs of Dover, England, the “White Cliffs of Arkansas” (pictured above) are composed of chalk. Chalk is a marine sedimentary rock that forms of calcite-rich mud that accumulates in semi-deep marine environments. The mud is composed of the accumulated skeletal remains of algal microorganisms called coccolithophores. These algae grow and shed skeletal parts called coccoliths which they arrange around them, in life, in a structure called a coccosphere. Below is a scanning electron microscopic image of some coccospheres (borrowed from news.algaeworld.org).
Chalk in Arkansas is found in the Annona Formation, which formed in the late Cretaceous Period, and crops out in southwest Arkansas as well as parts of Texas. In addition to being mined to make blackboard chalk, this resource is also used in brick, and cement manufacture.
100 million years ago, during the Late Cretaceous Period, a preponderance of igneous activity occurred in the continental region now known as Arkansas. In fact, all of the igneous rocks discovered in the state were emplaced around that time. Some of them are well known, such as Magnet Cove, located east of Hot Springs, or the diamond-bearing intrusion near Murfreesboro. There are also lots of smaller igneous intrusions like the one shown in the picture above.
Small igneous intrusions are found throughout the Ouachita Mountains. There are so many small intrusions that new ones are regularly discovered. Weathering at the earth’s surface has typically destroyed the original rock’s characteristics and what remains is mostly soft clay because the minerals that make up the intrusion are unstable under surface conditions.
If you happen to notice an unusual-looking body of rock that cuts across the strata of a road cut or other rock outcrop when you’re exploring the Ouachita Mountains, it’s likely that you have seen a Cretaceous igneous dike.
The Geologic Road Guide to Arkansas State Highway 10, a Geotour of the Southern Arkoma Basin Fold Belt and Related Ouachita Mountain Tectonic Zones by Drs. Richard Cohoon (Emeritus), Jason Patton (Associate), and Victor Vere (Emeritus), Professors of Geology at Arkansas Tech University, is now available for download on the Arkansas Geological Survey’s website. Here’s the link:
The route begins at Petit Roche Plaza in the River Market District of downtown Little Rock. “Petit Roche” was the name given to the first rock outcrop early explorers encountered on their way up the Arkansas River. It is near this outcrop that the eastern end of Arkansas State Highway 10 (AR-10) is now located. From here, you will tour the 139-mile length of AR-10 to its western terminus at the Oklahoma state line, just past Hackett. This route traverses a beautiful and geologically diverse cross section through the mountains of western Arkansas. The stretch from Ola to Hackett is designated as an Arkansas Scenic Byway.
An overview of the physiography of Arkansas, the concept of geologic time, and the rock formations and structural regions encountered along AR-10 introduce the reader to the detailed Road Guides that follow. The Road Guides describe the rock outcrops and geologic features along particular sections of the route. They contain many wonderful color photographs and color-coded geologic maps to help travelers understand the landscape passing outside their windows. Travelers are encouraged to get out of their vehicle at several places to have a look at the rocks, perhaps gaining a new appreciation of their significance. An illustrated glossary defines words and concepts that may be unfamiliar to those without an earth science background. Appendices direct the traveler to several interesting side trips just off the main route and detail the characteristics of the gas and coal resources in the Arkoma Basin.
This Geotour is written to be of interest to the general public, to students of geology, and to professional geologists who want to gain a more in-depth understanding of this beautiful and geologically complex region. So the next time you’re thinking of taking a scenic drive through the mountains of western Arkansas, consider traveling AR-10. And don’t forget to take along the Geologic Road Guide to make your drive more enjoyable and informative.
Above are several images of the same rock sample: a highly deformed quartzose siltstone collected from the Womble Formation, Ouachita Mountains, Arkansas. The uppermost image shows a cut and polished surface. The green line that’s been added to the picture defines a fracture that split the sample after it was cut. Ordinarily, that would be a bitter turn of events but, in this case, it was a fortunate accident. The fracture provides a rare, multi-dimensional view inside a tightly folded rock (lower photo). Luckily, the fracture propagated across the bedding rather than breaking along a bed, which makes the beds of the fold appear to fan out like a deck of cards showing a lot of the detail of the structure.
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.
In this photo we are looking at rock beds, tilted till they are nearly vertical, and exposed in three levels in a quarry near Kirby, Arkansas, Ouachita Mountains. Like a humongous 3-step staircase, each ascending level of the outcrop provides a deeper view into the rock formation. An outcrop like this one illuminates a couple of basic but important concepts of geology: key beds, and strike and dip.
The key beds or beds that can be traced across the outcrop, such as the one marked with red dots above, appear to shift to the right as your eyes ascend the steps. These are not faults! It’s an optical illusion. If our view were aligned parallel with the sides of the beds, they would appear aligned, but our view is actually diagonal to the bedding. To illustrate this, here is the same picture with a drawing of the key bed as if it were jutting out of the outcrop.
This is why geologists measure the orientation of rock beds – known as the bed’s strike and dip. Knowing how a bed is oriented in one place can help you to predict where it will be in another, perhaps inaccessible, place such as deep in the subsurface. If that bed is full of oil, gas, or other precious commodity, predicting where it is becomes very important.