A sinkhole is an area of ground that has no external surface drainage. Water that enters a sinkhole exits by draining into the subsurface. Many people are leery of sinkholes because of the damage they sometimes cause. Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake.
Not every kind of sinkhole is the dangerous kind though. The picture above shows a solution sinkhole. Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression. Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground.
In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone. These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.
Travertine is a common feature in the northern Ozarks and along the Buffalo River due to the abundance of soluble limestone there. Common in caves (stalactites, stalagmites), travertine forms by the precipitation of minerals from ground water. In the example above, it formed on the face of a bluff, giving the bluff a melted appearance.
Like limestone, travertine is composed of the mineral calcite which dissolves if exposed to acid. When rain falls, it picks up CO2 from the atmosphere and soil, and becomes slightly acidic. It then flows underground through the bedrock dissolving some limestone along the way. When the groundwater re-surfaces at a spring or seep, The pressure drops, forcing the CO2 out of the water. The loss of CO2 lowers the waters acidity; It can no longer hold the calcite in solution, and calcite precipitates as the sedimentary rock travertine.
Turkey fat smithsonite is the common name for the variety of zinc-ore mineral pictured above. It derives its name from its yellow color and globular crystal habit (called botryoidal which is Greek for bunch of grapes). The yellow is due to the presence of cadmium. Early miners likened its appearance to the fat of a delicious turkey.
Smithsonite, along with other lead- and zinc-bearing minerals, was mined in the lead/zinc districts of the northern Ozark Plateaus and the western Ouachita Mountains. Production started in 1857 and ended in 1962, with the peak occurring during WWI. Though mining has ceased, it’s estimated that 110,000 short tons of shallow resources remain, and significant deeper deposits may, as of yet, have gone undiscovered.
Happy Thanksgiving from the Arkansas Geological Survey!
Above is the fossil remains of a marine organism called a conical nautiloid. These were common in the shallow sea that covered Arkansas during much of the Paleozoic era 540 to 250 million years ago. This one was collected from shale of the Pitkin Formation in pieces over a period of years as it weathered slowly out of the outcrop.
Conical nautiloids are extinct now, but they are the evolutionary forerunner of the sleeker, more deft coiled nautiloids that thrive in the ocean today. They were marine predators, akin to squid, except that they had a shell. They used their shells for protection, buoyancy, and as a means of propelling themselves through the water by squeezing a stream of water out of the empty chambers like a jet.
It’s common to find conical nautiloid fossils that are smaller than a pinky finger; however, some have been unearthed in Arkansas that were as long as 8 feet.
Above is a commonly misunderstood geologic feature called an Asterosoma. Almost everyone, upon first seeing an Asterosoma, thinks it’s either a fossilized plant, flower, or some kind of fossilized animal – usually an octopus.
Asterosomas are actually trace fossils left behind by ancient marine animals (most likely worms or shrimp) that burrowed through mud in a delta or tidal-flat. This one was found in the Carboniferous section of north Arkansas and is roughly 300 million years old. These trace fossils are called Asterosoma because of their star-like shape.
In cross-sectional view, multiple Asterosomas sometimes overlie one another connected by a central vertical tube – like a garland of Asterosomas. This suggests that, as new sediment was periodically washed into the environment, the animal may have burrowed its way back to the top of the mud and wallowed out another home for itself. The animal itself was too soft-bodied to be preserved in the rock record.