The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.
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
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.
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.