Why do rocks have beds? Are rock beds where geologists sleep? Sometimes, but that’s not the point of this article. The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers). The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.
Here’s an example; when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes. Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit. That weathered surface will eventually differentiate the flood deposits into distinct beds of rock.
Bedding can also form as a result of flowing water gaining or losing velocity. The size of sediment that water carries (and eventually deposits) is directly related to flow rate. A sudden change in flow rate creates bedding distinguished by differences in grain size.
Everyone in the photo above was eventually air-lifted to safety… Just kidding! They’re still up there
Ooids are tiny grains that are typically composed of CaCO3 either as calcite or aragonite. They precipitate from seawater in concentric bands around a nucleus (for instance a fragment of rock or fossil) in turbulent shallow conditions.
Once ooids form, they can accumulate and be cemented to form a sedimentary type of limestone called oolite. The above picture is a magnified and tumbled piece of oolitic chert collected fromgravel on Crowley’s Ridge in northeast Arkansas. The difference between this and typical oolite is that it came into contact with silica(SiO2)-rich ground water after it formed. The SiO2 then replaced the CaCO3 the rock was initially composed of. The polished surface provides an ideal view of the internal structure of the spherical ooids.
If you live in Arkansas, chances are you’ve heard of the Ozark Mountains. Actually, the correct geologic term is Ozark Plateaus. Unlike typical mountains in which the bedrock has been squashed and folded, the Ozarks are one broad dome-like structure made up of flat-lying sedimentary bedrock. The hills and valleys of the Ozark topography are the result of rivers carving into this dome, rather than compression or deformation.
The picture above was taken overlooking the Buffalo River. The various hills, from the foreground to the distance, are roughly the same height. Of course they are! If not for this and other rivers, the landscape pictured here would be one solid flat surface, as tall as the highest peaks in the picture, stretching to the horizon.
Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years. Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.
In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures. These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment. The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.
In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock. This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.
To see more views of skolithos traces from Arkansas click here
A tempestite, like the one pictured, is a rock composed of debris deposited by a storm. It’s mostly a sandstone but also contains various fossils, pebbles, and other clasts that were picked up and tossed about by the waves.
Waves are generated as wind energy is transferred to water. Naturally, during a storm, waves are bigger and more energetic. This increased energy allows the waves to pick up, and in some cases rip up, various relatively large clasts and fossils and transport them. The large elongate fossil above is an extinct squid-like creature known as a conical nautiloid. Other marine fossils in this sample include gastropods, and crinoids. It also contains plant material.
The presence of tempestites in a rock outcrop indicate the area was a shallow marine environment when those rocks were being deposited. This sample was collected in Northwest Arkansas from the Pennsylvanian Prairie Grove Member of the Hale Formation.
Above is several pictures of an unidentified plant fossil found in NW Arkansas this past week in the Dye Shale Member of the Bloyd Formation. The fossil is mostly pyrite with an outer coating of calcite (gray crust). It was found in a shale unit and the original plant, or tree, has been squashed by the weight of sediment above it.
At just over 6 feet long and less than an inch thick, it’s an unusually well preserved fossil, especially considering the Dye Shale isn’t known to contain many fossils. It’s also a marine unit and this is certainly a terrestrial plant. Perhaps it was washed in to the environment during a storm and rapidly buried, which led to its preservation. There are no obvious places where branches or leaves might have attached to the trunk and it has a distinct bark pattern that is unlike the well-known plants of the Pennsylvanian Period, such as lycopods, Lepidodendron, or Calamites.
If any fossil savvy readers have a suggestion for its identity, feel free to pass it along. Otherwise, we’ll keep looking into it.