Any rockhound worth their salt knows that the best place to hunt for interesting minerals is in the void spaces in rock. Void spaces come in two types; vugs and veins. Vugs are usually found in igneous rock and result from trapped gas bubbles. Veins, on the other hand, can be found in any type of bedrock.
Veins are fractures, that have been plugged with minerals, typically by precipitation from circulating water. The above picture was taken in the Ron Coleman quartz mine, near Hot Springs, Arkansas. The near-parallel white streaks that riddle the sandstone are quartz-filled veins. The fractures resulted from the intense deformation of the Ouachita Mountains, by plate tectonic forces, around 300 million years ago. That deformation opened up space for quartz to grow in, and the tremendous heat and pressure from the mountain-building generated the mineral-rich fluid that deposited the crystals.
Pictured above is sandstone displaying classic herringbone cross-beds. Cross-bedding results from either sediment transport by flowing water, such as in this example, or by wind flow, as in the case of dunes.
Cross-beds form by the migration of sediment, and tilt in the direction of flow. As sediment grains are carried by the current, they migrate up the gentle ramp of previously deposited cross-beds. When they reach the end, they tumble down the steeper face there and are deposited to become part of the next cross-bed. In this way the sediment migrates in the downstream direction.
Each group of similarly tilted cross-beds is known as a set. In herringbone cross-bedding, the sets are oriented contrarily, which gives the outcrop a fishbone appearance. These differently oriented cross-bed sets indicate changing flow directions.
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.
Oncolite is a limestone made of oncoids, the roundish, tan things in the picture (average size less than an inch). Oncoids are made by microbes called cyanobacteria. Cyanobacteria, which also form larger mounds called stromatolites, are thought by many scientists to be one of the earliest forms of life to evolve on Earth.
The microbes attach to a nucleus – in this case fossil fragments – and encrust it in layers of calcium carbonate. The bacteria gather energy by photosynthesis and, thus, require access to the sun. Because they are easy to recognize and mostly limited to shallow marine environments, oncolites are useful to geologists, both as a stratigraphic marker and as an indicator of the depositional environment of the rock they are preserved in.
These were photographed in the Kessler Limestone Member of the Bloyd Formation, northwest Arkansas.
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.
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.
Even though they might look like it, those crystals in the picture above didn’t come out of a dog’s mouth. They are crystals of dogtooth calcite. Calcite (CaCO3) is the primary mineral that makes up limestone. It occurs in several crystal shapes. The two most commonly found in Arkansas are 6 sided rhombohedrons and the scalenohedral shape you see above. When it forms in this scalenohedral crystal structure it is called “dogtooth spar”.
Calcite is a very common mineral, but this particular crystal form of the mineral is typically only found in Arkansas in conjunction with the minerals sphalerite (zinc ore) and galena (lead ore) in the lead and zinc districts. Calcite is also a polymorph, like the mineral brookite from a previous geo-pic. This means calcite has “sister” minerals with the same chemical composition, but differing crystal structures. The three polymorphs of CaCO3 are: calcite, aragonite, and vaterite.