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.
(FOV approx. 2 mm, photo courtesy of Stephen Stuart)
The wedge-shaped crystal in the photo above is the mineral titanite. This calcium titanium silicate (formula CaTiSiO5) is commonly found as an accessory mineral in igneous intrusions similar to those present at 3M and Granite Mountain quarries near Sweet Home. This sample was collected from 3M Quarry.
Titanite gets its name from its titanium content, but it was more commonly known by the name “sphene” until 1982 when the new name was officially adopted by the International Mineralogical Association. Sphene was derived from the Greek word “sphenos”, meaning wedge.
Crystals of titanite have a higher dispersion than diamonds. Dispersion is the measurement of refractive properties of a gemstone. The higher the dispersion, the more “sparkle” from the gem. However, gem quality samples of titanite are very rare, and the mineral is relatively soft compared with other gemstones.
Pictured above is a little piece of geologic history known as a basal conglomerate. that’s a rock formed after a period of erosion that marks the boundary between two geologic time periods: in this case, the Mississippian (359-318 million years ago) and the Pennsylvanian (318-299 million years ago).
318 million years ago sea level subsided, bedrock was exposed, and the Mississippian Period came to an end. When exposed to erosion at the earth’s surface, pieces break off from bedrock. Flowing water in rivers, streams and oceans wears the edges of those rock fragments till they’re rounded. Once ocean level rises and deposition resumes, the rounded gravel gets mixed with newly accumulating sediment and forms a rock which is made partly of fragments of the older bedrock. Geologists call this type of rock a basal (at the base) conglomerate (containing round gravel) because it is the first bedrock signaling the beginning of a new period of geologic time.
The photo above illustrates surface iridescence. Iridescence is a play of colors caused by the interference of light waves. This phenomenon is also called thin film interference. You have probably seen this effect on soap bubbles and oil sheens. Light reflecting from a thin coating of iron oxide on the piece of novaculite above is producing the play of colors. Light waves are reflected from the top of the iron coating and the base of the iron coating producing multiple waves. A color is seen when the waves interfere constructively. The resultant color is dependent on the thickness of the coating and consequently, streaks and bands of differing color develop since the thickness of the iron oxide coating varies. The colors also change when the angle of reflection is changed.