Category Archives: #geoblog

Geo-pic of the week: Pennsylvanian plant fossil

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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.

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Geo-pic of the week: Folded Rock In 3 Dimensions

 

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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. 

Geo-pic of the week: Dogtooth Calcite

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(FOV approx. 10 cm, photo by Corbin Cannon)

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.

Geo-pic of the week: Titanite

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(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.

Geo-pic of the week: Basal Conglomerate

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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.

Geo-pic of the week: Slickensides

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The grooved surface pictured above is a slickenside.  Slickensides indicate the relative direction of movement between fault blocks (hanging wall moved up, down, laterally, etc..).  

Slickensides form when fault blocks move against each other.  The natural irregularities on each scratches grooves into the other.  The grooves are parallel to movement;  for instance in this example, movement was either to the right or the left.  To tell whether it was right or left, you can rub your hand along the slickensides.  They feel smooth in the direction the fault moved and rough in the opposite direction – it’s like petting a dog from tail to head.  Slickensides are a valuable tool because determining fault movement can be a challenge when there are no easily-recognized beds that can be correlated across the fault to show the sense of offset.

The shale above was photographed in Big Rock Quarry, North Little Rock, AR.  It’s a part of the Jackfork Formation (Pennsylvanian).

Geo-pic of the week: Syn-depositional Faulting

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Pictured above is one of many faults, closely spaced together, in an outcrop of the Atoka Formation, near Lake Fort Smith, Arkansas.  The fault pictured extends from the upper right to the lower left and is highlighted.  This type of faulting is called syn-depositional faulting, meaning it occurred at about the same time the rock was being deposited.  It results in disturbed-looking outcrops like this one.

Around 300 million years ago, plate tectonic forces were deforming the Ouachita Mountains in south central Arkansas. Those forces also caused faulting in the southern Ozark Plateaus, as the sediment that composes this rock outcrop was being deposited.  The freshly deposited sediment wasn’t fully consolidated when the faulting took place and the rock surrounding the fault got contorted by the stress. 

Some of the deformed features of the outcrop are labeled above.  The Zone of Soft-Sediment Deformation is the area surrounding the fault where the rock has been deformed by shearing: there is no recognizable bedding in that zone.  The soft clay-rich Deformed Shale was squeezed plastically between the fault blocks in that soft sediment deformation zone.  The bedding orientations surrounding the deformation zone (indicated by magenta lines) vary greatly, because the soft bedrock was broken and heaved around by the fault.