Geo-pic of the week: Pyrite

pyrite

(FOV approx. 3 mm, photo by Stephen Stuart)

Pyrite, also known as Iron Pyrite (FeS2), is the most common sulfide mineral. Its most frequent crystal structure is cubic, as seen in the picture above. It also forms octahedral (8 sided) and dodecahedral (12 sided) structures. Its brassy-yellow color and metallic luster can sometimes cause it to be mistaken for gold, hence the nickname “fool’s gold”. While it may look like gold, it is much lighter and harder. Typically pyrite cannot be scratched with a knife.

Pyrite is found in many counties in Arkansas. It is used in the production of sulfuric acid, although its use is declining. The primary value of this mineral currently is as a collectible specimen. Individual crystals are commonly found up to 1 inch in diameter.

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Geo-pic of the week: Veins

Ron Colemans Quartz Mine, quartz veins, truck, CStone, 18 Jun 02

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.      

Geo-pic of the week: Herringbone Cross-Bedding

 

Crossbedding

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.    

Geo-pic of the week: Tempestite

tempestite

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.

Geo-pic of the week: Oncolite

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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.

Sandstone Paleokarst

If you have spent any time on Beaver Lake in northwestern Arkansas, then you have probably seen sandstone paleokarst features.  Some stand tall like towers while others appear to be irregular to rounded masses.  It is common to see only the tops of these features when the lake level is low to normal.

 ss paleokarst photo    Top of sandstone mass in Beaver Lake.  Photo taken in October, 2016.

ss paleo 2-01  Sandstone mass along Beaver Lake.  Photo taken in October, 2016.

These features have been in geology literature since 1858 when David Dale Owen made his first geological reconnaissance of the northern counties.  He described a mass of isolated sandstone within adjacent magnesian limestone (now called dolostone) which stands out forming a conspicuous feature in the landscape.  Purdue, 1907, called them cave-sandstone deposits and was the first to consider them paleokarst.  Purdue and Miser, 1916, noted many of these deposits and concluded several were ancient sinkholes that had been filled with sand.  Two theses that pre-date the construction of Beaver Lake (Arrington, 1962, and Staley, 1962) mention numerous sandstone bodies within the Powell.  One very large sandstone mass was seen in the White River (Arrington, 1962).  It is approximately 45 feet tall!  Unfortunately, it is now covered with water.

photo       Sandstone mass in Carroll County.  From Owen, 1858

photo2 Sandstone mass in the White River near Hwy 12 access to Beaver Lake.  From  Arrington, 1962.

So how did these features form?  First, let’s define paleokarst.  Paleokarst consists of karst features that formed in the geologic past and were preserved in the rock record.  Karst features include sinkholes, springs, and caves.  These features form when acidic rain and ground water dissolves carbonate rocks (mainly rocks that contain calcium carbonate – calcite, or calcium-magnesium carbonate – dolomite).

The majority of sandstone masses are surrounded by dolostone, composed of dolomite, in the Powell Formation.  The Powell is Lower Ordovician in age, meaning it formed around 470 million years ago (mya).  It is likely that this formation was exposed to weathering at that time.  Depressions of various size, called sinkholes, developed on the exposed land surface.  Later, sand filled the depressions and eventually became rock called sandstone.  The age of the sandstone masses ranges from Middle Ordovician (approx. 450 mya) to Middle Devonian (approx. 390 mya).  Therefore, there is a gap in the rock sequence, between dolostone in the Powell and the sandstone, called an unconformity, lasting from 20-80 million years.

ss mass 3-01Sandstone mass (center) surrounded by Powell dolostone along Beaver Lake.  Photo taken in September, 2016.

Why is paleokarst important, other than being interesting features to observe?  Paleokarst provides clues to former geologic conditions and changes in climate and sea level (Palmer and Palmer, 2011).  We know that sea level was high in the Lower Ordovician and shallow seas covered all of northern Arkansas.  But, in the Middle Ordovician, sea level lowered and the sandstone paleokarst features provide additional evidence supporting this change.

Many sandstone paleokarst features were located while mapping the War Eagle quadrangle.  Fifty-two sandstone masses were located around Beaver Lake.  This is not a complete list, however, since the main focus of mapping was not a paleokarst inventory.

paleokarst points    Sandstone masses (yellow) located from recent geologic mapping around Beaver Lake.

The War Eagle quadrangle was mapped in preparation for State Park Series 4 – Geology of Hobbs State Park.  Follow the link below to see the geologic map of the War Eagle quadrangle:  http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/War%20Eagle.pdf.

Until next time,

Angela Chandler

 

References:

Arrington, J., 1962, The geology of the Rogers quadrangle:  University of Arkansas M.S. Thesis, 61 p.

Palmer, A.N., and Palmer, M.V., 2011, Paleokarst of the USA:  A brief review:  in U.S. Geological Survey Karst Interest Group Proceedings, Fayetteville, Arkansas:  U.S. Geological Survey Scientific Investigations Report 2011-5031, p. 7-16.

Owen, D.D, 1858, First report of a geological reconnaissance of the northern counties of Arkansas made during the years 1857 and 1858:  Little Rock, 256 p.

Purdue, A.H., 1907, Cave-sandstone deposits of the southern Ozarks:  Geological Society of America Bulletin, vol. 17, pp. 251-256.

Purdue, A.H., and Miser, H.D., 1916, Geologic Atlas of the United States, Eureka Spring-Harrison Folio, Arkansas-Missouri:  U.S. Geological Survey Folio No. 202, 82 p.

Staley, G.G., 1962, The geology of the War Eagle quadrangle, Benton County, Arkansas:   University of Arkansas M.T. Thesis, 56 p.

 

 

 

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.