Category Archives: Sedimentary Features

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.

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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.

Geo-pic of the week: Mineralized Vug in Chert

mineralized vug in chert

Pictured above is a mineralized vug (approximately 3 inches long) in chert.  A vug is a void or open space in a rock.  Many vugs are filled with minerals after water that is saturated with a certain mineral flows through the rock. This mineralization can happen in multiple stages. The vug above was initially filled with silica-rich fluid therefore quartz precipitated out of solution and lined the walls of the vug.   Afterwards calcite precipitated, as is evident from the larger crystal on the interior left of the vug. 

This vug is present in a section of ornamentally banded chert.  Chert is a sedimentary rock made up of microcrystalline quartz.  It can be a variety of colors or banded and quite beautiful.  The chert above is Devonian age (416-359 million years ago) from northwest Arkansas.

Geo-pic of the week: Calcite-filled Tabulate Coral

tabulate coral

Continuing with our previous theme “Sharkansas”,  this week’s geo-pic is on Arkansas corals.  Of course, corals don’t live in Arkansas today, but from about 480 million years ago, up until roughly 40 million years ago, coral would have been a fairly common sight in the natural state.

The picture above is of a tabulate coral: a now-extinct variety of colonial coral.  Each hexagonal corallite chamber housed a simple, individual animal, called a polyp, that could protrude and retract to filter food from the water.  The chambers in this fossil are in-filled with the mineral calcite, but that occurred after the coral died and was incorporated into the rock.  It was photographed in the Ozark Plateaus, in the Prairie Grove Member of the Hale Formation.

Other varieties of coral are found in the rocks of Arkansas.  For more views of Arkansas corals click here

Geo-pic of the week: Pebble Molds

pebbles-great(photo courtesy of Angela Chandler)

The sedimentary rock in the picture above is a sandstone with pebble molds. If the pebbles were present, this rock would be considered a conglomerate. Conglomerates consist of 2 mm or larger rounded fragments of rock, or clasts, surrounded by finer-grained sediment which geologists call “matrix”. The clasts in the rock above were pebble sized, 2-64 mm, and the matrix is sand sized.

Even though many of the clasts have been removed by erosion, we can tell that they were primarily shale pebbles. The sandy matrix was more resistant to erosion than the softer shale pebbles, so we are left with cavities where the pebbles were (pebble molds) on the rock’s surface. This creates an interesting optical illusion. Did you see the cavities as pebbles or as molds when you first looked at the picture?

This type of conglomerate is deposited by energetic and dynamic water, such as is found in rivers and waves. During higher flow periods, only large clasts are deposited. When flow is lower, finer-grained sediment settles in between the larger clasts.

Geo-pic of the week: Zinc Ore, Rush Creek Mining District

sphalerite and dolomite (1)

Zinc ore collected in 1943 from the Rush Creek Mining District, Marion County, Arkansas.  The brown mineral is sphalerite: an ore of zinc.  The pink mineral is dolomite – it’s pretty, but not economically valuable.   They were both deposited on the gray dolostone; you can just make it out on the right, in back. 

Zinc deposits are found throughout northern Arkansas, commonly with the lead mineral, galena.  They’re most abundant in Marion County, in a two mile stretch of rugged terrain, along Rush Creek, where 4 faults come together.  That area was mined for lead and zinc in the late 1800s and early 1900s.

It’s typical to find rich ore deposits in rock that’s been fractured by faulting.  The fractures facilitate migration of mineral-rich ground water which deposits the ore minerals in the fractures.  It’s hard to see in the picture, but the fractured dolostone rock, in this specimen, is bound together by the sphalerite and dolomite minerals.

Geo-pic of the week: “The Great Little Rock Silver Rush”

 

Argentiferous galena enhanced

In September of 1982, this 800 lb. boulder was excavated by a backhoe operator during construction of the La Quinta Inn on Fairpark Boulevard (currently Day’s Inn), Little Rock, AR. Another worker on site recognized it as galena (ore of lead) and, wanting to score some quick cash, the finder began contacting local geologists, hoping to sell. Eventually, then State Geologist, Bill Williams, heard about it and sent another geologist from the Arkansas Geological Survey (AGS), Ben Clardy, to investigate. Clardy bought the boulder for $100 and the backhoe operator loaded it onto Clardy’s truck for transport back to the AGS office.

At the office, an engine lift was rented to remove it. The agency’s chemist, Gaston Bell, assayed a piece for silver, determining it contained 1 – 2 %, making it high-grade silver ore. Feeling he had cheated the seller, Clardy contacted him with the results but the seller was happy with the $100 deal. The State Geologist reimbursed Clardy and placed the specimen on display in the lobby of the AGS office.

News of the find spread quickly, as the story was picked up by local newspapers. Someone claiming to be the hotel property owner announced that the backhoe operator had stolen the rock and came to the AGS office demanding it back. It was now property of the state, but Bill Williams told him he could have it, as long as he could bring some large guys to carry it off; he didn’t want heavy equipment in the office lobby. The man left and never raised his claim to the rock again. A couple days later another piece was found on the property in the same mineralized pocket which was at the intersection of two quartz veins. The property owner took possession of that piece and sent it to Colorado where it was smelted and produced a substantial silver bar.

Around the same time, as the public became aware of the find, some midnight rock poachers began sneaking onto the property, after hours. Small chunks of galena appeared around town for sale, being marketed as “Little Rock Silver Ore”.

At least one silver company took an interest in the find, conducting a series of soil tests over several blocks surrounding the La Quinta property. They soon abandoned the effort due to the difficulty of mining in such an urbanized area. Results of their tests were never disclosed. Eventually, construction of La Quinta was completed, the lot was paved over, and thus ended the “Little Rock Silver Rush”.

The original 800 – pound chunk is still on display in the lobby of the AGS office in Little Rock. Part of the other piece, which was not melted down, was displayed in the lobby of the La Quinta Inn on Fairpark Boulevard before the property changed hands.

Based on written correspondence with Michael J. Howard