Category Archives: Sedimentary Features

Geopic of the week: Skolithos

 

st. pete skolithos

Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years.  Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.

In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures.  These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment.  The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.

In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock.  This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.

To see more views of skolithos traces from Arkansas click here

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: 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.