Category Archives: Sedimentary Features

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

Geo-pic of the week: Pyritized Ammonoid

ammanoid cropped

Pictured above is the internal mold of an ammonoid fossil – a group of invertebrate marine animals abundant in the world’s oceans from 416 – 66 million years ago.  They died during the same mass extinction that killed the dinosaurs.

Ammonoids were not stationary bottom dwellers, but had an interesting way of getting around in the water.  Their shells were partitioned into chambers, which are evident in the picture above.  The squid-like ammonite only occupied the final chamber of the shell.  The rest were empty so that the animal could control its buoyancy, and swim by taking in and expelling water.

Because ammonoids were abundant, widespread, and evolved new species quickly, geologists use their fossils to correlate rock units of similar age worldwide.  This one was collected from the Fayetteville Shale in northwest Arkansas.  Its gold color is due to the original organic material having been replaced by pyrite – also known as fool’s gold.

Geo-pic of the week: Zebra Weathering

Zebra weathering enhanced

Pictured above is an exposure of Prairie Grove Sandstone near Durham, Arkansas, southeast of Fayetteville.  The ribbed, planar faces that are central in the photo resulted from a weathering phenomenon called zebra weathering.

Zebra weathering occurs in sandstones cemented with calcite – a soluble mineral.  Calcite is common in marine sediment and, in the tidal environment where this rock was deposited, marine sediment mixed with insoluble sand from the continent.   The ratio of marine sediment to sand changed continuously in that environment due to seasonal and climatic cycles.  Today, the beds of sandstone weather at different rates depending on their calcite content.  As the rock weathers, the sandier beds stand out in relief since they wear away more slowly than the soluble beds between them.  Hence, the banded zebra pattern.

Geo-pic of the week: Frankenstein scar on stylolite

Frankenstein boxwork on stylolitic surfaceBoris-Karloff-Frankenstein

I was working near Lake Fort Smith State Park this last week when I came across a peculiar mineral deposit resembling Frankenstein’s Scars (Fig. 2).  It was just in time for Halloween!  The resemblance is uncanny.  Despite the horror, there is a lot of geology illustrated in this rock. 

The mineral that forms the “scars” seen in the photo is called limonite, and it was deposited within a cavity in a stylolite.  A stylolite is a surface, typically a bedding plane, that has recrystallized due to pressure from the weight of overlying rock material.  Stylolites can be recognized by their rough, jagged appearance (it’s difficult to see in this photo, but trust me – it’s there).  The limonite “scars” formed in a pattern called boxwork and, surrounding the boxwork, limonite is also present in botryoidal form: a crystal shape resembling small round globs (the orange goosebumps around the scars).

At this time, rocks are not thought to celebrate Halloween, although more work needs to be done to verify that.

Geo-pic of the week: Conostichus trace fossils

Conostichus topConostichus bottom

Above are two pictures of a trace fossil, Conostichus, from the Ozark Plateaus region of Arkansas.  Like other trace fossils, Conostichus are structures found in sedimentary rock that represent the spot where an animal lived, fed, or travelled.  Despite their abundance, especially in rocks of the Carboniferous period (299 to359 million years ago), it’s not certain what kind of animal made Conostichus, because the animal’s body wasn’t preserved. 

The upper picture is the top of the Conostichus and shows the hole through which the animal entered or exited the structure.  The lower picture is the same Conostichus with the top facing down.  As you can see, they taper and come to a rounded point at the base, vaguely resembling a badminton birdie.  

At present, the most widely accepted theory for their origin is that Conostichus are burrow traces left by Sea Anemone.