Tag Archives: Geology

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.

Buttress on Little Buffalo River near Parthenon

Notes From the Field: Boone Buttresses

Buttress on Little Buffalo River near Parthenon

Buttress on Little Buffalo River near Parthenon, Newton County

The photo above shows an unusual rock column located near Parthenon in Newton County.  Judging from the man standing at the base, it is probably over 100 feet tall.  Recently, I was asked what to call these impressive features.  The term we’ve used at the Survey is buttress, which is defined by the Glossary of Geology as a protruding rock mass on, or a projecting part of, a mountain or hill resembling the buttress of a building; a spur running down from a steep slope.  Example: a prominent salient produced in the wall of a gorge by differential weathering.  We’ve used the term buttress, instead of other terms like pinnacle or rock pillar, because these terms refer to a free-standing column of rock, whereas a buttress is, at least nominally, attached to the bluff line.  The term also differentiates these particular features from others that are similar in shape, such as pedestals or hoodoos, which typically form in clastic rocks like sandstone and siltstone.  Their development is controlled by joints, which are planar fractures with no displacement, and by the presence of a resistant caprock, which acts to protect the underlying, less-resistant rock from weathering as quickly.  This process leads to a characteristic shape that is wide at the top and narrower below.

Sandstone pedestal at Pedestal Rocks, Pope County

Sandstone pedestal at Pedestal Rocks, Pope County

A buttress, on the other hand, is typically either uniform in diameter or may taper slightly towards the top, probably because they develop in fairly homogeneous rock.

Buttresses are known to be present in two locations in Arkansas: along the Little Buffalo River near Parthenon in Newton County and along Bear Creek near Silver Hill in Searcy County.

dscn6238

Buttresses on Bear Creek, Searcy County

They are all developed in the Mississippian Boone Formation which averages about 320 feet in thickness, and is composed of interbedded limestone and chert.  Limestone is dissolved by slightly acidic surface and groundwater, and over time this process leads to many unusual surface and subsurface features known as karst.  Buttresses are one such feature.

The exact mechanism for their development is poorly understood, but some of the factors that contribute to their formation are known.  First, dissolution of limestone can produce similar shapes on a small scale, as seen in this photo of coarsely crystalline Fernvale Limestone in a creek bed.

Dissolutioned limestone in creek bed, Stone County

Dissolutioned limestone in creek bed, Stone County

This process may be all that is needed to produce the buttresses at a larger scale.  Second, all rock units have planes of weakness due to the regional history of tectonic stress.  This stress is usually expressed as a joint system, and is one of the most commonly observed structural features in an outcrop.  Observations at these two sites have shown that jointing is poorly exposed, but as you can see from the aerial photograph on Bear Creek, weathering of the buttresses roughly aligns with the most prominent joint trends in the area (N/S and NE/SW) as indicated by the joint diagram from the Geologic Map of the Marshall Quadrangle.

Aerial view of buttresses on Bear Creek showing prominent regional joint orientationsMarshall Rose

Aerial photo of buttresses on Bear Creek showing prominent regional joint orientations

So even though the joints are poorly developed, one can interpret that pathways for water preferred these orientations, enlarged them over time, and left the buttresses as erosional remnants. 

However they occur, they are certainly beautiful rock formations and worthy of further study.

Buttresses on Bear Creek, Searcy County

Buttresses on Bear Creek, Searcy County

Many thanks to Angela Chandler for the featured image!

Richard Hutto

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: Pinnacle Mountain

Pinnacle closeup edited (1)

This is a picture of Pinnacle Mountain – one of several steep-sided hills up to about 1000 feet tall, located at Pinnacle Mountain State Park, northwest of Little Rock, Arkansas.  Though its appearance may be misleading and its origin is debatable, Pinnacle Mountain is not a volcano!

What’s intriguing about Pinnacle and the smaller nearby hills is that they’re sandstone lenses surrounded by shale.  That, from a geologic perspective, is difficult to explain since sedimentary rocks are suppose to form in layers, not lenses or blocks.  This has led geologists to a variety of interpretations for their origin ranging from giant undersea landslides, to sand that got trapped in the empty gouges left by large undersea landslides, to beds of sandstone caught up and scrambled with shale beds along huge thrust faults.

Whatever their origin, because they are sedimentary rock and contrary to some satirical publications I’ve seen circulating online, they are not volcanoes and they are not going to blow up!